Study On Adsorption Behavior Of Activated Sludge Extracellular Polymer For Heavy Metals

Activated sludge extracellular polymer（EPS）is a new type of biological adsorbent with the advantages of non-toxic and biodegradable.EPS surface contains rich functional groups such as hydroxyl,amino,carboxyl and phosphate groups,which can provide adsorption sites for heavy metal ions,so it has high application value in heavy metal pollution control and other fields.Due to the limitation of current extraction methods,the extraction efficiency of EPS is generally not high.For this reason,this topic proposes to use surfactant combined with ultrasonic method to extract EPS in order to improve the extraction efficiency.On this basis,the extracted components were analyzed,the adsorption behavior of EPS on heavy metals was investigated,and the adsorption mechanism was studied by using modern analytical technology.It lays a foundation for the application of EPS in the treatment of heavy metal pollution in the future.The results of the study are as follows.（1）The experimental conclusion of extracting EPS from activated sludge is as follows:the best liquid phase environment is phosphoric acid buffer solution with Na Cl dosage of9g,and the extraction amount of EPS is CTAB>SDS>betaine.The extraction conditions were optimized by response surface experiment and the regression model was established.The optimal conditions were as follows:the dosage of CTAB was 54.801mg,the ultrasonic time was 8.882 min,and the extraction time was 56 min.The total amount of EPS and nucleic acid content extracted under these conditions did not differ significantly from the results of three parallel experiments conducted under optimal conditions,demonstrating the reliability of the model.（2）The composition of EPS was characterized and analyzed by FTIR,EMM,Zeta,XPS,etc.The conclusion is as follows:FTIR shows that EPS contains a large number of functional groups such as hydroxyl,carboxyl,carbonyl and amino groups.EEM showed that EPS contained tyrosine and tryptophan proteins.Through Zeta potential,EPS is found to be negative at different p H,which indicates that EPS is electronegative as a whole.The BET experiment results show that the total pore volume of the single-point method is0.027 66cm³/g,and the specific surface area is 16.78 m²/g.The analysis shows that the adsorption of heavy metals by EPS is not mainly dependent on physical adsorption.Combined with the XPS analysis results,it is found that the C element in EPS mainly exists in the form of C-C,C-（N/O/S）,O-C-O,O=C-O（H/R）,while the O element mainly exists in the form of O=C and C-O-（C/H）,while the N element has two functional groups,one from amide and amino group,and the other from amino acid and polysaccharide.（3）The experimental results of EPS adsorption of Cu²⁺and Pb²⁺are as follows:The optimum adsorption conditions of EPS adsorption of Cu²⁺and Pb²⁺are:p H5～6,dosage m(M²⁺)∶m（EPS）=1:2,adsorption time 60 min,adsorption temperature 45℃.Moreover,the adsorption of Cu²⁺and Pb²⁺by EPS was better fitted by Langmuir equation.At adsorption equilibrium,the theoretical maximum adsorption capacity of EPS for Cu²⁺and Pb²⁺is 572mg/g and 581 mg/g respectively.Compared with the pseudo-first-order kinetic equation,the pseudo-first-order kinetic equation can better reflect the process of adsorption of metal ions,which shows that adsorption is mainly achieved by chemical action.（4）FTIR,XRD and SEM-EDS were used to analyze the mechanism of adsorption of Cu²⁺and Pb²⁺by EPS.The experimental results were as follows:FTIR analysis showed that the functional groups of EPS changed before and after adsorption of Cu²⁺and Pb²⁺,which was mainly related to sulfuric acid,phosphoric acid,carbonyl and other groups.XRD analysis shows that the structure of EPS will change before and after adsorption to produce new crystals,which indicates that the phosphate group will react with metal ions to form hydrated phosphate during the adsorption process.SEM-EDS not only showed that the two metal ions were indeed adsorbed on the surface of EPS,but also showed that there was an ion exchange reaction in the adsorption process according to the change of element content before and after adsorption,and the oxygen-containing group,amine group and phosphate group all participated in the reaction.