Interactions of Organic Contaminants with Mineral-Adsorbed Surfactants LIZHONG ZHU* AND BAOLIANG CHEN Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China SHU TAO Department of Urban and Environmental Sciences, Peking University, Beijing 100871, China CARY T. CHIOU U.S. Geological Survey, Box 25046, MS 408, Denver Federal Center, Denver, Colorado 80225 Sorption of organic contaminants (phenol, p-nitrophenol, and naphthalene) to natural solids (soils and bentonite) with and without myristylpyridinium bromide (MPB) cationic surfactant was studied to provide novel insight to interactions of contaminants with the mineral-adsorbed surfactant. Contaminant sorption coefficients with mineral-adsorbed surfactants, K ss , show a strong dependence on surfactant loading in the solid. At low surfactant levels, the K ss values increased with increasing sorbed surfactant mass, reached a maximum, and then decreased with increasing surfactant loading. The K ss values for contaminants were always higher than respective partition coefficients with surfactant micelles (K mc ) and natural organic matter (K oc ). At examined MPB concentrations in water the three organic contaminants showed little solubility enhancement by MPB.At low sorbed-surfactant levels,the resulting mineral- adsorbed surfactant via the cation-exchange process appears to form a thin organic film, which effectively “ adsorbs” the contaminants, resulting in very high K ss values. At high surfactant levels, the sorbed surfactant on minerals appears to form a bulklike medium that behaves essentially as a partition phase (rather than an adsorptive surface), with the resulting K ss being significantly decreased and less dependent on the MPB loading. The results provide a reference to the use of surfactants for remediation of contaminated soils/sediments or groundwater in engineered surfactant-enhanced washing. Introduction Sorption ofan organic pollutant to soilhas a major influence on its transport, bioavailability, and fate in natural environ- ments.Thesorption ofnonionicorganiccompounds(NOCs) from water to most soils is due primarily to the partitioning into soil organic matter (SOM) (1-4). Adsorption of NOCs from water by relatively polar soil minerals is suppressed by the strong competitive adsorption of water. Extensive research (1-3, 5-7) has shown that the sorption of NOCs increases with increasing soil organic matter content and decreasing water solubility of NOCs. Soils with high organic matter serve as large sinks of NOCs, limiting the NOC bioavailability; low organic-content soils and clays are ineffective in acceptingorganic contaminants.The potential for NOCleachingthrough the latter solids is high.As a result, many water-soluble organic chemicals are found as ground- water contaminants and pose a serious threat to human health. Clays or soils with cation-exchange capacities (CECs)may be modified by ion exchange with large organic cations (e.g., [(CH3)3 N R] + ) to increase their organic matter contents and thus to provide organophilic moiety favorable for sorption of organic contaminants (8-11). This simple modification has been performed in several previous studies to improve the retardation capacities of low organic matter soils and clays (12-17). According to Boyd and co-workers (12-13, 15, 16),the sorption ofNOCs to the long-chain organic moiety of the exchanged cations occurs by partition, analogous to the process with soil organic matter (1, 2). The surfactant- derived organic matter is 10-30 times more effective on a unit weight basis than naturalsoilorganic matter for sorbing nonpolar or weakly polar organic contaminants. Boyd et al. suggested that soils could be modified in situ with injections ofcationic surfactant solution (14) to attenuate the mobility of organic contaminants and thus to prevent further down- gradient groundwater and aquifer contamination (16-20). Recently, it is also suggested that organic pollutants sorbed to surfactants may be directly available for degradation by attached cells (21-25). Therefore, surfactant-enhanced re- tention oforganicpollutantsand the subsequent degradation by microbes has become one of the promising schemes for contaminated soil and groundwater remediation. Sorption of NOCs to sorbed surfactants (i.e., to hemimi- celles, admicelles) has been investigated in a few studies (26-34). Holsen et al. (26) examined the sorption of several NOCs on SDS-coated ferrihydrite and found that the ones with lowest solubility have the highest sorption. They also found a linear relationship between NOC sorption and the amount of SDS on the ferrihydrite, suggesting that the adsorbed SDSwasprimarilyresponsible forthe NOCuptake. Su n an d Jaffe (28)foundthatthepartitioningofphenanthrene with adsorbed dianionic surfactant phases (as monolayers and bilayers) is generally 5-7 times more effective than with the corresponding surfactant (as monomers and micelles) in water. Likewise, Nayyar et al. (29) reported partition coefficientsofseveralorganiccontaminantswith SDSsorbed on alumina that were higher in magnitude than the corre- spondingmicellar partition coefficients.The reason for these differenceshasnot been elucidated.Similarexperimentswere performed by Sun et al. (30) to obtain partition coefficients for three chlorinated NOCs to silt soils with a sorbed nonionic surfactant (Triton X-100). They observed that the sorbed surfactant enhanced NOC partitioning relative to the un- treated soils. However, when the surfactant concentration in water was greater than the CMC (the critical micelle concentration), the aqueous micelles competed more ef- fectively against the sorbed surfactant for NOC partitioning, leading to a net decrease in NOC distribution coefficients. Ko et al. (31, 32)investigated the partitioningofphenanthrene and naphthalene to aqueous surfactant micelles (SDS and Tween 80)and kaolinite-sorbed surfactants.Theyfound that the carbon-normalized solute sorption coefficients with the sorbed surfactants (K ss) were the highest at low sorbed- surfactant levels and were reduced sharply with increasing sorbed-surfactant level. That is, the NOC distribution coef- ficients between the immobile and mobile phases varied *Corresponding author phone: 86-571-8827-3733; fax: 86-571- 8827-3450; e-mail: lzzhu@mail.hz.zj.cn. Environ. Sci. Technol. 2003, 37, 4001-4006 10.1021/es026326k CCC: $25.00  2003 American Chemical Society VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4001 Published on Web 07/26/2003