Simultaneous hexavalent chromium removal, water reclamation and electricity generation in osmotic bio-electrochemical system

For the first time, an osmotic microbial fuel cell (OsMFC) was employed for simultaneous hexavalent chromium [Cr(VI)] removal, organic contaminant removal, energy production, and water reclamation. In this system, draw solutions were prepared from 0 to 1 M NaCl mixed with 12 mg/L Cr(VI). Cr(VI) reduction and power generation at different pH were studied. In terms of power generation, the results revealed that the highest power density of 6.6 mW/m² (170 mW/m³) was obtained using the 1 M NaCl catholyte at pH 7. However, the greater Cr(VI) reduction of 44.5% and 97.6% was achieved using the 0.2 M NaCl/Cr(VI) based catholyte at pH 7 and 2, respectively. This can be attributed to the competition between Cr(VI) and oxygen reduction due to the high rate of dilution at higher concentrations of NaCl. Additionally, the highest open-circuit voltage of 0.752–0.845 V was recorded at 0.2 M NaCl/Cr(VI) as a proof to support that 0.2 M NaCl was a favorite condition for reducing Cr(VI). Moreover, through cyclic voltammetry, the signals of oxidation-reduction peaks were observed in the case of 0.2 M NaCl/Cr(VI), while, reduction peaks mostly disappeared in other cases. Additionally, more precipitate [Cr(OH)_3(s)] was observed to be deposited on the surface of the cathode electrode when 0.2 M NaCl was used as the draw solution. Hence, 0.2 M NaCl was the optimal catholyte concentration for Cr(VI) reduction. The Cr(VI) removal efficiency was 97.6% for an initial pH of 2 with the 0.2 M NaCl catholyte because low pH is thermodynamically favored for Cr(VI) reduction.