MOVEMENT AND ABSORPTION 0F ZINC BY A WISNER SILTY CLAY LOAM SO=fL Thesis for the Degree of Ph. D. MICHIGAN STATE UNIVERSITY BOB G. VOLK 1970 I!"WI!“MINIMUM/JUNIIIII(JIWIWTT(II 969 8457 This is to certify that the thesis entitled MOVEMENT AND ABSORPTION OF ZINC BY A WISNER SILTY CLAY LOAM SOIL presented by Bob G. Volk has been accepted towards fulfillment of the requirements for .RhJL__ degree in _S.Q:Ll_S_cience yMajor professor Date June 81 1970 0-169 --.- mum”. _ "i Lips/IR} hue} €753“ State "£9 I University amoma av ‘5‘ “MB & SUNS' 300K BINDERY INC. ' “"“ “V BINDERS 7. ”CM“! MOVEMENT AND ADSORPTION OF ZINC BY A WISNER SILTY CLAY LOAM SOIL By Bob G; Volk AN ABSTRACT OF A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Crop and Soil Sciences 1970 ABSTRACT MOVEMENT AND ADSORPTION OF ZINC BY A WISNER SILTY CLAY LOAM SOIL By Bob G. Volk A Wisner silty clay loam soil was selected for study due to its extreme Zn deficiency for several field crOps. Very little work has been conducted on the chemistry of Zn native to the soil or on the fate of Zn after it has been applied to the soil in fertilizers. The first objective was to determine the relationship of pH, time of soil contact, and temperature of incubation on the relative movement of Zn carriers. The second objective was to determine the maximum amount of Zn that the Wisner soil can retain, bonding energies of the soil for Zn, and the stability of these metal soil relationships. The final objective was to study effects of 002 equilibrium levels on controlling the solubility of Zn in the soil solution. Movement of Zn applied as 65ZnSOh, 65ZnEDTA, and 65ZnNTA under the force of leaching was examined in a soil column experiment. Variables studied were soil pH and time and temperature of incubation of Zn carrier with soil. Bob G. Volk Results indicated that: 1) Zn from ZnSOh moved very little (less than 10 mm) in the soil columns; 2) neither temperature of incubation (5 - 35 C) nor time of incubation (0 — 28 days) produced a significant effect of movement of any Zn carrier; 3) at a soil pH of 3.65, Zn as ZnNTA was much more mobile than Zn as ZnEDTA, (ZnEDTA appeared to be almost completely dissociated at this pH); and h) Zn as either ZnEDTA or ZnNTA was mobile at pH's of 6.3, 7.8, and 8.3 but the EDTA form was more mobile. Stability constants and adsorption isotherms were determined for a Zn-clay and a Zn-H202 treated clay. Using an ion exchange resin method with 65Zn to determine stability constants, it was found that organic matter present increased stability constant (log K) values from 2.62 to 3.06 at pH 3.5 - 4.0 and 3.22 to 3.48 at pH's h.5 - 5.0. It was concluded from Langmuir adsorption isotherms plots that: 1) H202 treated soil has a higher bonding energy for Zn than untreated soil and; 2) adsorption maximums were 2.76 to 5.20 me/lOOgm for the untreated soil and 0.31 me/lOOgm for the H2O2 treated soil. Experiments were conducted by equilibrating ZnEDTA with a Wisner silty clay loam soil with pH's adjusted from 6.88 to 8.06. Carbon dioxide concentrations of 0.03, 0.3, and 3.0% were bubbled through the soils until equilibrium was reached. Little influence of 002 level on Zn in solution was found. Postulated compounds such as ZnCO3, Zn(0H)2, and Bob G. Volk ZnSi03 are too soluble to account for the low levels of Zn in the soil solution. No one compound has yet been found to control Zn in soil solutions. MOVEMENT AND ADSORPTION OF ZINC BY A WISNER SILTY CLAY LOAM SOIL By Bob G.LVolk A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of CrOp and Soil Sciences 1970 To SHARI This thesis is dedicated to my wife for without her inspiration, unfailing interest, and patience, this study could not have been completed. ii ACKNOWLEDGEMENTS The author expresses sincere appreciation to his major professor, Dr. B. G. Ellis, for his interest, assistance, and guidance during this investigation. His enthusiasm in all phases of research and teaching has been a true inSpiration. Thanks are also due to Dr. A. E. Erickson, Dr. B. D. Knezek, Dr. C. J. Pollard, and Dr. E. C. Doll, who served on my guidance committee. Appreciation is also expressed to Dr. R. L. Cook and other members of the Soil Science Department who provided many opportunities for intellectual growth. iii TABLE OF INTRODUCTION . . . . . . . LITERATURE REVIEW . . . . . Movement of Zinc . . . Adsorption of Zinc . . Soil Reaction . . . . Adsorption Isotherms Stability Constants . . MATERIALS AND METHODS . . . Soil Preparation for Zin Soil Column Preparation Fractionation of Soil Samples . Organic Matter Removal Organic Carbon Determination . Cation Exchange Capacity CONTENTS C Chelate Stability Constant Determination Isotherms - Zinc Adsorption Maximum CaC03, ZnCOB, 002 Equilibrium . . . RESULTS AND DISCUSSION . . . Movement Movement of Zinc With Different Carriers . Stability Constant Determinations . . . . . Zinc Adsorption . . Equilibrium Relationships of. Zinc With EDTA 80115 O O O O O O O 0 CONCLUSIONS . . . . . . . . LITERATURE CITED . . . . . . iv Page LIST OF TABLES TABLE l. 10. 11. Percent organic carbon-organic matter deter- minations on a Wisner silty clay loam soil. . Cation exchange capacities of a Wisner silty Clay loam 8011. O O O O O O O O O O O O O O 0 Influence of incubation time,6§emperatug§ and Z water leached on movement 0 Zn from. nSOh at pH's 3.6, 6.3, 7.7, and .3. . . . . Influence of incubation time, gemperatuge , and water leached on movement of 6 Zn from g5ZnNTA atpHBooéoooooooo co. co Influence of incubation time,6§emperatu§ , and water leached on movement of Zn from anNTA at pH 6.3 O O O O O O I O O O O O O 0 Influence of incubation time,6gemperatug§, and water leached on movement of Zn from ZnNTA at pH 7.7 o o o o o o o o o o o o o o 0 Influence of incubation time,6%emperatufi , and water leached on movement of Zn from anNTA at pH 8.3 O O O O O O O O O O O O O O 0 Influence of incubation time, temperatugg, and water leached on movement of 6 5Zn from ZnEDTA at pH 3.6 O O O O O O O O O O O O O O 0 Influence of incubation time,6§emperatu$ , and water leached on movement of Zn from anEDTA at pH 6.3 o o o o o o o o o o o o o 0 Influence of incubation time, temperatu e, and water leached on movement of 65Zn from 5ZnEDTA at pH 7. 7 O O O O O O O O O O O O O O O O O 0 Influence of incubation time,6gemperatufi , and water leached on movement of Zn from anEDTA at pH 8.3 O O O O O O O O O O O O O O O Page 25 30 35 36 37 38 39 4O 41 42 LIST OF TABLES - Continued Page 12. Data for stability constant determination of a Wisner silty clay loam soil at pH 3.5 - 4.0, clay + organic matter . . . . . . . . . . . . . 53 13. Data for stability constant determination of a Wisner silty clay loam soil at pH 4.5 — 5.0, clay + organic matter . . . . . . . . . . . . . 54 14. Data for stability constant determination of a Wisner silty clay loam soil at pH 3.5 — 4.0, Clay -' H202 treated. o o o o o o o o o o o o o o 55 15. Data for stability constant determination of a Wisner silty clay loam soil at pH 4.5 - 5.0, Clay "' H202 treated o o o o o o o o o o o o o o 56 16. Concentration of a Wisner silty clay loam soil. 59 17. Stability constants for a Wisner silty clay loam SOil in mOles O O O O O O O O O O O O O O O O O 59 18. Stability constants for a Wisner silty clay loam 8011 in grams 0 O O O O O I O O O O O O O O I 0 60 19. Langmuir adsorption parameters for a Wisner Silty Clay loam SOil. O O O O I O O O O O O O O 66 20. Langmuir adsorption parameters for a Wisner silty clay loam soil treated with H202. . . . . 67 21. Calculated adsorption maximum and E values. . . 68 22. Effect of 002 level and time on pH, ZnEDTA, and Ca concentration for soil A . . . . . . . . . . 76 23. Effect of 002 level and time on pH, ZnEDTA, and Ca concentration for soil B . . . . . . . . . . 79 vi OF FIGURES Schematic drawing showing equipment used for Zn in soil columns . . . . . . . as ZnSOh in a soil and 140 as ZnEDTA in soil column at pH 6.3. . . . . . . . . . . . . as ZnNTA in a soil as ZnEDTA in a soil as ZnNTA in a soil column as ZnEDTA in a soil column Partition of Zn between resin and clay + organic matter suSpension. . . . . . . . . . . Partition of Zn between resin and clay - H202 Langmuir plot of Zn adsorption data for a Wisner silty clay loam soil. . . . . . . . . . Langmuir plot of Zn adsorption data for a H202 treated Wisner silty clay loam soil . . . Loss of Zn from ZnEDTA on soil A at pH's LIST FIGURE 1. determining 2. Distribution of 65Zn column . . . . . . . 3. Distribution of 65Zn 4. Distribution of 65Zn column at pH 3.65. . 5. Distribution of 65Zn column at pH 3.65. . 6. Distribution of 65Zn at pH 6.30 o o o o o 7. Distribution of 65Zn at pH 6.30 o o o o o 8. 9. treated suspension . 10. ll. 12. 7.29, 7.47, and 7.71 13. Loss of Zn from ZnEDTA on soil B at pH's 6.91, 7033’ and 8001+ vii Page 23 31 34 43 44 45 46 57 58 69 70 82 83 LIST OF FIGURES 14. 15. — Continued Reaction of ZnEDTA with soil pH's . Reaction of ZnEDTA with soil pH's . viii A B at different at different Page 84 85 INTRODUCTION Maximum plant growth can only occur if all essential nutrients are present in ample concentrations. The role of micronutrients in plant nutrition has been increasingly emphasized over the last few years. As early as the 1920's, Zn was recommended for field crOps and fruit trees; however, only since 1960 has much emphasis been placed on Zn ferti- lization. Higher plants such as corn, soybeans, navy or pea beans, and many vegetables have shown an absolute requirement for Zn in Michigan as well as in other states. Most studies on Zn deficiency have been devoted to deter- mining the effectiveness of various Zn fertilizers, possible interactions of Zn with other ions, and the develOping of effective methods for delineating Zn deficiencies in plants or in the soil. Very little work has been conducted on the chemistry of Zn native to the soil or on the fate of Zn after it has been applied to the soil in fertilizers. The Wisner silty clay loam soil in Michigan has been associated with extreme Zn deficiency for several field crOps. At one location in Saginaw county, pea beans (Sanilac variety) will not grow on this soil without applications of Zn. Even though total analysis show as much as 40 ppm Zn in this soil, the addition of just 1.5 ppm Zn will eliminate Zn deficiency during a growing season. Addition of 12.5 ppm Zn in a broadcast application has been shown to elimi- nate Zn deficiency for at least five years. This investiga— tion was conducted to create overall knowledge concerning possible factors affecting this lack of Zn availability on a Wisner silty clay loam soil. The first objective was to determine the relationship of pH, time of soil contact, and temperature of incubation to the relative movement of Zn chelates in artificially prepared soil columns. The second objective was to determine the maximum amount of Zn that the Wisner soil can retain, bonding energies of the soil for Zn, and the stability of these metal soil relationships. The final objective was to study effects of CO2 equilibrium levels on controlling the solubility of Zn in the soil solution. LITERATURE REVIEW Movement of Zinc There are two basic forms of Zn applied to soils to correct Zn deficiency--inorganic Zn, which is generally applied as ZnSOI+ or ZnO, and organic bound Zn, which is generally applied as ZnEDTA (ethlenediamine tetraacetic acid) or ZnNTA (nitrilotriacetic acid). Jurinak and Thorne (1955) studied the movement of various ionic forms of Zn in columns of calcareous, clay soil. Zn was applied as ZnC12, a Zn-NH3 complex, and a zincate anion. After large quantities of tap water were leached through the soil columns, they found a maximum movement of 3 cm for all forms of Zn. Zn applied as ZnCl2 moved a maximum of only 2 cm. They attributed these slight movement differences to: (l) a possible reaction with the CaCOB; (2) neutralization of the hydroxyl ions which would bring instability to the complex; and (3) a greater sorp- tion bond between Zn and soil than between Zn and the coordinated hydroxyl ions and water molecules, which would cause Zn adsorption by the soil. Brown, Krantz, and Martin (1962) also showed that Zn applied as ZnO or ZnSO did not 4 move appreciably in columns of sandy loam or silt loam soils even when heavily leached with water. Barrows, Neff and Gammon (1960), however, found that surface applied ZnSOh was leached to a depth of 46 cm in either a Lakeland fine sand or a Red Bay fine sandy loam. Movement of applied Zn was inversely related to the soil P content and organic matter and was also affected by the type and amount of clay minerals in the soil. Vermiculites seemed to decrease mobility and kaolinite allowed more Zn movement. Mortvedt and Giordano (1967) found that Zn movement from fertilizer granules varied from 0.5 to 2.0 cm with various fertilizer carriers. The mean movement of Zn from the fertilizer granules did not exceed 2.0 cm in any case. Other studies by Giordano and Mortvedt (1966) and Alben (1955) have also indicated that inorganic Zn is essentially immobile in soils. 