Environ. Sci. Techno/. 1995, 29, 685-692
Introduction
Contaminants to SinQle-and Dual Organic Cation Bentonites from J A M E S A. SMITH* A N D A D I N A G A L A N + Department of Civil Engineering and Applied Mechanics, University of Virginia, Charlottesville, Virginia 22903-2442
Wyoming bentonite was modified by the exchange of one or two classes of quaternary ammonium cations for inorganic ions. Sorption of tetrachloromethane, trichloroethene, and benzene to the resulting organobentonites from water at 10 and 30 "C was studied. The "first class" of quaternary ammonium cations studied are characterized by short-chain alkyl or aryl functional groups and include tetramethyl-, tetraethyl-, and benzyltriethylammonium cations. The "second class" of quaternary ammonium cations studied are characterized by the following structure: (CH&-N+-R where R is a lo-, 12-, 14-, 16-, or 18-carbon alkyl chain. Nonionic solute sorption to bentonite exchanged with cations from the first class (in an amount equal to 40% of cation-exchange capacity) is caused primarily by adsorption. When a second organic cation from the second class is also exchanged onto this sorbent in an amount equal to 20, 40, or 60% of cation-exchange capacity, the longchain alkyl functional groups cause t w o effects. First, the alkyl chains interfere with nonionic solute adsorption to the bentonite mineral surface modified by the tetramethyl-, tetraethyl-, or benzyltriethylammonium cations. Second, the alkyl chains create a partition medium for sorption of the nonionic solutes. Solute uptake by adsorption decreases, and solute uptake by partition increases as the amount of organic cations from the second class exchanged onto the bentonite increases from 0 to 60% of cation-exchange capacity. Solute uptake by adsorption decreases, and solute uptake by partition increases as the length of the alkyl chain of the quaternary ammonium cation increasesfrom 12to 14 carbon atoms. In general, the reduction in solute uptake by adsorption caused by the long-chain alkyl functional groups is greaterthan the increased solute uptake by partition. Therefore, the net effect of the exchange of the second class of organic cations to the bentonite mineral surface is a reduction in nonionic solute sorption.
Wyoming bentonite is composed primarily of Na+-montmorillonite, an expandable, aluminosilicate clay mineral with a unit layer formula ofA13,5Mgo,sSis020(0H)4.It is a 2:l type of mineral, and its unit layer structure consists of one A13+ octahedral sheet placed between two Si4+tetrahedral sheets (1). In the tetrahedral sheets, A13+ can substitute for Si4+;in the octahedral sheets, Mg2+can substitute for A13+ (1). These isomorphous substitutions in the mineral lattice cause a net negative charge at the clay-mineral surface ( 1 ) . Typically, inorganic cations (e.g.,H+, Na+, Ca2+)offset this charge imbalance on the external and internal surfaces of montmorillonite crystals (2). In aqueous systems, water is intercalated into the interlamellar space of the montmorillonite, resulting in expansion (swelling)of the mineral (1). Adsorption of nonionic organic solutes from water to Wyoming bentonite is relatively weak because of the preferential attraction of polar water molecules to the mineral surfaces (3, 4). Wyoming bentonite typically has a low organic carbon content; therefore, nonionic solute sorption by partition into the soil organic matter is also relatively weak (3, 4). As a result, Wyoming bentonite is not a strong sorbent for the uptake of nonionic organic contaminants from water. Organobentonites are produced by the exchange of organic cations (typicallyhaving a quaternary ammonium structure) for inorganic ions (e.g., H+, Nat, Ca2+)on the internal and external mineral surfaces of bentonite (5).The mineral surfaces of the resulting organobentonite are organophilic (Le., the organic functional groups of the quaternary ammonium cations are not strongly hydrated by water). As a result, organobentonites are powerful sorbents for nonionic organic pollutants relative to conventional bentonite (3, 4, 6, 7 ) . Because of their unique sorption capabilities, organobentonites have been investigated for a wide variety of environmental applications. Smith et al. ( 7 ) have studied the use of organobentonites as components of earthen landfill liners and have determined that they can significantly improve the liner performance relative to conventional liners with regard to the transport of aqueous-phase nonionic organic contaminants. Harper and Purnell (8) have investigated the use of organoclays as adsorbents for the air sampling of airborne organic contaminants. Several researchers have studied the sorption of pollutants to natural soil modified by quaternary ammonium organic cations and have found that the sorption capacity of the soil for nonionic organic solutes can be increased significantly (3, 9). Other researchers have investigated in laboratory and field studies the feasibility of using soil exchanged with dodecylpyridinium cations (10) and hexadecyltrimethylammonium (HDTMA)cations (11)to retard the transport of nonionic organic contaminants in groundwater. Still other researchers have studied the modification of aluminum oxide (12) and ferrihydrite (13)with anionic surfactants. The modified sorbents were found to be effective at removing organic contaminants from solution and have been suggested for use in wastewater treatment + Present address: Hydrogeologic, 1165 Herndon Parkway, Herndon, VA 22070.
