Preferable Individual Rather Than Sequential Feedings Under Air Exposure Conditions for Deposition of W and Mo in Stacked Bioelectrochemical Systems

Abstract

The stacked bioelectrochemical systems (BESs), composed of microbial electrolysis cells (MECs) driven by microbial fuel cells (MFCs), provide an alternative approach for the recovery and separation of mixed W(VI) and Mo(VI) without input of external energy. Herein, the stacked BESs, assembled with three parallel MFCs (2#, carbon rod cathode) connected to an MEC (1#, stainless steel mesh cathode) in series were advanced to be operated, where mixed W(VI) and Mo(VI) were continuously flowed either in parallel (influent to each individual cathodes, individual feeding) or serial (sequentially through the cathodes of the 2# to the 1# units, sequential feeding) modes. Individual feeding outperformed sequential feeding at an hydraulic retention time (HRT) of 4 h and under air exposure conditions, achieving deposition of 29.1-0.8% to 33.8-0.5% (W) and 65.2-0.4% to 71.8-0.8% (Mo) in the 1# units with separation factors of 4.3-5.9, higher than those in the 2# units (W: 13.4-0.6% to 14.9-0.3%; Mo: 31.7-0.4% to 39.6-0.3%; separation factor: 2.8-3.9). Anaerobic conditions or a shorter HRT of 2 h disfavored system performance. Electrochemical impedance spectroscopy data, scanning electronic microscopy observation, and X-ray photoelectron spectroscopy analysis supported the optimum system performance. The elucidation of such effects deepens our understanding of these operational parameters for optimization of the metallurgical self-driven BESs for W and Mo deposition and separation, which could ultimately lead to industrial application.