The Role Of Extracellular Polymeric Substances On Microbial Reduction Of Fe(Ⅲ) In Cd-loaded Polyferric Sulfate Flcos

Sudden Cadmium(Cd)pollution incidents occur from time to time,posing a serious threat to the ecological environment of the local water bodies and the drinking water safety of urban residents downstream.Poly ferric sulfate(PFS)is often used for emergency treatment of cadmium pollution accidents in rivers.PFS can rapidly flocculate aqueous Cd2+ to form Cd-loaded polyferric sulfate flocs(Cd-PFS)into the sediments,thereby reducing the cadmium concentration in the water.Dissimilatory iron-reducing bacteria(DIRB)are commonly found in anaerobic environments such as sediment.Our previous study found that the Cd-PFS can be reduced and dissolved by DIRB,which caused the release of cadmium to solution.Extracellular polymeric substances(EPSs)connecting microorganisms and minerals are considered as the key components that determine the physicochemical and biological properties of cell surface microenvironment.EPSs play an important role in the metal migration and mineral formation.In this study,we investigate the effects of extracellular polymeric substances on the secondary mineral formation and the transfer and transformation of Cd during the process of iron reduction by Shewanella oneidensis MR-1.The main conclusions of this study are as follows:(1)EPS can promote the reduction of Fe(Ⅲ)in the flocs by MR-1.The mechanism may be that the protein of EPS tends to adsorb on the floc surface,resulting in the uneven charge on the flocs surface,enhancing the dispersion of particles,increasing the interfacial electron transfer active surface sites,and thus accelerating the electron transfer efficiency between particles and microorganisms;EPS contains MtrB,MtrC and some other redox proteins,which may participate in electron transport and promote Fe(Ⅲ)reduction in flocs;The flavin of EPS may serve as electron shuttle in the process of MR-1 driven reduction of Fe(Ⅲ)in the flocs,which can greatly accelerate the electron transfer speed.EPS can not only facilitated the reduction of Fe(Ⅲ)mediated by microorganisms,but can also directly reduce the flocs.(2)It was found for the first time that EPS could provide inorganic phosphate to induce the formation of vivianite.The protein and DNA of EPS could serve as a phosphorus source and be degraded into inorganic phosphate by phosphatase and nucleotidase secreted by microorganisms,and the inorganic phosphate reacted with the reduced Fe2+ to form vivianite.During the process of dissimilatory iron reduction,EPS secreted by the microorganism may be maintained at a high enough concentration to induce the formation of vivianite under poor diffusion conditions.EPS-induced formation of vivianite may occur in natural anaerobic environments such as sediment.(3)EPS can provide nucleation sites to promote the formation of lepidocrocite and magnetite.Phosphate group deriving from nucleic acid or phosphorylated proteins of EPS can act as nucleation sites to form P-O-Fe bonds with iron atoms,anchoring and accumulating the aqueous Fe3+and Fe2+,accelerating iron ions precipitate and promoting the formation of iron minerals.(4)EPS promotes iron reduction as well as the release of loaded cadmium into the solution.However,as the reaction proceeds,EPS promotes the formation of secondary minerals such as vivianite,lepidocrocite and magnetite.These secondary minerals have a strong adsorption capacity for Cd2+due to their abundant hydroxyl functional groups.The Cd2+released into the solution is re-bind by complexation,which reduces the mobility and biodegradability of cadmium.