Optimizing the parameters of microbial fuel cells using response surface methodology to increase Cr(VI) removal efficiency and power production

Abstract

Cr(VI) is present naturally as Cr2O72- and its negative charge repels electrons (e-), preventing direct reduction to Cr(III). The Cr(VI) reduction process consumes H+ , suggesting that conventional Cr(VI) reduction techniques a strictly require a low pH, increasing their cost. Therefore, the development of a process that enables Cr(VI) reduction at neutral pH conditions is of interest. In this work, Cu(II) was used as an electron shuttle medium in a microbial fuel cell (MFC). The effects of MFC operating conditions, such as Cu(II)/Cr(VI) ratio, substrate (sodium acetate) concentration and external resistance on the power density (PD) and Cr(VI) removal efficiency were investigated using the response surface methodology (RSM). The maximum Cr(VI) removal efficiency (73%) and power production capacity (30.57 mW/m2) were achieved under pH-neutral conditions with a Cu(II)/Cr(VI) ratio of 1.65, a substrate concentration of 1.36 g/L, and an external resistance of 1360 Ω. The Cr(VI) removal efficiency of the MFC system without added Cu(II) was only 43%, and the power production capacity (15.85 mW/m2) was only half of that under optimal conditions. The removal efficiency of Cu(II) was 100% after seven days. Chromium and copper deposits were observed on the cathode surface, indicating that Cu(II) was deposited on the cathode surface following Cr(VI) removal, eliminating the secondary contamination that is caused by the addition of excess Cu(II). An RSM-optimized MFC system exhibits improved chemical oxygen demand removal, Coulombic efficiency and normalized energy recovery (NER). Its NER is at least 1.5 times higher than that of an unoptimized MFC system, indicating that the MFC most efficiently converts substrates into electrons when the operating conditions are optimized. This work demonstrates that Cu(II) plays a critical role as an electron-shuttle mediator in the removal of Cr(VI). The electron transfer function of Cu(II) increases the efficiency of the reduction that involves Cr(VI), electrons and H+, and eliminates the cost of adding acidic agents, as required in the conventional method in wastewater treatment. The obtained Cu(II)/Cr(VI) ratio is used to determine the maximum amount of Cr(VI) that can be removed by a scaled-up MFC system.