Reduction of Cr(VI) to Cr(III) by Activated Carbon Cloth through Adsorption and Electrochemical Processes

Effective electrode materials are required to eliminate high concentrations of toxic Cr(VI) contaminants from industrial effluents. Kynol activated carbon cloth (CC) is a commercial, high-surface-area, and mechanically stable carbon material that can be used for adsorption and/or electrochemical reduction processes. Here, the mechanisms of Cr(VI) adsorption and reduction by CC in the absence or presence of an applied potential were investigated using X-ray photoelectron spectroscopy, scanning electron microscopy/energy-dispersive spectroscopy, Raman, cyclic voltammetry, and chronocoulometry experiments. The pH change close to the CC electrode during the electrochemical process was monitored using a solution containing Bromo-cresol green as an indicator of the solution pH. The role of surface hydroxyl groups (−OH) on CC in (1) the adsorption of Cr(VI) and (2) the reduction of Cr(VI) to Cr(III) was elucidated. As found, redox reactions between Cr(VI) and surface −OH groups on CC led to the chemical reduction of Cr(VI) to Cr(III). Without an applied potential, the chemical reduction of Cr(VI) to Cr(III) was limited by the number of surface −OH groups on CC. With an applied potential of −0.6 V on CC, the Cr(VI) adsorption/reduction was 2.1 times faster and 1.5 times higher than that without an applied potential after 7 h. With an applied potential, the CC adsorbed Cr(VI) and chemically and electrochemically reduced Cr(VI) to Cr(III). This work indicates that for Kynol CC, the electrochemical reduction process is superior to adsorption when optimizing an electrochemical system to achieve a faster and higher reduction of Cr(VI) to Cr(III).