Environ. Sci. Technol. 2001, 35, 1523-1530
Evaluation of Shale and Organoclays as Sorbent Additives for Low-Permeability Soil Containment Barriers RICHARD W. GULLICK† AND WALTER J. WEBER, JR.* Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
A natural shale and four synthetic organoclays were compared as potential sorbent additives to containment barriers at hazardous waste sites. Trimethylphenyl ammonium bentonite (TMPA-bent) was shown in batch experiments to have the greatest sorption capacities for 1,2,4trichlorobenzene, trichloroethylene, and methyl isobutyl ketone, followed by the shale and a commercial organoclay. Sorption capacities were lowest for hexadecyltrimethyl ammonium bentonite (HDTMA-bent) and hexadecyl pyridinium bentonite (HDP-bent). Operative sorption mechanisms for the organoclays depended on the size of the organic modifier, i.e., uptake by the TMPA-bent occurred via adsorption onto mineral surfaces, while that for the HDTMA-bent and HDP-bent took place by absorption into organic phases formed by their long hydrocarbon tails. The shale was found to be by far the most cost-effective sorbent, an important factor for large scale applications. Solids concentration effects (i.e., higher apparent sorption capacities at lower experimental sorbent concentrations) were exhibited by HDTMA-bent and HDP-bent. This can be attributed to aggregation of sorbent particles as a result of interactions among their hydrocarbon chains. Solids effects were observed to decline and eventually disappear as sorbent concentrations were increased. Such effects must be considered in applying batch sorption results to flowthrough systems.
Introduction Contaminant transport through low-permeability soil barriers (e.g., soil-bentonite slurry cutoff walls and clay landfill liners) involves a combination of advection and molecular diffusion, the latter being dominant at sufficiently low flows (1-3). Natural soils generally exhibit only modest capacities for sorption of most organic contaminants, and the transport of these substances through containment barriers can therefore usually be reduced through addition of an appropriate sorbent to barrier materials (4-5). A variety of materials have been suggested as potential sorbent additives to soil-bentonite cutoff walls and/or clay liners (6), including activated carbon (7), fly ash (5, 8), modified clays (9-13), natural geosorbent materials (6, 13), * Corresponding author phone: (734) 763-2274; fax: (734) 7632275; e-mail:
[email protected]. † Present address: American Water Works Service Company, Inc., 1025 Laurel Oak Rd., Voorhees, NJ 08043. 10.1021/es0015601 CCC: $20.00 Published on Web 02/22/2001
2001 American Chemical Society
and shredded scrap tires (14). The sorptive capacity and geotechnical compatibility of coal fly ash for this application have been demonstrated in our laboratories (5, 8, 15), and we are aware of at least one full-scale soil-bentonite-fly ash cutoff wall having been installed at a hazardous waste site in Michigan in 1996. Shale, a sedimentary rock formed from silt and clay, is a natural geosorbent exhibiting particularly good potential for inclusion in both confinement and permeable reactive barriers. Weber and co-workers (16) have shown that the sorptive capacities of certain subsurface soils are in fact dominated by relatively small quantities of shale materials. Investigations with assorted shales have shown them to be generally good sorbents for organic contaminants (6, 13, 17, 18). The effective sorption of organic compounds by a variety of modified clays, including organoclays, inorganoclays, and inorgano-organoclays, is well documented (10, 11, 19), particularly for BTEX compounds (benzene, ethylbenzene, toluene, and xylene) and chlorophenols. Organoclays are prepared by exchanging cationic organic quaternary ammonium compounds (QACs) for the mineral cations normally associated with negatively charged natural clays. Sorption of hydrophobic organic contaminants (HOCs) by organoclays can occur via different mechanisms and to differing degrees depending on the type of QAC used, the base clay type and its cation exchange capacity (CEC), the fraction of the CEC satisfied by the QAC, and such sorbate characteristics as size, shape, and solubility/hydrophobicity (10, 20-22). The solids concentration effect, or simply “solids effect”, wherein apparently higher sorption capacities are observed for lower solids concentrations, has not previously been investigated for organoclays. Any such effects could potentially have adverse impacts when using experimentally determined parameters for full-scale modeling and design. This paper focuses on comparing the sorption of organic chemicals by organoclays and by a natural shale and on solids concentration effects observed in the organoclay sorption experiments.
