Microencapsulation technologies for iron sulfides: Principles and applications

The surface-exposed sulfur atom of pyrite possesses an unshared electron pair which produces a slightly negatively charged surface that can attract cations, e.g., Fe2+. This surface Fe2+, however, depending on pH, coordinates OH. We proposed that this surface-Fe2+-OH when in the presence of CO2 is converted to -Fe-CO3 or -Fe-HCO3. In this study, using Fourier Transform Infrared spectroscopy (FT-IR) we demonstrated that such complexes form on the surface of pyrite. FT-LR spectra also showed the presence of two carbonyl absorption bands (1682 cm(-1) and 1653 cm(-1)) on the surface of pyrite upon exposure to CO2. It suggested that pyrite surface carbonate complexes existed in two different chemical environments, suggesting two potential mechanisms of pyrite surface-CO2 interactions. One potential mechanism involved formation of a pyrite surface-Fe(II)-HCO3 complex whereas a second potential mechanism involved formation of a pyrite surface-carboxylic acid group complex [-Fe(II)-S-COO-Fe(II)]. We hypothesized that these pyrite surface-CO2 complexes could promote abiotic oxidation of pyrite by accelerating the abiotic oxidation of Fe Iron (III) would oxidize the disulfide (-S-2) by accepting its electrons. Using a miscible displacement technique, oxidation of FeS2 with H2O2 was carried out in the absence or presence of 10 or 100 mM NaHCO3. The data showed that 100 mM NaHCO3 significantly increased the oxidation rate of FeS2. Feasibility for controlling pyrite oxidation by creating on pyrite surfaces an impermeable coating with Si was studied through leaching pyrite with solution composed of H2O2 or hypochlorite, H4SiO4, and sodium acetate (NaOAc) or limestone. It was postulated that during leaching with H2O2 pyrite is oxidized and Fe3+ is produced so that iron silicate will precipitate as coatings on pyrite surfaces. The results showed that indeed iron silicate coatings are established on pyrite by leaching it with a solution composed of H2O2 and, Si buffered at pH 5 to 6. Leachates from large field column experiments also showed that silicate coatings produced the least amount of sulfate relative to the control, limestone and/or phosphate treatments. On the other hand, limestone maintained the leachate near neutral pH but produced more sulfate than the control.