[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

16 Effects of Humic Acid on the Adsorption of Tetrachlorobiphenyl by Kaolinite

Gregory A. Keoleian and Rane L . Curl

Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109

Humic acids (HA) can affect the partitioning of organic contaminants between aqueous and sediment phases by complexation in solution and adsorption of the contaminant-HA complex to a mineral surface. The objective of this work was to elucidate these and other mechanisms, using 14C-radiolabeled 2,2' ,4,4'-tetrachlorobiphenyl (TeCB), a filtered humic acid preparation, and two natural kaolinites (KGA). The binary interactions, TeCB with KGA, HA with KGA, and TeCB with HA, were studied experimentally at 25 °C and pH 6.9. Isotherms were measured for the TeCB-HA-KGA multicomponent system to evaluate adsorption partition coefficients. The data were fitted satisfactorily, within estimated limits of uncertainty, by a model that assumes noncompetitive TeCB and HA adsorption and the same binding constant between free TeCB and dissolved or adsorbed HA.

H UMIC SUBSTANCES PLAY AN IMPORTANT ROLE as macromolecular vectors

for the transport of hydrophobic organic contaminants (HOC) in groundwater and surface water. In natural waters, H O C can exist free or in a bound state associated with dissolved humic substances, aquatic organisms, suspended particulates, and other colloidal matter. The binding or association of H O C with dissolved humic substances has been measured for many systems (1-12).

The equilibrium binding constant, K A B , relates the concentration of the pollutant (A) in the bound state, C A B ; the concentration of A in the free state,

0065-2393/89/0219-0231$06.00/0 © 1989 American Chemical Society

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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

232 AQUATIC HUMIC SUBSTANCES

C A ; and the concentration of humic substance (let Β represent fulvic acid and/or humic acid) as dissolved organic carbon, C B .

C A B = K A B C B C A (1)

The humic substances can enhance the total concentration of H O C in solution by as much as the factor KABCB and, consequently, can strongly influence H O C migration from hazardous waste sites, agricultural runoff, and other sources.

The fate of this H O C - h u m i c acid (AB) complex has not been fully investigated. McCarthy and Jimenez (8) studied the dissociation of the A B complex and observed that the binding of benzo[a]pyrene to dissolved humic material was completely reversible. Other studies were concerned with the degradation of the pollutant that is solubilized or complexed by humic substances (J, 3).

An important mechanism that can immobilize H O C in an aqueous environment is adsorption of a H O C - h u m i c (AB) complex on a solid surface. The effect of humic substances and other dissolved organic matter on the sorption of H O C by natural sorbents has been studied by Hassett and Anderson (13), Caron et al. (14), Brownawell and Farrington (15), and Baker et al. (16). They observed a reduction in the solid-phase partition coefficient as dissolved organic carbon was added to HOC-natural sorbent systems. This effect was attributed to an enhancement in the aqueous phase H O C chemical activity by the factor K A B C B , which leads to the following relation for the apparent partition coefficient:

1 + K A B C B

where K A is the linear adsorption constant in the absence of dissolved organic carbon. The mechanism of A B complex immobilization, however, was not studied experimentally by the investigators cited. It is a major objective of the present work.

Generalized Mechanistic Description of Interactions The possible mechanisms of interaction between H O C , humic acid, and natural sorbent systems are illustrated in Figure 1. This interaction represents a complex network of equilibria further complicated by the heterogeneous nature of the sorbent and humic acids. Naturally occurring organic matter indigenous to soils and sediments can desorb or dissolve into the aqueous phase and complex with H O C (17). These "implicit adsorbates" (18) may also compete for sites on the sorbent surface; in both scenarios the true sorption behavior may be misrepresented if these effects are not identified.

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16. KEOLEIAN & CURL Adsorption of Tetrachlorobiphenyl by Kaolinite 233

Sorbent Aqueous Phase

adsorption sites

Figure 1. Possible mechanisms of interaction between a model sorbate, humic acid, and a natural sorbent.

The mechanism for the interaction of H O C with soils is still unresolved; Chiou et al. (19) and others support an absorption or partitioning process, whereas Mingelgrin and Gerstl (20) and Maclntyre and Smith (21) conclude that the process is adsorption. The unclear distinction between absorption and adsorption arises in part from the difficulty in distinguishing between a sorbent surface and sorbent phase for the organic matrix of soil.

