Electrochemical reduction of hexavalent chromium on titania nanotubes with urea as an anolyte additive

The present study reports an energy efficient electrochemical strategy to remediate hexavalent chromium Cr(VI) to less toxic Cr(III). The novelty in this system is the use of titania nanotubes (TNT) synthesized by anodization as cathode for the reduction of Cr(VI) with alkaline urea as an anolyte additive. The electrochemical reduction of Cr(VI) was evaluated using bare Ti and porous TNT/Ti cathodes, both with and without urea as an anolyte additive. The experimental results showed that the dual combination of alkaline urea oxidation at anode and acidic Cr(VI) reduction on TNT cathode showed the best performance, with enhanced degradation efficiency up to 97% at 5 V within 15 min for an initial concentration of 100 mg L−1 Cr. The optimized urea-TNT combination was evaluated for the effect of catholyte pH, temperature, initial metal ion concentration and the effect of other anions, such as Cl−, NO3−, PO42− and CO32−, which are commonly present in wastewater discharged from tannery or electroplating industries. Furthermore, the performance when evaluated with cow urine as an anolyte additive confirmed that the dual combination could be a promising in-situ treatment technique for simultaneous urine oxidation and Cr(VI) reduction.