TY - JOUR A1 - Eiby, Simon H. J. A1 - Tobler, Dominique J. A1 - Voigt, Laura A1 - van Genuchten, Case M. A1 - Bruns, Stefan A1 - Jensen, Kirsten M. Ø. A1 - Stawski, Tomasz M. A1 - Wirth, Richard A1 - Benning, Liane G. A1 - Stipp, S. L. S. A1 - Dideriksen, Knud T1 - Topotactic Redox-Catalyzed Transformation of Iron Oxides N2 - Fe oxides frequently exist in systems containing both Fe(II) and Fe(III), where their reactivity is enhanced and where interfacial electron transfer from Fe(II) adsorbed to the solids causes the transformation of metastable Fe oxides. Here, we contribute to the understanding of such a transformation using green rust sulfate (GR) synthesized in the presence or absence of Si or Al as the starting material. X-ray diffraction (XRD) and pair distribution function (PDF) analyses showed that (i) rapid oxidation by Cr(VI) caused transformation to Fe oxyhydroxide with short-range ordering, with a pattern identical to that reported for the oxidation of isolated GR hydroxide sheets (i.e., a trilayer of Fe with both edge- and corner-sharing polyhedra) and (ii) goethite formed at the expense of the short-range-ordered Fe oxyhydroxide when residual Fe(II) was present, particularly when Si was absent. This is consistent with the Fe(II)-catalyzed transformation of the short-range-ordered Fe oxyhydroxide. High-resolution transmission electron microscopy (TEM) showed that the two oxidation products coexisted within individual particles and that particle shape and the crystallographic orientation of both products were inherited from the original GR crystals, i.e., they had formed through topotactic transformation. We interpret that the structural reorganization to goethite occurred either in response to distortions caused by polaron movement or as a result of electron transfer reactions occurring at internal surfaces. Once nucleated, goethite growth can be sustained by dissolution–reprecipitation. KW - Iron oxide KW - Electron microscopy KW - Pair distribution funvtion KW - Total scattering PY - 2025 DO - https://doi.org/10.1021/acsearthspacechem.5c00220 SN - 2472-3452 SP - 1 EP - 12 PB - American Chemical Society (ACS) AN - OPUS4-64924 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Perez, Jeffey Paulo H. A1 - Tobler, Dominique J. A1 - Thomas, Andrew N. A1 - Freeman, Helen M. A1 - Dideriksen, Knud A1 - Radnik, Jörg A1 - Benning, Liane G. T1 - Adsorption and reduction of arsenate during the Fe2+-induced transformation of ferrihydrite N2 - Iron (oxyhydr)oxides play an important role in controlling the mobility and toxicity of arsenic (As) in contaminated soils and groundwaters. However, dynamic subsurface geochemical conditions can potentially impact As sequestration since this is highly dependent on the dominant iron mineral phases present and the pathways through which they form. In this study, we investigated the Fe2+-induced transformation of As(V)-bearing ferrihydrite (As-FH) to more crystalline phases under relevant anoxic subsurface conditions. Specifically, we examined the influence of varying Fe2+(aq)/Fe(III)solid¬ ratios on the behavior and speciation of the mineral-bound As species during the mineralogical transformation of As-FH at pH 6.5 for 24 h. At lower Fe2+(aq)/Fe(III)solid¬ ratios (0.5 to 1), goethite, green rust sulfate (GR¬SO4) and lepidocrocite formed within the first 2 hours of the reaction, but only goethite and some unreacted FH remained after 24 h. At Fe2+(aq)/Fe(III)solid¬ ratio = 2, GRSO4 remained stable throughout the 24 h reaction, alongside goethite and unreacted FH. Despite >82% of the As-FH being transformed to goethite  GRSO4 in these reactions, no significant As release (>99.9% removal) was observed. However, while As remained mineral-bound, partial oxidation of the initially added As(V) was reduced to As(III), most likely, by the goethite-Fe2+(aq) redox couple. The extent of As(V) reduction increased from ~40% to ~50%, as the Fe2+(aq)/Fe(III)solid¬ ratio increased from 0.5 to 2. Overall, these results provide important insights into transformation pathways of iron (oxyhydr)oxide minerals in As contaminated, anoxic soils and sediments, and also demonstrate the great impact these can have on As oxidation state and, hence, toxicity and mobility in these environments. KW - Ferrihydrite KW - Mineral tranformation KW - XPS KW - Green rust KW - Goethite KW - XAS PY - 2019 DO - https://doi.org/10.1021/acsearthspacechem.9b00031 SN - 2472-3452 VL - 3 IS - 6 SP - 884 EP - 894 PB - ACS AN - OPUS4-48436 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -