An experimental study of Cobalt (II) complexation in Cl− and H2S-bearing hydrothermal solutions

The speciation of cobalt (II) in Cl− and H2S-bearing solutions was investigated spectrophotometrically at temperatures of 200, 250, and 300 °C and a pressure of 100 bars, and by measuring the solubility of cobaltpentlandite at temperatures of 120–300 °C and variable pressures of H2S. From the results of these experiments, it is evident that CoHS+ and CoCl42- predominate in the solutions except at 150 °C, for which the dominant chloride complex is CoCl3−. The logarithms of the stability constant for CoHS+ show moderate variation with temperature, decreasing from 6.24 at 120 °C to 5.84 at 200 °C, and increasing to 6.52 at 300 °C. Formation constants for chloride species increase smoothly with temperature and at 300°C their logarithms reach 8.33 for CoCl42-, 6.44 for CoCl3−, 4.94 to 5.36 for CoCl20, and 2.42 for CoCl+. Calculations based on the composition of a model hydrothermal fluid (Ksp-Mu-Qz, KCl = 0.25 m, NaCl = 0.75 m, ΣS = 0.3 m) suggest that at temperatures ⩽200 °C, cobalt occurs dominantly as CoHS+, whereas at higher temperatures the dominant species is CoCl42-.