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.