Combined effects of temperature, mineral type, and surface roughness on chlorite dissolution kinetics in the acidic pH

Abstract Many studies reported the chlorite dissolution kinetics in acidic pH—resulting in the development of chlorite dissolution rate law covering the pH and temperature dependence—but they rarely discussed how various intrinsic factors, including chlorite types, structural complexity, and surface roughness (refers specifically to the ratio between Brunauer–Emmett–Teller surface area, BET SA, and geometric surface area, GSA, in this paper), influenced the chlorite dissolution rate or the mechanism for dissolution congruency. In this study, results obtained from mixed-flow reactor experiments performed on a Mg-rich chlorite, over a pH range of 2–6 at 25 °C and 95 °C, indicated that pH and temperature control not only the chlorite dissolution rate but also the congruency of element release. Low pH facilitated the dissolution of Fe from chlorite interlayers and caused incongruent element release rates at 25 °C, while higher temperature experiments at the same pH overcame the differences in surface reactivity between the interlayer and the tetrahedral-octahedral-tetrahedral (TOT) layer, and finally evolved into congruent dissolution. The lower pH also made chlorite dissolution more resistant to varying flow rates. A dissolution rate constant of 10–10.51 mol/m2/s, a reaction order of 0.32, and activation energy of 42.03 kJ/mol were determined for the Mg-rich chlorite dissolution kinetics at 25 °C, in an acid-enhanced mechanism. Longer-term reactive transport modeling (RTM) on the chlorite dissolution demonstrated that low pH and large specific surface area (SSA) decreased the chlorite percentage, bulk surface area, and saturation index quicker, and in a non-linear fashion. Chlorite with higher SSA lost more bulk surface area (BSA) than that with lower SSA, when the same mass of chlorite was dissolved. Through tabulating and recalculating a series of data from the literature, the combined effects of intrinsic and extrinsic factors—including chlorite type, BET SA/GSA ratio, and temperature—were explored. Fe-rich chlorite dissolves faster than Mg-rich chlorite in acid and neutral pH, due to the oxidative dissolution mechanism. The chlorite dissolution rate constant is linearly and positively correlated to BET SA/GSA for the same type of chlorite (Fe-rich and Mg-rich). Higher temperatures help reduce the effects of extrinsic and intrinsic factors on chlorite dissolution rates.