The Study On The Mechanism Of Action Of Extracellular Polymeric Substances Of Citrobacter Freundii On Stainless Steel In Circulating Cooling Water System Of Power Plant

In this paper,the circulating cooling water system of thermal power plant is taken as the research background,Citrobacter freundii and its extracellular polymeric substances is taken as the research object,three kinds of stainless steel SS304,SS316L and ss317l are taken as the research materials,and the reclaimed water from the secondary effluent of a sewage treatment plant in Beijing and its three times and four times concentrated reclaimed water are taken as the experimental medium.The growth rule of Citrobacter freundii in the reclaimed water environment was studied by biochemical method,the p H range of Citrobacter freundii growth was determined,and the harm of bacteria growth to the circulating cooling water system was revealed.The effect of Cl^-,SO₄^2-and NH₄⁺on the electrochemical corrosion behavior of SS316L biofilm was studied by electrochemical impedance spectroscopy?EIS?.SEM,XPS,FTIR and HPLC were used to qualitatively and quantitatively analysis the morphology,element distribution and content,functional groups,specific components of polysaccharides and opal derivatives of Citrobacter freundii EPS.The electrochemical behavior of EPS on three kinds of stainless steel was studied by electrochemical method.The mechanism of the interaction between EPS and metals was further revealed.It plays a guiding role in the corrosion control of the circulating cooling water system of the thermal power plant.The research results are as follows:?1?The results showed that Citrobacter freundii could survive in the circulating cooling water with p H of 8.2-8.8,and the p H suitable for the growth of the bacteria was 7.5,8.2,8.5,8.8 and 9.0 from strong to weak.In the water environment with initial p H value of 8.2 and inoculated microorganism,the range of p H change was 7.85-8.66with the extension of time.With the increase of concentration ratio,the influence of p H on the growth of bacteria becomes smaller and smaller,which is no longer the main factor affecting the growth of bacteria.?2?In the presence of Citrobacter freundii,the index control of the circulating water system of the power plant should be Cl^-<460 mg·L^-1,SO₄^2-<430 mg·L^-1,NH₄⁺<15mg·L^-1,exceeding the critical concentration,The corrosion resistance of the SS316L surface passivation film decreased significantly.?3?The surface analysis of EPS showed that after freeze drying,the EPS of Citrobacter freundii showed a flow gel structure,which made EPS easy to bond on the metal surface.XPS results show that EPS contains C=O,C-H bonds,which are mostly in polysaccharide structure,O=C-NH bonds,C-N bonds,mostly in protein structure,PO₄^2-?P₂O₇^4-mostly in nucleic acid.These negative functional groups can be complexed with metal ions.FTIR results showed that the EPS of Citrobacter freundii contains carboxyl,hydroxyl,phosphoryl,sulfate group,amino group and other groups,which have the ability of complexation and coordination.Nitrogen,oxygen,phosphorus and sulfur can form covalent bond with metal ions as coordination atoms to achieve the purpose of metal ions adsorption.?4?The specific components of polysaccharide derivatives in EPS of Citrobacter freundii are composed of eight monosaccharides,including mainly glucose,glucuronic acid,arabinose and rhamnose.The content of hydrophilic ketose is much higher than that of hydrophobic aldose,so the polysaccharide in EPS is hydrophilic;the specific components of protein derivatives in EPS are composed of 17 amino acids,among which aspartic acid,glutamic acid,lysine and alanine are the most,accounting for47.5%of the total protein.There are more anionic groups in the polysaccharide,and the side chain of amino acid also carries negative groups,which makes the whole EPS appear negative.The content of protein and polysaccharide in EPS of Citrobacter freundii was 346.12mg/g,617.46mg/g,and PS/PN was about 1.7,which indicated that polysaccharides in EPS played an important role in the adsorption process of microbial membrane.?5?The polarization curve test results show that the corrosion resistance of the three kinds of stainless steel surfaces in the three times recycled water decreases with the increase of time?1-14d?.SS304 is strongly corroded,while SS317 still shows good corrosion resistance,followed by SS316L.The experimental results show that the corrosion resistance of stainless steel surface is improved in the presence of EPS,indicating that EPS has a certain degree of corrosion inhibition.?6?The electrochemical impedance measurement further revealed that in the presence of EPS,the electrochemistry parameters of three kinds of stainless steel surface transfer resistance(R_ct)and film resistance?R_f?increased,indicating that the EPS organic film formed on the stainless steel surface can slow down the corrosion rate of the metal surface to a certain extent.The EPS organic film formed on SS317 surface has higher resistance and lower conductivity.The film formation on SS317 is faster and more stable,followed by SS316L and SS304.?7?The proteins in EPS contain a large number of hydrophobic regions,which can make EPS more easily adsorbed on the surface of stainless steel.The anionic groups such as glucuronic acid,phosphoric acid and carboxyl in polysaccharide of EPS react with metal ions to form a complex and adsorb on the surface of stainless steel,forming an organic film layer.To some extent,the mass transfer process on the electrode surface is hindered,which can inhibit the corrosion of the metal surface.The amount of EPS adsorbed on SS317 surface is the most,followed by SS316L and SS304.