Enhanced organic contaminant sorption on soil treated with cationic surfactants

Batch sorption and column transport laboratory experiments were performed to investigate the use of cationic surfactants to enhance sorption of dissolved organic contaminants in aquifer material with low organic carbon content. The experiments used sand from the Borden aquifer in Ontario, Canada, treated with hexadecyltrimethylammonium (HDTMA(+)) surfactant. Test results indicate that organic cations are strongly adsorbed by cation exchange reactions on the solid surface. Further adsorption occurs in admicelles, which form by hydrophobic interactions between adsorbed and mobile HDTMA monomers. The nonlinear adsorption Isotherm may be described by Langmuir adsorption theory. The retardation factor for HDTMA(+) on Borden aquifer material is approximately 460 when aqueous surfactant concentrations are less than 100 mg/l, while HDTMA(+) is relatively mobile at higher concentrations. Column experiments suggest that tbe adsorption reaction is very rapid, and thus independent of ground-water flow velocity. A column treated with HDTMA(+) equivalent to 33% of CEC and flushed with 325 pore volumes of surfactant-free water showed that although the HDTMA(+) modified zone was smeared downgradient, all HDTMA(+) mass remained adsorbed within the column. The results indicate that an essentially stationary surfactant-modified zone can be created. Adsorbed HDTMA(+) mass increased f(oc) from 0.02 wt% on natural Borden aquifer material to 0.18 wt% on the treated sediments. Organic matter derived from the adsorbed HDTMA(+) is an effective partitioning medium for dissolved HOC, and results indicate that it may sorb HOC more efficiently than natural f(oc) on a unit weight basis. Batch sorption tests showed that sorption coefficients (K-d) for trichloroethylene (TCE), perchloroethylene (PCE), and 1,2,4-trichlorobenzene (TCB) were 13 times higher on HDTMA(+) treated Borden aquifer material compared to the natural sediments. Retardation factors estimated from column transport tests confirmed these results, and suggested that sorption of moderately hydrophobic organic contaminants on the HDTMA(+) modified sediments was a rapid and reversible process. Data from these laboratory-scale experiments indicate that it may be feasible to develop an enhanced sorption zone in situ in a low f(oc) aquifer as part of a ground-water remediation scheme for dissolved organic pollutants.