Electron Transfer Improved by Nanoscale Zero-Valent Iron to Enhance Sulfate Reduction of the Anaerobic System under a Low Chemical Oxygen Demand-to-Sulfate Ratio

Insufficient availability of electron donors during the biological treatment of sulfate-rich wastewater can hinder the efficient and targeted removal of sulfate. Nanoscale zerovalent iron (nZVI) has been shown to enhance the anaerobic biological treatment of sulfate reduction as an electron donor; however, the exact mechanism of electron transfer remains unclear. Thus, this work delved into the impact of nZVI on enhancing the performance of an anaerobic biosystem, elucidating the underlying improvement mechanism systematically. The sulfate removal efficiency of an anaerobic biosystem could be enhanced by 30% through the use of nZVI under a chemical oxygen demand-to-sulfate ratio (COD/SO42–) of 0.3. The introduction of nZVI through electrochemical experiments has been shown to enhance the electrical conductivity and capacitance of anaerobic sludge. In addition, it could also stimulate the secretion of c-type cytochrome and extracellular polymeric substances. These findings collectively confirmed that the addition of nZVI enhanced electron transfer among microbial communities within the system. Moreover, nZVI facilitated the transition of important sulfate-reducing bacteria, specifically from Desulfobulbus to Desulfomicrobium, which can engage in direct interspecies electron transfer. Within a specific range, nZVI enhanced assimilatory sulfate reduction, leading to a heightened conversion of sulfate into sulfite, thereby enhancing sulfate reduction.