0013-936x/95/0929-0685$09.00/0
@ 1995 American Chemical Society
VOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
685
applications (12,131. Organoclays have also been used as chromatographic media for the separation of mixtures of organic vapors (14-16) and for hazardous waste stabilization (1 7, 18). Recently, Haggerty and Bowman (19) have studied the sorption of several inorganic anions, benzene, toluene, andxylene to zeolite modified by HDTMA cations. The environmental applications described above rely on an understanding of the mechanism of nonionic solute sorption to organically modified soil and clay. Recent research indicates that the molecular structure of the quaternary ammonium cations used to modify soil and clay affects both the magnitude and mechanism of nonionic solute sorption. Smith et al. (4) demonstrated that tetrachloromethane sorption to bentonites modified by small organic cations (e.g., quaternary ammonium cations with methyl, ethyl, or benzyl functional groups) was characterized by nonlinear isotherms, strong solute uptake, and competitive sorption, whereas tetrachloromethane sorption to bentonites modified by relatively large organic cations (e.g., quaternary ammonium cations with dodecyl, tetradecyl, or hexadecyl functional groups) was characterized by linear isotherms, relatively weak solute uptake, and noncompetitive sorption. These authors concluded that the observed differencesbetween the two groups of sorbents was attributable to different sorption mechanisms (adsorption for bentonite modified with quaternary ammonium cations with small functional groups and partition for bentonite modifiedwith quaternary ammonium cations with relatively large functional groups). The small organic cations (e.g., benzyltriethylammonium) create a relatively rigid, nonpolar surface amenable to nonionic solute uptake by adsorption (4, 7, 20-22). The larger organic cations (e.g.,HDTMA) create an organic partition medium through the conglomeration of their flexible alkyl chains (3-5, 7). Both of the classes of organobentonites described above (e.g.,organobentonites that remove organic contaminants from solution primarily by adsorption and organobentonites that remove organic contaminants primarily by partition) have advantages and disadvantages when used as sorbents for environmental applications. Organobentonites that function primarily as adsorbents generally exhibit strong solute uptake for a wide variety of organic contaminants, particularly at aqueous solute concentrations that are low relative to solubility (4,22). However, because the uptake mechanism is adsorption, the sorption of a single solute will be reduced in the presence of additional solutes (which is a common scenario for environmental contamination problems) as a result of competition between multiple solutes for the available adsorption sites. Another disadvantage is that certain organobentonite adsorbents (e.g.,tetramethylammonium-bentonite]only sorb certain organic contaminants (e.g.,benzene, tetrachloromethane) to an appreciable extent (20,221. Also, because adsorption isotherms are nonlinear, the ratio of sorbed-phase to aqueous-phase concentrations decreases as the aqueousphase solute concentration increases. Organobentonites that primarily sorb contaminants by a partition process do not exhibit competitive sorption effects ( 4 , 3 , and the ratio of sorbed-phase to aqueousphase concentrations does not decrease with increasing aqueous-phase solute concentration (owing to the approximate linearity of the isotherm). These characteristics make this class of organobentonites suitable for environmental applications involving multiple dissolved-phase contaminants at high concentrations relative to aqueous 686
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 3, 1995
solubility. However, for environmental applications involving only a small number of contaminants at low concentrations relative to aqueous solubility,the magnitude of solute sorption to these organobentonites is small relative to organobentonites that function primarily as adsorbents. Given that both classes of organobentonites described above have advantages and disadvantages, it is useful to consider the properties of a “hybrid”organobentonite that shares the characteristics of both classes. This hybrid organobentonite consists of a sample of Wyomingbentonite wherein some fraction of its cation-exchange capacity is satisfied by quatemary ammonium cations with short-chain alkyl or aryl functional groups (e.g.,tetramethylammonium, benzyltriethylammonium, etc.) and another fraction of the cation-exchange capacity is satisfied by quaternary ammonium cations with one long-chain alkyl functional group (e.g., dodecyltrimethylammonium, hexadecyltrimethylammonium, etc.). The resulting hybrid, or dual-cation organobentonite, would function primarily as a powerful adsorbent at low solute concentrations (relativeto solubility) and as a partition medium at high concentrations (relative to solubility) and would be useful for contamination scenarios involving only a few contaminants or a wide range of contaminants. This paper presents experimental results and analyses that quantify the equilibrium sorption of three nonionic organic contaminants to a variety of dual-cation organobentonites from water. The objective of this study is to determine how the molecular structure and amount of quaternary ammonium cations exchanged onto Wyoming bentonite affect the magnitude and mechanism of nonionic solute sorption.