0n the other hand, chelated Zn is less readily inacti- vated in most soils. A relatively small amount of chelated metal is normally needed to supply deficient plants on most soils, Alben (1955). The effectiveness of chelates as metal carriers in soils depends on their ability to keep these metals in soluble, mobile forms. To do this, the ligand must remain in solution and continue to complex the applied metal ion, Norvell and Lindsay (1969); Hodgson (1969); and Brown (1969). Adsorption by soil particles is the primary cause of the loss of EDTA from a soil solution. The adsorption of EDTA onto soils from ZnEDTA application is less serious than from FeEDTA, Anderson (1964); Wallace and Lunt (1956). But in most soils the majority of EDTA remains in solution at least several weeks and probably much longer, Norvell and Lindsay (1969). Wallihan and Heymann-Herschberg (1956) found that ZnEDTA penetrated the soil and moved much faster than ZnCl2. Hodgson, Lindsay and Kemper (1967) conducted an experi- ment whereby Zn was allowed to diffuse from a ZnCO3 pre- cipitate through an agar-agar gel to a stream of continuously flowing water. In one treatment, part of the agar-agar was replaced by Ca-polygalacturonate to incorporate fixed nega— tive charges in the gel in the form of carboxyl groups. In a second treatment all phases of the system were kept in equilibrium with CaCOB. The transport of Zn in the system was increased more by the mobile complexing agent than it was by the fixed charges. Soil temperature has been found to affect Zn avail- ability to the plant. Two groups of workers have found slow growth related to Zn deficiency several weeks after emergence when the soil was wet and cold, Bauer and Lindsay (1965); Burleson et al. (1961). Decreasing soil temperature from 750 to 500 F decreased Zn content from 310 to 73‘pg per pot and decreased yields of pea beans, Ellis et al. (1964). Work by Martin et al. (1965) also showed that a decrease in air temperature resulted in 2H1 deficiency symptoms on tomatoes. Langin et a1. (1962) attributed Zn deficiency in cold, wet growing conditions to: (1) decreased Zn availability as such, (2) decreased microbial activity, (3) restricted root growth and development. No work was found relating Zn movement in soil to temperature. The time effects of Zn-soil contact were studied by Giordano and Mortvedt (1966) who found that movement of Zn from Zn0 or ZnSOI+ extended to 1 cm after one day but did not move beyond this distance even after four weeks. A later study by Mortvedt and Giordano (1967) indicated that most Zn movement from P carriers occurred during the first week, except in an extremely acid soil. They also found that the amount of Zn recovered from limed soil less than 0.5 cm from the granule by extraction with 2N MgClZ, varied with the carrier after one week, but was similar for all carriers after 8 weeks. In general, when granular carriers containing Zn are applied to soil, the volume of Zn-affected soil is related to the number of granules and the distance of Zn movement of the individual granules. Reactions of Zn with other components of the macronutrient carrier may result in the formation of insoluble compounds which would restrict Zn movement from the granule. Adsorption of Zinc The clay fraction in soils exerts an important influence on the availability of Zn. Elgahfly (1950) called the frac— tion of Zn that was non-exchangeable with ammonium acetate "fixed zinc" and stated that Zn can replace Mg and Al in the octahedral position of many layer silicates. Earlier Elgabfly et al. (1943) stated that Zn—clay has a mosaic surface capable of independent cation and anion exchange, and that fixed Zn is inside empty oxygen and hydroxyl positions of the octahedral layer of montmorillonite. They also found that Zn was adsorbed partly as a monovalent complex ion after which it became part of the inner electri— cal double layer. De Mumbrum and Jackson (1956) determined that Zn reacted with the octahedral hydroxide in layer silicates but did not react with kaolinite. Jurinak and Thorne (1955) prOposed that the unavailability of Zn was from its chemical nature because Zn forms many chemical and clay adsorption complexes. Hibbard (1940) suggested that acid extractable Zn may be held in the crystal lattice of the clay minerals and that only the H ion would be small enough to enter and replace it. Nelson and Melstead (1955) found that with symmetry additions of Zn to a H soil system, practically all of the Zn adsorbed by the soil was replaceable with NHAOAC. The longer the period of time of contact between the Zn and the Ca-clay, the less was the Zn removed by extraction. They also found that after Zn adsorption had occurred, the exchange capacity was not altered, showing that Zn was not occupying cation exchange positions nor being adsorbed in the double layer as a complex ion. Mangaroo et a1. (1965) found that pretreatment of clays with Ca, K, and Cu solutions resulted in Zn adsorption in the order: K-clay> Ca-clay> Cu-clay. Ammonium acetate could replace Zn added to a H saturated soil system but could only partially replace Zn added to a Ca saturated soil system. Work by Nelson and Melstead (1955) indicated the amount of Zn extracted with NHAOAC decreased with time when Zn was applied to a Ca saturated soil. Jurinak and Bauer (1956) in studying the adsorption of ZnCl2 by calcite, dolomite, and Ca substituted magnesite crystals reported that approximately ten percent of the adsorption sites available on calcite are occupied by Zn when the equilibrium Zn concentration is 9 x 10-7 M at 250 C. The Ca magnesite shows a somewhat greater affinity for' Zn. ions than calcite, while dolomite was intermediate. The possibility that lime minerals in the soil may con- stitute a potential adsorptive phase for certain cations was recognized by Leeper (1952) who postulated that, in a calcareous soil, CaCO3 may be an important adsorbent of heavy metals. Canals et al. (1949) have shown that Zn is adsorbed from solution by CaCOB. They found that when the temperature was raised from 20°C to 100°C the adsorption of Zn by CaCO3 was greatly increased. Considerable work has been done on the interaction of organic matter and Zn in the soil. Zn deficiencies have been reported on soils to which high levels of organic materials have been added, Thorne and Warm (1950). DeRemer et al. (1964) found that the recovery of Zn added to a soil was decreased when the soil was incubated with sugar beet tops. It was concluded that Zn is immobilized by microorganisms during decomposition of organic materials. Ark (1936) reported that Zn deficient soils which were steam sterilized released sufficient Zn to correct the deficiency--implying microbial fixation. It should be noted here that steam sterilization will cause the reduction of organic matter to colloidal forms which might complex Zn, keeping it in an available form. Many workers have indicated that some type of coordi- nation compound exists between the micronutrients and soil organic matter, Bremner and Lees (1949); Lees (1950); Peech (1945). Investigations have led researchers to con— clude that Zn deficiencies are caused by the formation of organic matter-Zn complexes. De Mumbrum and Jackson (1956); Miller and Ohlrogge (1958); and Hodgson et a1. (1966) found that from 28 to 99 percent of the Zn in soil solution was complexed with organic matter. Chelation in soil organic matter mainly involves the phenolic groups, Himes et al. (1963); Broadbent and Bradford (1952). According to Himes and Barber (1957) destruction of organic matter by hydrogen peroxide treatments destroyed the Zn-chelating ability of the carboxyl and phenolic groups. Infrared techniques by Randhawa and Broadbent (1965) indi- cated that Zn saturation of peat fractions resulted in numerous shifts in the double bond region, showing a che- lation with 0:0 and N=0 groupings. According to Bould (1963) Zn is adsorbed as a divalent cation on clays or complexed by organic matter after release from the minerals. Mortensen 10 (1963) concluded that soil organic matter complexes Zn by ion exchange, surface absorption, chelation, and peptidiza- tion. He also stated that hydroxyl, carboxyl, and amide groups of soil organic matter are probably responsible for chelating metals. Soil Reaction In considering all the factors related to Zn chemistry in the soil, pH is one of the most important, Barrows and Gammon (1960). As the pH increases due to liming, Zn availability to plants decreases, Boawn et al. (1960); Camp (1945); Lott (1938); Thorne (1957); Wear (1956). The optimum availability of Zn to plants was reported by Camp (1945) to be in the pH range of 6.0 to 6.5. Non-chelated Zn was practically unextractable at pH 7.0 to 8.5 using the dithizone extractant, Shaw and Dean (1951). Jurinak and Thorne (1955) showed Zn solubility in Na and K bentonite clay systems to be lowest between pH 5.5 and 6.7, but increased at higher pH values. Solubility of Zn in a Ca bentonite system did not increase at higher pH values and was lowest at pH 7.6. Differential solubility of alkali zincates and of Ca zincate was proposed as possible solu- tions to the problem. Ca ions have been reported to have no effect on Zn availability, Viets et a1. (1957); Wear (1956). Randhawa and Broadbent (1965) found that humic acid complexed very little Zn at pH values less than 3.6 but ll complexing ability increased rapidly with an increasing hydroxyl ion concentration up to pH 8.5. Water-soluble chelating agents in organic material were found by Miller and Ohlrogge (1958) to complex more Zn at higher pH's. Because of the basic properties of most ligands, they are usually associated with H ions over a fairly wide range. The formation of a metal chelate compound frequently involves a displacement of H ions from the ligand according to the following scheme: M+n + HA—>MAn_l + H+ MAn‘l + HA—aMAZn"2 + H+ By decreasing the pH of a soil or solution which contains the chelate, a dissociation of the metal complex will result through the reversal of the above reactions, Chabrek and Martell (1959). The effect of pH probably results from the differences in solubility of the various pH dependent forms of Zn occurring in the soil, De Mumbrum and Jackson (1957); Elgaufly and Jenny (1943); Nelson and Melstead (1955); and Peech (1941). Recently, Norvell and Lindsay (1969) found that when ZnEDTA was reacted with soils, the fraction of added EDTA that remained associated with Zn was pH dependent and reached a maximum near pH 6.7. Examination of their findings suggests that ZnSiO3 in equilibrium with amorphous SiO2 12 could account for the level of ZnEDTA remaining in solution at each pH. Similar studies with ZnDTPA (diethlenetriamine pentaacetic acid) also showed close correspondence between the observed and predicted levels of chelated Zn in solution. However, Norvell and Lindsay (1970) published a retraction of the above findings after discovering the solubility constant for ZnSiO3 was in error. Their calculations now indicate that agreement between equilibrium levels of Zn in solution and ZnSiO3 is only coincidence. Adsorption Isotherms The process by which atoms or molecules on one material become attached to the surface of another is called adsorp- tion. Adsorption is a means of neutralizing or satisfying the forces of attraction that exist at a surface. The unfilled forces at the surface can be satisfied by the adsorption of atoms or molecules of another Species. This reduces the attraction of the surface atoms