Troilite oxidation by hydrogen peroxide

The kinetics and mechanism of troilite oxidation by H2O2 was studied at temperatures of 25 and 45 degrees C. Solutions within the ran-e 0.1-0.85 mol L-1 H2O2 in HClO4 (0.01-0.1 mol L-1) were used as dissolution media. The experimental amount of dissolved iron was plotted versus t(n), with n ranging from 0.25 to 1.55. The theoretical interpretation of this dependence suggests that the troilite oxidation involves several processes: acidic troilite dissolution, FeS + 2H(+) reversible arrow /SH2/ + /Fe2+/, where /SH2/ and /Fe2+/ are H2S and Fe2+ at troilite/sulfur rich layer (SRL) interface; /Fe2+/ migration into solution across SRL, and its rapid oxidation by hydrogen peroxide into ferric iron, 2Fe(2+) + H2O2 + 2H(+) = 2Fe(3+) + 2H(2)O; oxidation of /SH2/ sites to elemental sulfur, a process that contributes to sulfur enrichment of troilite surface, /SH2/ + 2Fe(3+) = S + 2Fe(2+) + 2H(+); oxidation of elemental sulfur to sulfate, a sulfur-consuming process, S + 3H(2)O(2) = SO42- + 2H(2)O + 2H(+). Both experimental results and theoretical considerations illustrate the importance of temperature, pH, and [H2O2] for the kinetics and mechanisms of troilite oxidation. The amounts of dissolved iron strongly increase with temperature and [H+], whereas an increase of H2O2 concentration seems to reduce the troilite oxidation. The reaction orders with respect to [H+] are variable, pointing out notable modifications of reaction mechanism with experimental conditions. The estimated value E-a = 25.4 +/- 0.9 kJ mol(-1) ([H2O2] = 0.4 mol L-1 and pH 1) points to dissolution kinetics controlled by a mix regime of surface reaction and diffusion. (c) 2006 Elsevier Inc. All rights reserved.