| Summary: | Dissolution of the (104) surface of magnesite (MgCO3) was studied as a function of bulk solution pH over the range 2.0 < pH < 5.0 at 60 °C using atomic force microscopy (AFM) with well-defined hydrodynamics. The experimental data and corresponding solution of the convective-diffusion equation for the system revealed that magnesite dissolution is kinetically inhibited by a factor of 102-104 relative to the proton mass transport limit. The dissolution flux was found to vary nonlinearly with the surface concentration of H+, [H+]y=0, and that inclusion of the homogeneous chemical kinetics of H+ consumption in the system to form carbonic acid was unnecessary. The nonlinear behavior was best represented by a Langmuir isotherm for proton adsorption in which the adsorbed entity consists of a surface complex containing more than one proton. The apparent surface kinetic detachment coefficient, k′n, for this surface complex was determined to be 5 × 10-12 mol cm-2 s-1, but the determination of a particular coordination number, n, of this detachment complex was not possible based on the experimental data. The velocity of dissolving + steps was found to be constant, within error, over the entire experimental pH range, whereas the dissolution flux varied by over an order of magnitude in this same range. The AFM images revealed a dramatic increase in step density associated with a large increase in dissolution flux that was attributed to the protonation of terrace-adsorbed carbonate sites (i.e., adions). We propose that the intrinsic protonation constant, Kint, differs for adions, kink, step, and terrace sites based on the AFM observations of surface and step morphology as a function of pH.
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