Removal of hexavalent chromium by membrane-based hybrid processes

The presence of the toxic metal Cr(VI) in wastewater is a major concern from an environmental point of view, and its complete removal from waste solutions is a difficult task to accomplish. Membrane-based hybrid processes have been developed to maximize its efficient removal from aqueous wastes. One such process comprises emulsion liquid membrane (ELM) extraction and reduction-precipitation. ELM is a one-step process involving extraction and stripping, simultaneously. The liquid membrane constitutes of an extractant and surfactant dissolved in the organic diluent. The addition of strip phase into the organic phase results in the formation of an emulsion. The selection of surfactant is decisive in the ELM extraction process, as the concentration of the target metal inside the strip phase heavily relies on it. Aqueous waste containing Cr(VI) (300 mg/L) was treated with emulsion to gain higher concentrations of Cr(VI) inside the strip solution in one step. Important parameters affecting the ELM process of Cr(VI) concentration, such as feed phase pH, role of extractant, and surfactant and stripping concentration were studied. The preconcentrated Cr(VI) (> 15 times) solutions obtained after breaking of emulsions were further reduced to nontoxic Cr(III) form using a FeSO4 catalytic method at pH 2. Effects of reactant ratio and temperature on the reduction process were examined. A complete reduction of Cr(VI) was achieved using a 10% stoichiometric excess of FeSO4 and 100 degrees C temperature. Further, the nontoxic metal Cr(III) is precipitated with alkaline solutions. The dependence on pH of the solubility of Cr(Ill) was identified. Finally, it was observed that the membrane-based hybrid processes minimized the concentration of Cr(VI) far below the level of the discharge limit.