A Study On The Mechanism Of Sulfate-reducing Bacteria Constrained Decomposition And Transformation Of Nano-magnetic Iron Oxides

Nanomagnetite and nanomaghemite are important components of iron oxides that determine the magnetic properties of rocks,soils and sediments.In order to study the potential stability of these two magnetic minerals in sulfate diagenetic environment,the decomposition and transformation experiments of calcine-forming porous nanomagnetite and nanomaghemite under the action of sulfate reducing bacteria?SRB?were carried out,focusing on the decomposition and transformation processes and mechanisms of minerals in the short-term?31 d?and long-term?165 d?and with/without sulfate.Preliminary results are as follows:In the short-term experiments with SRB,the magnetic susceptibility of magnetite and maghemite systems continued to rise.At the end of the 31-day experiment,the magnetic susceptibility of magnetite system reached the maximum,and the magnetic susceptibility of maghemite system had decreased slightly.In the experiments of magnetite and maghemite containing SO₄^2-,the reduction of SRB resulted in the conversion of 22.4%and 5.1%sulfates to acid volatile sulfide?AVS?,respectively.X-ray photoelectron spectroscopy?XPS?analysis showed that Fe?III?in part of maghemite was reduced to Fe?II?by AVS and SRB,resulting in mineral dissolution,particle size reduction,and the formation of sulfide precipitation of various forms.X-ray diffraction?XRD?and M?ssbauer spectrum analysis showed that nanomaghemite has the tendency of solid phase transformation into nanomagnetite through electron transfer of SRB,and superparamagnetic phase is formed in the process.In the long-term experiments with SRB,the magnetic susceptibility of magnetite system increased rapidly in the first 25 days,and then remained stable;the magnetic susceptibility of maghemite system increased rapidly in the first 20 days,then decreased slowly,and reached stability in 140 days.In the experiments of magnetite and maghemite containing SO₄^2-,the reduction of SRB resulted in the conversion of 28.1%and 7.1%sulfates to AVS,respectively.Transmission electron microscopy?TEM?and XPS analysis showed that in the magnetite experiment,the main secondary sulfides were monosulfide mackinawite?FeS??greigite?Fe₃S₄?and polysulfide.In the maghemite experiment,besides the secondary phase mentioned above,there was a high proportion of pyrite?FeS₂?.XPS analysis showed that both AVS and SRB could reduce Fe?III?on the surface of magnetite and maghemite to Fe?II?.XRD and M?ssbauer spectrum showed that nanomaghemite could be completely transformed into nanomagnetite by the reduction of solid phase Fe?III?under the medium of SRB.This transformation was also reflected in the changes of magnetic susceptibility and hysteresis loop characteristics of maghemite,which were different from those of magnetite.The results show that the structural similarity between magnetite and maghemite is the precondition for solid phase transformation of maghemite into magnetite under the medium of SRB.At the same time,magnetic iron oxides can be used as electron acceptors of SRB growth,which has a certain restriction effect on microbial ecology in sulfide environment.