Calculation of Solubility for Metal Oxide in Supercritical Water Environments

Abstract

H⁺ and OH^- concentrations and solubilities of Fe₂O₃ and NiO were calculated in supercritical aqueous electrolyte solutions of HCl, NaCl, NaOH and H₂SO₄ at 400°C (30 and 60MPa) and concentrations ranging from 10^-5 to 0.5mol·kg^-1 for the purpose of understanding corrosion behavior at supercritical water condition. At 30MPa, in 0.1mol·kg^-1 NaCl solution, OH^- concentration was about one order of magnitude higher than that in pure water, namely NaCl solution acted as a base. Calculation made by introducing NaCl into the HCl solutions decreased H⁺ concentration. At 30MPa, coexistence of 0.1mol·kg^-1 NaCl in 0.01mol·kg^-1 HCl solution reduced H⁺ concentration by about 60%. Further, solubilities of metal oxide increased in acid solution and decreased in NaCl and NaOH solutions with increasing pressure. Solubilities also increased with increasing NaCl concentration. At these low dielectric conditions, association of dissolved species consisting of metal oxides in the presence of Na⁺, Cl^- and SO₄^2- were significant in addition to dissociation of acid and base and metal oxide dissolution.