Review
Surfactant–soil interactions during surfactant-amended remediation of contaminated soils by hydrophobic organic compounds: A review

https://doi.org/10.1016/j.jenvman.2008.08.006Get rights and content

Abstract

Surfactants are amphiphilic molecules that reduce aqueous surface tension and increase the solubility of hydrophobic organic compounds (HOCs). Surfactant-amended remediation of HOC-contaminated soils and aquifers has received significant attention as an effective treatment strategy – similar in concept to using soaps and detergents as washing agents to remove grease from soiled fabrics. The proposed mechanisms involved in surfactant-amended remediation include: lowering of interfacial tension, surfactant solubilization of HOCs, and the phase transfer of HOC from soil-sorbed to pseudo-aqueous phase. However, as with any proposed chemical countermeasures, there is a concern regarding the fate of the added surfactant. This review summarizes the current state of knowledge regarding nonionic micelle-forming surfactant sorption onto soil, and serves as an introduction to research on that topic. Surfactant sorption onto soil appears to increase with increasing surfactant concentration until the onset of micellization. Sorbed-phase surfactant may account for the majority of added surfactant in surfactant-amended remediation applications, and this may result in increased HOC partitioning onto soil until HOC solubilization by micellar phase surfactant successfully competes with increased HOC sorption on surfactant-modified soil. This review provides discussion of equilibrium partitioning theory to account for the distribution of HOCs between soil, aqueous phase, sorbed surfactant, and micellar surfactant phases, as well as recently developed models for surfactant sorption onto soil. HOC partitioning is characterized by apparent soil–water distribution coefficients in the presence of surfactant.

Introduction

Surfactants are surface-active agents that alter the properties of fluid interfaces. The amphiphilic nature of surfactant molecules arises from their possession of both a strongly hydrophilic group and a strongly hydrophobic group (Rosen, 1989). In aqueous solution, surfactant molecules aggregate at the air–water interface resulting in reduced solution surface tension. As the concentration of surfactant increases, there is a critical concentration beyond which surfactant monomers start aggregating to form self-assemblies called micelles. The minimum surfactant concentration at which micelles start to form is called the “critical micelle concentration” (CMC). CMC is a function of surfactant structure, the composition, temperature, ionic strength, and the presence and types of organic additives in the solution (Rosen, 1989, Edwards et al., 1994b). At supra-CMC surfactant concentrations, any further surfactant addition contributes to the formation of additional micelles. The micelle in aqueous solution consists of surfactant molecules oriented so that their hydrophobic non-polar regions are in maximum contact with one another forming a hydrocarbon-like core, and their hydrophilic polar regions are in maximum contact with water (Yalkowsky, 1999).

Hydrophobic organic compounds (HOCs) tend to partition into the hydrocarbon-like micellar core, giving micelles the capacity to solubilize HOCs. This characteristic has resulted in significant interest in the use of surfactants for the treatment of recalcitrant organic contaminants in the soil–water environment by facilitating HOC desorption from soil and increasing apparent aqueous phase HOC concentrations. Several studies have looked into the use of surfactant-enhanced remediation for HOC-contaminated soils including soil washing applications and bioremediation (Fountain et al., 1991, Fountain et al., 1995, Liu et al., 1991, Laha and Luthy, 1992, West and Harwell, 1992, Guha and Jaffé, 1996, Bramwell and Laha, 2000, Grasso et al., 2001, Ussawarujikulchai et al., 2008). Mechanisms for promoting HOC remediation in soil–water systems include the reduction in interfacial tension, micellar solubilization, and phase transfer from soil to the pseudo-aqueous phase.

In the presence of soil, the surfactant dose required for micelle formation is greater owing to surfactant partitioning onto soil – a consequence of their amphiphilic nature. This results in a higher measured CMC for soil–water systems compared to the CMC in aqueous solutions; and this higher surfactant dose in soil–water systems is referred to as the elevated CMC or effective CMC (CMCeff). Losses of surfactant due to sorption need to be considered when selecting surfactant doses for soil/aquifer cleanup operations. The degree of surfactant sorption onto soil depends primarily on the organic carbon fraction of soil and the chemical nature of the surfactant (Harwell et al., 1999, Brownawell et al., 1997). The partitioning of an HOC on soil is almost exclusively into the organic carbon fraction of the soil (foc) if organic carbon constitutes at least 1% of the soil on a weight basis (Karickhoff et al., 1979). This suggests that the higher the organic content of soil, the greater the surfactant dose required for contaminant solubilization. Furthermore, in addition to reducing micelle formation, surfactant sorption also increases soil organic carbon content with implications on the partitioning behavior of target HOC contaminants.

Section snippets

Micellar solubilization of organic compounds

The molar solubilization ratio (MSR), which is defined as the ratio of the moles of solute solubilized to the moles of surfactant present as micelles, measures the effectiveness of a particular surfactant in solubilizing a contaminant. The MSR can be calculated as:MSR=CmicCCMCCsurfCMCwhere Cmic is the total apparent solubility of the HOC (in moles per liter) in micellar solution at a particular surfactant concentration greater than the CMC, CCMC is the apparent solubility of the HOC (in moles

Studies on surfactant sorption by soils

Liu et al. (1992) studied sorption of nonionic surfactants onto soil using surface tension measurements, spectrophotometry and chemical oxygen demand methods. They observed that whereas nonionic surfactant molecules sorbed onto soil, micelles did not adsorb onto soil, and surfactant sorption attained a maximum value as the surfactant dose approached that required for micelle formation. As the soil/water weight-to-volume ratio increased, higher amounts of surfactant had to be added to decrease

Surfactant sorption theory

The number of moles of surfactant sorbed per gram soil, Qsurf, may be estimated by the following relationship (Zheng and Obbard, 2002, Liu et al., 1992):Csurf,sorb=Csurf,soilCsurf,aQsurf=(Csurf,soilCsurf,a)(VaWsoil)=Csurf,sorb(VaWsoil)where Va is the volume of aqueous solution (L), Wsoil is the mass of soil (kg), Csurf,soil is the bulk surfactant dose in the soil/aqueous system that produces a surface tension value of σ in the supernatant (mol/L), Csurf,a is the corresponding surfactant

Conclusions

The ability of surfactants to promote contaminant mobilization by interfacial tension reduction, micellar solubilization, and phase transfer from soil-sorbed to the aqueous pseudo-phase has led to a range of studies with different organic contaminants both in the laboratory and the field. This paper attempts to summarize much of the work reported on the use of surfactants in the treatment of soils or aquifers contaminated by organic pollutants particularly as it relates to the fate of the added

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