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3 1293 01019 20

This is to certify that the

thesis entitled
Organoclay treatment to reduce leachable concentrations
of organic pollutants in contaminated soils

presented by

Julia M. Brixie

has been accepted towards fulfillment
of the requirements for

MS degree in Cr0p and Soil Sciences

 

 

 

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

 

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MSU Is An Affirmative Action/Equal Opportunity Institution
cmmS-M

ORGANOCLAY TREATMENT TO REDUCE LEACHABLE CONCENTRATIONS
OF ORGANIC POLLUTANTS IN CONTAMINATED SOILS
By

Julia M. Brixie

A THESIS

Submitted to

Michigan State University

in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Crop and Soil Sciences

1 992

ABSTRACT
ORGANOCLAY TREATMENT To REDUCE LEACHABLE CONCENTRATIONS
OF ORGANIC POLLUTANTS IN CONTAMINATED SOILS
By

Julia M. Brixie

The Toxicity Characteristic Leaching Procedure (T CLP, US EPA
method 1311) was used to evaluate the eflectiveness of organoclays in reducing the
leachability of organic contaminants fiam contaminated soils. Organoclays are
smectite clays exchanged with quaternary ammonium cations of the form [ (CH ,) ,NR‘ ]
or [ (CH ,) ,NRJ’ where R is an aliphatic or aromatic hydrocarbon. Organophilic
organoclays were compared to adsorptive organoclays. The organophilic clays were
generally found to be more effective at reducing the leachabilty of benzene, toluene,
ethylbenzene and xylenes fi'om a gasoline contaminated soil than the adsorptive clays.
The adsorptive organoclays were not eflective in reducing leachable
pentachlorophenol (PCP) in soils contaminated by a wood treatment facility.
However, the organophilic clays dramatically reduced the leachabilty of PCP fi'om all
of the soils. A 20% addition of organophilic clay to the soil resulted in leachable

PCP levels below our detection limit (0.2 ppm).

Dedication

This thesis is dedicated to Woof Brixie, a good dog.

Acknowledgements

I would like to thank my advisor, Dr. Stephen A. Boyd for financial support during
my graduate work and for the opportunity to obtain a valuable education in
environmental soil science. The assistance my advisor and all my committee
members gave me throughout my stay at Michigan State was appreciated. I would
especially like to thank Dr. James Crum, for helping me gain admittance to the
program. Dr. Randall J. Schaetzl gave me continued support during every possible
aspect of this thesis, and during my entire graduate career. Dr. William F. Jaynes
gave me tremendous assistance with the experimental design and execution of this
research and with the interpretation of results and editing of the thesis. Dr. Sharon
Anderson allowed me generous use of her laboratory, assistance with various analyses
required for this thesis and continued daily moral and technical support, as well as
many helpful technical discussions. Dr. Ines Toro-Suarez shared her valuable
instrumentation expertise with me in a patient and thorough manner. Dr. Shaobai Sun
gave me assistance with research design, data interpretation and many helpful
discussions. Kellie Ellis allowed me to complete my experiments in a timely fashion
with her dishwashing assistance. Linda Schimmelpfennig was helpful with waste

management, extraction assistance and frequent moral support.

ii

TABLE OF CONTENTS

LIST OF TABLES ............................................. iv
LIST OF FIGURES ............................................. v
Chapter I .................................................... 1
ABSTRACT ............................................ 1
INTRODUCTION ......................................... 2
Background on Organoclays ............................. 2

MATERIALS AND METHODS ............................... 7
Organoclay Preparation ................................ 7

Soil Treatments .................................... 10

Soil Amendment .................................... 12

TCLP Extractions ................................... 13

Analytical Methods .................................. 14

RESULTS AND DISCUSSION .............................. 15
CONCLUSIONS ......................................... 3 1
LITERATURE CITED ..................................... 32
Chapter 2 .................................................. 35
ABSTRACT ............................................ 35
INTRODUCTION ........................................ 36
MATERIALS AND METHODS .............................. 40
Organoclay Preparation ............................... 40

Soil Properties ..................................... 44

Soil Treatments .................................... 46

TCLP Extractions ................................... 47

Batch Sorption Isotherrns .............................. 48

Analytical Methods .................................. 49

RESULTS AND DISCUSSION .............................. 49
CONCLUSIONS ......................................... 64
LITERATURE CITED ..................................... 65

1.0
1.1
1.2
1.3

1.4

2.0
2.1
2.2
2.3

2.4

2.5

LIST OF TABLES

Structural formulae and organoclay designation. ......................
Organic carbon content and (1001 spacings of organoclays. ...............
Particle size, CBC, and carbon content of soil. ......................
Concentration of BTEX compounds in the TCLP leachate of

recontaminated soil treated with adsorptive and organophilic clays“. .....
Concentration of BTEX compounds in the TCLP leachate of the

unamended soil treated with adsorptive and organophilic clays and

Sorbond and grouped by individual experiment date" ................

Structural formulae and organoclay designation. .....................
Organic carbon contents and doc, spacings of organoclays ...............
Some natural and anthropogenic properties of soils H, C and J. ..........
pH and concentration of PCP in TCLP leachate of soil H after different
treatments. .............................................
pH and concentration of PCP in the TCLP leachate of soil I after different
treatments. .............................................
pH and PCP concentration in the TCLP leachate of soils C and H after
treatment with different amounts of organoclay and Sorbond. .........

iv

8
9
ll

17

18

41
43
45

1.0

1.1

1.2

1.3

1.4

1.5

1.6

2.0

2.1

2.2

2.3

LIST OF FIGURES

Decreases in the concentration of ethylbenzene and xylenes in the TCLP

leachates of an untreated gasoline contaminated soil ................

Concentrations of BTEX in the TCLP leachates from organoclay and
Sorbond treated soils, untreated soils and from mixed, untreated

(control) soils ...........................................

Concentrations of BTEX in the TCLP leachates from organoclay and

Sorbond treated soils and from mixed, untreated (control) soils. .......

Effects of three different treatments on the concentration of BEX
compounds in the ”modified" TCLP leachates of a gasoline

contaminated soil. .......................................

Leachability of BTEX compounds in the TCLP leachates of a

recontaminated soil treated with adsorptive clays. .................

Leachability of BTEX compounds in the TCLP leachates of a

recontaminated soil treated with organophilic clays. ...............

Leachability of TEX compounds in the TCLP leachate of a contaminated

soil treated with organoclays and Sorbond .......................

Sorption isotherm of PCP on DICOCO C clay with and without Sorbond. . .

Effects of the amount of added DICOCO C organoclay on the

concentration of PCP in TCLP leachates of soil H. ...............

Effects of the amount of DICOCO C organoclay added on the

concentration of PCP in TCLP leachates of soil C. ................

Effects of the amount of DICOCO C organoclay on the concentration of

PCP in TCLP leachates of soil C mixed with Sorbond. .............

16

20

21

22

25

27

30

55

62

63

Chapter 1

ABSTRACT

Organoclay Treatment to Reduce the Leachable Concentrations of Organic Pollutants

in a Gasoline Contaminated Soil.

The Toxicity Characteristic Leaching Procedure (T CLP, US EPA method 1311)
was used to evaluate the efl'ectiveness of organoclays in reducing the leachability of
benzene, toluene, ethylbenzene, and xylenes (BTEJO in a sandy, low organic matter
soil contaminated with gasoline. The organoclays were smectites exchanged with
quaternary ammonium cations of the form [(CH’),NR]+ or [(CH,),NRz]+ where R is an
aliphatic or aromatic hydrocarbon. Organoclays made using organic cations with
large aliphatic R groups (organophilic organoclays) were compared to those made
using organic cations with small aromatic or aliphatic groups (adsorptive
organoclays). The organophilic clays were generally more eflective at reducing the
leachabilty of organic pollutants than were the adsorptive clays. The DICOCO clay
was found to be the most effective at reducing the leachability of toluene, ethylbenzene
and m,o,and p-xylenes while the MSU TMPA was found to be most eflective at

reducing the leachability of benzene fiom the T CLP leachate.

INTRODUCTION

Soil contamination is a widespread and serious environmental problem.
Contamination can result from improper disposal of industrial wastes, accidental
chemical spills, overflow of storage tanks or lagoons, leaks from underground tanks,
and land application of wastes. Leaking, underground, gasoline storage tanks are one
of the most prevalent sources of soil contamination in the U. S. Cleanup costs for
underground tanks have already reached an excess of 3.2 billion dollars (Palacios,
EC, 1992).

Both Michigan (State of Michigan, 1979) and the United States government
mandate the use of the Toxicity Characteristic Leaching Procedure (TCLP; FR 55,
1990) to determine if soils at a known spill or contamination site are to be classified
as hazardous or nonhazardous soils. The TCLP method estimates the concentrations
of specific chemicals that leach from the soil into aqueous media If the
concentration of pollutants in the TCLP leachate are greater than or equal to the
groundwater standards for those pollutants, then the soil is considered to be
contaminated. Obviously, the costs for soil restoration and disposal will be influenced
greatly by these regulatory designations. As a result, there is a great deal of interest in
technologies for reducing the leachability of pollutant chemicals in soils so that a
hazardous or contaminated classification can be avoided. Reducing the leachability
will minimize the potential for transport of contaminants into groundwater.
Background on Organoclays

The movement of organic contaminants in hydrologic environments is

3

predominantly controlled by their interaction with soils and subsoils. The uptake of an
organic contaminant by soil is dependent on the chemical properties of the compound,
the physical and chemical characteristics of the soil, and the properties of the medium
from which the compound is sorbed. In dry soils, the sorption of organic solutes is
controlled by adsorption on mineral surfaces (Chiou et al., 1985). In soil-water
systems, the primary uptake mechanism of nonionic organic contaminants (NOCs) is
considered to be solute partitioning into the soil-humic phase (Chiou et al., 1979;
1983; 1985). In these systems, adsorption of the organic solutes by soil minerals is
insignificant compared to the uptake by solute partitioning into the soil-humic phase
because water is preferentially adsorbed by the minerals. Extensive research has
shown that NOC sorption increases as the soil organic matter content increases and as
the water solubility of the NOC decreases (Chiou 1989; Chiou et al., 1985).

Smectite clays (e.g. sodium bentonite) have been used widely in landfill liners,
waste treatment lagoons and slurry walls because of their ability to disaggregate upon
hydration ("swell") and form a dispersed phase of very small particles that impedes
water movement. Although smectite clays impart useful hydraulic properties to clay
liners and slurry walls, they have little sorptive capability for removing NOCs from
water. Smectite clays have a net negative charge that in nature is satisfied by
inorganic cations (e.g., Na” and Ca2+) on their surfaces. The hydration of these metal
exchange cations imparts a hydrophilic nature to the clay surfaces and interlayers.
This hydration may cause displacement of NOCs from adsorption sites on the clay
mineral surface resulting in mobilization of the NOCs. Brown et al. (1990) found

that dry smectite clays, which are sometimes used as sorbent materials for spill

4

cleanup of NOCs at contaminated sites, can release large amounts of these same NOCs
when placed in landfills because of the preferential adsorption of water by the
smectite.

