Adsorption of Low Molecular Weight Halocarbons by Montmorillonite Thomas J. Estes, Rajlv V. Shah, and Vincent L. Vllker"
Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, California 90024
m Montmorillonite clay from Clay Spur, WY, was found to adsorb several low molecular weight, hydrophobic halocarbons from aqueous solution at sub-parts-per-million levels. The halocarbons studied were trichloroethylene, tetrachloroethylene, hexachloroethane, and dibromochloropropane. When the niontmorillonite was treated with sodium citrate-bicarbonate-dithionite (CBD), it adsorbed higher levels of halocarbons than the untreated clay. In addition, the CBD-treated clay exhibited a maximum in halocarbon adsorption around pH 4, while untreated clay showed little variation in adsorption over the pH range 2-10. Adsorption of trichloroethylenewas inhibited by low concentrations of sodium chloride (0.01 M or greater) in solution. Aging the CBD-treated clay in water decreased its capacity to adsorb trichloroethylene. Desorption studies showed that the sorption of tetrachloroethylene to CBDtreated clay is an irreversible process when compared to sorption by fumed silica. The ability of montmorillonite to adsorb halocarbons and the instability of the clay in water are postulated to involve changes in the oxide surface coating on the clay.
Introduction In recent years there has been increasing public and scientific concern with the problems of hazardous waste disposal and contamination of groundwater (1-3). Among the most widely distributed and troublesome organic wastes are the low molecular weight halocarbons (LMHs). This group includes common industrial solvents such as trichloroethylene (TCE) and perchloroethylene (PCE) and pesticides such as 1,2-dibromo-3-chloropropane (DBCP). These compounds are of interest for environmental research because of their potential as a threat to human health (41,their ubiquity in several environmental compartments (5), and the lack of reliable data on their fate and transport at the interfaces among water, soil, and air. These compounds can be released into the environment, either deliberately as in the case of pesticides or unintentionally as in the case of industrial spills or leakage from waste landfills. The transport of these compounds in the environment is a complex process, depending on such factors as solubility, air-water partition coefficient, chemical and biological reactivity, and sorption by soil particles. Previous work from this laboratory has dealt with reduction of LMHs in the vapor phase above humic acid solutions (6). An important factor in soil-water transport of LMHs, especially as it relates to groundwater protection, is their adsorption by soil particles, including both organic and mineral solids (7). There is a large body of work on the 0013-936X/88/0922-0377$01.50/0
adsorption of organic compounds by clay minerals (8-10). Most of this work, however, deals with adsorption of ionic or polar molecules or of large hydrophobic molecules. On the other hand, there has been little study of the interaction between clays and small, volatile, hydrophobic compounds such as LMHs. Results from the few previous studies on LMH-clay interactions are highly variable. For example, Singhal and co-workers (11, 12) reported that 1,2-dibromo-3-chloropropaneand 1,3-dichloropropeneare adsorbed by montmorillonite in extraordinarily high amounts. However, Rogers and McFarlane (13) reported that adsorption of trichloroethylene, carbon tetrachloride, and ethylene dibromide (EDB) by montmorillonite is variable, ranging from zero to moderate amounts of adsorption depending on the particular clay used. We undertook this study in order to more carefully define the conditions under which LMHs are adsorbed by clays. In this paper, we report on the adsorption of four LMHs (trichloroethylene, perchloroethylene, hexachloroethane, and dibromochloropropane) from water by montmorillonite. We have investigated a number of factors that affect LMH uptake by the clay from water. These factors include pretreatment of the clay to alter its surface, pH and ionic strength of the system, and the changes in the clay surface due to aging in water.
