"2!! . ‘ 4.“: 21.41:?» m ~ y ‘ . . Mm: Rafi * . dumsby, .. Y . , iii" ,. {1.9.5 3.1.513»? 2w. .. , . ‘b «I .I . in “2:56;. 1).» . KEEHI‘Tv y.fi'.-):li .\ 5.... .. . ’ s‘. I. . . . ‘ , V . I .1 :1 I uptfir. . 3...!!! ‘ . .522. we ‘ . A . . .... its .. . V . i lb 3|! .4! .‘i 5%! p 1: 3:...“ u . c . at 1:!" "mm. u an: ”Hun-t" ”.51 . )1 3 p .f‘ but! 5.1.1.4 $3516! I" «Not... . . x: V I .Lvt 90.“: Jun. 1 av...» a Rama“. Lin! Axis: Koch. . ,. . .ssnakflu. llllllll\lllllllllllllllllllll llllllll 1293 01420 This is to certify that the dissertation entitled The Use of Gaseous Ozone to Remediate the Organic Date Contaminants in the Unsaturated Soils presented by I-Yuang Hsu has been accepted towards fulfillment of the requirements for Ph. D . degree in Environmental Engineering ajor professor November, 1995 MS U i: an Affirmative Action/ Equal Opportunity Institution 0-12771 LIBRARY Mlchigan State University PLACE ll RETURN BOXtomnovothbcMckomtmmywtoootd. TO AVOID FINES return on or before dd. duo. DATE DUE DATE DUE DATE DUE WM! THE USE OF GASEOUS OZONE TO REMEDIATE THE ORGANIC CONTAMINANT S IN THE UNSATURATED SOILS By I-YUAN G HSU A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Civil and Environmental Engineering 1995 ABSTRACT THE USE OF GASEOUS OZONE TO REMEDIATE THE ORGANIC CONTAMINANT S IN THE UNSATURATED SOILS By I-YUAN G HSU The use of gaseous ozone for the remediation of unsaturated soils contaminated with organic chemicals was investigated in this study. Using laboratory soil columns, it was found that when air stripping was first applied, followed by treatment with gaseous ozone, the removal efficiencies of trichloroethylene (TCE) from Ottawa sand were not improved compared to that obtained when only air was used. On the contrary, the removal eficiencies of TCE fiom Borden aquifer material and of naphthalene from Ottawa sand columns, were significantly improved when ozonation was used as a polishing step. The kinetics of the heterogeneous reaction of gaseous ozone with soil water, soil organic matter and phenanthrene were investigated in a cycling batch reactor. The rate of ozone decomposition in soil water is described as a first order rate expression with respect to gaseous ozone concentration. The rate constant observed for this reaction is consistent with that determined for ozone self-decomposition in the aqueous phase. The rate expression for the reaction of gaseous ozone with organic matter in moist soil can be described as second order overall, first order with respect to both gaseous ozone and soil organic matter concentrations. The rate of reaction for ozone with the soil organic matter was observed to be independent of the ozone gas pore velocity, indicating that the overall reaction rate is dominated by the chemical reaction rather than by interfacial mass transfer. A shallow packed bed reactor was used to determine the rate at which gaseous ozone reacts with a phenanthrene-contaminated soil Two pseudo-first order kinetic regions with respect to the concentration of phenanthrene were observed. This is thought to be due to the nonuniformity of the surface distribution of solid phenanthrene. A mathematical model was developed to descn’be the in-situ ozonation process in one-dimensional space. The numerical simulations fit reasonably well with the data obtained from the soil column experiments. The simulations suggest that gaseous ozone should be able to penetrate a soil as long as the soil pH was <10. It was also determined that in-situ ozonation should be allowed to continue until the concentration of ozone in the eflluent gas stabilizes, rather than stopping the ozone flow at the point where the contaminant concentration was, theoretically, zero. To my grandfather and grandmother ACIOIOWLEDGMENTS I would first like to express the appreciation to my advisor Dr. Susan Masten for her patience, encouragement, advises and fiiendship for this research and through the period of time I stayed at MSU. I would also like to thank the members in my committee; Dr. Roger Wallace, who encourage me to "look forward and don't be stuck by the current problems which may be easier when you look back later"; Dr. Simon Davies, who guided me to conduct experiments during the early stage of this study; Dr. Sharon Anderson, who always answers my questions for soils. I would also like to thank all my colleagues who have made my studies more enjoyable including; Xianda Zhao, who always has different views in discussions and is also a good tennis partner; I. J. Yao, my oflice mate whom I always borrow beakers fiom; Chien-Chun Shih, a good fiiend and has the same period of time stayed at MSU as mine. I also appreciate Dr. Paul Loconto, Mr. Yan-Lyang Pan and Mr. Joseph Nguyen who helped me to use several instruments during this study. I also like to thank Angela Bandemehr and James Day who work with me in the early stage of this study and help me to understand "English". I would like to express my greatest appreciation to my parents who are so understandable and so kindly to make loan to me, and my grandfather who teaches me from my childhood. Finally, I have to thank my wife, Lin, and the kids, Amy and Andy, who accompanied with me to go through the really very special period. Thanks to all. TABLE OF CONTENTS LIST OF TABLES ......................................................................................................... xi LIST OF FIGURES ....................................................................................................... xii NOMENCLATURE ..................................................................................................... xiv CHAPTER 1 INTRODUCTION AND THE PROBLEM SCOPE 1.1 Introduction ............................................................................................................. 1 1.2 Objective and Scope of the Study ............................................................................. 3 1.3 List of References .................................................................................................... 5 CHAPTER 2 THE USE OF GASEOUS OZONE FOR THE REMOVAL OF RESIDUAL VOLATILE ORGANIC CONTAMINANT S FROM UNSATURATED SOILS 2.1 Introduction ............................................................................................................ 6 2.2 Experimental Work .................................................................................................. 7 2.2.1 Column Preparation .............................................................................................. 7 2.2.2 Venting Procedures ............................................................................................... 8 2.2.3 Analytical methods .............................................................................................. 10 2.2.3.1 Determination of the Concentration of Organic Chemicals in the Ofllgas .......... 10 2.2.3.2 Determination of Residual Concentrations in Sand ........................................... 11 2.2.3.3 Analysis of Residual Contaminants Concentrations in Soils afler Ventings ........ 12 vii 2.3 Results and Discussion ........................................................................................... 15 2.3.1 The Variations of VOCs Concentrations in the Eflluent Gas from Soil columns during the Air Stripping ...................................................................................... 15 2.3.2 The Variations of TCE Concentrations in the Efiluent Gas from Soil columns during the Ozone Venting .................................................................................. 17 2.3.3 TCE Residual Concentrations in Ottawa Sand after Applying Vapor Stripping 21 2.3.4 TCE Residual Concentration in Borden Aquifer Material afler Vapor Stripping... 23 2.3.5 Naphthalene and Toluene Residual Concentration in Ottawa Sand Afier Vapor Stripping ............................................................................................................ 28 2.4 Conclusions ........................................................................................................... 35 2.5 List of References .............................................................................................. 36 CHAPTER 3 THE KINETICS OF THE REACTION OF OZONE WITH PHENATHERENE IN UNSATURATED SOILS 3.1 Introduction ........................................................................................................... 38 3.2 Experimental Methods ........................................................................................... 40 3.2.1 Materials ............................................................................................................. 40 3.2.2 Analytical Methods ............................................................................................. 40 3.2.3 Soils Preparation ................................................................................................. 43 3.2.4 Kinetic Experiments ............................................................................................ 44 3.3 Results and Discussion ........................................................................................... 47 3.3.1 Rate of System Self-Decomposition .................................................................... 47 3.3.3 Rate of Reaction of Gaseous Ozone with Soil Organic Matter ............................. 54 3.3.4 Rate Eexpression for the Reaction of Gaseous Ozone with Phenanthrene ............ 60 3.4 Conclusions ........................................................................................................... 65 3.5 List of References .................................................................................................. 66 viii CHAPTER 4 USE OF GASEOUS OZONE TO REMEDIATE PHENANTHRENE CONTAMINATED UNSATURATED SOILS: MATHEMATICAL MODELING AND COLUMN STUDIES 4.1 Introduction ............................................................................................................ 69 4.2 Experimental Materials and Methods ...................................................................... 71 4.2.1 Materials .............................................................................................................. 71 4.2.2 Experimental Method .......................................................................................... 72 4.3 Model Development ............................................................................................... 75 4.3.1 Mass Balance Equations ...................................................................................... 75 4.3.2 Gas Flow Equation ............................................................................................. 78 4.4 Results and Discussion ........................................................................................... 80 4.4.1 Transport of Gaseous Ozone through Organic—Matter Free Soil .......................... 80 4.4.2 Transport of gaseous ozone through natural soil ................................................. 83 4.4.3 Transport of Gaseous Ozone through Phenanthrene Contaminated Soil ............... 90 4.5 Conclusions ........................................................................................................... 94 4.6 List of References .................................................................................................. 95 CHAPTER 5 SUMMARY AND RECOMMENDATIONS 5.1 Summary ................................................................................................................. 97 5.2 Recommendations and Future Developments ....................................................... 101 APPENDIX A NUMERICAL CODE FOR GASEOUS OZONE PASSING THROUGH UNSATURATED SOIL .............................................................................................. 102 ix APPENDIX B GASEOUS OZONE DEGRDATION IN OVEN-DRIED METEA SOIL .................... 105 APPENDIX C THE DETERMINATION OF THE STOICHIOMETRIC COEFFICIENT FOR THE REACTION OF OZONE WITH PHENANTHRENE .................................................. 106 TABLE 3-1. 3-2. 3-3. 3-4. LIST OF TABLES Experimental results for the treatment of TCE in Ottawa sand using air or gaseous ozone ......................................................................................... 16 Residual TCE concentrations in Ottawa sand after venting ................................ 22 TCE contaminated Borden aquifer material treated by air or ozone venting ............................................................................................................. 24 Residual concentrations of toluene in Ottawa sand after venting ........................ 30 Residual concentrations of naphthalene in Ottawa sand alter venting ............................................................................................................. 34 Characterization of Metea soil .......................................................................... 41 System decomposition of gaseous ozone in the cycling batch reactors ............................................................................................................ 50 Rate constants for the degradation of gaseous ozone in soil water ..................... 53 The rate constants for the reaction of gaseous ozone with soil organic matter .................................................................................................. 59 Soil colunms propeties and operating conditions for ozonation ......................... 73 xi FIGURE 2-1. 2-3. 2-4. 2-5. 3-1A. 3-lB. 3-2. 3-3. LIST OF FIGURES Experimental apparatus for soil columns vented with air or gaseous ozone .......................................................................................... 9 TCE residual concentration in Ottawa sand column along axial direction across column .................................................................. l3 Concentration of TCE in the ofl‘ gas and the accumulated mass of TCE purged fi'om dry Ottawa sand ............................................. 18 Ozone consumption and breakthrough curve for dry Ottawa sand containing TCE ............................................................................... 19 Concentration of TCE in the ofl‘ gas and the accurrmlated mass of TCE during ozone venting. ........................................................ 20 Concentration of TCE in the ofllgas fi'om Borden aquifer material during secondary venting by air. ................................................ 26 Concentration of toluene in the off gas from dry Ottawa sand. ....................................................................................................... 3 1 Residual toluene concentrations in the sand at different venting times. ......................................................................................... 32 Concentrations of toluene and naphthalene in the 011‘- gas fi'om dry Ottawa sand during air venting. ................................................ 33 Experimental apparatus for cycling batch reactor. ................................... 45 Experimental apparatus for shallow packed bed reactor .......................... 48 The kinetics of the degradation of gaseous ozone in contaminant- and organic matter free Metea soil ..................................... 52 The kinetics of the degradation of gaseous ozone in contaminant-free Metea soil .................................................................... 58 xii 3-4. The kinetics of the reaction of phenanthrene with ozone determined by shallow packed bed reactor .............................................. 62 The kinetics of the reaction of phenanthrene with ozone in cycling batch reactor ............................................................................... 63 Experimental apparatus used for the transport of gaseous ozone through soil columns .................................................................... 74 The breakthrough curves of gaseous ozone passed through pre-ozonated 10 cm long soil column ...................................................... 82 The breakthrough curves for the transport of gaseous ozone through Metea soil Column length was 10 cm ....................................... 86 The breakthrough curves for the transport of gaseous ozone passed through Metea soil Cohrmn length was 30 cm ............................ 87 The breakthrough curves for the transport of gaseous ozone through Metea soil Column length was 10 cm. ....................................... 88 The determination of the gas conductivity ............................................... 89 The break through curves for the transport of gaseous ozone through phenanthrene contaminated soil ....................................... 91 Simulation results of the phenanthrene, ozone, and soil organic matter breakthrough curves at different soil organic matter concentrations in 10 cm long soil cohrmns .................................... 