Environ. Sci. Technol. 2000, 34, 4341-4347
Binary Desorption Isotherms of TCE and PCE from Silica Gel and Natural Solids C H A R L E S E . S C H A E F E R , * ,† CHRISTOPH SCHU ¨ TH,‡ CHARLES J. WERTH,§ AND MARTIN REINHARD† Environmental Engineering and Science Program, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, Geological Institute, Applied Geology Group, University of Tu ¨ bingen, Sigwartstrasse 10, D-72076 Tu ¨ bingen, Germany, and Department of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Binary solute desorption isotherms of trichloroethylene (TCE) and tetrachloroethylene (PCE) at 100% relative humidity from silica gel and two well-characterized natural solids were investigated. Results indicated that the ideal adsorbed solution theory (IAST) was able to describe desorption isotherms for the silica gel. For the natural solids, IAST was not able to describe desorption isotherms for the full concentration range examined. Failure of IAST was greatest for the most heterogeneous sorbent, even when considering multiple sorption domains. In addition, IAST predictions worsened as nonlinear uptake mechanisms began to dominate. Several possible explanations for the failure of the IAST are given, including the possibility that complex interactions between the sorbing solutes and the sorbent may exist, causing deviations from ideal sorption behavior.
Introduction Efforts to remediate subsurface groundwaters may often be hindered by contaminant sorption to aquifer solids. The slow release of organic contaminants from soils has been shown to reduce the effectiveness of many cleanup strategies (1, 2). Although many studies have indicated that the rate of release of contaminants from soils is often controlled by diffusion (3-6), the equilibrium phase behavior of desorbing contaminants must still be understood in order to determine the concentration gradient necessary for the diffusion driving force. Thus, desorption isotherms are of particular interest. Several researchers have investigated adsorption of organic contaminants from water on to soils. Many models, both linear (7, 8) and nonlinear (9-12), have been proposed to describe sorption of a single solute to natural solids. Studies have shown that adsorption on mineral surfaces, condensation in micropores, and partitioning into both hard and soft components of soil organic matter may contribute to the overall uptake of organic contaminants (13-17). The Freundlich model (18) has been used to characterize contaminant uptake via these mechanisms (11, 13, 17) and may be described by * Corresponding author e-mail:
[email protected]; phone: (650)723-6348; fax: (650)725-2099. † Stanford University. ‡ University of Tu ¨ bingen. § University of Illinois at Urbana-Champaign. 10.1021/es000875d CCC: $19.00 Published on Web 09/15/2000
2000 American Chemical Society
C S ) K FC n
(1)
where CS is the sorbed solute concentration (mol/g), C is the aqueous solute concentration (mol/mL), and KF and n are Freundlich parameters. The Freundlich exponent n is a measure of the degree of isotherm nonlinearity. An exponent value of unity would indicate a linear isotherm, while values of n < 1 indicate marginal decreases in sorption energy with increasing concentration. Values of n ≈ 1 have been observed for sorption on to water-wet mineral surfaces (17, 19) and sorption into soft soil organic matter (20). Values of n < 1 have been observed for sorption on to water-wet soils and sediments (12, 17, 21), on to microporous silica gel (11, 17, 22), and into glassy or hard natural organic matter (14, 22). The Freundlich isotherm may not accurately describe contaminant uptake for sorbents consisting of both linear and nonlinear sorption domains. For example, LeBoeuf and Weber (14) found that sorption isotherms for phenanthrene on soils with both soft (linear) and hard (nonlinear) components were not log-log linear. Uptake in both linear and nonlinear domains has been described by the Distributed Reactivity Model (DRM) (12, 22):
CS ) CS,linear + CS,nonlinear ) KDC + KFCn
(2)
where CS is the total sorbed contaminant concentration (mol/ g), CS,linear and CS,nonlinear are the sorbed concentration (mol/ g) associated with linear and nonlinear domains (respectively), and KD is the linear partition coefficient (mL/g). Recent studies by Huang and Weber (19) have also shown that the DRM model may be applied to desorption processes. Other researchers have substituted a Polanyi-type isotherm for the nonlinear portion of the DRM to describe pore filling in micropores (23). For the purpose of discussion, sorbents with multiple sorption domains (as described by the DRM) will be considered heterogeneous with respect to sorption. The ideal adsorbed solution theory (IAST) has been used to describe competitive adsorption in multicomponent solute systems (22, 24, 25). The two primary assumptions in this model are that each solute has access to all the same sorption sites and that the adsorbed phase fugacity is equal to its adsorbed phase mole fraction (i.e., a Raoult’s law approximation). IAST may fail if significant solute-solute interactions exist (in either the liquid or sorbed phase) or if one of the sorbates blocks access to sorption sites for the other sorbate (i.e., steric factors). For example, one solute may sorb strongly to a particular site that could block access of a second solute to a different sorption site. This type of blockage could occur more easily in micropore regions where the pore diameters are of the same order of magnitude as molecular diameters. Such phenomena could cause nonideal sorption, and thus the ideal behavior assumed in the derivation of IAST will not be appropriate. For a binary solute system in which the single solute systems are described by Freundlich isotherms, IAST sorption may be described by (24, 20)
