CONNECTIONS BETWEEN SURFACE COMPLEXATION AND GEOMETRIC MODELS OF MINERAL DISSOLUTION INVESTIGATED FOR RHODOCHROSITE

Abstract

Mineral dissolution rates have been rationalized in the literature by surface complexation models (SCM) and morphological and geometric models (GM), and reconciliation of these conceptually different yet separately highly successful models is an important goal. In the current work, morphological alterations of the surface are observed in real time at the microscopic level by atomic force microscopy (AFM) while dissolution rates are simultaneously measured at the macroscopic level by utilizing the AFM fluid cell as a classic flow-through reactor. Rhodochrosite dissolution is studied from pH = 2 to 11 at 298 K, and quantitative agreement is found between the dissolution rates determined from microscopic and macroscopic observations. Application of a SCM model for the interpretation of the kinetic data indicates that the surface concentration of >CO3H regulates dissolution for pH < 7 while the surface concentration of >MnOH2+ regulates dissolution for pH > 7. A GM model explains well the microscopic observations, from which it is apparent that dissolution occurs at steps associated with anisotropic pit expansion. On the basis of the observations, we combine the SCM and GM models to propose a step-site surface complexation model (SSCM), in which the dissolution rates are quantitatively related to the surface chemical speciation of steps. The governing SSCM equation is as follows: R = χ1/2(kco + kca)[>CO3H] + χ1/2(kmo + kma)[>MnOH2+ ], where R is the dissolution rate (mol m-2 s-1), 2χ1/2 is the fraction of surface sites located at steps, [>CO3H] and [>MnOH2+ ] are surface concentrations (mol m-2), and kco, kca, kmo, and kma are the respective dissolution rate coefficients (s-1) for the >CO3H and the >MnOH2+ surface species on obtuse and acute steps. We find kco = 2.7 s-1, kca = 2.1 x 10-1 s-1, kmo = 4.1 x 10-2 s-1, kma = 3.7 x 10-2 s-1, and χ1/2 = 0.015 +/- 0.005. The rate coefficients quantify the net result of complex surface step processes, including double-kink initiation and single-kink propagation. We propose that the SSCM model may have general applicability for dissolution far from equilibrium of flat mineral surfaces of ionic crystals, at least those that dissolve by step retreat.