Adsorption of dissolved copper and zinc on sand and iron oxide-coated sand (IOCS) for urban stormwater treatment: effects of pH, chloride, and sulfate

Abstract

This study investigated the adsorption efficiencies of dissolved copper (Cu(II)) and zinc (Zn(II)) on sand and iron oxide-coated sand (IOCS), which were considered as potential filter materials for treating urban stormwater runoff. Through a series of batch adsorption experiments, we compared adsorption efficiencies between sand and IOCS and examined the effects of varying stormwater conditions including pH, the coexistence of copper and zinc, and concentrations of chloride and sulfate ions. Surface area and point of zero charge of the sand media largely increased by coating iron oxides. IOCS always showed greater and stable removal of Cu(II) and Zn(II) with greater removal efficiency for Cu(II) than Zn(II). The effect of pH on the removal efficiency of Zn(II) by IOCS was minimal at pH 6-8 whereas the removal efficiency of Cu(II) by IOCS profoundly increased at pH > 7 likely due to the precipitation of copper hydroxide. The coexistence of Cu(II) and Zn(II) decreased their adsorption efficiencies by IOCS due to their competition for the available sites on the IOCS surface but with less decrease compared to sand. Adsorption efficiencies of sand and IOCS decreased as the concentrations of the Cl- and SO42- increased with smaller effects on IOCS than on sand. Competitions of the heavy metal ions with other cations such as Ca+ and Na+, the counter-ions for Cl- and SO42-, might be the main cause of the decreased adsorption. The results of this study suggest that the use of IOCS as a filter media for the urban stormwater treatment can be potentially beneficial compared to conventional filter media such as sand because it can provide more stable and efficient removal of heavy metals contained in the stormwater runoff under varying chemical conditions.