The capacity of clays for removing NOCs from water can be greatly enhanced
by a chemical modification process, whereby the naturally occurring inorganic
exchange cations are replaced with organic cations via ion-exchange reactions (Boyd et
al., 1988a, b, c; Lee et al., 1989a, b, 1990; Jaynes and Boyd 1990, 1991a, 1991b;
Mortland et al., 1986). The organic cations most commonly used for this purpose are
quaternary ammonium ions of the general form [(CH3),NR]” or [(CH,)2NRR']" where
R,R' is an alkyl or aromatic hydrocarbon. The displacement of inorganic cations by
organic cations results in organoclays with high affinities for organic pollutants (Jaynes
and Boyd 1990; Boyd et al., 1988b). The considerable ability of organoclays to sorb
and retain organic contaminants suggests the use of organoclays in combination with
solidifying agents, may be effective in sequestering pollutant chemicals contained in
hazardous and industrial wastes and contaminated soils (Evans and Pancoski, 1989;
Montgomery et al., 1988; Boyd et al., 1991; Lee et al., 1989a).

Organoclays have been grouped into two general types: adsorptive and
organophilic clays (Boyd et al., 1991). These classes of clays are distinguished by
their sorptive mechanisms and resultant sorptive properties for NOCs. Adsorptive
organoclays are smectites modified with relatively small organic cations such as
tetramethylammonium (TMA) and trimethylphenylarnmonium (TMPA). These small
cations exist as discrete entities in the clay interlayers and are separated by uncovered

clay surfaces (Lee et al., 1989b, 1990; Boyd et. al., 1991; Jaynes and Boyd, 1990,

5

1991b). The uptake of aromatic hydrocarbons such as benzene, toluene, ethylbenzene,
and xylenes (BTEX) from water by adsorptive organoclays is attributed to a surface
adsorptive mechanism (Lee et al., 1989b, 1991; Jaynes and Boyd, 1990, 1991b).
Adsorption is commonly defined as a process through which a net accumulation of a
substance occurs at the interface of two contiguous phases (Hassett and Banwart,
1989). In these clays the organic cations act as compact, non-hydrated pillars that
prop open the clay layers exposing the abundant siloxane surface area for adsorption
of organic contaminants. The adsorptive clay (Boyd et al., 1991; Lee et al., 1989b,
1990) TMA-smectite, has high affinity for benzene, but exhibits shape and size
selectivity resulting in progressively less uptake for larger substituted benzenes. This
characteristic was not evident for TMPA-smectite where the high affinity for benzene
was extended to larger aromatic hydrocarbons (Jaynes and Boyd, 1990, 1991b). Solute
removal by an adsorptive mechanism is characterized by nonlinear isotherms, high
exothermic heats of sorption, and sorptive competition between solutes (Chiou, 1990).

Organoclays formed using quaternary ammonium cations containing one or two
large (CIo or larger) alkyl hydrocarbon moieties are referred to as organophilic clays. A
well studied organophilic clay is HDTMA-smectite. Once such large, hydrophobic
organic cations are fixed on the exchange sites of smectites, the surface properties
change from hydrophilic to organophilic. This is due to the formation of an organic
phase composed of the alkyl hydrocarbon chains of the organic cations, and because
the exchanged cations are less hydrated (Jaynes and Boyd, 1991a; Boyd et al., 1988b,
c). The tails of the large cations can interact with each other to form a bulk

partitioning phase (Boyd et al., 1988b; Jaynes and Boyd, 1991a) which is similar to,

6

but 10 to 30 times more effective (on a unit mass basis) than soil organic matter in the
removal of NOCs from water (Boyd et al., 1988a; Lee et al., 1989; Jaynes and Boyd,
1991a).

The partition mechanism for the sorption of NOCs by organophilic clays can be
described as the uptake of a solute by dissolution of the sorbed organic compound into
the sorptive phase (Boyd et al., 1988a; Jaynes and Boyd, 1991a), analogous to the
distribution of an organic compound between water and a bulk organic solvent phase
such as hexane or octanol (Chiou, 1989). A partition mechanism is characteriwd by
linear sorption isotherms, an inverse linear relationship between a compound's partition
coefficient (KOM ) and water solubility, low and constant heats of sorption, and by the
lack of competitive sorption in multisolute systems (Boyd et al., 1988b; Chiou et. al,
1979, 1981, 1983; Chiou, 1986, 1989).

Although the uptake of NOCs from aqueous solutions by organoclays has been
extensively studied ( Boyd et al., 1988b; Jaynes and Boyd, 1990, 1991a; Lee et al.,
1989b; Mortland et al., 1986; Cadena et al, 1990), the uptake of organic contaminants
from soils or wastes by organoclays has not recieved as much attention. However,
previous studies have shown that organoclays used with cement products show
promise in effectively stabilizing and solidifying previously difficult to treat wastes
that contain a significant organic fraction (Sheriff et al., 1987; Montgomery et al.,
1988; Evans et al., 1989).

The purpose of this study was to determine the effectiveness of nine
organoclays in reducing leachable concentrations of BTEX in a gasoline-contaminated

soil. Leaching of BTEX was evaluated using the TCLP. Laboratory-prepared

7

organoclays were compared to commercially-prepared organoclays. In addition, a
solidification reagent was added to the soil to determine the effects, if any, on the
leachability of BTEX in contaminated soils. The study was designed to simulate
solidification and stabilization treatment of soils. Solidification and stabilization
techniques frequently involve removing the soil from the ground and then mixing it
with various additives in large equipment on site. After the soils are stabilized and
solidified (by cement treatment) they can either be removed from the site and placed
in a landfill or put back into the ground on the site. The low organic matter content
and sandy nature of soil frequently encountered near leaking underground petroleum
storage tanks provides conditions where NOCs are highly mobile and can often leach
into the groundwater. Consequently, sandy-loam soils with low organic matter were
used in this study.
MATERIALS AND METHODS
Organoclay Preparation

The organoclays studied in this experiment were modified with a number of
different quaternary ammonium cations. A summary of the names, abbreviations and
structural formulae of the cations are shown in Table 1.0. The organic C contents and
d001 spacings of the organoclays are listed in Tablel.l. The smectite clay used for the
laboratory-prepared clays was a Wyoming bentonite (primarily Na-smectite) provided
by American Colloid Company, Arlington Heights, IL. Twenty-five g of clay were
dispersed in 5 liters of water and allowed to stand several hours to allow settling of
sand, silt, and heavy minerals. The suspension was decanted and the appropriate

quantity of an aqueous solution of the bromide or chloride salt of the organic cation

Table 1.0 Structural formulae and organoclay designation.

 

 

Name and Abbregation Structural Formulae
0”.
l
Tetramethylamnoritm Cfi-N-CH,
TMA“ (EH3
(EH.
TnmetIMphenytanmoriun CH?" O
TNPA“ en,
9'".
Benzylrimethytanmriun 3”,."
arm“ (EH
‘5”.
Heradecylrimethytamron‘un CH;N‘—(CH,),CH3
I-DTMA“ H3
9“.
Dioctadecyflimethytarrmriun 9'14." "(CHLOE
DoDMA“ EMF".
9“.
Ditalowdimethytammriun Cl't,—t'J‘—(<:H,)1,CHa
DrrALLow (CH),CH,
9H:
Dicocodimethylarrmriun CH3—N’—(CH2)11CH3
Dicoco I

(CH2)flCH3

9

was added while stirring on a magnetic stirrer. Salts of the organic cations HDTMA
and DODMA were added in an amount equal to the cation exchange capacity (CEC)
of the clay. Salts of the organic cations TMPA and BTMA were added in an amount
equal to five times the CEC of the clay. The chloride salt of DODMA was dissolved
in a 50:50 v:v mixture of methanolzwater and the HDTMA, TMPA, and BTMA

chloride or bromide salt, were dissolved in water before addition to the clay

suspension .

Table 1.1 Organic carbon content and doc, spacings of organoclays.

 

 

Organoclay Organic Carbon doc, spacings
% A
TMA 3.60 13.8
MSU TMPA 8.84 14.0
AC TMPA 10.00 14.8
MSU BTMA 10.43 14.1
AC BTMA 11.02 14.0
HDTMA 18.70 17.5
DODMA 33.66 34.0
DICOCO S 23.76 22.8
DICOCO C 23.85 36.9
DITALLOW 28.11 35.4

 

After addition of the organic cation, the solution was mixed for approximately

4 hours and allowed to settle. The clear supernatant was poured off and the remaining

10

clay suspension was placed in dialysis bags that were dialyzed against distilled water
until the water was free of Cl'and Br‘ as determined by a silver nitrate test. The clay
solution was later quick-frozen and freeze-dried. The TMPA and BTMA clays
prepared in the laboratory at Michigan State University (MSU) will hereafter be
referred to as MSU TMPA and MSU BTMA.

The commercially-prepared clays used in this study were provided by American
Colloid Company and were made by a ”dry-mix” process. In this process, an
isopropanol solution containing the organic cation was added to the dry clay. The
mixture was extruded as a paste and was subsequently oven-dried and ground. The
quaternary ammonium cations used were TMPA, BTMA, DICOCO, and DITALLOW
(Tables 1.0, 1.1). The DICOCO cation is a derivative of coconut oil and the alkyl
chains consist of approximately 70% C12 chain lengths with a range of C8 to C16 chain
lengths. The DITALLOW cation is a derivative of beef fat and the alkyl chains
consist of approximately 70% C1. chain lengths with a range of CNS to CH3 chain
lengths. Each cation was mixed with a Wyoming sodium bentonite to make the
commercial clays; DICOCO was also mixed with a calcium bentonite. The
American Colloid TMPA and BTMA will hereafter be referred to as AC TMPA and
AC BTMA. The DICOCO clay prepared with the sodium bentonite will be referred to
as DICOCO S; the DICOCO prepared with a calcium bentonite will be referred to as
DICOCO C.

Soil Treatments
Contaminated soil was obtained from a mid-Michigan environmental consulting

firm. It had been taken from below a leaking, underground, gasoline storage tank.

11

The soil is sandy and low in organic matter (Table 1.2).

Table 1.2 Particle size, CBC, and carbon content of soil.