Experimental Methods Clay Purification. The clay used in these experiments, a montmorillonite from Clay Spur, WY, was obtained from Wards Natural Science Establishment, Rochester, NY, The clay purification methods that we followed and their effects on clay impurity levels are described by Jackson (14). Separate portions of the clay were treated with (1) 1M sodium acetate, pH 5 (removes surface carbonates), (2) 20% hydrogen peroxide (oxidizes organic material), or (3) sodium dithionite in sodium citrate-bicarbonate buffer (reduces surface iran and removes amorphous metal oxides). After these chemical treatments, the clay was saturated with sodium by suspending the clay in 1M NaC1, then washed with deionized water, and centrifuged until the excess ions were removed. Finally, dilute suspensions of the clay were size-fractionated by centrifugation to obtain the 0.2-2-pm fraction. Another portion of the clay was not chemically pretreated but was sodium-saturated and size-fractionated identically with the other clay samples. All of the clays were freeze-dried for storage. Halocarbons. Trichloroethylene (TCE), perchloroethylene (PCE), and hexachloroethane (HCE) were obtained from Aldrich Chemical Co., Milwaukee, WI. 1,2Dibromo-3-chloropropane(DBCP) was kindly supplied by
0 1988 American Chemical Society
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Professor Charles Castro of the University of California, Riverside. The purity of these compounds exceeded 99%, and they were used without further purification. Saturated solutions of the halocarbons were made by mixing an excess of the halocarbon with distilled-deionizedwater. (The deionized water contained trace levels of volatile organics. These volatile organics were removed by boiling the water for 15 min.) The saturated halocarbon solutions were diluted prior to their use in the adsorption experiments. Adsorption Experiments. All halocarbon adsorption measurements were made in batch experiments. Freezedried clay was suspended in deionized water or electrolyte solution (deionized water, NaC1, HC1, or NaOH) and put into 7-mL glass-stoppered centrifuge tubes. A small volume (0.5 mL) of an aqueous solution of halocarbon was then added to the clay tubes. The tubes were stoppered and mixed continuously end-over-end at room temperature (22 "C). The tubes contained a minimal vapor space to minimize volatilization of the halocarbon from the liquid phase. The final clay concentration was 1.39% (w/w), and the halocarbon concentration was varied from 10 to 100 ppb (w/w). After 1 to several days of mixing, the tubes were centrifuged at 5000 rpm (2800g) for 1h to sediment the clay. This condition has been shown to be more than adequate for clearing all clay particles from the solution (15). A total of 4 mL of the supernatant was extracted with 1 mL of hexane, and the extract was analyzed for halocarbon concentration by gas chromatography. The gas chromatograph was a Varian Model 3700 with a 63Ni electron capture detector. Samples were injected into a 6 ft X 1/8 in. i.d. column packed with 10% OV-101 on Chromosorb W-HP, 80/ 100. The injector and detector temperatures were both 150 "C. The column temperature was 60 "C for TCE, 85 "C for PCE, and 120 "C for HCE and DBCP. The carrier gas was nitrogen, at a flow rate of 30 mL/min. Control tubes containing only electrolyte solution and halocarbon, without clay, were run simultaneously with the adsorbing solutions. The amount of adsorption was calculated by comparing the halocarbon concentration in the adsorbent supernatant C, with the concentration in the control solution Cc. The control solution contained the same amount of halocarbon that was initially added to the adsorbent suspension. The amount of halocarbon adsorbed r, in g of halocarbon/g of adsorbent, was then calculated as
r = Vsoln(Cc - Cs)/Wada
(1)
and the fraction adsorbed f , in g of halocarbon adsorbed/g of initial halocarbon as
where Vsoh is the volume of solution and Wad is the weight of adsorbent. In all experiments, duplicate tubes were run for both the adsorbent suspension and the control solution. Uncertainties were estimated as the standard deviations from the duplicate runs. Typically, the uncertainty in the amount of halocarbon adsorbed was about 5% or less of the total halocarbon added to the adsorbent. Control samples using clay suspension supernatant behaved the same with respect to TCE concentration as control samples using electrolyte solution or deionized water. To measure the pH of the clay suspensions, a parallel set of tubes containing clay in the electrolyte solutions was prepared. The pH was measured after the same mixing time as the clay suspensions, which were analyzed for halocarbon concentration. The pH of the clay suspensions in the adsorption tubes was not measured directly because 378
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Table I. Properties of Montmorillonite Clay Pretreated with Sodium Citrate-Bicarbonate-Dithionite Buffer (CBD) clay
CBD measured property cation exchange capacity: mequiv/100 g of clay electrophoretic mobility at 25 OCtb (rm/s)/ ( V/cm) surface area: m2/g of clay organic carbon content,d wt %
Na+ saturated
treated
97
96
-0.45
-0.45
846 0.29
854 0.18
uMethod given in ref 16. bMeasurements made with a ZetaMeter, Inc., New York, NY, instrument using the molybdenum electrode. Clay concentration 70 mg/L. Results found to be independent of NaCl concentration over 0-400 ppm and pH over 4.0-7.8. CEquilibrium ethylene glycol method given in ref 17. dMethod given in ref 18.
