The enhancement effect of n-Fe3O4 on methyl orange reduction by nitrogen-fixing bacteria consortium.

Although the anaerobic reduction of azo dyes is ecofriendly, high ammonia consumption remains a significant challenge. This work enriched a mixed nitrogen-fixing bacteria consortium (NFBC) using n-Fe 3 O 4 to promote the anaerobic reduction of methyl orange (MO) without exogenous nitrogen. The enriched NFBC was dominated by Klebsiella (80.77 %) and Clostridium (17.16 %), and achieved a 92.7 % reduction of MO with an initial concentration of 25 mg·L−1. Compared with the control, the consortium increased the reduction efficiency of MO, cytochrome c content, and electron transport system (ETS) activity by 11.86 %, 89.86 %, and 58.49 %, respectively. When using 2.5 g·L−1 n-Fe 3 O 4 , the extracellular polymeric substances (EPS) of NFBC were present in a concentration of 85.35 mg·g−1. The specific reduction rates of MO by NFBC were 2.26 and 3.30 times faster than those of Fe(II) and Fe(III), respectively, while the enrichment factor of the ribosome pathway in NFBC exceeded 0.75. Transcriptome, carbon consumption, and EPS analyses suggested that n-Fe 3 O 4 stimulated carbon metabolism and secreted protein synthesized by the mixed culture. The latter occurred due to the increased activity of consortium and the content of redox substances. These findings demonstrate that n-Fe 3 O 4 promoted the efficiency of mixed nitrogen-fixing bacteria for removing azo dyes from wastewater. This innovative approach highlights the potential of integrating nanomaterials with biological systems to effectively address complex pollution challenges. [Display omitted] • N 2 gas was the only source of N needed for the bioreduction of methyl orange. • The enriched consortium XL1, dominated by Klebsiella and Clostridium had a high reduction rate for methyl orange. • n-Fe 3 O 4 accelerated the release of redox-active substances by the mixed nitrogen-fixing bacteria. [ABSTRACT FROM AUTHOR]