 

Particle Size Fraction

 

 

 

% Sand % Silt % Clay
65.6 25.3 9.1
Carbon Content
% Carbonate C % Total C °/o Organic C
2.09 2.73 0.64

 

Five soil treatments were performed in this study. In the first treatment, only
organoclay was added to the soil. In a 4° C cold room, 2.5 g of the organoclay was
added directly to the top of 10 g of contaminated soil in a small glass jar. The jar was
quickly capped and 1 ml of distilled water was added on top of the organoclay-soil
mixture through a 1.5 cm hole in the cap. The organoclay-soil-water mixture was
thoroughly mixed (~45 seconds) with a stainless steel spatula inserted through the hole
in the cap. The hole in the cap was then sealed with an aluminum foil-covered rubber
stopper and the organoclay-soil-water mixture was allowed to cure for 24 hours.

In a second treatment, organoclay and the commercial solidification agent
Sorbond (American Colloid Co., Arlington Heights IL) were added to the soil. The
organoclay and soil were first mixed as described above for the first treatment, with
the exception that the organoclay-soil-water mixture was allowed to cure for one hour

(instead of 24 hours). Then, 2.5 g of Sorbond was added to the organoclay-soil-

12

mixture. After thoroughly mixing, the cap was sealed and the Sorbond-organoclay-
soil-water mixture was allowed to cure for 23 additional hours.

A third treatment was designed to serve as a control, and is referred to as the
control treatment. In a 4° C cold room, 10 g of the contaminated soil was placed in a
small glass jar and capped as described above. The unamended soil was thoroughly
mixed through the hole in the cap as in the previous two treatments. The cap was
then sealed and the soil was allowed to cure for 24 hours. This was an attempt to
measure losses due to volatilization during the mixing procedure.

In a fourth treatment, 10 g of the contaminated soil and 2.5 g of Sorbond were
treated as described for the first treatment. This was an attempt to measure BTEX
losses while amending the soil with Sorbond.

In a fifth treatment, the contaminated soil was removed from storage in the
cold room and used immediately. This treatment will subsequently be referred to as
the untreated soil.

Soil Amendment

Due to losses of gasoline components caused by volatilization, the
contaminated soil was amended with gasoline after 11 weeks storage. Exactly 50 ml
of regular, unleaded gasoline was added to ~1000 g contaminated soil and mixed
thoroughly with a stainless steel spatula This amended soil was used for experiments
involving both adsorptive and organophilic clay amendments to the soil without
Sorbond. The unamended soil was used for the experiments that involved Sorbond

and organoclays.

1 3
TCLP Extractions

Following treatment, the soils were extracted according to the TCLP procedure
described for volatile organic contaminants (FR 55, 1990). The TCLP test for
volatiles involves placing the sample in a stainless steel, zero headspace, extraction
vessel (ZHE, Analytical Testing and Consulting Services, Inc., Warrington, PA; FR
55, 1990). For these experiments, the entire sample of soil and additives was added to
the ZHE. The ZHE is equipped with a movable piston that was raised to remove
excess headspace from the vessel. Once the ZHE was sealed with the sample inside,
the piston was lowered and at the same time, the ZHE was charged with a sodium
acetate buffer solution (pH 4.95 1.05). In each extraction, the quantity of buffer
solution used was equal to 20 times the weight of the sample. Each sample jar was
weighed before and after the sample was added to determine the actual weight of
sample to be extracted. Because the TCLP requires a 20 to one extraction of the
sample, and is based on the weight of the soil sample, those soils that have been
treated (and thus diluted) with additives will be diluted additionally during the TCLP.

After the acetate buffer solution was added, the ZHE was shaken end-over-end
for 18 hours on a DC-20 rotary agitator (Analytical Testing and Consulting Services,
Inc., Warrington, PA) at 30 rpm. The leachate was then filtered inside the ZHE with a
Whatrnan glass fiber filter (Whatrnan, England), removed from the outlet valve of the
ZHE, and stored in a 10 ml serum bottle with a teflon-lined cap. This leachate was
later used for duplicate and triplicate analysis. Another 1 m1 of leachate was removed
from the ZHE with a Hamilton, gas-tight, luer-tip syringe and was directly analyzed

for BTEX contaminants by a purge and trap gas chromatograph. All samples

14

including replicates were analyzed within 24 hours of removal from the ZHE.

A "modified TCLP" extraction was also performed. This procedure was identical to
the TCLP in all respects except that the extrath fluid was pentane instead of
buffered water. The modified TCLP was performed to determine the total mass of
BTEX in the gasoline-contaminated soil.

Each individual experiment in this study consisted of six treatments. The
experiments were set up in this manner because of the limited number of ZHEs (6)
that were available.

Analytical Methods

After the leachate was removed from the ZHE with a Hamilton gas-tight
syringe, it was diluted with Milli-Q water to reduce the contaminant concentration to
the appropriate analytical range. The diluted sample was placed in the sampler tube of
a Tekmar model ALS autosampler and purged with a Tekmar model 4000 dynamic
headspace concentrator (Tekmar Company, Cincinnati, OH). The sample was
subsequently analyzed for BTEX compounds using a Tracor model 540 gas
chromatograph (GC) and a Tracor model 703 photoionization detector (Tracor
Instruments, Austin, TX). The GC was equipped with a 50 m x 0.73 mm ID Vocol
column (Supelco, Bellefonte Pk, PA) . The mobile phase was 99.999% pure helium
and the temperature program was started at 50 °C and increased to 180 °C by
6°C/minute with a final hold of 8 minutes. Both the TCLP and the modified TCLP
leachate were analyzed in this manner.

Each leachate sample was analyzed in duplicate or triplicate on the GC and the

peak areas were determined with a Hewlett Packard 3392A (Hewlett Packard,

15

Corvallis, OR) integrator. Concentrations in mg 1’1 (ppm) were calculated from peak
areas using a standard regression curve obtained from external standards. Averages of
the duplicate or triplicate analyses were reported.

RESULTS AND DISCUSSION

Concentrations of leachable ethylbenzene and xylenes in the untreated sample
leachate were monitored over an eleven week period to assess the stability of the soil
sample under the storage conditions used. A gradual decline (36 to 81%) in the
amount of ethylbenzene and xylenes in the TCLP leachate occurred as the soil sample
aged (Figure 1.0). This decline was presumably caused by volatilization losses and
indicates that samples of this type have a relatively short life span. Because of the
dynamic nature of the sample, the absolute concentrations of BTEX in the TCLP
leachate should only be compared within an individual experiment and not among
experiments. At the end of eleven weeks, the soil was amended with gasoline and
used in subsequent experiments.

BTEX leaching from soils amended with the commercially-prepared and the
laboratory-prepared adsorptive organoclays, with and without Sorbond, was evaluated
using the TCLP. These data are summarized in Tables 1.3 and 1.4. The data reported
in Table 1.4 are from individual experiments performed on different dates which
included three treatments: untreated soil, soil amended with organoclay and Sorbond,
and a control. These experiments used the unamended gasoline contaminated soil.
These samples (except those in the experiment on 10-6-91) were subjected to the
standard TCLP using acetate buffer as the leaching solution, and to the "modified"

TCLP where pentane was used rather than acetate buffer. Pentane is a much more

16

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effective solvent for removing BTEX both from the untreated soil and the organoclay
treated soil than the TCLP solution. It was assumed that pentane would extract the
majority of the BTEX from all the soils, and therefore pentane was used to gain a

mass balance of BTEX in the system.

Table 1.3 Concentration of BTEX compounds in the TCLP leachate of
recontaminated soil treated with adsorptive and organophilic clays".

 

 

Treatment BENZ. TOLUENE ETHYL- M,P- O-
BENZ. XYLNE XYLNE
(ppm)/ (ppm) (ppm) (ppm) (ppm)

Adsorptive clays
UNTREATED 19.72 62.82 6.97 12.77 10.69
AC TMPA 5.42 32.60 5.42 11.27 12.63
MSU TMPA 3.33 21.90 3.61 7.26 9.00
AC BTMA 1.70 24.86 5.08 9.97 11.88
MSU BTMA 3.66 34.15 6.08 11.22 11.75

Organophilic clays
UNTREATED 6.903 21.817 5.142 8.551 8.274
HDTMA 1.293 14.081 3.895 6.679 6.177
DODMA 2.005 10.245 2.749 3.937 3.996
DICOCO S 1.903 12.199 2.521 4.108 3.649
DICOCO C 1.865 9.485 1.635 2.723 2.280
DITALLOW 1.652 10.441 2.064 3.406 2.032

 

" The soil used in these experiments was amended with additional gasoline before use.

18

Table 1.4 Concentration of BTEX compounds in the TCLP leachate of the

unamended soil treated with adsorptive and organophilic clays and Sorbond and

grouped by individual experiment date".

 

 

 

 

 

 

 

Treatment BENZ. TOLUENE ETHYL- M, P- O-
BENZ. XYLNE XYLNE
(PPm) (PP!!!) (PP!!!) (PP!!!) (PP!!!)
Adsorptive and organophilic clays with Sorbond
10-06-91
UNTREATED 0.062 0.731 2.503 4.432 4.227
CONTROL 0.028 0.109 0.390 1.367 1.401
SORBOND1 0.032 0.107 0.677 1.550 1.396
MSU TMPA 0.042 0.797 0.307 0.964 1.867
MSU BTMA ND§ 0.087 0.304 1.260 1.608
DICOCO S ND 0.245 0.546 0.961 0.940
6-25-91
UNTREATED 0.083 0.659 0.654 0.919 0.751
CONTROL 0.037 0.221 0.384 0.599 0.514
AC TMPA 0.055 0.335 0.232 0.370 0.557
7-22-91
UNTREATED 0.010 0.626 0.155 0.803 0.898
CONTROL ND 0.058 0.095 0.492 0.515
AC BTMA ND 0.037 0.040 0.280 0.348
7-08-91
UNTREATED 0.007 0.364 1.066 2.333 2.147
CONTROL ND 0.01 1 0.053 0.429 0.462
HDTMA ND 0.049 0.208 0.846 0.892
6-30-91
UNTREATED 0.109 0.592 0.590 0.977 0.853
CONTROL ND 0.248 0.457 0.759 0.640
DODMA 0.096 0.364 0.325 0.390 0.365

 

I-T11e soil used in these experiments was not amended with additional gasoline.
1 This treatment was the Sorbond control treatment.
ND§ The sample leachate was below the method detection limit (0.005 ppm)

19

Selected results from Table 1.4 are displayed in bar graph form in Figures 1.1-
1.3. In these graphs, the height of each bar represents the concentration in the sample
leachate for the different treatments; bars are grouped by compound. Each graph
shows the results of a single experiment and includes an untreated sample and one or
more of the other treatments. The performance of each organoclay (in the treated
samples) was evident from the concentrations of BTEX compounds in the leachate.
High concentrations in the leachate indicate contaminant release, whereas low
concentrations indicate effective sequestering of contaminants by the organoclay.
The graphs in Figures 1.1 and 1.2 show the concentrations of BTEX in the TCLP
leachates from organoclay- and Sorbond-treated soils and from the control soils. The
results of the control treatment from the different experiments were inconsistent
because the concentrations of BTEX compounds in the control TCLP leachate were
higher than, equal to, or lower than the organoclay-Sorbond-treated samples. The
control treatment never resulted in a clear trend. The inconsistent results of the
control treatments were probably due to volatilization losses during the mixing
process. The control samples could have undergone volatilization losses during the
mixing step. However, in the organoclay-treated samples, the soil was covered with
organoclay before being thoroughly mixed. The organoclay may have acted as a
barrier in these treatments hence reducing the loss of any compounds that would have
volatilized during the physical mixing.

