Sorption of arsenic by surfactant-modified zeolite and kaolinite

In this study, the feasibility of using surfactant-modified zeolite and kaolinite to remove arsenic from water was evaluated from batch experiments. The results showed that a significant increase in arsenate sorption capacity could be achieved as the loading level of hexadecyltrimethylammonium, a cationic surfactant, on zeolite and kaolinite surfaces exceeded monolayer coverage. At surfactant bilayer coverage, the arsenate sorption capacity reached up to 7 mmol/kg for surfactant-modified zeolite (SMZ) compared to almost none for the unmodified counterpart. Arsenite sorption on SMZ also increased although to a less degree. Similar results were observed for surfactant-modified kaolinite. Desorption of counterion bromide accompanying arsenate sorption followed 1.5:1 relationship, indicating that the sorption of arsenate on SMZ surfaces was due to anion exchange with the sorbed As(V) species being H2AsO4- and HAsO42- at neutral pH. In contrast, the ratio of bromide desorption to arsenite sorption followed 4.5:1 relationship, revealing that besides anion exchange, other mechanisms may also contribute to arsenite sorption on SMZ, since the As(III) species is H3AsO30 at neutral pH. Solution pH had a less effect on arsenate and arsenite sorption. Solution ionic strength had a significant effect on arsenate sorption but a minimal effect on arsenite sorption. The results show that surfactant-modified zeolite and kaolinite are better sorbents for arsenate rather than arsenite removal due to differences in sorption mechanisms.