Sorption of ionizable organic compounds on HDTMA-modified loess soil

A natural loess soil was modified using a cationic surfactant, hexadecyltrimethylammonium (HDTMA) bromide. Sorption of ionizable organic compounds (IOCs), 2,4-dichlorophenol (DCP), p-nitroaniline (NA) and benzoic acid (BA), on the modified soil was determined under different pH conditions. The objective of this study was to examine the sorptive characteristics of IOCs on HDTMA-modified loess soil as a function of pH in an attempt to establish the sorptive models and mechanisms for predicting the sorptive behaviors of IOCs on the HDTMA-modified loess soil. The sorption isotherms of DCP, NA and BA with the soil were obtained using the batch equilibration method. Results indicated that the sorption isotherms of IOCs, regardless of ionic or neutral forms, were non-linear and obeyed to the Freundlich equation. A model describing the sorption of IOCs on the HDTMA-modified loess soil was derived from the experimental data. The model well predicted the sorption of DCP from individual sorption of both ionic and neutral species of the IOC. In binary solute systems, sorption of NA was reduced in the presence of DCP or BA, which indicated that DCP and BA had a competitive effect on the sorption of NA on the HDTMA-modified loess soil. The effect of DCP on the sorption of NA gradually increased with decreasing pH from 10.8 to 6.7, suggesting a stronger effect of neutral DCP than that of the ionic species on the sorption of NA. Modification of loess soil may effectively immobilize ionizable organic contaminants in soil environment.