The Process And Mechanism Of The Reductive Decomposition Of Magnetic Iron Oxides By Anaerobic Bacteria

Nanoporous magnetite and maghemite with different surface area were prepared by calcining synthetic goethite under suitable conditions, after which abiotic and biotic experiments for these two minerals were conducted:monitoring the phase transformation of nanomagnetite under aerobic condition with fixed temperature; investigating the changes of magnetism and phase of magnetite and maghemite with the addition of anions; studying the decomposition process and mechanism of magnetite by anaerobic microorganisms (methanogens and sulfate-reducing bacteria)with magnetite. The following results have been obtained.At 70℃ magnetite converts to maghemite rapidly and the reaction reached equilibrium after 52 d. At the end of the experiment (70 d), a 26% reduction of the ratio of Fe2+to total iron was arrived. The volume of mineral was increased and crystalline particle decreased during the ageing. The magnetic susceptibility and frequency susceptibility of the sample decreased with conversion. The results provide a reasonable explanation for the forming of natural maghemite through combustion:in the burning stage nanomagnetite is formed and in the cooling stage it is transformed to nanomaghemite.Anions are effective to the magnetic susceptibility of both nanomagnetite and nanomaghemite. All anions in this study reduced the susceptibilities of magnetite, with the degree:CO32>PO43->Cl->H2O>SO42-. However, the susceptibilities of maghemite were enhanced by SO42- and Cl-, with the degree:SO42>Cl-. Others anions, especially CO32- and PO43-, reduced the susceptibilities of maghemtie. The phenomena can be explained by the crystal field theory:the ligand ions adsorbed on mineral surface change the state of Fe ions, thereby change the spin state of electron in d-shell and magnetic susceptibility.The magnetism of nanomagnetite was changed by microorganisms in methane-producing experimental system:magnetic susceptibility increased rapidly during the first 6 days, then declined slowly and got to stabilization eventually; the smaller the particle size of mineral, the greater the change of magnetic susceptibility (approaching 10%). More CO2 but less CH4 were produced in experiments with smaller magnetite particle. The phenomena indicate that magnetite can promote the activities of microbes, however, the strengthening mechanisms are different for different microorganisms.Sulfate-reducing bacterium can also change the magnetism of magnetite. The magnetic susceptibility was decreased rapidly in the first 6 d and stabilized at last. The decrease of susceptibility is more strong for experiment with smaller magnetite. Besides, more CO2 and less H2S were produced in experiments with smaller magnetite size. The phenomena indicate that sulfate-reducing bacterium can use magnetite as electron acceptor for growth.