Mechanisms of galena dissolution in oxygen-saturated solutions: evaluation of pH effect on apparent activation energies and mineral-water interface

This work describes a laboratory investigation of processes ruling the dissolution of galena (PbS) under conditions that mimic oxidising Earth surface environments. Freshly cleaved (001) galena surface was partially dissolved in solutions at different pH (HCl, pH between 1.2 and 5.8) and temperature (298 K) levels for several weeks in a flow-through reactor. Analysis of the galena surface after exposure to oxygen-saturated solutions was performed by XPS. The estimated thickness of surface layer formed after interaction for four weeks with solution at PH 5.8 was similar to 0.5 nm. After the same time of interaction with solution at PH 1, the surface layer was at least 9 nm thick. Stationary dissolution rates were measured at different PH (between 1 and 5.8) and temperatures (298, 323, 348 K). Reaction order with respect to hydrogen ion averages 0.6 and slightly decreases with temperature. Dissolution rates, plotted vs. the reciprocal values of temperature, follow Arrhenius behaviour. Activation energies show a sharply discontinuous variation with PH. They are 13.5 +/- 2 kJmol(-1) at pH = 1.2 and 2.87, whereas at PH of 4.08, 4.7, and 5.8, they show, respectively, values of 48.8 +/- 3.5, 45 +/- 4, and 51 +/- 4 kJmol(-1). The apparent activation energy was also investigated by starting the experiment from PH 5.8 and then decreasing to PH 1.2. In this case, the apparent activation energies were 44 4 kJmol-1 and 21 4 kJmol(-1), respectively. Apparent activation energies indicate a diffusion-controlled kinetic regime for PH < 3, and a mixture of surface and diffusion control for PH between similar to 4 and 5.8. A strong effect of solution PH on the mineral-water interface is revealed also by the observed PH dependence of surface-layer thickness. On the basis of the high-resolution XPS signals, the dependence of activation energies on solution PH should reflect a change of structure, and also chemical composition, of the mineral-water interface.