93 xiii Ci ci+l NOMENCLATURE the phenanthrene concentration in soil [M/M] the phenanthrene concentration in soil at the i th point of time frame in using finite difference calculation [M/M] the phenanthrene concentration in soil at the i+1th point of time frame in using finite difference calculation [M/M] the longitudinal dispersion coefficient the gaseous ozone flux through x-y plane [M/Lz/T] second order rate constant for the reaction of ozone with phenanthrene [(lVI/If)’l Tl] pseudo first order rate constant with respect to phenanthrene for the reaction of ozone with phenanthrene [ III] the gas conductivity [UT] first order rate constant of gaseous ozone system decomposition [l/I'] rate constant of ozone self-decomposition in aqueous phase under different pH [l/T] rate constant of gaseous ozone with soil organic matter [T1 (M/L3) (M/MYI] first order rate constant of the reaction of gaseous ozone with soil water [1/1] the length of gas flow path [L] xiv [03] [OH] om At §§< ,2 Aw (lswo3 )orn the total porosity of soil aqueous ozone concentration [M/L3] aqueous concentration of hydroxide ion [M/L3] the soil organic matter content as total organic carbon [M/M] the initial soil organic matter content as total organic carbon before reaction [M/M] the gas pressure [M/LTZ] the inlet gas pressure at the inlet end [M/LTz] the gas pressure at the outlet end [M/LTZ] the reaction rate of gaseous ozone with soil water [(1/M)(M/T)] the reaction rate of gaseous ozone with soil organic matter [(1/M) (M/T)] the mass of ozone consumed due to the reaction of system decomposition divided by the mass of ozone consumed due to the reaction of ozone with soil organic matter the degree of saturation of gas phase time [T] the reaction time interval between each experimental point [T] the pore gas velocity [L/T] total mass of gaseous ozone in the system [M] weight of soil water [M] weight of dried soil [M] the weight of the dry soil in the control vohrme [M] the mass of gaseous ozone consumed by the reaction of gaseous ozone with soil organic matter [M] XV (Awe, ).y. (AWO, ). AxAyAz 0: Greek letters Pb the mass of gaseous ozone consumed by system decomposition [M] the total mass of gaseous ozone consumed by system decomposition and the reaction of gaseous ozone with soil organic matter [M] volume of gas phase in cycling batch reactor [L3] the control vohrme of soil the extent of reaction of ozone the position along flow direction [L] the bulk density of soil [M/L3] gaseous ozone concentration [M/L3] gaseous ozone concentration at the i th point of time frame in using finite difi‘erence calculation [M/L3] initial gaseous ozone concentration before reaction [M/L3] water content [L3/M] ratio of the volume of gas phase to the weight of dry soil in cycling batch reactor the demand of ozone to phenanthrene [M/M] the demand of ozone to phenanthrene [M/M] CHAPTER 1 INTRODUCTION AND THE PROBLEM SCOPE 1.1 Introduction The leakage of chemical compormds or hazardous wastes from tanks, dmms, or pipes results in the accumulation of those chemicals residing in the soil or groundwater. Organic chemicals that are insoluble water, e. g., gasoline or TCE, form non-aqueous liquids (N APLs) that cab become trapped in the unsaturated zone of soil. These chemicals can migrate downward by gravity serving as a continued source of contamination. When those NAPLs reach the water table, the dense NAPLs (DNAPLs) will continue to move downward to reach the bottom of water table where it spreads by the flow of grormdwater. The light NAPLs (LNAPLs), on the contrary, will float on the t0p of groundwater and become trapped at the fiinge zone. Numerous techniques are available to treat soils contaminated with organic chemicals, but all have their limitations. For contaminants in the saturated zone, pump and ex-situ treatment is used extensively. This process is not without its limitations, resulting mainly from the slow mass transfer of contaminants from the soil pores or from slow desorption of the contaminants. As such, it can take many years to treat an aquifer using extraction is the widely used technology to treat the volatile organic chemicals (VOCs). The air as it moves through the soil, picks up the volatilized chemical out of the soil For the non-volatile compounds, air stripping is ineffective due to their very low vapor pressures. Excavation followed with thermal destruction by incineration is the current technology for those non-volatile compormds; however, the cost is expensive when compared to the soil venting for the removal of VOCs. Although there have been several attempts to solubilize the contaminants in water (1) or in surfactant / water mixtures (2), the success of these technologies has not been demonstrated. In-situ bio-remediation has been used in the unsaturated soil (3); however, it is applicable only for the remediation of biodegradable chemicals, such as benzene and toluene, and is highly affected by the field conditions like soil pH, soil porosity, and nutrients availability (4). A novel approach to treat imsaturated soils that are contaminated with organic chemicals is through the use of ozone. Ozone is a powerful oxidant with E° = 2.07 volts. Ozone can react directly with olefinic compounds by a 1,3-dipolar cych addition across the double bond or by electrophillic attack on an aromatic ring. The products formed from the reaction of ozone with olefins are usually carboxylic acids, aldehydes, and ketones. Although ozone can react with saturated hydrocarbons, the rates of these reactions are very slow in aqueous solutions. Although numerous studies have shown that ozone is an effective oxidant of many organic chemicals, especially olefinic and aromatic compounds with electron donating groups (5,6), only a few studies have investigated the effectiveness of using ozone for the treatment of organic chemicals in soils (7,8). Yao and Masten (7) investigated the treatment of organic chemicals in soils (7,8). Yao and Masten (7) investigated the effectiveness of soil ozonation by passing gaseous ozone through air-dried soils contaminated with PAHs. It was observed that over 95 % of phenanthrene and pyrene were oxidized with the ozone dosage at 25 - 30 mg 03 / mg PAH, but only 40% of chrysene was removed with 216 mg 03 / mg PAH Masten (9) studied the ozonation for several chlorinated VOCs in soil slurries, and found that cis-dichloroethene could be completely degraded. These studies indicate the potential of using gaseous ozone for the remediation of soils contaminated by recalcitrant organics. 1.2 Objective and Scope of the Study The overall objective of this research project was to study the feasibility of using gaseous ozone to treat volatile and non-volatile organic contaminants in the unsaturated soils. Three phases were included in this study. In the first phase of this study (Chapter 2), the efficiency of the removal of residual VOCs was investigated by comparing the efiiciency of ozone gas and air following air stripping. For volatile contaminants, although the conventional air stripping is a very efl‘ective remediation process, the removal rate of VOCs becomes very slow alter most of the contaminants have been removed during the closure period, which is conventionally called tailing stage. In this study, it was attempted to use gaseous ozone as the polishing step to shorten the closure time to reduce the residual contaminant concentration in soil. A series of experiments using soil columns were conducted by first air stripping the soils until the tailing stage begin to occurred, then air or gaseous ozone was used as a secondary treatment process. The effectiveness of the soil. TCE was selected as the model volatile compound, and naphthalene, dissolved in toluene, was selected as the semi-volatile compound. In order to avoid the reaction of ozone and soil organic matter, Ottawa sand and Borden aquifer material were used as the soil matrix In the second phase in this study (Chapter 3), the kinetics of the reaction of gaseous ozone with a non-volatile organic contaminant, phenanthrene, was evaluated. A cycling batch reactor was selected to determine the rate expression for the reaction of soil organic matter with gaseous ozone. A shallow packed bed reactor, where homogenous gaseous ozone concentration can be assumed through the soil reactor, was also used to determine the rate expression for the reaction of gaseous ozone with phenanthrene contaminated soil The kinetic result was further verified by comparing the predicted phenanthrene concentrations obtained from the rate expression in shallow packed bed reactor with the observing experimental data obtained from cycling batch reactor. Due to the heterogeneity nature of soil, the unclear distributions of soil organic matter and contaminant in the moist soil, lumped rate expressions were used. In the last phase of this study (Chapter 4), a series of soil column were conducted to simulate the use of in-situ ozonation for the non-volatile organic compounds in the field and to verify the results of the rate expressions obtained fi'om the Chapter 3 with a mathematical model. The l-D mathematical model, which included the mass balances of gaseous ozone, soil organic matter, and contaminant, was developed to describe the transport of gaseous ozone in the unsaturated soil and the variation of residual contaminant with time and space. 1.3 List of References (1) Annable, M.D., Wallace, RB, Hayden, NJ, and Voice, T.C. Journal of Contaminant Hydrology 1993, 12, 151-170. (2) Pennel, KD., Abriola, L. M., Weber, W. J. Environ Sci. T echnol. 1993, 27, 2332- 2340. (3) Madsen, E. L. Environ. Sci. & T echnol. 1991, 25,1663-1673. (4) Bioremediation: Innovative Pollution Treatment technology: 1993,; U. S. Environmental Protection Agency. Oflice of Research and Development. U. S. Government Printing Ofice: Washington, DC; EPA/640/K-93/002. (5) Hoigné, J.; Bader, H Water Res. 1983, 17, 173-183. (6) Masten, S. J.; Davies, S. H R. Environ Sci. T echnol. 1994, 28, 181A-l85A (7) Yao, J. J .; Masten, S. J. In Proc. of 46th Purdue Industrial Waste Conference; Purdue University: West Lafayette, ID. 1992; pp 642-651. (8) Day, J. E. M. Sc. Thesis, Michigan State University, 1993. (9) Masten, S.J. Ozone: Science and Engineering 1991, 13, 287-312. CHAPTER 2 THE USE OF GASEOUS OZONE FOR THE REMOVAL OF RESIDUAL VOLATILE ORGANIC CONTAMINANT S FROM UNSATURATED SOILS 2.1 Introduction Soil vapor extraction (SVE) is a conventional method for the remediation of unsaturated soils contaminated with volatile organic chemicals (VOCs). It has been widely applied to remediate sites contaminated with VOCs, such as benzene, tohrene, ethylene benzene and xylene (BTEX), and for TCE. The principle of SVE is that the liquid organic contaminants vaporize into the gas phase, then the clean air carries the vapors out of soil. Several investigators (1-5) have studied the transport and phase transformation of contaminants on a laboratory scale. It was found that the local equih’brium assumption is valid during most of the venting period. However, when air stripping is applied in the field, " rest and start" cycles are usually used during at the last stage of venting before site closure (6). Ozone, a very strong oxidant, can react with many organic chemicals, such as olefins, monocyclic aromatics, and PAHs (polycycfic aromatic hydrocarbons). Ozone reacts directly with olefinic compounds via a 1, 3 - dipolar cyclic addition across the double bond (7 ). The ozonated products of olefinic compounds are aldehydes, ketones or organic acids (7 ). Ozone can also autocatalytically decompose to form the hydroxyl radical (8,9), which is a non-selective and powerfirl oxidant. Very few studies have been conducted to investigate the reactions of ozone in soil systems. Yao and Masten (10) applied gaseous ozone to phenanthrene contaminated dry Metea soil and obtained removal efficiencies of over 95%. Masten (1 1) also obtained effective treatment emciencies for VOCs in soil slurries treated with ozone. These studies suggest that it may be feasible to use ozone to remediate soils contaminated with organic chemicals despite the presence of organic matter, which, itself; would be expected to exert an ozone demand. In this study, we attempt to use gaseous ozone as a final polishing step after conventional air stripping to treat volatile organic chemicals in contaminated geological material The removal emciencies of volatile- and semi-volatile organic chemicals fiom sand by conventional air stripping and " ozone venting " were compared. 2.2 Experimental Work 2.2.1 Column Preparation All columns used in this study are the same as those of Annable (12), which are 10 cm long and 5.3 cm inside diameter, Pyrex made glass columns. The end caps are fabricated of stainless steel or Teflon to avoid losses of the chemicals due to sorption. These columns were packed either with Ottawa sand ( ELE International Co., Soiltest Division), mono-dispersed particle with an average diameter of 0.51 mm, or Borden aquifer material ( CFB Borden, Ontario, Canada). The Borden aquifer material contains 3.6 % clay, 0% silt, 96.4 % sand and 0.2 % organic matter (as total organic carbon). A 1 bar ceramic pressure plate was placed at the bottom of column to control the liquid saturation in column while it was drained. The packed columns were vacuumed to -0.8 bar to avoid trapping air in the pores. For the columns containing dry sand, pure 1, 1, 1- trichloroethylene (TCE, Aldrich Chemical Co., 99 % +) was introduced to the bottom of column through the ceramic plate. When the column was fully saturated with liquid, it was pressurized by introducing cylinder air from the top of column to drain the liquid from the soil. The draining pressure was maintained at 6 psi for 2 days to reach residual saturation. Columns with toluene and naphthalene were prepared using the same procedures as described above except that toluene was used as the non-wetting fluid. One gram of naphthalene (Aldrich, 99% +) was dissolved in 100 ml toluene (Aldrich, 99% +). For the soil columns of Borden aquifer materials, the column preparation methods were the same as described above. All ceramic plates were replaced with stainless steel screens before vapor stripping was performed. 2.2.2 Venting Procedures Columns containing organic contaminants were treated at room temperature using vapor stripping. Figure 2-1 illustrates the apparatus used for air stripping and ozone venting. During air stripping, the air, which was controlled at a rate of 50 ml/min using a mass flow controller (Sierra Co., Model #840 ), was introduced from the top of the column. The 011‘ gas flowed through a 10-port valve (Valco Co.) contained in a heating box, maintained at 60 0C, to prevent condensation of the organic vapor. Afler purging the air flow ozone flow % ‘ soil colunms ‘ 5:213:22 E-I OO ozone hot generator oil bath [(1)] , 10 port . valve hehum Figure 2-1. Erqrerimental apparatus for soil columns vented with air or gaseous ozone. 10 off-gas through a 25 [.11 sample loop for 1 minute, the valve was switched to inject the gaseous sample into a gas chromatograph (GC, SRI Co., Model #8610 ) to monitor the contaminant concentration in the ofllgas. The valve rotation and GC operation were controlled by a personal computer. Alter cessation of soil venting experiments, all columns were allowed to cool in a 4 0C storage room for 1 day, thus preventing chemical loss during sampling. After sampling, these columns were allowed to equihhrate in the room temperature for two days prior to secondary venting by air or ozone. For soil cohrmns vented by ozone, the gaseous ozone was prepared from dried oxygen using an ozone generator (Polymetric Co., Model # T-408). The concentration. of ozone was monitored by passing the off-gas through a flow cell in a UVNis spectrophotometer ( Hewlett Packard, model# 8452A). An extinction coefficient of 3000 M‘1 cm'1 at 260 nm was used to calculate the gaseous ozone concentration. In order to prevent the GC capillary column being oxidized, the eflluent gaseous ozone fi'om the soil columns was thermally destroyed by passing the gas through an oil bath (250 0C). Following this step, the ofllgases were directed into the gas chromotograph through a 10 port valve to detect the VOCs concentration in the effluent gas. 2.2.3 Analytical methods 2.2.3.1 Determination of the Concentration of Organic Chemicals in the Ofllgas The concentration of organic vapor in the eflluent gas from soil columns was determined using gas chromatography as descn'bed above. The gaseous TCE and toluene concentrations analyzed by GC was cah'brated by bubbling air through a gas washing 11 bottle which contained pure TCE or toluene. The flow rates of air flowed through the gas washing bottle was gradually reduced until the GC peak area of the VOCs reached a constant, which corresponds to the contaminant's saturated vapor pressure. The published saturated vapor pressures can be obtained from literature (13,14). Therefore, the cahhration factor for converting the GC area into the gaseous VOCs concentration can be determined. Because naphthalene is a crystalline solid, the calibration procedure for the naphthalene concentration in the efliuent gas was the same as that for VOCs except that the naphthalene was packed in a 10 cm long, 1/ " diameter stainless steel tubing. 2.2.3.2 Determination of Residual Concentrations in Sand Because TCE, toluene and naphthalene are volatile or semi-volatile compounds, a static headspace method (15) was applied to determine the residual contaminants concentrations in the sand alter the air or ozone venting. A GC (Perkin Elmer, AutoSystem, 0.53 mm capillary column) and headspace analyzer( Perkin Elmer HS-40) were used for the analysis. FID detector was used for the analysis TCE, toluene and naphthalene in Ottawa sand, and PID detector, which has lower detection limit for TCE was used for the analysis of TCE in the Borden aquifer material. After 5 'ml of de-ionized water were added into a 22 ml headspace vial, the contaminated sand samples taken from the soil columns were added into the water and the vial was capped quickly with a Teflon coated septum and an aluminum ring. The settings used in headspace analyzer were as followings: thermosetting time was 50 min, time for pressurization was 3 min, injection time was 0.10 second, and the thermosetting temperature for TCE was 80 0C, for toluene and naphthalene was 90 0C. 12 The sorption of TCE, toluene and naphthalene onto Ottawa sand was investigated. Vials containing 0.5 g clean Ottawa sand, 1.12 mg/l toluene, and 1.48 mg/l naphthalene solution were placed into a vortex type shaker for 2 days at room temperature before GC analysis. Other vials with the same toluene/naphthalene concentration, but containing only water were placed in the shaker. As was observed, no statistical difl‘erence between the two data sets. The sorption of toluene and naphthalene onto Ottawa sand is small enough to be neglected. Sorption of TCE onto Ottawa sand was determined by the same procedures as above. Results also show the sorption onto Ottawa sand to be negligible. For Borden aquifer material, the same procedure was conducted and the results also show that the presence of Borden aquifer material didn't affect the headspace method. 2.2.3.3 Analysis of Residual Contaminants Concentrations in Soils after Ventings For the experiments in which the soils were to be treated by two sequential treatments, it was necessary to know the concentration of contaminants remaining in the soil after each venting. Because it is virtually impossible to take soil samples along the axial direction without changing the soil column properties, several soil columns were sacrificed after applying the air stripping to remove most of the contaminated VOCs until they reached the final stage to conrpare the samples taken from along axial directions with those taken at both ends of soil colurrms. The concentrations of TCE in the soil cohrmn of Ottawa sand, the axial direction distribution is shown in Figure 2-2. Results indicate that no significant difference was observed between the average of the samples obtained from top and bottom of the column and the average of the samples obtained from the axial directions. Similar results were observed for TCE in Borden aquifer material and toluene l 2 b .s 14 (a) 5 l l g .. 