Oxidation of FeS by oxygen-bearing acidic solutions

Oxidation of FeS in oxygen-bearing acidic solutions was investigated at different temperatures (25 to 45 degrees C) and pH (2.75 to 3.45). The rate of the oxidative dissolution of FeS is strongly dependent on pH. The reaction order with respect to hydrogen ions has been found to be 1.03+/-0.02 at 25 degrees C, and the apparent activation energy (E(a)) is 41.6 +/- 10.7 kJ mol(-1) at initial pH 3.00, suggesting that the FeS oxidative dissolution is controlled by the diffusion of oxidant species across a sulfur-rich layer (SRL) that undergoes chemical transformations leading to an increase in the mean number of sulfur atoms in polysulfide chains and the rearrangement of these chains. Fourier transform infrared spectroscopy and X-ray diffraction results obtained for the FeS samples reacted for 72 h at 25 degrees C and pH between 2.75 and 3.45 indicate the formation of goethite, of lepidocrocite, and of poorly ordered solid phases (assigned as SRL) on initial surfaces. The experimental data suggest a mechanism based on the protonation of FeS surfaces followed by oxidation of FeS by dissolved oxygen to produce Fe(2+), S(0), and S(2-)(n). Fe(2+) is unstable under oxidative conditions and transforms into Fe(OH)(3(s)), goethite and lepidocrocite.