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Dion, P. W.; Bruce, W. R. Mutat. Res. 1983,119,151-160. Lederman, M.; Van Tassell, R.; West, S. E. H.; Ehrich, M. F.; Wilkins, T. D. Mutat. Res. 1980,79, 115-124. Ames, B. N.;McCann, J.; Yamasaki, E. Mutat. Res. 1975, 31. 347.
deserres, F.J.; Shelby, M. D. Science (Washington, D.C.) 1979.203. 564.
Ma, T.Hi;Sparrow, A. H.; Schairer, L. A.; Naumann, A.
F.Mutat. Res. 1978,58, 251. Ma, T.H.Mutat. Res. 1980,64, 307.
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Received for review August 18,1983. Revised manuscript received April 11,1984.Accepted May 29,1984. This work was supported by U.S. Environmental Protection Agency Grant R807009 and by Grant S-095-ILLfrom the Water Resources Center. Although supported by the Agency, this report has not been reviewed by it, and no official endorsement of this report should be inferred.
Impliclt-Adsorbate Model for Apparent Anomalies with Organic Adsorption on Natural Adsorbents Rane L. Curl’ and Gregory A. Keolelan Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109 Adsorption-desorption hysteresis, the adsorbent-concentration effect, and other apparent anomalies that have been observed in studies of adsorption of organic chemicals on natural adsorbents can be explained by an implicitadsorbate model. The model is based on the competitive adsorption, between an adsorbate under study, A, and an implicit adsorbate, B, initially on the adsorbent. During batch adsorption-desorption experiments B desorbs and uncovers sites for binding A, increasing the apparent partition coefficient. The model agrees well with several sets of experimental data from the literature, but ranges of model parameters such as the true adsorption partition coefficient for A give equivalently good agreement. The true adsorption parameters are not determinable uniquely from presently available data, in the presence of strong hysteresis or adsorbent-concentration effects. They are determinable if the implicit adsorbate could be identified and characterized.
Introduction The interaction between organic pollutants and soils and other natural adsorbents is important in determining the fate and distribution of organic pollutants in the environment. Numerous adsorption-desorption experiments have been performed and reported to characterize this interaction. In many of these experiments an aqueous solution of the organic chemical has been equilibrated with the soil or sediment and the equilibrium concentration of each phase determined. The distribution of the adsorbate between the adsorbed phase and the aqueous phase for linear partitioning is defined by the apparent partition coefficient as
CA! KA* = (1) CA where CAtand CA are the adsorbed and aqueous equilibrium concentrations of A, respectively. In studies conducted to measure the apparent partition coefficients and characterize adsorption-desorption behavior, several apparent anomalies have been discovered, which have not yet been adequately explained. The three most significant anomalies are the adsorbent-concentration effect, adsorption-desorption hysteresis, and endothermicity of the adsorption process. Adsorption studies conducted by Grover and Hance ( I ) , O’Connor and Connolly (2),Di Tor0 et al. (31, and Voice et al. ( 4 ) have demonstrated the adsorbent-concentration 916
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effect. They observed linear adsorption isotherms but a decrease in the apparent partition coefficient with increasing adsorbent concentration. Figure 1 shows this dependence for typical data from O’Connor and Connolly (2) and Huang and Liao (5). The figure shows that the dependence of the apparent partition coefficient on the adsorbent concentration follows a variety of patterns. The maximum effect appears to be a near reciprocal dependence of KA* on m / V . O’Connor and Connolly (2) correlated the data with an empirical relationship but did not explain the effect. Grover and Hance (1)suggested that the adsorbentconcentration effect can be attributed to the variation in the degree of soil aggregation with soil concentration. The first quantitative model of the adsorbent-concentration effect was recently proposed by Voice et al. (4). This will be contrasted with the model developed here under Discussion. A variety of organic chemical-soil systems have been shown to display adsorption-desorption hysteresis or “nonsingularity”. Rao and Davidson (6) and Di Toro and Horzempa (7) describe a number of experimental studies demonstrating this phenomenon. In these studies