AN EXPERIMENTAL STUDY OF MAGNESITE DISSOLUTION AND PRECIPITATION RATES

The goal of this study is to provide fundamental physical parameters enabling the accurate geochemical modeling of carbonate precipitation in natural and industrial processes. Towards this goal, steady state dissolution and precipitation rates of magnesite have been determined in mixed flow reactors as a function of solution saturation state, aqueous Mg and HCO₃^- concentration, and temperature from 25 to 150° C. Measured dissolution rates depend strongly on temperature; pH 7 ‘far from equilibrium’ dissolution rates increase from 10^-13.3 to ~10^-11 mol/cm²/sec with increasing temperature from 25 to 150° C. Rates are interpreted using the rate model originally proposed by Pokrovsky and Schott (1999; Geochim. Cosmochim Acta, 63, 881-897) which is based on combining a surface complexation model with transition state theory to generate a single equation describing magnesite dissolution and precipitation rates over a wide range of chemical affinity and solution composition. Within this model, magnesite dissolution and precipitation rates at neutral and alkaline conditions are assumed proportional to the concentration of a Mg rich precursor complex of the form >MgOH₂⁺; aqueous species that react with this surface species such as bicarbonate ions, are found to inhibit rates. Due to the double affects of reverse reaction and bicarbonate inhibition, near to equilibrium dissolution and precipitation rates are surprisingly slow compared to dissolution rates measured in highly undersaturated solutions. Because magnesite precipitation is limited to relatively low degrees of supersaturation due to the likely precipitation of other Mg-bearing minerals, such as brucite, these slow near to equilibrium magnesite precipitation rates may be rate limiting in numerous natural and industrial processes.