Data from the pentane extractions (modified TCLP) support this explanation.
Figure 1.3 shows the results of a control treatment which was extracted with pentane

using the modified TCLP. The organoclay-treated soil in this experiment had

20

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23

approximately the same concentration of BTEX compounds as the untreated soil, yet
the BTEX concentration in the pentane leachate of the control sample was reduced by
~50-70 percent. These results indicate that the mass of BTEX was approximately the
same in the untreated and treated samples, if we assume that the pentane extracted the
majority of the BTEX in all the samples. However, the BTEX mass was significantly
lower in the control samples, possibly indicating volatilization losses during mixing.
This loss of BTEX in the control samples led us to believe the control was invalid,
and it was abandoned in further experiments. Unfortunately this left us without a
control for the remaining experiments.

If indeed adding organoclay on the top of the soil reduced volatilization during
the soil mixing, then there are some important implications that should be addressed.
Volatilization of gasoline components can be a source of air pollution during
solidification and/or stabilization of contaminated soils. There also may be a
flammability hazard during the solidifications and/or stabilization, and during transport
of the solidified material from the contaminated site to the landfill. Because the
organoclays seem to reduce the volatilization of some of the gasoline components
during soil mixing, they may prove to be useful to reduce the flammability hazard
and to reduce air pollution during solidification, stabilization, removal, or transport of
the gasoline contaminated soil from the site. They might prove especially useful in
regions with strict air quality requirements.

Each experiment performed was limited to a maximum of five treatments and
one untreated sample, where absolute concentrations of BTEX can be compared

directly. However, due to the changes in the BTEX concentration in the soil sample

24

over time, it was difficult to compare absolute concentrations among the experiments .
The percent leachability of the treatment can, however, be compared among
experiments. The percent leachability (L) of the treatment can be expressed as:
L={[T]/[U]}"'100

where [T] is the concentration of BTEX in the treated sample leachate and [U] is the
concentration of BTEX in the untreated sample leachate. Thus high leachability would
indicate that a large amount of the BTEX was leached out (treatment was ineffective),
and a small leachability would indicate that a small amount of BTEX was leached out
after treatment. By calculating the percent leachability it was possible to compare the
clays used in the different experiments. However, this equation should only be used
when studying a single soil. Soil type differences could result in misleading
leachability comparisons.

Figure 1.4 shows the leachability of the 4 different adsorptive organoclays.
The MSU TMPA treatment resulted in the lowest concentration of every compound
except benzene in the TCLP leachate. This result is consistant with previous results
on the adsorptive capabilities of organoclays that showed TMPA smectite to be the
most effective adsorbent of aromatic hydrocarbons (Boyd et al., 1991; Jaynes and
Boyd, 1990). The AC BTMA treatment resulted in the lowest concentration of
benzene in the TCLP leachate, with MSU TMPA being second. The leachability of
BTEX compounds after treatment with the adsorptive organoclays (Figure 1.4) was
greater than 50 percent for almost every organoclay and compound except benzene for
which the leachability was less than 30 percent for all organoclays. Figure 1.4 shows

that as the size of the compound increased from benzene to o-xylene, the clays

25

 

 

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ILP

Figure 1.4 Leachability of BTEX compounds in the TCLP leachates of a

recontaminated soil treated with adsorptive clays.

26

were progressively less effective in reducing leaching. This may result from
competitive adsorptive effects among solutes, with the smaller more compact aromatic
hydrocarbons being preferentially adsorbed. Lee et al., (1990) observed such
competitive effects in binary solute systems for TMA-smectite. In that study, the
presence of benzene diminished the uptake of toluene, whereas toluene had no effect
on benzene sorption.

Figure 1.4 also shows that the laboratory prepared TMPA performed somewhat
better than the commercially prepared TMPA in reducing the leachable concentrations
of BTEX compounds. The slightly better results of the MSU TMPA clay may result
from a more complete exchange of the cation on the clay during its preparation. The
smaller cations used to prepare the adsorptive clays do not exchange as efficiently as
the larger cations used to make the organophilic clays. Because of this, the wet mix
laboratory preparation method for adsorptive clays uses excess cation to insure
complete saturation of the CEC. The dry-mix commercial method of preparation only
uses one equivalent of the cation and may therefore result in incomplete cation
exchange. The commercially prepared BTMA performed slightly bettter than the lab
prepared BTMA in reducing the leachable concentrations of BTEX compounds. Thus,
the wet method of preparation appears to work somewhat better for the TMPA-clay
but the method of preparation does not seem to have a significant effect on the
properties of the BTMA clays.

Figure 1.5 shows the leachability of 5 different organophilic organoclays. The
DICOCO C clay treatment resulted in the lowest concentration of each compound

except benzene in the TCLP sample leachate. The HDTMA treatment

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resulted in the lowest concentration of benzene in the TCLP sample leachate. In
addition, every organoclay treatment except HDTMA resulted in a leachability of the
BTEX compounds of 50% or less (Figure 1.5). The observation that the HDTMA clay
did not sorb as much of the contaminants (except benzene) as the other organophilic
clays may be because HDTMA+ is a monalkyl cation (a cation with a single large
hydrocarbon chain) whereas the other organic cations are dialkyl (cations with two
large hydrocarbon chains). The monoalkyl cation results in an organoclay with less
organic carbon and hence lower overall sorption capability (Boyd et al., 1988b). Also
the dialkyl cations are likely result in greater interlayer expansion of smectite than a
monalkyl cation. This property has been shown to be important in the sorption of
alkyl substituted benzenes by organoclays by Jaynes and Boyd (1991). They observed
that organoclays with greater doc, spacings were more effective sorbents for substituted
benzenes, but that benzene sorption was not strongly affected by basal spacings
between 18 and 28 A The larger basal spacings were believed to be necessary to
more effectively accomadate the bulkier substituted benzenes, consistent with the
results shown in Figure 1.5.

The DICOCO C clay treatment resulted in the lowest leachability of BTEX of
the organophilic clays tested. This is most likely because the DICOCO C clay was
made with a calcium smectite and all the others were made with a sodium smectite.
The calcium smectitie has a higher charge than the sodium smectite clay. High charge
clays have been shown to be more effective at sorption of organic contaminants once
they have been modified with organic cations because the resultant organoclay has

more organic carbon and/or greater basal spacings (Jaynes and Boyd, 1991a).

29

In general, the organophilic clays as a group performed much better than the
adsorptive organoclays in reducing the leachability of toluene, ethylbenzene, and the
xylenes in the contaminated soil. However, the adsorptive clays performed slightly
better than the organophilic clays in reducing the leachability of benzene in the
contaminated soil. This can be explained by two factors. First, organophilic
organoclays exhibit increased sorption with decreased water solubility of a compound.
Benzene has the highest water solubility of the five compounds studied hence the
lowest sorption coefficient. Second, the adsorptive clays studied generally have high
affinities for benzene (Boyd et al., 1991; Jaynes and Boyd, 1990).

The evaluation of six different organoclays plus the solidification agent
Sorbond is shown in Figure 1.6. Benzene data is not included in this figure because
the concentrations were too low to calculate the leachability. Both adsorptive (MSU
TMPA, AC TMPA, MSU BTMA and AC BTMA) organoclays and organophilic
(HDTMA and DICOCO S) clays were studied in this treatment, and resulted in less
than 50 % leachability for every clay and compound except for o-xylene and the AC
TMPA clays. In this experiment, the soil was diluted by an additional 20% from the
addition of the Sorbond, and the leachability values should be lower than those in

Figures 1.4 and 1.5 by at least 20%.

30

 

 

 

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Figure 1.6 Leachability of TEX compounds in the TCLP leachate of a

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contaminated soil treated with organoclays and Sorbond.

3 1
CONCLUSIONS

In conclusion, organoclays have been shown to reduce the leachability of
BTEX compounds in soils contaminated with gasoline. The leaching of BTEX from
soil treated with organophilic clay at a 5:1 ratio was reduced to between 10 and 50%
of that in an untreated soil. The organophilic clays were more effective than the
adsorptive organoclays in reducing the leachability of toluene, ethylbenzene, and
xylenes in this complex soil system. The DICOCO C clay was the most effective of
the organophilic clays in reducing the leachability of toluene, ethylbenzene, and
xylenes. The adsorptive organoclay MSU TMPA was most effective in reducing the
leachabilty of the benzene in this contaminated soil. Thus, adsorptive clays may be
useful in a system where benzene is the primary pollutant of concern. Adsorptive
clays might be useful when mixed with an organophilic clay. The organoclay
approach to remediation of a gasoline-contaminated soil might be especially useful in
regions where air quality regulations are strict because organoclay addition appears to

reduce volatilization losses during the mixing process.

32
LITERATURE CITED

Boyd, S. A., Jaynes, W. F., and B. 8. Ross. 1991. Immobilization of organic
contaminants by organo-clays: Application to soil restoration and hazardous waste
containment. In R A. Baker (ed), Organic Substances and Sediments in Water. Vol 1,
pp 181-200. CRC Press, Boca Raton, FL.

Boyd, S. A., Lee, J. F. and M. M. Mortland. 1988a Attenuating organic contaminant
mobility by soil modification. Nature 333:345-347.

Boyd, S. A., Mortland, M. M. and C. T. Chiou. 1988b. Sorption characteristics of
organic compounds on hexadecyltrimethylammonium-smectite. Soil Sci. Soc. Am. J.
52:652-657.

Boyd, S. A., Shaobai, S., Lee, J. F., and Mortland, M. M. 1988c. Pentachlorophenol
sorption by organo—clays. Clays Clay Minerals 36:125-130.

Brown, K. W., Thomas, C. J., Holder, M. and J. F. Artiola. 1990. The Ability of
Sorbent Materials to Adsorb and Retain Organic Liquids Under Landfill Conditions.
Hazardous Waste & Hazardous Materials 7:361-372.

Cadena, F ., Garcia, R, and R. W. Peters. 1990. Adsorption of benzene from aqueous
solutions by bentonite treated with quaternary amines. Environmental Progress 9:245-
253.

Chiou, C. T. 1989. Theoretical considerations of the partition uptake of nonionic
organic compounds by soil organic matter. In B. L. Sawhney and K. Brown (eds),

Reamgns and Movement of Organic Chemicals in Soils. Soil Science Society of
America, Special Pub. No.22, Madison, WI. pp 1-29.

