Introduction

The widespread occurrence of hydrophobic organic contaminants (HOCs) in soils, sediments, and aquifers has led to intensive studies of the mobility and fate of these compounds in subsurface environments. Because of their low solubilities and slow dissolution/desorption rates, many HOCs are associated with the solid phase or exist as nonaqueous phase liquids (NAPLs). Depending on the particular site conditions, two complementary remediation alternatives are often considered: (1) HOC sorption to an immobile phase that subsequently decreases HOC mobility, or (2) HOC partitioning to a mobile phase that results in an increase in HOC mobility (and apparent solubility) in water. For the first approach, use of organoclays (e.g., Wagner et al., 1994; Gullick et al., 1994; Xu and Boyd, 1995) or organooxides (e.g., Holsen et al., 1991; Kibbey and Hayes, 1993; Sun and Jaffe, 1996) to remove HOCs from water has received much attention. For the second alternative, in-situ surfactant-enhanced aquifer remediation (SEAR) has been suggested as an economically and technically feasible remediation approach (e.g., Kile and Chiou, 1989; Abdul et al., 1990; Pennell et al., 1993). To date, most field-scale SEAR applications have focused on the removal of NAPLs from contaminated aquifers by utilizing NAPL mobilization (i.e., brought about by lowering the NAPL-water interfacial tension) as well as enhanced solubilization of NAPL components in surfactant micelles. In contrast to NAPLs, however, we are aware of only one field-scale SEAR application that has been used to remove HOCs existing in a sorbed state for the purpose of aquifer restoration or risk mitigation (Smith et al., 1997; Sahoo et al., 1998).

Enhanced HOC solubility in surfactant systems generally has been quantified by a distribution coefficient that only considers HOC partitioning to surfactant micelles that exist above the critical micelle concentration (CMC). Although surfactants can form a mobile micellar pseudophase that leads to the facilitated transport of solubilized HOCs, they also can be adsorbed by the solid matrix and thereby lead to HOC partitioning to immobile sorbed surfactants and, thus, enhanced HOC retardation. Therefore, the effectiveness of a remediation scheme utilizing surfactants depends on the distribution of an HOC between immobile compartments (e.g., subsurface solids, sorbed surfactants) and mobile compartments (e.g., water, micelles).

Although the partitioning of HOCs to surfactant micelles has been well studied, HOC partitioning to sorbed surfactants (e.g., hemimicelles, admicelles) has received much less attention. Holsen et al. (1991) examined the sorption of several HOCs on ferrihydrite coated with sodium dodecyl sulfate (SDS) and found that the HOCs with the lowest water solubility showed the highest sorption. Holsen et al. also found a linear relationship between HOC sorption and the amount of SDS on the ferrihydrite, suggesting that the SDS coating was primarily responsible for HOC sorption. Sun and Jaffe (1996) investigated the partitioning of phenanthrene to dianionic monomers and micelles and to the same surfactants sorbed on alumina, and found the sorbed surfactants to be generally 5 to 7 times more effective as a partitioning medium than the aqueous-phase micelles. Likewise, Nayyar et al. (1994) reported partition coefficients for several organic contaminants to SDS sorbed on alumina that were higher in value than the corresponding micellar partition coefficients. Similar experiments were performed by Sun et al. (1995) to obtain partition coefficients for three chlorinated HOCs to a silt loam with a sorbed nonionic surfactant (Triton X-100). Sun et al. observed that the sorbed surfactant increased HOC partitioning relative to the untreated soil; however, when the aqueous surfactant concentration was greater than the CMC, the micelles competed against the sorbed surfactant for HOC partitioning and led to an overall decrease in HOC distribution coefficients.

Many previous studies of HOC partitioning to sorbed surfactants examined conditions favorable for the formation of surfactant bilayers resulting from high adsorption densities; in these studies, a single partition coefficient was often observed. However, for many expected surfactant remediation applications, a surfactant solution would likely be pumped into or near the contaminated subsurface environment, and thus aqueous surfactant concentrations would vary spatially and temporally from zero to the applied concentration. Also, because soil-surfactant and surfactant-surfactant interactions lead to highly nonlinear sorption isotherms, the transport of surfactant monomers and micelles would exhibit very complex behavior. Correspondingly, the HOC distribution between immobile and mobile phases would also be expected to show complex behavior depending on the surfactant mass in each phase. Finally, it is likely that the varying soil solution chemistries found in different subsurface environments will affect surfactant sorption to solid phases and subsequent HOC partitioning to micelles and sorbed surfactants. Therefore, a quantitative evaluation of any potential surfactant remediation approach must consider the distribution of surfactant and subsequent HOC partitioning to each phase as a function of solution chemistry to maximize efficiency and minimize remediation costs.

The objectives of this work were to (1) study the equilibrium sorption characteristics of an anionic surfactant (SDS) and a nonionic surfactant (Tween 80) to kaolinite, a common soil mineral, as a function of solution chemistry; (2) examine the equilibrium partitioning of two HOCs (phenanthrene and naphthalene) to the surfactant micelles and sorbed surfactants for varying solution chemistry conditions; and (3) quantify overall HOC distribution coefficients that consider sorbed surfactant amounts and the presence of micelles as a function of surfactant dose and aqueous chemistry. Results from this investigation can be used to elucidate the role of sorbed surfactants in HOC partitioning in contaminated subsurface environments and to provide a framework for evaluating HOC removal efficiencies for alternative surfactant remediation applications.

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