Long-term exposure of Y2O3 nanoparticles on reactor performance and microbial community of activated sludge

The present study focuses on the potential effect of Y2O3 nanoparticles (NPs) on the stability of activated sludge (AS) after long-term exposure. Significant NO3--N and NO2--N accumulation were observed in AS with the presence of high-strength Y2O3 NPs, while the activities of the key functional enzymes of the nitrification process, ammonia monooxygenase (AMO) and hydroxyammonia oxidase (HAO), were reduced, and the release of lactate dehydrogenase (LDH) was increased, indicating that the nitrification performance of AS had been adversely affected. Besides, the decrease of polysaccharide (PS) and protein (PN) contents and the increase of microbial metabolites in extracellular polymeric substance (EPS) proved the cytotoxicity of Y2O3 NPs. Elemental analyses indicate that the blockage of PN in different channels also contributes to the decrease in biological nitrogen removal capacity. However, FTIR analysis showed that low concentration of Y2O3 NPs stimulated the production of beta-glucan with protective activities. Meanwhile, the variation trend that Proteobacteria and Chloroflexi also increased in relative abundance, suggesting that AS was inclined to maintain the normal performance and to resist the cytotoxicity of Y2O3 NPs. In the LEfSe method and redundancy analyses, increased abundance of Proteobacteria and Actinobacteria partially led to poorer nitrification performance, and high concentrations of Y2O3 NPs significantly affected microbial community succession.Additionally, prediction based on the PICRUSt2 metabolic pathway showed that the abundance of genes related to carbon and nitrogen metabolism was significantly decreased in AS at high concentrations of Y2O3, which led to the weakening of nitrification and affected the normal metabolism of microorganisms.