1.2 .8 1 E E 1 f, A ‘E 0 ‘c‘ u 0.3 o '" 0 i g g 0.8 g 2' ° 0 o ,_ 06 3 E 0.6 2 '0 g ‘5 g 5 04 E E 0.4 :9 In ' 3 u. 3 a 0.2 a 0-2 '9 0 9 0 l 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 Position along axial direction top and bottom of column AVERAGE STDEV RSTDEV MEAN TOP 1.05 0.06 0.06 1.09 BOTTOM 1.14 0.03 0.03 AVERAGE ALONG AXIAL DIRECTION = 1.09 Figure 2-2. TCE residual concentration in Ottawa sand column along axial direction across column. ( Values were determined after air stripping ) (a) The samples were taken from column along axial direction with sample 1 being from the bottom and sample 10 from the top of soil cohnnn. (b) Six samples were taken from either the top or bottom of the same Ottawa sand cohrmn as used in (a). Sample 1-3 were taken from the top and samples 4-6 from the bottom of column. 14 in Ottawa sand. As such, the two-end sampling method is valid to represent the whole column residual concentration for TCE or toluene alter preliminary air stripping. After taking the soil samples were taken from either sides of the soil column, the caps were immediately re-assembled and the soil columns were placed in the room temperatures for 2 days to restore the soil temperature before the application of secondary treatment by air or gaseous ozone. The distribution of naphthalene along the axial direction of the soil column was not as uniform as those contaminated for TCE or toluene where the inlet end of soil column always had less naphthalene concentration after using the air as the preliminary treatment. Because it is virtually impossible to take soil samples along the axial direction without changing the column properties, the two-end sampling method was still used to analyze the naphthalene concentration after preliminary air stripping, however; the values for top and bottom samples would be reported respectively as a qualitative estimate of the naphthalene concentration in the soil columns alter preliminary treatment by air stripping. Soil samples from columns after secondary treatment by air or ozone were taken along the axial direction by the following procedures. The top cap from the column was replaced by a Teflon made cap with a 1/2" diameter hole. Then a stainless steel tubing equipped with a plunger was inserted through the hole of the top cap to the bottom of the soil column. The stainless steel tubing, containing the soil sample, was withdrawn slowly out of the column, then the phmger was applied to push the sand samples into the headspace vials, which contained 5 ml of de-ionized water. 15 2.3 Results and Discussion 2.3.1 The Variations of VOCs Concentrations in the Effluent Gas fiom Soil cohrmns during the Air Stripping The effluent gaseous VOCs concentrations from soil columns during using air stripping as a preliminary treatment were investigated and the column properties are listed in Table 2- 1. The eflluent concentrations of TCE from the columns of dry Ottawa sand (Cohrnm A) vs. venting time is shown in Figure 2-3. Three phases of the TCE concentrations in the eflluent gas could be categorized: (1) Phase 1: The VOCs vapor concentration and its mass removal rate are constant. The TCE concentration in the ofllgas before it began to decrease in Column A was 0.481 1: 0.004 (at 95% CI) mg/ml, and its theoretical saturated vapor pressure was 0.484 mg/ml under the same temperature, 23 0C (14). Therefore, it is apparent that the local equilibrium between the organic liquid- and gas phases was reached at this phase, which is conventionally assumed when the soil venting is applied in the field. (2) Phase 2: The VOCs vapor concentration reduces sharply, which indicated that the local equih'brium assumption can not be hold from the beginning of this phase. ( 3) Phase 3: The VOCs concentration gradually levels off to nearly zero. Similar three phases of vapor TCE concentrations in the effluent gas of wet Ottawa sand column was also observed (column B). After cohrmn A had been repeatedly allowed to rest overnight at room temperature and purged by air, it was observed that the TCE vapor concentration in the ofllgas increased initially and then very quickly dropped to nearly zero again as shown in Figure 2-3. This indicates that there is still a small amount 16 3., s an M: Sod mom 9m Red mm _.a o2 gene wmd 06 9.30 3., .s as nd mood m3 «.6 Bed own ed on gene ed - 0.30 ad mood on mm 3.3 ovm «5 an an and ad 930 ed Sod on n.m weed m3 as on an 33 - «r30 3 Aiuav £5 3 EEE 3.5 E Easav 3., .x. V as: was: .3» mun. .280 2:: mob .250 2:: mo.—. 3.: 3m 8880 a 593 393 woman mph mama—3 womb:— moh wave? Exam 26¢ 2% mafia:— bmmeeea ~39: Ede—co N 98.3 ~ 9433‘ gene 258% no he mean can «335 E mob me =5ng 2% .8.“ £38 anon—«Beam .TN 033. TCE concentratic in off-gas (mg/rt l7 0.5 4 5 0.45 - l» 4 0.4 * Colurm OW-A ”” 3-5 '8 0.35 —~ E T 3 '8 0.3 _,__ +m (HISUY 2) +ooc. mass ~~ 2.5 a 0.25 T "U -_ 2 a 2 0.2 ~ 3 -. 1.5 g 0.15 a" 0 0 -. 1 U 0.1 “t 0.05 “r l l r 0.5 o r l i i l ' — o 50 lm 150 2C!) 250 31) venIirgiimemin) Figure 2-3. Concentration of TCE in the off gas and the accumulated mass of TCE purged from dry Ottawa sand. The arrows indicate that the air stripping was resumed afier rest overnight. 18 of TCE trapped in the soil columns. This phenomena implies that the local equihbrium assumption is not valid at the final stage and the removal rate of VOCs is dominated by the mass transfer from the VOCs trapped in the immobile zones that the bulk air cannot reach or the TCE sorbed on the soil surface rather than the interfacial mass resistance between the gas- and NAPL phases. 2.3.2 The Variations of TCE Concentrations in the Emuent Gas from Soil columns during the Ozone Venting Column C was packed with dry Ottawa sand and contained TCE. It was vented using ozone at a flow rate of 50 ml/min. The initial gaseous ozone concentration was 32.8 mg/l, and then increased to 79.3 mg/l alter 71 minutes. The ozone breakthrough and consumption curves are shown in Figure 2-4. The TCE ofl‘ gas concentration and accumulated mass of TCE removal are shown in Figure 2-5. The initial TCE amount in the column was 9.6 g, and the mass of TCE removed by stripping was 5.4 g. As such, 44 % of the TCE reacted with ozone, assuming that the residual amount of TCE in sand was negligible. Theoretically, if TCE and ozone reacts via the direct reaction mechanism, the molar ratio of ozone consumed to TCE reacted would be 1.0. However, the calculated ratio was 0.78 mol ozone/mol TCE. One possible reason to explain why the stoichiometric ratio is less than 1 might be due to the experimental error in the determination of the mass of TCE purged fiom the cohrmn. Nevertheless, the result does suggest that the experimentally determined stoichiometric ratio is close to that which one would expect for a direct reaction of ozone and an olefin. l9 70 : 1.2 60 a 1 .Q +— o. 50 —~ E u: + 0.8 3 E E ‘7 o 0 40 T U C 0') 8 Column owc .. 0.6 O o 'o c 30 “r 9 o .9 N 3 ° +OGomc (r004) ~~ 0.4 E 20 - | +000. @3me 8 o o 10‘ 1 1 1 L T - I I I 0 100 200 300 400 500 600 Venlirgtimemin) Figure 2-4. Ozone consumption and breakthrough curve for dry Ottawa sand containing TCE. 20 0.5 0.45 1* m0.— UomSQ U033E300< Colurm OW-C is L + 4. «w 3. O 0. O <95 30 to E 0:00 mop i 2 O 0.25 “r 0.15 t b q 1 o O 0.05 T ,mom m mov ,mmm {mom I mmm snow In? Tmop 1mm limelrrin) mv venii Figure 2-5. Concentration of TCE in the off gas and the accumulated mass of TCE during ozone venting. 21 2.3.3 TCE Residual Concentrations in Ottawa Sand alter Applying Vapor Stripping Although ofi‘ gas monitoring is a convenient way to estimate how much TCE has been removed, sand analysis is still the most direct way to determine the level of soil remediation. To accomplish this, samples were taken from sand columns for analysis after venting. Five columns of dry Ottawa sand and two columns of wet Ottawa were contaminated with TCE and treated with the two-stage venting procedure. All columns were designed to be vented by air as a preliminary step. Venting was ceased when the 03 gas concentration was reduced to 4 % of its saturated vapor pressure, which was approximately at the end of Phase 2. Air or gaseous ozone was then used as a second treatment method. The column properties, operating conditions, and treatment results are shown in Table 2-2. The average TCE concentration remaining after preliminary air stripping from dry Ottawa sand columns was determined to be 1.00 i 0.20 ( at 95% CI ) mg TCE / kg sand. For all columns, the removal efliciencies were greater than 99.99%. This high removal emciency may be due to the uniform nature of Ottawa sand whose pore size is much larger than that of natural soil The residual TCE concentration in the wet Ottawa sand after preliminary air stripping was 0.07 :t 0.17 mg/kg sand. The residual concentration in wet sand, which was expected to be higher than in the dry sand due to the lower vapor pressure for the TCE dissolved in water, was actually found to be lower than that in the dry sand. Because water is a wetting fluid and TCE is a non-wetting fluid (16), water has a tendency to cover the sand surface and displaces free TCE from small pores, i.e., less TCE is trapped in the small pores in wet sand. The result again implies that the 22 Table 2-2. Residual TCE concentrations in Ottawa sand alter venting initial conditions preliminary venting secondary venting air; 50 ml/min 20 ml/min cohtmn porosity water TCE venting residual gas residual # content wt. time TCE type TCE (% VM (8) (min) (mg/kg) (mg/kg) OW-l 0.35 0 6.74 320 1.07 air 0.105 OW-3 0.35 0 7.76 360 0.72 air 0.096 OW-4 0.35 0 5.31 240 0.99 - - OW-S 0.35 0 7.65 375 0.93 ozone 0.103 OW-6 0.35 0 9.9 430 1.28 ozone 0.225 OW-9 0.35 0.6 10.21 440 0.11 air < 0.01 Ow-lO 0.35 1.8 2.66 195 0.03 - - 23 dominant mechanism governing the efl‘ectiveness of air stripping is by the mass transfer of pollutants from the pores rather than from aqueous TCE vaporized from water phase to gas phase. Prior to conducting experiments using gaseous ozone for secondary treatment, it was thought that since ozone can react with TCE, ozone might be able to diflirse into the small pores where it would react with TCE, thereby, reducing the residual TCE concentration. However, comparison of the residual TCE concentrations obtained in Ottawa sand experiments after using air and gaseous ozone during secondary venting, revealed that there was no difference in residual TCE concentrations alter treating sands with these two gases. This result indicates that the rate determining step for the removal of ”9 ~ contaminants in sand is the mass transfer step of TCE from the pores and ozonation does not appear to increase the removal rate of TCE from Ottawa sand. It also implies that the rate of diffusion of TCE from the pores is slower than that for the reaction of ozone and TCE. 2.3.4 TCE Residual Concentration in Borden Aquifer Material alter Vapor Stripping Five columns containing dry Borden aquifer material and two columns containing wet Borden aquifer material were contaminated with TCE and were treated using two- stage venting, as described above. These results are summarized in Table 2-3. After preliminary air stripping, the residual T CE concentration in dry sand was found to be 4.6 d: 2.1 (95% CI) mg/kg. The higher residual TCE in Borden aquifer material (as compared to Ottawa sand ) is thought to be due to the (1) presence of smaller pores in Borden aquifer material due to a more heterogeneous distribution of grain size, ( 2) 24 38:8 on 003 88 3.5 ha 2E. 0 - nooo n .2 on 2.30 - fin own - es sun—m - 36 OS om b0 - od mam .. fie odm - vmoo WE om 0:80 - m.n mum . o6 nA—m Em hmoo 92 on an no 2 .o o3. NR Eb Tam né Rod 02 on 283 ad and 2:. m6 «.3 mA—m - omoo or om 0:80 - ~.n ooo - v.2 Ndm - bag on om ha - m0 mom - 9N— 79m 9? so @005 to 3an $3 .5 Romeo 35 9s .5 E .280 .280 H083 wok. 250 83 mam 833 mph. 08w 25250 .03 a 2:200 .3300.— oBEmou mean“; 26w 2% wager, 13202 .3302 manque, 2055 m0“. mew—20> new 5303 enigma he SEEM—02— 283—280 E wane? 2.30 8 an .3 330: 3.8008 00%? cocoon 300E088 NOB .m-~ 030,—. 25 presence of organic matter which can sorb the TCE. In moist Borden aquifer material, the average residual TCE alter preliminary venting was determined to be 0.27 mg/kg which is much less than in dry sand. As with Ottawa sand, the wetting and non-wetting theory can be used to explain this phenomena. The TCE residue calculated by multiplying the measured soil concentration by the soil weight is 0.15 mg for column BD-3, a wet cohrmn. The mass of TCE that would be dissolved in the pore water if it were saturated with TCE can also be calculated1 to be 12.2 mg, which is about 80 times of calculated TCE residue. The results show that the concentration of TCE in the pore water is well below the solubility limits, thus suggesting that the mass transfer rate for TCE fi'om aqueous phase to bulk gas phase is faster than that for pure TCE which is trapped in the soil pores. For secondary venting, cohrmns BD-l and 2 contained dry Borden aquifer material were vented for 76 hours using 20 ml/min by air and ozone, respectively. Figure 2-6 depicts the variation in the concentration of TCE off-gas from column BD-l during the secondary venting by air. The column was allowed to "rest overnight and then was restarted" several times. Every time the flow was started, the TCE off-gas concentration rose and then quickly dropped ofl‘Z As with Ottawa sand, this phenomena is consistent with the theory previously proposed that the non-equih‘brium conditions exist during the final stage. Unlike the Ottawa sand in which no TCE was observed in the off-gas after half hour of secondary venting, there was still a very small concentration of TCE observed in the off-gas after 76 hours of secondary venting for the Borden aquifer material In addition to the higher residual TCE in Borden aquifer material after preliminary air stripping, the 1. The solubility of TCE is assumed to be 1100 mg TCE /1 water, using the percent of moisture content, and porosity, the mass ofwater in the column is calculated to be 11.2 g 26 0.035 0.03 ~~ _ CduntD-l p 0 off gas conc. (mg/ 8 o G S 0.01 t 0.005 ‘r veniirgiimeflr) Figure 2-6. Concentration of TCE in the oflT-gas from Borden aquifer material during secondary venting by air. Each peak represents the initial TCE concentration alter the “rest and start”. 27 more heterogeneous texture, which might have more small pores to causes slower mass transfer rate would be another possibility. The residual TCE concentration obtained was 1.27 mg/kg after air venting and 0.02 mg/kg alter ozone venting. The ozone vented soil cohrmn contained a much lower residual concentration than observed in the air vented column, implying that the chemical reaction of ozone / TCE increased the rate of volatile contaminant removal during the secondary venting stage. For the other Borden aquifer material columns, the gas flow rate used for secondary venting was increased to 50 ml/min and the venting time was reduced to 12. 5 hours. Again, the TCE residues in ozone vented column were less than the air vented cohrmns. The average TCE residues obtained in dry aqueous material were 0.35 mg/kg for air venting and 0.05 mg/ml for ozone venting. The reason that less venting time can reach the same TCE removal of remediation (as described above) is due to the higher gas flow rate. According to the empirical equation for a fixed bed reactor ( 17), the overall mass transfer coeflicient for TCE vaporized from liquid to gas phase is proportional to the gas velocity in the pore space. Therefore, higher gas flow rate results in a larger overall mass transfer coeficient, thus increasing the contaminant vaporization rate. The TCE residuals remaining in dry Borden aquifer material after secondary venting with air and ozone is significantly different. After 76 hours of treatment at a gas flow rate of 20 ml/min, the TCE residuals were 1.27 mg/kg when using air and 0.02 mg/kg when using ozone venting. When the flow rate was increased to 50 ml/min and vented for 12.5 hours, there were 0.35 mg/kg and 0.05 mg/kg for air and ozone venting, respectively. However, with dry Ottawa sand, there was no difference observed between air and ozone ll 28 venting after half hour of venting at a flow rate of 20 ml/min. It is apparent that the chemical reaction of the contaminant and ozone is significant only when a very long venting time is required. The results suggest that in addition to the diflirse of TCE out of small pores, the gaseous ozone can also difl‘use into the pores where diffusion rate is enhanced by the chemical oxidation of TCE with ozone the small pores. However, the contaminant removal rate is far less than its chemical reaction rate in bulk organic phase. This implies that mass transfer appears to be the limiting mechanism in the final venting stage rather than the chemical reaction. The reason why ozone does not enhance the removal of TCE in Ottawa sand is because the secondary venting time is not long enough to significantly afl‘ect the results. 2.3.5 Naphthalene and Tohrene Residual Concentration in Ottawa Sand After Vapor Stripping To compare the removal efficiency of ozone venting for a semi-volatile chemical with that of TCE, a highly volatile chemical, erqreriments were conducted with naphthalene. The naphthalene was applied to the Ottawa sand using toluene as the solvent. Five Ottawa sand cohrmns were packed and drained with a naphthalene / toluene sohrtion. These columns were vented using air at a flow rate of 50 ml/min. The air flow was stopped when the toluene vapor concentration dropped to 10 % of its saturated vapor concentration. After taking sand samples from top and bottom of the cohrmn for analysis, these columns were vented using air or gaseous ozone for secondary treatment. Table 2-4 lists the column properties, operating conditions and treatment results, and Fig. 2-7 is a typical curve showhrg the concentration of toluene in the off gas during air stripping. 29 E058 oN 00>» 0:: 30a 00» 2E. A8 ease on as as 3.00 :0 2: 5 - - - go no 3 .n o Rd 03th moo v fin E0 omo 3 mod m.~ omo 7300.2 moo v Nam 00000 and mm com. 00 So $5052 a .o NM 0030 mod mm 56 o end «.36; moo v wan E0 and mm and o 30 73th A305 05 @005 no 3 o<> ago 00033 2E0 25 000200 2E0 .03 0000000 in E02000 wEE0> 0.0» 130000 mew—20> 00028 00003 $00.80 08200 LBW—flag 00w 5:008 oomEaEhm E0 EEE—oa 000E300 BEE wEE0> 0a.. 3.8 «BED E 00023 m0 000005000000 10203— .v..