Experimental Section Sorbents. The sorbents studied included three laboratoryprepared organoclays, a commercial organoclay, and a natural shale. The laboratory-prepared organoclays, selected on the basis of the different chemical structures of the QAC modifying agents employed (Figure 1), included trimethylphenyl ammonium bentonite (TMPA-bent), hexadecyltrimethyl ammonium bentonite (HDTMA-bent), and hexadecyl pyridinium bentonite (HDP-bent, or cetyl pyridinium bentonite). HDTMA-bent has an aliphatic headgroup (trimethyl ammonium) with a long 16-carbon aliphatic hydrocarbon chain attached, HDP-bent has an aromatic headgroup (pyridinium) with a similar 16-carbon aliphatic hydrocarbon chain, and TMPA-bent has an aromatic (phenyl) headgroup without a long hydrocarbon chain. A standard commercial organoclay manufactured by modifying bentonite with a dialkyl dimethyl QAC was used for comparison purposes (SM-399, also known as Adogen 442, Colloid Environmental Technologies Company, Arlington Heights, IL). Commercial organoclays are typically prepared with a fairly simple ion exchange procedure. A more elaborate procedure that included several purification steps was used for the organoclays prepared in the laboratory for reasons of consistency and scientific reproducibility. The laboratoryprepared organoclays were produced by exchanging the modifying QAC for the natural cations of a Wyoming sodium VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Sorbate Properties TCE molecular formula mol. wt. (g/mol) densitya at 20 °C (g/mL) aqueous solubility at 25 °C (mg/L) log KOW
C2HCl3 131.39 1.4642 1100b
TABLE 1. Sorbent Properties
sorbent Bentonite (SWy-1) TMPA-bentd HDTMA-bentd HDP-bentd Ohio shaled
% % organic carbona nitrogena 0.24 7.01 14.2 16.1 2.50
0.04 0.89 1.00 1.13 0.14
N2-BET surface area (m2/g)b 36.5 109 20.6 27.4 14.8
basal spacing (d001) (Å)c
H2O contact angle (deg)
14.8 19.6 14.7 45.8e 17.8 58.9e 17.5 55.4e not appl NAf
a Perkin-Elmer 2400 CHN analyzer. b Quantachrome Corp. Autosorb-1 sorption system, model AS-1 (Syosset, NY). c Philips XRG 3100 X-ray generator. d Sorbent particle size ) 38-53 µm. e Sample preparation technique similar to that of Norris et al. 1992 (26). f Relatively hydrophilic (H2O readily soaked into sample). not appl. ) not applicable. N.A. ) not analyzed.
bentonite clay (SWy-1, Source Clays Repository, Columbia, MO) having a CEC of 76.4 mequiv/100 g (23) using a procedure detailed elsewhere (24). HDTMA-Br (99%) and TMPA-Cl (98%) salts were obtained from Eastman Kodak Co. (Rochester, NY), and HDP-Cl (98%) was obtained from Aldrich Chemical Co., Inc. (Milwaukee, WI); all three QACs were used as received. Upon completion of the substitution procedure the organoclays were rinsed three times with methanol (99.9+%; Aldrich Chemical Co., Inc., Milwaukee, WI) to remove excess H2O, vacuum freeze-dried for >24 h, ground with mortar and pestle, sieved to a particle size range of 38-53 µm (270/400 U.S. Standard mesh sizes), and stored in a desiccator in sealed glass bottles until use. The shale studied is a sedimentary rock obtained from a natural outcrop near the southern border of Lake Erie in northwestern Ohio. It is part of the Ohio Shale Formation from the Upper Devonian time period and is approximately 370 million years old (25). This material was rinsed with Nanopure water, ground with mortar and pestle, and sieved to the same particle size as the organoclays (38-53 µm). Sorbent Characterization. Each sorbent was analyzed for organic carbon (OC) and nitrogen content, surface area (nitrogen-BET), basal spacing (except for shale), and water contact angle. Results of these analyses are presented in Table 1. The OC analysis was used to confirm the percentages of the SWy-1 CEC utilized by the modifying QACs, which for all three organoclays were fairly close to 100% (92.2% to 101.5%). Water contact angles were used as a relative measure of sorbent hydrophobicity. The Ohio shale is an organic-rich, finely laminated, black shale. A semiquantitative mineralogical analysis was performed using X-ray diffraction, and approximate clay mineral contents were calculated based on response intensity and estimations of clay mineral response factors. The results showed that the clay fraction (roughly 60%) of the shale contained ∼15% smectite, ∼55% illite, and ∼30% kaolinite or chlorite. Sorbate Solutions and Analytical Techniques. Three nonionic organic solutes of environmental concern were used 1524
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C6H3Cl3 181.45 1.4542 31.3c; (30-48.8) d 4.00e
2.42e; (2.37-3.30)d 9.58g 2.32h
Henry’s constant (KH) at 25 °C (L‚atm/mol) CAS registry number 79-01-6 U.S. EPA MCL j (µg/L) 5
FIGURE 1. Chemical structures of QAC clay modifiers.
