Rate law for galena dissolution in acidic environment

Abstract

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: R Gn , pH < 2 = 10 − 5.7 ± 0.4 e − 23 ± 3 RT a H + 0.43 ± 0.05 where R Gn 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 a H + is the activity of hydrogen ion in the solution. Galena dissolution rate law for pH between 2 and 3 can be expressed as: R Gn , pH = 2 − 3 = 10 − 8.5 ± 0.4 e − 15 ± 2 RT a H + − 0.78 ± 0.04 a O 2 ( aq ) 0.30 ± 0.03 where a O 2 ( 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.