it was necessary to minimize losses of the halocarbons by volatilization. In order to assess the reversibility of adsorption, PCE desorption from the CBD-treated montmorillonite and from fumed silica (Alfa Chemicals, Danvers, MA; 400 m2/g surface area) was studied. PCE adsorption was first performed in deionized water at pH 4 and an initial PCE concentration of 10 ppm for 4 days. After 4 days, 1 mL from either the 7-mL control sample or the 7-mL suspension sample was transferred to a second tube containing 5.9 mL of a desorbing solution (deionized water, 1M NaCl solution, 50% methanol in water, 50% acetone in water, or hexane). After 3 more days of mixing at room temperature, the desorption samples were centrifuged, extracted, and analyzed as described previously. The amount of PCE desorbed was then determined to be the difference r (at 4 days) - r (at 7 days), where adsorbent concentration was 3.0 wt % for the silica and 0.1 wt % for the clay. Results We performed experiments to determine the effect of various pretreatments on the ability of montmorillonite to adsorb trichloroethylene (TCE) from aqueous solutions at different pH levels and ionic strength. The most active form of montmorillonite for TCE adsorption was the clay that had been treated with citrate-bicarbonate-dithionite (CBD), which removes or alters the surface coating of amorphous metal oxides. Table I compares our measurements of several properties for the CBD-treated clay with those of clay that had only been sodium ion saturated. Literature values for the same properties of comparable samples of montmorillonite are as follows: cation exchange capacity, 97 mequiv/ 100 g (19);electrophoretic mobility, -0.9 (bm/s)/( V/cm) (20); ethylene glycol surface area, 820-840 m2/g (21). The measurements shown in Table I are close to these results of others and show no apparent differences between the two differently prepared clay samples. The very low organic carbon contents are especially noteworthy since this essentially rules out interactions of the LMHs with a residual organic fraction in the adsorption studies. Figure 1shows the effect of the CBD treatment on TCE adsorption over a wide range of pH (pH was adjusted with HC1 or NaOH). In these experiments, TCE concentration was 50 ppb (w/w), and the adsorption time was 26 h. The CBD-treated clay adsorbed more TCE than the untreated clay at all pH levels, except for the most acidic conditions where both clays adsorbed about the same small amount of TCE. Furthermore, there exists a definite optimum pH (pH 4)for adsorption to the CBD-treated clay, while the
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Flgure 1. Effect of clay pretreatment by citrate-bicarbonate4thbnite on TCE adsorption in deionized water at various pH levels: untreated (A)and CBD-treated (0)montmorilionlte. I n both cases adsorption was measured at 26 h, 1.39 wt % clay, and 50 ppb initial TCE concentration. 1.0
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Figure 3. Adsorptlon by CBD-treated clay at various pH levels of trich!oroethylene (0), tetrachloroethylene (O), dibromochloropropane (A), and hexachloroethane (0).Adsorption was measured at 67 h, 1.39 wt YO clay, and 10 ppb initial halocarbon concentration.
1
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Flgure 2. Effect of ionic strength on TCE adsorption by CBD-treated montmorillonite. Adsorptlon was measured at 26 h, 1.39 wt % clay, and 50 ppb Initial TCE concentration in deionized water (solid curve, redrawn from Figure 1): 0.01 M NaCl (O), 0.1 M NaCl (A),and 1.0 M NaCl (m).
untreated clay adsorbed only a small, constant amount of TCE over the entire pH range tested. Adsorption of TCE by CBD-treated montmorillonite was inhibited by increasing concentrations of NaC1. Figure 2 shows that as the concentration of NaCl was increased the TCE adsorption decreased. For clay in 0, 0.01, or 0.1 M NaC1, the optimum pH for adsorption remained at around 4-4.5. For clay in 1.0 M NaC1, adsorption was small over the entire pH range, and no optimum pH was found. In other experiments (not shown), we treated the montmorillonite with sodium acetate, pH 5, to remove carbonates or with H202to destroy organic material. The sodium acetate treated clay adsorbed more TCE than the untreated clay, but not as much as the CBD-treated clay. H20ztreatment, on the other hand, did not significantly alter the amount of TCE adsorbed compared to that of the untreated clay. CBD-treated montmorillonite also adsorbed a number of other low molecular weight halocarbons from water. Figure 3 shows the variation of adsorption with pH for perchloroethylene (PCE),hexachloroethane (HCE), and 1,2-dibromo-3-chloropropane(DBCP), as well as for TCE. For all compounds, the halocarbon concentration was 10 ppb, the clay concentration was 1.39%, and the adsorption time was 67 h. Every halocarbon tested exhibited a maximum in adsorption around pH 4. At this pH, the amount of haloalkenes (trichloroethylene and tetrachloroethylene) adsorbed was more than 4 times larger than the amount of haloalkane (dibromochloropropane and hexachloroethane) adsorption.
