Comparison of Co(II) reduction on three different cathodes of microbial electrolysis cells driven by Cu(II)-reduced microbial fuel cells under various cathode volume conditions

Abstract

Reduction of aqueous Cu(II) and Co(II) is one critical step for simultaneous recovery of copper and cobalt, and recycle of spent lithium ion batteries, but suffers from consumption of large amount of energy and chemicals. Here we report Co(II)-reduced microbial electrolysis cells (MECCo) can be driven by Cu(II)-reduced microbial fuel cells (MFCCu) for simultaneous Cu(II) and Co(II) recovery with no external energy consumption, and system performance was heavily dependent on cathode material of MECCo and cathode volumes in both MECCo and MFCCu. Either titanium sheet (TS) or stainless steel mesh (SSM) cathode achieved efficient Co(II) reduction whereas carbon rod (CR) cannot proceed this occurrence. While smaller cathode volumes in MFCCu led to appreciable Co(II) reduction (41.4±3.8%) on the CR cathode, the highest Co(II) reduction using TS (45.0±0.3%) or SSM (39.7±3.6%) was obtained under smaller cathode volumes in both MFCCu and MECCo. Moreover, when a mixed Cu(II) and Co(II) catholyte was deliberately used as the influent of MFCCu and the effluent of MFCCu was subsequently imported into the connected MECCo for tentatively simultaneous Cu(II) and Co(II) recovery from simulated mixed wastes, this so-called sequential MFCCu and MECCo operation achieved Cu(II) reduction of 100% and Co(II) reduction of 65.3–72.0% using either TS or SSM cathodes. These results illustrate cathode material of MECCo and cathode volumes in both MECCo and MFCCu were critical for efficient Co(II) reduction in MECCo driven by MFCCu with achievements of simultaneous copper and cobalt recovery as well as no external energy consumption.