C i,mix )
CS
CS j,mix
i,mix
+ CS j,mix
(
CS i,mix + CS j,mix KFi
() ni nj
)
1/ni
(3)
where C i,mix is the aqueous concentration (mol/mL) of solute i in the binary solute system, and CS i,mix and CS j,mix are the sorbed concentrations (mol/g) of solutes i and j, respectively, in the binary solute system. KFi, ni, and n j are the Freundlich parameters obtained from single solute data. Component j VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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is calculated analogous to component i. Thus, the IAST model allows for prediction of sorption in mixtures based on single solute isotherms. The IAST model has been shown to give good predictions of adsorption for a wide variety of model organic solutes and soils (20, 22, 26). However, other studies have indicated that IAST does not accurately describe competitive sorption for organic solutes (27, 28). It has been shown that three chlorinated contaminants, while displaying linear isotherms in single solute systems, all showed a competitive reduction of about 50% in corresponding binary sorption experiments on to natural sediments (28). Sorption of these three contaminants was attributed to the mineral domain of the sediments rather than to organic domains. The IAST model would have predicted no competitive behavior in the binary system and thus would have overpredicted sediment uptake for all three contaminants for each sediment. While the IAST and DRM models have been combined and used to investigate aqueous-solid adsorption processes on to soils and sediments (20), the IAST and DRM model have hitherto not been applied to aqueous-solid desorption processes. It has been shown that equilibrium desorption isotherms may differ significantly from their corresponding adsorption isotherms (13, 17, 29). It is currently unknown if the IAST model will apply to desorption isotherms for sorbents containing multiple sorptive domains (i.e., sorbents that may be described by the DRM in single sorbate systems). Since the removal of contaminants from contaminated soils and sediments is a desorption process, understanding desorption mechanisms is needed in order to predict contaminant release from solids. The purpose of this research is to examine the applicability of the IAST and DRM models to multisolute desorption systems and to investigate the relation between single solute and multisolute desorption isotherms on both homogeneous and heterogeneous sorbents. By examination of binary solute desorption isotherms, insight can be obtained into both fundamental adsorption and desorption processes in aqueous-solid systems.
Model Simulations using the IAST model were calculated by use of eq 3, with the sorbed concentration determined by species mass balance as described in the Experimental Methods section. Freundlich desorption isotherm parameters of trichloroethylene (TCE) and tetrachloroethylene (PCE) in single solute systems were taken from Farrell and Reinhard (17). The Freundlich isotherm is shown to describe the single solute data well (17) and is used within the IAST and DRM models to examine the binary solute experiments. Sorbed amounts (mol/g) of TCE and PCE taken by mass balance from the binary solute experiments (described in the following sections) were used for the mixture terms in eq 3 and plotted as a function of the aqueous solution concentration (mol/ mL). For sorbent systems exhibiting multiple sorption domains, the DRM must first be applied using the single solute data before the IAST model for the binary system may be incorporated. Since the IAST model in eq 3 only applies to one domain, the nonlinear portion of the DRM must be separated (as in eq 2). The linear portion of the DRM indicates that sorption in that particular domain is not competitive if the linear sorption is controlled by the “soft” organic matter (14, 20), thus use of the IAST model to describe multi-solute effects for that domain is not necessary. The nonlinear (Freundlich) portion of the DRM in the single solute data along with the mixture solid concentration terms (CS i,mix and CS j,mix obtained directly from the experimental data) may then be inserted in the IAST equation to obtain the model aqueous concentrations (C i,mix and C j,mix) for the binary solute desorption isotherm. Use of a simple IAST versus a combined 4342
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TABLE 1. Sorbate Propertiesa aq solubility at 20 °C (mol/mL) vapor pressure at 30 °C (KPa) log Kow at 23 °C (-) av molecular diameter (Å) a
TCE
PCE
8.3 × 10-6 12.5 2.42 6.8
9.1 × 10-7 4.28 2.53 7.2
Data are taken from Farrell and Reinhard (17) and references therein.
TABLE 2. Sorbent Propertiesa
sorbent
organic content (foc) (wt %)
BET-N2 surface area (m2/g)
column water saturation (mL of water/mL of void)
Norwood silica gel A Santa Clara
1.4
