The performance of Cu2+ as dissolved cathodic electron-shuttle mediator for Cr6+ reduction in the microbial fuel cell

Abstract The study investigates the performance of Cu2+ as dissolved cathodic electron-shuttle mediator (dcESM) for simultaneous Cr6+ reduction and electricity generation in a microbial fuel cell (MFC) at pH 2 and 4 conditions. The dcESM behavior of Cu2+ on carbon cloth (CC) catalyzes the reduction of Cr6+ into Cr3+ at pH 2 by undergoing redox reactions. However, at pH 4, a simultaneous reduction of Cu2+ and Cr6+ was observed. Cyclic voltammetry studies were performed at pH 2 and 4 to probe the dcESM behavior of Cu2+ for Cr6+ reduction on CC electrode. Also, at pH 2, increasing the concentration of Cu2+ from 50 to 500 mg L-1 favors the Cr6+ reduction by reducing the reaction time from 108 to 48 h and improving the current production from 3.9 to 6.2 mA m-2, respectively. Nevertheless, at pH 4, the efficacy of Cr6+ reduction and electricity generation from MFC is decreased from 63 to 18% and 4.4 to 1.1 mA m-2, respectively, by increasing the Cu2+ concentration from 50 to 500 mg L-1. Furthermore, the performance of dcESM behavior of Cu2+ was explored on carbon felt (CF) and platinum (Pt) electrodes, and compare the results with CC. In MFC, at pH 2, with an initial concentration of 100 mg L-1, the reduction of Cr6+ in 60 h is 9.6 mg L-1 for CC, 0.2 mg L-1 for CF, and 51.3 mg L-1 for Pt cathodes. The reduction of Cr6+ (initial concentration of 100 mg L-1) at pH 4 in 120 h is 44.7 mg L-1 for CC, 32.1 mg L-1 for CF, and 70.9 mg L-1 for Pt cathodes. Maximum power densities of 1659, 1509, and 1284 mW m-2 were achieved when CF, CC, and Pt, respectively were employed as cathodes in the MFC.