Scope of This Investigation

The complete set of mechanisms that are represented in Figure 1 would be difficult to resolve through experimental investigation. Selecting a non-

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swelling and nonporous mineral such as kaolinite precludes absorption of the model sorbate and reduces the complexity of the investigation. In addition, implicit adsorbate effects are not possible with a mineral adsorbent free of organic carbon.

Criteria for selection of a model adsorbate included nonionic organic compound, relatively high aqueous-phase activity coefficient, chemical stability (resistance to chemical or microbial degradation), and environmental significance. On the basis of these criteria, 1 4C-radiolabeled 2,2',4,4'-tetrachlorobiphenyl (TeCB) was chosen for this study.

Aldrich humic acid was used after purification by membrane filtration. Malcolm and MacCarthy (22) have reported, on the basis of 1 3 C N M R spectra, that commercial humic acids are not representative of soil and aquatic humic acids. Gauthier et al. (12), however, measured partition coefficients (normalized to the fraction of organic carbon) for pyrene and 14 different humic substances, including untreated Aldrich humic acid (HA). They found that K A B varied by as much as a factor of 10, depending on the source of humic material. The K A B measured for the Aldrich humic acid was within the range of values reported for the other soil humic acids studied.

Sorption data at 25 °C were measured and isotherms were deduced for the interactions between the TeCB-humic acid, TeCB-kaolinite, and humic acid-kaolinite systems. Equilibrium parameters for these isotherms were used in multicomponent sorption models to describe TeCB partitioning in the TeCB-humic acid-kaolinite system. Model predictions are compared with experimental multicomponent sorption data to choose between models and provide a basis for further refinement.

Adsorption of Tetrachlorobiphenyl by Kaolinite Anomalous results for adsorption partition coefficients have been observed for many HOC-c lay mineral systems (23, 24). The adsorbent-concentration effect (dependence of the partition coefficient on the clay concentration of a suspension) and adsorption-desorption hysteresis are the two most significant apparent anomalies examined in the soil science and environmental science literature. In some cases that exhibit an adsorbent-concentration effect and adsorption-desorption hysteresis, experimental artifacts and other complex physicochemical phenomena may have been misinterpreted as anomalous sorption behavior (17, 25, 26). With this perspective in mind, the adsorption experiments were designed with a special emphasis on control and TeCB mass accountability.

Experimental Details. The 14C-radiolabeled 2,2'4,4'-tetrachlorobi-phenyl (TeCB) was purchased from Pathfinder Laboratories with a specific activity of 10.5 mCi/mmol and a chemical and radiochemical purity of greater than 98%. Two natural kaolinites (KGA) from the Source Clays Repository at the University of

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Missouri, one well crystallized (KGA-1) and the other poorly crystallized (KGA-2), were studied. Surface properties and other characteristics of these kaolinites are given in Table I; additional physical and chemical characteristics are available from van Olphen and Fripiat (27). Type I deionized water, which was produced from a microfilter system (Millipore Corporation, Milli-R/Q), was glass-distilled from permanganate. This reagent water was buffered with 0.0025 M KH 2P0 4 and 0.0025 M NaHP04 to maintain a pH of 6.9. All glassware was baked in an oven at 600 °C to remove residual organics, except for volumetric glassware and 50-mL and 150-mL Corex centrifuge tubes used as sorption vessels. These items were cleaned with chromic-sulfuric acid cleaning solution and thoroughly rinsed with deionized water. The centrifuge tube caps were lined with aluminum foil discs that were cleaned with acetone and baked.

