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 log10Kex = 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 Kex. 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.