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 O 2 and H 2O has been examined in both the gaseous and aqueous environments. In agreement with previous ab initio quantum chemical calculations, the surface induced dissociation of H 2O in either environment was found to be energetically unfavourable. However, the dissociative adsorption of O 2 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 Pb 2+ 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. Pb 2+ 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 H 2S (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 H 2SO 4 and Pb 2+•6H 2O is energetically viable. The dissolution of one of the protonated S atoms neighbouring the Pb 2+ vacancy, resulting from the anaerobic dissolution, to form H 2SO 4, is also possible.