An experimental study of magnesite dissolution rates at neutral to alkaline conditions and 150 and 200 °C as a function of pH, total dissolved carbonate concentration, and chemical affinity

Steady-state magnesite dissolution rates were measured in mixed-flow reactors at 150 and 200 degrees C and 4.6 < pH < 8.4, as a function of ionic strength (0.001 M <= I <= 1 M), total dissolved carbonate concentration (10(-4) M < Sigma CO(2) < 0.1 M), and distance from equilibrium. Rates were found to increase with increasing ionic strength, but decrease with increasing temperature from 150 to 200 degrees C, pH, and aqueous CO(3)(2-) activity. Measured rates were interpreted using the surface complexation model developed by Pokrovsky et al. (1999a) in conjunction with transition state theory (Eyring, 1935). Within this formalism, magnesite dissolution rates are found to be consistent with r(d) = k(Mg) {> MgOH(2)(+)}(4)[1 - exp (-4A/RT)] where r(d) represents the BET surface area normalized dissolution rate, {> MgOH(2)(+)} stands for the concentration of hydrated magnesium centers on the magnesite surface, k(Mg) designates a rate constant, A refers to the chemical affinity of the overall reaction, R denotes the gas constant, and T symbolizes absolute temperature. Within this model decreasing rates at far-from-equilibrium conditions (1) at constant pH with increasing temperature and (2) at constant temperature with increasing pH and Sigma CO(2) stem from a corresponding decrease in {> MgOH(2)(+)}. This decrease in {> MgOH(2)(+)} results from the increasing stability of the > MgCO(3)(-) and >MgOH degrees surface species with increasing temperature, pH and CO(3)(2-) activity. The decrease in constant pH dissolution rates yields negative apparent activation energies. This behavior makes magnesite resistant to re-dissolution if formed as part of mineral carbon sequestration efforts in deep geologic formations. (C) 2010 Elsevier Ltd. All rights reserved.