Sulfidation of Ni-bearing goethites to pyrite: The effects of Ni and implications for its migration between iron phases

Goethite and pyrite are common iron minerals in oxic or anoxic environments, respectively, both minerals being major reservoirs for Nickel, a bio-essential element. Mineral transformation between goethite and pyrite is frequent owing to the alternation of oxic and anoxic conditions in sulfate-rich environments. This mineral transformation has been amply studied, but the effect of Ni on this transformation and its fate along it remain poorly understood. Sulfidation of Ni-free and Ni-containing (through adsorption or isomorphic substitution) goethites was thus studied experimentally by reacting goethite with dissolved S(-II) (molar Fe:S≈1:1). X-ray diffraction and associated Rietveld refinement, thermogravimetric analysis, scanning/transmission electron microscopy, X-ray absorption spectroscopy, and wet chemistry were used to monitor mineralogical evolutions and unravel Ni association with reaction products. After 44 days of sulfidation, about half of initial goethite converted to iron sulfides: thermodynamically stable pyrite (67%–93%) with minor contents of mackinawite (2%–15%) and greigite (5%–25%). Although the overall content of iron sulfides formed was essentially independent of Ni presence, Ni hampered the conversion from metastable iron sulfides (i.e., mackinawite and greigite) to pyrite (67%–78% vs. 93%, in the presence and absence of Ni, respectively). Pyrite formation from metastable sulfide precursors yielded a uniform Ni distribution in newly formed pyrite, regardless of the initial Ni association with goethite. Although no Ni was released to solution during pyrite formation, Ni incorporation to pyrite results in an increased risk of release to the environment as iron sulfides will be oxidized when exposed to air and water in supergene environments, leading to highly acidic conditions favoring Ni solubility and mobility.