Chiou, C. T., Peters, L. J. and V. H. Freed. 1979. A physical concept of soil-water
equilibria for nonionic organic compounds. Science 206:831-832.

Chiou, C. T., Peters, L. J., and V. H. Freed. 1981. Soil water equilibria for nonionic
organic compounds. Science. 213:684.

Chiou, C. T., Porter, P. E. and D. W. Schmedding. 1983. Partition equilibria of
nonionic organic compounds between soil organic matter and water. Environ. Science
Technol. 18:295-297.

Chiou, C. T., Shoup, T. D. and P. E. Porter. 1985. Mechanistic roles of soil humus
and minerals in the sorption of nonionic organic compounds from aqueous and organic
solutions. Org. Geochem. 8:9-14.

33

Daley, P. S. 1989. Cleaning up sites with on-site process plants. Environmental
Science and Technology 23:912-916.

Day, P. R 1965. Particle fractionation and particle-size analysis. In C. A. Black (ed),
Methods of Soil Analysis. Am. Soc. Agron, Madison, WI. pp 545-567.

Evans, J. C. and S. E. Pancoski. 1989. Organically modified clays. 68th Annual
Transportation Research Board Meeting, 880587.

Federal Register 55. June 1990. Method 1311, Toxicity characteristic leaching
procedure. 11798-11877.

Grumbly, TR 1989. Environmental Progress. 8(1), F2-F3.

Hassett, J. J. and W. L. Banwart. 1989. The sorption of non-polar organics by soils
and sediments. In B. L. Sawhney and K. Brown, (eds), Reaotions and movomont of
organio ohemicals in soils. Soil Science Society of America, Special Pub. No.22,
Madison, WI. pp 31-44.

Jaynes, W. F. and S. A. Boyd. 1990. Trimethylphenylammonium-smectite as an
effective adsorbent of water soluble aromatic hydrocarbons. J. Air Waste Mgmt.
40:1649-1653. '

Jaynes, W. F. and S. A. Boyd. 1991a. Clay mineral type and organic compound
sorption by hexadecyltrimethylarnmonium-exchanged clays. Soil Sci. Soc. Am. J.
55:43-48.

Jaynes, W. F. and S. A. Boyd. 1991b. Hydrophobicity of siloxane surfaces in
smectites as revealed by aromatic hydrocarbon adsorption from water. Clays Clay
Minerals. 39:428-436.

Lee, J. F., Crum, J. R. and S. A. Boyd. 1989a Enhanced retention of organic
contaminants by soils exchanged with organic cations. Environ. Sci. Technol. 2321365-
1372.

Lee, J. F., Mortland, M. M., Boyd, S. A. and C. T. Chiou. 1989b. Shape-selective
adsorption of aromatic molecules from water by tetramethylammonium-smectite. J.
Chem. Soc. Faraday Trans. 1. 85:2953-2962.

Lee, J. F., Mortland, M. M., Chiou, C. T., Kale, D. E. and S. A. Boyd. 1990.
Adsorption of benzene, toluene, and xylenes by two tetramethylammonium smectites
of different charge densities. Clays Clay Minerals. 38:113-120.

34

Montgomery, D. M., Sollars, C. J., Sheriff, T. S., and R. Perry. 1988. Organophilic
clays for the successful stabilisation/solidification of problematic industrial wastes.
Environ. Technol. Letters 911403-1412.

Mortland, M. M. 1986. Mechanisms of adsorption of nonhumic organic species by

clays. In P. M. Huang (ed), Interactions of Soil Minerals with Natural Qrganioa and
Miorobeg. Soil Sci. Soc. Am. Spec. Pub. no. 17, 59-76.

Mortland, M. M., Shaobai, S., and S. A. Boyd. 1986. Clay-organic complexes as
adsorbents for phenol and chlorophenols. Clays Clay Minerals 34:581-585.

Palacios, E. C. 1992. An assessment of relative environmental risk in Michigan.
Planning and Zoning News 10:10:5-13.

Sheriff, T. S., Sollars, C. J., Montgomery, D., and R Perry. 1987. Modified clays for
organic waste disposal. Environmental Technology Letters 8:501-514.

State of Michigan, Department of Natural Resources. 1979. Hazardous Waste
Management Act (64). &299.501-299.551.

Chapter 2
ABSTRACT
Treatment of contaminated soils with organoclays to reduce leachable

pentachlorOphenol.

The Toxicity Characteristic Leaching Procedure (T CLP, US EPA method 1311)
was used to evaluate the eflectiveness of 9 organoclays in reducing the leachability of
pentachlorophenol (PCP) from three highly (3000-5000 mg kg" ) contaminated soils
from wood treatment facilities. The organoclays were smectite exchanged with
quaternary ammonium cations of the form [(CH),NRf or [(CHJWRJ where R is an
aliphatic or aromatic hydrocarbon. The soils had difi'erent anthropogenic and natural
properties (% oil and grease, particle size, and % organic carbon). A cement-based
solidification agent (Sorbond) was also added to the soil to evaluate its eflects on the
leachability of PCP. Organoclays with large ( > C ,0) aliphatic R groups (organophilic
organoclays) were compared to those with small aromatic or aliphatic groups
(adsorptive organoclays). The adsorptive organoclays were not particulary eflective in
reducing PCP levels in the T CLP leachate but the organophilic clays dramatically
reduced the leachabilty of PCP from all of the soils. The most efl'ective clay tested
was the dimethyldicocoammonium-modified smectite (DICOCO); a 20% addition of
the organoclay to the soil resulted in leachable PCP levels below the detection limit
(0.2 mg L"). The addition of Sorbond to the soil increased both the pH (from 4.95 to
12.00) and the PCP concentrations (from 6.8 mg L" to 100.8 mg L”) in the leachate.

However, when Sorbond was added with an organophilic clay, the PCP concentration

was decreased below detection limits despite the increase in pH. These results
demonstrate the efl'ectiveness of organoclays for reducing the leaching of PCP from

highly contaminated soils.

INTRODUCTION

Soils contaminated with pentachlorophenol (PCP) are frequently found at wood
treatment facilities. PCP is a biocide that has been widely used as a wood
preservative because it is toxic to bacteria, fungi, and algae. In humans, PCP can
cause adverse health effects including chloracne and liver cancer. PCP has been
designated as a priority pollutant by the United States Environmental Protection
Agency (US. EPA) (Keith and Tillard, 1979). In the US, 70 of the 2000 wood
preserving sites in need of remediation are on the National Priorities List (Stinston et
al., 1991) and about 500 of the sites have PCP contamination (Cirelli, 1978). There
are several treatment technologies that have been tested on PCP contaminated soils
including soil washing (Stinson, et al., 1992), anaerobic and aerobic microbial
degradation, solar irradiation, (Dupont et al., 1990), cement solidification and
organoclay treatment (Sherif et al., 1987; Montgomery et al., 1988; Sell et al., 1992;
Bates et al., 1992). Many of these treatment technologies have limitations; soil
washing is only applicable for soils containing less than 25 % fines (silt and clay)
(Stinson et al., 1992), microbial degradation can be inhibited by high PCP
concentrations, and solar irradiation is only suitable for aerial contamination confined
to relatively thin surface layers (Dupont et al., 1990).

Both Michigan and the United States government mandate the use of the

36

37

Toxicity Characteristic Leaching Procedure (FR 55,1990) to determine if the soils at a
known spill or contamination site are to be classified as hazardous or nonhazardous
wastes or soils (State of Michigan, 1979). The TCLP method determines the
concentrations of specific chemicals that leach from the waste into aqueous media As
a result of these government regulations, there is a great deal of interest in
technologies that reduce the leachability of pollutant chemicals in soils so that a
hazardous or contaminated classification can be avoided. Reducing the leachability
will minimize the potential for transport of contaminants into groundwater.

The transport of organic contaminants in aqueous environments is
predominantly controlled by their interaction with soils and subsoils. The uptake of an
organic contaminant by soil is dependent on the composition of the soil and the
medium from which the compound is sorbed. In soil-water systems, the primary
uptake mechanism of nonionic organic contaminants (NOCs) is thought to be solute
partitioning into the soil-humic phase (Chiou et al., 1979, 1983, 1985). Extensive
research has shown that NOC sorption increases as the soil organic matter content
increases and as the water solubility of the NOC decreases (Chiou 1989; Chiou et. al.,
1985). In soil-water systems, adsorption of the organic solutes by soil minerals is
insignificant compared to the uptake by solute partitioning into the soil-humic phase.
The hydration of metal exchange cations in clays creates a hydrophilic environment at
the clay surface and in the clay interlayer regions which greatly reduces or eliminates
their ability to sorb NOCs and pesticides (Jaynes and Boyd, 1990, 1991). As a result
of these factors, natural clays and low organic matter soils and subsoils are ineffective

as sorbents for NOCs; the potential for NOC leaching from these materials is high.

38

Although smectite clays are ineffective in removing NOCs from water, they
have been used widely in landfill liners, waste treatment lagoons and slurry walls to
reduce hydraulic conductivity. This property is dreived from their ability to
disaggregate upon hydration (”swell”) and form a dispersed phase of very small
particles. These particles fill void spaces between larger soil particles and thus impede
water movement.

The sorptive capacity of clays for removing NOCs can be greatly enhanced by
a chemical modification process, whereby the metal cations naturally occurring in
clays are replaced with organic cations via ion-exchange reactions (Boyd et al., 1988a,
b, c; Lee et al., 1989a, b; Lee et al., 1990; Jaynes and Boyd, 1990, 1991, 1992;
Mortland et al., 1986). The organic cations most commonly used for this purpose are
quaternary ammonium ions of the general form [(CH3)3NR]+ or [(CH,)2NRR']+. The
ion exchange of inorganic cations by organic cations results in organoclays with high
affinities for organic pollutants (Jaynes and Boyd, 1990, 1991; Boyd et al., 1988b).
The capacity of organoclays to sorb and retain organic contaminants suggests that the
addition of organoclays along with solidifying agents may effectively sequester
pollutant chemicals that are contained in hazardous wastes, industrial wastes, and
contaminated soils (Evans and Pancoski, 1988; Montgomery et al., 1988; Boyd et al.,
1988a, 1991; Lee et al., 1989a).

Organoclays have been grouped into two general types: adsorptive and
organophilic clays (Boyd et al., 1991). These classes of clays are distinguished by
their sorptive mechanisms and resultant sorptive properties for NOCs. Organophilic

clays are synthesized using quaternary ammonium cations containing large alkyl

39

hydrocarbon R groups. This results in the formation of a paraffin-like organic phase
which is fixed on the clay surfaces and interlayers. The organic phase essentially
solubilize organic pollutant chemicals and thereby immobilizes them (Jaynes and
Boyd, 1991). Adsorptive organoclays are synthesized using small quaternary
ammonium cations, e.g., where R is a methyl group and R' is a phenyl group. In these
organoclays, the organic cations act as non-hydrated pillars that prop open the clay
layers exposing the abundant silioxane surface area. Aqueous phase organic
contaminants are adsorbed by the mineral (siloxane) surfaces between the organic
pillars (Jaynes and Boyd, 1992) demonstrating the inherent hydrophobicity of these
surfaces (Jaynes and Boyd, 1992). Each of these two general classes of organoclays
has its unique properties and potential environmental applications (Boyd et al.,
1991b).