or molecules of the solid or liquid toward its neighbors of the same kind, and reduces surface tension. Thus, the process of adsorp- tion continues until the free surface energy of the system, due to the imbalance of surface forces, has reached a mini- mum value. Adsorption of a gas may be a relatively weak physical adsorption or a strong interaction of a chemical nature--called chemisorption. Van der Waals forces are considered to be responsible for physical adsorption while chemisorption involves an energy of activation and heats 13 of adsorption on the order of chemical reactions. The forces that are responsible for adsorption from solutions have been discussed by Giles (1959). They may be classified as (a) non—polar Van der Waals attraction, (b) formation of hydro- gen bonds, (0) ion exchange, and (d) covalent bond formation. The adsorption equation developed by Langmuir (1919) and (1940) was one of the first and most important equations based on theory. Langmuir postulated that adsorption occurred as a monomolecular film. He envisioned a dynamic equilibrium such that the rate of adsorption equaled the rate of desorption. Fowler (1935) has emphasized that three important conditions are implied in the kinetic and sta- tistical derivations of the Langmuir isotherm. These are the following: (a) Adsorption is localized and takes place only through collisions of gas molecules with vacant sites. (b) Each site can accommodate one and only one adsorbed particle. (c) The energy of an adsorbed particle is the same at any site on the surface and is independent of the presence or absence of nearby adsorbed molecules. One form of the Langmuir equation is: 9 = m 01“ [1] “fig-ms [2] 14 where 9 is the fraction of the surface covered, K1 and K2 are proportionality constants, P is the pressure of the gas, and K is equal to 52 and is sometimes called the adsorption coefficient. 1The simple Langmuir isotherm gives two limiting types of behavior. At very low pressures, where KP<§1, the isotherm reduces to Q = KP which corres- ponds to the initial steep rise of the sorption curve, and at very high pressures, where KP>>1, 9 approaches the con— stant maximum value of unity. The amount of gas adsorbed (X/m) at a pressure of P and the amount of gas (b) needed for monolayer formation are related to 9 by: Kemee [3] and equation [2] becomes: KbP M“ = W [4] upon rearrangement gives: P l P I75 = Kb'+ 5' [5] If the data agrees with the Langmuir theory, plotting this equation will give a straight line with the intercept l/Kb and the slope with the constant l/b. This equation can apply to adsorption from a solution, but the theoretical treatment is not well developed, Boyd et a1. (1947) and Graham (1953). For liquid solutions the gas pressure term 15 P is replaced by a concentration term (C) giving: Tm=fi+g W] Using the equation - F0 = RT ln K, the standard free energy of adsorption may be calculated, which constitutes a measure of the strength of the adsorption bond. In soils, use of adsorption isotherms has been restricted to mainly P adsorption. Langmuir isotherms have been applied very successfully to P adsorption on soil by Olsen and Watanbe (1957) and Fried and Shapiro (1956). Constants calculated from Langmuir isotherms have permitted a success- ful theoretical approach to some of the problems of P sorp- tion in soils. Use of isotherms for other elements has been extremely limited. Himes and Barber (1957) suggested that Langmuir adsorption equations could be successfully used for study- ing the retention of Zn by soil. Udo, Bohn and Tucker (Agronomy Abstracts, 1968) reported that at low Zn concen- tration, the Langmuir equation was followed. They found absorption capacities varied from 0.90 to 5.1 me/lOOg of calcareous Arizona soil compared to cation exchange capaci— ties of 3.3 to 17.3 me/lOOg. Zn adsorption capacities were related to total surface area, organic matter, and quantity of CaCO3 and clay in the soil. 16 Stability Constants In general, the term "stability" describes the amount of association that occurs in solutions containing two or more component species in equilibrium. The more stable the resultant complex, the greater is the probability that association will occur under a given set of conditions. Ideally, stability constants should be true thermodynamic constants expressed in terms of the activities of the species in equilibrium. In practice, since it is often difficult or impossible to determine activities, concentra- tions are_used instead. In this equation: K (MChX) E 3 _ (M)(c:h)x 7 (K) is an expression of the stability of the complex in solution, (M) is the concentration of the metal ion, (Ch) is the concentration of the complexing agent, and (x) is the number of moles of the complexing agent which combines with one mole of metal ion to form the metal complex. The principle of ion exchange equilibrium can be used to determine the stability constant of the complex formed. This determination is based on the fact that the quantity of metal bound to a known weight of resin at equilibrium is proportional to the concentration of free ions in solution. 17 If one assumes that only a single complex species occurs in appreciable concentrations, one can solve for the partition of this species between resin and solution. In other words, a stability constant (K) may be calculated from the formula: P "' qB K = 0 [8] where P0 = the distribution ratio of the metal in the absence of ligand, qB = the distribution ratio of the metal in the pre- sence of the ligand, P1 = the distribution ratio of the metal-clay complex between the resin and solution, Ch = the ligand concentration in moles. If the formula is arranged to the form of P - q _2Chf_§'= K(qB - Pl) then the stability constant can be determined from the slope of a line Po - qB vs. qB. Chapter 11 of Rossotti and Rossotti (1961) gives a very comprehensive description and develOpment of the above theory on stability constant determination by the ion exchange method. A second method for determination of stability con- stants can also be used. The stability constant of the complex or complexes can be determined from the following l8 relationship given by Martell and Calvin (1952). log (3§_ — l) = log K + x log (Ch) [9] In this equation K, Ch, and x are the same values as defined in equations [7] and [8]. ‘10 = the distribution constant of the metal in the absence of ligand. 2A: the distribution constant of the metal in the pre- sence of ligand. The distribution constants,:]O and A are the coefficients by which the concentration of free ions or free plus complex ions in solution must be multiplied to obtain the quantity of that cation bound to a definite weight of cation resin at equilibrium. The slope and intercept of a graph log (71- — 1) vs. log (Ch) are the values of x and log K, respectively. Possible sources of error in stability constant deter- mination include: (1) analytical, (2) adsorption of the ligand or complex Species by the exchange resin and (3) two or more complexing agents or complex Species being present. If more than one complex species occurs in solution, the method of Fronaeus (pages 246 and 247 of Rossotti and RossottL 1961) may be used to calculate the various K values. Almost all work on stability constants has been based on the organic fraction or constituents of the organic l9 fraction in the soil. Miller and Ohlrogge (1958); Randhawa and Broadbent (1965); and Schnitzer and Skinner (1966) have successfully used ion exchange equilibrium to determine stability constants for Zn-organic matter, Zn-fulvic, or Zn-humic acid preparations. Values obtained for stability constants ranged from 1.7 to 7.8 depending on the pH at which the constants were determined. Log K values for Zn-fulvic acid complexes reported by Schnitzer and Skinner (1966) were considerably lower than those reported by Randhawa and Broadbent (1965). Himes and Barber (1957) used two methods for deter- mination of the strength of bonding of Zn by soil organic matter. By assuming that only one type of adsorption site existed, they were able to construct a Langmuir type curve which could be extrapolated to give the concentration of the complexing sites. They were able to obtain stability constants of log K = 3.4 to 5.6 for a Zn-organic matter complex. By a second method, they found that the soil-Zn log K complex values were between 5.2 and 10.4. MATERIALS AND METHODS Soil Preparation for Zinc Chelate Movement Bulk samples of a Wisner silty clay loam soil, pH 7.73, were ground to pass a 2 mm plastic sieve. A pH of 8.3 was obtained by adding small increments of saturated Ca(0H)2 and allowing the moist soil samples to equilibrate over a period of several months. To achieve lower pH's, small portions of 0.5 N HCl were added and the soil allowed to come to equilibrium with the acid. In this manner, stable pH's of 6.3 and 3.64 were obtained. These values of pH were constant over several cycles of drying and rewetting. Zn chelates (0.1 M) were prepared by mixing equal molar quantities of ZnCl2 with either EDTA or NTA. During this procedure 65Zn as ZnCl2 was added to give a 65Zn con- centration of lO/uc/ml of solution. The prepared Wisner soil of various pH's was next moistened to slightly less than field capacity (15% moisture on an oven—dry basis) with appropriate amounts of the prepared 65Zn chelate and well mixed. Beakers of these chelate treated soils were covered with parafilm and placed under various conditions of temperature and times of incubation. These beakers were aerated daily and kept at 15% moisture by distilled water additions. 20 21 Soil Column Preparation Columns, manufactured from plexiglass, were con- structed 7 x 7 cm square with a length of 20 cm. A hole was drilled in the flat base to allow water passage. Glass wool was placed in the bottom of the column followed by about 7 cm of fine sand, and about 7 cm of prepared soil. Approximately 650 gm of air-dry soil were required for the 7 cm depth. The soil was packed as uniformly as possible using a glass stirring rod to aid in removing air Spaces in the soil. The top of the soil in the column was leveled using a rubber stopper as a tamp. ‘Water was added to the soil until the water reached about 1% - 2 cm from the sand and then the soil column was allowed to equilibrate. After equilibrium the percent moisture was 14% in the soil. If the soil was kept at field capacity, 18%, it became difficult to handle by becoming extremely muddy. After appropriate incubation time and temperature, 40 gm of treated soil were weighed and very carefully placed on top of the soil column previously prepared with the same soil except lacking the 65Zn chelate. Great care was taken to make sure that the soil was placed in an even layer no more than 4 mm deep. After the 65Zn chelate soil was added, the columns were counted for 65Zn as described later. Twenty—five ml portions of distilled water were leached through the soil column every other day until a total of 125 m1 had been added. Parafilm was used to cover the 22 tOpS of the columns to limit evaporation of water from the soil surface. A Nuclear Chicago Scintillation Counter Model 8725 with a NaI crystal was used for counting 65Zn. The crystal detector was placed behind a 2.5 mm Opening or Slit in a column of lead bricks. The soil column was placed on the opposite side of the lead and moved up and down by means of screw type jack (Figure 1). Thus the 65Zn chelate move— ment pattern could be determined by counting the radiation that passed through the narrow slit. The soil columns were marked with 2.5 mm gradations allowing the columns to be placed in the same position each time. Fractionation of Soil Samples The clay fraction.(<2/0 of Wisner silty clay loam was separated from the sand and silt by differential sedimenta- tion. DiSperSion was obtained by adding 2% Na2003 and shaking overnight. The clay suSpenSion was allowed to settle 10 cm for a time interval calculated according to Stoke's Law. The settling and syphoning process was repeated ten times. Clay suSpenSions collected were floc- culated with calcium chloride. Excess Salts were removed by washing the clay three times with distilled water, three times with 75% (V/V) aqueous methanol and finally two times with methanol alone until a chloride free test was obtained using AgN03. 