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

• Air injection and increasing background electrolyte enhanced olivine dissolution.
• Olivine dissolution rates are enhanced in the presence of constant CO2 dissolution.
• Mg release from mineral surface is enhanced under constant air injection.
• Increasing NaNO3 and total alkalinity affected olivine protonation mechanisms.

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 4 g of olivine, and 0, 0.001 and 0.01 M NaNO3. Experiments were performed for 40,440 and 36,048 min 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 40,440 min. In the absence of air flow, an alkaline pH of 9.6 to 9.3 was observed for 36,048 min. 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 0.25 mM, compared to 0.9 mM with air flow at 0.01 M 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.

Plot of the log of the rate of olivine dissolution (log ROL) calculated as per Eq. (1) as a function of pH, in closed (A) and partially closed (B) batch reactors as a function of NaNO3 concentration. Squares, diamonds and circles, for both (a) and (b) represent electrolyte concentrations of 0, 0.001 and 0.01 M respectively for all experiments. Open symbols (□, ◊, ○) in (a) describe results for experiments in the absence of air flow. Filled symbols (■, ♦, ●) in (b) correspond to measurements in sample aliquots under constant air injection at 3 L/min.Figure optionsDownload as PowerPoint slide