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Hexavalent chromium, Cr(VI), is a highly toxic carcinogen occurring in natural and industrial environments. Pathways to economical reduction to the more benign trivalent form, Cr(III), are necessary for treatment of contaminated groundwater. Magnetite’s (Fe₃O₄) mixture of Fe(II) and Fe(III) make it a promising material for remediation. This study investigated the mechanisms for reduction of Cr(VI) catalyzed by Fe₃O₄as a redox mediator in the presence of oxalic acid in HClO₄and SO₄²⁻solutions, a system where the interactions among these species are not fully understood. The reduction of Cr(VI) in different anion environments is first measured on an Au rotating disk electrode. SO₄²⁻inhibits the formation of a passivation layer and Cl^-partially inhibits passivation. The reduction of Cr(VI) on Fe₃O₄is limited by the availability of Fe(II) surface sites. Addition of oxalic acid works synergistically through liberation of Fe(II)-oxalate and soluble Cr(III)-oxalate products. A combination of Fe₃O₄activated by exposure to oxalic acid and use of an oxalic acid solution as a medium for reduction of Cr(VI) produces over 97% removal of Cr(VI). These results provide relevant insights regarding interactions of Fe₃O₄with organic acids and the anion environment which lead to the effective reduction of Cr(VI). 

Sealing of oil and gas production wells is done to protect overlying aquifers and the land surface from vertical migration of drilling fluids, produced water (PW), and natural gas. The integrity of the contact between the well casing and cement seal is especially important as both the steel and the cement are subject to reactions caused by exposure to very high salinity PW. The objective of this study was to identify corrosion and precipitation products that form at this contact and determine their effect on the microannular space between the two materials. Steel cylinders were embedded in Type G Portland cement to simulate a sealed wellbore. They were then exposed to simulated and PW sampled from the Permian Basin in the southwestern United States. Solid phases in the cement were identified by X-ray diffraction and electron microprobe analyses and included portlandite, a calcium silicate, and brownmillerite. Gas flow measurements were used to estimate the aperture of the microannulus between the steel surface and the cement. A decrease in the aperture with increasing reaction time was detected for all experiments. The findings suggest that exposure to PW has the potential to reduce the microannular space between the casing and the cement seal as a result of precipitation of calcium- and magnesium-carbonate as dominant phases, with the co-occurrence of sulfate and silicate minerals. These results have implications related to the long-term integrity of annular seals used to seal oil wells exposed to very high salinity PW. 

Perovskite materials are used for high temperature electrochemical applications such as solid oxide fuel cells (SOFC) and electrolyzers due to their tunable conductivity and catalytic activity. However, high temperature operation poses significant challenges in both fabrication and durable operation that is further complicated by the operating environment. We studied barium niobates with various A and B site dopants. These doped niobates showed enhanced thermochemical stability in SOFC relevant conditions and catalytic activity towards methane activation. The redox behavior of the Nb^4+/5+couple seem to be at a key reason behind this redox stability while the size and electronegativity of the dopants affect the electrical properties. The chemical stability was analyzed by TGA measurements followed by analysis of the perovskite powders using PXRD measurements. Impedance measurements were utilized to analyze their electrical conductivity. Our results demonstrate doped barium niobates as a promising candidate for stable operation in high temperature electrochemical applications. 

Doped perovskite metal oxide catalysts of the form A(B_xM_1-x)O_3-δhave been instrumental in the development of solid oxide electrolyzers/fuel cells. In addition, this material class has also been demonstrated to be effective as a heterogeneous catalyst. Co-doped barium niobate perovskites have shown remarkable stability in highly acidic CO₂sensing measurements/environments (1). However, the reason for their chemical stability is not well understood. Doping with transition metal cations for B site cations often leads to exsolution under reducing conditions. Many perovskites used for the oxidative coupling of methane (OCM) or the electrochemical oxidative coupling of methane (E-OCM) either lack long term stability, or catalytic activity within these highly reducing methane environments. The Mg and Fe co-doped barium niobate BaMg_0.33Nb_0.67-xFe_xO_3-δshown activity in E-OCM reactors over long periods (2) (>100 hrs) with no iron metal exsolution observed by diffraction or STEM EDX measurements. In contrast, iron decorated BaMg_0.33Nb_0.67O₃showed little C2 conversion activity.