Chrysotile dissolution rates: Implications for carbon sequestration

Abstract

Serpentine minerals (e.g., chrysotile) are a potentially important medium for sequestration of CO2 via carbonation reactions. The goals of this study are to report a steady-state, far from equilibrium chrysotile dissolution rate law and to better define what role serpentine dissolution kinetics will have in constraining rates of carbon sequestration via serpentine carbonation. The steady-state dissolution rate of chrysotile in 0.1m NaCl solutions was measured at 22°C and pH ranging from 2 to 8. Dissolution experiments were performed in a continuously stirred flow-through reactor with the input solutions pre-equilibrated with atmospheric CO2. Both Mg and Si steady-state fluxes from the chrysotile surface, and the overall chrysotile flux were regressed and the following empirical relationships were obtained: FMg=-0.22pH-10.02;FSi=-0.19pH-10.37;Fchrysotile=-0.21pH-10.57 where FMg, FSi, and Fchrysotile are the log10 Mg, Si, and molar chrysotile fluxes in mol/m2/s, respectively. Element fluxes were used in reaction-path calculations to constrain the rate of CO2 sequestration in two geological environments that have been proposed as potential sinks for anthropogenic CO2. Carbon sequestration in chrysotile tailings at 10°C is approximately an order of magnitude faster than carbon sequestration in a serpentinite-hosted aquifer at 60°C on a per kilogram of water basis. A serpentinite-hosted aquifer, however, provides a larger sequestration capacity. The chrysotile dissolution rate law determined in this study has important implications for constraining potential rates of sequestration in serpentinite-hosted aquifers and under accelerated sequestration scenarios in mine tailings.