COUPLED DISSOLUTION AND PRECIPITATION REACTION RATES AT MINERAL SURFACES: CALCITE IN SIMULATED AMD SOLUTIONS

Limestone is frequently added to acid mine waters to neutralize their acidity and increase the net alkalinity. During this treatment, secondary mineral coatings form on the limestone and slow its dissolution rate, which reduces the effectiveness of the treatment. Coupled reaction rates were measured for calcite dissolution in conjunction with gypsum precipitation on the surface of calcite from a sodium sulfate solution over time. These results help us to understand the mechanisms of coupled reactions, which will lead to improved methods of limestone remediation

A series of experiments were carried out in an externally-recycled mixed flow reactor using calcite as a primary phase on which the formation of secondary phases was induced. The composition of the initial solutions was varied using (1) nitric acid and (2) sodium sulfate solutions to simulate the range of pH and [SO₄^2-] observed in AMD. The flow rate was high enough that the reaction was surface-reaction-limited rather than transport-limited. The rate (r, mol/m²sec) of dissolution of calcite inhibited by gypsum precipitation on the surface based on H⁺ consumption and Ca²⁺ release measured over time fit power law expressions where dn is the change in the concentration of H⁺ and Ca²⁺ (mol) and t is time (sec). The experiments used an initial 2 liter solution of nitric acid with a 0.5 molal sulfate concentration and a pH of 3.11. Compared to values of calcite dissolution in pure nitric acid done in this and previous studies, it was found that calcite dissolution rates slow with time as the calcite surface becomes coated.

Scanning electron microprobe was used to monitor the distribution of the coatings on the calcite surface and these data provide a basis for a quantitative model of coupled dissolution and precipitation reactions. Rate and coating data was used to construct a general model of coupled dissolution and precipitation reactions that links their chemical and geometric behaviors.