Experimental study to better understand factors affecting the CO2 mineral trapping potential of basalt

The CO2 mineral trapping potential of basalt is reduced when smectites, zeolites, and various oxides form. To better understand the reactions competing for the divalent metal cations, we performed a systematic batch-reactor experimental study varying pH, temperature (from 80 to 150 degrees C), CO2 pressure, and initial aqueous metal ion composition. From the experiments we could not see any mineral carbonate formation at pH < 8 at 80 degrees C and pH <7 at 100 and 150 degrees C. Smectite, identified by XRD as a nontronite, was formed in all experiments. At higher pHs various forms of Ca-carbonates formed together with the smectite. The effect of the smectite growth to deplete solutions of metal cations (Mg and Fe) appears not to be the main reason for the lack of predicted MgFe-carbonates. Instead, data suggest that the lack of observable growth may come from a combination of inhibition of carbonate nucleation, and slow carbonate growth rates. The slow growth is mainly caused by a very large deviation of the aqueous Me2+/CO32- activity ratio (r) from the stoichiometric ratio of the secondary carbonates, leading to a 45 order of magnitude drop in precipitation rates compared to stoichiometric solutions. Using transition state theory (TST)-based rate laws to model basalt carbonatization will lead to corresponding orders of magnitude over-predictions. In addition to reducing the carbonatization potential, smectites may also deteriorate permeability and CO2 injectivity if forming in pore throats and narrow fractures. (c) 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.