Dissolution Of Nano-ZnO And Its Effects On The Removal Efficiency Of Phosphorus By Chlorella Vulgaris

Algae,which can absorb nitrogen and phosphorus and convert them into algal biomass,have been foused on for nutrients removal in the advanced wastewater treatment in recent years.However,hazardous contaminants in aquatic environment show adverse effects on algal removl of nutrients.Nano-materials,due to their increasing production and widespread application,will release into the environment inevitablely.Therefore,the environmental behavior and bioeffects of nanoparticles have been studied by reasearchers.Previous studies reported that nanoparticles,such as nano-ZnO,have obvious toxic effects on the growth of algae.Thus,the effects of nano-ZnO on the removal of nitrogen and phosphorus by algae should be investigated.In this study,nano-ZnO was selected as the representative of nanoparticles,a green alga Chlorella vulgaris was employed as the research object.On the basis of investigating the effects of nano-ZnO and the dissolved Zn²⁺ on the biomass of C.vulgaris,their effects on phosphorus removal efficiency by the algae were discussed in detail.Furthermore,we employed Fourier transform infrared（FTIR）,transmission electron microscopy combined with energy dispersive X-ray spectroscopy（TEM-EDX）,X-ray diffraction analysis（XRD）and X-ray photoelectron spectroscopy（XPS）to analyze the properties of crystallites formed by nano-ZnO and phosphates.The following results have been obtained in this paper:（1）The original particle size of nano-ZnO used in the experiment were confirmed to be about 20-30 nm after being characterized by TEM,XRD etc.We found that Nano-ZnO can partially dissolve and release Zn²⁺,but the dissolution of nano-ZnO was quite different in different media conditions.In ultrapure water,the concentrations of dissolved Zn²⁺ increased with the increase of initial nano-ZnO concentration.When the initial concentrations of nano-ZnO ranged form 0.005 to 0.15 mM,the equilibrium concentrations of Zn²⁺increased from 0.6 to 44.8 μM.Because high ionic strength can promote the dissolution of nano-ZnO,the concentrations of dissolved Zn²⁺ were higher in the modified OECD medium than that in ultrapure water（0.005-0.02 mM）.However,when the concentrations of nano-ZnO increased to0.04-0.15 mM,the dissolved Zn²⁺ was lower compared to that in ultrapure water.When the concentration of nano ZnO was 0.005 and 0.15 mM,the equilibrium concentrations of Zn²⁺were 2.2 μM and 9.4 μM,respectively.（2）Nano-Zn O showed significant toxic effects on the growth of C.vulgaris.When the initial concentrations of nano-ZnO were 0.005 μM and 0.01 μM,the algal biomass were 76.2% and 60.0% of the control（without nano-ZnO）after 7-days culture,respectively,whereas it was only 27.4% of the control with 0.15μM nano-ZnO.The dissolved Zn²⁺showed similar effects on the biomass of C.vulgaris compared with nano-ZnO.However,the effects of nano-ZnO on phosphorus removal efficiencies were different from that on the algal biomass.With the increase of nano-ZnO concentrations（0.005-0.15 mM）,the removal efficiencies of PO₄^3-decreased at first and then gradually increased after 7-days culture.The lowest removal efficiencies of PO₄^3-were obtained at the presence of 0.04 mM nano-ZnO or its released Zn²⁺among the tested groups,which were 28.1% and 35.1% respectively.However,the removal efficiency of PO₄^3-increased to 36.1% with 0.15 mM nano-ZnO,and reached the maximum value of 62.1% at the presence of dissolved Zn²⁺from 0.15 mM nano-ZnO,which was slightly higher than that of the control（58.5%）.We hyperthesized that Zn²⁺ dissolved from nano-ZnO can react with PO₄^3-and then form crystallite precipitation,thus promoting PO₄^3-removal efficiencies.（3）FTIR results showed that the characteristic absorption band of PO₄^3-emerged in the tested crystallites;TEM-EDX results showed that the crystallites contained Zn,O elements and P element;XRD results showed that Zn₃（PO₄）₂ appeared in the crystallites;The XPS peak of Zn and P showed that Zn₃（PO₄）₂ was contained in the crystallites.With the analysis of the above methods,we confirmed that crystallites containing Zn₃（PO₄）₂ were generated when nano-ZnO coexisted with PO₄^3-.Zn₃（PO₄）₂crystallites can be formed in the pH range of algal growth（pH 7.1 to 8.8）,which were accerlated in lower pH value.As a result,the formation of Zn₃（PO₄）₂ crystallites was the main reason to accelerate the PO₄^3-removal efficiencies by C.vulgaris,and to decline the concentrations of Zn²⁺in the modified OECD medium.