Metal oxide nanoparticles resonate to ammonium removal through influencing Mg2+ absorption by Pseudomonas putida Y-9.

Most metal oxide nanoparticles (NPs) can impact ammonium removal, but the underlying mechanism remains unclear. In this study, high doses of NiO, CuO, ZnO and TiO2 (>1 mg/L) inhibited the ammonium removal performance of Pseudomonas putida Y-9. Interestingly, low levels of CuO NPs (0.1, 0.5 mg/L) and NiO NPs (0.1 mg/L) enhanced ammonium removal efficiency. Moreover, a decrease in Mg2+ levels was significantly positively correlated with ammonium removal efficiency, while negatively correlated with the Ti2+, Zn2+, and Cu2+ release of NPs. Further research on effect of NPs and their corresponding cations on ammonium removal revealed that four NPs affected Mg2+ absorption in Y-9 via different routes, thus impacting NH4+ removal efficiency, i.e., the effect of NiO NPs was caused by itself, TiO2 NPs' impact was solely due to the release Ti4+, while the influence of CuO NPs and ZnO NPs was based on both the particles and released ions.