TeCB-KGA adsorption isotherms were measured by both a modified aqueous difference method and a direct extraction method. For both methods, TeCB material balance calculations were corrected for adsorption of TeCB by the vessel. Stock aqueous TeCB solutions were prepared by injecting TeCB dissolved in hexane into a glass bottle, allowing the hexane to evaporate, adding phosphate buffer solution, and agitating overnight. The 50-mL Corex tubes were charged with 50 mL of stock TeCB solution, with a Class A volumetric pipet. The tubes were agitated in a ther-mostatted shaker bath (150 cycles/min) at 25.0 ± 0.5 °C for 24 h to achieve vessel adsorption equilibrium, and then two approximately 2-mL initial TeCB concentration (CAo) samples were collected. An Oxford pipetter was adapted to fit glass disposable Pasteur pipet tips, and sample volume was calibrated gravimetrically. The kaolinite was weighed and added to make a suspension of nominally m IV = 2.0 g/ L for all isotherm measurements, except for the TeCB-KGA-1 isotherm measurement, where m/V - 1.0 g/L. Vessels were agitated for 24 h, although equilibrium was achieved in 2 h or less, as shown in Figure 2. The aqueous and solid phases were separated by centrifugation at 3500 X g for 2 h in a thermos tatted centrifuge, and two 2-mL equilibrium (CA) samples were taken from the supernatant liquid. For the extraction method, 35 mL of supernate was carefully removed and 20 mL of hexane added to extract both the TeCB adsorbed by the kaolinite and the vessel and the TeCB dissolved in the residual supernate (approximately 7 mL). The vessels were agitated 24 h for extraction and centrifuged for 15 min; then two 5-mL samples of the hexane phase were collected. Both aqueous and hexane samples were added to 15 mL of scintillation cocktail (Safety-Solve) and counted with a scintillation counter (either Packard 4430 or LKB 1219). Calibrations were performed for quench correction.

The adsorbed TeCB concentration, C' A , was determined for the modified difference method by

V(C A 0 -C A ) + M A V 0 - M A V

where M AVo and M A V were the initial and final equilibrium TeCB mass adsorbed by

Table I. Surface and Other Characteristics of the Kaolinites KGA-1 KGA-2

Characteristic {well crystallized) (poorly crystallized) BET surface area (m2/g) 10.05 ± 0.02 23.50 ± 0.06 Cation-exchange capacity (meq/100 g) 2.0 3.3 Median (mass basis) particle diameter (μπι) 1.59 1.59

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AQUATIC HUMIC SUBSTANCES

ο

t Ο > 2

Ο

ο CO Û

< ο

o° 0)

<

χ

TeCB/KGA-2 Adsorption (EX)

Phosphate buffer: pH = 6.9 Adsorbent cone: m/V - 2 g/L

1.00 2.00 3.00 4.00 TIME PERIOD FOR ADSORPTION: (DAYS)

Figure 2. Demonstration of equilibrium for TeCB adsorption by kaolinite (KGA-2).

the vessel. M A V = f[CA) was determined through a series of control studies. For the extraction method,

Ma - V n p n , C A - M, (4)

where Mn was the total TeCB mass extracted, and V ^ t was the volume of residual supernatant liquid. Uncertainty estimates for the adsorbed concentration, C' A , were propagated from uncertainties in each of the measured quantities (19 variables and parameters) used to determine C\. Uncertainty bars representing the 99% confidence intervals from these propagations are shown on isotherm plots.

Results and Discussion. The TeCB adsorption data for K G A - 1 and K G A - 2 measured by the extraction method are shown in Figures 3 and 4, respectively. The data were fitted with a one-parameter linear isotherm model (fixed at the origin) and gave equilibrium adsorption constants, K A ,

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AQUEOUS TeCB CONC: CA (uG/L)

Figure 3. Adsorption isotherm for TeCB and well-crystallized kaolinite (KGA-1) measured by the extraction method.

of 129.4 ± 17.2 L / k g with K G A - 1 and 224.2 ± 20.6 L / k g with K G A - 2 . Adsorption constants measured by the modified difference method agree within 4%, which provides verification of these two independent methods. A one-parameter model was found to be acceptable; the two-parameter least squares regression yielded intercepts not significantly different from zero.

The differences in magnitude of the adsorption constants of the two kaolinite samples may be resolved if the adsorption constants are expressed on the basis of adsorbent surface area instead of mass,

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ο ο

AQUEOUS T e C B C O N C : CA ( u G / G )

Figure 4. Adsorption isotherm for TeCB and poorly crystallized kaolinite (KGA-2) measured by the extraction method.

