Efficient W and Mo deposition and separation with simultaneous hydrogen production in stacked bioelectrochemical systems

- W and Mo were efficiently deposited in stacked bioelectrochemical systems (BESs).
- W and Mo were separated from one another in the various units of the stacked BESs.
- A single BES unit serially matched with three parallel connected units was optimum.
- The circulation of the catholyte after acidification enhanced W and Mo deposition.
- Complete separation of W and Mo was achieved after the 7th circulation cycle.

Efficient deposition and separation of W(VI) and Mo(VI) with simultaneous hydrogen production, without external energy input, is achieved in stacked bioelectrochemical systems (BESs) composed of microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). The stacked BES-3-1 made of one MEC unit (1#) serially connected with three parallel connected MFC units (2#) outperformed other modules, achieving depositions of 27.6 ± 1.2% (W) and 75.4 ± 2.1% (Mo) with a separation factor of 8.1 ± 0.2 and hydrogen production of 0.34 ± 0.01 m3/m3 d in the 1# unit, compared to 12.3 ± 0.9% (W), 52.6 ± 2.2% (Mo) and 7.9 ± 0.5 (separation factor) in the 2# unit. The control experiments with W(VI) only deposited 6.8 ± 1.3% in 1# and 3.3 ± 0.4% in 2#, compared to 65.1 ± 3.2% in 1# and 45.2 ± 0.9% in 2# with Mo(VI) only. The control experiments with either the 1# or 2# unit and a mixture of W(VI) and Mo(VI) deposited 15.3 ± 1.7% (W) and 60.1 ± 1.6% (Mo) (1# only), and 12.9 ± 1.3% (W) and 56.1 ± 2.0% (Mo) (2# only). Reuse of the catholyte after acidification achieved complete separation of W and Mo from one another. This study demonstrates the feasibility of stacked BESs for W and Mo deposition and separation with simultaneous hydrogen production. The dual benefits of W(VI) and Mo(VI) species, and the favorable impact of the 2# unit on the 1# unit in the stacked BES module were critical to achieve efficient performance.

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