The presence of ionizable functional groups, (e.g. PCP, pKa 4.75), complicates
the sorptive behavior of organic solutes in both soil-water systems and organoclay-
water systems. Schellenberg et al. (1984) showed that PCP sorption in sediment-water
systems could be accounted for, to a first approximation, by considering the
partitioning of PCP into sediment organic matter and neglecting sorption of
pentachlorophenolate. Hence, the pH of the system and the pKa of the compound are
of the utmost importance when determining the sorptive behavior of ionizable organic
solutes.

The objectives of this study were to l) evaluate the effectiveness of nine
organoclays in reducing leachable concentrations of PCP from contaminated soils, and

2) determine the effects of adding an alkaline solidification agent, with and without

40

organoclay, on the leachable concentrations of PCP from contaminated soils. The
influence of natural and anthropogenic soil properties (such as pH, percent organic
carbon, oil and grease, particle size, and cation exchange capacity) on the leachability
of PCP from contaminated soils was also evaluated. Laboratory- and commercially-
prepared organoclays were evaluated to determine the effect of preparation method.
The ability of organoclays to reduce the leachate concentrations of PCP in
contaminated soils was evaluated using the TCLP. We hypothesized that the addition
of organoclays to PCP contaminated soils would dramatically increase the sorptive
capacities of the soils, and, hence, reduce the PCP concentrations in the TCLP
leachate.
MATERIALS AND METHODS
Organoclay Preparation

The organoclays studied in this experiment were modified with a number of
different quaternary ammonium cations. A summary of the names, abbreviations and
structural formulae of the quaternary ammonium cations are shown in Table 2.0. The
smectite clay used to make the laboratory-prepared clays was a Wyoming bentonite
(primarily Na-smectite) provided by American Colloid Company, Arlington Heights
IL. Twenty-five g of clay were dispersed in 5 liters of water and allowed to stand
several hours to promote settling of sand, silt, and heavy minerals. The clay-water
suspension was decanted and the appropriate quantity of an aqueous solution of the

bromide or chloride salt of the organic cation was added to the suspension. Both

41

Table 2.0 Structural formulae and organoclay designation.

 

Name and Abbreviation Structure
9H:

Tetramethylammriun cr-g—W-CH3
TMA“ 4H,

CH.
Trimethybhenylamnon‘un org—it
* H

TWA

CH.
Benzytnmethytarrmoriun CH 4".
BTMA“ ’ é“

9”.
I
Heradecyttrimethylamriun CHs—N’—(CH2)“CH3
HDTMA“ AH:
CH
' 3
Dioctadecyflimethytamroriun CH,-t;i'—(CH1)'CH3
DODMA" (CHI)VCH3
(EH.
Ditalowdimettrylarrmonlun Cit—hr—(CHLCi-g
DITALLOW (CHILCH’
<5".
Dicocodimethylarrmriun Cit—f—(CHgflmia

00000
(0112111011:

42
HDTMA and DODMA salts (Table 2.0) were added in an amount equal to the CEC of

the clay , TMPA and BTMA salts (Table 2.0) were added in an amount equal to 5
times the CEC of the clay. A salt of DODMA was dissolved in a 50:50 v:v mixture
of methanol and water and HDTMA, TMPA, BTMA and TMA salts were dissolved in
water before addition to a stirred clay suspension. The solution was mixed for
approximately 4 hours and then allowed to settle. The clear supernatant was poured
Off and the remaining clay suspension was placed in dialysis bags that were left in
distilled water until the water was free of Cl‘and Br‘ as determined by a Ag(NO,)2 test.
The resulting clay-water mixture was later quick-frozen and freeze-dried. The TMPA
and BTMA clays prepared in the laboratory at Michigan State University will hereafter
be referred to as MSU TMPA and MSU BTMA.

The commercially-prepared clays used in this study were provided by American
Colloid Company and were made by a "dry-mix" process. In this process, the
quaternary ammonium cations dissolved in an isopropanol solution were added to the
dry clay. The mixture was extruded as a paste and was subsequently oven-dried and
ground. The quaternary ammonium cations used were TMPA, BTMA, DICOCO, and
DITALLOW (Table 2.0). The DICOCO cation is a derivative of coconut oil and the
alkyl chains consist of approximately 70% C,2 chain lengths with a range of C, to Cl6
chain lengths. The DITALLOW cation is a derivative of beef fat and the alkyl chains
consist of approximately 70% Cl. chain lengths with a range of C16 to CH3 chain
lengths. A Wyoming bentonite (predominantly Na-smectite) was used to make all but
one of the commercial clays; a calcium bentonite (smectite) was used to prepare a I

DICOCO commercial clay. The American Colloid TMPA and BTMA will hereafter

43
be referred to as AC TMPA and AC BTMA. The DICOCO clay prepared with the

sodium bentonite will be referred to as DICOCO S; the DICOCO clay prepared with a
calcium bentonite will be referred to as DICOCO C.

The organic C contents and doc, spacings of the organoclays are listed in Table
2.1. Organic carbon contents of the organoclay samples (50 mg) were determined
with a DC-190 carbon analyzer (Dohrmann, Santa Clara, CA.). Organoclay samples
(30 mg) were washed with 5 ml of 95 percent ethanol, ultrasonically dispersed in 2 ml
of 95 percent ethanol, and dried as oriented aggregates on glass slides. Basal X-ray
diffraction spacings were recorded using a Philips APD 3720 automated X-Ray

diffractometer (Philips Electronic Instruments Inc., Mahwah, NJ)

Table 2.1 Organic carbon contents and doc, spacings of organoclays.

 

 

Organoclay Organic Carbon doc, spacings
% A
TMA 3.60 13.8
MSU TMPA 8.84 14.0
AC TMPA 10.00 14.8
MSU BTMA 10.43 14.1
AC BTMA 11.02 14.0
HDTMA 18.70 17.5
DODMA 33.66 34.0
DICOCO 8 23.76 22.8
DICOCO C 23.85 36.9

DITALLOW 28.11 35.4

 

44
Soil Properties

Three PCP contaminated soils were used in this study. The soils were obtained
from an analytical lab which acquired the soil samples from wood treatment facilities.
Some soil properties are listed in Table 2.2. Particle size analysis of the three soils
was determined following the method of P. R Day (1965) as described in Methods of
Soil Analysis. Organic carbon contents were determined on the three soils with a
DC-190 carbon analyzer (Dohrmann, Santa Clara, CA.). Natural organic carbon was
determined on soils extracted with freon as described below. Total organic carbon
was determined on the soils with no pretreatment. CEC was determined following the
method of Day (1965).

To determine the total PCP concentration in the untreated contaminated soils,
10.0 g of each soil were extracted overnight with ~ 250 ml of methylene chloride
(CHzClz) in a Soxhlet apparatus. The CHzCl2 soil extract was extracted into water
three times with ~ 30 ml of 0.1 N aqueous potassium carbonate (K2C03) for a final
volume of 100 ml. The KZCO3 was filtered by gravity filtration with a Whatrnan #1
filter (Whatman, England), diluted with acetate buffer and analyzed to determine total
PCP concentrations in the soils (Table 2.2).

The oil and grease content of the duplicate samples of the contaminated soils
was determined by the method of Boyd and Sun (1989). Exactly 20.0 g of soil was
placed in a beaker and four drops of concentrated hydrochloric acid and 05-30 g of
magnesium sulfate (for moisture removal) were added to the soil and mixed
thoroughly. The soil was then transferred to cellulose thimbles (Whatman, England)

and extracted with ~ 250 ml of 1,1,2-trichlorotrifluoroethane (freon, Aldrich,

 

45

Milwaukee, WI) in a Soxhlet apparatus overnight. The soil extract was then back
extracted three times with ~ 30 ml of 0.1 N aqueous K2C03 to remove PCP.
Magnesium sulfate was added to the freon to remove moisture and then the freon was
filtered by gravity filtration with a Whatrnan #1 filter paper (Whatman, England) and
transferred to a preweighed flask. The freon was then evaporated under vacuum on a
rotary evaporator at 41 C and the remaining oil and grease were weighed. The flasks
were left open in a hood for 2 weeks and periodically weighed to insure no additional
weight loss over time. The oil and grease contents of the three soils are listed in

Table 2.2.

Table 2.2 Some natural and anthropogenic properties of soils H, C and J.

 

 

 

Property units Soil --------------
H C J
Sand' (2.0-0.05 mm) % 82.3 65.8 87.2
Silt (005-0002 mm) % 8.8 30.7 9.3
Clay (<0.002 mm) % 8.9 3.5 3.6
Natural1 O.C. % 0.38 0.48 0.924
CEC mol kg " 1.5 .7 1.1
Oil & Grease % 0.63 6.47 6.81
Total O.C. % 1.21 4.97 4.86
PCP§ mol kg " 3285 4327 4989

 

'particle size determined by pipette method (Day, P. R, 1964)
fidetermined on ”clean” freon extracted soil
§determined by CH2C12 soxhlet extraction method

46

Soil Treatments

Four types of soil treatments were performed on the three different soils. In
the first treatment, organoclay was added to the soil according to the following
procedure: in a 4°C cold room, 2.0 g of the organoclay was added directly to the top
of 10.0 g of contaminated soil in a small glass jar. The jar was quickly capped and
1.0 g of distilled water was added to the top of the organoclay-soil mixture through a
1.5 cm hole in the cap. The organoclay-soil-water mixture was thoroughly mixed with
a stainless steel spatula inserted through the hole in .the cap. This hole was then
sealed with an aluminum foil-covered rubber stopper and the organoclay-soil-water
mixture was allowed to cure for 24 hours at 4°C.

In a second treatment, the organoclay and a solidification agent (Sorbond,
obtained from American Colloid Company) were added to the soil. The organoclay,
soil and water were mixed as described above and allowed to cure for one hour.
Then, 2.0 g of Sorbond was added and mixed into the organoclay-soil mixture. After
the cap was sealed, the whole mixture was allowed to cure for 23 more hours.

In a third treatment, only Sorbond was added to the soil. In a 4°C cold room,
10.0 g of the contaminated soil was placed in a small glass jar and capped as
described above. Two g of Sorbond and 1.0 g of water were added and then mixed
and sealed as in the previous treatments. Then, the Sorbond-soil-water mixture was
allowed to cure for 24 hours.

