Oxidation State and Structure of Fe in Nontronite: From Oxidizing to Reducing Conditions

The redox reaction between natural Fe-containing clay minerals and its sorbates is a fundamental process controlling the cycles of many elements such as carbon, nutrients, redox-sensitive metals, and metalloids (e.g., Co, Mn, As, Se), 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, less is known about their behavior under anoxic and reducing conditions, thereby impeding a full understanding of the mechanisms of clay-driven reduction and oxidation (redox) reactions especially under reducing conditions. Here we investigate the structure of a ferruginous natural clay smectite, nontronite, under different redox conditions, and compare several methods for the determination of iron redox states. Iron in nontronite was gradually reduced chemically with the citrate-bicarbonate-dithionite (CBD) method. 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% Fe(II) 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.