Adsolubilization of anthracene on surfactant covered silica in dependence on pH: indications for different adsolubilization in admicelles and hemimicelles

The enhanced sorption of contaminants on solid phases caused by surfactant adsorption is an important phenomenon for surfactant based environmental technologies. We have studied the distribution of anthracene between aqueous solution and silica gel covered with the cationic surfactant hexadecyltrimethylammonium (HDTMA) in batch experiments as a function of the adsorption density of HDTMA and in dependence on pH. It was observed, that the partitioning constants Koc, normalized on the content of organic carbon in the surfactant coverage, are not only a function of the surface concentration but also depend on pH. At low concentrations of adsorbed HDTMA no adsolubilization of anthracene was measured. The critical adsorption density CAD, at which adsolubilization is enabled, depends on pH. At pH 5 the CAD is higher than at pH 8, 1×10−8 mol m−2 versus 5×10−9 mol m−2, approximately. Beyond the CAD at pH 5 the Koc values of anthracene steeply increase with increasing adsorption density of HDTMA and reach a plateau of about 1.9×105 l kgoc−1 which is about two times higher compared to 9×104 l kgoc−1 reached in the plateau at pH 8. The increase of the Koc values beyond the CAD at pH 8 is less steep compared to pH 5. We explain the dependence of the adsolubilization on pH by the formation of different types of surfactant aggregates at the silica surface due to the influence of the pH on the surface charge density. We suggest that in the determined range of surfactant concentration at the surface the low surface charge density at pH 5 leads to the formation of admicelles beyond the CAD while the high surface charge density at pH 8 cause the formation of hemimicelles.