Materials and Methods Wyoming bentonite was obtained from the American Colloid Company (sample TFS-81) and has been described previously (4). Briefly, the bentonite is 3.6% sand, 7.3% silt, and 89.1% clay. The predominant clay mineral is Namontmorillonite. The natural organic carbon content of the bentonite is 0.1%, and its cation-exchange capacity is 78.5 mequiv/ 100 g. The bromide salts of eight quaternary ammonium compounds were obtained from Aldrich Chemical Company and were used as received. Their molecular structures, abbreviations, and percent chemical purities are given in Figure 1. Tetrachloromethane, trichloroethene, and benzene were used as received from Aldrich Chemical Company and have chemical purities of 99%. Three 250pCi aliquots of [l4C1tetrachloromethane (DuPont NEN), [l4C1trichloroethene (Sigma), and [l4C1benzene (Sigma) were obtained. Their specific activities (and percent radiochemical purities in parentheses) are 3.4 mCi/mmol ( >99%)for [14Cltetrachloromethane,6.2 mCi/mmol(>98%) for [14Cltrichloroethene,and 53.4 mCi/mmol (’98%) for [14C]benzene. The 250-pCi aliquots of each radioisotope were combinedwith 3 mL of the corresponding nonlabeled organic liquid for use in all sorption experiments. The aqueous solubilities of tetrachloromethane, trichloroethene, and benzene are 800, 1100, and 1780 mg/L, respectively (23). Organobentonites were synthesized by combining Wyoming bentonite with an aqueous solution containing one or two quaternary ammonium cations. The volume of the aqueous solution was chosen to ensure complete solubilization of the quaternary ammonium cations. The total mass of each cation in the aqueous solution was determined
TABLE 1
Organohentonite Sorbents and Their Theoretical Organic Carbon Contentsa Tetramethylammonium W A )bromide. 99 Tevaethylammonium(TEA) bromide.99 percent purity percent punty
Benzyltriethylammonium(BTEA) bromide, 99 percent punty
[.3-"'H3]
C12H25
'Br.
~ c y ~ t r i m e t h y h m o " (DmA) bromide, 99 percent punty
[..;"'.I C14H29
'81.
Dodecyltrimethylammonium (DDTMA) Temdecyltrimethylammonium("4) bromide, 99 percent purity bromide. 99 percent punty
k3-qyCH3]
'91-
[..3-r>H3] C18"37
'Br.
C16H33
Hexadecyltrimethylammonium (HDTMA) Octadecyltrimethylammonium(ODTMA) bromide, 99 percent purity bromide, 99 percent purity
FIGURE1. Molecular structure diagrams, abbreviations,and percent chemical purities of eight quaternary ammonium salts.
based on the desired organic carbon content of the final organobentonite and the assumption that exchange of the organic cations onto the bentonite is essentially 100% if the number of added cations is less than or equal to the cation-exchange capacity of the Wyoming bentonite. The bentonite-water slurrywas mixed for 30 min; the bentonite and water were separated by evaporation, and the resulting organobentonite was gently groundwith amortar and pestle to break up aggregate particles. The organobentonite was stored in a glass container until needed for use in isotherm experiments. A total of 20 organobentonites was synthesized as described above. In some cases, only a single type of organic cation was exchanged onto the bentonite. The resulting organobentonite is identified by a prefur that states the percent of the bentonite's cation-exchangecapacity satisfied by the organic cation followed by the abbreviation for the specific type of organic cation (Figure 1). For example, 40BTEA identifies an organobentonite that has 40% of its cation-exchange capacity satisfied by benzyltriethylammonium cations. In other cases, dual types of cations were exchanged simultaneously onto the bentonite. The resulting organobentonites are identified by two percentages of cation-exchange capacity and two abbreviations of the specific types of organic cations. For example,40BTEA/ 20DDTMA identifies an organobentonite that has 40% of its cation-exchange capacity satisfied by benzyltriethylammonium cations and an additional 20% satisfied by dodecyltrimethylammonium cations. Because the molecular structures of the organic cations exchanged onto the Wyoming bentonite are precisely known, the theoretical organic carbon contents of the organobentonites can be calculated assuming complete exchange of the organic cations onto the bentonite. Table 1 lists the single- and dual-cation organobentonites used in this study along with their associated theoretical organic carbon contents. The total carbon and inorganic carbon contents of a subset of these organobentonites and the untreated Wyoming ben-
a
organobentonite sorbent
Ql~ organic carbon
40TEA 40TMA 40BTEA 40DDTMA 40HDTMA 40TEN20TDTMA 40TEN40TDTMA 40TEN6OTDT M A 40TMN20HDTMA 40TMN40HDTMA 40TMN60HDTMA 40BTEN20DTMA 40BTENZODDTMA 40BTEN40DDTMA 40BTEN60DDTMA 40BTEN20TDTMA 40BTEN20HDTMA 40BTEN40HDTMA 40BTEN60H DTMA 40BTEN200DTMA
2.93 1.53 4.60 (4.83) 5.22 6.53 (6.69) 5.97 8.72 11.22 4.92 7.96 10.70 6.60 (6.75) 6.91 (7.63) 9.02 (9.55) 10.97 (11.65) 7.21 (6.57) 7.49 (7.58) 10.15 (9.72) 13.77 7.81 (8.35)