KA,s = - (5) «s

where as is the specific particle surface area (m2/g). The B E T surface area measurement with N 2 gives the total external surface area and can be used to normalize adsorption constants for nonporous, nonexpanding minerals. For expanding clay minerals and microporous soils, normalization by B E T surface area is not useful. This conversion gives equilibrium adsorption constants, K A s of0.01288 ± 0.00171 L / m 2 with K G A - 1 and 0.00954 ± 0.000877 L / m 2 with K G A - 2 . The factor of 2 difference in total specific surface area may be accounted for in the particle size distributions of these kaolinite samples. The distribution for K G A - 2 includes a much higher fraction than

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K G A - 1 of particles less than 0.5 μιη; 17.0% for K G A - 2 vs 7.6% for K G A - 1 . Kaolinite platelets in this size range would be expected to have a higher specific surface area than particles in larger size fractions.

The low adsorption constants for kaolinite and the linearity of the isotherms indicate a nonspecific physical adsorption mechanism. The surface coverage of TeCB can be estimated from the total surface area of the TeCB molecule, reported as 259.56 Â 2 (28). At the maximum C ' A = 2.4378 μg /g (C A = 19.06 |xg/g) measured for K G A - 2 , the total projected surface coverage of TeCB is 0.0130 m 2 / g . This value is orders of magnitude less than the total surface area available for monolayer coverage, 23.50 m 2 /g . Apparently, the hydration of the aluminum oxide surface and hydrogen bonding of water to the silanol groups of kaolinite is energetically a more favorable interaction than the van der Waals energy of attraction between TeCB and these surface functional groups. Water can strongly compete with H O C for mineral surfaces (29).

Adsorption of Humic Acid by Kaolinite Adsorption of the A B complex to the surface of kaolinite should be governed by the factors affecting humic acid adsorption by kaolinite. Humic acid adsorption by kaolinite was studied previously by Evans and Russell (30) and Davis (31). Mechanisms for the adsorption of humic substances to clay mineral surfaces have been reviewed (32-34).

Experimental Details. A 200-mg/ L suspension of Aldrich sodium humate (lot no. 01816HH) was prepared in a 250-mL volumetric flask with phosphate buffer. The suspension was stirred with a poly(tetrafluoroethylene)-coated (Teflon) magnetic stir bar for 20 min, then filtered through a glass fiber prefilter (Gelman Type A/E) and a 0.2-μπι pore size membrane filter (Gelman HT-200). The filtration removed insoluble components of the Aldrich humic acid. A "standard" humic acid solution at a concentration of 60 mg of carbon/L could be reproduced for each experiment, following this protocol. The humic acid concentration was measured by UV spectrophotometry (Varian DM S 90) at 285 nm. This method is very sensitive and can be used to analyze humic acid solutions containing radioactive TeCB, which cannot be safely treated in a total carbon analyzer. A Beers law relation, calibrating absorbance against total carbon, was measured using a carbon analyzer (Ionics 1270) with combustion at 900 °C.

The HA-KG A adsorption experiments were conducted according to the method given for TeCB-KGA adsorption experiments. The kinetics of adsorption were studied, and a 4-day agitation period was satisfactory to achieve equilibrium. A complete isotherm was measured for HA-KGA-2. For the HA-KGA-1 system, triplicate adsorption measurements were made using the same stock humic acid solution.

Results and Discussion. The average for the adsorbed concentrations obtained from the triplicate H A - K G A - 1 adsorption measurements are C ' B = 0.21 ± 0.059 mg of carbon/g at an average aqueous concentration

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240 AQUATIC H U M I C SUBSTANCES

of C B = 12.38 ± 0.12 mg of carbon/L. A Langmuir model fit of H A - K G A - 2 isotherm data is presented in Figure 5. The Langmuir equation is

n* _ ^ B C S ^ B ( A \

where K B is an adsorption constant and C s is the adsorbed concentration at saturation. The adsorption data were regressed by using a nonlinear fitting program giving K B = 0.6844 L / m g of carbon and C s = 1.3483 mg of carbon/g. Originally a linear regression of the data was performed with the reciprocal form of the Langmuir equation, but this fitting method favors the adsorption data in the lower concentration range.