23 Lead bricks /////// sandman. ///// Son ///// {-23.2}: /////// Figure 1. Schematic draping showing equipment used for determining Zn in soil columns. 24 The concentration of the Ca-clay was then determined by weighing aliquots of the clay suSpenSionS. Clay con- centration was found to be 4.92% for clay + organic matter suSpenSionS and 4.6% for the clay suspensions with the organic matter destroyed. Organic Matter Removal It was necessary to first remove Ca salts, including CaCO3 before treating the soil with H202 in order to avoid formation of CaCZOA. NaOAc buffer was used to dissolve carbonates and soluble salts in a method described by Grossman and Millet (1961). Organic matter was oxidized with H202 by a method des- cribed by Kunze (1965). Ogganic Carbon Determination Determinations of organic carbon were performed on a Leco 70 Second Carbon Analyzer made by Laboratory Equipment Corporation in St. Joseph, Michigan. The following data were obtained: Table 1. Percent organic carbon - organic matter determin- ations on a Wisner silty clay loam soil. Treatment Organic Carbon Organic Matter‘a 7o % H202 0.34 0.59 0.23 0.40 None 2.15 3.71 ’ ’ 2.02 3.48 aPercent organic matter = percent organic carbon x 1.724. 25 Cation Exchangngapacity Samples of soil and clay were Ca saturated by use of 1 N CaClZ. Excess salt was removed by washing with 75% (V/V) aqueous ethanol until a chloride free test was obtained using AgN03. The Ca ion was displaced with Mg by washing with l N MgCl2 four times. Ca concentration was determined by a Perkin Elmer Model 303 Atomic Adsorption Spectrophotometer. Cation exchange capacities were as follows: Table 2. Cation exchange capacities of a Wisner silty clay loam. Soil Fraction T at t re men <2/u Soil --- me/lOOgm --- None 46.2 15.2 H202 35.2 10.2 Stabilitnyonstant Determination Quantities (0.2 — 2.0 gm) of K-saturated Dowex-50 resin, 50-100 mesh (analytical grade, AG50W purchased from Bio-Rad Laboratories) were weighed into 125 m1 Erlenmeyer flasks. The K—saturated form of the resin was obtained by washing the H form with l N KCl and removing the excess KCl by dis— tilled water washings. It was also necessary to neutralize the system with KOH to obtain a near neutral pH due to the low pH found when chlorides were washed free. 26 The swelling factor for the resin was previously determined by Dr. B. G. Ellis and found to be 0.29 ml/gm resin which is considered negligible. Forty ml of the clay suSpenSions (approximately 5% by weight), 5 ml of l N KCl, and 5 ml 65Zn (40,000 cpm) were pipetted into the Erlenmeyer flasks. The pH of the solution was adjusted to 3.5 or 5.0 using less than 10 drops of dilute HCl or KOH. A set of blanks was also prepared that contained everything but the clay suSpenSionS. The flasks were shaken for 72 hours on a gyroscopic shaker at 24 I 20 C. Flasks were removed from the shaker at 2 minute inter- vals, thus allowing the resin to settle to the bottom of the flask, and then 2 ml of clay suspension were pipetted from the top portion of the liquid. Great care was taken to avoid pipetting any resin from the flasks. The pipetted samples were placed in a Packard Auto-Gamma Spectrometer and the 65Zn counted for one minute intervals. Calculations performed on the data are described in Results and Discussion. The techniques used for stability constant determination are similar to those used and des- cribed by Rossotti and Rossotti (1961), Schnitzer and Skinner (1966), and Dr. B. G. Ellis (personal communication). Isotherms - Zn Adsorption Maximum Ten gram soil samples were equilibrated for 6 hours with 50 ml of 0.1 N KCl of varying Zn concentrations from 5 to 27 625 ppm Zn. The pH of the soil—0.1 N KCl suSpenSion was adjusted to approximately 4.5 before Zn additions. The samples were shaken on a gerSCOpic shaker, filtered, and Zn determined by atomic adsorption. The data was calculated according to the method of Langmuir (1918) as discussed by Olsen and Watanabe (1957). CaCOB, ZnCOB, C02 Equilibrium Samples of a Wisner silty clay loam soil were prepared in two ways. The pH of the natural soil (7.73) was adjusted with HCl or Ca(OH)2 to pH's of 7.47 and 7.29. A second set of soil samples was first washed with 0.5 N HCl and the pH adjusted with Ca(0H)2 to values of 6.88, 7.30, and 8.06. A stock solution of ZnEDTA was prepared by mixing equimolar quantities of reagent grade ZnSO’+ with Na2H2EDTA. Aliquots of the ZnEDTA were adjusted to pH's approximately equal to the soil pH's. Tracer quantities of EDTA tagged with lLi'C-labeled carboxyl groups were added, and the solu- tions were diluted to a final concentration of 1.5 x lO-BM. All chelate-soil reactions were carried out in small polyethylene bottles containing 15 g soil and 30 m1 of solution. Soil suSpenSions were aerated continuously with air—CO2 mixtures of 3% C02, 0.3% 002, and 0.03% 002. The bottles were shaken continuously at a temperature of 25 i 20 C. Several times daily the bottles were hand shaken to resuspend adhering or settled particles. All suSpenSions were shaken 28 for two days before 2 ml of ZnEDTA chelate was added, bringing the final volume of the solution to 30 ml. Thus, the initial concentration of the ZnEDTA chelate added to the soil was 1 x 10_5 M or 13.1 ppm Zn expressed on a soil weight basis. Reaction periods, following EDTA chelate additions were varied from 6 hours to 30 days. At the end of each reaction period, a set of bottles was removed and the pH of the suSpenSions was immediately determined. The suspensions were centrifuged and the supernatant solutions filtered to remove floating organic matter. The concentration of th tagged EDTA in each solution was measured by liquid scintillation using aqueous stand- ards with a Similar pH and salt content. Bray (1960) solution was used as the solvent for the aqueous samples. Solutions were analyzed for Ca, Fe, and Zn by atomic adsorp- tion SpectrOphotometry. RESULTS AND DISCUSSION Movement of Zn With Different Carriers Little movement of 65Zn from anO4 occurred under leaching as shown in Table 3. Approximately 1-3% of the Zn moved a maximum of 7.5 mm as illustrated in Figure 2, where counts per minute from 65Zn are plotted against depth. Time or temperature of incubation had no effect on the 65ZnSOA movement. The applied Zn must readily be adsorbed by the organic matter and/or clay thus rendering the Zn immobile. Other workers have also found the same results—- Brown, Krantz, and Martin (1962) and Barrows, Neff, and Gammon (1960). Soil columns were marked in numbered 2.5 mm sections to insure repeatable 65Zn counting measurements between water leachings. The gamma radiation counting was begun 5.0 mm above the surface of the soil in the column because after several additions of water, the soil surface became irregular, and it was difficult to determine the exact starting position of the soil surface. The gamma radiation penetrated the 10 cm. lead brick shielding to some extent and gave a tailing off effect in graphing of results when the highest radiation level was close to the counting slit. 29 30 Table 3. Influence of incubation time, temperatu e, and water leached on movement of 65Zn from 5ZnSOh at pH's 3.6, 6.3, 7.7, and 8.3. Incubation Period (days) 50 C. Water Leached Total 0 7 14 21 28 Avg. ml ;:i% __ 0a 4.8 4.1 5.9 6.1 6.8 5.5 25 5.2 5.9 4.8 6.7 5.§ 5.6 50 4.1 6.4 5.4 6.9 6. 5.9 75 6.8 6.2 6.4 4.1 5.4 5.8 100 5.7 5.3 6.2 5.8 6.3 5.9 125 5.5 4.3 5.0 6.3 6.5 5.5 Incubation 150 C. O __ O 3.9 7.4 3.4 6.5 3.3 4-9 25 4.8 5.9 5.4 4.1 6.3 5.3 50 5.6 4.6 4.1 7.2 5.4 5.4 75 4.1 7.2 5.7 6.9 4.2 5.6 100 6.1 6.5 4.5 5.7 3.8 5.3 125 5.2 4.4 3.2 6.5 7.2 5.3 Incubation 250 C O 4.8 5.9 6.4 6.8 3.7 5.5 25 5.6 5.9 6.1 3.2 7.4 5.6 50 7.2 8.0 3.1 5.6 4.7 5.7 75 4.9 5.2 5.3 5.8 3.7 5.0 100 3.4 5.4 5.6 6.1 6.4 5.4 125 5.3 7.1 6.4 3.2 5.8 5.6 Incubation 350 C 0 3.1 7.2 6.8 4.3 5.1 5.3 25 5.7 6.4 6.2 4.1 6.8 5.8 50 g.3 5.1 5.8 4.9 4.1 4.8 75 .l 3.8 5.1 6.2 4.7 5.2 100 5.5 7.1 4.2 5.5 5.9 5.6 125 4.5 6.2 7.1 6.3 6.1 6.0 aDue to some penetration of gamma rays through the lead bricks, the top 2.5 mm band of 5Zn-chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. 31 8000 P T 7000 6000 T 5000 I #— 4000~ I 65Zn(countS/minute) I 3000 2000 I 1000” O 25 ml water 1 1 I 1 l 7.5 15.0 22.5 30.0 Depth (mm) Figure 2. Distribution of 65Zn as ZnSOLF in a soil column. 32 For this reason the 65Zn chelate vs. depth graphs Show a peak width of around 7.5 mm with no water leaching instead of 3—4 mm, which was the depth of the 65Zn chelate soil added to the column. Several soil columns of each 65Zn chelate treatment were prepared where the total soil was of the same pH under study. However, the same movement characteristics were found for the Zn if the lower 7 cm of soil was pH 7.7 for all soil pH's added to the top of this 7 cm. Therefore, it was not necessary to change the pH of the soil in the column below the added 3-4 mm of treated soil. Apparently any effect of pH on the Zn chelate soil movement took place very rapidly and movement of the soluble Zn chelate into soil of a different pH wasogo consequence to the Zn solubility. The possibility of the 65Zn moving in a form other than ZnEDTA or ZnNTA was also considered. To study this possibility, EDTA and NTA were obtained from Geigy Chemical Company which was 150 labeled. The radiation from the 65Zn and th could not be determined separately by means of liquid scintillation counting due to their similarity of energy Spectrum (150 and 65Zn emitt beta energies of 0.155 and 0.32 MEV respectively, which could not be separated by pulse height analysis). Therefore, separate columns were prepared where 65ZnEDTA or NTA were used on one column and ZnEDTA or NTA - 150 labeled were used on the other column. Equal quantities of water were leached through each column. The 65Zn radiation could easily be determined by gamma 33 scintillation counting. Samples of soil were taken from the 1["C chelate column and counted for 150 by liquid scintillation. Figure 3 shows that the peak position and moving front of 65Zn and th correlate well giving a good indication that the Zn was moving in the chelated form. After the data for the various chelate and pH treat- ments was plotted, an arbitrary vertical line was drawn at thele5 mm position (eg. Figure 4). This was the point at which the initial peak tailed off to the background counting for the zero water leaching. A planimeter was then used to measure the areas under the reSpective water leaching lines. The area to the left of the vertical line was that portion of 65Zn chelate which did not move or was dissociated and that portion of area to the right of the vertical line was the 65Zn chelate which was mobile. The percent of mobile or chelated Zn would be the area to the right of the verti- cal line divided by the total area under the line. Data obtained from the Zn chelate movement is presented in Figures 4-7 and Tables 4-11. The movement characteristics of the Zn chelates was quite different depending upon the pH of the soil and the type of chelate studied. First considering a soil pH of 3.65 for ZnNTA and ZnEDTA (Figure 4 and 5), it can be seen that the ZnNTA moved farther than the ZnEDTA. ZnNTA moved 22.5 mm for 125 ml water leaching, while ZnEDTA moved only 7.5 mm. Using area measurements, a maximum of 44.9% (39.3% if 5.6 is sub- tracted for 0 water leaching - Table 4) of the total ZnNTA 34 6000 P 7.; L p z E. 33 4000 U) 4.3 n 5 o e \ 5‘ 200° ’ fl‘ :8 ’ O 2 5O 7' 12‘ ml. 0 n 1 I 1 n 1 water 15 30 45 60 5000 r ’33 .p 5 a '8 h 3000 .p 5 o 3 429.) vi 1000 0_ Depth (mm) . . . . 65 14 . Flgure 3. Dlstrlbutlon of Zn and C as Zn EDTA 1n a soil column at pH 6.3. 35 Table 4. Influence of incubation time, temperatu e, and water leached on movement of 5Zn from 5ZnNTA at pH 3.6. Incubation Period (days) 50 0. Water Leached Total 0 7 14 21 28 Avg. mI - — o ——— — — 0a 4.1 6.2 7 2 6.2 4.2 5.6 25 18.2 15.8 16.9 18.8 11.1 16.2 50 28.1 25.1 27.2 29.2 27.2 27.4 75 31.2 33.1 34.5 36.9 34.5 34.0 100 45.6 41.2 38.2 43.6 42.0 42.1 125 46.1 42.1 41.6 47.8 44.5 44.4 Incubation 15° C. O 4.3 6.9 7.4 5.8 3.7 5.6 25 17.0 16.1 16.8 17.4 15.0 16.5 50 27.6 26.2 30.2 25.9 27.0 27.g 75 35.9 33.8 35.5 34.3 38.5 35. 100 42.9 40.0 40.1 40.9 40.8 40.9 125 46.0 43.7 45.0 43.9 45.7 44.9 Incubation 250 C. O .3.9 7.4 7.3 6.1 3.4. 