Azo dyes as electron transfer mediators in the electrochemical reduction of Cr(VI) using boron-doped diamond electrodes

Abstract

Reducing hexavalent chromium in industrial wastewater to its less toxic trivalent state is an important but challenging problem. Among the available techniques, electrochemical reduction has many advantages. Within electrochemical cells, boron-doped diamond (BDD) electrodes are advantageous in that they are inert and have a high overpotential. However, even at optimal pH of 2 with a supporting electrolyte, a BDD electrochemical cell could only achieve a reduction of 36% of aqueous Cr(VI). However, small amounts of azo dyes enhanced the process by serving as an electron transfer mediator which increases the electrochemical effectiveness. The three azo dyes thionine (Th), methyl orange (MO), and methylene blue (MB) all increased the effectiveness of the electrochemical reduction, with MO increasing it to about 90%. The electrochemical process also oxidized 98% of the azo dye. Other than electrolytes and acid, the only chemical reagent added was the MO, which was decomposed in the process, and no sludge was produced, so the process is quite environmentally friendly.