0.0
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Time of Mixing of Clay and TCE (Days1
Figure 4. Long-term kinetics of TCE adsorption by CBD-treated montmorillonite clay In deionized water suspension, pH -9.5, 1.39 wt % clay, and 50 ppb initial TCE concentration. Error bars are standard deviations. 0.5
0
5
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Time of mixing Clay and TCE (Oayrl
Flgure 5. Effect on TCE adsorption of aging CBD-treated clay in deionized water, 1.39 wt % clay, and 50 ppb initial TCE concentration. Clay was aged in deionized water (before mixing with TCE) for 0 (O), 1 (U), 2 (0),5 (m), 10 (0) or , 19 days (A).
We have also investigated the kinetics of TCE adsorption to the CBD-treated montmorillonite in deionized water for periods of up to 1 month. Figure 4 shows that the uptake of TCE continues over 28 days and never achieves equilibrium. There was an initial rapid uptake of TCE for the first few days and then a slow, constant rate of uptake over the remaining time. This failure to attain equilibrium was also observed for more acidic pHs, where the amount of TCE adsorbed is higher (see Figure 1). In fact, the uptake of TCE was often observed to continue until the TCE concentration decreased below the minimum detectable level (-0.1 ppb). The kinetics and extent of TCE adsorption also depended on the time of aging the clay in deionized water Environ. Scl. Technoi., Vol. 22, No. 4, 1988 379
Table 11. Tetrachloroethylene (PCE) Desorption from Fumed Silica or CBD-Treated Montmorillonite Clay
desorbing solvent deionized water 1 M NaCl 50% methanol in water 50% acetone in water hexane
PCE desorbed from silica: % from clay* 60 58 100 100 100
ndc nd nd nd nd
“After 4 days of adsorption conditions when r(at 4 days) = 9.2 g of PCE/g of silica. bAfter 4 days of adsorption conditions when r(at 4 days) = 5.4 X g of PCE/g clay. CNotdetected at the sensitivity level of 5% of the amount of PCE adsorbed. X
before TCE adsorption experiments were begun. Adsorption to clays that were aged from 0 to 19 days is shown in Figure 5. Again, adsorption continues to increase with time of mixing with the TCE solutions for all the clay samples. The extent of adsorption decreases as the time of clay aging increases. Furthermore, the fresher clay suspensions exhibited an initial rapid adsorption of TCE, followed by a period of slow adsorption (as in Figure 4). The older clay suspensions showed only the slow uptake of TCE. The desorption of tetrachloroethylene (PCE) from fumed silica and CBD-treated montmorillonite was studied in order to investigate the reversibility of halocarbon adsorption. The desorbing solvents used included water, 1 M NaCl, 50% (v/v) methanol in water, 50% (v/v) acetone in water, and hexane. In these experiments, the adsorbents were allowed to adsorb LMH from water for several days. Then the desorbing solvent was added to the adsorbentwater-LMH mixture, and the desorption was allowed to take place over several more days. The amount of the initially adsorbed halocarbon that was subsequently desorbed by the desorbing solvents was then measured. The results are shown in Table 11. These results show that adsorption of PCE by silica was reversible. With a silica concentration of 3.0% (w/w) and an initial halocarbon concentration of 10 ppm (w/w), the g of PCE/g of silica. halocarbon adsorption was 9.2 X When the silica was diluted into water or 1M NaC1, some of the PCE was removed from the silica and returned to solution. The amount of halocarbon adsorbed was not affected by the presence of NaCl in solution. And when methanol, acetone, or hexane was added to the silica, the adsorbed halocarbon was completely desorbed, probably because these solvents competed with the halocarbons for adsorption on the silica, Adsorption by montmorillonite, on the other hand, was completely irreversible. In these experiments, the clay concentration was 0.1% (w/w), the pH was 4, and the initial halocarbon concentration was 10 ppm (w/w). After 4 days of mixing, the montmorillonite adsorbed 5.4 X g of PCE/g of clay. None of the desorbing solvents, however, was able to remove the PCE once it was adsorbed. Discussion In addition to the clay minerals, natural clays contain numerous mineral and organic impurities, probably located mostly on the surface. Our results showing small affinity for TCE adsorption by untreated clay or clay pretreated with H202but increased affinity for clays pretreated with sodium acetate or CBD buffer solutions suggest that surface mineral coatings are an essential feature of LMH adsorption. The reversibility of adsorption of LMHs to the more hydrophobic surfaces of silica suggests that a different mechanism of interaction is involved than that 380