Values in parentheses are the measured organic carbon contents. ~~
tonite were measured by Huffman Laboratories, Golden, CO. Organic carbon contents were calculated by subtracting the inorganic carbon content from the total organic carbon content. These values are also reported in Table 1. Sorption of tetrachloromethane, trichloroethene, and/ or benzene to the organobentonites listed in Table 1 was measured using a batch equilibration method. Varying amounts of sorbent, water, and 14C-labeledorganic solute were combined in 15-mL(nominalvolume)glass centrifuge tubes with Teflon-lined caps. Sorbent masses ranged from 1.0 to 3.0 g and were selected so that greater than 30% of the total solute mass in each centrifuge tube was sorbed to the organobentonite at equilibrium. Water volumes ranged from 14 to 15 mL and were chosen so that the headspace in each centrifuge tube was less than 0.25 mL. Organic liquid volumes ranged from 2 to 90 p L and were chosen so as to result in equilibrium aqueous solute concentrations ranging from approximately 10 to 80% of aqueous solubility for each solute/sorbent pair. Following the combination of sorbent, water, and solute in centrifuge tubes, the tubes were shaken at either 10 or 30 "C in the dark for 48 h. Previous kinetic experiments using equilibration times ranging from 15 min to 30 d have shown that equilibrium is reached in approximately 12 h (4). The tubes were centrifuged at 2000g (at a temperature equal to the incubation temperature) for 45 min to separate the water and soil phases; 0.5 mL of the aqueous supematant in each tube was transferred to 5 mL of scintillation cocktail, and the sample was analyzed with a Packard 1900TRliquid scintillation analyzer. The measured radioactivity was related to aqueous concentration by a standard curve. The sorbed concentration of the solute was then calculated by the difference. For each sorption isotherm, three additional centrifuge tubes were handled similarly to the above-described tubes for quality assurance. Two of these tubes contained water, 14C-labeledsolute, and no organobentonite. These tubes were used to quantify solute losses caused by processes other than sorption of the organoclay (volatilization, VOL. 29, NO. 3,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1687
TABLE 2
Noniirear Begression Data for Sorption Isothermsa sorbent
solute
temp ("C)
40TEA
TCE
10
40TEN20TDTMA
TCE
10
40TEN40TDTMA
TCE
10
40TEN6OTDTMA
TCE
10
40BTEA
TCE
10
40BTEN20HDTMA
TCE
10
40BTEN40HDTMA
TCE
10
40BTEN60HDTMA
TC E
10
40TMA
TCE
10
40TMN20HDTMA
TCE
10
40TMN40HDTMA
TCE
10
40TMN60H DTMA
TCE
10
4OBTENZODDTMA
TCE
10
40BTEN40DDTMA
TCE
10
40BTENGODDTMA
TCE
10
40BTEN20DTMA
TCE
10
40BTEN200DTMA
TCE
10
40BTEA
benzene
10
40BTEN20DDTMA
benzene
10
40BTEN40DDTMA
benzene
10
40BTENGODDTMA
benzene
10
40BTEN20DTMA
benzene
10
nonlinear regression parameters
asymptotic standard error
K = 736.7 n = 0.5415 K = 43.21 n=0.7114 K = 50.29 n = 0.5445 K = 232.4 n = 0.4325 K = 1616 n = 0.3905 K = 341.4 n = 0.4069 K = 22.52 n = 0.8205 K = 47.50 n = 0.7901 K = 1.003 n = 1.568 K = 4.422 n = 0.9794 K = 2.705 n = 1.035 K = 9.770 n = 0.9619 K = 964 n = 0.3403 K = 9.782 n = 0.9382 K = 4.994 n = 1.096 K = 359.3 n = 0.521 1 K = 56.72 n = 0.7399 K = 3707 n = 0.2867 K = 1667 n = 0.334 K = 35.46 n = 0.8820 K = 2.456 n = 1.325 K = 2670 n = 0.2492
132.1 0.0343 27.17 0.1033 43.58 0.1384 114.1 0.08228 405.6 0.04516 156.2 0.07424 14.77 0.1043 34.77 0.1216 0.3073 0.055 17 3.459 0.1248 1.210 0.07101 5.681 0.09839 224.7 0.03868 9.663 0.1444 0.8885 0.02825 82.02 0.04084 18.94 0.0537 765.4 0.03363 215 0.0 1959 10.22 0.04303 1.220 0.07564 207.5 0.01 18
temp sorbent
solute
(OCl
40BTEN20TDTMA
benzene
10
4OBTENZOHDTMA
benzene
10
40BTEN200DTMA
benzene
10
40HDTMA
benzene
10
40BTEN40HDTMA
benzene
10
40DDTMA
benzene
10
40BTEN40DDTMA
benzene
10
40BTEA
CCl4
30
40BTEN20DDTMA
CCl4
30
40BTEN40DDTMA
cc14
30
40BTENGODDTMA
CCl4
30
40BTEA
cc14
10
40BTEN20DDTMA
CCl4
10
40BTEN40DDTMA
CCl4
10
40BTEN6ODDTMA
cc14
10
40BTEN20HDTMA
CCl4
10
40BTEN40HDTMA
cc14
10
40BTENGOHDTMA
CCll
10
40BTEN20DTMA
CCl4
10
40BTEN20TDTMA
CCl4
10
40BTEN200DTMA
CCll
10
nonlinear regression parameters
asymptotic standard error
K = 3.661 n = 1.209 K = 13.36 n = 1.060 K = 57.22 n = 0.866 K = 3.176 n = 1.169 K = 29.50 n = 0.9789 K = 0.5034 n = 1.093 K = 35.46 n = 0.882 K = 1578 n = 0.4137 K = 635 n = 0.3947 K = 0.4989 n = 1.401 K = 2.718 n = 1.208 K = 3836 n = 0.2775 K=948 n = 0.3463 K = 86.23 n = 0.529 K = 0.9263 n = 1.3456 K = 124.1 n = 0.5153 K = 7.028 n = 1.034 K = 1.279 n = 1.438 K = 699.9 n = 0.3761 K = 218.9 n = 0.426 K = 111.2 n = 0.5776
4.723 0.1774 10.79 0.1135 30.51 0.07564 1.194 0.06131 12.44 0.06619 1.288 0.3497 10.22 0.04303 182.3 0.0203 79.86 0.021 1.638 0.4998 3.969 0.2333 1300 0.05907 158.3 0.0284 29.35 0.05541 1.037 0.1753 42.43 0.05449 16.82 0.3129 0.8268 0.1069 100.8 0.02393 138.5 0.1013 54.35 0.07729
