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7. Oxidation state and structure of Fe in nontronite: From oxidizing to reducing conditions

Author

Qian, Y., (0000-0002-6608-5428) Scheinost, A., Grangeon, S., Greneche, J.-M., Hoving, A., Bourhis, E., Maubec, N., Churakov, S. V., Marques Fernandes, M., Qian, Y., (0000-0002-6608-5428) Scheinost, A., Grangeon, S., Greneche, J.-M., Hoving, A., Bourhis, E., Maubec, N., Churakov, S. V., and Marques Fernandes, M.

Abstract

The redox reaction between natural Fe-containing clay minerals and its sorbates is a fundamental process controlling the cycles of carbon, nutrients, and inorganic as well as organic pollutants in Earth’s critical zone. While the structure of natural clay minerals under oxic conditions is well known, this is not true for anoxic conditions, thereby impeding a full understanding of the mechanisms of clay-driven redox reactions especially under reducing conditions. Here we investigate the structure of an Fe-rich natural clay mineral, nontronite, under different redox conditions, and compare several methods for the determination of iron redox states. Nontronite was gradually reduced chemically with the Citrate-Bicarbonate-Dithionite (CBD) method. All methods used to determine the Fe redox state, i.e. 57Fe Mössbauer spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) spectroscopy including its pre-edge, extended X-ray absorption fine structure (EXAFS) spectroscopy, and mediated electrochemical oxidation and reduction (MEO/MER), provided consistent Fe(II)/Fe(III) ratios. By combining X-ray diffraction (XRD) and Transmission electron microscopy (TEM), we show that the long-range structure of nontronite at the highest obtained reduction degree of 44% is not different from that of fully oxidized nontronite except for a slight basal plane dissolution on the external surfaces. The short-range order probed by EXAFS spectroscopy suggests, however, an increasing structural disorder and Fe clustering with increasing reduction of structural Fe.

Published
2023

13. Surface engineering of titanium by multi-interstitial diffusion using plasma processing

Author

Drouet M., Pichon L., Dubois J.B., Le Bourhis E., and Christiansen T. L.

Subjects
Engineering (General). Civil engineering (General), TA1-2040
Abstract

Titanium and its alloys possess a range of highly interesting properties such as excellent corrosion resistance, high specific strength and biocompatibility, but suffers from poor wear resistance. The present work addresses plasma assisted surface treatment of CP 2 titanium using various combinations of oxygen and nitrogen, i.e. mixed interstitials. The sequence of controlled plasma nitriding and oxidizing treatments plays a significant role for the evolution of the hardness depth profiles and the development of the surface compound layer and the underlying diffusion/transition zone. Composition profiles of oxygen and nitrogen are obtained by GDOES; Mixed interstitial solubility of nitrogen and oxygen is found in both h.c.p. α titanium and in the compound layer. The combination of interstitials leads to larger case depth, in particular for the diffusion zone (expanded h.c.p. α titanium). Therefore, it highlights the advantages of combined nitriding and oxidizing compared to single nitriding treatments on the mechanical properties.

Published
2020
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