Direct measurement of CO2 drawdown in mine wastes and rock powders: Implications for enhanced rock weathering
Enhanced rock weathering (ERW) sequesters CO2 via solubility and mineral trapping and can be implemented by the mining industry to reduce their net greenhouse gas emissions. Kimberlite residues from Venetia Diamond Mine in South Africa, as well as powdered forsterite, serpentinite, wollastonite skarn, and 10 wt.% brucite mixed with quartz sand, were tested as potential feedstocks for ERW. A CO2 flux system directly measured CO2 removal rates and sensors tracked laboratory conditions and pore water saturation during a series of 2-week experiments. With respect to kimberlites, unweathered residues achieved the greatest drawdown rate of -870 g CO2/m2/yr at 48% saturation. In contrast, fine and coarse residues previously exposed to mine process water achieved fluxes of -150 and -160 g CO2/m2/yr at 60% saturation. Brucite reached -2940 g CO2/m2/yr at 14% saturation compared to forsterite, serpentinite, and wollastonite that achieved fluxes of -500, -260, and -190 g CO2/m2 /yr, respectively, at higher saturations of 53–60%. Mineralogical composition had the greatest effect on CO2 fluxes, followed by water content which drives carbonation reactions and affects permeability. Solid inorganic carbon increased in the brucite, wollastonite, and unweathered kimberlite, indicating that CO2 was stored via mineral trapping as opposed to solubility trapping in the other experiments. Increasing the exposure of unweathered residues, expanding dispersal area, and optimizing water saturation would lead to greater CO2 removal at mine sites.