Experimental study on plagioclase dissolution rates at conditions relevant to mineral carbonation of seafloor basalts

Over the last two decades, all scenarios projected to achieve the goals of the Paris climate agreements have required negative emissions of greenhouse gases. Mineral carbonation of basalt is a promising negative emission technology for long-term storage of carbon dioxide (CO2). During mineral carbonation, dissolved CO2 is converted into solid carbonate minerals through reaction with silicate minerals. Plagioclase feldspars are the most abundant primary silicate minerals in basalts readily available for water-rock interactions. Despite numerous recent laboratory studies, the rate at which plagioclase dissolution occurs under the required conditions for large-scale carbon storage remain poorly constrained. In this study, we present new flow-through experiments quantifying the apparent dissolution rates of plagioclase in sodium chloride solutions with elevated concentrations of dissolved CO2 at temperatures between 25 °C and 125 °C and pressure of 200 bars. The mildly acidic conditions produced by carbonic acid yield apparent rates that are slower than those previously reported for plagioclase under more acidic conditions and alkaline conditions. We used these apparent rates to develop new temperature-dependent rate equations for plagioclase dissolution in solutions buffered by carbonic acid: k Ca = 10 − 9.811 ± 0.664 exp − 22.27 ± 2.08 R ∙ 1 T − 1 T r k Si = 10 − 10.334 ± 0.445 exp − 33.99 ± 1.40 R ∙ 1 T − 1 T r where k is the rate constant (mol m−2 s−1) at any temperature (T in K), R is the universal gas constant (8.3145 KJ/mol/K)and T r is the reference temperature (298.15 K). Utilizing the new Ca rate equation into a geochemical model, we estimated that the reaction time to achieve carbonate saturation in a closed system with high CO2 ranges from a few days to a few years depending on water-to-mineral ratios. These results could have significant implications for required monitoring on projects or achieving gigaton-scale of carbon storage and mineralization annually, where planned injection rates of million(s) of tons of CO2 per well per year could overwhelm aquifer alkalinity, lower pH, and reduce the efficiency of carbon mineralization.