Aragonite Kinetics in Dilute Solutions

• Published in: Aquatic geochemistry 2011-09, Vol.17 (4-5), p.339-356
• Main Authors: Romanek, Christopher S, Morse, John W, Grossman, Ethan L
• Format: Article
• Language: English
• Subjects: activation energy; Alkalinity; Aqueous chemistry; aragonite; calcium; carbon; Carbon dioxide; Careers; Chemical precipitation; Earth and Environmental Science; Earth Sciences; Geochemistry; Hydrogeology; Hydrology/Water Resources; ionic strength; Kinetics; Original Paper; Precipitation; sodium; Solute movement; solutes; temperature; Water Quality/Water Pollution
• ISSN: 1380-6165; 1573-1421
• DOI: 10.1007/s10498-011-9127-2

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 102.8 to 103.8, 102.3 to 104.0, and 102.5 to 104.1 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 101.3 ± 0.12, 101.9 ± 0.06, and 102.6 ± 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).