5.6 25 16.7 16.4 16.2 18.1 13.2 16.1 50 27.4 27.3 28.4 27.9 25.8 27.4 75 35.6 34-1 34.6 35.7 35.7 35.1 100 42.7 42.8 39.2 41.8 40.0 41.3 125 45.7 44.8 43.5 44.6 44.9 44.7 Incubation 350 C. O .3.2 7.6 7.1 6.3 3.3 5.5 25 15.9 16.9 16.1 18.4 13.0 11.1 50 26.8 27.9 28.4 28.3 27.0 27.7 75 31.0 36.0 31.2 35.9 34.9 33.8 100 40.2 45.0 38.0 44.0 43.0 42.0 125 43.4 42.1 43.2 49.0 41.0 43.7 aDue to some penetration of gamma rays through the lead bricks, the top 2.5 mm band of 5Zn-chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. 36 Table 5. Influence of incubation time, temperatuge, and watgr leached on movement of 65Zn from 5ZnNTA at pH .3. Incubation Period (days) ‘59 C. Water Leached Total 0 7 14 21 28 Avg ml -------- ——— w —— - 0a 5 3 6.6 6.4 5.8 6.1 6.0 25 32 8 27.1 29.9 31.2 35.8 31.4 50 47.0 49.0 45.9 48.7 50.1 48.1 75 60.4 55.1 58.5 62.6 60.2 59.4 100 61.5 69.0 61.5 64.6 60.0 63.3 125 69 5 64.3 66.5 69.2 69.1 67.7 Incubation 150 C. O 5 5 6.4 6.5 5.6 6.4 6.1 25 31 6 28.0 29.5 32.5 33.9 31.1 50 49.6 46.1 47.6 49.0 50.3 48.5 75 59.2 58.2 57.7 59.9 59.5 58.9 100 63.0 65.2 62.2 65.7 62.2 63.7 125 68 3 67.3 64.7 68.1 67.7 67.2 Incubation 253 C - —— 3 O 5.4 6.5 6.1 6.9 6.3 6.2 25 27.3 32.2 28.3 35.0 32.0 31.0 50 48.9 47.9 45.9 49.9 50.1 48.5 75 58.9 59.3 57.6 63.0 59.4 59.6 100 62.5 65.7 61.9 66.7 61.1 63.6 125 68.6 67.9 62.0 69.1 65.9 66.7 Incubation 350 C. 0 5.6 6.6 5.1 5.5 6.6 5.9 25 34.0 29.0 23.2 31.5 35.0 30.5 50 52.0 47.9 45.8 48.9 51.5 49.2 75 60.5 59.5 55.3 59.0 60.6 59.0 100 64.3 65.5 60.0 65.5 62.7 63.6 125 69.1 68.2 62.3 68.0 69.3 67.4 aDue to some penetration of gamma rays through the lead bricks, the top 2.5 mm band of 65Zn-chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. Table 6. pH 7.7. 37 Influence of incubation time,6§emperatu water leached on movement of Zn from €52. NTA at Incubation Period (days) 50 C. Water Leached approximately 7.5 mm deep. are also due to this gamma ray penetration. bricks, the top 2.5 mm band of Total 0 7 14 21 28 Avg. m1 %’— — 0a 5.5 6.8 6.6 3.2 6.2 5.7 25 32.1 28.5 27.2 33.5 31.9 30.6 50 49.5 48.0 46.5 49.1 52.0 49.0 75 59.1 52.0 59.0 60.4 58.5 57.8 100 60.4 64.3 62.2 63.7 62.9 62.7 125 68.1 66.4 64.9 69.0 68.1 67.3 Incubation 159 70 0 3.2 6.6 5.5 7.9 6.8 6.0 25 32.8 29.0 28.1 31.5 35.9 31.5 50 48.6 45.3 43.1 49.9 52.0 48.0 75 60.5 57.5 57.7 58.9 60.9 59.0 100 61.2 63.0 61. 64.5 58.4 61.6 125 69.4 65.0 67.8 69.1 69 1 68.1 Incubation 25° c. 0 5.9 6.3 6.8 5.6 6.3 6.2 25 31.8 27.9 29.1 31.5 34.2 30.9 50 49.8 46.0 47.8 48.9 51.2 48.7 75 58.9 59.3 57.8 58.8 59.9 58.9 100 63.2 64.3 62.1 66.2 63.8 63.9 125 67.9 67.2 63. 68.1 66.4 66.5 Incubation 35° C. O 5.6 6.3 6.3 5.6 6.9 6.1 25 31.8 27.2 34.2 32.5 34.1 32.0 50 49.5 46.5 49.5 45.0 50.3 48.2 75 59.6 58.7 58.7 60.3 57.2 58.9 100 63.8 66.2 66.2 66.1 61.4 64.7 125 68.7 65.3 68.1 68.3 67.7 67.6 8Due to some penetration of gammg rays through the lead 5Zn—chelate appeared to be The values for 0 water leached 38 water leached on movement of 65Zn from NTA at Table 7. Influence of incubation time, temperatugg, and Zn pH 8.3. . Incubation Period (days) 50 C. Water Leached Total 0 7 14 21 28 Avg m1 — ‘3%“ 0a 5.9 6.0 6.7 5.§ 6.6 6.1 25 32.5 27.6 30.2 31. 3g.1 31.2 50 48.8 49.2 45.2 51.0 4 .8 48.6 75 60.2 57.8 59.7 57.6 59.2 58.9 100 65.0 63.2 64.6 64.5 62.9 64.0 125 69.1 66.2 64.9 67.9 66.2 66.9 Incubation 150 C. __ 7o _ 0 5.1 6.6 3.1 7.9 6.3 5.8 25 30.2 29.3 25.0 35.0 34.0 30.7 50 48.5 46.4 44.3 49.8 51.? 58.1 75 59.1 58.5 57.1 62.0 59. 59.3 100 62.8 65.9 61.1 63.7 62.3 63.2 125 68.9 67.6 64.1 69.1 65.3 67.0 Incubation 250 C % _ _ 0 3.1 7.2 6.5 5.9 6.8 5.9 25 33.4 27.9 32.0 31.1 35.4 32.0 50 50.6 45.3 48.4 48.6 52.1 49.0 75 60.5 59.1 56.6 58.2 60.5 59.0 100 64.1 66.3 61.1 63.2 64.1 63.8 125 69.0 66.0 64.0 68.1 67.9 67.0 Incubation 350 C. 0 5.1 6.5 6.6 5.8 6.3 6.1 25 30.6 28.1 29.6 32.4 31.0 30.3 50 49.6 46.5 47.9 49.1 48.7 48.4 75 58.8 59.3 57.9 59.8 55.4 58.2 100 61.8 66.1 62.5 65.7 62.1 63.6 125 68.1 67.9 65.4 68.1 66.8 67.3 aDue to some penetration of gamma rays through the lead bricks, the t0p 2.5 mm band of 5Zn-chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. 39 Table 8. Influence of incubation time,6gemperatug§, and water leached on movement of Zn from ZnEDTA at pH 3.6. Incubation Period (days), 5° 0. Water Leached Total 0 7 14 21 28 Avg mi *% 0a 3.0 7.3 6.9 4.2 5.0 5.3 25 5.8 6.3 6.1 4.8 6.9 6.0 50 4.2 5.3 5.9 4.7 4.0 4.8 75 6.3 3.1 5.3 6.1 4.9 5.1 100 5.4 7.2 4.3 5.6 5.9 5.7 125 4.6 6.1 7.0 6.2 6.1 6.0 Incubation 150 C _______ % _ _ 0 4.0 7.2 3.6 6.6 3.3 4.9 25 4.7 5.8 5.1 4.5 6.4 5.3 50 5.7 4.7 4.9 7.5 5.5 5.7 75 4.2 7.4 5.9 6.1 4.6 5.6 100 6.3 6.6 4.9 5.3 4.9 5.6 125 5.4 4.1 .1 6.2 7.8 5.3 Incubation 250 C 0 4.9 5.4 6.5 6.8 3.7 5.5 25 5.8 5.8 6.2 3.0 7.4 5.6 50 7.3 7.1 4.8 5.8 4.2 5.8 75 5.0 5.1 5.5 5.7 3.8 5.0 100 3.6 5.3 5.7 6.6 6.5 5.5 125 5.5 7.1 6.0 3.7 5.9 5.6 Incubation 350 C ______ __- % _ 0 5.0 4.8 5.8 6.0 6.7 5.7 25 5.8 5.3 4.7 6.9 5.5 5.6 50 4.4 6.6 5.4 4.1 6.8 5.5 75 6.9 6.2 6.4 6.9 4.1 6.1 100 6.0 505 6.2 305 702 507 125 6.2 4.1 5.2 6.3 6.5 5.7 3Due to some penetration of gamma rays through the lead bricks, the top 2.5 mm band of SZn-chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. Table 9 40 Influence of incubation time, temperature, and water leached on movement of 5Zn from 65ZnEDTA at Incubation Period (days) 5? C. Water Leached Total 14 21 28 Avg ml ‘7% 0a 4.7 7.2 5.8 6.3 6.2 6.0 25 2.1 45.0 43.1 51.0 45.1 45.3 50 9.1 72.1 68.5 72.0 68.2 70.0 75 80.5 81.9 78.2 83.0 79.0 80.5 100 84.8 86.4 83.1 85.0 82.8 84.4 125 85.2 87.2 84.0 88.1 85.4 86.0 Incubation 15° c. 0 5.2 5.9 6.8 6.1 6.2 6.0 25 44.0 43.7 45.0 48.1 45.3 45.2 50 70.1 68.8 70.2 73.0 69.3 70.3 75 81.6 78.8 81.0 81.8 80.0 80.6 100 85.7 8 .1 85.0 84.3 84.6 84.7 125 87.8 8 .7 85.0 87.0 86.3 85.7 Incubation 250 C 0 4.9 6.9 6.2 6.2 6.3 6.1 25 2.7 44.9 24.3 49.3 45.2 45.3 50 9.4 70.2 9.7 74.0 68.3 70.3 75 80.8 80.2 80.7 82.1 79.0 80.6 100 85.2 84.9 84.4 85.7 83.0 8 .6 125 86.7 86.8 84.1 87.4 85.5 8 .1 Incubation 350 C. —% 0 4.9 7.1 5.2 8.1 5.1 6.1 25 43.2 42.6 45.8 47.3 41.9 44-2 50 70.2 68.9 71.3 72.0 70.1 70.5 75 81.3 79.1 78.1 81.3 78.5 79.7 100 85.1 81.2 80.3 85.7 84.0 83.3 125 86.6 86.9 83.2 86.5 86.2 85.9 aDue to some penetration of bricks, the top 2.5 mm band of approximately 7.5 mm deep. are also due to this gamma ray penetration. gammg rays through the lead 5Zn—chelate appeared to be The values for 0 water leached 41 Table 10. Influence of incubation time, mEemperatu and water leached on movement of6 Zn from g5ZnEDTA at pH 7.7. Incubation Period (days) 5° C. Water Leached Total 0 7 14 21 28 Avg. m1 = _ ——-——————-—;i%“ _ —_— — — 0a 4.8 7.1 6.1 6.2 5.2 5.9 25 42.6 45.3 40.2 49.5 47.0 44.9 50 69.1 71.3 67.3 76.2 68.1 70.4 75 78.3 80.5 78.2 82.9 81.3 80.2 100 84.1 86.3 81.3 86.3 79.6 83.5 125 84.5 87.7 81.3 87.6 87.1 85.6 Incubation 15 C _ _ _________ % _ _ __________ 0 5.2 7.3 6.1 6.9 6.1 6.3 25 44.9 42.7 49.1 44.5 46.3 45.5 50 70.1 71.2 69.3 74.2 68.2 70.6 75 82.1 79.0 80.1 81.2 80.0 80.5 100 84.8 85.9 84.1 83.2 84.1 84.4 125 87.4 86.9 84.0 86.5 85.3 86.0 Incubation 250 C 0 6.2 6.3 6.8 5.1 3.2 5.5 25 22.8 42.1 27.3 25.1 6.4 45.1 50 8o]. 7 03 8o]. 803 905 7001 75 79.2 80.7 80.1 82.1 83.0 81.0 100 84.9 83.6 84.6 85.7 85.6 8 .9 125 86.1 87.9 86.8 86.7 85.7 8 .6 Incubation_35o C 0 6.3 6.1 6.9 3.1 6.6 5.8 25 49.3 44.3 43.2 42.1 45.0 44.8 50 73.1 69.2 71.4 69.4 68.3 70.3 75 82.3 79.8 82.3 79.8 74.3 79.7 100 85.7 84.4 84.9 84.7 83.2 84.6 125 87.9 84.6 85.3 85.8 86.9 86.1 aDue to some penetration of gammg rays through the lead bricks, the t0p 2.5 mm band of 5Zn-chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. 42 Table 11. Influence of incubation time, temperatuge, and water leached on movement of 65Zn from 5ZnEDTA at pH 8.3. Incubation Period Ldays) 50 C. Water Leached Total 0 7 14 21 28 Avg. ml ------------------- 7. ---------------------- 0a 6.4 6.1 6.5 5.8 4.2 5.8 25 45.3 48.7 45.7 41.7 41.7 44.6 50 69.9 73.2 70.2 68.2 69.3 70.2 75 79.2 81.9 82.7 78.3 80.8 80.6 100 84.1 83.6 85.9 83.1 85.1 84.4 125 85.9 86.5 89.1 80.2 85.3 85.4 0 6.9 4.2 6.2 6.5 6.1 6.0 25 42.9 43.2 40.5 50.2 45.3 44.4 50 70.3 68.9 68.3 75.3 69.6 70.5 75 81.0 79.1 80.7 83.6 79.3 80.7 100 86.3 83.2 84.4 85.6 84.2 8%.? 125 86.9 85.7 84.1 88.2 88.7 8 .7 Incubation 250 C 0 4.3 4.2 6.9 6.4 6.6 5.7 25 38.7 39.7 46.2 50.5 49.0 44.8 50 65.0 70.1 70.3 73.2 66.2 69.0 75 78.2 79.8 83.0 85.5 74.3 80.2 100 83.6 84.8 86.0 87.1 85.4 85.4 125 86.5 85.8 89.0 88.0 85.5 87.0 Incubation 359 C ___________________ % ______-_________-_____ o 6.1 6.5 6.9 4.2 6.9 6.1 25 5.3 45.7 41.7 45.1 50.9 45.7 50 9.8 70.2 68.9 75.3 70.3 70.9 75 79.3 81.8 78.3 82.7 80.8 80.6 125 85.3 80.2 89.1 89.5 88.6 86.5 aDue to some penetration of gamm rays through the lead bricks, the t0p 2.5 mm band of 5Zn—chelate appeared to be approximately 7.5 mm deep. The values for 0 water leached are also due to this gamma ray penetration. 65Zn (counts/minute) 8000 r 7000 6000 ' 5000 ‘ 4000 ' 3000 ' 2000 1000 0 50 12 ml water J 0 7:5 15:0 22.5 30.0 37.5 Depth (mm) Figure 4. Distribution or652n as ZnNTA in a soil column at pH 3.65. 8000* 7000- 6000' 5000’ 4000- 65Zn (counts/minute) 3000- 2000’ 10004 25 ml water l l _L 7.5 15.0 22.5 Depth (mm) Figure 5. Distribution of 5Zn as column at pH 3.65. 30.0 ZnEDTA in a soil 65Zn (counts/minute) 45 8000 r 7000 - 6000 t 5000 ‘ 4000 b 3000 - 2000 ‘ 1000 ' 0 50 12 ml water I l L l L l J 7.5 15.0 22.5 30.0 37-5 45.0 52.5 Depth (mm) Figure 6. Distribution of 5Zn as ZnNTA in a soil column at pH 6.30. 65Zn (counts/minute) 46 800fir 7000- 6000* 5000r 4000‘ 300C- 200C- 100C” 7.5 22.5 37.5 52.5 67.5 Depth (mm) Figure 7. Distribution of 65Zn as ZnEDTA in a soil column at pH 6.30. 47 moved into a region of the soil below the top 7.5 mm, while only 6.0% (0.7% if 5.6 is subtracted for 0 water leaching - Table 8) of the ZnEDTA moved into new soil. Because the only mobile form of Zn is in the chelated form, the Zn must have dissociated almost immediately from the EDTA at this low pH and thus remained in the t0p portion of the column. For the ZnNTA, almost half of the Zn remained in the chelated or mobile form with 125 ml of water leaching. Incubation times as well as incubation temperatures had essentially no effect on the solubility of these Zn chelates (Tables 4 and 8). The log K value for HZEDTA is 6.16 while for HZNTA it is 2.49. The dihydrogen form (which is expected when divalent ions dissociate from a chelate) of the EDTA is considerably more stable and would be formed more readily than HZNTA. Calculations show that approximately 5x103 as much NTA is available to chelate with Zn than EDTA at a pH of 3.65. This fact would account for the immediate dissociation of Zn from EDTA at low pH's. It also should be noted here that at low pH's FeEDTA and FeNTA have sta- bility constants of 25.1 and 15.9 reSpectively. These log K values are very high and would thus compete very favorably with the H for the chelate. The FeEDTA would be expected to form before the FeNTA due to its higher stability con- stant, (approximately 108 times higher). The pH effect on chelate solubility brings another important aspect to correcting Zn deficiency on acid soils. 