The Langmuir model assumes that individual adsorbate molecules interact at localized sites on the adsorbent surface, whereas humic acids are polyfunctional, with several segments of a macromolecule attaching at multiple surface sites.

An alternative approach is to treat humic acids as polymer molecules having an average number, v, of adsorbable segments per molecule. An isotherm model was derived for this case by Simha, Frisch, and Eirich (35):

where θ is the surface coverage ( C ' B / C S ) , Kx is an interaction parameter, and K2 is an effective adsorption isotherm constant. For Kl = 0 and ν = 1, equation 7 reduces to the Langmuir equation. The Simha, Frisch, and Eir ich isotherm model requires two parameters more than the Langmuir equation, which could improve data fitting. Humic acids, however, are not well-defined polymeric molecules of repeating monomelic units. For this reason the Langmuir model was used as an empirical model, because it could fit the asymptotic approach of the adsorbed concentration to a maximum, as is observed for the data.

Another feature of humic acid systems is their wide distribution of molecular weights. Preferential adsorption of a particular molecular size fraction wil l depend on the conformation of the molecule at the solution-solid interface (36).

Several mechanisms have been proposed to describe humic acid adsorption by clay mineral and oxide surfaces (31, 32, 34, 37, 38). These mechanisms include cation bridging, water bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Identification of the surface functional groups and origin of the surface charge is necessary to understand the mechanism of humic acid adsorption. Kaolinite is a 1:1 layer clay mineral consisting of alternating sheets of gibbsite and silicate, with a low cation ion-exchange capacity (<10 meq/100 g).

(7)

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16. KEOLEIAN & C U R L Adsorption of Tetrachlorobiphenyl by Kaolinite 241

κ

m

8 1 , , , , , , , , , , , , °0.00 10.00 20.00 30.00 40.00 50.00 60.00

AQUEOUS HUMIC ACID CONC: CB (MG/L)

Figure 5. Humic acid and kaolinite (KGA-2) adsorption data fitted by a Langmuir isotherm model.

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Davis (31) described the adsorption of natural organic matter by alumina with a ligand-exchange mechanism. The reaction between organic anions (A) and surface metal hydroxyls (MeOH) was written as

x M e O H + t / H + + A 2 - = [ M e O H / - H A ] * ' 2 (8)

where ζ is the charge on the anion. For kaolinite, the aluminol groups at the edges would be expected to form complexes with the carboxylic acid groups of the humic acid. The aluminol groups of the basal surface are believed to be coated with silica (39) and may not participate in the com-plexation reaction. Parfitt et al. (40) demonstrated that adsorption of oxalate and benzoate occurred at the edges of gibbsite crystals, although the plate faces were unreactive. The siloxane surface of kaolinite is hydrophobic and would also be unreactive.

Binding of Tetrachlorobiphenyl with Humic Acid Binding coefficients for the interaction of H O C and humic acids have been measured by several methods, including equilibrium dialysis (J), ultrafiltration method (4), a reverse-phase separation method (3), gas purge method (7), water solubility enhancement method (10), and equilibrium headspace technique (5). The equilibrium dialysis method of Carter and Suffett (1) was used in this study, with some modifications given in the procedure to follow.

Experimental Details. Dialysis membrane tubing (Spectra/Por 6, 24.2-mm diameter, preserved in 0.1% sodium azide) with a molecular weight cutoff of 1000 daltons was used to measure the TeCB-HA binding constant. Binding experiments were conducted using 150-mL Corex centrifuge tubes with aluminum foil lined caps. Dialysis tubing was filled with 25 mL of humic acid and closed at both ends by knotting the tubing itself and also tying several knots with a piece of cotton string (cleaned by soaking in acetone) near the knot in the tube. The dialysis "bag" was placed into the 150-mL tube containing 100 mL of TeCB solution prepared as described previously. The tubes were agitated for 4 days in a thermos tatted shaker bath at 25 °C and covered to prevent photoexcitation of the humic acid. Quadruplicate 2-mL dialysate samples were collected from the solution outside the bag, C' A , 0 , and an aliquot was taken to measure the concentration of humic acid that leaked through the membrane, C B o . Likewise, triplicate 2-mL samples were withdrawn from inside the bag, C A,<, and an aliquot was taken to determine C B.<. The binding constant K A B

was calculated from the following expression:

Κ - C / M ~ C

A o (Q)

^ A B . a q ~ ~^ Γ 7^ Γ ^ '

The term C B,0C A.« in the denominator corrects for the enhanced TeCB activity in the dialysate due to humic acid that has escaped from the dialysis bag. This correction assumes that the binding constant and extinction coefficient used to determine the humic acid concentration in milligrams of carbon per liter is applicable to the humic acid that has escaped from the dialysis bag.

