Quantifying the kinetics of olivine dissolution in partially closed and closed batch reactor systems

Abstract

This study quantified the rates of olivine dissolution under constant air injection, in partially closed batch reactors and in control closed batch reactors (in the absence of air flow), in the presence of 4g of olivine, and 0, 0.001 and 0.01M NaNO3. Experiments were performed for 40,440 and 36,048min in partially closed and closed reactors respectively. Under constant air injection, the solution pH ranged from 6.8 and 7.3 for all conditions at the start of the experiments, to 7.5 and 7.8 at 40,440min. In the absence of air flow, an alkaline pH of 9.6 to 9.3 was observed for 36,048min. This pH difference is attributed to the constant buffering the solution through the formation of dissolved carbonate species (H2CO3/HCO3−). The alkaline pH, measured in the presence of olivine, implied a mechanism of pH control caused by the presence of Si(OH)4 species from the mineral surface. This effect was suggested by a drop to near neutral pH of the same aliquots, upon filtration of the olivine through a N2 degassed filter. A decrease in total alkalinity was measured for experiments under constant air injection, as a function of increasing NaNO3, suggesting a mechanism of Na+/CO32− interaction is involved in the enhancement of olivine dissolution rates through proton/Mg exchange. In the absence of air flow, the total alkalinity reached a maximum of 0.25mM, compared to 0.9mM with air flow at 0.01M NaNO3. Magnesium concentration increased in the presence of NaNO3 and permanently dissolving CO2 in partially closed batch reactors. This strongly suggests a key role of Na+ (coupled to CO2 dissolution) for enhancing olivine dissolution. Mg/Si ratios for all conditions showed incongruent olivine dissolution, suggesting a combination of two dissolution mechanisms at alkaline and near neutral pH values, in closed and partially closed batch reactors. The rate of olivine dissolution (ROL) was calculated as a function of mineral surface area and forsterite equilibrium solution parameters. In the presence of constant air injection, highest log ROL values range from −11 to −10.7 at the acidic to near neutral conditions (pH=6.8 to 7.2). Slower dissolution rates, with log ROL values of −11.5 to −11.4 were determined for experiments in the absence of air flow at alkaline pH values (9.6 to 9.8). Constant air injection (i.e. constant dissolution of CO2 at atmospheric pressure) in the presence of Na+ has proven effective for enhancing olivine dissolution.