Kinetic isotope effects during CO2 hydration: Experimental results for carbon and oxygen fractionation

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Kinetic isotope effects (KIEs) during the hydration of carbon dioxide (CO2) in aqueous solution are key to understanding various geochemical, physicochemical, and biological processes. Despite the growing interest in KIEs in natural systems (e.g., speleothem calcites, skeletal formation of corals, formation of cryogenic carbonates, and more), the currently available experimental data is limited to only a few studies. Here, we report results from laboratory experiments to constrain kinetic isotope fractionation (KIF) during the hydration of CO2. The experimental setup consisted of a stock solution vessel containing a NaHCO3 solution, a reaction chamber in which BaCO3 was rapidly precipitated from a Tris-buffered BaCl2 solution, and a pump to internally circulate gas throughout the system that was isolated from the ambient laboratory air. Resultant BaCO3 samples, as well as the NaHCO3 and deionized H2O used for our experiments were analyzed for stable carbon and oxygen isotopes by mass spectrometry. Our experimental results were grouped into four different batches, which were obtained during discrete time periods, while some adjustments to the experimental setup and protocols were implemented to improve the data quality. We discuss possible experimental errors among the four batches and conclude that Batch-4 provided the most reliable results. The average δ13C and δ18O values of the Batch-4 BaCO3 samples produced at 25 °C and pHNBS = 8.0 are −29.7 ± 0.7‰ (VPDB) and 18.8 ± 0.6‰ (VSMOW), respectively, which is strongly depleted relative to gaseous CO2 in the system (δ13CVPDB ≅ −10.7‰, δ18OVSMOW ≅ 37.9‰) . From the isotope data, we calculate our experimental 13KIF and 18KIF, which refer to the 13C/12C and 18O/16O fractionation between CO2(g) and BaCO3, where the δ13C and δ18O values of CO2(g) were calculated using known equilibrium fractionation factors. From the mean of Batch-4 results, we conclude 13KIF = 19.6 ± 0.8‰ and 18KIF = 18.8 ± 0.6‰ at 25 °C and pHNBS = 8.0. If full carbon KIF of ∼ 2.0‰ is assumed between CO2(g) and CO2(aq), then the mean 13KIF relative to CO2(aq) is 17.6 ± 0.8‰. Our KIFs are the largest values compared to previously reported experimental KIFs (except for one study), suggesting that our values are closest to the full isotope disequilibrium during CO2 hydration, as any partial re-equilibration tends to reduce observed KIFs.