batch adsorption experimentswere conducted, followed by either single or consecutive desorption experiments. After initial adsorption and centrifugation steps, a portion of the supernatant was removed and replaced with adsorbate-free solution. The equilibrium concentrations of both phases were then determined. This procedure was repeated for consecutive desorption experiments. The hysteresis effect is exhibited when the equilibrium desorption points are not coincident with the apparent adsorption isotherm. Figure 2, from Swanson and Dutt (8),shows typical adsorption-desorption hysteresis, here for atrazine on Mohave soil. In the consecutive desorptions shown, each starting with a different initial atrazine concentration, more atrazine is retained by the soil than is predicted by the initial adsorption isotherm. Three suggestions for causes of hysteresis presented by Rao and Davidson (6)are (1)artifacts due to some aspects of the method, (2) nonattainment of equilibrium during adsorption, and (3) chemical and/or microbial transformation of the adsorbate during the experiment. Di Toro et al. (9) modified adsorption and desorption experiments to test these factors and concluded that they were not the cause of hysteresis. Many investigators have suggested that hysteresis is produced by irreversible adsorption (10-13, 7). Di Toro and Horzempa (7) modeled the ad-
0013-936X184/0918-0916$01.50/0
0 1984 American Chemical Society
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Heptochior on Kaolinite ( 5 ) Kepone on James River Sediment (2) Kepone on Range Point Sediment (2)
Theory If two species A and B adsorbed on an adsorbent exhibit a competitive Langmuir isotherm, as has been observed in some systems (20), the adsorbate concentrations for the species are given by
and
10 10
I o4 m / V (mg/L)
lo3
102
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Figure 1. Adsorbent concentration effect data and implicit adsorbate model regressions. Open symbols show the points at which the model was fitted.
where C A and C B = equilibrium solution concentration of adsorbates (mg/L), CA’ and CB’ = equilibrium adsorbed concentrations (mg/kg), a and b = “site”, or maximum adsorbed concentrations (mg/kg), and KAand KB = adsorption equilibrium constants (L/mg). While eq 2 and 3 do not apply to many observations of competitive adsorption, they are sufficient to demonstrate the effect of an implicit adsorbate. We define A to be the adsorbate under study and B the implicit adsorbate. It is assumed that B is a single species. From eq 1and 2, the apparent partition coefficient KA* (L/kg) is given by
aKA KA* = 1 + KACA + KBCB
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ADSORPTION ISOTHERM
DESORPTION ISOTHERM
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Flgure 2. Adsorption-desorption isotherms for atrazine on Ca2+ Mohave soil. Reproduced with permission from Swanson and Dutt (8). Copyright 1973 Soil Science Society of America.
sorption-desorption of hexachlorobiphenyl on lake sediment and montmorillonite clay by defining reversible and resistant components for the adsorbed state. Irreversible adsorption, however, does not account for the curvature of the consecutive desorption data toward the origin, shown in Figure 2 and also observed by Saltzman et al. (141, van Genuchten et al. (15, 16),Wahid and Sethunathan (17), and Di Tor0 et al. (9). In several organic chemical-solid systems an increase in the apparent partition coefficient with increasing temperature has been found (4, 9, 18, 19). Application of a van’t Hoff relationship would give a positive heat of adsorption for such a temperature dependence. This is contradictory to the expected exothermicity of the adsorption process. The purpose of this paper is to present a model that can account for the anomalies discussed above. The adsorbents displaying anomaious adsorption-desorption behavior are usually natural materials such as soils, clays, and sediments. In their natural environmentsthese adsorbents will adsorb various organic substances to which they are exposed. These unidentified organic substances are referred to here as the “implicit adsorbates”. The model is based on the competitive adsorption between the adsorbate studied and the implicit adsorbate. After a presentation of the theory, the model will be applied to the experimental data shown in Figures 1 and 2.
(4)
The adsorption of trace amounts of organic compounds often exhibits linear isotherms, and many adsorbates have been characterized by the constant KA* (6). This is possible here if KACA