~ 030,—. 30 0J6 0J4“ _ 0.12 \ 3’ ~—' OJ ‘ i g EICIOB-t AL I) A g 006 y Column mow-2 38 A 0'04 + +vqxr density ‘ (102‘ o i + 4. l 0 5 10 15 20 25 Venlirgiime (tr) Figure 2-7. Concentration of toluene in the off gas from dry Ottawa sand. 31 After preliminary air stripping of the columns containing dry sand, the residual toluene concentration was determined to be 0.77 :t 0.77 mg/ kg sand, approximately the same order of magnitude as that obtained for TCE. However, for wet sand cohrmns, the residual toluene concentration was 0.29 d: 0.02 mg/kg. Though the difi‘erence between dry and wet columns is not large, there tends to be a lower residual toluene concentration in the wet columns (as compared to the dry cohrmns). This phenomena can be explained by the wetting, non-wetting concept as described in the earlier section on TCE removal. A dry column (NT OW-S) was treated with air for an extended time, 87 hours of continuous air stripping, at a flow rate of 50 ml/min. As shown in Fig. 2-8, the residual toluene concentration was 0.26 mg/kg sand. Thus, there comes a point where the length of time does not significantly improve the toluene removal when naphthalene is present. The concentration variation of naphthalene in the ofl” gas fiom column containing dry Ottawa sand, NTOW-S, is shown in Fig. 2-9. Venting was ceased when the concentration of naphthalene in the off gas was reduced to 2 % of its saturated vapor pressure. After 87 hours of air stripping at a flow rate of 50 ml/min, samples were then taken along axial direction of the cohrmn for analysis. As shown in Table 2-5 the residual naphthalene concentration in the sand was 4.47 mg/kg sand. Cohrmns NTOW-l and NTOW-4, which contained dry and wet sand, respectively, were secondary vented by air at a rate of 15 ml/min for 5.2 hours, the residual concentrations were 23.1 mg/kg sand and 9.78 mg/kg, respectively. For columns secondary vented with gaseous ozone, the residual naphthalene concentration were lowered than vented by air. Naphthalene concentration in column NTOW-2, a dry Ottawa sand column vented with ozone ( 134 mg/l ) for 3.2 hours, was 0.65 mg/kg sand. For column NTOW-3, a column containing wet sand vented Toluene residual cone. in sand (mg 32 .0 so 1 .0 co l NT OW-2 .0 \1 + 0.6 “r NTOW—l NT OW -5 verrfirglimefl'r) Figure 2-8. Residual toluene concentrations in the sand at different venting times. (Experimental conditions are provided in Table 2-4) 33 0.16 0.0005 0.14 E m ‘0- ‘1..— E Cdurm NTOW—S 0‘000' 8 ~— 0.12 0) U’ I 0 a: 9) 01 O :3: . r a .E o +Tduene looooo 0' C ._ + C 0' 0.08 ‘ Nqfi 0 0 5 0 0 0.06 ~~ ‘* 00002 5 a) _ c o 0 £ 2 0.04 -~ .c ,9 » 0.0001 g- 0.02 ~~ z 0 — 0 0 20 40 60 80 100 ventingfimeflr) Figure 2-9. Concentrations of toluene and naphthalene in the off-gas from dry Ottawa sand during air venting. 34 38:8 2 23 33 Ben 8w 2:. A3 ES...— 9. was 3.: Bow he 25. A: - - - :3. S 886 o a: «.382 a; 2 a 8.3.2.8 ”.3 R «n2; 3 a: 732.2 3 v N.“ 2.8.. A3: .3; e2 2 335 3 ”3 $332 3o 2 288 $3 .3: New 2.. 38.0 o o: 352.2 12 Na a 8.3 .3: SN 3 «~86 c a: 7332 3&5 3:55 3:8 we \ may €85 E 3., «L one—«5.3a: 0:5 as one—efiame: 08¢ .25 2833 gnaw * 133m"... wave? new 2323.. mama?» one—wanna: .883 “=3 33—3 A ”maze? new 5808 comet—:5 he Egan—moan 283:8 39: mate? node .58 e385 3 one—33%: we 30958280 1333— .n-N 035. 35 by gaseous ozone ( 119 mg/l ) for 5.2 hours, its residual naphthalene concentration was less than the detection limit of 0.1 mg/kg sand. These results suggest that ozone can efi‘ectively reduced the naphthalene residual concentration in sand. Using the data obtained during the venting of column NTOW-3, the stoichiometric ratio of ozone / naphthalene was estimated. The amount of naphthalene purged during ozone venting was 2.8 mg. Ifit is assumed that both the ozone consumption for toluene, and naphthalene residual concentration after ozone venting are negligible, the molar consumption ratio is calculated as 16 mol O3/mol naphthalene. In the same way, the ratio is calculated as 14 mol O3/mol naphthalene for cohrmn NTOW-2. The theoretical molar consumption ratio of naphthalene for complete oxidation is 8 mol O3/m01 naphthalene . Though this is just an rough estimation of the amount of ozone consumed, it does imply that a significant percentage of the ozone actually reacts with the naphthalene. 2.4 Conclusions For sands which are contaminated by volatile compounds, such as TCE 0r tohrene, air stripping is an effective remediation process. Due to mass transfer limitations, gaseous ozone does not improve the removal of residual volatile contaminants in Ottawa sand. But it does increase the level of treatment, as observed from natural soil, after a longer venting time. The smaller the overall mass transfer coefficient and the longer air venting time required to achieve a certain level of remediation, the more significant is the impact of ozone venting. Nevertheless, mass transfer appears remain to be the dominant mechanism during the final venting stage, irrespective of whether ozone or air is used. 36 It was also found that the presence of water increased the level of treatment achieved as compared to the dry sand. This phenomena might result from the water, a wetting fluid, replacing the trapped organic phase from small pores. It appears that the mass transfer for contaminants from bulk aqueous phase is larger than that for the free contaminant difi‘using out of small pores. It was also noticed that the TCE residual found in wet Borden aquifer material was as low as that observed in the dry Borden aquifer material that had been treated by gaseous ozone. It is apparent that if the time is long enough to let the water replace the free contaminant from small pores, it is not necessary to use ozone venting to remove volatile contaminants. For semi-volatile compounds, such as naphthalene, although air venting can remediate the soil to some level of cleanness; ozone venting, can significantly shorten the remediation time and lower the residual concentration by an order of magnitude. Since the ozone can react with the PAHs, it would be more valuable to apply ozone to PAH contaminated soils, which could not be remediated by conventional air stripping. From the ozone consumption data for TCE and naphthalene, we can hypothesize that most of the ozone applied will actually react with target contaminants rather than being degraded by the soil. 2.5 List of References (1) Marley, M.C., and Hoag, G. E, In Proc. the National Water Well Association / American Petroleum Institute Conference on Petroleum hydrocarbons and Organic Chemicals in Groundwater, Houston, TX, 1984, 473-503. (2) Baehr, AL, Hoag G.E., Marley, M.C. Journal of Contaminant Hydrology 1989, 4, 1-26. 37 (3) Johnson, P.C., Kemblowski, M.W. and Colthart, J.D. Ground Water 1990, 28, 413- 429. (4) Gierke, J. S., Hutzler, N.J., Crittenden, J.C. Water Resources Research 1990, 26, 1529- 1547. (5) Rathfelder, K, Yeh, W.W., Mackay, D. Journal of Contaminant Hydrology 1991, 8, 263-297. (6) Stinson, M.K. JAPCA, 1989, 39, 1054-1062. (7) Bailey, P.S. In Ozone in water and wastewater treatment; Evans, F. L., Ed.; Ann Arbor Science: Ann Arbor, MI, 1972, pp 29-59. (8) Hoigné J., Bader, H Water Res. 1976, 10, 377-386. (9) Staehelin J. and Hoigné J. Env. Sci. T echnol. 1985, 19, 1206-1213. (10) Yao, J. J.; Masten, S. J. In Proc. of 46th Purdue Industrial Waste Conference; Purdue University: West Lafayette, II). 1992; pp 642-651. (11) Masten, S.J. Ozone: Science and Engineering 1991, 13, 287-312. (12) Annable, M.D., Wallace, RB., Hayden, N.J., and Voice, T.C. Journal of Contaminant Hydrology 1993, 12, 151-170. (13) Mackay, D. and Shin, W.Y. J. Phys. Chem. Ref Data 1981, 10, 1175-1198. (14) David, ed. CRC handbook of chemistry and physics, 73rd ed. 1992-1993, pp 6-99. (15) Voice, T. C., and Kolb, B. Environ. Sci. T echnol 1993, 27, 709-713. ( 16) Corey, AT. Mechanics of immiscible fluids in porous media; Water Resource Publication: Colorado, 1990, pp 1-22. (17) Carberry, J. J. A.I.Ch.E. Journal 1960, 6, 460-463. CHAPTER 3 THE KINETICS OF THE REACTION OF OZONE WITH PHENATHERENE IN UNSATURATED SOILS 3.1 Introduction The contamination of groundwater with petroleum products is of significant environmental concern because of the presence of carcinogenic compounds (1) and the widespread nature of the. problem (2). Remediation efforts using “pump and treat”, soil venting and biological degradation have been used with limited success, especially with regard to the polycych aromatic hydrocarbons (PAHs) (3-5). Although the PAHs are found at lower concentrations in petroleum products than the single-ring aromatic compounds such as benzene, their carcinogenic properties have resulted in legislation demanding their removal from soils (6) and drinking waters (7). As such, the development of cost-efl‘ective technologies for the removal of PAHs from soils and aquifers is imperative. Ozone is a strong oxidant (E° = +2.07 V) and has been used in drinking and wastewater treatment for the oxidation of a number of organic chemicals, inchrding trichloroethylene, phenol, and atrazine (8- 10). As ozone is highly reactive with PAHs (l l, 38 39 12), the use of in-situ ozonation in the vadose zone could potentially remediate sites that might be cleaned up only by very expensive means involving excavation and processes such as thermal destruction. During in-situ ozonation, gaseous ozone would be pumped through an unsaturated soil in a manner similar to that used for conventional soil venting. As ozone moves through the soil, some of the ozone would be expected to react with the contaminants, some would likely react with the soil organic matter and some would dissolve into the pore water, where it could decompose to form hydroxyl radicals (13). As the hydroxyl radical (OOH) is a non-selective and very strong oxidant (E° = +3.06 V), OOH could also contribute to the oxidation of the target chemical. Few studies have been conducted to evaluate the reactivity of ozone with target organic chemicals in soil systems. Yao and Masten (14) have demonstrated the ability of ozone to oxidize pyrene, chrysene and phenanthrene in soil columns. Day (15) showed that ozone could be readily transported through several soils, such as Ottawa sand, Metea and Wmtsmith soil Some pilot scale applications of in-situ ozonation has also been successful for the treatment of PAHs and halogenated VOCs (volatile organic compomrds) in the field (16, 17). In order to design in-situ ozonation systems, it is important that we understand the reaction mechanisms that would occur and develop models to predict the transport of gaseous ozone and the reactivity of the target chemicals in the in-situ soil environment. In order to accomplish this, one must understand the influence of soil organic matter in these reactions. In this study, we have focused on investigating the kinetics of the reactions of gaseous ozone with soil pore water, soil organic matter and phenanthrene in unsaturated 4O soils. The kinetic expressions were developed to describe these reactions using a cycling batch reactor and a shallow packed bed reactor. 3.2 Experimental Methods 3.2.1 Materials Phenanthrene, the model PAH used in this study, has at 25 °C an aqueous sohrbility of 1.18 mg/l and a vapor pressure of 2.67x10'5 kPa (18). It was obtained from Aldrich Chemical Co. at a purity of 98%. Toluene was selected as the model solvent, representing the lower molecular weight fiaction present in petroleum products. It was obtained from Sigma Chemical Co. at a purity of 99.8%. The aqueous solubility and vapor pressure of toluene are 515 mg/L and 3.8 kPa, respectively, at 25 °C (18). This study was conducted using Metea soil which was obtained from the southern end of Michigan State University at a depth of approximately 4 it below ground surface. The excavated soil was sieved through a No. 16 screen (1.19 mm) and air-dried at room temperature. The properties of this soil are listed in Table 3- 1. Ozone gas was produced from dried oxygen (99.9%) using an ozone generator (Model # T-408, Polymetrics, Inc.). Ozone gas was moisturized by passing the gas stream through a gas washing bottle containing deionized water, acidified using phosphoric acid to a pH of 2. This prevented the soil in the columns fiom drying out. 3.2.2 Analytical Methods The concentration of gaseous ozone was determined using UV spectroscopy (Model # 1201, Shirnadzu, Co.) at 254 cm. Quartz flow cells having a width of 0.2 cm 41 Table 3-1. Characterization of Metea soil Paramegrs M organic matter content‘ (wt %) 0.14 pHb 5.6 specific surface area" (m2 / g) 2.17 soil texturecl clay (%) 5.8 % silt (%) 17.6 % sand (%) 76.6 % ' Soil Sample was pretreated using 30% phosphoric acid to remove inorganic carbon, dried and analyzed by CNH analyzer. The organic matter content is expressed as the percent total organic carbon. b Determined by glass electrode method (ref 36). ° Determined by BET sorption method (ref 37). ‘ Pretreated with Na-hexametaphosphate as the dispersant and determined by sieving method (ref 38). 42 were used. A molar extinction coeflicient of 3000 (mol/L)'1cm'l (21) was used to convert absorbance units to concentration units. Phenanthrene was extracted fiom the soil using toluene. The wet contaminated soil samples were prepared for analysis by first drying the soil in a desiccator containing silica gel Once the mass of the soil samples was constant, a subsample was placed into a 12 mL centrifuge tube, to which 2 mL of toluene was added. The soil samples were sonicated (Sonic Dismembrator 550, Fisher Scientific) for 30 sec and then centrifuged for 5 min at 5000 rpm The supernatant was withdrawn from the vial using a glass pipette and placed into 2 mL glass vials. Phenanthrene was analyzed by gas chromatography (Autosytem, Perkin-Elmer Co.) using a flame ionization detector. A 0.53 mm capillary column (DB-5, J&W Co.) was employed along with helium as the carrier gas. The oven temperature was set at 100 °C for 3 minutes then increased at a rate of 20 °C/ min until a temperature of 240 °C was attained. During method development, it was determined using spike and recovery experiments that phenanthrene was not lost during either the drying or sonication steps. The recovery of phenanthrene fiom the soil was determined to be 93.3% using 0.2-1.0 g of soil The detection limit was determined to be 0.2 mg/L or 1.3 mg/kg soil based on 0.3g of dried soil. The residual tohrene concentration in phenanthrene contaminated soil after removed by air striping was examined by static headspace method with the same procedures as Hsu and Masten (19). Soil moisture was determined gravirnetrically afier drying the soil at 105 °C for 30 hours (22). The soil organic matter content was determined using CHN analyzer ( Model 43 # 2400, Perkin-Elmer Co.) and was expressed as total organic carbon (TOC). The methods used for other tests are provided in Table 3-1. 3.2.3 Soils Preparation The soil was prepared in glass columns (10 cm in length, 5.5 cm in diameter) equipped with a ceramic pressure plate (1 bar) at the bottom of the column. The air-dried soil was first packed into the column, then saturated with water by imbibing water from the bottom of the column until water ponded at the top of the column. The water- saturated soil was allowed to sit in the column overnight, afier which it was drained by applying cylinder air to the top of the column at 6 psi for 2 days. After draining the water from the soil, the cohrmn was opened and approximately one-third of the soil vohrme was removed from the column, homogenized by mixing with a spatula, sealed and placed in a desiccator. Water had been previously added to the bottom of the desiccator to ensure a relative humidity of 100% in the desiccator. The water and the soil organic matter contents of the wet soil were checked periodically during the period in which experiments were conducted. For experiments conducted in the 3/8” o.d. glass columns, the soil samples were in-situ prepared in the 3/ 8” id. columns with the same procedures described in the 5.5 cm id. column. A glass fiber filter was used in place of the ceramic pressure plates and the drainage pressure of 2 psi was employed. The contaminated soils were prepared by first saturated with water using the method described above. Following this, the water saturated ceramic plate was replaced with a clean, dry ceramic plate or, for 3/8” o.d. columns, with a Teflon” filter paper. Then the cohrmns were reassembled, and tested for leaks. After that, the soil was then saturated with a 10,000 mg/L solution of phenanthrene in toluene by imbibing the solution from the bottom of the column and the soil was drained for 2 days under 3 psi and 1 psi in the 5.5 cm i.d. colurrm and 3/8” o.d. cohrmn, respectively. Soil venting, as described by Hsu and Masten (19), was then applied to remove the toluene by passing cylinder air through the soil column. After toluene was no longer detected in the off-gas from soil column, the air flow was halted and the soil cohrmns were allowed to rest overnight. The air flow was then resumed and continued until toluene was again no longer present in the ofllgas. Approximately one-third sample of the contaminated soil was removed from the top of the 5.5 cm i.d. column, sealed in a glass vial and stored 1mder 100% relative humidity. Organic matter-free soil was prepared by treating the air-dried Metea soil first with a few drops of l M hydrochloric acid to lower the pH of the soil slurry to 2, in order to remove any carbonate. Following this, 30 % of hydrogen peroxide (H202) was added in increments of 5 mL until the slurry ceased to froth (20). Sodium hydroxide (1 N) was then added to the soil to adjust its pH to 5, which was slightly lower than the initial pH of the soil, 5.6. The hydrogen peroxide-oxidized soil was then further treated by passing gaseous ozone (ca. 3%, 100 mUmin) through the soil for 48 hours. Although no ozone demand was exerted by the soil after then first few minutes, the longer ozonation time ensured complete oxidation of all oxidizable organic matter. 