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Results and Discussion. A one-parameter regression of [ 1 4 C]-T e C B - H A isotherm points gave a binding constant, K A B = 48 ,300 ± 15,000 L /kg . Although the binding constant increased slightly with increasing humic acid concentration, there was no significant dependency of K A B on C B in the range of humic acid concentration studied, 11-53 mg of carbon/L. The isotherm data for C B i = 27 mg of carbon/L are shown in Figure 6.

The colloidal structure of humic acid must be identified before the mechanism for the association between TeCB and H A in solution can be understood. Aggregate or micelle structures have been proposed for humic acid by Senesi et al. (41) and Wershaw et al. (42). However, a critical micelle

AQUEOUS TeCB CONC: CA (uG/L)

Figure 6. TeCB and humic acid binding isotherm measured by equilibrium dialysis partitioning.

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concentration (cmc) has not yet been determined within the concentration range of humic acid normally encountered in natural waters (< 100 mg of carbon/L). Piret (43) reported a cmc of 16-21 g /L , which is well above this concentration range. Chiou et al. (10) found that high-molecular-weight poly(acrylic acids), unlike humic acid, were unable to enhance D D T solubility. This finding was attributed to the inability of poly(acrylic acids) to form a hydrophobic environment that could solubilize D D T . On the basis of evidence of Chiou et al. and Wershaw et al., the likely mechanism for T e C B - H A association is solubilization in the hydrophobic interior of a humic micellelike structure.

Multicomponent Sorption for TeCB, Humic Acid, and Kaolinite

The measurement of the simultaneous adsorption of TeCB and H A on kaolinite is necessary to test the hypothesis of competition between TeCB and H A , and also to examine the mechanism of T e C B - H A complex adsorption.

Experimental Details. Both the adsorbed and aqueous TeCB and HA concentrations were measured for each multicomponent sorption experiment. Stock aqueous TeCB-HA solution was prepared by adding a humic acid solution (15, 30, or 60 mg of carbon/L) to a stock botde, either 250 mL or 650 mL, containing TeCB residue left after evaporation of hexane transfer solvent. The stock TeCB-HA solution bottles were wrapped in aluminum foil to prevent photoexcitation of HA by exposure to UV light. The remaining steps are identical to those outlined for TeCB-KGA and H A - K G A single-sorbate adsorption experiments, with a 4-day agitation period for adsorption.

Results and Discussion. The effect of humic acid on the apparent T e C B - K G A - 2 adsorption isotherm is shown in Figure 7 at three humic acid concentrations. The apparent adsorption partition coefficient is given by

Kt = %± (10) W

where C ' A is the total TeCB adsorbed concentration and C A is the total dissolved concentration, each including both free and HA-complexed TeCB. K% is evaluated from the slope of each isotherm, with their uncertainty estimates, as given in Table II. The linearity of the__multicomponent isotherms indicates that K% is independent of the total C A , and consequently is independent of free C A . The average K% of the three experimental cases for T e C B - K G A - 1 are also included in Table II.

The decrease in the partition coefficient with increasing humic acid concentration can be attributed to competitive adsorption between TeCB and H A or to a decrease in the free TeCB concentration due to complexation

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16. KEOLEIAN & C U R L Adsorption of Tetrachlorobiphenyl by Kaolinite 245

ο ο ( 0

0.00 4.00 8.00 12.00 16.00 20.00 24.00 TOTAL. AQUEOUS TeCB C0NC: (uG/L)

Figure 7. The effect of humic acid on TeCB adsorption by kaolinite (KGA-2). Three additional isotherm data points for CB = 60.5 mg of carbon/L are not

shown and lie outside the domain of this plot.

