Re-evaluation of the environmental hazards of nZnO to denitrification: Performance and mechanism.

Numerous studies have shown that zinc oxide nanoparticles (nZnO) have an inhibitory effect on wastewater biotreatment, where doses exceeding ambient concentrations are used. However, the effect of ambient concentrations of ZnO (<1 mg/L) on anaerobic digestion processes is not clear. Herein, this study comprehensively explored the impact of nZnO on the denitrification performance and core microbial community of activated sludge under ambient concentrations. Results showed that only 0.075 mg/L nZnO had shown a beneficial effect on nitrogen removal by activated sludge. When nZnO concentration reached 0.75 mg/L, significant enhancement of nitrate reduction and mitigation of nitrite accumulation were observed, indicating a remarkable stimulatory effect on nitrogen removal. Simultaneously, nZnO could weaken the sludge surface charge and improve the secretion of extracellular polymeric substances, thus enhancing sludge flocculation for denitrification. Microbial community analysis revealed that nZnO exposure increased the relative abundance of denitrifying bacteria, which could contribute to the reinforcement of traditional denitrification. Furthermore, exogenous addition of NH4+ significantly inhibited the accumulation of nitrite, implying that nZnO had a potential to improve the denitrification process via a partial denitrification-anammox pathway. Considering current ambient concentration, the stimulatory effect shown in our work may better represent the actual behavior of ZnO in wastewater biotreatment.

Evaluation of antibacterial activities of silver nanoparticles on culturability and cell viability of Escherichia coli.

Nanoparticles released into the environment are attracting increasing concern because of their potential toxic effects. Conventional methods for assessing the toxicity of nanoparticles are usually confined to cultivable cells, but not applicable to viable but non-culturable (VBNC) cells. However, it remains unknown whether silver nanoparticles (AgNPs), a typical antimicrobial agent, could induce bacteria into a VBNC state in natural environments. In this work, the viability of E. coli, an indicator bacterium widely used for assessing the antibacterial activity of AgNPs, was examined through coupling plate counting, fluorescence staining and adenosine triphosphate (ATP) production. AgNPs were found to have a considerable antibacterial ability, which resulted in less than 0.0004% of culturable cells on plates. However, more than 80% of the cells still maintained their cell membrane integrity under the stress of 80 mg/L AgNPs. Meanwhile, the residue of ATP production (0.6%) was 1500 times higher than that of the culturable cells (< 0.0004%). These results clearly demonstrate that when exposed to AgNPs, most of cells fell into a VBNC state, instead of dying. Environmental factors, e.g., Cl- and illumination, which could change the dissolution, hydrophilicity and zeta potential of AgNPs, eventually influenced the culturability of E. coli. Inhibition of dissolved Ag+ and reactive oxygen species was found to facilitate the mitigation of the strain into a VBNC state. Our findings suggest the necessity of re-evaluating the environmental effects and antibacterial activities of AgNPs.