Effects of Linear Alkylbenzene Sulfonate on the Sorption of Brij 30 and Brij 35 onto Aquifer Sand SHWETA TRIPATHI AND DERICK G. BROWN* Department of Civil & Environmental Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 Received August 21, 2007. Revised manuscript received November 21, 2007. Accepted November 23, 2007. Surfactant sorption is of considerable importance to environ- mental applications, including surfactant flushing to mobilize hydrophobic contaminants; effects of surfactants on the transport of dissolved contaminants, microorganisms, and colloids through porous media; and bioremediation of hydrophobic organic compounds, as well as understanding the fate and transport of surfactants as environmental contaminants themselves. Although most sorption studies consider pure surfactants, commercial detergent formulations typically consist of mixtures of nonionic and anionic surfactants. In this study, the effects of varying concentrations of the anionic surfactant linear alkylbenzene sulfonate (LAS) on micelle formation and sorption behavior of the two commonly used nonionic surfactants Brij 30 and Brij 35 onto aquifer sand were examined. A strong linear relationship was observed between the critical micelle concentration (CMC) of the Brij surfactants and the concentration of LAS in the mixture, with the CMC decreasing with increasing concentration of LAS. The relative change in CMC as a function of the LAS concentration was identical for the two Brij surfactants, indicating that LAS interacted with their common alkyl chains. Sorption isotherms were developed for Brij 30 and Brij 35 present as single surfactants in an aqueous solution as well as when present with LAS. Although LAS had minor effects on the maximum sorption plateaus of the Brij surfactants, Brij sorption at was significantly enhanced as a function of the LAS concentration for Brij aqueous concentrations below the CMC. Application of a multi-interaction isotherm model indicated that the formation of surface aggregates (e.g., hemimicelles) decreased with increasing LAS concentration. Overall, these results provide insight into the complex sorption behavior of surfactant mixtures. Introduction Surfactants are molecules that have both hydrophobic and hydrophilic moieties. This amphiphilic structure bestows surfactants with properties that are used in both industrial applications (e.g., agricultural crop applications, industrial cleaning, and textiles) and consumer goods (e.g., laundry and dishwashing detergents, bath soaps, shampoos, and bathroom and kitchen cleaners) (1). A key result of surfactant’s amphiphilic structure is that surfactant molecules gather at interfaces in an aqueous system, including air/water and solid/water interfaces. This sorption can have beneficial uses, such as with the reme- diation of contaminated soil and water. For example, surfactant sorption can be used to reduce interfacial surface tensions and to mobilize nonaqueous phase liquids trapped in the subsurface (2), and surfactant sorption on the bacterial cell surface can result in the formation of hemimicelles, which can enhance the bioavailability of hydrophobic compounds (3–5). Surfactant sorption can also affect the movement of microorganisms through groundwater via alteration of the interfacial properties (6–9). This process can be detrimental where surfactant release into the subsurface, such as through septic systems, can enhance the transport of pathogens to drinking water wells. It can also be beneficial, such as the use of surfactants with bioaugmentation schemes to facilitate transport of bacteria into contaminant zones (8, 10). Sorption of pure surfactants is fairly well understood, and it is known that surfactants often exhibit an S-shaped isotherm. This isotherm occurs as surfactant monomers sorb onto the surface at low aqueous surfactant concentrations, followed by the formation of surface aggregates (e.g., hemimicelles) as the aqueous surfactant concentration increases, with a sorption plateau ultimately occurring as the aqueous surfactant concentration increases beyond the critical micelle concentration (CMC) (3, 11–16). A few isotherm models have been developed that capture this complex shape (3, 17), and one has been successfully applied to elucidate the impact of surfactant sorption on the biodegradation of hydrophobic compounds (3, 18). Although surfactants are typically studied as pure com- pounds, in application they are predominantly used in mixtures because this often provides better performance over a single surfactant (1, 19–22). For example, in household detergents, which are typically mixtures of anionic and nonionic surfactants, the anionic surfactants are used to increase solubility, and nonionic surfactants are used to increase tolerance to hardness in water (23). Despite the wide use of surfactant mixtures, the sorption behavior of surfactant mixtures remains relatively unex- plored. Kibbey and Hayes (24, 25) explored the sorption of polydisperse ethoxylated nonionic surfactants to aquifer materials, and they developed a thermodynamic model that accurately represented this system. Wieslaw et al. (26) investigated the effects of propanol on the sorption of linear alkylbenzene sulfonate (LAS) at the air–water interface. Yang et al. (27) examined the impact of sodium dodecyl-benzene sulfate on the sorption of Triton X-100 onto calcium-enriched montmorillonite, and they observed a reduced sorption of individual components within the surfactant mixtures as compared to individual components. In contrast, Zhang et al. (28) observed cooperative sorption of the two nonionic surfactants NP-10 and n-dodecyl--D-maltoside on silica and alumina surfaces. To elucidate potential synergistic effects with surfactant mixtures, the current study focused on binary systems consisting of an anionic surfactant and a nonionic surfactant. Three surfactants commonly used in commercial products were examined, including the nonionic surfactants Brij 30 and Brij 35 (11, 29–31) and the anionic surfactant LAS (1, 26, 32–34). The goal was to identify the effects of LAS on the sorption of the nonionic surfactants onto an aquifer sand. To achieve this goal, there were two main objectives. The first objective was to experimentally develop sorption isotherms for the single surfactants and surfactant mixtures on aquifer sand. The second objective was to apply the isotherm model developed by Brown and Al Nuaimi (3) to * Corresponding author: phone: 610-758-3543; fax: 610-758-6405; e-mail: dgb3@lehigh.edu. Environ. Sci. Technol. 2008, 42, 1492–1498 1492 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 5, 2008 10.1021/es0720964 CCC: $40.75  2008 American Chemical Society Published on Web 01/25/2008