Electrically conductive membranes for contemporaneous dye rejection and degradation

Ultrafiltration (UF) and nanofiltration (NF) membranes have attracted great research interest to treat dye wastewaters. However, UF membranes suffer from low dye rejection and NF membranes have low salt recovery. There is a tradeoff between the dye rejection and salt recovery in the UF and NF membranes. A novel membrane is required for dye rejection with high salt recovery. Herein, we report a novel UF conductive ceramic membrane made from nano-zeolite and carbon nanostructures (CNS) combined with an external electric potential to treat crystal violet (CV) dye/NaCl-Na₂SO₄ salt solution. The membrane showed trimodal hierarchical porosity, a water contact angle of ≈40°, good flexibility and high electrical conductivity. Voltages from 2 to 15 V were applied to the membrane acting as a cathode in a cross-flow filtration setup. High dye rejections ≈100% with a flux of 210 LMH at a voltage as low as 3 V were achieved. Contemporaneous dye degradation was observed with several intermediate compounds, identified through mass spectroscopy. It was observed that higher potentials produced nitrates/nitrites from organic intermediates as deduced from ion chromatography results. Donnan steric repulsion increased with higher potentials, leading to increased ion transfer resistance for anions and improved permeation for cations. Various permeate properties such as pH and conductivity were monitored, along with high salt recoveries, hence providing a huge advantage of using such a membrane for treating dye wastewaters with selective dye/salt rejection. The versatile properties, together with its facile fabrication process indicates tremendous prospect of zeolite/CNS membranes for multipurpose applications treating wastewaters containing charged molecules and ions.