Table II. Comparison of Multicomponent Model Predictions of Adsorption Partition Coefficients

CB C.' K J (observed) K? (eq. 17) KJ (eq. 18) Kaolinite (mg CIL) (mg Clg) (Llkg) (Llkg) (Llkg) K G A - 1 0.0 0.0 129.4 (±17.2)· K G A - 1 12.4 0.21 88.2(±12.1)FE 87.3 (±43.5)£

K G A - 2 0.0 0.0 224.2 (±20.6) K G A - 2 10.9 1.06 157.8 (±7.5) 50.7 (±13.6)C 180.3 (±17.4) K G A - 2 29.5 1.29 100.9 (±9.4) 30.0 (±4.1) 118.1 (±16.1) K G A - 2 60.5 1.35 77.1 (±6.8) 17.9 (±2.2) 73.7 (±13.1)

"The 99% confidence intervals are shown for Kf observed, except for in footnote b. lThree standard deviations for η — three data points. The 99% confidence intervals for predicted K% from an uncertainty analysis propagation.

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with H A or to both. The extent of this latter interaction is known a priori from measurements of the T e C B - H A binding in solution. The importance of A and Β competitive adsorption was tested using multicomponent sorption models that include the measured binary interactions between TeCB, H A , and K G A .

Model Formulation and Evaluation of Multicomponent Sorption Data Humic acids do not have a defined molecular structure, and their colloidal nature in water is unknown. Initially, however, it is assumed that humic acids, which are polydispersed (distributed in molecular weight and functional groups), can be treated as a homogeneous set with unique sorption properties. There may be a dependence of K A B on molecular size (44), but the effect is uncertain. An average binding constant wil l be used here.

The total A concentration in solution is

C A = (1 + K A B , a q C B ) C A (11)

For the adsorbed phase, assume A/Β competitive adsorption and also the binding of A to the adsorbent via adsorption of the A B complex. Many H O C adsorbates exhibit a linear single-sorbate adsorption isotherm. For this case, competition can be modeled with the simplified competitive Langmuir equation proposed by Cur l and Keoleian (18):

c ' « - r f ^ <12>

As written here, K A represents the product of the Langmuir adsorption constant and the maximum adsorbed concentration. In the absence of B, the equation will predict linear adsorption. The adsorption of the A B complex to the adsorbent, which can be thermodynamically equivalent to the binding of A to adsorbed B, can be expressed by

C ' A B = ^ A B ^ s C B C A (13)

which is analogous to equation 1 for the aqueous phase. This relation assumes that A B complex adsorption is dominated by the adsorption properties and behavior of the Β macromolecules or that binding of A to adsorbed Β does not change the chemical activity of adsorbed Β or that both are true. For the multicomponent sorption experiments conducted in this investigation, TeCB and H A were agitated for 24 h before kaolinite was added; this favors complexation equilibrium prior to adsorption of the complex. Combining

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16. KEOLEIAN & CURL Adsorption of Tetrachforobiphenyl by Kaolinite 247

equations 12 and 13 gives the following expression for the total adsorbed concentration:

^ = (rrb;+ K^c'°) Ca (14)

The partition coefficient, found by substituting equations 11 and 14 into equation 10, is given by

, , t Γ

+ K A B a d s C ' B

K * =

1 + K B C B ( 1 5 )

1 + ^ A B a q C B

In this model Kf is independent of C A , which is observed experimentally. No direct measurement of K A B a d s was done, so the following assumption will be made:

ÂB.ads = ÂB.aq (16)

Equation 16 assumes that the conformational changes occurring in Β upon adsorption do not affect A / B association. Kf now becomes

K a + Κ C + *MB,aq^ Β 1 + KKC,_

^ A B . a q ^ Β κ * =

b;v r— (17)

The alternative hypothesis is that A and Β can adsorb on independent sites, in which case the competitive interaction coefficient Kh is zero and K% becomes

Kf = K a + K a b ^ C ' b (18) 1 + ÂB.aqCfi

Model Predictions A l l of the parameters given in equations 17 and 18 can be evaluated from T e C B - H A , T e C B - K G A , and H A - K G A sorption experiments. With these parameters, the model predictions for Kf, considering both competitive (equation 17) and noncompetitive (equation 18) A adsorption, are given in Table II. The uncertainties in the predicted partition coefficients were propagated from the uncertainties in the parameters K A , K B , K A B , and the concentration data C B and C u . The competitive model predictions for K% clearly

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do not agree with data for Kf, indicating that competition between A and Β may not be significant. In all cases, the predicted Kf underestimates the measured Kf data. The competition model was not tested for K G A - 1 because the complete isotherm was not measured and K B was unknown.

