Effects and mechanisms of polypropylene nanoplastics on biological denitrification and functional genes
Polypropylene nanoplastics(PP NPs)were selected to research their effects on biological nitrogen removal.The water quality,nitrogen transformation functional gene,and biological activity were detected.The effects and mechanisms of different concentrations of PP NPs(1～100mg/L)on biological denitrification and functional genes were explored by examining the water quality indexes,the abundance of functional genes for nitrogen transformation,and the metabolism content of substances in the process of denitrification in the sludge system in each reactor,and by employing the methods of Pearson's correlation analysis and linear regression modeling analysis.The results showed that the inhibition effect increased with the increase of PP NPs concentration.The effects of low concentration of PP NPs on the nitrogen removal performance were weak,and the effluent COD,effluent ammonia nitrogen,and nitrate nitrogen removal at the aerobic end were reduced to 85％,91.4％,and 66％,respectively,under the influence of 100mg/L PP NPs.The qPCR showed that the abundance of nitrogen transformation function genes,except nirK and norB genes,decreased gradually with the increase in the concentration of PP NPs,among which 16S rRNA,amoA,nxrA,narG,napA,nirS,nosZ genes showed the highest decreases of 6.82％,23.78％,26.25％,14.56％,respectively.24.90％,26.37％,21.04％.With the increase in the concentration of PP NPs,the intracellular ROS and LDH contents increased up to 33.36％and 13.55％,respectively,and the sludge cells were in a state of severe oxidative stress,which led to the damage of the cell membrane structure and the decrease of cellular activity.Pearson's correlation showed that there were strong correlations between physicochemical parameters and some functional genes.The negative correlation between nitrate-nitrogen at the aerobic end and functional genes changed to a positive correlation under the stress of 1mg/L PP NPs.Positive correlations of varying degrees were observed between ammonia-nitrogen of effluent and nxrA,nirK and norB genes under the stress of 1～100mg/L PP NPs.Linear regression modeling indicated that the amoA and nxrA were the key driving factors for ammonia oxidation and nitrification,nirS was the key driving factor for nitrite reduction,and nosZ and narG were the key driving factors for denitrification.In this study,the effects and mechanisms of PP NPs on biological nitrogen removal were revealed from the perspective of molecular biology.
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