Thermodynamics and kinetics of the reaction of copper(II) and iron(III) with ultra-small colloidal chalcopyrite (CuFeS2)
The kinetics of oxidation of colloidal chalcopyrite (CuFeS2) have been examined in the presence of aqueous FeIII and CuII at pH 2. 3 and in the presence of radiolytically generated Fe(OH)2+ at neutral pH. The reaction of CuII with CuFeS2 has been ascribed to an ion-exchange reaction of copper for iron in the CuFeS2 lattice. The initial copper sulfide phase that forms, as a result of the exchange, is rich in CuI and thermally converts to the known covellite (CuS) phase. Oxidation of CuFeS2 by FeIII at low pH leads to the dissolution of the chalcopyrite phase and the formation of Fe2+(aq), Cu2+(aq) and S°(s). The formation of an unstable intermediate CuSy phase is suggested from thermodynamic calculations. Under neutral pH conditions, CuFeS2 is oxidized by Fe(OH)2+ adsorbed on the particle surface. Oxidation is restricted to the first monolayer of the particles due to the formation of an FeIII hydroxide layer at the particle surface. The oxidation of CuFeS2 by Fe(OH)2+ is slower than by Fe3+(aq), presumably due to the lower redox potential of the former species.