Aragonite Kinetics in Dilute Solutions

Abstract

Aragonite was synthesized inorganically using a seeded-growth technique to characterize precipitation kinetics for the heterogeneous growth of solid from dilute solutions (ionic strength: 0.05–0.07 mol l⁻¹). The concentration of all aqueous constituents, including Ca (~5–15 mmol l⁻¹), Na (~10–35 mmol l⁻¹), Cl (~30–35 mmol l⁻¹), and carbon (as total alkalinity: ~10 to 17 meq l⁻¹), was held constant by the addition of titrants that contained excess solute concentrations to balance the growth of solid phase during the precipitation reaction, and a CO₂/N₂ gas mixture (0.009–0.178) was bubbled through each solution to facilitate mass exchange between gaseous and aqueous carbon species. Forty-three experiments were conducted at 10° (n = 13), 25° (n = 21), and 40°C (n = 9), over a range of average saturation states with respect to aragonite from 8.3 to 28.5, 2.9 to 19.6 and 2.0 to 12.2, and average precipitation rates from 10^2.8 to 10^3.8, 10^2.3 to 10^4.0, and 10^2.5 to 10^4.1 micromol m⁻² h⁻¹, respectively. Reaction orders averaged 1.7 ± 0.10 at 10°, 1.7 ± 0.07 at 25° and 1.5 ± 0.06 at 40°, and they were independent of temperature while rate constants averaged 10^1.3 ± 0.12, 10^1.9 ± 0.06, and 10^2.6 ± 0.04 micromol m⁻² h⁻¹, respectively, increasing one-half order of magnitude for each 15°C rise in temperature. From these data, an Arrhenius activation energy of 71.2 kJ mol⁻¹ is calculated for the heterogeneous precipitation of aragonite. This value is comparable to a sole independent measurement of 80.7 kJ mol⁻¹ reported for the solid-solution recrystallization of monohydrocalcite to aragonite (Munemoto and Fukushi in J Mineral Petrol Sci 103: 345–349, 2008).