TCB
120-82-1 70
MIBK C6H12O 100.16 0.7978 19000b 1.09f 0.0149i 108-10-1 no MCL
a CRC, 1993-1994 (27). b Verschueren, 1983 (28). c Banerjee, 1984 (29). d Range reported in the literature found by Montgomery and Welkom, 1990 (30). e Schwarzenbach et al., 1993 (31). f Hansch et al., 1968 (32). g Gossett, 1987 (33). h Valsaraj, 1988 (34) (cited in 30). i Montgomery and Welkom, 1990 (calculated) (30). j 40 CFR part 141 (MCL ) maximum contaminant level for drinking water).
as sorbates, including trichloroethylene (TCE), 1,2,4-trichlorobenzene (TCB), and methyl isobutyl ketone (or 4-methyl2-pentanone; MIBK). The properties of these sorbate solutes are given in Table 2. TCE (99+% purity), TCB (99+%), and MIBK (99.5+%) were used as received (Aldrich Chemical Co., Inc., Milwaukee, WI). Stock solutions were prepared by dissolving the pure chemical into methanol, and working solutions were prepared by dilution of the stock solutions with Nanopure water. The presence of small amounts of residual methanol in the aqueous working solutions is not expected to have affected the sorption properties of the synthetic organic chemicals (35). Aqueous-phase MIBK was analyzed directly by gas chromatography (GC), while TCE and TCB were extracted with 5 mL:5 mL hexane (pesticide free grade; J. T. Baker, Inc., Phillipsburg, NJ) prior to GC analysis. Concentrations were determined using external standards with each analysis, and all samples were analyzed in duplicate. Ranges of initial sorbate concentrations (Co, M L-3) were set so that residual equilibrium values (Ce, M L-3) after sorption covered ranges similar to those commonly detected at hazardous waste sites (36). Target Ce ranges were ∼505000 µg/L for TCE and TCB, and ∼1000-100 000 µg/L for MIBK, the upper values corresponding to only 0.455% of solubility (at 25 °C) for TCE, 16.0% for TCB, and 0.526% for MIBK. Equilibrium Sorption Isotherms. Isotherms were determined for sorption of each solute on TMPA-bent, HDTMAbent, HDP-bent, Ohio shale, and unmodified bentonite; TCE and TCB isotherms were measured for the commercial organoclay. The common bottle-point completely mixed batch reactor (CMBR) method was used. Solutions of varying Co were added to 30 mL glass centrifuge tubes containing a known mass of sorbent, and the vials were sealed with no headspace using Teflon-lined septa. All background water was Nanopure grade; no buffer or other pH control was used. The vials were tumbled end over end for 3 to 4 days, a period long enough to reach apparent equilibrium (i.e., subsequent solution-phase concentration changes were too small to measure over a reasonable subsequent period of elapsed time) yet short enough to avoid significant system losses of solute (i.e., other than by sorbent uptake). Preliminary sorption rate studies showed that apparent equilibrium typically was attained in less than 24 h, a result consistent with sorption rate studies performed by others for a variety of organic compounds and different organoclays (22, 37-40). The solid and aqueous phases were separated after mixing by centrifugation at 8000 rpm (7649 g) for 60 min. Supernatant solution was then drawn from the tubes via pipet, and Ce was determined by GC analysis.
TABLE 3. Linear and Freundlich Model Isotherm Parameters sorbate TCB
TCE
MIBK
sorbenta TMPA-bent HDP-bent HDTMA-bent SM-399 Ohio shale TMPA-bent HDP-bent HDTMA-bent SM-399 Ohio shale TMPA-bent HDP-bent HDTMA-bent Ohio shale
KDb (mL/g) 4530 531 291 906 303 14.2 14.2 62.1 71.8 4.90 5.90 3.82
log KOCc (mL/g-OC) 4.81 3.52 3.31 4.56 3.64 1.95 2.00 3.40 3.01 1.48 1.62 2.18
KF d
KF (95% C.I.)d
nd
n (95% C.I.)d
R2 d (Freundlich)
22.7 0.44 0.35 0.70 19.8 0.17 0.016 0.013 0.022 1.44 0.20 0.00083 0.018 0.31
16.6-31.0 0.25-0.77 0.21-0.59 0.43-1.15 15.3-25.5 0.10-0.30 0.0082-0.031 0.0059-0.027 0.0077-0.063 0.94-2.21 0.14-0.30 0.00046-0.0015 0.0083-0.041 0.21-0.45
0.81 1.02 0.98 1.13 0.62 1.06 0.98 1.00 1.11 0.63 0.92 1.16 0.90 0.61
0.77-0.85 0.94-1.10 0.91-1.05 1.06-1.20 0.58-0.66 0.98-1.14 0.89-1.07 0.90-1.10 0.97-1.26 0.58-0.69 0.88-0.96 1.10-1.22 0.82-0.98 0.57-0.65
0.985 0.954 0.960 0.991 0.985 0.961 0.970 0.910 0.988 0.988 0.983 0.997 0.998 0.971
a TMPA-bent, HDTMA-bent, HDP-bent, and Ohio shale were all sieved to 38-53 µm. b K was obtained by linear regression of q vs C data over D e e the full concentration range of the isotherm, and forced through (0, 0), using Microsoft Excel. c KOC ) KD/fOC, where fOC ) the fraction of organic carbon in the sorbent. d Freundlich isotherm model linear regressions were performed on log-transformed Ce and qe data using Microsoft Excel. The values of KF correspond to units of µg/L for Ce and µg/g for qe, i.e., (µg/g)(µg/L)-n.
