Capacitive Bimetallic Redox Cycles and Ligand-to-metal Charge Transfer to Boost Denitrification with NiII/FeII-Gallic Acid Phenolic Networks

Biological denitrification is limited by slow nitrate (NO3-) reduction due to low electron transfer efficiency, unsatisfactory community functional efficiency and insufficient metabolic activity of microbial communities. To overcome these challenges, Ni2+ and Fe2+ were incorporated with gallic acid (GA) to form bimetallic polyphenol networks (NiFeGA BPNs) with low-cost and high-biocompatibility. NiFeGA BPNs exhibited capacitive Ni(II)/Fe(II) redox cycles and excellent ligand-to-metal charge transfer capabilities to enable complete degradation of 200 mg/L NO3- within 8 h. All these improvements could be ascribed to that NiFeGA BPNs significantly improved electron transfer efficiency and stimulated microbial metabolic activity, which were proved by extracellular polymeric substances electrochemical analysis and electron transport chain inhibitors experiments. More importantly, metagenomic sequencing analysis confirmed that NiFeGA BPNs improved community structure by directionally enriching Pseudomonas. Consequently, NiFeGA BPNs significantly improving denitrification, which provides both theoretical guidance and technical frameworks for the continuous and efficient treatment of nitrate in wastewater.