The Complexation Of Extracellular Polymeric Substances With Cadmium In Alleviating The Toxicity On Chlorella Vulgaris

The Extracellular polymer substances?EPS?refers to a polymer that are secreted by biological cells such as algae and bacteria.It is mainly composed of protein?PN?,polysaccharide?PS?and humus,etc.And the EPS chemical functional group mainly includes hydroxyl and carboxyl groups.As a high molecular substance attached to the surface of algae cells,EPS can not only provide energy and carbon sources to cells,but also coordinate external environmental protection cells to ensure the growth of algae under external environmental stress.Heavy metals are the most common contaminants in the environment.The chemical functional groups in EPS can alleviate the toxic effects of heavy metals on organisms through heavy metal reactions.However,the interaction mechanism between EPS and heavy metals is still unclear.In this paper,the Chlorella vulgaris?C.vulgaris?,which is common in water environment,was studied.The cadmium?Cd?was used as a heavy metal to investigate the mitigation effect of EPS on the toxic effects of Cd.On this basis,the adsorption efficiency and adsorption mechanism of EPS for Cd were studied,and the mechanism of complexation between EPS and Cd was analyzed by fluorescence excitation emission spectroscopy-parallel factor analysis?EEM-PARAFAC?,two-dimensional Fourier transform infrared correlation spectroscopy?2D-FTIR-COS?and X-ray Photoelctron Spectroscopy?XPS?.The following conclusions were obtained:?1?Cd stress promoted the secretion of EPS,polysaccharides and proteins in EPS,and EPS alleviated the inhibition of Cd on biomass,cell activity and photosynthesis of The correlation coefficient between Cd²⁺.?2?EPS alleviate metal toxicity by adsorbing Cd prevented it from entering algal cells,and the complexation of protein in EPS with Cd is the mainly adsorption mechanism.After removing the EPS from C.vulgaris,there was no significant difference in the total biosorption amount of Cd,but the content of Cd adsorbed on the surface of algae cells decreased by 52.67%,while the content of Cd absorbed increased by 68.30%;EEM results show that Cd can cause static quenching of EPS fluorescence,which is caused by the complexation of tryptophan-like substances with metals.When the concentration of Cd²⁺was 2 mg/L,the hydraulic diameter of EPS was 2.26 times that of the control group,which further proved the occurrence of complexation.?3?The complexation of EPS with Cd is mainly responsible for the O atom in the carboxyl terminus of the amino acid.The EEM-PARAFAC analysis result showed that the Fm/F0 values of tryptophan and humic substances were 52.31±2.37%and76.25±5.34%,respectively.The conditional stability constant k of tryptophan is3.23*10⁴ L/mol which is significantly higher than that of humus?k=0.52*10⁴ L/mol?;2D-FTIR-COS results show that the binding sequence of EPS component to Cd is protein Substance>polysaccharide>humic-like,and the carboxyl group has the largest contribution.In addition,the complexation constant of deprotonated carboxyl group C-O?k=4.88*10⁴ L/mol,R²=0.98?is larger than all other EPS functional groups;XPS analysis further prove the contribution of O atoms in carboxylic acids.