Controls for each isotherm experiment included (i) background water only, (ii) sorbent and background water only (no sorbate), and (iii) a series of vials containing different concentrations of sorbate and no sorbent. The latter controls were used to assess sorbate system losses during the course of the experiment. Percentage system losses for each solute were usually found to be approximately the same regardless of initial solute concentration and were typically ∼6-8% for TCB, ∼2-4% for TCE, and ∼0-1% for MIBK. These losses were most likely due to volatilization and/or sorption to system components (e.g., Teflon-lined septa), with volatilization probably more likely for TCE and sorption onto system components more likely for TCB. To account and adjust for these system losses, equilibrium solid-phase concentrations (qe, M M-1) were determined for each experiment using a mass balance between the amount sorbed at equilibrium (qe, M M-1) and the initial and equilibrium solution-phase concentrations (Co and Ce, respectively, M L-3) and sorbent dose (Do, M L-3)
100 Co - Ce × 100 - % system loss qe ) Do
(
)
(1)
Solids Concentration Effects. Sorption isotherm experiments were performed at varying sorbent dosages to explore potential sorbent concentration effects on observed sorption capacities. Differences in capacities were often observed for different relatively low values of Do for the long-chain organoclays (HDTMA-bent and HDP-bent). The magnitude of this apparent “solids effect” decreased as Do increased, and it eventually disappeared when Do was increased sufficiently. Experimental Do values were therefore increased for most sorbent/solute systems until no solids effect was observed, and typically only the results for the highest Do experiments were used for analysis of relative equilibrium sorption capacities. This conservative approach is appropriate in that these conditions would be closest to the soil solution ratios found in typical cutoff wall applications. Because increasing sorbent concentrations increase the amount of solute sorbed, they also decrease the magnitude of data adjustment for system losses and generally decrease data scatter as well (24). The upper limit sorbent/solution ratios used varied from 1:2000 to 1:30 (Do ) 0.5 to 33.3 g/L); lower sorbent/solution ratios were employed for more strongly sorbing solute/ sorbent systems to avoid removal of too much solute to allow
accurate determination of residual values. Achieving the desired solute removal was not possible in all cases, however, especially for nonsorbing bentonite and for weakly sorbing HDTMA-bent, the latter of which showed an apparent solids effect for MIBK at the soil solution ratios used.
Results and Discussion Sorption Capacities and Mechanisms. The equilibrium sorption data were examined by plotting them in the form of sorption isotherms (qe vs Ce) and fitting each set with linear, Freundlich, and Langmuir isotherm models having the respective forms (41):
q e ) K DC e
(2)
qe ) KFCen
(3)
qe )
QobCe 1 + bCe
(4)
The term KD in eq 2 is the linear sorption distribution coefficient (L3 M-1); KF and n in eq 3 are the Freundlich capacity parameter ([M M-1][M L-3]-n or [L3 M-1]n) and the dimensionless Freundlich site energy heterogeneity factor (or linearity factor), respectively; and Qo and b in eq 4 are the Langmuir capacity parameter (M M-1) and the Langmuir enthalpy parameter (L3M-1), respectively. Values given in Table 3 for the Freundlich parameters KF and n, their 95% confidence intervals, and model fit R2 values were obtained by linear least-squares regression of logtransformed experimental data (Ce, qe) using Microsoft Excel. The Freundlich model fit most of the isotherm results reasonably well. The Langmuir model provided reasonable fits for only four of the isotherms generated. Calibrated Langmuir isotherm parameters for these four isotherms, along with their 95% confidence intervals and model fit R2 values, are presented in Table 4. The model fits were obtained by linear least-squares regression of Ce/qe vs Ce experimental data (Microsoft Excel). KD values obtained by linear regression (Microsoft Excel) of qe vs Ce data over the full concentration range of the isotherm and forced through the joint axis origin (0, 0) are also presented in Table 3. The organic-carbon normalized linear distribution coefficients (KOC, L3 M-1) given in the table were calculated from KD and sorbent OC content (Table 1) results. It is noted that the use of KD and KOC inherently VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Langmuir Model Isotherm Parameters sorbate TCB TCE MIBK
sorbenta
Qo (µg/g) × 10-3 b,c
Qo (95% C.I.) × 10-3 b,c
b (g/µg) × 106b,c
b (95% C.I.) × 106 b,c
TMPA-bent Ohio shale Ohio shale TMPA-bent
45.3 3.73 0.58 17.4
37.6-56.8 3.33-4.23 0.51-0.66 14.3-22.24
165 817 244 6.14
151-183 703-973 215-282 5.76-6.57
R2
c
0.811 0.949 0.972 0.678
a TMPA-bent and Ohio shale were sieved to 38-53 µm. b Values of Qo and b correspond to units of µg/L for C and µg/g for q . c Isotherm linear e e regressions were performed on Ce/qe vs Ce Langmuir model linearizations of isotherm data using Microsoft Excel.