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which causes irreversible adsorption to montmorillonite. The observation of the sensitivity of LMH adsorption on CBD-treated clay to solution pH and ionic strength further suggests involvement of surface mineral oxides. Oxides of several of the elements composing the montmorillonite mineral, such as silica, alumina, and iron oxides, are known to exhibit amphoteric surface charge, which depends primarily on pH and to a lesser extent on the concentration of “indifferent”ions such as Na+ and Cl- (22, 23). The pH of maximum LMH adsorption could correspond to the pH at which an optimal surface charge exists given a mixture of the different kinds of surface oxides. Most likely such an optimal surface charge is close to zero, although this cannot be firmly established without a better understanding of the relative contributions of surface charge-halocarbon dipole (permanent or induced) interactions and hydrophobic interactions (including clay surface hydration) to the overall adsorption process. The decrease in TCE adsorption with increasing ionic strength indicates an effect of electrolyte shielding of the surface charge, while the constancy of the pH at which optimum adsorption occurs with respect to ionic strength indicates H+ is the only surface potential determining ion in our solutions. We also note that only a small fraction of the total clay surface is required to be coated with the amphoteric oxides in order to give the effects that we have observed. We estimate that at the maximum TCE adsorption shown in Figure 1, the fractional coverage of the clay surface, calculated with I’ from eq l and 800 m2/g of clay, is only about 6 X lo4. Since most of the montmorillonite surface has a permanent net negative charge (which is pH independent) (24))the trace quantities of surface oxides, which are responsible for halocarbon adsorption, would be difficult to detect by usual methods. Our electrophoretic mobility measurements of whole clay particles did not reveal such differences. The adsorption affinity to CBD-treated clay for the four LMHs, which are shown in Figure 3, appears to correlate best with the presence or absence of an unsaturated carbon-carbon bond. Trichloroethylene and tetrachloroethylene (PCE) showed the greatest adsorption at the optimal pH, and both compounds have a carbon-carbon double bond. Other properties such as water solubility or octanol-water partition coefficient showed no relationship to adsorption affinity. The lack of equilibrium for TCE adsorption by clay shown in Figure 4 could be the result of the following: (1) the clay surface is slowly changed during contact with the solution, so that new adsorption sites are continually being exposed; on (2) the overall process of TCE mass transfer to adsorption sites and the adsorption reaction are very slow. Although both effects may be involved, we believe the first to be more important because of our results showing the effect of clay aging on TCE adsorption (Figure 5). Apparently our fresher clays have more adsorption sites initially than aged clays, but the surface chemical changes leading to exposure of new sites are the same regardless of clay age. This follows from the similarity in the slopes of the curves of Figure 5 for times greater than about 5 days. Hamaker and Thompson (25)have also postulated that slow changes in the clay surface were the reason for the failure to achieve equilibrium even after 30 days in several cases of adsorption of organics to clays. Other studies (26-28) have addressed some of the detailed chemical changes that occur with montmorillonite in aqueous suspensions over comparable time periods. Some of these results have led to a model of montmorillonite