3.2.4 Kinetic Experiments The rates of reaction of gaseous ozone with the organic matter-free soil and the native soil were determined using a cycling batch reactor. As shown in Figure 3-1A, the moisturized ozone gas was allowed to flow through a glass column (15 cm in length, 5 cm 45 ozone generator OQDG 0:: 0:: vent (to KI) 3-wa valvey :'O D circulatin water ba Figure 3-1A Experimental apparatus for cycling batch reactor. Syxrllii'e’r 46 in diameter) which was used as a reservoir. After the gaseous ozone concentrations at the inlet and outlet to the reservoir were equal, ie., steady state conditions were attained, the ozone generator was shut down and two three-way valves were switched to form a closed system The ozone gas was pumped using a diaphragm pump (Cole-Parmer, Inc.) fiom the reservoir through the soil reactor (containing approximately 2 g soil), then through a flow meter and back into the reservoir. The gas was allowed to flow through the top of the soil The pump and flow meter were equipped with Teflon seals and 1/8” diameter Teflon tubing was used throughout the reactor. The reactor was periodically checked for 1 leaks by gas leak detector (Model # 21-150, Gow-MAC Co.). In order to investigate the effect of gas flow velocity on the rate of ozone decomposition, experiments were conducted in glass colurrms of different diameters. The following three glass columns were used: (1) 1 cm (id.) with variable bed height, (2) 3/8” (o.d.) x 8 cm length equipped with Swagelock stainless steel fittings and Teflon ferrules, and (3) 2.5 cm (id.) x 15 cm length with Teflon caps. Each column was equipped with glass fiber filter paper at the inlet and outlet of the soil cohrmn to more uniformly distribute the gas flow. The soils used in the cycling batch experiments of 1- and 2.5 cm id. columns were obtained from the soils previously prepared in 5.5 cm id. columns, and those conducted in 3/8” id. cohrmn were in-situ prepared as described above. The void space in the 3/8” and 1 cm columns (between the soil and the top of the cap) were negligible; however, the void volume in the 2.5 cm column was reduced by placing 3 mm glass beads (pre-ozonated for 24 hours) below the soil The rate at which gaseous ozone reacted with the phenanthrene-contaminated soil was determined using a shallow packed bed reactor (illustrated in Figure 3-1B). 47 CD 6) H l] < __—l‘ _> O O o 0 ozone generator mass flow vent helium gas troller (to KI) +— V vent . . (to KI) — sorl packed bed reactor UV/V' 1s Figure 3-1B. Experimental apparatus for shallow packed bed reactor 48 Approximately 1 g of wet contaminated soil was placed in the 2.5 cm diameter cohrmn, which was pro-filled with 3 mm glass beads below soil samples. The flow rate of the moisturized gaseous ozone was controlled using a mass flow controller (Model #840, Sierra Co.) at a rate of 50 mL/min. Initially, the ozone gas bypassed the soil column and was vented into a potassium iodide (KI) solution until the concentration of ozone reached steady-state. Once steady-state conditions had been reached, the ozone gas was directed through the shallow packed bed reactor from top of the soil column. After the desired reaction time, the ozone gas was redirected to bypass the soil cohrmn and helium gas was immediately applied to the soil for approximately 2 minutes to purge all remaining ozone from the column. The soil was then removed from the column and dried in a desiccator containing silica gel until the weight no longer changed; after which the soil was analyzed for the residual phenanthrene concentration. The variations of the weights of soil and the gaseous ozone concentration used in all experiments were controlled to be less than 1%. 3.3 Results and Discussion 3.3.1 Rate of System Self-Decomposition Self-decomposition of ozone within the system was measured since, even in the empty cohnnns, approximately 20% of the ozone gas degraded in the cycling batch reactor after 5 hours of operation. The extent of decomposition was essentially independent of column size. This degradation may be the result of the self-decomposition of aqueous ozone in the moisturized gas (13), sorption or reaction of ozone on the glass or tubing surfaces, or the decomposition of ozone by UV light during monitoring. It was found that system decomposition did not fit simple first order kinetic or Langmuir sorption equations 49 during the initial reaction stage. To describe this decomposition, an empirical rate expression is proposed: dpo3 . — — = k t" = 3-1 dt .1 p03 opo, ( ) I 11" where k; = ks!" , p,3 is the ozone gas density (mg/L), k; is the first order rate constant (sec'l) and t is the time (sec). Table 3-2 lists the values for n and k., which were determined for different soil columns. 3.3.2 Rate of Decomposition of Ozone in Soil Water As ozone decomposes at pH>3 to form the OOH radical (8,13) and the pH of the Metea soil is 5.6, it was imperative that the rate of decomposition of ozone in the soil water be determined. This reaction has been shown to be first order with respect to ozone (8). At steady state conditions, the rate expression for the decomposition of aqueous ozone based upon the unit weight of soil water can be written as: _ .1 We, _ —V8 dpo3 "3 W, dt -W,6, dt = 16.10., (3-2) where Ww is the mass of the soil water (g), W,3 is the weight of gaseous ozone in the system (g), VB is the volume of the gas phase in the system (L), W, is the weight of dry soil (g), 6,, is the water content (g/g), and kw is the first order rate constant (sec'l). Combining eq 3-1 and 3-2, yields the total gaseous ozone decomposition rate: dpo3 _ ' ngw _ 4 — kopo3 +(V_g-)kwp03 (3-3) The kinetics of ozone degradation in organic matter-free soil and in contaminant-flee soil are illustrated in Figure 3-2. From the plot in which the system decomposition was 50 Table 3-2. System decomposition of gaseous ozone in the cycling batch reactors column [03]. mg]! k. n r2 type 318" 61.1 7.97 x 10'5 -0.2689 0.998 25.3 7.89 x 10‘5 -0.2689 0.987 53.9 7.89 x 10'5 -0.2689 0.984 avg. (7.92 :l: 0.05 ) x 10" 1 cm 50.86 7.89 x 10'5 -0.2266 0.992 78.12 8.89 x 10'5 -0.2266 0.994 77.35 1.11 x 10“ -0.2266 0.984 avg. (9.31 :l: 1.64) x 10‘5 2.5 cm 72. 9 1.78E-04 -0.3017 0.999 51 O 50 lOO 150 200 250 300 Time (min) Figure 3-2. The kinetics of the degradation of gaseous ozone in contaminant- and organic matter free Metea soil (moisture content = 8.4 %). Experiments were conducted in 3/ 8” o.d. cycling batch reactor. Solid circle (O) is the observed overall gaseous ozone degradation, and the circle (0) is the ozone degradation in soil water where system decomposition is being subtracted. Solid line is the pseudo-first order regression line for ozone degradation in soil water at steady state. 52 subtracted from the total decomposition, it can be observed that ozone decomposition is linear with time after approximately 60 minutes. It is believed that the initial (and higher) ozone decomposition rate is due to the dissolution of gaseous ozone into the soil water. As initially, there would be no ozone dissolved in the soil water, the initial mass transfer rate of ozone would be higher than that obtained once equih'brium conditions were obtained. However, as the initial stage is fairly long (ca. 60 minutes), this non-eqrrihbrium may also be due to the diffusion of aqueous ozone from the bulk water into the aggregate (immobile) water or to the water in the small pores. As ozonation times were generally long (>> 1 hour), the steady-state region of the curve can be used to represent the ozone-soil water reaction. Table 3-3 lists the values determined for the calculated rate constant, kw. Hoigné (23) reported that the aqueous ozone decomposition rate can be expressed as: -d[03]dt : kg” [03] = (175: 10) x[o,] x [OH'] (3-4) where [03] is the aqueous ozone concentration (mg/L). Because the mass of ozone consumed in the aqueous phase is equivalent to that degraded in the gaseous phase (after that due to system decomposition is subtracted) and the weight of soil water equals its volume, the rate constants in eqs 3-2 and 3-4 have the same physical meaning. Substituting 8 pH of 5.0 into eq 3-4, yields an ozone decomposition rate of constant of (1.75 .4.- 0.10) x 10‘7 sec". This value is comparable to the value of(l.02 i 0.15) x 10'7 sec], which was obtained in this study. As such, eq 4 can be used to describe ozone self- decornposition in the soil water once steady state conditions have been reached. 53 Table 3-3. Rate constants for the degradation of gaseous ozone in soil water“ dry soil soil water water content gas vel. k. r2 wt. (g) wt. (g) (vlw) % (cm/sec) 1.708 0.143 8.4 23.3 9.77E-08 0.993 2.021 0.138 6.8 17.5 8.88E-08 0.986 1.938 0.157 8.1 15.3 1.18E-07 0.997 avg. 1.02E-07 :t 0.15E-07 * All the experiments were conducted in 3/8" soil columns. 54 3.3.3 Rate of Reaction of Gaseous Ozone with Soil Organic Matter The observed degradation rate of ozone in the cycling batch reaction can be described as the sum of the rates of ozone reaction due to decomposition in the system and due to reaction with the soil organic matter: 4‘1"“! dpz‘: =(—d"°3 (1,)... +(—d”°/d,).. (3-5) The reactions of gaseous ozone with soil organic matter are extremely complicated as reactions may involve (1) gaseous ozone reacting with bare soil organic matter on the soil surface, (2) or in the aqueous phase as aqueous dissolved ozone reacts with dissolved and soil surface sorbed soil organic matter, or both. Since the distribution of aqueous and soil surface sorbed soil organic matter is unclear, and the mass of ozone consumed, in respective of whether it occurred in the gaseous or aqueous phases, is supplied from the gaseous phase (alter the system decomposition is subtracted), an rate expression based on gaseous ozone and overall soil organic matter concentrations (as total organic carbon, TOC) can be used to describe these complex reactions. Because the reaction of ozone and organic matter is an overall second order with respect to ozone and organic matter in the aqueous phase (24, 25), it is assumed that the reaction of gaseous ozone with soil organic matter is second order, overall; first order with respect to each reactant. The rate of degradation of ozone, normalized to the weight of soil, can be written as: _ dW —V d A 03 = 8 pa: = kompo 0’" W, dt W, dt 3 (3-5) (_r03 )0": = where om is soil organic matter concentration (mg/kg), reported as TOC, k0," is the 55 second order rate constant, sec“(kg Lx’"%g)“. Combining the rates of ozone decomposition due to reactions with organic matter and system decomposition, yields: dp W 03 _ ' s ‘7 - koPo, +V—kompo3 am (3'7) Because the decomposition of gaseous ozone in the system is significant and changes in the soil organic matter during the reaction can not be quantified, it is advantageous to introduce a term referred to as the selectivity of a competition reaction (26). The selectivity is the mass of ozone consumed due to system decomposition divided by the mass of ozone consumed due to the reaction of ozone with the soil organic matter. The selectivity is also the ratio of the reaction rates (26). Mathematically, the selectivity, S, can be represented as =.(dp°3 flit)!” = k;p03 = fl“; =(AW03)’J” -(dp.,/dt)... (M/V,)k...omp., k...om (M.,)... (3-8) V where ,6: 3 W S and (A W,3 )m and (A W,3 )0,” are the masses of gaseous ozone consumed by system decomposition and soil organic matter, respectively. As the total mass of gaseous ozone consumed in the reactor is the sum of that due to system decomposition and the reaction of the gaseous ozone and soil organic matter, (Am: ), = (AW03 )0, +(AW03 )0", = V800; — p03) (3-9) where pf,3 is the initial gaseous ozone density. By combining eqs 3-8 and 3-9, the mass of gaseous ozone consumed by the soil organic matter, (A was )0", , can be written as: 56 Vg(p2, - 1)..) AW = ( 0,)”. 1+S (3-10) A stoichiometric coefiicient based on mass consumption can be defined as the mass of ozone consumed relative to the mass of soil organic matter which reacts with ozone: (AW )0... 60m: 0’ %AWm) (3-11) where (A W”) is the mass of soil organic matter reacted with ozone. Combining eq 3-10 and eq 3-11, yields: V p" —p _ 0_ ___ 8 03 03 _ (AWom)—W,(0m 0m) 4 ( 1+S ) (312) where om° is the initial soil organic matter concentration. Eq 3- 12 can be re-written as: om=om ———————-——-:———- (3-13) because the fractional conversion of ozone, X ,3 , is defined as: : p3, - pa, p3, X 03 (3-14) Substituting eqs 3-13 and 3-14 into eq 3-7, the overall rate of ozone degradation can expressed as: “W . k flp" X °’=k ° l—X + 0'" ° l—X m°————°’—"3 3-15 dt opa,( 0,) fl p.,( 0,)(0 4 1+S) ( ) p3, As the selectivity, S, is the function of k; , om°, 4m, ,6, and X ,3 , one cannot obtain an analytical sohition for eq 15. Carberry (26) pointed out that in such cases, the selectivity, like the fractional conversion, can be determined based upon values obtained at the completion of the reaction, or based upon point values, which are those obtained for any 57 time during the reaction. Therefore, the selectivity, S, for each experimental datum can be assumed to be a constant and be calculated using the experimentally observed value X ,3 , k; which was calculated using eq 3-1, and assuming an initial k0,, value. Once this is done, eq 3-15 can be integrated to yield: 1 (A—BX.,)/(A—BX:,> 1=a wfiv nmd end .w>a 5”: 55.3 32:33 have?» can 58 man fl do v.3 név mdm 35.5 .8. 3:5 ad «in N6 N6 0.2 0.2 s. 33 ommd now; So; 3a; was.“ meA 3 .5on 05m $82. 8m wads—co m8 “55% me .055; 55 525%.: .5956 2C. . E9 m.~ :5— .o”\" 252.8 .535 .93 :3 E .03 5.5.8 535: cameo =8 .23 283 macaw» me 2:58.. 2: 8m 355:8 88 2:. 41m 053. 60 gas velocities for conventional soil venting that are conventionally applied in the field are between 0.5 and 12.7 cm/sec (30), the rate constant for the reaction of gaseous ozone with soil organic matter in natural soil can be assumed to be a constant. As such, the reaction rate constant would not need to be adjusted to account for spatial or temporal changes in velocity during field application. 3.3.4 Rate Eexpression for the Reaction of Gaseous Ozone with Phenanthrene Ifthe reaction of gaseous ozone with phenanthrene is second order overall, first order with respect to both gaseous ozone and phenanthrene, then the rate expression for this reaction can be written as: dC ——=k C 3-18 where C is the phenanthrene concentration in soil (mg/kg). For experiments conducted in the shallow packed bed reactor, the inlet gaseous ozone concentration is a constant and the concentration of ozone can be assumed to be uniform throughout the depth of soil in the shallow packed bed reactor. As such, eq 18 can be written as dC . d! C 9 ( ) where k; = kept,3 or, 41% )= kLt (3-20) The rate expression determined from results obtained using the shallow packed bed reactor was also verified using the data from the cycling batch reactor. Using the rate constant obtained from the shallow packed reactor and the measured p03 , which is 61 independent of the type of reaction, ie., whether the reaction is a parallel or series reaction, the phenanthrene concentration can be calculated by forward finite difference from eq 3- 18: C‘+1 = c‘[1— kcpis At] (3-21) where At is the reaction time interval between each experimental point, and the superscript i and i+l denotes the time flame. The results obtained using the shallow packed bed reactor are shown in Figure 3-4. Two kinetic regions were observed. For phenanthrene concentrations greater than about 40 mg/kg, a first order rate constant of 0.0037 sec'1 was obtained A second order rate constant of 7.56 x 10'5 (mg /L)'1 sec'l can be obtained by dividing the first order rate constant by the gaseous ozone concentration. For phenanthrene concentrations less than 40 mg/kg the first order rate constant was determined to be 0.00067 sec'1 , while the second order rate constant was 1.57 x 10'5 (mg /L)'1 sec". The results obtained for the decomposition of phenanthrene in the cycling batch reactor are shown in Figure 3-5. The phenanthrene concentrations were calculated using eq 21. As observed with the shallow packed bed reactor, two kinetic regions exist with 60 mg/kg seeming to be the dividing point. This deviation (in the breakpoint) is attributed either to the experimental error in determining the residual phenanthrene concentration or to the limited phenanthrene data points available for use in the forward difl‘erence calculation. Although in the literature, pseudo-first order rate expressions with respect to PAHs have been reported for the reactions of ozone and PAHs in the aqueous phase (1 l) 62 0.04 L [03]=47 mg/l 500 1000 1500 Time (see) Figure 3-4. The kinetics of the reaction of phenanthrene with ozone determined by shallow packed bed reactor. Experiments were conducted by passing gaseous ozone through phenanthrene contaminated Metea soil (moisture content (v/w) = 6.4 %, soil organic matter content = 0.14 %) in 2.5 cm i.d. packed bed reactor. The ozone gas flow rate was controlled at 50 mL/min. Lines represent the regression by using the pseudo first order rate expression (see eq 3-20 ). 63 [O3]o=84 mg/l Time (min) Figure 3-5. The kinetics of the reaction of phenanthrene with ozone in cycling batch reactor. Experiments were conducted by passing gaseous ozone through phenanthrene contaminated Metea soil (moisture content = 6.4 %, soil organic matter content = 0.14 %) in 2.5 cm id. cycling batch reactor. The ozone gas flow rate was controlled at 50 mL/min. Lines represent the calculated phenanthrene concentrations by using eq 21 and the rate constants obtained fiom Figure 3-4. and gaseous ozone with solid PAHs on uniformly coated surfaces (31-33), the presence of two kinetic regions has not been reported. The two kinetic regimes observed in the systems used in this study may be due to variations in the contacting surface area between gaseous ozone and solid phenanthrene during the course of the experiment. The solid phenanthrene is unlikely to be uniformly coated on the soil surface prior to reaction and the exact nature of the distribution of phenanthrene in the soil afler the application of soil venting to remove toluene is also rmknown. It is widely accepted that NAPLs distribute themselves in unsaturated soils as a continuous phase that floats on the surface of soil water and as material that is trapped at the pores by capillary force (3 8). Therefore, it is possible that after the tohrene was removed after soil venting, the residual phenanthrene became distributed in the soil as small particles at the pores or as a continuous solid layer on the surface of the soil water, or as both. Since ozone is likely to react only with the outermost layer of PAHs, the reaction rate would be expected to be constant until the contacting surface area began to decrease (31,33). Thus, it is hypothesized that the faster kinetic region occurs during the time when the irrterfacial surface area is larger and phenanthrene is both floating on the soil water and trapped in the pores. As the phenatherene that is floating on the soil water being reacted, only the phenanthrene that is trapped at the pores remains. The reaction that occurs during this time may account for the slower kinetic region. In addition to the two kinetic regions, some residual phenanthrene was observed for reaction times where the predicted value was zero. The non-zero residual phenanthrene may be due either to experimental errors in the analysis of phenanthrene or it 65 may be attributed to the characteristic that the residual contaminant is more difiicult to be removed from a soil, as is commonly observed with VOCs during soil venting (34), or during the desorption of groundwater contaminants from soil particles (39). 