a The table gives regression parameters and their asymptotic standard errors for the Freundlich equation. TCE = trichloroethene; CCI4 = tetrachloromethane.
biodegradation, etc.). The concentrations of tetrachloromethane, trichloroethene, and benzene in these tubes were approximately400,400, and 1100 mg/L, respectively. In almost all cases, solute recovery from these tubes was greater than 94%. If solute recovery was less than 90%, the isotherm experiment was repeated. Percent recovery data from these blanks were used to adjust the added mass of solute in tubes containing soil in order to calculate the sorbed solute concentration. The third tube contained water, organobentonite, and no 14C-labeledsolute. This tube was used to quantify the background radiation and to identify if the glassware, organobentonite, or water was contaminatedwith radioactivity. In all cases, the measured radiation in these tubes was at normal, background levels (less than 30 dpm).
Results Statistical comparison of the theoretical and measured organic carbon contents inTable 1 using a one-sided paired 688
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 3, 1995
t-test indicated that essentially 100%of the added organic cations are exchanged onto the bentonite ( p = 0.05). This observation is consistent with results reported by other researchers for similar clay-organic cation systems (4,24, 25).
All experimental batch sorption data were fit to the Freundlich sorption model using nonlinear least-squares regression. The Freundlich equation is empirical and has the following form: C, = KC,"
where C, is the equilibrium sorbed concentration of the solute (mg/kgl , C, is the equilibrium aqueous concentration of the solute (mglL), and Kand n are the fitted parameters determined from nonlinear regression. Table 2 presents the results of the nonlinear regression analyses for the Freundlich isotherm model for the different solute-organobentonite combinations considered in this
QOBTEA
40BTEA
X QOBTEAl40HDTMA
8t 40BTEN40DDTMA
o 40BTEN20DDTMA
i)
A 40BTEN60DDTMA
25000 2
m
15000
loo00 5000 0 0
200 400 600 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
800
0
200 400 600 800 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
1000
0
300 600 900 1200 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
1500
100 200 300 400 500 600 700 800 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
0
FIGURE 2. Benzene sorption to I B T E A , I H D T M A , and I B T W 40HDTMA from water at 10 "C.
research. The Langmuir equation was also used to quantitatively describe each isotherm, and the goodness-of-fit of each isotherm model (Freundlich and Langmuir) was evaluated by comparison of the magnitude of the residual sum of squares (RSSI (26). In general, both models provided equally good fits of the experimental data and therefore only the Freundlich parameters are reported in Table 2. Table 2 also gives the asymptotic standard error for each fitted parameter. Figures 2-5 graphically depict a subset of the isotherm data from Table 2. The isotherms in Figure 2 quantify benzene sorption to 40BTEA, 40HDTMA, and 40BTEAI 40HDTMA from water at 10 "C. Benzene sorption to 40BTEA is characterized by relatively strong solute uptake and isotherm nonlinearity. By contrast, benzene sorption to 40HDTMA is much weaker, and the isotherm is approximatelylinear over awide range of equilibrium aqueous concentrations. The magnitude of benzene sorption to 40BTEA/40HDTMA is intermediate to the other two isotherms, even though the organic carbon content of the sorbent is greater than 40BTEA or 40HDTMA. Results similar to those shown in Figure 2 were also observed for benzene sorption to 40BTEA, 40DDTMA, and 40BTEAI 40DDTMA (Table 2). Figure 3 shows isotherm data for tetrachloromethane (A), trichloroethene (B), and benzene (C) sorption to 40BTEA, 40BTEA/20DDTMA, 40BTEA/40DDTMA, and 40BTEA160DDTMA from water at 10 "C. Sorption of all three solutes to 40BTEA is characterized by relativelystrong solute uptake and isotherm nonlinearity. As the amount of DDTMA cations exchanged onto the bentonite increases from 0 to 40% of cation-exchange capacity, the magnitude of solute sorption decreases, and the isotherm shape becomes increasingly linear or slightly concave up. When the amount of DDTMA cations is increased from 40 to 60% of cation-exchange capacity, the magnitude of sorption is increased slightly. Results similar to those shown in Figure 3 were also observed for tetrachloromethane sorption to dual-cation organobentonites exchanged with BTEAI HDTMA cations and for trichloroethene sorption to dualcation organobentonites exchanged with the followingpairs of organic cations: TEAITDTMA,BTEAIHDTMA,andTMAI
16000 12000 8000 4000 0
3oo00 25000 2 m 15000
loo00 5000 0
FIGURE3. Tetrachloromethane(A), trichloroethene(B),and benzene
(Cl sorption to 40BTEA. 40BTEmDDTMA. IBTEA/40DDTMA, and 40BTEWDDTMA from water at 10 "C.
