A QUANTUM CHEMICAL INVESTIGATION OF THE OXIDATION AND DISSOLUTION MECHANISMS OF GALENA

Abstract

The oxidation and dissolution mechanisms of galena (PbS) remain uncertain with a wide variety of possible mechanisms having been proposed in the literature. In this study, the thermodynamic viability of some possible mechanisms has been tested using semi-empirical quantum chemical calculations applied to a perfect (001) galena surface.The adsorption of O2 and H2O has been examined in both the gaseous and aqueous environments. In agreement with previous ab initio quantum chemical calculations, the surface induced dissociation of H2O in either environment was found to be energetically unfavourable. However, the dissociative adsorption of O2 was found to be possible and resulted in two O atoms bonded to diagonally adjacent S atoms with the O atoms oriented along the diagonal.The adsorption of H+ and possible subsequent dissolution mechanisms have been examined in the aqueous environment. An anaerobic mechanism leading to the dissolution of hydroxylated Pb2+ was identified. The mechanism involves the protonation of 3 surface S atoms surrounding a central surface Pb atom followed by substitution of this Pb by a further H+. The activation energy of this mechanism was estimated to be ~100 kJ mol-1. Pb2+ dissolution could only occur with vacancy stabilisation by a H+. The analogous mechanisms for systems comprising H+ adsorbed on either 2 or 4 of the S atoms surrounding a central surface Pb were not found to be energetically viable. Subsequent dissolution of one of the protonated S atoms to form H2S(g) was not found to be possible thus indicating the likely formation of a Pb-deficient S-rich surface under acidic anaerobic conditions.Acidic aerobic dissolution has also been examined. Congruent dissolution to form H2SO4 and Pb2+*6H2O is energetically viable. The dissolution of one of the protonated S atoms neighbouring the Pb2+ vacancy, resulting from the anaerobic dissolution, to form H2SO4, is also possible.