Reaction of FeS with Fe(III)-bearing acidic solutions

The reaction of FeS with Fe(III)-bearing acidic solutions (Fe(III)BAS) was probed at 25 °C and pH between 2 and 3. Initial dissolved Fe3 + ([Fe3 +]init) was varied from 0.1 mM to 1 mM, and the length of the experiments was 240 min. Except for the experiment at initial pH 2, total dissolved iron ([Fe]total) decreased immediately (within 1 min) after contact of FeS and Fe(III). Afterwards, [Fe]total increased smoothly. A progressive increase in pH values and an Eh decrease within 240 min of reaction time were also observed. The reaction order of FeS dissolution in Fe(III)BAS with respect to [H+] is estimated to 0.65 at initial pH 3.0, and increases up to 1.0 with decreasing initial pH, indicating that [H+] is an important parameter of FeS dissolution in Fe(III)BAS. In contrast, changes in [Fe3 +]init have only a limited effect on the rate of FeS dissolution in Fe(III)BAS. Raman spectra of initial and reacted FeS samples reveal the accumulation of α-S8-like material on FeS surface. These results support a mechanism of FeS dissolution in Fe(III)BAS starting with the protonation of mineral surface and Fe3 +(aq) adsorption. Adsorbed protons subsequently accelerate Fe2 + release from FeS matrix into solution. The adsorbed Fe3 + may oxidize sulfur moieties and generate insoluble species, presumably polysulfide and elemental sulfur. The subsequent migration of Fe2 + into solution is controlled by the formed sulfur rich layer.