The effect of high applied voltages on bioanodes of microbial electrolysis cells in the presence of chlorides

While most microbial electrolysis cell (MEC) tests and other bioelectrochemical tests use applied voltages (E_ap) of 1 V or less, higher voltages are used in some tests that could lead to the generation of free chlorine species (FCS), from chloride ions, and hydroxyl radicals. To examine the impact of high E_ap on bioanodes, MECs were acclimated at E_ap = 1 V, tested for one cycle at an E_ap of 3 or 4 V until the same total coulombs were achieved as E_ap = 1 V, and then returned to cycles of E_ap = 1 V. All biotic MECs with chloride ions showed severe biofilm damage based on the absence of current production, lack of acetate oxidation, and the absence of hydrogen gas production in subsequent cycles at 1 V. Abiotic tests conducted at E_ap = 4 V, with same amount of total coulombs transferred as that which occurred in biotic tests at E_ap = 4 V, showed 1.8-fold higher acetate removal than biotic cells at 4 V, suggesting 43% of generated coulombs could have contributed to microbial inactivation. FCS generation, rather than hydroxyl radical production, was concluded to be the major contribution to oxidation of organics due to small changes in acetate oxidation in the presence of a hydroxyl radical scavenger, and the measurement of FCS. These results demonstrated that high applied voltages should be avoided if bioanodes are needed in bioelectrochemical systems when chloride species are present in the solution.