Proton/calcium ion exchange behavior of calcite

Abstract

The characterization of the proton sorptive properties of calcite in aqueous solutions at 25 ± 1 °C over a relatively wide range of chemical conditions (7.16 ≤ pH ≤ 9.7; 3 × 10⁻⁵ M ≤ΣCa ≤ 5 × 10⁻³ M; 10⁻⁴ M ≤ΣCO₂≤ 1.7 × 10⁻³ M) and solid : solution ratios (0.4 to 12.3 g L⁻¹) was performed using a novel surface titration technique. A large net proton uptake, coupled with a significant release of Ca²⁺ ions is consistently observed, greatly exceeding the theoretical number of reactive surface sites. These observations are interpreted as a fast proton/calcium exchange equilibrium between the solution and “exchangeable cation sites” (e.g., lattice positions) at and/or beneath the calcite surface (species identified by “(exc)”), (CaCO₃)₂(exc) + 2H⁺⇔Ca(HCO₃)₂(exc) + Ca²⁺, that leads to a transient, “apparent” incongruent dissolution regime and the formation of a stable calcium-deficient, proton-enriched layer within the calcite lattice under circum-neutral and alkaline regimes at standard conditions. The 2H⁺/Ca²⁺ ion exchange is quantitatively described by the Langmuir-power exchange function under the Vanselow convention: where n = 1 and log₁₀K_ex = 13.0 ± 0.3. This calcite behavior, never reported before, masks surface equilibria and directly impacts the aqueous speciation of carbonate-rock systems with poor CO₂(g) ventilation (e.g., aquifers, pore and deep sea waters, industrial reactors) via the buffering of pH and calcite dissolution. In contrast, at fixed pCO₂ conditions, aqueous speciation remains unaffected upon CO₂(g) sequestration resulting from ion exchange-induced calcite precipitation: (CaCO₃)₂(exc) + CO₂(g) + H₂O ⇔Ca(HCO₃)₂(exc) + CaCO₃(s). Accordingly, reliable predictions of aqueous speciation in natural or engineered calcite-containing systems at variable pCO₂ conditions must consider this exchange reaction and the associated K_ex. The postulated proton/calcium exchange may have far-reaching implications on the interpretation of kinetic and equilibrium data and can partly explain the anomalous solution chemistry observed in some field and laboratory carbonate studies.