Sorption of chromate and PCE by surfactant-modified clay minerals

Negatively charged clay mineral and zeolite surfaces have a strong affinity for cationic surfactants. Surfactant-modified clay minerals enhance sorption of nonionic organic compounds (NOCs) from aqueous solution due to their increased organic carbon content, Sorption of hexadecyltrimethy ammonium (HDTMA) cation by clay minerals and zeolite can far exceed the cation exchange capacities (CEC) of these minerals, up to 200% of the CEC for nonswelling clay minerals such as kaolinite and illite, and up to 250% of the CEC for swelling clay minerals such as smectite, The sorption of HDTMA by clay minerals follows a Langmuir isotherm, At the sorption plateau, the sorbed HDTMA molecules form a bilayer on the surface of the clay minerals and reverse the surface charge, which is responsible for the sorption of oxyanions such as chromate, The sorption of chromate by HDTMA-modified clay minerals also follows a Langmuir isotherm, while the sorption of perchloroethylene (PCE) follows a linear isotherm, The ratio of the amount of chromate sorbed to that of counterion (bromide) desorbed is close to 1:2, indicating that chromate sorption is mainly due to anion exchange, The organic carbon based partition coefficient K-oc for PCE varies with the surface density of the surfactant bilayer, which is controlled by the mineral's surface charge density, The dual sorption of chromate and PCE by surfactant-modified clay minerals extends their application to the removal of anions as well as hydrophobic organics from contaminated water.