ENHANCED SORPTION OF ORGANIC CONTAMINANTS BY SMECTITIC SOILS MODIFIEDWITH A CATIONIC SURFACTANT

Soils, subsoils, and aquifer materials can be modified with hydrophobi c cationic surfactants to increase their sorptive capabilities for org anic contaminants. In this study, we evaluated the adsorption! desorpt ion of hexadecyltrimethylammonium (HDTMA) by smectitic soils, and the sorptive characteristics of the resultant organo-modified soils for tr ichloroethylene and chlorobenzene, Adsorption of HDTMA with loading le vels up to 70% of the cation exchange capacity (0.70 CEC) was nearly q uantitative and resulted in an equivalent release of Gale from the soi ls, indicating ion exchange as the sole mechanism over this range. At higher loadings, HDTMA is adsorbed by both ion exchange and hydrophobi c bonding, The selectivity coefficients for HDTMA replacing Ca2+ were very high (10(9) - 10(7) between 0.1 and 0.8 CEC). indicating the high chemical stability of HDTMA-soil complexes at these loadings. Desorpt ion is more significant for HDTMA adsorbed via hydrophobic bonding tha n via ion exchange. Sorption coefficients for trichloroethylene and ch lorobenzene on HDTMA-modified soils (0.7 CEC) were 20 to 60 and 100 to 350 times higher, respectively, than those on the corresponding unmod ified 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, respecti vely. A synergistic effect on sorption of trichloroethylene and chloro benzene in binary solute systems was observed. The sorptive characteri stics of HDTMA modified smectitic soils for organic contaminants are s imilar to those of pure HDTMA-smectites.