Mobile Organic Sorbent Affected Contaminant Transport in Soil: Numerical Case Studies for Enhanced and Reduced Mobility

cleanup. Major compounds that have been shown to increase the solubility and thus the mobility of nutrients Mobile organic sorbents (MOS) such as dissolved and colloidal and contaminants are surfactants (Edwards et al., 1991; phase organic matter control flow of water and transport of solutes in soils. We studied the effect of MOS on contaminant fate by systematic Haulbrook et al., 1993; Rahman et al., 1994; Liu and numerical case studies. The scenarios considered were (i) enhanced Chang, 1997), cosolvents (Nkedi-Kizza et al., 1985, 1987; mobility, (ii) reduced mobility due to cosorption, and (iii) reduced Brusseau et al., 1991), (hydr)oxides, clay and other minmobility due to cumulative sorption. The enhanced mobility and coerals (Saiers and Hornberger, 1996; Karathanasis, 1999; sorption scenario require contaminant sorption to the MOS. The Degueldre et al., 2001), humic substances (Kan and cosorption and cumulative sorption scenario require sorption of MOS Tomson, 1990; Ding and Wu 1997; McCarthy and Zato the immobile sorbent. Simulations were run for physicochemically chara, 1989; Dunnivant et al., 1992; Totsche et al., 1997; different fractions of dissolved organic matter and two model contamiMcCarthy, 1998; Kinniburgh et al., 1999) and huminnants. Mobile organic sorbents mobility is characterized by a wide coated inorganic colloids (Amirbahman and Olson, range of retardation parameters. Continued import of MOS to subsoil 1993; Kretzschmar and Sticher, 1997). In contrast to material high in MOS-specific sorption sites will increase the solid phase organic matter content. Enhanced mobility was observed for aquatic environments (Buffle et al., 1998; Wilkinson et soils without MOS-specific sorption sites or for situations where MOS al., 1999; Degueldre et al., 2000; Jin and Flury, 2002), the are in sorptive equilibrium with the immobile sorbent. Cosorption majority of dissolved and colloidal-size solution phase resulted in reduced contaminant mobility. The extent to which reduced constituents in soils is either biotic or organic in nature, mobility was observed depended on the ratio of the affinities of the or composed of inorganic matter with organic coatings. free contaminant and the MOS-bound contaminant. As the sorption Due to their chemical properties (i.e., chemical diverof the MOS-bound contaminants is controlled by the properties of sity and high concentration of functional groups), these the MOS, the characterization of these properties is a crucial step substances interact both with components of the immofor the estimation of the effect of MOS on contaminant mobility. bile solid phase and with other dissolved and colloidal Cumulative sorption resulted in reduced contaminant mobility as well. phase components of the liquid phase. Contaminant However, this result is the consequence of the increase of the sorption capacity due to the sorption of MOS, a long-term process that may facilitated transport in the presence of MOS applies to last for years to decades. such environmental conditions where MOS have to be considered a nonreactive mobile-phase constituent; that is, neither filtration nor sorption or partitioning of MOS to the immobile solid phase occurs. This is the case for M organic sorbents such as dissolved and colporous media that is either in sorptive equilibrium with loidal phase organic matter affect flow of water MOS or has unfavorable conditions for MOS sorption and transport of solutes in soils (McCarthy and Zachara, or attachment. In soils, however, MOS have to be con1989; Murphy and Zachara, 1995; Kögel-Knabner and sidered reactive and are subject to immobilization due Totsche, 1998; Totsche, 2001). Major processes between to sorptive interactions with the immobile soil (Murphy the solution and the solid phase, such as sorption, partiand Zachara, 1995; Kaiser et al., 1997; Weigand and tioning, speciation, and ion exchange, are influenced by Totsche, 1998). Common MOS sorbents are iron-, manthe interactions with MOS. Their presence affects the ganese-, and aluminum-(hydr)oxides; clay minerals; and solute solubility due to complexation, solubilization, organic matter (Kaiser et al., 1996; Jardine et al., 1989; carrier association, and the solvophobic effect. In recent McCarthy et al., 1993; Weigand and Totsche, 1998; Kaiyears, research has focused on processes leading to moser and Zech, 1998). In such environments, MOS may bility enhancement of organic and inorganic pollutants cause reduced overall contaminant mobility due to im(Magee et al., 1991; Kan and Tomson, 1990; Grolimund mobilization of MOS and thus immobilization of the et al., 1996; Elimelech and Ryan, 2002; Kretzschmar et MOS-associated contaminants (Lee et al., 1989; Totsche al., 1999; Baumann et al., 2002; Saiers, 2002). The issue et al., 1997). The underlying process has been described was to understand to what extent colloids may facilitate as cosorption. Also, mobile organic sorbents sorption contaminant transport in porous media with respect to can lead to an increase of the organic C content of the risk assessment, soil and groundwater reclamation, and bulk soil, thus increasing potential contaminant binding K.U. Totsche and I. Kögel-Knabner, Lehrstuhl für Bodenkunde, sites, a process referred to as cumulative sorption Department für Ökologie, Wissenschaftszentrum Weihenstephan, (Totsche et al., 1997). Technische Universität München, D-85350 Freising-Weihenstephan, In this paper, we analyze the effect of MOS on the Germany. Received 5 July 2003. Special Section: Colloids and Colloidmobility of contaminants in different environmental setFacilitated Transport of Contaminants in Soils. *Corresponding author ([email protected]). Abbreviations: BTC, breakthrough curve; HM, heavy metals; HOC, Published in Vadose Zone Journal 3:352–367 (2004).  Soil Science Society of America hydrophobic organic compounds; MOS, mobile organic sorbents; PAH, polycyclic aromatic hydrocarbons. 677 S. Segoe Rd., Madison, WI 53711 USA