Influence of organic ligands on the crystal growth of magnesite (MgCO3) : Mechanistic aspects and implications for the mineral sequestration of CO2

The efficiency of Mg-silicates mineral carbonation processes is limited by the slow dissolution of silicate minerals and subsequent formation of a stable Mg-carbonate phase. It is therefore necessary to enhance the whole process to make it economically and energetically viable. It has recently been suggested that organic ligands, whose ability to significantly enhance the rate of dissolution of silicate minerals is well known, could be used in this purpose (Park et al., 2003). Studies on calcite have shown that most organic ligands have an inhibitory effect on its crystal growth (e.g Reddy & Hoch, 2001). However, their effect has not yet been assessed on Mg-carbonates precipitation, and might be different due to the highly hydrated state of the Mg²⁺ ion in aqueous solution. We will present results of magnesite crystal growth experiments performed in mixed-flow reactors (MFR) as well as under an Hydrothermal Atomic Force Microscope (HAFM). Three different types of ligands have been chosen for their different chemical properties and chelating effect: Oxalate, Citrate and EDTA. Experiments have been performed at temperatures between 80 and 120°C, alkaline pH, and dissolved CO₂ concentration ~ 0.02 mol/L. Chemical speciation was calculated for all experiments, thus allowing to monitor saturation states and to determine magnesite growth rates as a function of solution chemistry. MFR experiments results show that the addition of variable amounts of Oxalate to the solutions leads to a limited decrease of the precipitation rate (up to ~30% decrease with 10^-2 M Oxalate). Nonetheless, another set of experiments, performed at constant Oxalate concentration and changing saturation state, revealed an increase of the kinetic constant of magnesite precipitation. Furthermore, HAFM experiments, performed at constant saturation states with Oxalate, Citrate and EDTA, showed an increase of the kinetic constant of the magnesite step advancement rate, which is positively correlated to the Mg-ligand complexation constant. These results indicate that investigated organic ligands have an overall inhibitory effect on magnesite precipitation. However, the observed increase of kinetic constants suggests that by complexing the Mg²⁺ ion, organic ligands enhance the reactivity of the cation, which is likely due to an increase of water exchange rates. This effect partly compensates the reduction of the saturation state, which explains that the overall precipitation inhibition remains relatively small. Nevertheless, the use of organic ligands to enhance direct mineral carbonation should be limited to Mg-silicates whose dissolution is significantly slower than magnesite precipitation (e.g serpentine or talc, and not forsterite). References: Park, A.-H. A.; Jadhav, R.; Fan L.-S., CO2 mineral sequestration: chemically enhanced aqueous carbonation of serpentine. The Canadian Journal of Chemical Engineering 2003, 81, 885-890. Reddy, M. M.; Hoch, A. R., Calcite crystal growth rate inhibition by polycarboxylic acids. Journal of Colloid and Interface Science 2001, 235, 365-370.