Rate law for galena dissolution in acidic environment

A dissolution rate law for galena in acidic environment was derived from the steady-state dissolution rates using flow-through experiments. The influence of temperature, dissolved oxygen concentration and pH between 1 and 3 was assessed. This rate law can be used for predicting galena dissolution behavior in a wide range of conditions analogous to Acid Rock Drainage.For pH below 2, the dissolution rate law can be expressed as:RGn,pH<2=10−5.7±0.4e−23±3RTaH+0.43±0.05where RGn is the galena dissolution rate (mol m− 2 s− 1), R is the gas constant (kJ mol− 1 K− 1) , T   is the temperature (K) and aH+aH+ is the activity of hydrogen ion in the solution.Galena dissolution rate law for pH between 2 and 3 can be expressed as:RGn,pH=2−3=10−8.5±0.4e−15±2RTaH+−0.78±0.04aO2(aq)0.30±0.03where aO2(aq)aO2(aq) is the activity of dissolved oxygen.XPS (X-ray Photoelectron Spectroscopy) examination of the reacted galena samples shows the formation of a lead-deficient and sulfur-rich surface layer, consistent with the observed non-stoichiometry between dissolved sulfur and lead in all the studied solutions.Based on the S/Pb ratio observed in solution and the reacted surfaces and the pH and dissolved oxygen dependence of the rates, two possible reactions for galena dissolution in acidic aqueous solution are proposed; (1) at pH ≤ 2 the rate seems to be determined by the protonation of surface sulfur atoms, and (2) at pH ≥ 2 the rate seems to be controlled by the attachment of oxygen to surface sulfur atoms.The values obtained for the activation energies (15 ± 2 kJ mol− 1 at pH 3 and 23 ± 3 kJ mol− 1 at pH 1) suggest that galena dissolution is controlled by diffusion processes or mixed-controlled by diffusion of reactants and products between the bulk solutions and the reacting surfaces.