3.4 Conclusions A rate constant consistent with that determined for the self- decomposition of ozone in the aqueous phase was observed for the decomposition of gaseous ozone in soil water. The reaction of gaseous ozone with soil organic matter could be described as a second order reaction, first order with respect to both gaseous ozone and soil organic matter. A lumped rate constant was used since the exact distribution of the soil organic matter in soil water or on the soil particles is not clear. The rate of reaction of ozone with the soil organic matter was found to be independent of the ozone gas velocity, indicating that the chemical reaction rate is the dominant process rather than interfacial mass transfer rate. The results also indicate that the rate expression for this reaction would not have to be adjusted to account for difi‘erences in the gas velocity, at least within the range between 0.18 to 23.4 cm/sec, which covers range of gas velocities used in conventional soil venting. Although in the literature it has been shown that for PAHs that are coated uniformly in rmrltiple layers on a solid sru'face, the reaction of gaseous ozone with PAHs can be described as a first order reaction with respect to the surface area of PAH, our results indicate a more complicated relationship involving two kinetic regimes. This observation may be attributed to the nonuniformity of the surface distribution of solid phenanthrene, which may occur as different sized crystal particles and films of difl‘erent 66 thickness floating on the soil water. The results of this study demonstrate a theoretical basis for applying gaseous ozone to remediate unsaturated soils contaminated with residual PAHs. 3.5 List of References (1) Carter, J. C. In Petroleum Contaminated Soils; Kostecki, P. T., Calabrese, E. J, Eds; Lewis Publishers: Chelsea, MI 1990; Vol. 3, pp 3-7. (2) Must for US Ts, a summary of the new regulations for underground storage tank;U. S. Environmental Protection Agency, EPA/600/M- 86/020, 1988. (3) MacDonald, J. A.; Kavanaugh, M. C. Environ. Sci. T echnol., 1994, 8, 362A—368A. (4) Gibson, T. L.; Abdul, A. S.; Glasson, W. A; Ang, C. C.; Gatlin, D. W. Groundwater 1993, 31, 616-626. (5) Madsen, E. L. Environ. Sci. T echnol. 1991, 25, 1663-1673. (6) Bell, C. E. ;Kostecki, P. T.; and Caiabrese, E. J. In Petroleum Contaminated Soils; Calabrese, E. J., Kostecki, P. T., Eds; Lewis Publishers: Chelsea, MI 1989; Vol 2, pp 73-94. (7) US Environmental Protection Agency, 54 Federal Register, 1989, 22141. (8) Hoigné, J.; Bader, H. Water Res. 1983, I 7, 173-184. (9) Joshi, M. G.; Shambraugh, R L. Water Res. 1982, 16, 933-938. (10) Hapeman-Somich, C.; Zong, G.; Lusby, W. L.; Muldon, M. T.; Waters, R J. A gric. Food Chem, 1992, 40, 2294-2298. (11)Butkovic’, V.; Klasnic, L.; Orhanovic’, M.; Turk, J. Environ. Sci. T echnol., 1983, 17, 546-548. ( 12) Andreozzi, R; Insola, A; Caprio, V.; D’Amore, M. G. Wat. Res. 1992, 26, 639-643. (13) Hoigné, J.; Bader, H Wat. Res., 1976, 10, 377-386. (1 4) Yao, J. J.; Masten, S. J. In Proc. of 46th Purdue Industrial Waste Conference; Purdue University: West Lafayette, 1]). 1992; pp 642-651. 67 (15) Day, J. E. M. Sc. Thesis, Michigan State University, 1993. (16) Nelson, C. H; Brown, R A Environmental Engineering / A Supplement to Chemical Engineering, Nov. 1994, 20-23. (17) PreuBer, M.; Ruhol], H.; Schindler, E.; WeBling, E.; Wortrnann, C. Wissenschaft und Umwelt, 1990, 3, 135-140. (18) Mackay, D.; Shin, W. N. J. Phys. Chem. Ref Data, 1981, 10, 1175-1198. (19) Hsu, 1.; Davies, S. H R; Masten, S. J. In Proc. of 48th Purdue Industrial Waste Conference; Purdue University: West Lafayette, ID. 1993; pp 642-651. (20) Kunze, G. W.; Dixon, J. B. In Methods of Soil Analysis Part 1: Physical and Mineralogical Methods Second Edition; Klute, A Ed.; American Society for Agronomy: Madison, WI, 1986; pp 95-97. (21) Masschelein, W. J. In Proceedings of the Ninth Ozone World Congress: Ozone in Wastewater Treatment & Industrial Applications, New York,; International Ozone Association: Zurich, Switzerland, 1989, Vol 2, pp. 562-565. (22) Gardener, W. H. In Methods of Soil Analysis Part 1: Physical and Mineralogical Methods, Second Edition; Klute, A Ed.; American Society for Agronomy: Madison, WI, 1986; pp 493-507. (23) Staehelin, J.; Hoigné, J. Environ. Sci. T echnol., 1982, 16, 676-681. (24) Staehelin, J.; Hoigné, J. Environ. Sci. T echnol., 1985, 19, 1206-1213. (25) Xiong, F.; Legube, B. Ozone Sci. Eng, 1991, 13, 349-363. (26) Carberry, J. J. Chemical and Catalytic Reaction Engineering, John Wiley & Sons: New York, 1976, pp 24-26. (27) Carberry, J. J. Chemical and Catalytic Reaction Engineering, John-Wilely & Sons: New York, 1976, Chapter 6. (28) Bird, R B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena, John Wiley & Sons: New York, 1960; Chapter 21. (29) Levenspiel, O. Chemical Reaction Engineering, John Wiley & Sons: New York, 1972, Chapter 11. 68 (30) Hutzler, N. J .; McKenzie, D. B.; Gierke, J. S. Presented at the International Symposium on Processes Governing the Movement and Fate of Contaminants in the Subsurface Environment, Stanford, CA, July 1989. (31) Wu, C.; Salmeen, I.; Niki, H. Environ. Sci. T echnol., 1984, 18, 603-607. (32) Cope, V. W.; Kalkwart, D. R Environ. Sci. T echnol., 1987, 21, 643-648. (33) Alebic’-Jurectic’, A; Cvitas, T.; Klasinc, L. Environ Sci. T echnol., 1990, 24, 62-66. (34) Stinson, M. K JAPCA 1989, 39, 1054-1062. (35) McLean, E. O. In Methods of Soil Analysis Part 2: Chemical and Microbiological Properties Second Edition; Klute, A Ed.; American Society for Agronomy: Madison, WI, 1986; pp 206-209. (36) Carter, D. L.; Mortland, M. M.; Kemper, W. D. In Methods of Soil Analysis Part 1: Physical and Mineralogical Methods Second Edition; Klute, A. Ed.; American Society for Agronomy: Madison, WI, 1986; pp 413-423. (37) Kemper, W.D.; Rosenau, R C. In Methods of Soil Analysis Part 1: Physical and Mineralogical Methods Second Edition; Klute, A Ed.; American Society for Agronomy: Madison, WI, 1986; p 425-442. (38) Corey, AT. Mechanics of Immiscible Fluids in Porous Media, Water Resources Publications, Littleton, CO, 1990; pp 7-22. (39) Brusseau, M. L. J. Contam. Hydrol, 1992, 9, 353-368. CHAPTER 4 USE OF GASEOUS OZONE TO REMEDIATE PHENANTHRENE CONTAMINATED UNSATURATED SOILS: MATHEMATICAL MODELING AND COLUll/IN STUDIES 4.1 Introduction Soil venting is the one of the most commonly used treatment technologies for the remediation of rmsaturated soils that are contaminated with volatile organic chemicals, such as benzene, toluene and the chlorinated solvents. However, soil venting is ineffective for the removal of semi- or non-volatile chemical compounds, e. g., the polycyclic aromatic hydrocarbons (PAHs). The PAHs, which are present in petroleum products, are an important group of environmental pollutants as several of the PAHs are regulated in drinking water (1) and soil (2) and are carcinogenic or potentially carcinogenic (3). As such, methods to remediate soils contaminated with the PAHs and other semi- or non-volatile chemicals must be developed. The use of gaseous ozone to oxidize these organic chemicals has significant potential, since ozone and its primary decomposition product, the hydroxyl radical (OOH), are very strong oxidants with redox potentials, E°, of 2.07 and 3.06 V, respectively. Since in-situ ozonation requires similar equipment as that necessary for conventional soil venting, the installation costs of in-situ 69 70 ozonation can be reduced if soil venting has been used as the preliminary procedure to remove the volatile organic chemicals. Based upon studies we have done with several soils (4), the costs of generating ozone to meet the ozone demand exerted by a soil can be estimated to be on the order of < $2.00 per ton soil Even with the additional costs of installing the distribution system, the total remediation cost would be expected to be much lower than that required for excavation followed by incineration or thermal stripping. While the reaction of ozone with PAHs (5-6) has been investigated in organic solvents and in the aqueous phase, few studies (7-10) have investigated the reaction of gaseous ozone with PAHs (or other organic chemicals) in contaminated soils. In laboratory studies, Yao and Masten (7 ) observed complete removal of pyrene and phenanthrene, both initial concentrations were 100 mg / kg, from a contaminated air-dried soil using 30- and 25 mg ozone/ mg PAHs, respectively. Hsu and Masten (8) showed that naphthalene could be completely removed by using gaseous ozone as a secondary treatment afler conventional soil venting. In the field, the PAHs have also been successfully removed from contaminated unsaturated soils using in situ ozonation (9,10). The formation of toxic by-products by the reaction of ozone with PAHs has also been investigated Upham et al. (11,12) who observed that the toxicity of the ozonation products formed fi'om the reaction of ozone with pyrene are all less than that of the parent compound if 4.5 mol ozone/ mol of pyrene was used. The purpose of this study is to model the one dimensional transport of ozone in- situ so as to predict the performance of ozone in the field. The mass balance equations for gaseous ozone, phenanthrene, and soil organic matter have been derived. Because 71 gaseous ozone is a compressible fluid, the equation of motion for gaseous ozone was also included to describe the variation in the gas velocity along its flow path. The terms to describe the reaction of gaseous ozone with phenanthrene, and soil organic matter are very complicated due to the inderterminant distribution of contaminant and soil organic matter in the moist soil. In this study, the rate expressions for the reactions of gaseous ozone with soil organic matter and with phenanthrene obtained from Chapter 3 were used in the mathematical simulation. Ozone gas transport through one-dirnensional soil cohrmns was monitored to verify the results of the sinnrlation. 4.2 Experimental Materials and Methods 4.2.1 Materials Phenanthrene, the model PAH used in this study, has, at 25 °C, an aqueous solubility of 1.18 mg/l and a vapor pressure of 2.67x10’5 kPa (13). It was obtained from Aldrich Chemical Co. Inc. (Milwaukee, WI) at a purity of 98%. Toluene was selected as the model solvent, representing the lower molecular weight fi'action present in petroleum products. It was obtained fiom Sigma Chemical Co. (St. Louis, Mo) at a purity of 99.8%. The aqueous solubility and vapor pressure of toluene are 515 mg/L and 3.8 kPa, respectively, at 25°C (13). This study was conducted using Metea soil which was obtained fiom the southern end of Michigan State University at a depth of approximately 4 it below ground surface. The excavated soil was sieved through a No. 16 screen (1.19 mm) and air-dried at room temperature. The properties of this soil are listed in Table 3-1 of Chapter 3. 72 Ozone gas was produced from dried oxygen (99.9%) using an ozone generator (Model # T-408, Polymetrics, Inc. ). Ozone gas was moisturized by passing the gas stream through a gas washing bottle containing deionized water, acidified using phosphoric acid to a pH of 2. This prevented the soil in the columns fiom drying out. 4.2.2 Experimental Method The soil columns were prepared according to the method described in Chapter 3. Air-dried Metea soil was packed in either 5.5 cm id., 10 cm or 30 cm long glass cohrmns. Water were imbibed from the bottom of the columns, allowed to sit overnight, and, then, drained to residual water saturation. For the contaminated soil column, a solution containing 10,000 mg/l phenanthrene in toluene was imbibed from the bottom of the soil column containing residual water. The column was then drained. Following this, conventional air stripping was applied to remove the toluene fi'om the soil (8). The properties of the prepared soil columns and the related parameters used for simulation are listed in Table 4- 1. As shown in Figure 4-1, ozone gas was passed through a mass flow controller (Sierra Co., Model #840), prior to being bubbled through the acidified aqueous solution. The gaseous ozone flowed through a UV/vis spectrophotometer (Hewlett Packard, Model #8452A) to continuously monitor the gaseous ozone concentration. Afler the gaseous ozone concentration had reached steady state, a three way valve was switched to direct the ozone gas into the soil column. During this time, the efiluent ozone concentration fiom the soil column was measured spectrophotometrically at a wavelength of 254 nm 73 Table 4-1. Soil colunms properties and operating conditions for ozonation Columns A B C 1)‘ column propgt’es “" : column length (cm) 10 30 10 10 bulk density (kg/L) 1.57 1.58 1.65 1.61 water content % (v/w) 10.2 7.9 8.8 5.1 total porosity 0.40 0.40 0.38 0.39 air porosity 0.25 0.28 0.24 0.31 opmmflmdibm gas flow rate" 175 195 192 194 pore gas velocity“ 0.531 0.529 0.603 0.467 inlet gaseous ozone concentratione 83.5 77.5 85.2 83.0 ' The soil column propeties were determined before ozonation b Soil organic matter concentration in each soil column was 0.14% as total organic carbon. ° The gas flow rate indicated were measured at the end of soil columns at 1 atm. pressure. " The gas velocity was determined at the efilent of soil colurm ° The inlet gaseous ozone concentration was measured at 1 atm f The phenanthrene concentration in the soil column before ozonation was 212 mg/kg. 74 ozone ® generator 0 :1 [I Lit L... a O o o ' S washin — o. I L- _§8ttle g is mist mass flow vent controller \ trap vent flow ureter UV/vis spectrophotometer Figure 4-1. Experimental apparatus used for the transport of gaseous ozone through soil columns. 75 An extinction coeflicient of 3000 M'1cm'1 (16) was used to convert the absorbance values to concentration rmits. The phenanthrene was extracted from the soil by sonication in toluene and analyzed by GC. For, details, please refer to Chapter 3. The differences in the gas pressures at the inlet and outlet of the 10- or 30 cm soil columns were small that they could not be accurately measured by conventional l-bar pressure gauge. A digital tensiometer was used to determine the air conductivity of two soil columns by measuring the gas pressures at the inlet of soil columns for different gas flow rates. Darcy’s equation (19) was then used to determine the gas conductivity of the columns. 4.3 Model Development 4.3.1 Mass Balance Equations Any mathematical model that is developed to describe the fate of ozone in a contaminated soil must incorporate the equations describing the transport of gaseous ozone, and the mass balance equations of soil organic matter and contaminants which involve the reactions with ozone. Because the length of in-situ ozonation time is expected to be long, it has been assumed that ozone transport during the first few bed volumes, which unsteady state conditions would likely exist, is unimportant. As such, no attempt was made to model the dissolution of gaseous ozone into the soil water during the initial unsteady state. The variations of volume in the gas phase resulting from the reactions of gaseous ozone with soil organic matter and phenanthrene is unknown. However, as the 76 gaseous ozone concentration used in this study is quite dilute (ca. 4%), it has been assumed that any volumetric changes in the gas phase between the inlet and outlet of soil column are small enough to be neglected. Any pressure variations that might exist during the initial unsteady state conditions have been neglected, either. The mass balance equation for gaseous ozone within an infinitely small control volume can be written for the transport of gaseous ozone through an uncontaminated moist l-D soil column as: 5p. nsa ——a—3(AxAyAz) = nsa J1x AxAy - nsa J|1+Ax AxAy — (4,3 )Aw, (4- 1) where n is the total porosity of soil; 5. is the degree of saturation of gas phase; p03 is gaseous ozone mass density (mg/L); (AxAyAz) is the control volume; J|x is the gaseous ozone flux through x-y plane; Aw, is the weight of the dry soil in the control volume; (—r,,3) is the rate of reaction of gaseous ozone with soil organic matter normalized to the mass of soil (see Chapter 3): -1 dw —r03 = w— d? = kmp03om (4-2) 3 where w,3 is the weight of ozone (mg), k0... is a second order rate constant (sec"(mg of organic carbon / kg of soil)'l(L of gas / kg of soil)); om is the soil organic matter concentration expressed as total organic carbon (mg/kg). Because AW, = pt, (AxAyAZ) (4-3) , where p, is the bulk density of soil (kg/L), eq 4-1 can be rewritten by substituting eq 4- 2, 4-3 as: 77 070 a1 0’ = ——-— (%)kmposom (4-4) 07 d The equation describing the ozone flux, J|z , which includes the bulk gas velocity and dispersion terms, is 5pc, 62 J]‘ = vzpo3 —D, (4‘5) where D2 is the logitutional dispersion coeflicient. Carberry (15) noted that the dispersion term in packed bed reactor is negligible if the ratio of the bed length to the particle diameter is greater than 50. Since the largest diameter of soil grain is 1.19 mm and the length of soil column was at least 100 mm in this study, the dispersion term was, justifiably, neglected. The dispersion term was also omitted from the equations describing the transport of gases, as is conventionally done in modeling air flow in soil venting (16,17) or in a gas absorption tower (18). Therefore, eq 4-5 can be simplified to: apos apt), pb a V 2 ( a ) ompo3 0’" ( ) Assuming that the soil organic matter is immobile and use the rate expression obtained in Chapter 3, the mass balance for soil organic matter can be written as: dam _ 1 apo3 d (”a = (k... Momma, om (4-7) where 50,, is stoichiometric coefficient ( g of ozone / g of soil organic carbon ). The concentration of soil organic matter is a function of time and the location in the soil column. For the contaminated soil, the equations describing the reactions of ozone and contaminant are the same as that given above (eq 4-7). Therefore, the mass balance for the contaminant, ie., phenanthrene in this study, can be written as: 78 _r_5po, 6.. 