Chemistry and mineralogy of carbonation in the (CO2 + NaHCO3 + H2O + olivine) system at high temperature and high pressure: what is the rate-limiting step for in-situ carbon mineralization?

We report experimental studies of carbonation in the (CO2 + NaHCO3 + H2O + olivine) system at high pressure and temperature. The study encompassed both low (0.01 mol/kg) and high (≈ 0.7 mol/kg) molalities of sodium bicarbonate, with the starting brine pH varying between 4.6 and 6.4 at pCO2 = 12.7 MPa and T = 458 K. Key aspects of olivine carbonation were investigated: olivine dissolution, morphological and mineralogical alteration of olivine particles, formation of magnesite and secondary phases, and the fractional extent of carbonation. Rate-limiting step was identified under different pH conditions. At the higher pH conditions, the rate-limiting step was the dissolution of olivine, whereas at the lower pH conditions, it was the precipitation of magnesite and consequent cementation of particles, reducing the surface available for reaction. There lower pH was more advantageous for long-term carbonation under the experimental conditions, which closely resembled reservoir conditions.