FIGURE 2. Sorption isotherms for 1,2,4-trichlorobenzene.
FIGURE 3. Sorption isotherms for trichloroethylene. assumes linear isotherms. However, as a relatively simple means to facilitate rough comparisons of relative sorption capacities to organic carbon contents for the different sorbents studied, “forced” values for these parameters are provided herein for several isotherms that are clearly nonlinear. Sorption isotherm results and corresponding calibrated Freundlich and Langmuir model fits are presented for TCB, TCE, and MIBK in Figures 2, 3, and 4, respectively. As expected, sorption by unmodified Na-bentonite was minimal for all three solutes. The following general trends were observed for the sorptive capacities of the various sorbents for the solutes studied:
TCB: TMPA-bent . Ohio shale ≈ SM-399 > HDP-bent J or ≈ HDTMA-bent . Na-bentonite. TCE: TMPA-bent >>> Ohio shale ≈ SM-399 . HDP-bent ≈ HDTMA-bent . Na-bentonite. MIBK: TMPA-bent .> HDTMA-bent ≈ Ohio shale ≈ HDP-bent > Na-bentonite. TMPA-bent clearly had the highest sorption capacity for all three solutes (Figures 2-4), followed by Ohio shale for TCB and TCE (Figures 2 and 3). While the sorption capacities of HDP-bent and HDTMA-bent for TCB and TCE were lower 1526
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FIGURE 4. Sorption isotherms for methyl isobutyl ketone. than those of Ohio shale (Figures 2 and 3), the sorption capacities of these three sorbents for MIBK were similar (Figure 4). The extent of sorption by the three organoclays and shale was observed to relate inversely to the solubility of the organic solutes (i.e., TCB > TCE > MIBK). The sorption capacities of the commercial organoclay (SM-399) for both TCB and TCE (Figures 2 and 3) lie between those of TMPAbent and the other two organoclays (HDTMA-bent and HDPbent). The degree of isotherm linearity can be used to partially assess operational sorption mechanisms, with nonlinear isotherms being generally indicative of adsorption processes and linear isotherms potentially indicative of absorption processes (41). The degree of isotherm linearity can be assessed by examination of the slopes of the isotherm plots in Figures 2-4 and the corresponding Freundlich n values in Table 3. Sorptions of all three solutes onto HDP-bent and HDTMA-bent were fairly linear (n ) 0.90-1.16). Sorptions of TCB and TCE onto SM-399 were also fairly linear (n ) 1.13 and 1.11, respectively). Sorption by TMPA-bent produced a nonlinear isotherm for TCB (n ) 0.81) but was more linear for MIBK (n ) 0.92) and TCE (n ) 1.06) over the concentration range examined. The relative linearity of the TCE/TMPAbent isotherms (n ) 1.06) is likely attributable to the relatively low solution-phase concentration examined. The shale isotherms were significantly nonlinear for all three solutes (n ) 0.61-0.631). As noted above, the nonlinear Langmuir isotherm model provided a reasonable fit only for select isotherms generated using either TMPA-bent or Ohio shale. Interestingly, the two most effective sorbents for TCE and TCB have much lower OC contents (TMPA-bent 7.01% OC; shale 2.50%) than do the less effective long-chain hydrocarbon organoclay sorbents (HDP-bent 16.1%; HDTMA-bent 14.2%). This difference is shown clearly when sorption is normalized by sorbent OC content; e.g., TMPA-bent and Ohio shale have much higher KOC values than do HDP-bent and HDTMA-bent (Table 3). Furthermore, experimental KOC values for TMPA-bent and Ohio shale (Table 3) were significantly higher than the corresponding solute KOW values (Table 2), while the measured KOC for HDP-bent and HDTMAbent were much closer to the corresponding KOW values.
