Aquatic Humic Substances - ACS Publications - American Chemical

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch016

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 C-radiolabeled 2,2' ,4,4'-tetrachlorobiphenyl (TeCB), afilteredhumic 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 werefittedsatisfactorily, 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. 14

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 , relates the concentration of the pollutant (A) in the bound state, C ; the concentration of A in the free state, A B

A B

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

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

232

AQUATIC HUMIC SUBSTANCES

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

B

C

A B

= K

C C B

(1)

A

The humic substances can enhance the total concentration of H O C in solution by as much as the factor K C 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 sub stances (J, 3). A n important mechanism that can immobilize H O C in an aqueous en vironment 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 A n derson (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 C , which leads to the following relation for the apparent partition coefficient: AB


A B

A B

B

B

1 + K

A B

C

B

where K 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. A

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 repre sents a complex network of equilibria further complicated by the hetero geneous 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. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

16.

KEOLEIAN & CURL


Sorbent

Adsorption of Tetrachlorobiphenyl by Kaolinite

233

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

234


swelling and nonporous mineral such as kaolinite precludes absorption of the model sorbate and reduces the complexity of the investigation. In ad dition, 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 sta bility (resistance to chemical or microbial degradation), and environmental significance. O n the basis of these criteria, C-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 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 (nor malized to the fraction of organic carbon) for pyrene and 14 different humic substances, including untreated Aldrich humic acid (HA). They found that K varied by as much as a factor of 10, depending on the source of humic material. The K 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 T e C B - h u m i c 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 T e C B - h u m i c acid-kaolinite system. Model predictions are compared with experimental multicomponent sorption data to choose between models and provide a basis for further refinement.


14

1 3

A B

A B

Adsorption of Tetrachlorobiphenyl by Kaolinite Anomalous results for adsorption partition coefficients have been observed for many H O C - c l a y 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 signifi cant 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 T e C B mass accountability. 14

Experimental Details. The C-radiolabeled 2,2'4,4'-tetrachlorobiphenyl (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


16.

KEOLEIAN & CURL

Adsorption of Tetrachlorobiphenyl by Kaolinite

235

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 producedfroma microfilter system (Millipore Corporation, Milli-R/Q), was glass-distilledfromper manganate. This reagent water was buffered with 0.0025 M KH P0 and 0.0025 M NaHP0 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 thermostatted 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 (C o) samples were collected. An Oxford pipetter was adapted tofitglass 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 (C ) samples were takenfromthe 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 cor rection. The adsorbed TeCB concentration, C' , was determined for the modified dif ference method by 2

4


4

A

A

A

V(C -C ) + M A0

A

A V 0

-M

A V

where M o and M were the initial andfinalequilibrium TeCB mass adsorbed by AV

AV

Table I. Surface and Other Characteristics of the Kaolinites KGA-2 KGA-1 {well crystallized) (poorly crystallized)

Characteristic 2

BET surface area (m /g) Cation-exchange capacity (meq/100 g) Median (mass basis) particle diameter (μπι)

10.05 ± 0.02 2.0 1.59

23.50 ± 0.06 3.3 1.59



ο

t

Ο

>2


Ο

χ

ο

CO Û

Aquatic Humic Substances - ACS Publications - American Chemical

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