Effects of Nonionic Surfactants on Bacterial Transport through Porous Media DERICK G. BROWN AND PETER R. JAFFE Ä * Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544 Nonionic surfactants of the form C x E y , where x is the number of carbons in the alkyl chain and y is the number of ethylene oxide units in the polyoxyethylene (POE) chain, were studied for their ability to alter the transport of Sphingomonas pacilimobilis through an aquifer sand. The surfactants C 12 E 4 (Brij 30) and C 12 E 23 (Brij 35) were the focus of this study. Through a systematic study, it was shown that these nonionic surfactants were able to enhance the transport of this bacterial culture through porous media. The magnitude of the enhancement increased with decreasing solution ionic strength and increasing POE chain length. The mechanism of this enhanced transport appears to be due to expansion of the electric double layer about the bacteria and aquifer sand through displacement of the counterions by the sorbed surfactant. This expanded electric double layer increases the electrostatic repulsion, with a resultant reduction in the collision efficiency and an increase in the Langmuirian blocking parameter. Application of the colloid filtration theory with the experimental parameters of this study shows that nonionic surfactants have the potential to significantly enhance the bacterial travel distance, especially for low ionic strength systems. Introduction Background. The introduction ofsurfactantsto groundwater has the potential to alter the transport of bacteria through porous media. Surfactants readily adsorb onto bacteria and aquifer sands (1). Due to their surface-active properties, the sorbed surfactant can affect bacterialattachment byaltering the electrostatic,hydrophobic,and steric interactions.Ifthis results in an increase in the repulsion between the sand grains and the bacteria, the surfactants will enhance the transport of bacteria. Environmental engineers are interested in the potential for surfactants to enhance bacterialtransport for two reasons. The first is related to the bioaugmentation of subsurface contaminants.Bacteria readilyadhere to porous media,and development of a means to disperse the bacteria is an important consideration when attemptingbioaugmentation of contaminated groundwater (2). Surfactants that have the potential to inhibit bacterial attachment onto the porous media would serve to reduce biofouling and enhance the transport of bacteria into a contaminant plume. This may allow bioaugmentation to remain a viable option for bio- degrading subsurface contaminants when unfavorable bac- terial transport conditions exist. The second reason is the potentialthreat to public health by surfactant-facilitated transport of pathogenic microor- ganisms from septic tanks and other waste streams into potable aquifers. Synthetic surfactants from commercial detergents are the major man-made organic contaminant in raw sewage wastewaters and are ofconcern due to the results of recent studies that have found commercial surfactants in measurable quantities in groundwater aquifers (3). The presence ofsurfactants in domestic waste is important based on the fact that the dominant source of pathogenic micro- organismsishuman and animalfeces(4)andthatthemajority of drinking water disease outbreaks between 1972 and 1998 associated with microbial pathogens have been due to consumption of untreated groundwater (4-7). By having a better understanding of how surfactants affect bacterial transport, we may gain an appreciation of how commercial detergents in waste streams may affect the transmission of diseases through groundwater aquifers. While there has been a fairly extensive study on the transport of bacteria through porous media, there is very little data on the effects ofsurfactants on bacterialtransport. A few studies have shown that ionic surfactants can alter bacterial and colloidal transport through surface charge modification by the charged surfactants (8-12). Biosurfac- tants are beginning to be studied for their potential in enhancing bacterial transport (13). And preliminary studies have shown that nonionic surfactants can enhance bacterial transport (11, 14, 15). While the mechanism for enhanced bacterial transport with charged surfactants is understood, the mechanisms for enhanced bacterial transport with nonionic surfactants are still unknown. It is the purpose ofthis paper to identifythe mechanisms for enhanced transport of bacteria through an aquifer sand with nonionic surfactants. The approach focused on a systematic examination ofthe effects ofnonionic surfactant structure and solution ionic strength on the transport of bacteria through sand columns.The resultsofthese bacterial transport experiments were interpreted through bacterial surfacepropertymeasurements(16)and through application of the colloid filtration and DLVO theories, which are discussed in the following section. Theory The colloid (clean-bed) filtration theory was developed to understand and predict colloidal removal in packed beds (17). It assumes a constant rate of removal of the form where C is the colloid concentration and λ is the filter coefficient. Solution of eq 1 gives the standard form of the colloid filtration theory where L is the thickness of the packed bed, C is the colloid concentration exiting the packed bed, and C o is the initial colloid concentration entering the packed bed. The filter coefficient is dependent on the porous media, colloid, and fluid properties and the fluid flow conditions. It has been defined as (17) *Correspondingauthor phone: (609)258-4653;fax: (609)258-2799; e-mail: jaffe@princeton.edu. ∂C ∂x )-λ (1) C C o ) exp[-λL] (2) λ ) 3 2 (1 - θ) d c Rη (3) Environ. Sci. Technol. 2001, 35, 3877-3883 10.1021/es010577w CCC: $20.00  2001 American Chemical Society VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3877 Published on Web 08/24/2001