Structures of OTMA- and DODMA-bentonite and their sorption characteristics towards organic compounds

Illuminating the factors that influence the organic carbon content normalized sorption coefficient (KocKoc) of organoclays towards hydrophobic organic compounds (HOCs) is meaningful for predicting and optimizing the sorption capacity of organoclay. In this paper, the structures and sorption characteristics towards HOCs of organobentonites synthesized with octadecyltrimethylammonium chloride (OTMAC) and dioctadecyldimethylammonium chloride (DODMAC) were studied in order to further account for the variation of KocKoc. The conformations of bentonite-sorbed OTMA+ and DODMA+ transformed from disorder to order as surfactant loading increasing. The packing densities of DODMA+ aggregates were higher than those of OTMA+ aggregates at low surfactant loadings. At high surfactant loading region (1.0–1.4CEC for OTMA-Bent and 0.5–0.7CEC for DODMA-Bent), similar paraffin-type bilayer arrangements were adopted by sorbed OTMA+ and DODMA+, and their packing densities were close under the same focfoc in dry state organobentonites. It was found that loading forms of surfactant onto bentonite had important effect on the structure of organobentonite in water-saturated state, and further to influence the sorption characteristics of organobentonite towards HOCs. When the loading exceeded 0.8CEC, OTMAC in salt molecule form appeared in the clay interlayer via hydrophobic interaction. The strong hydration of surfactant ammonium heads and the counterions (Cl−) in aqueous system interfered the hydrophobic interaction of the OTMA+ clusters and destroyed the close packing in clay galleries. As a result, the sorption capacity of organobentonite towards HOCs was sharply reduced.

Graphical abstractThe appearance of OTMAC molecules in clay galleries destroyed the close packing of alkyl chains (a), then the sorption capacity of OTMA-Bent was reduced sharply (b).Figure optionsDownload full-size imageDownload as PowerPoint slide