In a fourth treatment, subsequently referred to as the untreated soil, the
contaminated soil was removed from cold room storage and extracted immediately.

The effect of adding varying amounts of DICOCO C, to soils C and H with

47

and without Sorbond, was also evaluated using the protocols described above.
DICOCO C was added to soil H (10.0 g) in amounts of 0.0, 0.3, 0.6, 0.8, 1.2, and 1.5
g, and in amounts of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 g to soil C (10.0 g). The jars
were capped, 1.0 g of distilled water was added, and then the jars were sealed and
mixed as outlined for the soil treatments above. The organoclay-soil-water mixtures
were allowed to cure for 24 hours at 4°C. Additionally, DICOCO C organoclay was
added in amounts of 0, 0.6, 0.9, 1.2, and 1.5 g to soil C (10.0 g) along with 2.0 g
Sorbond. The organoclay, soil, and Sorbond were mixed and cured as described above
for the second treatment.
TCLP Extractions

Following treatment, the soils were extracted according to the TCLP procedure
for volatile organic contaminants (FR 55, 1990). The TCLP for volatiles involves
placing the sample in a stainless steel, zero headspace, extraction vessel (ZHE) (FR
55, 1990). For these experiments, the entire soil sample and additives were added to
the ZHE. The ZHE is equipped with a movable piston that was raised to remove
excess headspace from the vessel. Once the ZHE was sealed with the sample inside,
the piston was lowered and at the same time, the ZHE was charged with buffer
solution (buffered with sodium acetate at pH 4.95 1.05, FR55, 1990). In each
extraction, the quantity of buffer solution used was equal to 20 times the weight of the
sample. Each sample jar was weighed before and after the sample was added to
determine the actual weight of sample extracted.

After the acetate buffer solution was added, the ZI-IE was rotated end-over-end

for 18 hours on a DC-20 rotary agitator (Analytical Testing and Consulting Services,

48
Inc., Warrington, PA) at 30 rpm. The leachate was then filtered inside the ZHE with a

Whatrnan glass fiber filter (Whatman, England) and transferred to 10 ml serum bottles
with teflon-lined caps and 50 m1 erlenmeyer flasks. The pH of the solution in the
flask was measured on a pH meter. Solutions from the Sorbond experiments were
acidified with glacial acetic acid to ~pH 5.0. The samples were analyzed for PCP by
high performance liquid chromatography (HPLC) within 14 days of removal from the
ZHE as specified by the TCLP procedure (FR55, 1990).

Each experiment in this study consisted of an untreated sample and up to 5
organoclay and/or organoclay-solidification agent treatments. The experiments were so
designed because only 6 ZI-IEs were available.

Batch Sorption Isotherms

The ability of DICOCO C to sorb PCP from an aqueous solution was
determined in batch sorption isotherm experiments at 20°C. Exactly 10.0 mg of the
DICOCO C clay, with and without 150.0 mg Sorbond, were added to 25 ml Corex
(Corning Glass Works, Corning, NY) centrifuge tubes with 25 ml of TCLP buffer
solution. Different amounts (5 to 250 pl) of a 10,000 mg L'1 PCP stock solution in
0.1 N KzCO3 were added to the tubes with a microliter syringe to achieve equilibrium
aqueous phase concentrations ranging between 0 and 15 mg L". The tubes were
capped and shaken at 20°C for 24 hours. The tubes were then centrifuged at 6315 g
for 25 minutes. A 10 ml aliquot of the solution was removed and stored at 4°C for
later analysis. The pH of each supernatant solution was measured and the solutions
from samples treated with Sorbond were acidified to pH 5.5 with glacial acetic acid.

All of the samples were analyzed in duplicate within 7 days of extraction.

49
Analytical Methods

The concentration of PCP was determined by HPLC. The PCP was detected at
its absorbance maxima (300.5 nm) with a detection limit of 0.2 mg L". The samples
were injected onto a Supelco (Supelco Co., Bellafonte, PA) C18 column (4.6 by 25.0
cm) by using a Rheodyne 7125 sample injector (Rheodyne Corp., Cotati, CA.) with a
50 11L loop. Peak area values were obtained using a Waters data module integrator.
The mobile phase, which consisted of 60:40 v/v acetonitrile and 5% aqueous acetic
acid. The retention time of PCP was approximately 9.0 minutes at a flow rate of 1.5
ml min".

RESULTS AND DISCUSSION

The soil samples used in this study were obtained from former wood preserving
sites and had high levels of pentachlorophenol (PCP) contamination. The PCP
concentrations in these soils ranged from 3000 to 5000 mg kg" (Table 2.2). In
addition to PCP, the soils were contaminated with significant amounts of petroleum as
indicated by their oil and grease contents (Table 2.2). PCP was often dissolved in
petroleum (e.g. diesel) for wood treatment processes, which may account for the oil
and grease components. Soil J had the highest PCP contamination (~5000 mg kg"),
high total organic carbon (TOC; 4.86 %) , and highest oil and grease concentration
(~7%). Soil H had the lowest PCP contamination (~3000 mg kg"), lowest TOC (1.21 %)
and lowest oil and grease content (~0.6%).

A summary of the initial experiments performed on soil H is listed in Table
2.3. These experiments were designed to compare the ability of different organoclays

to reduce PCP leaching from the contaminated soil. The adsorptive organoclays were

50

Table 2.3 pH and concentration of PCP in TCLP leachate of soil H after different
treatments.

 

 

Treatment Amount Amount TCLP PCP
Organoclay Sorbond Soln cone.
(8) (a) pH (mg L")
Adsorptive organoclays
untreated 0.0 0.0 4.87 15.1
TMA 2.0 0.0 4.90 8.8
MSU TMPA 2.0 0.0 4.89 10.7
AC TMPA 2.0 0.0 4.91 12.8
MSU BTMA 2.0 0.0 4.90 14.3
AC BTMA 2.0 0.0 4.90 14.6
Organophilic clays
untreated 0.0 0.0 4.88 16.9
HDTMA 2.0 0.0 4.88 ‘bd
DODMA 2.0 0.0 4.88 bd
DITALLOW 2.0 0.0 4.94 bd
DICOCO S 2.0 0.0 4.91 bd
DICOCO C 2.0 0.0 4.09 bd
untreated 0.0 0.0 4.92 15.9
Sorbond control 0.0 2.0 12.10 67.7
HDTMA 2.0 2.0 12.11 bd
DODMA 2.0 2.0 12.05 bd
DITALLOW 2.0 2.0 12.05 bd
DICOCO S 2.0 2.0 11.95 bd
DICOCO C 2.0 2.0 11.84 bd

 

'bd below quantifiable detection limit of 0.2 mg L"

51

clearly less effective than the organophilic clays. Of the adsorptive organoclays added
to soil H, the MSU TMPA-smectite caused the most significant reduction of PCP
(~40%) in the TCLP leachate. Tthe lab-prepared adsorptive organoclays performed a
slightly better than the commercially-prepared clays in reducing the amount of
leachable PCP. This is may be due to the incomplete exchange of TMPA cations on
the clay surface of the commercially-prepared clays resulting in an organoclay that is
only partially saturated with TMPA cations. The laboratory-prepared clays used an
excess of TMPA (i.e. five times the CEC) whereas only one equivalent of TMPA was
used to prepare the commercial organoclays. The addition of BTMA clays to soil H
caused an almost imperceptible reduction of PCP in the TCLP leachate. For the
BTMA clays, both the laboratory- and commercially-prepared clays were ineffective.
Previous research (Sun et al., 1988) has shown that adsorptive clays, namely TMA and
TMPA-smectites, are poor adsorbents of PCP. Hence the relative ineffectiveness of
the adsorptive clays studied herein at reducing PCP leachate concentrations was not
suprising.

In contrast to the adsorptive clays, all of the organophilic clays dramatically
reduced the leachability of PCP from soil H (Table 2.3). Each organophilic clay
treatment resulted in leachate concentrations of PCP below the detection limit of the
instrument (0.2 mg L"). PCP concentrations in TCLP leachate from the untreated
contaminated soil were about 16 mg L", so that a reduction in PCP leaching of at
least 80 times was achieved.

Boyd and Sun (1989) defined the soil-water equilibria of PCP in soils

containing residual petroleum, which, together with natural organic matter acts as a

52

sorptive phase for organic contaminants, including PCP. The sorption coefficient (K)

of molecular (PCP) in these systems can be calculated using their equation:

K =f~Ku +fwKw (1)
where f0, and foil are the fractional organic matter and residual oil contents of the soil,
and K0,, and Koil are the soil organic matter normalized sorption coefficient and the oil-
water partition coefficient of PCP, respectively. The octanol-water partition
coefficient, K", was used to estimate K0,]. The actual soil-water distribution of PCP

(molecular and ionized species) is then equal to :

o-xo (a

Where Q is the fraction of PCP present as the molecular species (Schellenberg et al.,
1984). I

Using these relationships, it was possible to predict the concentration of PCP in
the TCLP leachate of untreated soil H. The value of Q, calculated using a pKa for
PCP of 4.74 and a pH of 4.98, is ~0.38. Using values of f0,“ and Km from Table 2.2,
and K0,, and KOil values of 16,450 and 173,780, respectively (Schellenberg et al.,
1984), yielded a D value of 440. Hence, a predicted PCP concentration in the TCLP
leachate of ~ 7 mg L" was calculated for the untreated soil. If the sorptive contribution
of the residual oil phase is ignored , a predicted value of ~ 56 mg L" is obtained. The
average measured PCP concentrations from four separate experiments was 15.2 mg L",
or slightly over twice the predicted value. Obviously, the oil and grease component of
this soil acts as a sorptive phase for PCP, but its effectiveness is less than predicted

assuming Koil equals K". The measured value of PCP in the TCLP leachate suggests

 

53
a Koil of ~ 70,000 (about 40 percent of K0,) .

The predicted PCP concentration in TCLP leachate from organoclay amended
soils can also be calculated using an expression similar to equation [1] with terms
added for the fractional organic matter content derived from the organoclay and the
Km value of PCP for the organoclay. Assuming that the organic-phase of the
organoclay acts as a partition medium for molecular PCP, and neglecting sorption of
pentachlorophenolate (Schellenberg et al., 1984), the K0,, value of HDTMA can be
used to estimate Km. Previous research has shown good correspondence between Km
and K0, values for NOC sorption by HDTMA-smectites (Jaynes and Boyd, 1991).
Using the log K0,, (=Kw) value of 5.24, an HDTMA (organic matter) content of 20%,
and a Koil value of 70,000, the predicted PCP leachate concentration is ~ 1 mg L". The
measured PCP concentration in the TCLP leachate for the HDTMA smectite treated
soil was below the quantifiable detection limit of 0.2 mg L" indicating that the
performance of the organoclay was better than predicted, based on the assumptions
stated above.

