Controls On Fe Reduction Of Cd-loaded Polyferric Sulfate Flcos By Dissimilatory Iron-reducing Bacteria

Coagulation is an effective treatment in dealing with accidental cadmium pollution in water.Polyferric sulfate?PFS?can deal with Cd²⁺ in water quickly and form Cd-loaded polyferric sulfate flocs into the sediments,so it is widely used as a flocculating agent in water treatment.Our previous study has showed that the Cd-loaded flocs in the sediments could be dissolved by the dissimilatory iron-reducing bacteria?DIRB?during reduction process,which caused the release of cadmium to solution.However,the presence of humic substances,heavy metal ions and phosphates can affect the reduction process of iron in flocs and release of cadmium.How these factors affect the reduction of Cd-loaded flocs and the formation of secondary minerals is complex,and the process still remain poorly understood.So the main objective of this study is to investigate the effects of disodium anthraquinone-2-sulfonate?AQS?,heavy metals Zn²⁺ and Cu²⁺,NaH₂PO₄×2H2O addition on the bioreduction of Fe?III?in Cd-loaded flocs by a model DIRB strain,Shewanella oneidensis MR-1.And the effects on the reduction of Cd-loaded flocs by Shewanella putrefaciens CN32 and secondary minerals formation were also explored.The main conclusions of this study were as follows:?1?The addition of AQS could enhance the reduction of Fe?III?and the release of Cd from the flocs,the effect of low AQS concentration was significantly higher than high AQS concentration.The decrease of Cd2 + in solution of the later reaction period was due to the surface adsorption or complexation of carboxyl group,alcoholic hydroxyl group and ether group on bacteria,goethite and magnetite.With the increase of AQS concentration,the crystallinity of formed secondary minerals including goethite and magnetite were becoming worse during bioreduction of Cd-loaded flocs.The direct and indirect release risk of cadmium would increase in the presence of AQS,while the direct release of cadmium was more risky at lower concentration of AQS.?2?The addition of heavy metal Zn²⁺ or Cu²⁺ inhibited the microbial reduction of Fe?III?in Cd-loaded flocs,the concentration of Fe?II?and Cd²⁺ were lower than those in the control group,and the inhibitory effect of Fe?III?reduction and Cd release were more obvious in the experimental group of Cu²⁺ addition.From the results of XRD,it was found that the addition of heavy metal Zn²⁺ or Cu²⁺ did not change the types of minerals,but inhibited the formation of magnetite to a certain extent.FTIR analysis results confirmed those of XRD.?3?The addition of NaH₂PO₄×2H2O significantly inhibited the reduction of Fe?III?and reduced the release of Cd²⁺ in solution at the first 120 h.The inhibitory effect was more obvious with the increase of Na H₂PO₄×2H2O concentration.But both the reduction of Fe?III?and release of Cd²⁺ increased in the late reaction,which might be that NaH₂PO₄×2H2O could be used as a nutrient for bacteria,thus allowing the continuous reduction of Fe?III?in the flocs and increasing release of Cd²⁺.The formation of vivianite was a primary transformation product observed after bioreduction of flocs in the presence of NaH₂PO₄×2H2O.And the crystallinity of vivianite became better with the increase of NaH₂PO₄×2H2O concentration.?4?The maximum concentration of Fe?II?and Cd2 + in the S.putrefaciens CN32 system were 1.24 times and 3.22 times compared to the S oneidensis MR-1 system,which indicated that CN32 had stronger ability to reduce Fe?III?and formed goethite with better crystallinity.