An experimental study of calcite dissolution rates at acidic conditions and 25 °C in the presence of NaPO3 and MgCl2

Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 °C in aqueous HCl solutions over the bulk solution pH range ?1 to 3 and in the presence of trace concentrations of aqueous NaPO₃ and MgCl₂. These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl₂ at pH 1 and 3. The presence of trace quantities of NaPO₃, which dissociates in solution to Na⁺ and H₂PO₄^?, decreased the overall calcite dissolution rate at pH≤2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg²⁺ may not adsorb, whereas aqueous phosphate, present as either the anion H₂PO₄^? or the neutral species H₃PO₄⁰, readily adsorbs on calcite surfaces leading to significant dissolution inhibition.