48 Even though the pH of the soil studied was extremely low, the dissociation of ZnEDTA must be considered when applied to acid soils which are Zn deficient. Figures 6 and 7 with Tables 5 and 9 show the data for the chelate movement at soil pH 6.3. No differences in movement were found for either Zn chelate at all temper— atures of incubation and for pH's of 6.3, 7.7, or 8.3 (Tables 5-7 and 9—11). Incubation times had no effect on movement of the Zn chelate with the possible exception of 28 days incubation time where l-2% less movement was noted for a few isolated cases. At pH 6.3 ZnNTA moved a maximum of 35 mm with 125 ml of water leaching while ZnEDTA moved by far the most of all treatments--62.5 mm. It is obvious that at the higher pH's the Zn remained chelated and thus soluble, allowing a much greater movement. In addition to moving farther, approxi- mately 80 — 85% of the ZnEDTA moved into lower regions of the soil column vs. 60 - 65% of the ZnNTA. This data can also be explained by differences in stability constants for the two chelates. ZnNTA and ZnEDTA have log K values of 10.45 and 16.50 respectively. The much larger value for ZnEDTA would allow for greater stability under conditions of neutral soil pH and thus greater movement under leaching. Quite different patterns of movement are also noted for the chelates at the higher pH's (Figure 6 vs. 7). The ZnNTA showed a tailing off effect when the various amounts of water were leached. Some of the Zn must have immediately 49 dissociated from the NTA upon addition of the chelate to the soil, and some is slowly dissociating over a period of time. For the same pH's, however, ZnEDTA shows only a very small portion of the Zn fixed upon addition of the chelate to the soil as illustrated by the small peak at 2.5 mm (Figure 7). Most of the Zn is mobile or in the chelated form throughout the period of study causing an actual broad moving peak (Figure 7). Stability Constant Determinations Data just presented showed that either ZnNTA or ZnEDTA were stable in the calcareous Wisner silty clay loam soil for at least one month. Yet the extreme deficiency observed in the field on pea beans grown on this soil even though total Zn is moderate in concentration suggests that the soil binds Zn strongly and might have been expected to compete with the chelate for Zn. For this reason, studies were conducted to determine the stability of Zn with the soil. Several factors complicate the determination of a stability constant with soil, namely: (1) A value for the concentration of soil is needed. (2) Many methods for determination of a stability constant can not be used in a soil system. (3) Only one complex species should be formed in appreciable concentrations. 50 (4) A scarcity of values in the literature makes comparison difficult. An ion exchange method was employed in this study for determination of stability constants. If resins are equilibrated with a solution containing metal ions, they may take part in an exchange reaction of the following nature: + + Nar + BAn : (BAn)r + Na [10] where r = resin phase, Na = sodium, central group (Zn), B: An = ligand (clay or clay + organic matter). From the above equilibrium, the distribution of B between the ion exchange resin and ligand phase can supply information concerning the Species present in the solution. The partition coefficient of BAn can be given as Na+r p = E (-—) [11] c Na+ and will be constant provided that the equilibrium or exchange constant (Ec) and the ratio of the Na ion concen- tration in the two phases is kept constant. The latter con— dition was easily met in this study by using a K-saturated resin and a high constant concentration of K in the aqueous phase. 51 Most early workers found that the exchange constant EC was dependent on the load on the resin; however, EC can be constant over a small range of (BAn)r concentration, if the load on the resin is very small (Rossotti and Rossotti, 1961). In the present study, tracer quantities of 65Zn were used to keep the load on the resin as small as possible. A strong acidic monofunctional resin (Dowex-50) was also employed to insure that the constant (EC) is as independent of H ion concentration as possible. Data obtained from the stability constant-ion exchange study is listed in Tables 12-15. The values listed for water and clay suSpenSions are in counts per minute of 65Zn in the solution which was in equilibrium with the amount of resin listed. The method of Rossotti and Rossotti (1961) was used for data calculation. Using their method, the equation: Ofiqug = K(qB "’ Pl) [123 was plotted with P0 - qB as the vertical exis and qB as the horizontal axis (Figures 8 and 9). In this equation: P0 = the distribution constant of metal in the absence of ligand, qB = the distribution constant of metal in the presence of ligand, P1 = the distribution ratio of the metal-clay complex between resin and solution (partition coefficient), 52 K = stability constant, Ch = ligand concentration in moles. The values of K and P1 can be obtained from the slope and intercept on the graph. Calculated values for Po’ qB, and PO - qB are given in Tables l2-15. The graphs (Figures 8 and 9) show that the data follows a linear relationship, enabling slopes to be easily evalu- ated. There is a deviation from linearity on Figure 9 for the larger weights of resin. Errors at these larger weights might be due to equilibrium not being attained or from not enough reactive sites on the clay to obtain the linear relationship. In order to make meaningful determination of stability constants, a value for the concentration of clay is needed (Ch). All stability constants reported in the literature are in terms of moles; however, as long as the units of the stability constants are given, the constants could be stated in terms of cation exchange or grams of soil. Con- version could then be made between the units given. Since cation exchange capacity is an important factor in deter- mining clay reactivity, an equation may be set up where: (C.E.C.)(clayconcentrationgggmszl) = moles of divalent ion [13] 2 x 105 1 of suspension Table 12. 53 Data for stability constant determination of a Wisner silty clay loam soil at pH 3.5 - 4.0, clay + organic matter. Medium Resin . (D) (c) Po - qB Weight H20 SuSpenSIOn PO qB PO - qB __E;Er—’ g counts/minute72 ml X 1072 0.2 48908 25,187 4.614 0.090 4.524 4.113 0.4 3300 22,282 7.318 0.232 7.086 6.442 0.8 2163 16,800 11.691 0.634 11.057 10.052 1.2 1550 13,579 16.691 1.022 15.688 14.262 1.6 1142 11,502 23.037 1.387 21.650 19.682 2.0 990 10,259 26.720 1.676 25.044 22.767 0 27,451 27,451 (a)Average of three replications (b)27,45l - H20 media H 0 media 2 (C)27,45l - suSpenSion (clay) media suSpenSion—(clay) media (d)Ch = 1.1 X 10‘2 M 54 Table 13. Data for stability constant determination of a Wisner silty clay loam soil at pH 4.5 - 5.0, clay + organic matter. R . Medium P _ qB nggfit H20 SuSpension Péa qéC) Po - qB _26gfir_' g counts/minute/2 ml X 10-2 0.2 67862 27,300 3.046 0.006 3.040 2.764 0.4 5400 27,007 4.084 0.016 4.068 3.698 0.8 4041 25.500 5.793 0.077 5.716 5.196 1.2 3000 24,271 8.150 0.131 8.019 7.290 1.6 2450 22,725 10.204 0.208 9.996 9.087 2.0 1950 21,517 13.077 0.276 12.801 11.637 0 27,451 27,451 (a)Average of three replications (b)27,451 - H20 media H20med1a (0)27,45l - suspension (clay)_media suSpension (clay) media -2 (d)Ch = 1.1 X 10 M Table 14. 55 Data for stability constant determination of a Wisner silty clay loam soil at pH 3.5 — 4.0, clay - H202 treated. R . Medium P _ qB nggHt H20 Suspension Péb) ng) o - qB -EEEBT—' g counts/minute/2 ml X 10-2 0.1 90352 25,079 2.01 0.085 1.93 2.383 0.3 4884 18,598 4.57 0.46 4.11 5.074 0.5 3241 12,932 7.39 1.10 6.29 7.765 0.7 2420 9,531 10.24 1.85 8.39 10.358 1.0 1805 7,648 14.07 2.77 11.30 13.951 1.5 1246 6,425 20.83 3.23 17.61 21.741 2.0 979 6,031 26.78 3.80 22.98 38.270 0 27,204 27,204 (a)Average of three replications (b)27,204 - H20 media H20 media (0)27 204 — su§pension (clay) media _L_ suSpenSion Tclay) media (d)Ch = 8.1 X 10"53 M 56 Table 15. Data for stability constant determination of a Wisner silty clay loam soil at pH 4.5 - 5.0, clay - H202 treated. R . Medium P _ 9B nggHt H20 Suspension P 0CD) qB(C) Po - qB £6337- g counts/minute/2 ml X 10‘2 0.1 10,3898) 27,500 1.62 0 1.62 2.00 0.3 6350 25,300 3.28 0.075 3.21 3.963 0.5 4900 23,375 4.55 0.16 4-39 5.420 0.7 4281 22,522 5.35 0.21 5.14 6.346 1.0 3525 20,700 6.72 0.31 6.41 7.914 1.5 2550 17,770 9.67 0.53 9.14 11.284 2.0 1984 13,808 12,71 0.97 11.74 14.494 0 27,204 27,204 (a)Average of three replications (b)27,204 — H20 media H20 media (C)275204 - sugpensiong(clay)media suSpenSion (Slay) media (d)Ch = 8.1 X 10'“3 M 57 .COHmsommsm hoppmaaowcmmso + beau Ucm Gamma coozpon GN mo sowpflphmm mo O.N ©.H N.H m. d. o 4.I 1 o.m I m.s mo .m ouswfim 02 a) r1 0 d Z_OI X uo/gb - \O r-i ON dm 58 o.m .QOquonSm topmohp No mo 0.: o.m N m I zmao 6cm gammy smozpon 0N Mo cowpwphmm .o madmflm 0 0.4 u m.m mo . o.m I m.e mo 0H ON 8N Z_OI X 90 /8b - 0d 59 Using this relationship, the following table is given: Table 16. Concentration of a Wisner silty clay loam soil. Clay + organic matter Clay - H202 treatment Clay Concentration C.E.C. Moles (gms7l)— me/lOOg M71 49.2 46.2 1.1 X 10‘2 46.2 35.2 8.1 X 10‘3 The value of moles of clay was then divided into the (P - q ) values giving the stability constants listed in o B Table 17. Table 17. Stability constants for a Wisner silty clay loam soil in moles. Clay + organic matter Clay — H202 treatment pH Slopea Log K X 10'2 3.5 - 4.0 11.429 3.06 4.5 — 5.0 30.435 3.48 3.5 - 44) 4.127 2.62 4.5 - 5.0 16.552 3.22 aSlope from Figures 8 and 9. Another method for expressing stability constants is in terms of a weight unit of soil or clay. The following table is a recalculation of K on this basis. 60 Table 18. Stability constants for a Wisner silty clay loam soil in grams. Clay pH Concentration Slope Log K (@1749 ml) Clay + organic matter 3.5 - 4.0 1.97 6.38 0.81 4.5 - 5.0 1.97 16.99 1.23 Clay - H202 treatment 3.5 - 4.0 1.84 1.82 0.26 4.5 - 5.0 1.84 7.29 0.86 It is readily obvious from Tables 16 and 17 that the value placed on (Ch) determines the stability constant obtained. Very few values have been found in the literature expressing stability constants in terms of grams of clay or soil, so comparison of the accuracy of these values is dif— ficult. Himes and Barber (1957) determined the stability constant of Maumee soil to be 5.6 at pH 7 and 3.4 at pH 4.5. Effects of pH on the determination of stability con- stants is similar to that found by Randhawa and Broadbent (1965) who reported log K values for Zn-humic acids at pH values 3.6, 5.6, and 7.0 to be 4.42, 6.18, and 6.80, reSpectively. However, Schnitzer and Skinner (1966) stated that pH had relatively little effect on the stability con- stants of Zn—fulvic acid fractions of organic matter. Their calculated log K values were 1.73 at pH 3.5 and 2.34 at pH 5.0. In the present study, relatively small differences (0.42 - 0.60, Table 17) were obtained in log K values for 61 the equilibrium pH of 3.5 - 4.0 and 4.5 - 5.0. The slight increase in stability due to pH is readily explainable in terms of H competition. Hydrogen ions compete with the metal ion for the ligand so that a decrease in pH results in a reduction of the free ligand concentration and a resulting decrease in the amount of metal complexed. In other words, at a higher pH more complexing sites on the complexing material were available for combination with Zn and thus gave an increase in the stability constant value. There is wide variability of data for stability con- stants reported in the literature, depending upon the pH at which the constants were determined and the type of organic material used. Log K constants ranged from 1.7 to 7.8. No constants were found using a soil clay as the ligand, so the accuracy and validity of the determined values is difficult to substantiate. With the organic matter destroyed, the equilibrium would be of the form: clay + Zn : Zn - clay If the organic matter is present, the equilibrium would approximate: clay + organic matter + Zn : Zn—clay + Zn-organic matter + Zn-clay-organic matter All previous work on stability constants has dealt directly with only the organic constituents of the soil. While the organic complexing fraction of the soil is very 62 important, it comprises only a small portion of the total Zn present in a soil system. 