Enhancing nitrogen removal for low C/N ratio wastewater through substrates with MnOx-loaded biochar in vertical flow...

Biochar and manganese oxides (MnOx) are effective functional substrates in constructed wetlands (CWs) for enhancing nitrogen removal from wastewater with low carbon-to-nitrogen (C/N) ratios. This study synthesized MnOx-loaded biochar (BC/Mn) to synergistically combine the advantages of both materials, so as to improve the nitrogen elimination efficiency. Over a 180-d operational period, CWs utilizing BC/Mn substrates (CW-BC/Mn) demonstrated superior performance compared to those of systems adopting conventional biochar (CW-BC), MnOx alone (CW-Mn), or sand (CW-Control). Specifically, compared to the CW-Control, CW-Mn, and CW-BC groups, the CW-BC/Mn system achieved 32.08 %, 4.19 %, and 8.68 % higher ammonia nitrogen removal rates and 21.18 %, 15.29 %, and 9.29 % higher nitrate nitrogen removal efficiencies, respectively. This enhanced performance could be attributed to the unique physicochemical properties of the MnOx-loaded biochar, which provided microbial carbon sources and, notably, promoted long-term electron transfer for microbial metabolic processes. High-throughput sequencing revealed that the BC/Mn substrates selectively enriched key functional microorganisms, including nitrifying bacteria, denitrifying bacteria, manganese-reducing and ammonium-oxidizing denitrifies, and manganese-oxidizing and denitrifying bacteria which facilitated coupled nitrogen biogeochemical cycles. These results show that the use of MnOx-loaded biochar as a substrate represents a promising strategy for optimizing CW performance when treating low-C/N wastewater through integrated physicochemical and microbial methods.