HDTMA (Table 2). (Each of the pairs of organic cations listed above correspond to four sorbents. For example, TEAITDTMA refers to 40TEA, 40TEA/20TDTMA, 40TEA/ 40TDTh4A, and 40TEA160TDTMA). The results depicted in Figure 3 were also observed for tetrachloromethane sorption to 40BTEA, 4OBTEA/ZODDTMA,40BTEA/40DDTMA, and 40BTEAI60DDTMA from water at 30 "C (Table 2). Figure 4 shows isotherm data for tetrachloromethane (A), trichloroethene (B), and benzene (C) sorption to five organobentonites fromwater at 10 "C. These sorbents have 40% of their cation-exchange capacities satisfied by BTEA cations and 20% of their cation-exchange capacities satisfied by one of the following organic cations: DTMA, DDTMA, TDTMA, HDTMA, or ODTMA. For tetrachloromethane and trichloroethene sorption isotherms, organobentonite sorVOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
m
689
40BTEM2ODTMA 0 40BTEM20DDTMA
40BTEN200DTMA
25000
40BTEM20TDTMA
1(
n 30°C
lDoo0
8000
I
l " " " "
A Tetrachloromethane
2
82
+
I
' t 10°C
A 40BTEN20HDTMA
8000
4000
0
P
-0
200
loo00
200 400 600 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
600
800 I
t 0
400
0
200
400
600
800
8o00,
I
n
800
1B Trichloroethene
2000 40B'IEMODDThfA 0
400 600 800 0 200 400 600 800 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
200
FIGURE 5. Comperison of tetrachloromethane sorption to MBTEA, 40BTEA/2ODDTMA, 40BTWMDDTMA. and 40BTEAMDDTMA from water at 10 and 30 "C.
0
T""""
3 2
200 400 600 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
800
7
m
from water. For 40BTEA and 40BTEA/20DDTMA, an increase in temperature from 10 to 30 "C results in a decrease in the magnitude of tetrachloromethane sorption. For 40BTEA/40DDTMA and 40BTEA/GODDTMA, an increase in temperature from 10 to 30 "C results in an increase in the magnitude of tetrachloromethane sorption.
Discussion
m
0 0
300 600 900 1200 1500 AQUEOUS CONCENTRATION, MILLIGRAMS PER LITER
1800
FIGURE4. Tetrachloromethane(A), trichloroethene(B), and benzene (C) sorption to MBTEALMDTMA, QOBTEA/20DDTMA, M B T W 20TDTMA, MBTEA/20HDTMA, and MBTEA/20ODTMA from water at 10 "C.
