Olivine dissolution at 25°C: Effects of pH, CO2, and organic acids

The dissolution rates of Fo100 and Fo91 in aqueous solutions in the pH range 2–12.4 at 25°C have been measured using fluidized bed and batch reactors. Rates depend upon the pH, the partial pressure of CO2, and the presence of organic ligands. At low PCO2 (≤10−4.5 atm) with no organic ligands present, the rate of olivine dissolution, R, is given by R (mol cm−2 s−1) = 9.07 × 10−12aH+0.54 + 5.25 × 10−15 + 2.33 × 10−17aH+−0.31, where aH+ is the activity of H+ in solution. However in basic solutions, when the partial pressure of CO2 is equal to atmospheric levels (PCO2 = 10−3.5 atm), olivine dissolution rates are nearly pH independent throughout the pH range 6–12 and are about equal to the minimum rate of dissolution under CO2 purged conditions. At pH 11 the presence of atmospheric levels of CO2 reduces the dissolution rate by over an order of magnitude (to 10−14.1 mol cm−2 s−1). Apparently, positive charge on the olivine surface can be neutralized by increasing PCO2. In contrast, experiments conducted in the acidic and near neutral pH ranges indicate that organic ligands chelate surface Mg causing an increase in the olivine dissolution rate when present. Organic ligand effects are greatest in the near neutral pH domain. For example, at pH 4 dissolution rates are increased by 0.75 log units (to 10−12.25 mol cm−2 s−1) in solutions of 10−3 molar ascorbic acid or 0.05 molar potassium acid phthalate over rates measured in organic free solutions. The chelation effect becomes less important as pH decreases. Rates at pH 2 in the presence of these organic ligands are indistinguishable from those measured without organics.