Potential use of two filter media in constructed wetlands for simultaneous removal of As, V and Mo from alkaline wastewater - Batch adsorption and column studies

Abstract

The potential to use water treatment sludge and bauxite as active filter media in constructed wetlands to remove As, V and Mo from alkaline drainage originating from seawater-neutralized bauxite processing residue was evaluated in laboratory batch and column studies. Batch adsorption studies showed that increasing the electrolyte concentration from 0.01 to 0.30 M NaCl (the typical electrolyte strength of the drainage) increased adsorption of all three oxyanions onto both media while increasing initial pH from 6.7 to 8.3 (the typical pH of drainage) and using granules (1–2 mm dia.) rather than ground material (<0.2 mm) both decreased adsorption. Kinetic studies showed that while ionic strength had little effect on the contact time required to reach maximum adsorption, increased initial pH increased the time to reach maximum adsorption for Mo on both media and increased particle size increased the time required for maximum adsorption of all three oxyanions onto both media. In batch experiments, at initial elemental concentrations of 1 and 50 mg L−1, adsorption from multi-element solutions (compared with single element ones) was reduced in the order: Mo » As > V. In continuous flow column studies from single element solutions (1 mg L−1), breakthrough curves for Mo occurred first and greater than three times more eluent passed through the columns before breakthrough of V and then As occurred. When multi-element solutions were used, less volume of eluent was required for breakthrough of all three anions and the volume required before breakthrough of As was greatly reduced compared to that for V. The possibility that the strong ability of V to compete with As and particularly Mo could cause desorption of previously adsorbed Mo and As and their movement through a wetland filter needs to be further investigated. It was concluded that molybdate is the least strongly held oxyanion and that a decrease in solution pH within the wetland would greatly improve Mo removal efficiency.