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Chromium is a redox active 3d transition metal with a wide range of valences (-2 to +6) that control the geochemistry and toxicity of the element. A variety of Cr-bearing minerals are found in meteorites of all petrographic classes lunar basalts, Martian rocks, and the Earth, recording valuable information concerning geochemical conditions of mineralization. Natural weathering of Cr- bearing minerals and fallout of volcanic ash release Cr into the terrestrial environment. Furthermore, Cr compounds are widely used in numerous industrial processes which can discharge toxic Cr complexes in soils/sediments and surface/ground waters, as well as the atmosphere in the form of aerosols. Therefore, techniques that measure Cr valence are important biogeochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report electron energy loss spectroscopy (EELS) techniques that, unlike other methods, can determine Cr valence at high-spatial resolution (nm-scale) over a wide valence range, Cr(O) to Cr(VI).

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Chromium is a redox active 3d transition metal with a wide range (-2 to +6) of possible oxidation states; however only Cr(III) and Cr(VI) are stable. The valence state of Cr is important because it controls the geochemistry and toxicity of the element. Hexavalent Cr species are strong oxidants which act as carcinogens, mutagens, and teratogens in biological systems [1]. The high solubility, bioavailability, and toxicity of Cr(VI) make it a particular environmental concern. In contrast, Cr(III) species have low toxicity, in part, because their bioavailability is limited by their low solubility and their tendency to form strong complexes with organics and hydroxo complexes. Hexavalent Cr(VI) can be readily reduced to the trivalent state by Fe2+, S2-, organic compounds, wetland plants, and several species of microorganisms [2]. The associated mechanisms of Cr(VI) reduction are technologically and biologically important because they convert a toxic, mobile element into a less toxic, immobile form. Furthermore, the geochemical and microbiological processes of Cr oxidation/reduction drive both precipitation/dissolution and ion adsorption/desorption reactions in the environment. The study of these reactions has been hindered by the lack of an analytical technique that can determine the oxidation state of Cr at high spatial resolution. For instance, little is known about unstable redox intermediates with intermediate valence states between the valances of the stable end members of the oxidation/reduction process.

The geochemistry and toxicity of chromium is controlled by its valence state. Chromium is a redox active 3d transition metal with a wide range (− to +6) of possible oxidation states; however only Cr(III) and Cr(VI) are stable. Hexavalent Cr(VI) can be readily reduced to the trivalent state by Fe2+, S2−, organic compounds, wetland plants, and several species of microorganisms. The associated mechanisms of Cr(VI) reduction are technologically and biologically important because they convert a toxic, mobile element into a less toxic, immobile form.Reduction of Cr(VI) by the bacterium, Shewanella oneidensis (previously classified Shewanella putrefaciens strain MR-1), was studied by absorption spectrophotometry and in-situ, environmental cell (EC) - transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). Shewanella oneidensis (S. oneidensis), a gram-negative, facultative bacterium is capable of respiring aerobically and anaerobically using a variety of compounds, including O2, Fe(III), Mn(IV), NO2−, NO3−, SO2, SO32−, thiosulfate (S2O32−), trimethyamine oxide, fumarate, U(VI), and Cr(VI) as terminal electron acceptors.

Microbiologically influenced corrosion (MIC) is of wide concern in marine and non-marine environments. Biofilms and corrosion products associated with microorganisms cause numerous problems in aqueous environments, such as increased fluid frictional resistance, reduced heat transfer, and many types of corrosion, all of which can lead to failure of materials. Corrosion of metals has been extensively examined using TEM, but examination of MIC with TEM has only just begun (Blunn, 1986; Chio, 1996). Previous studies examining microbial colonization of copper surfaces and distribution throughout corrosion products demonstrate copper immobilization by bacterial biofilms (Blunn, 1987). In the current study, Pseudomonasputida attachment to corroding iron particles was examined in a sealed environmental cell in a JEOL 3010 scanning transmission electron microscope (STEM).Iron filings were produced from carbon steel (C1010) using 600 grit sandpaper, collected with a teflon coated magnet, degreased in acetone and sterilized in ethanol. Filings were incubated in distilled water until corrosion was visible under a dissecting microscope.