Comparison of the KOC results suggests that the organic phases of the HDP and HDTMA organoclays are slightly better partitioning media than the sediment organic matter reported by Karickhoff et al. (42) and significantly better than that of the Woodburn soil reported by Chiou et al. (43). Examination of the relative sorption isotherm behaviors of the sorbents suggests different sorption mechanisms. The operative sorption mechanism for the long hydrocarbon QAC sorbents, HDTMA-bent and HDP-bent, appears to be absorption into an organic matrix formed by the hydrocarbon tails of the QACs. This partitioning behavior is analogous to absorption into hemi-micelles or highly amorphous soil organic matter. Evidence for absorption centers primarily on isotherm linearity, low solute uptake, and correlation of sorption capacity with sorbent organic carbon content, all of which are indicative of absorption-dominated processes. Linear absorption by HDTMA-bent has been shown previously (e.g., 9, 21, 40, 44-46). Conversely, the operative sorption mechanism for TMPAbent (short hydrocarbon QAC) and Ohio shale appears to be adsorption. The primary evidence for this conclusion includes relatively high solute uptake, nonlinear sorption isotherms, and sorption capacities much higher than expected for partitioning into sorbent organic matter. For TMPA-bent, adsorption appears to occur on mineral surfaces rather than on sorbent QAC organic matter. TMPA-bent has a much higher measured surface area (SN2 ) 109 m2/g) than the other organoclays, indicating that more of the internal clay surface is accessible for sorption if the sorbate is small enough to enter the interlammelar spacing. Definitive evidence that the sorption of neutral organic compounds by TMPA-bent is dominated by mineral surfaces rather than by TMPA molecules was presented by Jaynes and Boyd (47, 48), who concluded that small QACs exist as discrete entities on mineral surfaces; i.e., they do not cover those surfaces completely. It is likely that the QACs create a hydrophobic environment that precludes H2O from sorbing onto the siloxane surface, thus allowing the surface to be available for sorption of organic solutes via polar adsorbate-adsorbent interactions (e.g., van der Waals interactions between the adsorbates and the siloxane surface oxygen atoms). While smectites modified with long-chain QACs may have mineral siloxane surfaces that are not hydrated (due to the presence of the QACs), these large organic molecules may physically obscure the mineral surfaces, making these sites unavailable for adsorption of organic solutes. While the headgroups of the two organoclays having 16carbon QAC tails are quite dissimilar (i.e., ammonium and pyridinium cations; see Figure 1), both sorbents exhibited similar sorption capacities. TMPA-bent, however, with a phenyl headgroup that is reasonably similar to a pyridinium headgroup, exhibited much higher sorption capacity than HDP-bent. The sorption mechanism of an organoclay thus depends principally on the size of the QAC modifier. A similar conclusion was reached by Smith et al. (44) after examination of tetrachloromethane sorption by different organobentonites. The nonlinear isotherms and high sorption capacity relative to sorbent organic carbon content for Ohio shale is attributable to the highly condensed nature of the kerogen organic matter present in the shale material. In contrast, the organic matter of HDP-bent and HDTMA-bent is relatively amorphous, resulting in linear sorption and relatively low sorbate uptake. These observations are consistent with the concepts of soil organic matter structure/activity relationships advanced by Weber and co-workers (e.g., 16, 17, 49, 50). Steric Hindrance. At least in terms of entropic effects, uptake of solutes by sorption should generally be related to KOW and inversely related to solubility. However, in comparing
FIGURE 5. Solids concentration effects for TCE and HDP-bentonite. ratios of KOC to sorbate solubility (or KOW) (Tables 3 and 2), HDTMA-bent and HDP-bent show significantly higher relative values of KOC for MIBK than expected based on the results for TCB and TCE. It is possible that MIBK can enter the interlammelar space of these organoclays, while steric hindrance prevents the chlorinated organics from doing so. The organic phase formed by HDP and HDTMA will be both between adjacent clay platelets and on external clay surfaces. Accessibility to the internal organic phase depends on the spacing of the interlayers (Table 1) and the size of the sorbate. In that all three solutes studied are relatively planar molecules, their respective abilities to enter the organoclay interlayers are controlled by the largest element in their structures. TCE and TCB both contain chlorine, which is significantly larger than either carbon or oxygen, the two largest elements in MIBK. Evidence of steric hindrance effects on sorption of organic molecules in organoclay interlayers has been shown previously (22, 51), including the effects of aromatic substitutions (45). Additional evidence exists specifically for the potential steric hindrance of TCE for access to HDTMA-bent interlayers (40, 46). Given that the TMPA-bent interlayer spacing is ∼3 Å less than that of HDTMA-bent and HDP-bent (Table 1), it seems clear that TCE and TCB would also be sterically hindered from entering the TMPA-bent interlayers and that their uptake by this sorbent is by adsorption onto exposed exterior mineral surfaces. No definitive experimental evidence is available to either confirm or refute the possibility that MIBK is able to access the TMPA-bent interlayer