Experimental Study On Treatment Of Single And Mixed Heavy Metal Wastewater By Microalgae

The removal of heavy metals by microalgae is an efficient and environmentally friendly bioremediation method,and with a wide range of microalgae species and sources,some progress has been made in bioremediation in recent years.However,the research on heavy metals by microalgae mainly focuses on the effect of single heavy metal ions on the physiological characteristics and adsorption effect of microalgae,and the research on mixed heavy metal ions is less,which limits the application of microalgae in the treatment of actual heavy metal wastewater.In this paper,we firstly selected four species of algae,such as Chlorella pyrenoidosa,Tetradesmus obliquus,Microcystis aeruginosa and Cyclotella hebeiana,to conduct acute toxicity experiments,determine the EC50 of microalgae to single heavy metal ions,and screen out the better tolerated microalgae as experimental species;then the physiological responses of experimental microalgae to single and mixed heavy metals and their removal effects were studied;finally,the kinetic and adsorption isotherm models of heavy metal ion adsorption by microalgae were developed to explore the mechanism of heavy metal removal by microalgae.The main research results are as follows:（1）The tolerance to Cu²⁺,Zn²⁺,Cd²⁺and Pb²⁺varied among different algal species.The EC50 of Cu²⁺was 1.94 mg/L,2.42 mg/L,2.50mg/L and 1.36 mg/L for Chlorella pyrenoidosa,Tetradesmus obliquus,Microcystis aeruginosa,and Cyclotella hebeiana,and the tolerance of Tetradesmus obliquus was higher than that of other algal species,with EC50 of 2.42 mg/L,13.1 mg/L,4.99 mg/L and 2.72 mg/L for the four heavy metals,respectively.（2）Different species of heavy metal ions have different effects on the physiological properties of microalgae.Cu²⁺had the greatest impact on Tetradesmus obliquus growth,and after 72 hours,the average OD₆₈₀ of the20 mg/L group had fallen by 21.56%compared to the control group in the same time period,reaching just 0.679;Cu²⁺and Zn²⁺had stronger impacts on protein synthesis in Tetradesmus obliquus than Cd²⁺and Pb²⁺.At a Cu²⁺concentration of 5 mg/L,the protein content of the experimental group was18.47 mg/L,and the change in protein content was 113.0%;All four heavy metal ions inhibited the synthesis of chlorophyll a in Tetradesmus obliquus,and the strongest inhibition was Cu²⁺,and the chlorophyll a content of the experimental group at 1 mg/L,5 mg/L,10 mg/L and 20 mg/L was 33.69%,22.31%,15.73%and 11.47%of the control group at the same period,respectively;Cu²⁺had the strongest inhibition compared with other heavy metal ions on the microalgae EPS secretion was the most influenced by Cu²⁺compared with other heavy metal ions,when the EPS content of the experimental 48 h,5 mg/L Cu²⁺group was 74.19 mg/L.（3）The pattern of single heavy metal ion harmful effects on microalgae did not apply to the combined heavy metal system.In the mixed system,the trend of changes in microalgal growth in the mixed group with Cu²⁺addition of 5 mg/L was consistent with the single addition of Cu²⁺of20 mg/L;the effect of protein synthesis was less than that of the single addition of Cu²⁺,while the changes in chlorophyll a content were different from that of all four heavy metal ions alone,and the single Cu²⁺and the mixed group containing Cu²⁺may have a greater facilitation effect on the secretion of EPS from the Tetradesmus obliquus.（4）In the mixed system,the adsorption of Pb²⁺and Cd²⁺heavy metal ions by microalgae will be interfered by Cu²⁺and Zn²⁺,and the adsorption order is Pb²⁺>Cu²⁺>Zn²⁺>Cd²⁺.The adsorption of heavy metal ions by microalgae was mainly chemisorbed and belonged to unimolecular layer adsorption,and the pseudo-second-order kinetic model was more suitable to describe the adsorption process of heavy metal ions.The maximum adsorption amounts of Cu²⁺,Zn²⁺,Cd²⁺,and Pb²⁺by microalgae were 59.02 mg/g,151.69 mg/g,166.22 mg/g,and 179.87 mg/g,respectively,by Langmuir fitting.FT-IR patterns showed that-NH₂ as the main functional group was involved in the adsorption process of heavy metal ions adsorbed by microalgae.SEM images showed that the cell surface showed obvious wrinkling after the adsorption of heavy metals by Tetradesmus obliquus,while the percentage of C,N and O elements changed.XPS patterns showed that C-C,C-H,C-OH,C-O-C,C=O and O-C=O were involved in the biosorption process of heavy metal ion,and after adsorption of Pb²⁺,Pb₃（PO₄）₂ was formed on the surface of microalgae.