An experimental study of magnesite precipitation rates at neutral to alkaline conditions and 100-200 °C as a function of pH, aqueous solution composition and chemical affinity

Magnesite precipitation rates were measured at temperatures from 100 to 200 °C as a function of saturation state and reactive fluid composition in mixed flow reactors. Measured rates were found to increase systematically with increasing saturation state but to decrease with increasing reactive fluid aqueous CO32- activity and pH. Measured rates are interpreted through a combination of surface complexation models and transition state theory. In accord with this formalism, constant saturation state BET surface area normalized magnesite precipitation rates (rMg) are a function of the concentration of protonated Mg sites at the surface (>MgOH2+) and can be described using: rMg=kMg-KCO3KOHKCO3KOH+KOHaCO32-+KCO3aOH-n1-ΩMgn where kMg- represents a rate constant, KOH and KCO3 stand for equilibrium constants, ai designates the activity of the subscripted aqueous species, n refers to a reaction order equal to 2, and ΩMg denotes the saturation state of the reactive solution with respect to magnesite. Retrieved values of n are consistent with magnesite precipitation control by a spiral growth mechanism. The temperature variation of the rate constant can be described using kMg-=Aaexp(-Ea/RT), where Aa represents a pre-exponential factor equal to 5.9 × 10−5 mol/cm2/s, Ea designates an activation energy equal to 80.2 kJ/mol, R denotes the gas constant, and T corresponds to the absolute temperature. Comparison of measured magnesite precipitation rates with corresponding forsterite dissolution rates suggest that the relatively slow rates of magnesite precipitation may be the rate limiting step in mineral carbonation efforts in ultramafic rocks.