structure. However, research by others (47, 48) has suggested that many nonchlorinated planar neutral aromatic compounds (e.g., benzene, alkylbenzenes, and naphthalene) are not sterically hindered from accessing these sites. Solids Concentration Effects. The two long-chain organoclays (HDTMA-bent and HDP-bent) exhibited apparently higher sorption capacities at a given solution-phase residual solute concentration when lower solids dosages (Do) were used for the sorption isotherms; an example for sorption of TCE on HDP-bent is provided in Figure 5. Increasing the concentration of HDP-bent resulted in decreased measured sorption capacity for TCE. Once a sufficiently high Do was used, however, there was no solids effect for further increases in Do. This is illustrated by the coincident isotherms for Do values of 8.33 and 16.7 g/L in Figure 5. The effects of Do on measured sorption distribution coefficients are shown for TMPA-bent and Ohio shale in Figure 6 and for HDP-bent and HDTMA-bent in Figure 7. For sorbents that exhibit nonlinear isotherms (i.e., TMPAbent and Ohio shale), it is best to compare KD values obtained over similar residual solute concentration (Ce) ranges when comparing isotherms. For each of these two sorbents, KD was obtained for a single Ce value near the common maximum Ce observed, and it was calculated by obtaining VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Solids concentration effects for TMPA-bentonite and Ohio shale.
FIGURE 7. Solids concentration effects for HDP-bentonite and HDTMA-bentonite. the corresponding value of qe from the calibrated Freundlich model for each isotherm (i.e., KD ) qe/Ce, with Ce and qe obtained as described above). The Ce-specific KD values varied noticeably from full-concentration range KD values in only a few instances. The presence and magnitude of observed solids effects is primarily a function of the sorbent characteristics. Little or no solids concentration effect was observed for sorption of any of the sorbates by TMPA-bent or Ohio shale (Figure 6) over the range of Do values examined. The apparent decrease in KD for TCE/Ohio shale with increasing Do above 1.67 g/L is primarily an effect of data scatter, with outlying values at high Ce significantly biasing the value of KD. In contrast, significant solids concentration effects were observed at times for both HDTMA-bent and HDP-bent, as shown in Figure 7. This was most notable for sorption of TCE by HDP-bent and TCB by HDTMA-bent, both of which systems were examined over larger and lower ranges of Do than all but one (TCB/HDP-bent) of the other systems represented in this figure. In each of these two cases the apparent KD values can be seen to decrease as Do increases, until appearing to level off at relatively high sorbent dosages. Values of Do required to reach the solids effect plateaus for HDP-bent and HDTMA-bent varied with the solutes, in the order TCB < TCE < MIBK. The levels of Do for which no significant further solids effects were observed for sorption of TCB and TCE by the HDP-bent and HDTMA-bent were nearly the same (i.e., Do ∼1 to 3 g/L and Do ≈ 8.3 g/L, respectively). Sorption of MIBK on HDP-bent and HDTMAbent was examined only over small ranges of Do; wider ranges would be required to more adequately assess potential related solids effects. Several different hypotheses have been presented for explaining observed solids effects in experimental sorption data (e.g., 52-58), none of which apply to all sorbent/solute 1528
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systems for which solids effects have been reported (e.g., 24, 59-61). Although similar solids effects results may be observed in many different sorbate/sorbent systems, the underlying causes of this phenomenon may vary from system to system. Apparent effects may be the results of experimental procedures, sorbent or sorbate characteristics, and/or interactions between system components. Further, more than one contributing cause may be operative in any given system. Models proposed to explain solids effects include the three-phase dispersed solids model (52-54, 62-65), the solute complexation model (55), the implicit-adsorbate model (56), the particle interaction model (57), and the sorbent aggregation/coagulation model (58, 62, 64, 66). The explanations associated with these models focus primarily on impacts either from experimental procedures (i.e., experimental artifacts) or from inappropriateness of standard sorption equilibrium models (i.e., an assumption that the experimental results are correct and that the sorption model is not). The three-phase model attributes solids effects to the dispersal of sorbing materials from the solid phase in the liquid phase and inadequate removal therefrom during experimental phase separation procedures. The particle interaction model suggests a particle-induced desorption process which results in less sorption at higher solids concentrations than otherwise would occur. The three-phase and particle interaction models both predict relatively constant KD values at low solids dosages (i.e., a no solids effect range), with a log-log linear reduction of KD with increasing Do at higher Do. As such, neither model can explain the effects noted in this investigation; i.e., the leveling off of KD at sufficiently high values of Do. Mass balance analyses performed in the current work using a general form of the three-phase model showed that the solids effect cannot be explained as a result of inadequate separation of solid and aqueous phases, no matter what percentage of the sorbent material was nonsettleable (24). In