Still oxides run deep: studying redox transformations involving Fe and Mn oxides using selective isotope techniques

Reactions of aqueous Fe(II) with Fe and Mn oxides influence heavy metal mobility, transformation of trace organics, and important elemental cycles as Fe precipitates form or dissolve, and as electrons move between aqueous and solid phases. Our objective was to characterize reactions of Fe(II) with important metal oxides, using a suite of complementary tools to investigate the extent and underlying mechanisms of Fe(II)-metal oxide redox activity.

Nanoscale materials (1-100 nm) may have fundamentally different surface or electronic properties than larger solids. Goethite was synthesized with primary particle dimensions above or below the nanoscale. Despite large differences in particle surface area, goethite nanorods and microrods had similar net Fe(II) sorption and electron transfer properties. Experimental evidence suggested particle aggregation resulted in particle complexes of a similar size, meaning considerations of available reactive surface area could explain our results.

Kinetics and extent of Fe(II)-Fe(III) redox reactions between aqueous Fe(II) and goethite were examined using a stable isotope tracer approach. Aqueous Fe(II) that had been enriched in ⁵⁷Fe was mixed with isotopically-normal goethite. Convergence of Fe isotope ratios in aqueous and solid phases to values predicted by complete Fe atom exchange provided evidence that all goethite Fe(III) atoms could eventually react with Fe(II), despite no evidence for complete atom exchange from bulk measurements of the aqueous or solid phase. Fe isotope data at different experimental conditions was combined with theoretical considerations governing electron transfer in goethite to provide evidence for redox-driven atom exchange involving bulk conduction of electrons between spatially distinct Fe(II) sorption and release sites. Procedures for stable Fe isotope tracer studies have been adapted to investigate redox transformations of magnetite solids with different divalent cation content.

Evolution of aqueous Fe(II)-Mn(IV) redox reactions was examined using complementary techniques. After pyrolusite particles were exposed to aqueous Fe(II), aqueous Fe and Mn were analyzed, and X-ray diffraction was utilized with electron microscopy to assess solid phase evolution during continued exposure to Fe(II). Selective use of Fe isotopes during Fe(II) resuspensions allowed us to track chemical changes occurring to one particular Fe addition using ⁵⁷Fe Mössbauer spectroscopy.