Enhancing Sulfate Reduction Efficiency in Microbial Electrolysis Cells: The Impact of Mixing Conditions and Heavy-Metal Concentrations on Functional Genes, Cell Activity, and Community Structure in Sulfate-Laden Wastewater Treatment

Microbial electrolysis cells (MECs) are promising for the treatment of sulfate-laden wastewater. The performance of the MEC cathode biofilms is influenced not only by the wastewater quality but also by the hydrodynamic mixing condition. Yet, the combined effects of these combined conditions have seldom been explored. This study examines the effectiveness and operational patterns of MECs in treating sulfate-laden wastewater under varying heavy-metal (Cu²⁺ as representative) concentrations (0–80 mg L^–1) and different hydrodynamic conditions (complete-mixing (CM) and nonmixing (NM, as control)). Results showed that CM-MECs achieved higher sulfate reduction efficiency (51 to 76%) compared to NM-MECs (with 46–69% of sulfate reduction) across the range of Cu²⁺ concentrations. Kinetic analysis revealed that CM-MECs reduced sulfate faster due to increased expression of genes involved in sulfate reduction and electron transport. Furthermore, CM-MECs maintained intact cell structures, enhanced electron transfer, and increased the relative abundance of Desulfobulbus when treating wastewater with low Cu²⁺ concentrations (<40 mg L^–1). Microbial defense mechanisms against Cu²⁺ also contributed to the enhanced sulfate reduction efficiency in the CM-MECs. These findings offer new insights into the design MECs with flowing conditions and pave the way for their future application in the treatment of heavy metal and sulfate-laden wastewater.