0f the available or 0.1 N HCl extractable Zn present, organic matter has been found to complex anywhere from 25 to 95%. In the present study, organic matter was found to increase the log K stability constant value from 2.62 to 3.06 at pH 3.5 - 4.0 and from 3.22 to 3.48 for pH 4.5 - 5.0 (Table 17). The values repre- sent a very small increase due to organic matter. The percent organic matter found (3.65) was that which was associated with the clay after clay separation from the soil by differential sedimentation. After H202 treatments, approximately 0.5% organic matter remained associated with the clay fraction. Even though the organic matter content was decreased around seven fold, the contribution to the log K value of the remaining organic fraction held by the clay could have been considerable. If the organic fraction-- humic or fulvic acid—-is extracted from the soil, as most other investigators on stability constants have done, the extraction procedure is certain to change some of the properties of the organic matter and thus change some of the stability constant values. Therefore, differences in stability constants, for the organic fraction of soils are easily realized. Due to the lack of values to which the calculated stability constants can be compared, a general discussion on the limitation of the accuracy of these values would be appr0priate. Several major problems are encountered when 63 the determination of stability constants is carried out by the ion exchange method. First of all, in the calcu- lations it is assumed that only one complex (Zn—ligand) is formed in appreciable concentrations. The possibility of only this one Species being present in a soil clay is quite remote. In the soil, Zn has been found to adsorb onto calcareous materials (Jurinak and Bauer, 1956), form Zn(0H)2 (Jurinak and Thorne, 1955), and found in crystal lattices (De Mumbrum and Jackson, 1956). The second major difficulty is in the adsorption of clay onto the resin. According to Rossotti and Rossotti (1961), the value of the partition coefficient (Pl) can be determined from the (x-axis) intercept on Figures 8 and 9. In order to accomplish this, a value for (Ch) in the term p _ _2_Efif§_ needs to be given. The uncertainty of exactly what value should be used for (Ch)-—such as moles or grams of clay—-makes the partition coefficient determination very difficult. If the value is given for (Ch) in terms of moles of clay as in Table 17, the partition coefficient ranges from 5 to 16%, indicating a large amount of the Zn-clay Species has been adsorbed by the resin. If this is in fact true, the value calculated for the stability constant would be in error. However, if two or more complexing Species are present, the Simple intercept method for determining Pl can not be used. Work by Dr. B. G. Ellis on the partition coefficient of a Cu—gluconate, ion exchange equilibrium 64 substantiated the fact that if two complex Species are present, Pl values can not be determined from the x axis intercept. The large adsorption values-—5 to l6%—-ca1cu1ated in this study, indicate that either two or more complex Species of Zn-ligand are present or that the equation for determining Pl does not hold true for very low resin weights. In summary, all investigations to date, have primarily considered organic matter as being one of the most important sources of Zn to the plant, and thusly have only calculated stability constants for the organic fraction of soils. The plant undoubtedly can utilize Zn from more than one source in the soil. With this view, the author contends that an overall stability constant for a Zn-clay organic matter complex would help soil scientists to predict the behavior of Zn in the soil system. Zn Adsorption According to the Langmuir isotherm, the fraction of Zn adsorbed to the soil is related to the equilibrium Zn concentration. The main advantage of applying the Zn adsorption isotherm to soils is that it provides a method to describe the behavior of the adsorbed Zn. If data follows the Langmuir equation, two facts are known con- cerning Zn adsorption: (l) the adsorption maximum; (2) a constant related to the energy of adsorption. It may be 65 possible to relate adsorption maximum to various soil parameters eg. surface area, organic matter content, or C.E.C. The constant related to the energy of adsorption would be important in determining what fractions of the soil might be most important in Zn adsorption. Tables 19 and 20 give the data for the Langmuir adsorp- tion plots shown in Figures 10 and 11. Table 21 gives the adsorption maximum for the Wisner silty clay loam soil as well as (K), the constant pr0portional to the free energy of adsorption. Figure 11 is a typical Langmuir plot show- ing a straight line relationship, while Figure 10 deviates slightly from the normal plot in that the data follows two straight line relationshipS--one at low equilibrium and one at high equilibrium Zn concentrations. Adsorption maximum for the peroxide treated soil was 0.31 me/100g soil. Values of 2.76 or 5.20 me/100gm soil were obtained for the untreated soil for low and high Zn equilibrium concentrations reSpec— tively. This data agrees well with previously determined adsorption maximums. Udo et a1. (1968) found that adsorp- tion capacities varied from 0.90 to 5.1 me/100gm soil compared to C.E.C.'s of 3.3 to 17.3 me/100gm reSpectively. Soil factors affecting the adsorption maximum of Zn have been investigated by many workers. The most important ones include surface area, percent organic matter, CaC03 content, pH, and percent clay in the soil. In determination of adsorption capacities the ionic strength of the 66 Table 19. Langmuir adsorption parameters for a Wisner silty clay loam soil. Equilibr1gm Zn in (X/m) C Zn Added Solution Fixed Zn X75 (ppm) or (M/l) X 10“ (M/l) X 10“ (mg/100gm) 77— 5 0.769 0.051a 2.34 0.022 10 1.538 0.103 4.67 0.022 20 3.076 0.162 9.48 0.017 40 6.15 0.38 18.77 0.020 80 12.31 1.031 36.7 0.028 120 18.46 2.06 53.3 0.039 200 30.77 6.09 80.2 0.076 250 38.46 8.69 96.8 0.090 300 46.15 11.62 112.3 0.103 350 53.85 16.00 123.0 0.130 400 61.54 19.85 135.5 0.146 450 69.23 24.98 143.8 0.174 500 76.92 30.12 152.1 0.198 625 96.15 40.00 183.0 0.219 aValues are the average of three replications. 67 Table 20. Langmuir adsorption parameters for a Wisner silty clay loam soil treated with H202. (C) Zn Added Equi8813818nzn in F1§68)Zn X75 (ppm) or (M/l) X 104 (M/l) X 105 (mg/lOOgm) 5 0.769 0.24a 3.43 0.140 10 1.538 0.46 7.04 0.131 20 3.076 1.18 12.3 0.190 40 6.15 3.29 18.6 0.354 80 12.31 9.11 20.8 0.876 120 18.46 14.46 26.0 1.11 160 24.62 19.38 34.0 1.14 aValues are the average of three replications. 68 00.0 000.0 44.0 40.44 s000.0 4wmmooss Noam 00.0 040.0 0s.m 4.404 s000.0 4 some 04.0 440.0 00.m 0.m04 4m00.0 m sand HHom vopmonpSD $-04 40 404 4 44xzv 40004\osv 4m004\ws0 x pdoonmPSH Any ESEHNMS Goapmhom0< omoam .mosHm> m 0cm sssflxms soflpmhompm 00p0430amo .HN oHQmB 69 '22( .20- .18- I; 012‘ .10' .06 - ’ .04- . A .02 ,~ L l l l l l I 5 10 15 20 25 30 35 40 Zn Concentration C X 106 (M/l) Figure 10. Langmuir plot of Zn adsorption data for a Wisner silty clay loam soil. 70 I. \‘ 1.1’ 1.0. 0 .5- .4 h- C .3‘ .2- 0 (I .1 3 6 9 12 1'5 18 21 Zn Concentration C X 105 (M/l) Figure 11. Langmuir plot of Zn adsorption data for a H202 treated Wisner silty clay loam soil. 71 equilibrium solution (0.1 N KCl in the present study) is also important. Himes and Barber (1957) developed a regression equation where Zn adsorbed was related to Zn added, pH, and the ionic strength of the solution. In investigating the chelating ability of the soil, they also found that removal of organic matter by oxidation with H202 destroyed the ability of the soil to chelate Zn; whereby, removal of hydrous silicates did not influence the reten- tion of Zn by the soil. Another factor that Should be considered is the possible precipitation of Zn(0H)2 in determination of adsorption isotherms. The initial pH of the soil suSpenSions was adjusted to 4.5 and after equilibrium was reached, pH's of the suspensions ranged from 3.0 to 3.9. At these low pH's, it is highly doubtful that precipitation of Zn(0H)2 will occur. Bingham et a1. (1964) found in studies of metal retention in relation to pH of equilibrium solution that no retention of Zn occurred in excess of the C.E.C. provided the pH of the system was below 5.5 to 6.5. Solubility pro- duct calculations indicate that Zn additions (0.7 to 96.2 x 10"P M) to these soils are far below the Zn concentrations required for Zn(0H)2 precipitation (approximately 1 M at pH 5). Other work by Udo et a1. (1968) showed that below the Langmuir adsorption capacity, the Zn(0H)2 ion product in the solution phase increased with Zn additions, while above this point the Zn(0H)2 product remained relatively constant. 72 The values of the bonding term (K), Table 21, calculated from the Langmuir equations, provides an estimate of the average bonding energy of Zn on the major adsorbing surfaces. These energies have a large error term inherent in the method of calculation. The H202 treated soil has a higher energy of adsorption indicating that the Zn adsorbed to the sur- face of the clay is held very tightly in comparison to that Zn held or chelated by the organic matter in the soil. The linear fit of the data with the Langmuir isotherm suggests near uniformity in bonding energy of Zn for the concentra— tions of Zn used. The difference in adsorption maximums for the H202 treated and untreated soils is quite striking (Table 21). The untreated soil has adsorption maximums approximately 10 to 20 times that of the soil with the organic matter destroyed. This information would Show the importance of organic matter on Zn adsorption and would support the findings of Himes and Barber (1957) in that retention of Zn by soils is largely a function of the amount of organic matter present. It is interesting to note that the adsorption maximum for the H202 treated soil (0.31me/100gm) is much less than the C.E.C. for the Wisner soil. This fact might indicate that the isotherm plotted would only represent a small frac- tion of the Zn that is capable of adsorption onto the clay. Perhaps if a wide enough equilibrium concentration of Zn could be used, a series of lines could be drawn whose adsorp— tion maximum sum would approximate the C.E.C. The Langmuir 73 plot drawn in this work may only be the first very small fraction of Zn that is most strongly bound to the clay. The untreated soil may Show two of these possible adsorption maximum lines (Figure 10). Line (A) would approximate a small fraction of Zn that is tightly bound to the clay or clay-organic matter complex, while line (B) would represent a less strongly held Zn fraction eg. organic matter. Because the H202 treated soil gave a much higher bonding energy for Zn, it seems plausible to suggest that this Zn may not be as available for plant use as the more weakly held Zn in the untreated soil. This suggests that Zn-organic complexes may be very important in Zn fertility. The Langmuir surface adsorption equation can be used as a means for characterizing Zn as a solid-phase supply in relation to solution characteristics. While this characterization does not necessarily identify the soil phase, it does have the advantage of describing the system with a set of constants which, when known, can be used to predict the effect of placing a stress on the soil system. Equilibrium Relationships of Zn With EDTA in Soils EDTA is involved in equilibria with a variety of cations in the soil solution, each of which is concurrently involved in equilibria with one or more soil phases. The relative importance of each EDTA complex present could, in principle, be calculated from formation constants, solubility products, 74 and other appropriate equilibrium constants. This is not possible, in general, because of the heterogeneous, poorly defined nature of participating soil phases. For this reason, most investigators of EDTA reactions in soil have been primarily empirical, Lindsay and Norvell (1969) and Lindsay, Hodgson, and Norvell (1967). An equilibrium eXpression can be derived which will relate the amount of free Zn in solution with the amount of Zn complexed with the EDTA. In this manner, it was hoped that free Zn could be related to variables such as pH, Ca concentration, and CO2 levels in the soil solution. The hypothesis that ZnSi03 controls the solubility of Zn in soils was also to be investigated. However, just prior to writing of this thesis, the original authors of the paper proposing the ZnSi03 hypothesis, Lindsay and Norvell (1969), published a retraction showing the impossibility of forma— tion of this compound in soils. A small percentage of the EDTA, as determined by lL"C analysis, was lost from the solution by adsorption onto the soil. The total loss ranged from 0 to 21%. This agrees well with that found by other workers, Hill-Cottingham and Loyd-Jones (1957). Due to this loss, the results are expressed as the percentage of EDTA remaining in solution that was complexed with Zn. The Zn measured in solution was assumed to be present as an EDTA complex because Zn could not be detected when a 75 l x lO-A'M ZnCl2 solution was reacted with the soil suSpen- Sion for several hours in the absence of EDTA. In interpretation of ZnEDTA reactions, the Ca compe— tition must be considered. The