A comparison of the noncompetitive model predictions with data for Kf shows agreement within the 99% confidence intervals. This result supports the assumption that independent surface sites on kaolinite are available for TeCB and H A adsorption. The TeCB would be expected to adsorb on the most hydrophobic surfaces of the kaolinite. The siloxane surface of kaolinite has a low affinity for water and may accommodate TeCB adsorption. Conversely, H A , which is hydrophilic in nature, probably interacts with A l O H at the edges of the kaolinite crystal lattices. A detailed description of the surface characteristics of kaolinite is given in ref. 39.

The agreement between noncompetitive model predictions and data for K% supports the assumption made for K A B with equation 16, that K A B is the same for both the adsorbed and aqueous phases. Chiou et al. (JO) found approximate agreement between the binding constant for a dissolved soil humic acid extract (K&J and bulk soil adsorption constants (K^ for D D T and 2,4,4'-trichlorobipheny 1. Means and Wijayaratne (2), however, reported that Kdoc for colloidal organic matter was 10 to 35 times greater than literature values. Differences in the origin of the soil and the dissolved organic matter alone can account for the variability in Koc (45) and Kdoc values (12). Furthermore, the entire organic matrix of a soil may not be accessible to a sorbate, in which case Κχ < Kdoc would be expected.

If the micelle model for humic acid is accepted, it follows that TeCB solubilization in the hydrophobic interior of the micelle would not be altered upon adsorption to the kaolinite surface. The micelle structure may not be significantly disrupted during the adsorption reaction because the complex-ing functional groups are oriented toward the exterior of the micelle.

Summary

The binary interactions between 2,2',4,4' tetrachlorobiphenyl, humic acid, and two kaolinites were characterized by conducting T e C B - K G A , H A - K G A , and T e C B - H A isotherm experiments. Equilibrium sorption constants were measured for these interactions and used in alternative multicomponent sorption models to predict the partitioning of TeCB to kaolinite in the presence of humic acid. Humic acids were found to affect the partitioning by enhancing the TeCB aqueous-phase concentration through a complexation mechanism, and by immobilizing TeCB to the kaolinite via adsorption of the T e C B - H A complex to the mineral surface. This latter mechanism was previously unstudied. Adsorption of TeCB and humic acid were found to be noncompetitive. This work is applicable for the prediction of H O C mobility through clay liners of "secure" landfills and clay mineral subsurface zones.

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Other environmental implications of this investigation were discussed by Keoleian (46).

Acknowledgments This research was funded in part by the University of Michigan Memorial-Phoenix Project Grant. Gregory A. Keoleian was supported by the Chemodynamics of Toxic Substances Control Internship established at the University of Michigan, Department of Chemical Engineering, by the Jessie Smith Noyes Foundation.

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Technol. 1984, 18, 187-192. 4. Wijayaratne, R. D.; Means, J. C. Mar. Environ. Res. 1984, 11, 77-89. 5. Garbarini, D. R.; Lion, L. W. Environ. Sci. Technol. 1985, 19, 1122-1128. 6. Haas, C. N.; Kaplan, Β. M. Environ. Sci. Technol. 1985, 19, 643-645. 7. Hassett, J. P.; Milicic, E. Environ. Sci. Technol. 1985, 19, 638-643. 8. McCarthy, J. F.; Jimenez, B. D. Environ. Sci. Technol. 1985, 19, 1072-1076. 9. Whitehouse, B. Estuarine Coastal Shelf Sci. 1985, 20, 393-402.

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Hemond, H. F. In Peat and Water; Fuchsman, C. H., Ed.; Elsevier: New York, 1986; pp 133-157.

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RECEIVED for review December 1, 1986. ACCEPTED for publication July 24, 1987.

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