instability caused by the dissolution of elements from the clay lattice and redeposition of their oxides, such as silica and alumina, on the clay surface (28). Also, although the CBD buffer pretreatment that we have used was originally devised to remove natural oxide coatings on clay (14), others (29, 30) have shown that this treatment slightly affects the structure of the clay mineral as well. Acknowledgments
We thank Lisa Olson and Ian Kaplan for making the clay characterization measurements reported in Table I. Registry No. TCE, 79-01-6; PCE, 127-18-4;DBCP, 96-12-8; HCE, 67-72-1; NaCl, 7647-14-5; montmorillonite, 61029-13-8; sodium dithionite, 7775-14-6. L i t e r a t u r e Cited (1) Evans, R. B.; Schweitzer, G. E. Environ. Sei. Technol. 1984, 18, 330A-339A. (2) McCarty, P. L.; Reinhard, M.; Rittmann, B. E. Environ. Sci. Technol. 1981, 15, 40-51. (3) Page, G. W. Environ. Sei. Technol. 1981, 15, 1475-1481. (4) Fishbein, L. EHP, Environ. Health Perspect. 1976, 14; 39-45. (5) McConnel, G.; Ferguson, D. M.; Pearson, C. R. Endeavour 1975, 34, 13-18. (6) Callaway, J. Y.; Gabbita, K. V.; Vilker, V. L. Environ. Sei. Technol. 1984, 18, 890-893. (7) Guswa, J. H.; Lyman, W. J.; Donigian, A. S.; LOB,T. Y. R.; Shanahan, E. W. Groundwater Contamination and Emergency Response Guide; Noyes: Park Ridge, NJ, 1984; p 254. (8) Bailey, G. W.; White, J. L. Residue Rev. 1970, 32, 29-92. (9) Burchill, S.; Hayes, M. H. B.; Greenland, D. J. In The Chemistry of Soil Processes; Greenland, D. J., Hayes, M. H. B., Eds.; Wiley: New York, 1981; pp 221-400. (10) Hoffmann, R. W.; Brindley, G. W. Geochim. Cosmochim. Acta 1960,20, 15-29. (11) Singhal, J. P.; Kumar, D. Clays Clay Miner. 1976, 24, 122-126. (12) Singhal, J. P.; Singh, C. P. J. Agric. Food Chem. 1976,24, 307-310. (13) Rogers, R. D.; McFarlane, J. C. Environ. Monit. Assess. 1981,1, 155-162.
(14) Jackson, M. L. Soil Chemical Analysis-Advanced Course; Department of Soil Science, University of Wisconsin: Madison, WI, 1956. (15) Vilker, V. L.; Fong, J. C.; Seyyed-Hoseyni, M. J . Colloid Interface Sei. 1983, 92, 422. (16) Rhoades, 3. D. In Methods of Soil Analysis, 2nd ed.; Page, A. L., Miller, R. H., Keeney, D. R., Eds.; American Society of Agronomy and Soil Science Society of America: Madison, WI, 1982; Part 2, Chapter 8. (17) Mortland, M. M.; Kemper, W. D. In Methods of Soil Analysis; Black, C. A,, Ed.; American Society of Agronomy: Madison, WI, 1965; Part 1, Chapter 42. (18) Minagawa, M.; Winter, D. A.; Kaplan, I. R. Anal. Chem. 1984,56, 1859-1861. (19) Greenland, D. J.; Mott, C. J. B. In The Chemistry of Soil Constituents; Greenland, D. J., Hayes, M. H. B., Eds.; Wiley-Interscience: New York, 1978; Chapter 4. (20) Black, A. P.; Birkner, F. B.; Morgan, J. J. J. Colloid Interface Sci. 1966, 21, 626-648. (21) Bower, C. A.; Goertzen, J. 0. Soil Sci. 1959,87, 289-292. (22) Breeuwsma, A.; Lyklema, J. Discuss. Faraday Soc. 1971, NO.52, 324-333. (23) James, R. 0.; Parks, G. A. Surf. Colloid Sei. 1982, 12, 119-216. (24) Grim, R. E. Clay Minerology; McGraw-Hill: New York, 1968. (25) Hamaker, J. W.; Thompson, J. M. In Organic Chemicals in the Soil Environment; Goring, C. A. I., Hamaker, J. W., Eds., Dekker: New York, 1972; Vol. 1, pp 49-143. (26) Eeckman, J. P.; Laudelot, H. Kolloid-2. 1961,178,99-107. (27) Shainberg, I. Soil Sei. Soc. Am. Proc. 1973, 37, 689-694. (28) Schramm, L. L.; Kwak, J. C. T. Soil Sei. 1984,137, 1-6. (29) Brewster, G. R. Clays Clay Miner. 1980,28, 303-310. (30) Stucki, J. W.; Golden, D. C.; Roth, C. B. Clays Clay Miner. 1984,32, 191-197.
Received for review September 9, 1985. Revised manuscript received August 3, 1987. Accepted October 23, 1987. This research was financed in part by the US.Department of the Interior, Geological Survey, through the State Water Resources Research Institute, University of California Water Resources Center, Project UCAL- WRC-W652(CA06),and by the U S . Environmental Protection Agency under Assistance Agreement CR-812771 to the National Center for Intermedia Transport Research. This paper does not necessarily reflect the views of the U.S. E P A or the U.S. Department of the Interior, and no official endorsement should be inferred.
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