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ENVIRONMENTAL RESEARCH BRIEF: USE OF CATIONIC SURFACTANTS TO MODIFY AQUIFER MATERIALS TO REDUCE THE MOBILITY OF HYDROPHOBIC ORGANIC COMPOUNDS
Citation:
Westall, J. C., J. Wagner, J. Chen, AND B. J. Brownawell. ENVIRONMENTAL RESEARCH BRIEF: USE OF CATIONIC SURFACTANTS TO MODIFY AQUIFER MATERIALS TO REDUCE THE MOBILITY OF HYDROPHOBIC ORGANIC COMPOUNDS. EPA/600/S-94/002 (NTIS 95-111951), 1994.
Impact/Purpose:
Information.
Description:
Cationic surfactants can be used to modify surfaces of soils and subsurface materials to promote sorption of hydrophobic organic compounds (HOC) and retard their migration. For example, cationic surfactants could be injected into an aquifer downgradient from a source of HOC contamination to provide a temporary barrier against migration. A possible side effect of such a treatment could be the mobilization of previously adsorbed metal ions through cation exchange with the cationic surfactant. Batch and column experimemnts were performed to investiagte these phenomena. The cationic surfactant was dodecylpyridinium (DP); the HOC were chlorobenzene homologs; competing metal ions were Pb2+, Cd2+, and Cu2+; and sorbents were low-organic-carbon aquifer materials (Lula, EPA-12, Borden Sand) and pristine minerals (kaolinite, montmorillonite). The investigation covered three major topics: (i) adsorption of DP to particle surfaces; (ii)sorption of chlorobenzenes to DP-modified surfaces; and (iii) adsorption competition between DP and the metal ions. The adsorption isotherms of DP were distinctly nonlinear, even at very low surface concentrations. The distribution of DP was strongly dependent on the solution concentrations of Na+ and Ca2+, but virtually independent of solution pH. Amultisite adsorption model was developed to describe adsorption over a wide range of DP, NaCl, and CaCl2 concentrations. Two types of adsorption reactions were found to be significant: exchange of DP with a alkali-metal cation, and adsorption of pyridinium with chloride counter-ion. Distribution ratios of the chlorobenzenes varied nonlinearly with DP loading of the surface. The elution of chlorobenzenes from columns packed with DP-treated aquifer material showed significant retardation with only moderate amounts of DP on particle surfaces. A transport model based on results of the batch experiments and the local equilibrium assumption yields an acceptable approximation for the coelution of DP and HOC from the column. It is concluded that treatment of surfaces with cationic surfactants shows promise as a means of promoting HOC sorption in a variety of treatment processes. A comparison of the adsorption isotherms of DP, Pb2+,Cd2+,Cu2+ shows that DP is adsorbed more strongly than the metal ions from solutions at pH 5.5-6.0. Competition experiments suggest that DP and the metal ions adsorb to different types of sites, and that competition becomes significant only when about half the CEC of the material is occupied by DP. These results suggest that adsorbed metal ions could not effectively be removed by washing aquifer material with DP, but that aquifer materials could be treated with DP to retard HOC without danger of mobilizing adsorbed metal ions.