Electrochemical Study Of Biogenic Iron Mineralization Of Polyferric Sulfate Flocs By Dissimilatory Iron Reducing Bacteria

Heavy metal pollution emergencies in water have occurred frequently recently,which has brought great impact on the ecological environment and society.Polyferric sulfate（PFS）is widely used in emergency treatment of metal pollution accidents in water.After PFS dissolves in water and flocculated with heavy metal ions in the water,the formed aggregation will sink into the sediment of water body,thereby restoring the heavy metal concentration of the water body to a lower level.Our previous study found that Dissimilatory iron-reducing bacteria（DIRB）,which is widely found in natural water sediments,can use the unstable Fe（III）in the PFS in the sediment as an electron acceptor for dissimilatory iron reduction.The effect is accompanied by the transformation of microbial secondary minerals;the heavy metal ions and anions prevalent in contaminated water bodies can greatly affect the mechanism of the process.There have been many studies using electrochemical means for microbial redox reduction,which is simpler and more efficient than traditional microbial research methods.This study explored the possibility of using electrochemical methods to study the reduction of PFS dissimilatory iron,and applied it to study the effects of oxygenated anions on the process.We concluded as follows:（1）Impressed 0.2 V（vs.SCE）potential can significantly promote the expression of iron reductive protein,thereby enhancing the performance of dissimilatory iron reduction.The dissimilatory iron reduction process of PFS is more efficient than conditions without potential applied,which is characterized by faster accumulation of microbial reduced Fe（II）and faster formation and transformation of microbial secondary iron minerals.Besides,secondary minerals with better crystalline can be formed.At the same time,the Fe²⁺produced by the reaction can act as an electron shuttle from the bacteria to the electrode to construct an indirect electron transport path of the electron donor→Fe²⁺/Fe³⁺→electrode,and thus the microbial power generation performance is significantly improved.At the impressed-0.2 V（vs.SCE）condition,the above-mentioned promotion phenomenon did not occur due to the impact of the negative potential on the exoelectrogens and the suppression of electron transport.（2）In the 0.2 V（vs.SCE）potential impressed condition,the bio-current density can be used to characterize iron reduction rate in dissimilatory iron reduction process.There is a close relationship between the bio-current density and the iron reduction rate in dissimilatory iron reduction process in microbial electrochemical systems.In the early stage of rapid iron reduction,the bio-current density directly reflects the speed of the reduction rate,which shows a clear linear relationship.（3）Different buffer systems will cause PFS to form different secondary minerals eventually.In the PBS system,GR(SO₄^2-)and vivianite account for a large proportion,and magnetite also has a very small proportion.The formation of GR(SO₄^2-)originate from SO₄^2-and PO₄^3-in system,and vivianite form from the transformation of GR(SO₄^2-)or combination of Fe²⁺and PO₄^3-.Magnetite was deconstructed from GR(SO₄^2-)or formed through another way.In the PIPES system,magnetite is the main component,accompanied by a trace amount of vivianite caused by existed minor phosphate,which maintained the growth of the bacteria.In the carbonate buffer system,the main iron mineral species is siderite,accompanied by some ferrous hydroxide.