Fabrication of a novel conductive ultrafiltration membrane and its application for electrochemical removal of hexavalent chromium

The presence of Cr(VI) in the water/wastewater has serious environmental hazards due to its high toxicity. Ultrafiltration membranes are widely used in water/wastewater treatment due to their low cost and simple operation. However, conventional ultrafiltration membrane has low rejection of ions which limits its application for Cr(VI) removal. In terms of this, the CNT/PVDF/stainless steel mesh conductive ultrafiltration membrane was developed via nonsolvent induced phase separation (NIPS) method in this study. The properties of the membranes were characterized and the mechanism for the removal of Cr(VI) under different operational conditions was investigated. The results showed that the new generate membrane has good electrical conductivity and high mechanical strength. The conductive ultrafiltration membrane can realise the efficient removal of Cr(VI) under the application of voltage. It was found that the electrostatic repulsion between CrO42−/HCrO4− and negatively charged membrane plays a main role in Cr(VI) removal in the absence of electrolyte while electrochemical reduction plays an important role in Cr(VI) removal in the presence of electrolyte. Moreover, the removal efficiency of Cr(VI) was positively correlated with the ionic strength and residence time. The acidic condition was favorable for the removal of Cr(VI) when lower cell potentials (1 V, 3 V) were applied to the membrane and the neutral condition was beneficial for the removal of Cr(VI) removal when the applied cell potential increased to 5 V. Increase of the residence time by adjusting membrane operational pressure can further enhance the Cr(VI) removal. The highest removal of Cr(VI) (95.2%) was achieved under the best operation conditions (5 V, pH 7, 100 mM Na2SO4, 3.4 s residence time). It was demonstrated that the conductive membrane offers a promising way of Cr(VI) removal in the electrochemical filtration system.