Toxicity Effects And Mechanisms Of ZnO Nanoparticles On A Typical Ammonia Oxidizing Bacterium

As ZnO nanoparticles (n-ZnO) has been widely applied in a great variety of fileds, the manufacturing, transportation, application and disposal of them will inevitably lead to their release into wastewater treatment plants (WWTPs). The objective of this study was to investigate the effects of n-ZnO at different dosages on representlve ammonia oxidizing bacteria (Nitrosomonas europaea) in biological nitrogen removal system after short or long-term exposure under different conditions, together with the recovery performance of bacteria biochemical activity under n-ZnO stress.Acute (6 h) exposure were carried out with different dosages (O.lmg/L, lmg/L, 10mg/L and 50mg/L) of n-ZnO to assess the adverse impacts on stable growth N.europeae in chemostat bioreactor. The results showed that exposure to n-ZnO with increasing dosage led to the increase in cell membrane damage and the inhibition in cell density, ammonia-oxidizing activity and AMO enzymatic activity. Either n-ZnO is 10 mg/L or 50 mg/L, the activity of bacteria was shown the significant inhibitory effect. In addition, The significant toxicity of n-ZnO is due to the size effect of itself, because the cell membrane damage and ammonia oxidation inhibition caused by 50mg/L n-ZnO was significantly higher than its corresponding deliquescent Zn2+. And the biotoxicity of Zn2+ on N.europeae was higher than of n-ZnO at the same molar concentration.Chronic (45 d) exposure were also carried out with different dosages (1mg/L, 10mg/L and 50mg/L) of n-ZnO to assess the adverse impacts on on N.europeae with stable growth in chemostat bioreactor.1 mg/L and 10 mg/L n-ZnO had no significant effects on N.europeae. But with the stress time prolonging, ammonia oxidation activity, cell density, AMO activity, cell membrane intergrity and gene expression level of ammonia oxidation decreased obviously by 50mg/L n-ZnO, and 7d toxicity effects makes the complete loss of bacterial ammonia oxidation activity.The study of long-term toxicity of 50 mg/L n-ZnO on N.europeae by different way of loading-in found that both of shock load (both of influent and bioreactor got load of 50mg/L n-ZnO) and influent load (only influent got load of 50mg/L n-ZnO) exerted significantly toxicity stress on N. europaea. Ammonia oxidation activity, cell density, cell membrane intact rate, gene expression level of ammonia oxidation and AMO activity decreased obviously, and toxicity stress of n-ZnO after 7d makes the complete loss of activity of bacteria. Toxicity stress by shock load on N, europaea was stronger than the way of influent load, the system got significant toxicity effect by shock load after the 3h interaction, and it got significant toxicity effect by way of influent load from 30h interaction with accumulation of n-ZnO in bio-reactor. Ammonia oxidation rate, cell density, AMO activity, cell membrane intergrity got some recovery in 10%recovery experimental group (when ammonia removal effency got 10% inhibition degree, damaged bacteria was taken out from chemostat bioreactor for recovery experimental in fresh medium without n-ZnO, so was the 25%,50%,75%recovery experimental group) by different way of loading-in, but the recovery degree of indexes in influent load was obviously higher than shock load. Although the 25%recovery experimental group by different way of loading-in shows that the toxicity stress of shock load on N.europeae is more significant than those by influent load and bacteria activity got no significant increase in influent load experimental group with the recovery time increase, it can still maintain the existing ammonia oxidation. However, bacterial activity after stress of n-ZnO could not be restored in shock load experimental group, and it cannot maintain the existing ammonia oxidation. Both of shock load and influent load could not be restored in the 50% experimental group. So result of recovery experiment shows the toxicity stress of n-ZnO shock load mode on N.europeae is more significant than n-ZnO influent load mode, and damaged bacteria was irreversible earlier.The study of long-term toxicity of 50mg/L n-ZnO on N.europeae by low dissolved oxygen (DO= 0.5mg/L) and high DO (2.0mg/L) condition shows that 7d continuous effects makes the complete loss of bacterial ammonia oxidation activity.50mg/L n-ZnO biotoxicity stress under different DO concentration had no significant effect. As ammonia conversion rate got 10%inhibition degree, cell recovery found ammonia oxidation activity, cell density, AMO activity, cell membrane intergrity got some recovery with the recovery time prolonging, but the recovery degree of acquired cell in high DO was obviously higher than acquired cell in low DO. In 25%recovery experimental group, bacterial physiological and biochemical activity after recovery culture was found to show no significant improvement under either DO concentrations, but it can still maintain the existing ammonia oxidation. In 50%and 75% recovery experimental group, bacterial biochemical activity could not be restored, and it cannot maintain the existing ammonia oxidation ability. Therefore, high DO environment was conducive to cell activity against n-ZnO stress and the recovery of cell, but bacterial physiological and biochemical activity was irreversible and completely lost ultimately with the stress time prolonging,.In summary, the effect of n-ZnO at different concentrations on N.europaea in different acting time with different DO concentration or different way of loading-in was investigated in this thesis, and the recovery performance of damaged bacterial physiological and biochemical activities in different stress condition of n-ZnO was also discussed, which provide the necessary theoretical basis for the further study of the effects and mechanism of NPs on nitrogen removal, and emergency support theory for biological nitrogen removal recovery process under NPs stress.