Kinetics of chlorite dissolution at elevated temperatures and CO2 conditions

Abstract

Chlorite ((Mg4.29Al1.48Fe0.10)(Al1.22Si2.78)O10(OH)8) dissolution kinetics were measured under far from equilibrium conditions using a mixed-flow reactor over temperatures of 100–275°C at pH values of 3.0–5.7 in a background solution matrix of 0.05m NaCl. Over this temperature range, magnesium was released congruently with respect to silica. The effect of variable pCO2 levels representative of engineered geothermal systems with CO2 as a heat-exchanging fluid (CO2-EGS) was explored by reacting chlorite with solutions containing a range of dissolved CO2 concentrations (0.1–0.5M). The dissolution rate was insensitive to CO2(aq) concentration, with dissolved CO2 apparently affecting dissolution only through increased acidity. Over this range of far-from-equilibrium experimental conditions of elevated temperature, mildly acidic to moderately neutral pH, and CO2(aq) concentrations up to 0.5M, Mg-rich chlorite dissolution can be described as a surface area-normalized rate equal to:rate=kacid⋅exp−EacidR⋅1T−1298K⋅aH+n,where the apparent acid rate constant at 25°C is ka=10−9.91molm−2s−1, the reaction order n with respect to H+(aq) is 0.49, and the activation energy for the acid mechanism is E=25.1kJmol−1 (this value is significantly lower than previous estimates). This chlorite dissolution rate equation can be used with reaction affinity terms and kinetic laws for other minerals to estimate the impact of geochemical alteration within CO2-enhanced geothermal system operations or other higher-temperature subsurface systems. Over a 100–275°C temperature range, chlorite is 2–5 orders of magnitude less reactive than has been previously predicted.