0? g =p.,c (4'3) where c is the phenanthrene concentration in the soil (mg/kg). The distribution of phenanthrene in the moist soil will affect the reactions of ozone with phenanthrene and the soil organic matter. However, as the distribution of the phenanthrene in the moist soil after applying air stripping to remove the toluene is rmknown, two possible reaction mechanisms have been considered. Ifthe phenanthrene .L' were to completely coat the soil particles then the reactions of ozone with phenanthrene and with the soil organic matter would be sequential, with the reaction of ozone and phenanthrene predominate in the early stages of the reaction. On the contrary, if phenanthrene only partially covers the soil surface then the reactions of ozone with phenanthrene and with the soil organic matter would be parallel, both occurring from time zero. Using eq 4-6, the equation for the parallel reactions can be written as: fl:_v%_(m 0? 0'2 ns )po3 (kmom + kcc) (4-9) For the sequential reactions, the PAH would react first. Thus, the value of km. in eq 4-9 would be zero until the point that the organic matter begins to react with ozone. The initial conditions for the above mass balance equations are: t=0, c=c°, om=om°, p,1 = 0 (4-10) while the boundary conditions are z=0, c=0, om=0, p,3 = ,o‘;3 (4-1 1) 4.3.2 Gas Flow Equation 79 As the gas is a compressible fluid, the gas pressure would decrease along flow path, thereby, affecting the ozone gas velocity and its mass concentration. As the gas pressure has been assumed to reach steady state rapidly after the gas flow is initiated, the pressure distribution in 1-D soil column at steady state could be written fi'om the equation of motion (19) as: 10.2-P} P(Z) = [P3 — 21% (4-12) where P is the gas pressure (cm H20) along the flow path; P; and Pf are the inlet and outlet ends gas pressure, respectively; L is the length of gas flow path (cm). By substituting Darcy’s equation into eq 4-12, the gas velocity along the flow path can be described as: 2 2 2 2 =m[p2_flzf% (4-13) 2L ' V(Z) where kd is the gas conductivity (cm/sec). The gaseous ozone concentration used in the mass balance equations given above is erqrressed as a mass concentration rather than as the gaseous ozone partial pressure. Therefore, the ozone mass concentration needs to be corrected for the deviation in the pressure from atmospheric pressure. Using ideal gas law, the ozone mass concentration at any pressure can be written as: p..|, = Peal...<%,,> (444) where the subscription of atm indicates the atmospheric pressure (1033.6 cm H20). 4.3.3 Numerical Solution 80 The finite difference method is used to simultaneously solve the mass balance equations for gaseous ozone, soil organic matter and phenanthrene. Using the forward difference method, eqs 4-6 and 4-7 can written as: p1 l—p’ tom-p] p k fill.-- M__L_£ J J - At — v Az n 3,, ponom, (415) J I 0m —0m ”IA! i = —(%m ”com/71310)": (4-16) where i and j are the indices for the time and space coordinates. Using the initial and boundary conditions given in eqs 4-10 and 4-11, eqs 4-15 and 4-16 are explicit and can be calculated along the time direction first for a given point of space. The gaseous ozone concentration and velocity at each calculated data point were corrected for pressure variations using eq 4-12 to determine the pressure distribution, then using eqs 4-13 and 4- 14 to calculate the corrected gas velocity and ozone concentration for each specific temporal and spacial point. 4.4 Results and Discussion 4.4.1 Transport of Gaseous Ozone through Organic-Matter Free Soil Gaseous ozone was allowed to flow through a pre-ozonated soil column to determine the distance that the ozone gas would penetrate through an unsaturated soil which exerted essentially no ozone demand. Prior to the experiment, the soil was treated with gaseous ozone to completely oxidize any oxidizable matter present in the soil. As shown in Figure 4-2, gaseous ozone was first detected in eflluent afler ca. 1.1 bed volumes (BV). The concentration of ozone in the eflluent gas rose sharply and then began [ozone], mall 81 9o 80 g a? i i i i i a? . 70. 0.0..OOOOOOOOOOOOOOOOOOOO0000...... O O 60“” o o experimentd dab (pH=5.6) 1 o -kw=1.02e-7(pH=5.0) 50 -. x kw=1.02e<3(pH=9.0) . ° okw=1.029-2(pH=10.0) 4o .- o + kw=1.02e-1(pH=11.0) 301L 0 0 20* ++++++++++++++++++++++++++++++++++++ 0 10¢ o 0 OJ % l t + 4T o 50 100 150 200 250 300 time (see) Figure 4-2. The breakthrough curves of gaseous ozone passed through pre- ozonated 10 cm long soil column. The gas flow rate was 194 ml/min, obtained from the eflluent of soil column. The soil cohrmn properties were: bulk density = 1.53 g/cm3; total porosity = 0.42; degree of water saturation = 0.31; degree of gas saturation = 0.68. No pressure correction was made for those simulations. The time needed for gas to pass through one bed volume was 19 seconds. Except the open circles are experimental data, others are simulation results. 82 to level off at about 3 BV. By substituting the rate expression for the degradation of gaseous ozone in the organic-matter free moisture soil (see Chapter 3) -1 dWa3 _.r03 = W; —-—-—-dt : kwpa3 (4- 17) into eq 4-6, where Ww is the weight of soil water, the gaseous ozone transport equation can be described as : (T) o” a" , p“: ””3 >k..p., (4-18) . —=-v——( a 0'2 Swpb From this, the ozone breakthrough curve (BTC) can be calculated (also shown in Figure 2). Ifthe flow pattern can be described as an ideal plug flow, if no chemical reactions or physical sorptions occurred, and if the initial variation in the gas pressure change during the unsteady state conditions can be neglected, the concentration of ozone would be expected to increase vertically at 1 BV. Although the equation describing ozone degradation in the soil has been incorporated into eq 4-18, the rate expression, —r,‘: , only accounts for the reaction of ozone at steady state conditions. It does not consider the initial dissolution of gaseous ozone dissolution into the soil water, which is thought to be (,5. ' the main reason that the simulation results rise up sharper than the experimental data. The deviation in the simulations and the experimental results may be also attributed to the fact that we have neglected the to consider the initial unsteady state conditions, where the gas velocity would be slower as the gas pressure builds up. However, despite the deviations, the model and its numerical solution is considered to be valid to evahrate the transport of gaseous ozone passing through the soil organic matter or contaminant containing moist soils, where the chemical reaction rates of ozone reacted with the soil organic matter or 83 contaminant would be expected to be much faster than the transport gaseous ozone passing through the soil water ( details of these reactions will be discussed in the next sections). Because the rate constant, k,, , for the degradation of gaseous ozone in soil water has been shown to be equivalent to the rate of aqueous ozone self decomposition (see Chapter 3), the rate constants for ozone decomposition at different soil pHs can be calculated using the equation developed by Hoigné and Bader (19): k, = 1751: 10x [011‘] (4-19) This equation can be substituted into eq 4-18 to describe ozone transport through the pre- oxidized soil. The simulated BTCs for the transport of gaseous ozone through soils having different pH values are shown in Figure 2. It is observed at pH < 10, the aqueous ozone self-decomposition does not significantly affect the distance to which gaseous ozone is transported. This is important since the pH of most natural soils range fiom 5 to 9 (20), as such the depletion of ozone in a pre-oxidized unsaturated soil would be insignificant if the soil pH were below 10. As such, the ozone demand of an oxidized soil is finite, and once the ozone demand is met, the ozone gas would be able to move through the portion of soil. 4.4.2 Transport of gaseous ozone through natural soil The ability to describe the transport of gaseous ozone through a soil containing organic matter is imperative since ozone consumption in most naturally contaminated soils would be the result of the reaction of ozone with soil organic matter. This is true because 84 the soil organic matter content would, in almost all cases, be significantly higher than the concentration of the contaminants. Experiments were conducted in 10- and 30 cm long soil columns containing unsaturated natural Metea soil. The soil properties and operation conditions for each experiment are listed in Table 4-1. The experimental and simulated results of the ozone gas BTCs are shown in Figures 4-3 to 4-5. Unlike the transport of gaseous ozone through the organic matter-free soil, here, detectable concentrations of gaseous ozone were observed at a much later and the rising period was much slower. These phenomena are due to the consumption of ozone by the soil organic matter. The total ozone consumed by the soil containing organic matter can be obtained by graphically integrating the left-hand-side of the BTCs and multiplying this value by the gas flow rates as Awe, = QRPZ, -p.., M (4-20) where Aw,3 is the total mass of ozone consumed; Q is the gas flow rate (Umin) at the eflluent; dt is the time interval between each measured gaseous ozone concentration at the efiluent; pf,3 is the inlet gaseous ozone concentration of soil column which was determined by taking the average outlet ozone concentrations observed during the last 5 hours which have been leveled oflZ The average stoichiometric coefiicient (ozone to soil organic matter) was determined to be 5.44 i 0.15 g of ozone / g of soil organic matter as total carbon. (The uncertainty of the above value was 1 standard deviation). This value was obtained using four 10 cm long soil columns. The average stoichiometric value does not include the data obtained from the 30 cm long column because complete oxidation of 85 the organic matter could not be assumed as the period for which the emuent ozone concentration was constant was not as long as that obtained for 10 cm columns. Using the properties listed in Table 4-1, the average stoichiometric coeflicient, and the rate expression for the reaction of ozone with soil organic matter (obtained from Chapter 3), eqs 4-12 to 4- 16 can be used to sinnrlate the ozone gas transport through unsaturated soils. As shown in Figure 4-3 to 4-5, the sinmlated results adequately fit the experimental data, except that the initial simulated value has a slight tendency to be greater than the experimental data. This phenomena is most likely due to the fact that during the initial unsteady state conditions the actual gas flow velocity is less than that observed during the steady state conditions and that the initial dissolution of gaseous ozone into the soil water has been neglected. The gas conductivity in 10 cm long soil column was latterly determined by using a fine digital tensiometer to read the inlet gas pressures of an ozonated soil column at different gas flow rates. As shown in Figure 4-6 the determined gas conductivity was 0.08 cm/ sec for column A by applying Darcy's equation ( 19). As expected, the simulations obtained when the model was corrected for the gas velocities and gaseous ozone concentrations along flow path were not significantly different from that obtained when the model was not corrected for pressure changes through the 10 cm columns (see Figure 4-3). However, for field applications, where the flow path would much longer than 10- or 30 cm, the efl‘ect of pressure correction on the model simulation needs to be considered. [ozone] (mg/1 86 90 -u I I! “HE-3'" ” +14 80 -— ~. J . - .I '- 70 + A" ' J. $ -9 60 “T 'h 9 50 ~~ _.' Q.‘ .. cohnmA 40 + ." - . - expenmental data 9: x model prediction with pressure corr. 3O __ _' 0 model prediction wo/pressure corr. rr . 20 -— 10 —— ' 0 i?- l l t 0 5 10 15 20 time (hr) Figure 4-3. The breakthrough curves for the transport of gaseous ozone through Metea soil Column length was 10 cm. Average stoichiometric coeflicient, 5.44 g of ozone / g of total organic carbon, was used for simulations. For simulation with pressure corrections, a gas conductivity of 0.080 cm/sec was used and the inlet gas pressure was calculated as 1096 cm H20 by substituting the measured gas flow rate, 194 ml/min, into Darcy's equation. 87 80 70-+ 60~ 50~> 4o .. [0:] mg/l 30~> 20~~ Colurm B - experimental data a model prediction will zeta=4.80 x model prediction wih mta=5.44 L f 15 tiM901?) 10 20 25 30 Figure 4—4. The breakthrough curves for the transport of gaseous ozone passed through Metea soil. Column length was 30 our Model predictions were done without pressure correction. The stoichiometric coefficient, 4. 80 g/ g, was obtained directly fi'om the integration of this ozone breakthrough curve, and the vahre of 5.44 g/ g was the average stoichiometric coeficient obtained from 10 cm columns. 88 90 0 WWW 80 * M'nl' - I ‘6' 70 ~ i o J 60 —— 5 o .5 E 50 __ f0 COMO E s -experimentaldata §- 40 -- 1|; 0 modelprediction wo/pressure correction 30 -- 5 of 20 -- 10 ~ 5 0 5 10 15 20 time (hr) Figure 4-5. The breakthrough curves for the transport of gaseous ozone through Metea soil Column length was 10 cm Average stoichiometric coeflicient, 5.44 g/ g, was used for simulation. v (cm/sec) 89 0.8 3:098 0.7 -~ 0.6 -~ 0.5 -~ 0.4 4 0.3 4- 0.2 1} 0.1 «~ -dpldz Figure 4-6. The determination of the gas conductivity. Soil column A was used (properties see Table 4-1). The pressures were measured at the inlet of soil column and the outlet pressures were assumed to be one atmosphere. Gas velocity was converted from the gas flow rate measured from the outlet end of soil column and Darcy’s equation was used to determine the gas conductivity. 1O 90 4.4.3 Transport of Gaseous Ozone through Phenanthrene Contaminated Soil The BTC for the transport of gaseous ozone through a phenanthrene contaminated soil (column D) and the simulation results are shown in Figure 4-7. The initial phenanthrene concentration prior to ozonation was 212 mg/kg, and the soil organic matter was 0.14 % as total organic carbon. A second order rate expression was used to describe the reaction of gaseous ozone with phenanthrene in the soil (see Chapter 3). Other parameters used in the simulation are listed in Table 4-1. The stoichiometric coeflicients for the reactions of ozone with soil organic matter and phenanthrene were obtained independently. For soil organic matter, the averaged value, 5.44 g/ g, was used. However, the stoichiometric value for the reaction of ozone with phenanthrene was determined by spiking the phenanthrene into a pre-ozonated Metea soil column. By graphical integration using eq 20, the stoichiometric coefficient was determined to be 8.39 g of ozone / g of phenanthrene. The simulation data (Figure 4-7) fits the experimental results reasonably well at the initial stages but the predicted values are slightly lower than the experimental values during the latter rising stages. As explained in the soil organic matter section, the pressure re-distribution at the initial unsteady state may be the main reason for those deviations. There was no apparent difference in the simulation results obtained when the reactions of ozone with the soil organic matter and the phenanthrene were assumed to be parallel or serial (see Figure 4-7). With the serial reaction, it was assumed that ozone began to react with soil organic matter when the phenanthrene concentration was less than 40 mg/kg. This was based upon studies conducted using the phenanthrene contaminated ozone (mg/l) 91 80 4 70 «— -L _ 60 _ ‘ cohan .' U - experimental data 50 -_ 3' . o modelprediction w/serial reactions . a: model prediction w/parallel reactions ‘ I 40 “T -. I 30 + - O *- 20 ~— in 10 ~~ o 0 tin-13 l l 4 l 0 5 10 15 20 25 time (111') Figure 4-7. The break through curves for the transport of gaseous ozone through phenanthrene contaminated soil. Column length was 10 cm Model predictions were done without pressure correction. 