addition, control experiments showed >99% of HDP-bent sorbent was removed from solution by centrifugation, thus further ruling out this explanation. The implicit-adsorbate model suggests that solids effects in natural soils might be caused by competition between sorbing solutes and other solutes already present on the sorbent material. The solute complexation model relates KD to sorbent concentration, degree of complexation of solute by liquid-phase organic carbon, and the partitioning of free and bound solute to the sorbent material. Both the solute complexation and implicit-adsorbate models predict linear log-log relationships between KD and Do at low Do, and a leveling off of KD at high Do, the same trends observed for HDP-bent and HDTMA-bent in Figure 7. The implicit adsorbate model does not, however, apply to the systems investigated here in that it assumes competition between sorbing and sorbed solutes. Solute competition should not occur with HDP-bent and HDTMA-bent because both of these sorb via absorption-dominated processes. In addition, both HDP-bent and HDTMA-bent were prepared from clean sources of natural bentonite, which should not contain significant quantities of unknown bound sorbates. Finally, although it is physically possible for solute complexation to organic matter or other sorbent-derived material to have caused some of the solids effects observed, it is not likely to have been a consequential contributing factor in the systems examined. Further, attempts to calibrate the solute complexation model to the data for TCE/HDP-bent and TCB/HDTMA-bent using reasonable values for the model coefficients were unsuccessful (24). Any solute loss that is not accounted for can contribute to observed solids effects, especially for systems in which small amounts of sorption occur. An uncorrected mass balance for determination of qe assumes no system losses,
and any losses are thus calculated as contributions to sorption. As Do is increased, more solute is sorbed; and the effect of system losses on calculated qe values would decrease (as would their effect on resulting KD values), thus potentially causing apparent solids effects. The data correction used for the present analysis (eq 1), however, is designed to account for system losses and, therefore, to lessen such artifactual effects. It is likely that the solids effects observed for HDTMAbent and HDP-bent are attributable to aggregation of sorbent particles, resulting in decreases in the amounts of organoclay accessible for sorption. The relatively hydrophobic organoclays tend to aggregate in aqueous systems due to attractive interactions between the hydrocarbon chains of the modifying QACs. Aggregation phenomena were in fact visibly evident during the initial stages of isotherm experiments with these sorbents, and aggressive mixing was required to effect their dispersion. It is possible that, as Do increased, the size of the aggregated organoclay particles that could be maintained would be limited as a result of particle breakup resulting from shear forces caused by the mixing provided in the isotherm bottles. The aggregation concept does not account for the observation that the minimum Do required to eliminate further solids effects for the hydrophobic organoclays varied by sorbate. It is not clear if this trend was a result of specific solute characteristics that affect the degree of sorption and/ or system losses (e.g., via impacts on the accuracy of the observed isotherm data), differential chemical effects of the various solutes on the sorbents (e.g., on the ability of the hydrophobic organoclays to aggregate), or both. However, no distinct relationship was observed between Do values required to essentially eliminate further the effects and corresponding percent solute removals. TMPA-bent and Ohio shale are not nearly as hydrophobic as HDTMA-bent and HDP-bent, and thus they disperse much better in solution, perhaps explaining why no significant solids effects were observed for these materials. While TMPAbent is mildly hydrophobic and might therefore aggregate to some extent, it would do so to a much lesser degree than would the long-chain organoclays. TMPA-bent was not examined at Do values as low as those used for HDTMAbent, and it may be that modest solids effects might be observed for TMPA-bent as well at lower Do values. Potential solids effects are an important consideration when comparing sorption results from differently conducted batch experiments and when applying batch sorption results to systems in which effective solids concentrations are significantly different; i.e., those involved in flow-through column studies or field-scale subsurface contaminant transport scenarios. It is possible that batch experimental sorption coefficients observed only at relatively low Do values may greatly overestimate sorption for systems having higher sorbent concentrations. Sufficiently high sorbent concentrations were used (i.e., above those at which further increases in Do did not result in a change in observed sorption capacity) in the work described here to ensure that measured sorption capacities should be reasonably representative of those that would occur in high-solids systems such as soil barriers. Cost Factors. In terms of practicality as a sorbent for largescale containment barrier applications, the shale material would clearly be the most economical. Bulk quantities of organoclays can cost from about $0.50/lb to as much as $2.50/ lb or more. Shale can be purchased from as low as $6-$12 per ton up to $50 per ton (i.e., $0.003-$0.025/lb), even after grinding to a particle size of
