Enhanced Sorption of Organic Contaminants by Smectitic Soils Modified with a Cationic Surfactant

Abstract

AbstractSoils, subsoils, and aquifer materials can be modified with hydrophobic cationic surfactants to increase their sorptive capabilities for organic contaminants. In this study, we evaluated the adsorption/desorption of hexadecyltrimethylammonium (HDTMA) by smectitic soils, and the sorptive characteristics of the resultant organo‐modified soils for trichloroethylene and chlorobenzene. Adsorption of HDTMA with loading levels up to 70% of the cation exchange capacity (0.70 CEC) was nearly quantitative and resulted in an equivalent release of Ca2+ from the soils, indicating ion exchange as the sole mechanism over this range. At higher loadings, HDTMA is adsorbed by both ion exchange and hydrophobic bonding. The selectivity coefficients for HDTMA replacing Ca2+ were very high (109 −107 between 0.1 and 0.8 CEC), indicating the high chemical stability of HDTMA‐soil complexes at these loadings. Desorption is more significant for HDTMA adsorbed via hydrophobic bonding than via ion exchange. Sorption coefficients for trichloroethylene and chlorobenzene on HDTMA‐modified soils (0.7 CEC) were 20 to 60 and 100 to 350 times higher, respectively, than those on the corresponding unmodified soils. The HDTMA derived phase was 10 to 30 and 80 to 160 times more effective than natural soil organic matter (on a unit mass basis) as a sorptive phase for trichloroethylene and chlorobenzene, respectively. A synergistic effect on sorption of trichloroethylene and chlorobenzene in binary solute systems was observed. The sorptive characteristics of HDTMA modified smectitic soils for organic contaminants are similar to those of pure HDTMA‐smectites.