Influence Of Extracellular Polymeric Substances On Sorption Of Inorganic Arsenic By Green Algae

Extracellular polymeric substances?EPS?have a significant influence on sorption process of inorganic arsenic by microalgae.In this study,typical freshwater green algae Chlorella pyrenoidosa and Chlamydomonas reinhardtii were compared as research objects for short-term exposure of arsenite?As^??and arsenate?As^V?by analyzing the EPS prodcution and uptake capacity of inorganic arsneic,and C.pyrenoidosa was selected to further investigate its As^?and As^V sorption processes and the influence of EPS on arsenic sorption in C.pyrenoidosa.The Zeta potential of surface of algal cells,EPS production and its surface functional groups were detected to understand effect characteristics of EPS.The main findings are as follows:?1?The purification ability of C.pyrenoidosa to inorganic arsenic was better than that of C.reinhardtii,and the As^III and As^V uptake process of C.pyrenoidosa can be well described with Bio-dynamic model.When cells were exposed to 5-300?mol L^-1 As^V for 24 h,EPS production and arsenate sorption of C.pyrenoidosa were significantly higher than those of C.reinhardtii.EPS production of C.pyrenoidosa at300?mol L^-1 As^V exposure was 3.55 times as high as that of C.reinhardtii,and its arsenic sorption was 2.29 times as high as that of C.reinhardtii.Based on SEM morphological observation,C.pyrenoidosa exposed to 300?mol L^-1 As^III maintained its intact protoplasts rather than C.reinhardtii,which further confirmed that C.pyrenoidosa is more suitable for phycoremediation of arsenic.The As^III and As^V uptake process of C.pyrenoidosa can be well described with Bio-dynamic model?R²>0.9515?,with intracellular uptake of As^III and As^V by C.pyrenoidosa increasing in a concentration-dependent manner,and the maximal uptake capacities of As^III and As^V by algal cells was 3.03?mol g^-1 and 3.96?mol g^-1,respectively.?2?EPS enhanced As^III and As^V adsorption capacities of C.pyrenoidosa.Compared with EPS-cover?EPS-C?algal cells,the As^III and As^V uptake capacities of EPS-free?EPS-F?algal cells under 300?mol L^-1 As^III and As^V exposure increased from 2.53?mol g^-1 and 3.36?mol g^-1 to 3.36?mol g^-1 and 6.91?mol g^-1,while the As^III and As^V adsorption capacities of EPS-F cells decreased remarkably from 236.85?mol g^-1 and 297.39?mol g^-1 to 185.13?mol g^-11 and 219.29?mol g^-1.The As^III and As^V maximum sorption of EPS-F cells decreased by 21.15%and 24.81%in contrast to EPS-C cells.Based on the rate constants obtained from the bio-dynamic model,As^III and As^V adsorption rate constants of EPS-F cells decreased by 32.62%and 29.07%compared to EPS-C cells,and the As^III and As^V uptake rate constant increased by57.91%and 40.27%.The intracellular arsenic speciation and content at different pH values was further analyzed,and results showed that arsenic uptake by EPS-C and EPS-F cells were both decreased with pH increasing form 6.0 to 8.5,and As^IIIII oxidation,As^V reduction and methylation of EPS-F cells were significantly lower than those of EPS-C cells at same pH levels,meanwhile the average arsenic efflux rate of EPS-F cells was higher than that of EPS-F cells.?3?The EPS production of algal cells increased with increasing exposure concentrations of inorganic arsenic,and surface functional groups of EPS and surface zeta potential of algal cells were detected to be the key factors for effects of EPS on inorganic arsenic sorption by green algae.After exposure to 5-300?mol L^-1 As^III and As^V for 24 h,the EPS production of algal cells increased with arsenic exposure concentrations.Pearson correlation analysis demonstrated that the amount of EPS produced was positively correlated with adsorption of As^III and As^V by algal cells,and the pearson correlation coefficients were 0.972 and 0.919.It indicated that increasing arsenic exposure concentrations can promot EPS production,by which algae cells can increase the adsorption of inorganic arsenic.The surface functional groups of EPS involved in interacting with inorganic arsenic were identified using 3D-EEM,FTIR and XPS.3D-EEM spectra showed that the fluorescence peaks of tyrosine protein-like substances and tryptophan protein-like substances in EPS decreased by 63.5-77.6%when 300?mol L^-11 As^III and As^V added,and EPS surface functional groups including-COOH,-OH,-NH₂ were found to be the bounding sites for immobilizing inorganic arsenic through further analysis of FTIR and XPS.The Zeta potential of algal cells at different pH exhibited as the order of pH 6.0>pH 7.0>pH 8.5,which suggested that the negative charge of algal surfaces was increased when the pH varied from 6.0 to8.5 owing to the deprotonation of EPS,which finally led to decreased binding ability between EPS and As^III and As^V.