Nickel sulfide formation at low temperature: initial precipitates, solubility and transformation products

Environmental context Remediation technologies often rely on manipulation of redox conditions or natural redox processes to favour microbial sulfate-reduction and mineral sulfide formation for treatment of inorganic contaminants in groundwater, including nickel. However, few data are available on the structural properties, solubility and mineral transformation processes involving nickel sulfides. These data are needed in order to constrain the long term performance of groundwater remediation efforts. Abstract The formation of nickel sulfides has been examined experimentally over the temperature range from 25 to 60°C. At all conditions studied, hexagonal (a-NiS) was the initial precipitate from solution containing Ni2+ and dissolved sulfide. Freshly precipitated nickel sulfide possesses significant residual Ni–O coordination as revealed by X-ray absorption spectroscopy. With progressive aging, residual Ni–O coordination is replaced by Ni–S coordination. The formation of millerite (ß-NiS, rhombohedral) was not detected in any of the synthesis experiments. In the presence of elemental sulfur, hexagonal NiS converted to polydymite (Ni3S4) and vaesite (NiS2). Thus, conversion of nickel monosulfide to thiospinel and disulfide structures appears to be redox dependent, analogous to aging and transformation processes of iron sulfides. In the absence of elemental sulfur or with only hydrogen sulfide or bisulfide present, transformation of hexagonal NiS was not observed after 1680 h at 60°C. Low-pH solubility experiments yielded a solubility product for hexagonal NiS of log Ks0 = –2.69 ± 0.26. Solubility data at pH > 3 suggest that Ni–bisulfide complexation is important in controlling the solubility of Ni in sulfidic solutions.