92 Metea soil, which indicated that at this concentration the surface coverage of phenanthrene on the soil began to decrease ( see Chapter 3 ). However, as there is no evidence indicating that at concentrations used in this study, the PAHs can cover the entire soil water surface, it might be reasonable to assume that the reactions of ozone with phenanthrene and with soil organic matter are parallel These assumptions were fiuther analyzed by simulating the reactions that would occur in a soil contaminated with phenanthrene at a concentration of 1000 mg/kg. In this investigation, the soil organic matter content was varied from 0.1- to 5%, while the gas flow rate and the soil properties were assumed to be the same as those used in colunm D. As shown in Figure 4-8, there is no significant difference in the sirrmlations when the ozonation reactions were assumed to be parallel or serial for any of the organic matter contents investigated. As shown in Figure 4-8, it was also observed that the simulated phenanthrene concentration always decreased faster and earlier than that observed for the soil organic matter. As such, phenanthrene can be completely degraded before all of the soil organic matter has reacted. This result implies that ozone consumption could be reduced if the flow of gaseous ozone were stopped at the point where the theoretical contaminant concentration decreased to zero instead of halting the ozone flow when the concentration of ozone in the eflluent gas had stabilized. The residual phenanthrene concentrations were analyzed from two soil columns after ozonation. For the column (D) where the soil were ozonated for 20 hours and where the emuent ozone concentration had stabilized greater than 5 hours, the residual phenanthrene concentrations were below detection limit (1.3 mg/kg) along all axial direction. The soil from a second other phenanthrene contaminated 93 0 20 40 60 80 100 120 [ozone] mg/l 40 60 80 100 120 0 20 40 60 80 100 120 Figure 4-8. Simulation results of the phenanthrene, ozone, and soil organic matter breakthrough curves at different soil organic matter concentrations in 10 cm long soil columns. Lines and circles represent the sinnrlation results, assuming either serial- and parallel reactions, respectively. Curves from the left to the right side correspond to the soil organic matter contents were 0.1%, 0.5%, 1%, 2.5%, and 5%, respectively. The initial phenanthrene concentration in the soil was assumed to be 1000 mg/kg. Other soil properties and operation conditions were the same as those used for column D. 94 column (215 mg/kg initial phenanthrene concentration) was analyzed afler the eflluent gaseous ozone concentration reached 90% of the inlet ozone concentration. The residual phenanthrene concentration in soil column would be completely reacted as the emuent gaseous ozone concentration reached 74% of the inlet ozone concentration from the sinnrlation results. However, an average phenanthrene concentration of 2 i 5 mg/kg (some samples were below detection limit) was observed by taking samples along the axial directions. This slight discrepancy may be attributed to the non-uniform initial distribution of phenanthrene in that soil column; or the phenanthrene sorbed and / or trapped in immobile regions of the soil as is observed in soil venting (21). These indicate that this mathematical model, again, adequately describes this process; and it would be prudent to ozonate the soil for longer than the time required for the contaminant concentration to theoretically degrade to zero. This should ensure the removal of any contaminant that is non-uniformly distributed in the soil or trapped in the innnobile zone of soil. 4.5 Conclusions The use of in-situ ozone venting is a valid and economic technology to remediate the soils contaminated with organic chemicals. The model developed in this study was found to fit reasonably well with the experimental data obtained in one dimensional soil columns, using the rate expression obtained from Chapter 3. However, it was observed that the initial unsteady state conditions for the gas pressure resulted in minor deviation between the simulations and the experimental data. Based upon the sinnrlated results, the reactions of ozone with phenanthrene and organic matter could be modeled as either 95 parallel or serial reactions. Because the initial distribution of contaminant would be non- uniform and some contaminant would be sorbed or trapped in immobile regions, it is recommended that the in-situ ozone venting be ceased no sooner than when emuent ozone concentrations begins to stabilize. 4.6 List of References (1) US Environmental Protection Agency, 54 Federal Register, 1989, p 22141. (2) Bell, C. E. ;Kostecki, P. T.; and Caiabrese, E. J. In Petroleum Contaminated Soils; Calabrese, E. J., Kostecki, P. T., Eds; Lewis Publishers: Chelsea, MI 1989; Vol 2, pp 73-94. (3) Carter, J. C. In Petroleum Contaminated Soils; Kostecki, P. T., Calabrese, E. J, Eds; Lewis Publishers: Chelsea, MI 1990; Vol 3, pp 3-7. (4) Day, J. E. M. Sc. Thesis, Michigan State University, 1993. (5) Butkovic’, V.; Klasnic, L.; Orhanovic’, M.; Turk, J. Environ. Sci. T echnol., 1983, 17, 546-548. (6) Andreozzi, R; Insola, A; Caprio, V.; D’Amore, M. G. Wat. Res. 1992, 26, 639-643. (7) Yao, J. J .; Masten, S. J. In Proc. of 46th Purdue Industrial Waste Conference; Purdue University: West Lafayette, ID. 1992; pp 642-651. (8) Hsu, 1.; Davies, S. H. R; Masten, S. J. In Proc. of 48th Purdue Industrial Waste Conference; Purdue University: West Lafayette, 1]). 1993; pp 642-651. (9) Nelson, C. H; Brown, R A Environmental Engineering / A Supplement to Chemical Engineering, Nov. 1994, 20-23. (10) PreuBer, M.; Ruholl, H; Schindler, E.; WeBling, E.; Wortrnann, C. Wissenschaft und Umwelt, 1990, 3, 135-140. (11) Upham, B. L.; Yao, J. J.; Trosko, J. E.; Masten, S. J. Environ. Sci. T echnol., in press. (12) Upham, B. L.; Masten, S. J.; Lockwood, B. R; Trosko, J. E. Fundamental and Applied Toxicology, 1994, 23, 470-475. 96 (13)Mackay, D.; Shiu, W. N. J. Phys. Chem. Ref. Data, 1981, 10, 1175-1198. (14) Masschelein, W. J. In Proceedings of the Ninth Ozone World Congress: Ozone in wastewater treatment & Industrial Applications, New York,; International Ozone Association: Zurich, Switzerland, 1989, Vol. 2, pp. 562-565. (15) Carberry, J. J. Chemical and Catalytic Reaction Engineering, John Wiley & Sons: New York, 1976, Chapter 4. (16) Johnson, P. C.; Kemblowski, M. W.; Colthart, J. D. Ground Water, 1990, 28, 413- 429. (17) Baehr, A L.; Hoag, G. E.; Marley, M. C. Journal of Contaminant Hydrology, 1989, 4, 1-26. (18) Geankoplis, C. J. Transport Process and Unit Operations: Allyn and Bacon: Boston, MA, 1978; pp 419-432. (19) Wilson, D. J.; Clarke, A N.; Clarke, J. H Seperation Science and Technology, 1988, 23, 991-1037. (20) Forth, H D.; Turk, L. M. Fundamentals of Soil science; John Wilely & Sons: New York, Chapter 8, 1972. (21) Stinson, M. K JAPCA 1989, 39, 1054-1062. CHAPTER 5 SUMMARY AND RECOMMENDATIONS 5.1 Summary The use of gaseous ozone for the remediation of unsaturated soils contaminated with organic chemicals was investigated in this study. Soil venting is an effective and widely used technology for the removal of volatile organic chemicals (VOC s) fiom unsaturated soils. However, even afler long treatment times, a portion of the VOCs remain in the soil, either trapped as gas deeply in the pore spaces or sorbed on soil particles. Gaseous ozone was used in this study as a polishing step to clean up the residual VOCs after soil venting had been applied to strip the majority of the mass of the VOCs from soil. While the VOCs can be successfully stripped fi'om most soils using soil venting, it is dificult, it not impossible, to remove the semi- or non-volatile organic chemicals by this method. Alternatively, in-situ ozonation may be able to remediate these soils as many of the important contaminants would be oxidized by ozone due to the very strong reactivity of ozone which can oxidize most organic chemicals. A series of kinetic experiments were also conducted to determine the rate expressions for the heterogeneous reactions between gaseous ozone and non-volatile organic chemicals and the soil organic 97 98 matter. A mathematical model was developed and verified using the results obtained fiom a series of one dimensional soil columns. Trichloroethylene (TCE) and naphthalene, dissolved in toluene, were chosen as the representative target compormds for the VOCs and semi-volatile conrpounds. By continuously monitoring the efiluent TCE concentration by gas chromatography, it was found that the local equilrbrium assumption between the gaseous and the organic contaminant phases was valid during the initial stage when VOCs concentrations in gaseous phase can keep a constant. Afler applying conventional soil venting to remove the majority of the mass of TCE or toluene fi'om the unsaturated soils, it was observed that the subsequent application of gaseous ozone to the contaminated Borden aquifer material resulted in a further reduction in the soil TCE concentration as compared to when air was used during the polishing step. However, with TCE contaminated Ottawa sand, the use of gaseous ozone offered no significant improvement in removal efficiency as compared to air. This is thought to be due to the fact that Borden sand is finer, more heterogeneous, and has a higher organic matter content than Ottawa sand. As a result, the residual TCE in the Ottawa sand is, most likely, in the immobile pore regions or is tightly sorbed to the soil organic matter. On the contrary, in both Ottawa sand and Borden aquifer material, the use of gaseous ozone afler soil venting was effective for the removal of naphthalene, a semi-volatile organic contaminant, as ozonation resulted in a significant reduction in the residual naphthalene concentration and treatment times were shortened as compared to that required when air was used to strip the naphthalene out of the soil 99 A cycling batch reactor was used to determine the kinetics of gaseous ozone reacted with Phenanthrene, as a representative PAHs, in unsaturated Metea soil Due to the existence of numerous non-ideal and unknown conditions in the contaminated soil, a lumped rate expression was used to express the overall behavior of the three-phase heterogeneous reactions. It was formd that the rate expression of gaseous ozone reacted with soil organic matter in the moist soil can be expressed as an overall secondary rate equation, first order in both ozone and soil organic matter. The determined rate constant was 8.86 x 10*5 sec-1(k%x'"%g)-l . A shallow packed bed reactor was used to evaluate the rate expression for the reaction of ozone with phenanthrene. For these experiments, the rate expression was derived from the phenanthrene concentrations, rather than from the gaseous ozone concentrations, as was done with the cycling batch reactor. A lumped second order rate expression with two reactive regions was observed for the reaction of gaseous ozone with phenanthrene contaminated moist soil. When the phenanthrene concentration was greater than 40 mg/kg, the second order rate constant was 7 .56 x 10-5 (mg /L)'1 sec'l, while the rate constant was measured to be 1.57x10'5 (mg /L)'1 sec'1 for phenanthrene concentrations less than 40 mg/kg. Since the reaction rate for heterogeneous reaction is proportional to the contacting sruface area between phases, it was thought that the two kinetic regions are due to the variations of the surface coverage of phenanthrene with the extent of reactions. Phenanthrene would be a solid at room temperature in the absence of a solvent, and may form a multiple layered film on the surface of soil water and thicker layers would trapped in the soil pores due to the surface tension. Initially the total surface 100 area between the gaseous ozone and solid phenanthrene would remain constant, since only the outermost layer of phenanthrene can be reacted, causing the initial rate constant for phenanthrene concentrations greater than 40 mg/kg. As the phenanthrene continued to react, the contacting surface would be expected to decrease and only those regions with thicker layers remaining. This is thought to cause the second kinetic region. A mathematical model was developed to describe the in-situ ozonation in one dimensional space. The numerical simulations fit reasonably well with the data obtained from the soil column experiments under difl‘erent conditions. Based upon sinnrlations of the transport of gaseous ozone through the reactive organic matter and contaminant fiee soil at difl‘erent soil pH values, it was found that gaseous ozone should be able to penetrate the soil as long as the soil pH were less than 10. At pH values greater than 10, the rate of ozone self-decomposition in water becomes extremely large, preventing the ozone transport. The transport of ozone through phenanthrene contaminated moist Metea soil could be modeled equally as well using either the parallel or serial reaction models for the reaction of ozone with phenanthrene. From the simulation results, the effluent gaseous ozone concentration corresponding to complete disappearance of the phenanthrene were always during the rising portion of ozone BTC when the soil organic matter content varies from 0.1% to 5% as simulated in this study. The results implies that even the soils with a high organic matter content, there is little advantage in stopping the ozone flow at the point where the contaminant concentration would be zero as compared to the ozone flow when the eflluent ozone concentration stabilized. In either case, the total mass of ozone consumed were essentially equivalent. 101 5.2 Recommendations and Future Developments Based upon the results of this investigation, the use of gaseous ozone can be used to remediate unsaturated soils contaminated with organic chemicals, such as toluene, phenanthrene and trichloroethylene. Most of the sites contaminated with organic chemicals can not be efliciently remediated by conventional air stripping once the free product is removed and the tailing stage begins. Additionally, many sites are contaminated with petroleum-based products which contains numerous non-volatile constituents such as PAHs. As such, these sits would be good candidates for remediation with in-situ ozonation. Both the VOCs and the non-volatile chemicals could be removed at the same time. The recommendations for the future studies are listed below: 1. The pattern of ozone gas flow should be further investigated in 2- or 3-D domains to determine the configurations of wells required for field application of in-situ ozonation. 2. The toxicity of ozonated products that are produced in the soil environments and the effect of in-situ ozonation on soil microbial activity should be studied. 3. The efliciency of gaseous ozone for the treatment of VOCs in the soils bearing high organic matter and / or the high clay content should be determined. 4. The reactivity of different PAHs or other important non-volatile organic contaminant, especially those that are diflicult to bio-degrade, should be assessed. 5. The results obtained from bench scale and theoretical studies should be verified in the field. APPENDICES APPENDIX A NUMERICAL CODE FOR GASEOUS OZONE PASSING THROUGH UNSATURATED SOIL This numerical code was written in FORTRAN Language to solve the those equations derived in Chapter 4. This numerical code was run under MS-Windows. Ifit were nm under MS-DOS, the dimension in each array has to be reduced (about 200x200) to fit the memory limitation (640k). PROGRAM OZOVENT DEPENDENT VARIABLES: OZ: OZONE CONC. IN GAS PHASE; C: CONTAMINANT CONC. IN SOIL; OM: ORGANIC MATTER CONC. IN SOIL; PARAMETERS: V : GAS FLOW VELOCITY IN SOIL; Ktt GAS CONDUCT IVITY Kom: ORGANIC MATTER 2ND ORDER RATE CONSTANT; Kc: CONTAMINANT 2nd ORDER RATE CONSTANT; ZET A1: MASS STOICHIOMETRIC COEFFICIENT FOR OZONE/CONTAMINANT; ZETA2: MOLAR STOICHIOMETRIC COEFFICIENT FOR OZONE/ORGANIC MATTER; 00000000000000 REAL 02(2002,202),OM(2002,202),c(2002,202),Z(2002),L,P(2002) REAL KS,Kom,Kg,KC,DT,DZ,KK,KIl\IVIS,KD,v(2002) INTEGER L 1,L2,L3,L4,L5,L6,L7,Ll 1,M,MM,M1,M2,N,N1,I,J OPEN (UNIT=1,FILE='LL2.DAT',STATUS=’NEW‘) C READ PARAMETERS C KS=0.00005 KC=7.56E-5*ZETA1 Kom=8.86e—6 ZETA1=8.39 ZETA2=5.44 102 103 L=300. C RAED INLET AND OUTLET PRESSURE; IN CM OF H20 PIN=2405.0 PF=1033.6 READ THE CONDUCT IVITY OF METEA SOIL; CM/SEC KD=0.0798 READ TOTAL TIME OF OZONE VENTING, min TM=36000. TIME IN SECONDS T=TM*60. READ INITIAL VARIABLES CONC. DATA OZO,C0,0M0/83.02,1000.,10007 READ TIME INTERVALS M=2000 M1=M+ 1 M2=M+2 MM=M-l UT=T/M C READ AXIAL LENGTH INTERVALS N=50 N1=N+ l DZ=L/N C READ SOIL PROPETIES (AIR POROSITY & BULKDENSITY) APORO=0.240 BULKD=1.609 00000 C CALCULATE THE PRESSURE VARIATION ALONG AXIAL DIRECTION Z(1)=0. P(1)=PIN DO 14 L1 1=1,N1 P(Ll 1)=(PIN**2.-((P11\I**2.-PF"2.)*Z(L11)/L))**0.5 C CALCULATE THE VELOCITY VARIATION ALONG AXIAL DIRECTION V(L1 l)=KD*(PIN"'*2.-PF**2.)*(PIN"2.-(PIN**2.-PF**2.)*Z(Ll l)/L)** +(-0.5)/(2*L) Z(L11+1)=Z(L11}+DZ 14 CONTINUE WRITE(1,180) V(l),V(Nl),KD,PIN 180 FORMAT(1X,'INLET V =',F10.4,' cm/sec'l,’ OUTLET V=',F10.4, +' cm/sec'/,' Kd= ',F10.4,' cnr/sec'/,'INLET 1>=',1=10.4,' cm H20) C C READ INITIAL CONDITIONS DO 10 L1=1,Nl OZ(1,L1)=0. 10 CONTINUE DO 12 L2=1,N1 C(1,L2)=C0 OM(1,L2)=OMO 12 CONTINUE C READ BOUNDARY CONDITIONS DO 20 L3=2,M2 OZ(L3,1)=OZO 20 CONTINUE C C 104 C BEGIN THE FINIIE DIFFERENCE CALCULATION C CALCULATE THE CONC. PROFILE ALONG T DIRECTION AT A FIXED z FIRST, C THEN, VARYING THE 2 PROFILE. Z(1)=0. DO 30 J=1,N1 KC=7.56E-5*ZETA1 DO 40 I=1,MM OZ(I,J)=OZ(I,J)*P(J)/1033.6 C(I+l,J)=C(I,J)-DI‘*Kc"'OZ(I,J)*C(I,J)/ZE'1‘A1 IF(C(I+1,J).LT.0.) C(1+1,J)=0. IF(C(I+1,J).LT.40.) KC=1.57E-5*ZETA1 OM(I+l,J)=OM(I,J)-DT*Kom*OZ(I,J)*OM(I,J)/ZETA2 IF(OM(I+1,J).LT.0.) OM(I+1,J)=0. OZ(I+1,J+l)=OZ(I+1,J)-(DZ/V(J))*((OZ(I+2,J)-OZ(I+1,J))/UT + +(BULKD*OZO+1,J)/APORO)*(KC*C(I+1,J)+KOM*OM(I+1,1) + )) IF(Oz(I+1,I+1).LT.0.) OZ(I+1,J+1)=O. C RXN=KG*KK/(KG+KK) C WRITE (1,210) RXN,KK,C(I+1,J),OM(I+l,J),OZ(I+1,J+1) 40 CONTINUE Z(J+ 1)=Z(D+DZ 30 CONTINUE DO 50 L4=1,N1,10 c WRITE (1,110) Z(L4) WRITE (1,120) (OZ(L5,L4),L5=I,M1,50) WRITE (1,130) (C(L6,L4),L6=1,M1,50) WRITE (1,140) (OM(L7,L4),L7=1,M1,50) 50 CONTINUE C C FORMAT THE OUTPUT RESULT C10 FORMAT(1X,5F15.3) 110 FORMAT(/‘ AT 2 = ',F7.2,' CM‘) 120 FORMATU' THE OZONE CONC. ALONG TIME DIRECTION ARE: 7, + 101F102) 130 FORMATU‘ THE CONTAMINANT CONC. ALONG TIME DIRECTION ARE: 7, + 101F102) 140 FORMATU‘ THE ORGANIC MATTER CONC. ALONG TIME DIRECTION ARE: 7, + 101F102) STOP END APPENDIX B GASEOUS OZONE DEGRADATION IN OVEN-DRIED METEA SOIL 50.000 -. o a ._ 8 30.000 . “annulus!!!“ [0:] m9"- 10.(XX) r” o 0.000 0 l (130 2000 3000 4000 5CDO 6000 7000 time (sec) The comparison of gaseous ozone degradation for oven-dried and in moist Metea soils in cycling batch reactor. The symbols are: (0) is the inlet ozone concentration for oven dried soil; (0) is the outlet ozone concentration for oven dried soil; ( I) is the inlet ozone concentration for moist soil; ([3) is the outlet ozone concentration for moist soil Both soil weights for oven dried and moist soil were 2g (as air dried). Gas flow velocity were 1.8 cm/sec and 2.2 cm/sec for oven dried soil and moist soil, respectively. Both soil samples were without contaminant and with natural soil organic matter. The oven dried soil was dried in oven at 60°C for 2 days, and then dried in dessicator for 5 days. Water content in the moist soil was 6.2%. 105 APPENDD( C THE DETERMINATION OF THE STOICHIOMETRIC COEFFICIENT FOR THE REACTION OF OZONE WITH PHENANTHRENE ms 370- g) 60 _ [phenanthrene] =339 mg/kg 2; 5° ‘” —-— experimental data 5 40 ‘5 o simulation results 0 30‘ M O 20* 10 0 t1 tt111%t1%t11%1 012 3 4 5 6 7 8 91011121314151617181920 time(hr) The transport of gaseous ozone through phenanthrene contaminated Metea soil without reactive organic matter content in 10 cm soil column. The phenanthrene was spiked into the soil colurmr afler the soil was completely ozonated. The objective of this experiment was to determine the stoichiometric coeficient of the reaction of ozone with phenanthrene, which was determined by graphically integration of the left-hand side of ozone breakthrough curve. The gas flow rate was 194 ml/min, measured at the outlet of soil column. The simulation results were obtained by using the stoichiometric coeflicient determined from this soil column and the rate expression of the reaction of ozone with phenanthrene as described in the text of Chapter 4. The soil column properties were: bulk density = 1.53 g/cm3; degree of air saturation = 0.376. No pressure correction was made for the simulations. 106 MICHIGAN STATE UNIV. LIBRARIES lll"Willl"WWW"WI”llWIll“IINWIWIIIHW 31293014203164