It was not possible to evaluate the effect of the organophilic clay preparation
method on sorptive capacity because all of the organophilic clays reduced the PCP
below quantifiable limits. However, the DICOCO C clay treatment resulted in a
TCLP leachate that did not have any detector response (flat baseline), and was
believed to have caused the greatest reduction of PCP in the TCLP leachate.

The pH of the TCLP leachate increased from 4.9 to 12.0 after the addition of
Sorbond to the soil (Table 2.4). In the Sorbond control treatment, this increase in pH

produced a large increase in leachable PCP presumably because at pH 12 PCP exists

 

54

almost entirely as pentachlorophenolate which has a much higher water solubility than
molecular PCP. However, the use of the organophilic clays in conjunction with the
Sorbond still resulted in PCP concentrations below detection limits. This result
strongly suggests a sorptive mechanism distinct from solute partitioning because the
high water solubility of pentachlorophenolate would have very low sorption (partition)
coefficients (Boyd et al., 1988b; Jaynes and Boyd, 1991; Schellenberg et al., 1984).

To explore the sorptive capabilities of organoclays at acid and alkaline pH,
DICOCO C and DICOCO C plus Sorbond were used to obtain PCP sorption isotherms
at pH 4.95 (acetate buffer) and at pH 11, respectively (Figure 2.0). At both pHs, the
isotherms exhibit a sharp initial rise indicating strong PCP sorption. This is followed
by a gradual decrease in the slope of the isotherms resulting in distinctly non-linear
isotherms. At the acidic pH, the steep initial rise continues until the amount of PCP
sorbed is about 60 to 80 mg g"; at the alkaline pH it continues to about 30 mg/g. If
we assume that the entire mass of PCP is sorbed in the organoclay-treated soils (i.e.
the mass of solution phase PCP is essentially zero) then the PCP concentration in the
amended soil ranges from about 3 to 5 mg g". If we further assume that all sorption
occurs in the 20% organoclay portion of the clay soil mixture, then the PCP
concentration in the organoclay ranges from about 20 to 30 mg g". These
concentrations of PCP in the organoclay are within the steep initial region of the
isotherms. These results explain the strong reduction in PCP concentrations in the
TCLP leachate at both acid (pH 5) and alkaline (pH 12) pHs.

The nature of the apparent strong interaction between the organoclay and the

pentachlorophenolate is interesting, although at this point uncertain. One possible

55

.9558 Song; use 5.; 50 o 0000.0 :89. .o 5558. 52.58 on 9:9“.

.35. 8.8.5858 53.5.55
mm 0.0! mm cu mu 3

p - —

-m
C

 

 

020mm0w a 0 0000.0 I
{ mm... In

 

 

 

 

 

 

 

(Elfin!) Demos dOd tunowv

II
8

0 0000.0 , loom

56

mechanism is via a columbic interaction between pentachlorophenolate and the
positively charged N moiety of the quaternary ammonium cation. These positively
charged groups would be available for such an interaction if a portion of the organic
cations were associated with the clay by non-polar ("tail-tail”) interactions involving
the hydrophobic R groups of exchanged and non-exchanged quaternary ammonium
cations. We have recently observed that when HDTMA cations are added to soils in
an amount equivalent to the CEC of the soil, up to 50 percent of the HDTMA cations
are held by such ”tail-tail” interactions, i.e. are not ion exchanged (Xu and Boyd,
unpublished results).

A second possibility is the interfacial adsorption of pentachlorophenolate,
which is potentially an ampiphilic compound. The ionized functional group of PCP
may form an ion-dipole with water while the organic part of PCP interacts with the
organic phase of the organo-clay. These phases are thought to be similar
compositionally and functionally to a bulk hydrocarbon solvent phase (Boyd et al.,
1988b; Jaynes and Boyd, 1991). Hence, the water-organoclay interface may be similar
to a water-octane interface.

A second, more highly contaminated (4989 mg kg") soil (J) was studied to
determine the effectiveness of the different organophilic clay treatments and the effect
of the Sorbond. In spite of the high concentration of PCP in the soil, all of the
organophilic clay treatments effectively removed the leachable PCP from the soil to
levels below the detection limit of the instrument (Table 2.4).

Initially surprising was the low level of PCP (5.6 mg L") that leached from soil

J (untreated) during the TCLP (Table 2.4). However, after considering the oil and

57

grease content of the soil (6.81%, Table 2.2) it became apparent that the reduced
leachability of soil J as compared to soil H was the result of an oil and grease
component in the soil. The oil and grease acted as an additional sorptive phase.
Using equations 1 and 2, and the assumptions stated above, the predicted PCP

concentration in the untreated soil is ~1 mg L" as compared to the measured value of

5.6 mg L".

Table 2.4 pH and concentration of PCP in the TCLP leachate of soil J after different
treatments.

 

 

Treatment Amount TCLP PCP
Organoclay Leachate cone.
(8) pH (mg L")
Organophilic clays
Soil J untreated 0.0 4.96 5.6
Sorbond Control 0.0 11.85 104.1
HDTMA 2.0 4.94 bd'
DODMA 2.0 4.95 bd
DIT ALLOW 2.0 4.94 bd
DICOCO S 2.0 4.99 bd
DICOCO C 2.0 4.93 bd

 

' below quantifiable detection limit of 0.2 mg L"

Without considering the sorptive effects of the oil and grease phase, the predicted PCP
concentration is ~43 mg L" . Obviously, the oil and grease phase functions as a
sorptive sink for PCP as suggested by Boyd and Sun (1989), but here Koil (~32,000) is

lower than K0,.

58

The high total PCP in the soil resulted in the leaching of a large amount of
PCP (104.1 mg") from the soil with the Sorbond control treatment. The high pH (~12)
of this extraction lead to the leaching of large amounst of PCP from the soil. This is
because at pH 12 PCP exists almost entirely as pentacholorophenolate, which has a
high water solubility and does not significantly partition into natural (soil), or
anthropogenic (oil and grease) organic matter. With no sorption, the PCP leachate
concentration would be approximately 250 mg L"; about twice the observed value.

The experiments described above were all conducted using a soilzorganoclay
ratio of 5:1, which for the organophilic clays tested resulted in no detectable levels of
PCP in the TCLP leachate. Additional experiments were performed on soil H and a
third soil (C) using progressively lower amounts of organoclay to evaluate PCP
leaching as a function of the amount of organoclay added. These results are
summarized in Table 2.5. Soil H has the lowest PCP contamination and the lowest
oil and grease content of the three soils. Soil C has an intermediate PCP
contamination (4327 mg kg") and an intermediate oil and grease content (6.47%)
(Table 2.2). The data from Table 2.4 illustrate the high sorptive affinity that
DICOCO C has for PCP. The addition of only 3% organoclay (0.3 g organoclay per
10.0 g soil) resulted in an approximately 80% reduction of leachable PCP in soil H
and an approximately 60 % reduction in soil C. The data also illustrate the dramatic
effect that the increased pH (from adding Sorbond to the soil) had on the leachability
of PCP in the absence of organoclays. The PCP concentration in the TCLP leachate
of soil C increased from 6.8 mg L" at pH~5, to 100.8 mg L" at pH ~12. However, the

use of organoclays in conjunction with the Sorbond reduced PCP leachability

59

Table 2.5 pH and PCP concentration in the TCLP leachate of soils C and H after
treatment with different amounts of organoclay and Sorbond.

 

 

Soil" organoclay Sorbond Soln TCLP PCP
(g) (8) (1) pH (mg L")
H 0 0 .200 4.96 13.4
H 0.3 0 .226 4.94 2.4
H 0.6 0 .232 4.95 1.8
H 0.9 0 .236 4.99 1.5
H 1.2 0 .244 4.93 1.2
H 1.5 0 .250 4.94 0.9
H 2.0 0 .256 4.90 1bd
H 0 2.0 .257 12.10 67.7
C 0 0 .200 4.97 6.8
C 0.2 0 .214 4.97 3.8
C 0.3 0 .223 4.99 2.9
C 0.4 0 .224 4.98 2.1
C 0.6 0 .230 4.98 1.3
C 0.8 0 .232 4.98 0.9
C 1.0 0 .236 4.98 0.7
C 1.2 0 .241 4.93 0.3
C 0.0 2.0 .277 11.78 100.8
C 0.6 2.0 .287 12.12 15.6
C 0.9 2.0 .292 12.16 7.7
C 1.2 2.0 .301 12.13 3.5
C 1.5 2.0 .310 12.13 1.6

 

*each treatment used 10.0 g of contaminated soil
Ibd below quantifiable detection limit of 0.2 mg L"

 

60

substantially, as observed previously for soil H.

The dramatic effect that DICOCO C has on reducing the leachable PCP
concentrations from soil H can be seen in Figure 2.1 which shows the relationship
between the concentration of PCP in the TCLP leachate and the amount of DICOCO
added to soil H. A sharp drop in the concentration of PCP occurred with the addition
of a small (3%) amount of organoclay. The sharp drop was followed by a gradual
decline in PCP concentration eventually decreasing below detection limits with the
addition of DICOCO C at 20 %.

The effect of DICOCO C on reducing PCP concentrations from the more
highly-contaminated soil C can be seen in Figure 2.2. Figure 2.2 shows the
concentration of PCP in the TCLP leachate as a function of amount of the DICOCO C
organoclay added to the soil. In comparison to soil H, a more gradual decrease in PCP
concentration occurred when increased amounts of organoclay were added to the soil.
Despite the higher total PCP concentration in soil C, only 12% addition of the
organoclay was required to reduce the PCP concentration below detection limits. This
can be explained by the high oil and grease content present in soil C which acted as
an additional sorptive phase for PCP and prevented the leaching of PCP.

Figure 2.3 shows the concentration of PCP in the TCLP leachate as a function
of the amount of the DICOCO C organoclay added to soil C. Soil C was also
amended with the solidification agent Sorbond (2.0 g Sorbond per 10.0 g soil).
Although the initial PCP concentration was very high due to the high solubility of
pentacholorophenolate at pH 12, the PCP concentration declined dramatically as the

amount of organoclay added was increased. A 20% addition of organoclay was

61

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63

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cozmzcoocoo 05 :0 3.8590 0 0000.0 .o 2595 on. .6 Eootw QN 9:9“.

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64

required to reduce the PCP concentration in the TCLP leachate to below detection
limits.
CONCLUSIONS

This study clearly demonstrates that treatment of PCP contaminated soils with
organophilic clays can be an effective way to immobilize PCP in soils. Although the
addition of solidification agents such as Sorbond will increase both the pH and the
PCP concentration in the TCLP leachate, the PCP levels can be reduced below
detection limits of 0.2 mg L" if an organophilic organoclay is used in addition to the
solidification agent (Table 2.4). These results strongly indicate that organophilic clay

amendments can effectively reduce PCP leaching in contaminated soils.

65

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