Effect Of AgNPs On The Phosphorus Removal Efficiency Of Chlorella Vulgaris And Its Mechanism

The pollution of phosphorus in aquatic environment is the focus of Water Pollution Prevention Action Plan in China.Therefore,there is an urgent in developing economical and efficient phosphorus removal technologies.Algae-based biological phosphorus removal technology is green and efficient,but is susceptible to environmental pollutants.Silver nanoparticles?AgNPs?materials have been widely used in production and life due to their unique physical and chemical properties,thus AgNPs are the common nano-particle pollutants in environment.At present,studies on the toxic effects of nanoparticles such as AgNPs on algae have been reported.Nevertherless,to evaluate the role of nanoparticles in biotechnological water treatment,it is nessary to investigate the effect and it mechanism of AgNPs in algal phosphorus removal process.In this study,a single-cell green alga Chlorella vulgaris?C.vulgaris?was selected as the research object in the study.First,we investigated the toxic effects of AgNPs on C.vulgaris?the biomass,chlorophyll autofluorescence?,and then explored the effect of AgNPs on the removal efficiency of orthophosphate and total phosphorus.We focused on the effects of AgNPs on three phosphorus removal mechanism?assimilation absorption,surface adsorption and abiotic precipitation?in the process of phosphorus removal by the algal cells.The main results are as following:?1?AgNPs has toxic effects on C.vulgaris.The 96-h EC₅₀ of AgNPs was 0.13mg·L^-1,and Ag⁺EC₅₀0 was 0.02 mg·L^-1 by algae growth inhibition test.The chlorophyll autofluorescence intensity of algal cells in the presence of AgNPs was compared with the control group?no AgNPs?.In the 0.2 mg·L^-1 AgNPs treatment group,the fluorescence intensity increased by 24.6%at 0.5 h,and the 0 mg·L^-1 Ag⁺treatment group increased by 35.3%.The increase in chlorophyll autofluorescence indicates that the photosynthesis activity decreased of C.vulgaris.?2?AgNPs inhibited the growth of C.vulgaris,chlorophyll synthesis and photosynthetic activity under high phosphorus conditions(2 mg·L^-1),and reduced the removal efficiency of orthophosphate and total phosphorus.On the 4^th day,when the concentrations of AgNPs were 0.05,0.09,0.14 and 0.20 mg·L^-1,the algal cell concentration decreased by 9.7%,27.5%,46.0%and 58.1%,respectively,compared with the control group.The presence of AgNPs affected the chlorophyll content and chlorophyll fluorescence parameters of C.vulgaris.On the 7^th day,the removal rates of orthophosphate and total phosphorus in the control group were as high as 96.0%and 94.8%,while the removal rates of orthophosphate in the AgNPs treatment groups were 95.0%,71.0%,34.9%and 16.2%,the total phosphorus removal rates were85.7%,67.9%,47.1%and 31.9%,respectively.The removal efficiency of orthophosphate and total phosphorus decreased with the increase of AgNPs concentration.?3?The effects of AgNPs on the three phosphorus removal mechanisms of C.vulgaris are as follows:In the 0,0.09 and 0.20 mg·L^-1 AgNPs experimental groups,the phosphorus removal efficiency of C.vulgaris assimilation absorption was 88.8%,92.9%and 89.9%of the total phosphorus removal efficiency,respectively.The surface adsorption phosphorus removal ratio is stable between 1.3%and 8.0%,and the abiotic precipitation phosphorus removal ratio is between-1.9%and 5.6%.Surface adsorption phosphorus removal and abiotic precipitation phosphorus removal have weaker contribution to phosphorus removal by algae cells.AgNPs mainly affect the assimilation absorption in the process of phosphorus removal,thus reducing the phosphorus removal efficiency of C.vulgaris.AgNPs were found to reduce the content of energy substances?AMP,ATP and ADP?.The conversion of phosphorus in cells is inhibited,resulting,which further leads to a decrease in biomass.