Enhanced denitrification performance and biocatalysis mechanisms of polyoxometalates as environmentally-friendly inorganic redox mediators

• First report of polyoxometalates effectively enhanced the denitrification process. • The NO 3 − -N reduction rate was enhanced 3.93-fold with phosphomolybdic acid (PMo 12 ). • The activation energy was decreased by 51.84 kJ/mol in the PMo 12 -supplemented system. • PMo 12 stimulated microbial generation of extracellular polymeric substances. • PMo 12 stimulated denitrification electron transport system activity. Polyoxometalates (POMs) used in chemical catalysis field were first explored their effect on the denitrification process. Experiments demonstrated that NO 3 − -N reduction rate with 0.05 mM phosphomolybdic acid (PMo 12 ) was approximately 3.93-fold higher than the PMo 12 -free system. Simultaneously, PMo 12 also had positive effect on NO 2 − -N reduction. Compared with the PMo 12 -free system, the solution resistance and oxidation-reduction potential were decreased, and the activation energy ( E a ) was reduced by 51.84 kJ/mol. Besides, electron conductive substances in extracellular polymeric substances were stimulated by PMo 12 . NADH and riboflavin were enhanced to increase denitrification electron transport system activity. Higher microbial diversity and enrichment of Salmonella were observed in the PMo 12 -supplemented system. Based on the above analysis, the catalyzing mechanisms of PMo 12 are proposed that PMo 12 made it easier for electron transferring from electron donor to electron acceptor and shifted bacterial community structure. These findings may provide a promising strategy for nitrogen wastewater treatment.