importance of Ca competition ‘would be much easier to evaluate if CaEDTA could be measured directly. However, it is difficult to determine the concen— tration of the EDTA in the presence of excess Ca and various other EDTA complexes. In this work, any EDTA in solution which is not present as a ZnEDTA ligand will be assumed to complex Ca, due to the stable complex which Ca forms with EDTA (log K of 11.41) and due to the relative abundance of Ca in the soil solution (Tables 22 and 23). The reactions of ZnEDTA are summarized in Figures 12-15 and Tables 22-23. The difference in soils used for the experiments was in the manner of pH adjustment. Soil (A), Table 22, was adjusted to various pH's by adding small amounts of dilute HCl or saturated Ca(0H)2 thus leaving solid CaC03 in the soil. Soil (B), Table 23, was first washed with dilute HCl and then small increments of Ca(0H)2 were added to effect a pH change. By washing with acid, soil (B) had no native or solid CaCO3 present. Figures 12 and 13, Show major differences for the two soils with reSpect to percent EDTA complexed with Zn at 30 days equilibrium. Percentages ranged from 31 to 55 and 13 to 32 for soils (A) and (B) respectively. Reasons for these large differences can be attributed to the amounts of Ca present in the two soils. From Tables 22 and 23 at 30 days 76 44.0 04.0 44.0 04.4 00.0 00.0 00.4 04.0 00.0 2 m+04 x 00 44.4 44.4 40.4 44.4 04.4 44.4 44.4 40.4 00.4 s 4+04 4 400040 4440 0 04.4 04.4 00.4 04.4 00.4 40.4 04.4 40.4 40.4 40 440004 4.44 4.00 0.00 0.04 4.04 0.04 4.44 4.40 4.04 004 4 fi¢mwmqu 04.0 40.0 04.0 04.4 40.0 44.0 44.4 44.0 04.0 2 4+04 0 40 44.0 04.4 44.4 04.0 04.4 04.4 04.0 44.4 40.4 s 4+04 4 440000 44004 44 44.4 40.4 44.4 04.4 40.4 44.4 44.4 44.4 00.4 40 4cspo4 . . . . . . . . . _44000 0 04 4 04 0 04 0 04 4 04 4 44 0 04 4 44 m 04 004 N 440000 00.0 00.0 44.0 40.0 40.0 04.0 44.4 04.4 44.4 s 4+04 4 40 00.4 00.4 04.4 00.4 00.4 40.4 44.4 00.4 40.4 2 0+04 N 440000 44.4 44.4 00.4 44.4 44.4 00.4 44.4 44.4 00.4 40 4440404 44004 0 000 440.0 000 44.0 000 44 .< HHom pom coflpmppcoosoo mo 0cm .oH 000 mo poommm .00 mamas 77 40.4 00.0 04.0 44.4 04.4 04.0 04.4 00.0 44.0 0 4+04 4 40 44.4 44.4 04.0 04.4 04.0 44.4 00.4 40.4 04.4 0 4+04 4 440404 4440 04 44.4 44.4 04.4 04.4 04.4 44.4 44.4 00.4 40.4 404 : 4++040 00.4 40.4 40.4 04.4 40.4 44.4 44.4 00.4 00.4 40 44040040004 04.4 40.4 44.4 40.4 40.4 04.4 04.4 44.4 40.4 40 440004 0.44 4.44 0.44 4.44 0.04 4.44 0.04 4.04 4.04 004 4 4¢mmmqu 40.4 44.4 00.4 44.4 44.4 04.4 04.4 00.4 04.4 2 4+04 x 40 44.4 44.4 40.4 40.4 00.4 04.4 40.4 04.4 00.4 0 4+04 4 440404 44.4 44.4 00.4 44.4 44.4 00.4 44.4 44.4 00.4 40 4440404 4440 04 44.4 04.4 40.4 40.4 00.4 44.4 00.4 04.4 04.4 mm 440004 . . . . . . . . . 4 4 00 4 44 4 04 0 44 4 04 0 44 4 00 4 04 4 04 004 4 4¢mwmqu 000 440.0 000 44.0 000 44 0.0000 .44 04044 78 om.m :m.m ou.m Ho.m mn.m ¢¢.m 0:.m Hm.m mm.m moa u h++nwg om.m mo.m mo.m an.u $0.5 mm.n oa.m no.5 mo.n mm HmOHpmgooze m©.m mm.n no.5 mm.> mm.w mm.m mm.n ma.n 94.5 mm H¢dpo< fig 9.: ER 0.3 0.? Hi 5% 3R QR 02 x dEEimgN moo Rmo.o moo Rm.o moo Rm 6.9200 .mm magma 79 mm.m o¢.© mH.© d©.m mm.© mo.© mo.m 4m.b mo.b E m+OH N mo om.m 4m.¢ mm.q ©¢.¢ dm.m No.4 mm.4 dm.¢ ma.o z m+OH x ¢aamGN mw 00.0 mm.n om.o o¢.o om.o o¢.u z m+oa x ®H NOD .HO POQMQUW oMN QHDQB 80 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 2 0+00 0 00 50.0 00.0 00.0 00.0 00.0 00.0 05.0 00.0 00.0 2 0+00 0 000000 0000 00 05.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 000 n m++000 00.5 00.5 00.5 00.5 50.5 00.5 00.0 00.0 .00.0 00 00000000000 00.0 05.5 00.5 00.5 00.5 00.5 00.5 00.5 00.5 00 000000 0.00 0.00 0.00 5.00 5.00 0.00 0.00 0.00 0.00 000 x flwmwmmmq 00.0 00.0 00.0 00.0 50.0 00.0 00.0 mm.0 00.0 S m+oa x mo 00.0 00.0 00.0 05.0 05.0 00.0 05.0 00.0 00.0 s 0+00 0 000000 00.0 00.5 00.0 00.0 00.5 00.0 00.0 00.5 00.0 00 0000000 0000 00 00.5 00.5 00.5 50.5 00.5 00.5 50.5 00.5 00.0 mm HmSpo< 0.50 0.00 0.00 5.00 0.00 0.00 0.00 0.00 0.05 000 0 fiwmwmmmq 000 000.0 000 00.0 000 00 0:0000 .00 00000 81 00.0 50.0 00.0 00.0 00.0 50.0 00.0 50.0 00.0 000 u m++000 00.5 00.5 00.5 00.5 00.5 00.5 00.0 00.0 00.0 00 00000000000 00.5 00.5 50.5 00.0 00.5 00.5 00.5 05.5 00.0 00 000000 . . . . . . . . . 0 000 000 000 000 000 050 000 500 000 800% 000 000.0 000 00.0 000 00 0.0000 .00 00000 82 .05.5 .50.5 .00.5 0.00 00 0 0000 00 000000 0000 00 0o 0000 .00 000000 Amhmvv @809 cOHpommm Om ON 00 N I T 1- d 1 1 q d d d - fi 1 [d 4 O JON I uz UQIM pexetdwoo VLGE queoaed .OOH 83 .do.m .mm.n .Hm.o m.mg pm m HHom no «emch 50pm :N mo mmoq .mH ouswflm Amzmvv mafia GOflpommm //////////// uz HQIM paxetdmoo Vida aueoaed 8h 5 . 0.3% 002 6 . F 7r c: 52. hi) O :—l I 8 L of? o q: A A 0 [50 xx QA A n D 9 r o 3.0% 002 A 0.3% 002 D 0.03 % 002 10 L L 1% n L . P . L 1 7.0 7.2 7.4 7.6 7.8 pH Figure 1h. Reaction of ZnEDTA with soil A at different pH's. 85 D 5 F ZnCO3 0.3% 002 6 3.0% co2 f" a 7 ’ 53 no 0 r4 I 8 . o 0 0 0E! A93 U o A A 8:: Cl 9 r [3 AK 0 3.0% co2 A 0.3% 002 D 0.03% 002 10 1 J 4 1 J n 4 l J I J A 7.0 7.2 7.h 7.6 7.8 8.0 pH Figure 15. Reaction of ZnEDTA with soil B at different pH's. 86 equilibrium, approximately 2% to 6 times as much Ca was found in soil (B) as soil (A). Even though the stability constant for CaEDTA is smaller than that of ZnEDTA (11.41 vs. 17.08), the larger quantity of Ca present would be mass action cause the ZnEDTA to dissociate and thus give smaller amounts of Zn complexed with EDTA. In these suspensions the loss of Zn from solution was very rapid initially. Further losses occurred quite slowly and in most cases the percentages of ZnEDTA became nearly constant with time. The large majority of Zn lost from the EDTA must have been diSplaced by Ca because no other metals other than the applied metal and Ca could be found by atomic adsorption techniques. Competition from Ca was also sug— gested by the very rapid initial displacement of Zn from EDTA which indicated that the displacing cation was present in abundance. Lindsay and Norvell (1969) in studies of EDTA chelates found that ZnEDTA reached a maximum stability at pH 6.75 and tapered off at pH's on either side of this value. They stated that this maximum stability resulted primarily from decreased competition by Fe for the EDTA ligand as the pH increased to 6.75 and a more rapid decrease in the concen- tration of Zn than in that of Ca for pH's above 6.75. Therefore, this maximum stability pH represented a balance between the two opposing trends. In the present work, no Fe was found in the soil solution so less than one percent of the EDTA could have been complexed by Fe. Also, the pH 87 range considered was very narrow (6.9 - 8.1) compared to the pH range of Lindsay and Norvell (1969). Large differences were also found for the time it takes to reach equilibrium or a constant percent of ZnEDTA ligand. Soil (A) required around Z—A days for equilibrium while soil (B) required 8-10 days. This difference can be explained by two factors. First, since no CaCO3 was present initially in soil (B), much more Ca was available to react with the EDTA for the first few days or until equilibrium was obtained between Ca, 002, and CaCOB. The formation of CaCO3 would remove some of the Ca from solution. The second explanation is that the larger drop soil (B) must undergo to reach the lower percentage of ZnEDTA would normally take a longer period of time. Using the 002 and Ca concentrations as controlling the solubility of CaCO3 and thus the pH of the soil, theoretical pH values for 10 and 30 day equilibrium times are given in Tables 22 and 23. Correlation between the actual pH and theoretical pH is very poor; however, the trend of increasing Ca concentration along with decreasing pH is generally true for the actual pH values determined. A discussion of the analytical problems will follow later. The range for percentages of ZnEDTA ligand formed for each time period is quite variable but follows the trend of higher Ca concentrations causing lower amounts of Zn to be complexed with EDTA. Data from the experiment indicates that 88 the amount of Ca present in a soil system is very important in determining the amount of ZnEDTA in solution. The fraction of EDTA occupied by Zn approaches equili- brium after 10 to 30 days. This indicates that the Zn con- centration approaches a constant level and that a temporary equilibrium has been established between the Zn in solution and compounds or other phases in the soil. In order to calculate the amount of free Zn in solution, the stability constants of Zn and Ca need to be known. From these relationships: Zn2+ + EDTA: ZnEDTA K = 1017'08 [14] Ca2+ + EDTA: CaEDTA K = 1011'“1 [15] the following can be written: an. :(ZnEDTA)(lO_5°67)(Ca2+) [16] *(CaEDTA) Figures 14 and 15 show a plot of soil pH vs. - log (Zn2+) for free Zn values calculated from equation (16) at 10 or 30 days equilibrium as well as theoretical lines for ZnCO3 in equilibrium with 3 and 0.3% 002. The equilibrium reactions for the theoretical equilibriums are given below: 3 + 2H+;:_=- Zn2+ + 002 + H20 [17] (S) K = 10*7'5“ ZnCO 89 therefore 7.54 + 2 2 10 (H ) [Zn +] = Ltozj [18] At [002] = 0.0003 atmOSpheres At [002] = 0.003 atmOSpheres At [002] = 0.03 atmospheres [Zn2+] = 109'06 (H+)2 [21] All points for both soils fall below the theoretical lines for the ZnCO3. This compound is far too soluble to account for the free Zn concentrations reported in this paper. Other Zn compounds, such as Zn(0H)2 and ZnSi03, also would fall in the same general region of the graph as the ZnCOB, thus making these compounds too soluble to control the level of Zn in solution. In addition, the points of the graph seem to follow a trend that indicates a line of a different lepe than postulated compounds. This fact would possibly mean that the Zn solubility is not controlled by a divalent form of the ion. No trend for CO2 levels and Zn solubility was found. It is possible that the postulated compounds for controlling Zn solubility could form as initial reaction 90 products from Zn fertilizer, and then a more insoluble compound would form at a later time. Several problems were encountered in determining the equilibrium relationship of Zn with EDTA in soils. (1) It was very difficult to obtain precise pH measure— ments in the soil system. As soon as the soils were removed from the C02 bubbling apparatus, the pH would slowly begin to change. (2) It was impossible to obtain uniform C02 bubbling rates through the soil suSpenSions. (3) Changing the pH of the native soil may have altered chemical as well as physical properties which may be important in Zn solubility. CONCLUSIONS (1) Very little movement of ZnSOh occurred in a soil column after water leachings at all pH's. (2) At a soil pH of 3.65, hh.9% of the ZnNTA moved 22.5 mm while ZnEDTA immediately dissociated and was essentially immobile. (3) At soil pH's of 6.3, 7.7, and 8.3, 60 to 65% of the ZnNTA moved 35 mm while 80 - 85% of the ZnEDTA moved 62.5 mm with 125 ml of water leached. ZnEDTA at these higher pH's was the most soluble form of Zn applied to the soil columns. (4) Different patterns of movement were noted for the chelates at pH's of 6.3, 7.7, and 8.3. ZnNTA showed a tailing off effect for its movement while ZnEDTA moved in a broad peak. This indicates that more ZnEDTA must have remained in a chelated or more soluble form than ZnNTA. (5) Time or temperature of incubation had no notice- able effect on the movement of the Zn carriers studied. (6) Organic matter increased stability constant (log K) values for the Zn-clay from 2.62 to 3.06 at pH 3.5 - u.O and from 3.22 to 3.48 at pH's h.5 — 5.0. (7) As the pH of the clay was increased, the stability constants increased due to more availability of complexing 92 (8) Adsorption maximums for Zn were much greater in the untreated soil (2.76 to 5.20 me/lOOgm) than for the H202 treated soil (0.31 me/lOOgm). This information indi- cates the importance of organic matter on Zn adsorption by the soil. (9) The H202 treated soil has a higher bonding energy for Zn than the untreated soil. (10) CO2 equilibrium studies show no effect on the solubility of Zn compounds in the soil. (11) ZnCOB, Zn(OH)2, and ZnSiO3 are too soluble to account for the low levels of Zn in the soil solution. LITERATURE CITED LITERATURE CITED Alben, A. O. 1955. 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