bents exchanged with DTMA and DDTMA exhibit greater uptake of the solutes than sorbents exchangedwith TDTMA, HDTMA, or ODTMA. This observation also applied to benzene sorption to the five organobentonites at aqueous concentrations less than 500 mg/L. At aqueous benzene concentrations greater than 500 mg/L, isotherms for 40BTEA/20TDTMA, 40BTEW20HDTh4.4, and 40BTEA/ 200DTMA are concave up and the magnitude of benzene sorption is greater than for 40BTEA/20DTMA or 40BTEAI 20DDTMA. The graphs in Figure 5 quantlfy the effect of temperature on the sorption of tetrachloromethane to 40BTEA, 40BTEAI 20DDTMA, 40BTEA/40DDTMA, and 40BTEA/GODDTMA 690 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 3, 1995
The sorption isotherm data in Table 2 and the graphs in Figures 2-5 illustrate several important points about the sorption of nonionic organic solutes to dual-cation organobentonites. The isotherms in Figure 2 indicate that benzene sorption to 40BTEA is caused primarily by adsorption (as evidenced by the nonlinear isotherm and the relatively strong solute uptake). By contrast, benzene sorption to 40HDTMAis caused primarily by solute partition between water and the organic phase composed of the large alkyl functional groups of the HDTMA cations. The results of experiments quantifying benzene sorption to 40BTEA140HDTM.A (Figure 21 indicate that the two sorption mechanisms described above for BTEA-modified bentonite (adsorption) and HDTMA-modified bentonite (partition) are not additive. In other words, the ordinates of the 40BTEA and 40HDTMA isotherms in Figure 2 do not sum to give the ordinates of the 40BTEA140HDTMA isotherm (for a given abscissa value). The magnitude of benzene sorption to 40BTEA is reduced by the addition of HDTMA cations in an amount equal to 40% of cationexchange capacity, despite the fact that 40BTEA/40HDTMA has a greater organic carbon content than 40BTEA (Table 1). Presumably, the alkyl chains of the HDTMA cations interfere with adsorption sites created by the BTEA cations and thereby reduce solute uptake by adsorption. Although the alkyl chains of the HDTMA cations form a partition medium for solute uptake by adsorption,the magnitude of
the increase in solute uptake by partition is less than the magnitude of decrease in solute uptake by adsorption caused by the HDTMA cations. As a result, the magnitude of benzene sorption to 40BTEA/40HDTMA is less than the magnitude of benzene sorption to 40BTEA. At low aqueous concentrations relative to solubility (e.g., 0-250 mg/L), the isotherm shape for benzene sorption to 40BTEAl40HDTMA is slightly concave down, suggesting that solute adsorption is an important uptake mechanism in this concentration range. At higher aqueous concentrations, the sorption isotherm becomes more linear and even exhibits slight upward curvature. This behavior suggests that, in this higher aqueous concentration range, the relative contribution of solute uptake by partition to the total solute uptake has increased compared to lower aqueous concentrations. These concepts are further supported by the isotherm data in Figures 3-5. In Figure 3, sequentially increasing the fraction of the cation-exchange capacity of 40BTEA that is occupied by DDTMA cations from 0 to 40% results in a decrease in the sorption of tetrachloromethane, trichloroethene, and benzene and a increase in the linearity of the isotherm. As the amount of DDTMA cations exchanged onto the sorbent increases from 0 to 40% of cation-exchange capacity, the reduction in solute adsorption (caused by interference of the 12-carbonalkyl chains) is greater than the increased solute uptake by partition (caused by the increased mass of DDTMA cations). For 40BTEAl60DDTMA, there is a slight increase in solute sorption relative to 40BTEAl40DDTMA. The increased sorption observed by increasing the amount of DDTMA cations from 40 to 60% of the cation-exchange capacity indicates that the increased solute uptake by partition is greater than the reduction in solute uptake by adsorption for this case. The isotherm data in Figure 4 indicate that increasing the length of the long-chain alkyl functional group (from 10 to 18 carbons) also decreases adsorption of tetrachloromethane and trichloroethene (and benzene, at aqueous concentrations less than 500 mg/L) despite the fact that the number of long-chain cations remains constant (and equal to 20% of cation-exchange capacity). For example, tetrachloromethane sorption to 40BTEAl20DTMA and 40BTEAl20DDTMA is significantly greater than tetrachloromethane sorption to 40BTEA/20TDTMA,40BTEAl20HDTMA, and 40BTEAl200DTMA (Figure4, graph A). Therefore, quaternary ammonium cations with relatively long-chain alkyl functional groups (e.g., tetradecyl, hexadecyl, and octadecyl) cause a greater reduction in solute uptake by adsorption to BTEA-modified bentonite than quaternary ammonium cations with comparatively smaller alkyl functional groups (e.g., decyl and dodecyl). The precise mechanism for the reduction in solute uptake by the long-chain functional groups of DTMA, DDTMA, TDTMA, HDTMA, and ODTMA cations is uncertain. One possibility, however, is that the alkyl chains of the quaternary ammonium cations may compete for adsorption sites with the nonionic solute. Therefore, for organobentonites with lower surface coverage of quaternary ammonium cations, the interference of the alkyl chains on solute adsorption may be less pronounced. For example, at lower surface coverages, the quaternary ammonium cations will, on average, be spaced farther apart in the interlamellar space of the bentonite; the alkyl chain of an HDTMA cation may not be able to extend to an adsorption site created by one or more neighboring BTEA cations. For
