Bacteria-piezocatalyst for NO₃^- reduction to NH₄⁺ driven by hydraulic kinetic energy

In this study, we have constructed a microbial piezo-reduction system (MPRS) integrating inorganic piezoelectric materials (zinc oxide nanorods) with whole-cell biocatalysts (an electroactive bacterium, Shewanella oneidensis MR-1) as an innovative and sustainable way for NO₃^- reduction to NH₄⁺ driven by hydraulic kinetic energy. Based on the inward extracellular electron transfer capability of S. oneidensis MR-1, the MPRS reduced NO₃^- to NH₄⁺ through the dissimilatory nitrate reduction to ammonium (DNRA) pathway using the piezo-electrons from ZnO nanorods. Our MPRS achieved an average NO₃^- reduction efficiency of 97.97 % and an average NH₄⁺ production rate of 40.16 μmol·L^-1·h^-1 driven by hydraulic kinetic energy. The transient piezo-current (TPC) measurements illustrated that the S. oneidensis MR-1/ZnO biohybrid system possessed a fast piezogenerated carrier separation and transfer rate compared to the merely ZnO system. In addition, the kelvin probe force microscopy (KPFM) measurements found that the surface potential of ZnO was lower than that of S. oneidensis MR-1, providing evidence for the transfer of piezo-electrons from ZnO nanorods to S. oneidensis MR-1. The quantitative real-time polymerase chain reaction (real-time PCR) suggested that the reversing metal-reducing (Mtr) route in S. oneidensis MR-1 played a role in the piezo-electron transfer. This work demonstrates the feasibility of ammonium synthesis using piezo-electrons as electron donors in the MPRS, and maybe realize the in-situ ammonium recovery from nitrate-containing wastewater taking full advantage of using hydraulic kinetic energy converted from mechanical kinetic energy.