Pyrite and magnetite are major iron minerals in extreme natural anoxic (lake, ocean, groundwater) or engineered (geological radioactive disposal, water remediation) systems. Our study aims to investigate trace element – iron mineral binding features with two “reporter” showing contrasting relationships with sulfide in water, namely Selenium (Se) and Gadolinium (Gd). Se occurs as +VI/+IV oxyanions in natural oxygenated waters and may be reduced down to Se(-II) in reducing conditions. In contrast, gadolinium is only present as free metal (Gd3+) and its hydrolyzed species. Characterization of experimental products was conducted via spectroscopic techniques (RIXS, XPS), microscopic analysis (SEM/TEM) and ab-initio calculations. Our preliminary results indicate that sorption occurs as adsorption and co-precipitation for both elements. In presence of sulfide ions, the selenate (SeO42-) adsorption is increased. Oxygen (O) atoms progressive replacement by sulfur (S) atoms (as in SeO4-xSx2-) is slow in homogeneous-, but fast in heterogeneous- conditions. According to ab-initio DFT computations, it starts by an oxygen (O) atom sharing with a pyrite Fe site, followed by an increased charge density in the O atom further away from the mineral surface, which at some point is released in solution. A decreased charge density is observed for the other selenite O atoms. According to HERFD, the Se oxidation state shifts from +VI (selenate) to a final value close to zero, whether Se is adsorbed, coprecipitated or present in Se rich natural pyrites with no Se neighbor, while pure Se° nanowires are formed on magnetite. Gadolinium sorbs as free Gd3+ ion on magnetite or pyrite, or substitutes for Fe3+ in Gd-doped magnetite. But in contrast to Se, mixed S- and O- gadolinium coordination may co-exist, as e.g. in mixed lamellar GdO2S nanoparticles, where S, O and Gd layer alternate. These new data question the simple “chalcophile” concept and shed new light on extreme both present- and paleo- anoxic environments, opening the way to distinguish “sulfidic” vs. anoxic sulfide-poor environments.

Selenium and gadolinium sorption on magnetite and pyrite in presence of sulfide: Assessing the difference between thiometalate species and free metal ion

ACS Spring 2022

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