Magnetite (Fe3O4) is a mixed valence Fe mineral containing both Fe(II) and Fe(III). Biogenic magnetite can form as a result of dissimilatory Fe(III) reduction and microbial Fe(II) oxidation, whereas abiogenic magnetite can form from natural weathering and metamorphism or via chemical synthesis. Previous studies have shown that the reactivity of magnetite depends upon its stoichiometry (i.e. Fe(II)/Fe(III) ratio), particle size and the presence of organics. Here we present results which show how Cr-immobilization by magnetite is influenced by the effect of magnetite oxidation by Fe(II)-oxidizing bacteria, reduction by Fe(III)-reducing bacteria, or its association with humic substances. We synthesized two types of magnetite: biogenic magnetite produced by microbial reduction of the Fe(III) oxyhydroxide ferrihydrite and abiogenic magnetite via co-precipitation of an Fe(II)-Fe(III) solution. Each type of magnetite was exposed to three conditions; (1) Fe(III) reduction by Geobacter sulfurreducens, (2) Fe(II) oxidation by the phototrophic Rhodopseudomonas palustris strain TIE-1 and (3) a control which was not exposed to any Fe-metabolizing bacteria. This resulted in a total of 6 different magnetite samples with varying Fe(II)/Fe(III) ratios as a result of the activity of the Fe-metabolizing bacteria. The adsorption of hexavalent chromium (Cr(VI)) to the different types of magnetite, as well as the rates of removal from solution was quantified. Furthermore, the effect of humic substances on Cr immobilization was investigated for control magnetites. Our experiments have shown that microbial Fe(III) reduction and Fe(II) oxidation influence Cr-removal by magnetite. More specifically, microbially reduced magnetite displays the fastest Cr(VI)-removal rates and highest capacity to remove Cr(VI) while the microbially oxidized magnetite was surprisingly more efficient at Cr(VI) removal than the control magnetite. Furthermore humic substances decreased the Cr(VI)-removal compared with experiments conducted in the absence of humic substances. These results have clear implications for the use of magnetite for Cr remediation applications in environments where Fe-metabolizing bacteria and humic substances could be expected to be present.

Effect of Fe-metabolizing microbes and humic substances on Cr-immobilization by biogenic and abiogenic magnetite

253rd National Meeting (2017)

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