Impact of Spatial Heterogeneity Evolution in Cathodic Biofilm of Microbial Electrochemical Cell on Sulfate Reduction Performance with the C/S Ratio Changes

Microbial electrochemical cell (MEC) can supply electrons to sulfate-reducing bacteria (SRB) through the wastewater and cathode electrodes. Due to differences in their competitive ability with other bacteria when utilizing electron donors, SRB inevitably exhibits a heterogeneous distribution within the cathode biofilm, thereby affecting the removal of sulfate. This study reveals the driving mechanism of the spatial heterogeneity evolution within cathode biofilms, and its impact on sulfate reduction performance by changing the chemical oxygen demand-to-sulfate (C/S) ratio. We found that when the C/S ratio decreased from 5 to 3, the sulfate removal rate increased by nearly 10%, which indicates that the change of C/S has an impact on the desulfurization performance of MEC. The middle layer of the biofilm became an active region for SRB, with the abundance of Desulfobacterota and Desulfovibrio in this layer higher than that in the surface and inner layers. At C/S=5, Bacteroidota and Desulfovibrio were the dominant microbial groups, mainly distributed in the surface (23.8%) and middle (22.2%) layers of the cathodic biofilm, with dissimilatory sulfate reduction pathway prevailing. After the ratio decreased, Synergistota and Aminomonas were dominant, both concentrated in the middle layer, accounting for 28.5% and 21.8% respectively. Meanwhile, relative abundances of genes associate with assimilatory sulfate reduction pathway (CysJ, CysI, CysNC) also increased. This study demonstrates that after C/S ratio changes, the spatial stratification of the cathodic biofilm microbial communities evolves in a more efficient direction, enhancing the reduction efficiency of the system and providing a theoretical basis for the low-energy consumption biological electrochemical treatment of sulfur-containing wastewater.