these cases, the magnitude of solute sorption to dual-cation organobentonites may be predictable by adding the sorption isotherms for the two corresponding single-cation organobentonites. This hypothesis is consistent with the observation that longer alkyl chains (e.g., tetradecyl, hexadecyl, and octadecyl) create a greater reduction in sorption to 40BTEA than shorter alkyl chains (e.g.,decyl and dodecyl) (Figure 4). It is also interesting to note the isotherms in graph C of Figures 3 and 4 that describe benzene sorption to 40BTEAl GODDTMA, 40BTEA/20TDTMA, 40BTEA/20HDTMA, and 40BTEAl200DTMA. At aqueous benzene concentrations greater than about 500 mglL, the isotherms showa distinct upward curvature. This upward curvature was also observed for several tetrachloromethane and trichloroethene sorption isotherms. (These isotherms can be quickly identified in Table 2 as having values of the Freundlich exponent, n, greater than unity.) In general, the upwardcurving isotherms were associated with sorbents having large fractions of their cation-exchange capacity (e.g., greater than 40%) satisfied by quaternary ammonium cations with alkyl chains greater than 12 carbon atoms in length and/or sorbents with 20% or more of their cationexchange capacity exchanged with quaternary ammonium cations with alkyl chains greater than 14 carbon atoms in length. At high aqueous concentrations relative to solubility, the partition of the solute into the organic medium formed by the alkyl chains of the quaternary ammonium cations increases the solvency of the medium for the solute, resulting in upward curvature of the sorption isotherm. The presence of quaternary ammonium cations with small functional groups (e.g., TEA, BTEA) and solute that is adsorbed in the interlamellar space probably also contribute to the changing solvency of the organic medium. The benzene sorption isotherms in graph C of Figure 4 for 40BTEAl20TDTIvL4, 40BTEA/20HDTMA, and 40BTEAl 200DTMA are particularly noteworthy in that the magnitude of benzene sorption is among the highest values ever reported for organobentonites, particularly at aqueous concentrations greater than 50% of solubility. Finally, the temperature-dependent isotherm data in Figure 5 provide additional support to the hypothesis that the increased substitution of quaternary ammonium cations with relatively long-chain (greater than 10 carbon atoms) alkyl functional groups onto 4OBTEA, 40TEA, or 40TMA causes a reduction in solute uptake by adsorption and causes an increase in solute uptake by partition. For tetrachloromethane sorption to 40BTEA, the magnitude of sorption is reduced as the temperature is increased from 10 to 30 "C. The corresponding exothermic heat of sorption (approximately ranging from -7 to -5 kcallmol based on analysis using a Clausius-Clapeyron-type relation (27))is typical of a physical adsorption process. As the fraction of the bentonite's cation-exchange capacity satisfied by DDTMA cations increases from 0 to 60, the corresponding heats of sorption become less negative and eventually become weakly endothermic (approximately 1.0-2.1 kcall mol for 40BTEA/GODDTMA). Qualitatively, this is evident from the observation that the magnitude of tetrachloromethane sorption to 40BTEA/GODDTMAis increased from 10 to 30 "C. The low, slightly endothermic heats of sorption observed for this latter sorbent are characteristic of a partition process. VOL. 29, NO. 3, 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Conclusions The dual-cation organobentonites studied in this research have unique properties with regard to the sorption of nonionic organic solutes that have not been reported previously. These organobentonites have a two-component functionality with regard to nonionic solute sorption from water. Quaternary ammonium cations with short-chain alkyl or aryl functional groups (e.& TMA, TEA, or BTEA cations) create a component of the organobentonite that functions as an adsorbent in the uptake of nonionic solutes from water. Quaternary ammonium cations with longchain functional groups (e.g., DTMA, DDTMA, TDTMA, HDTMA, or ODTMA cations) create a component of the organobentonite that functions as a partition medium in the uptake of nonionic solutes from water. For the range of surface coverages of the organic cations studied (expressed as a percent of cation-exchange capacity),the adsorption and partition components of the dualcation organobentonite sorbents do not function independently. For example, for a given equilibrium aqueous benzene concentration, the concentration of benzene sorbed to 40BTEA/40HDTMA is less than the sum of the concentrations of benzene sorbed to 40BTEA and 40HDTMA. The long-chain functional groups of the DTMA, DDTMA, TDTMA, HDTMA, or ODTMA cations interfere with nonionic solute adsorption to the TMA-,TEA-,or BTEAmodified mineral surfaces of the bentonite, and the extent of interference increases as the fraction of the bentonite's cation-exchange capacity satisfiedby the long-chain cations increases from 0 to 60%. Although long-chain alkyl functional groups interfere with solute adsorption, they increase solute uptake by partition. Presumably, the conglomeration of the flexible alkyl chains creates a partition media for nonionic solute sorption. For many of the sorbents studied in this research, the decrease in solute uptake by adsorption is greater than the increase in solute uptake by partition caused by the long-chain alkyl functional groups of the DTMA, DDTMA, TDTMA, HDTMA, or ODTMA cations. For lower surface coverages of the quaternary ammonium cations on the Wyoming bentonite, the interfering effect of the long-chain alkyl functional groups on nonionic solute adsorption may be less pronounced. Dual-cation organobentonites hold promise as useful, general purpose sorbents for a variety of environmental applications. For dilute contaminant concentrations (relative to aqueous solubility)and only a few constituents, these organobentonites will act as powerful adsorbents relative to conventional bentonite. For high contaminant con-
692 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 3,1995
centrations (relative to solubility) and for multiple contaminants, the dual-cation organobentonite will function primarily as a partition media and will not be affected by competitive sorption limitations.
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Received for review J u n e 21, 1994. Revised manuscript received November 17, 1994. Accepted November 28, 1994.@
ES940403X @Abstractpublished in Advance ACS Abstructs, January 1, 1995.
