Real-time monitoring of the overall exchange of oxygen isotopes between aqueous CO32- and H2O by Raman spectroscopy

In this contribution we demonstrate that in situ   Raman spectroscopy is a powerful tool to study the exchange kinetics of oxygen isotopes between aqueous oxo-anions and water using the CO32-–H2O system as an example. In situ exchange experiments have been carried out using a 1 M Na2CO3 solution at 45, 60, 75, and 100 °C in a closed system using an in-house-made Teflon©-based fluid cell. The solution was prepared with H2O enriched with 97 at.% 18O. At the given pH from 10.92 to 9.65 at 45–100 °C, respectively, CO32- is the dominant DIC species (>99% CO32-). At the beginning of the reaction the Raman spectrum of the solution is characterised by an intense band near 1067 cm−1 that has been assigned to the ν1(CO3) symmetric stretching vibration of the CO32- molecule. With increasing reaction time three, well-separated bands successively appear near 1046, 1026, and 1006 cm−1. These bands result from mass-related (isotopic) splitting of the ν1(CO3) mode and reflect the symmetric stretching vibration of the oxygen-related isotopologues C16O3-n18On2- (n = 1, 2, and 3). The relative integrated intensities of these bands are related to the amount of 18O present in the aqueous CO32- molecule, which allowed monitoring the time-dependent distribution of the four isotopologues at each temperature. The measured distributions as a function of time were found to agree well with those modelled on the basis of the overall reactions ΣC18On16O3-n+3H218O⇌ΣC18On+116O3-n-1+3H216OΣC18On16O3-n+3H218O⇌ΣC18On+116O3-n-1+3H216O, where the sum sign represents the sum of all carbonate species in solution and n = 0, 1, and 2. Moreover, the measured equilibrium fractions of 18O agree well with those expected from mass balance and published fractionation factors. After correcting the observed exchange rates for the dependence of the exchange kinetics on OH− and CO2(aq) activities, the observed overall logarithmic oxygen isotope exchange rates describe a linear relationship with the inverse temperature along with published exchange data that were obtained by conventional means. From this relationship, we obtained an activation energy for the oxygen isotope exchange process of 63.4 ± 1.4 kJ/mol, which agrees very well with the activation energy for the CO2(aq)+OH-⇌HCO3- reaction, further verifying that the formation of bicarbonate from CO2 and OH− is the rate-determining step for the oxygen isotope exchange in strongly alkaline solutions. Accordingly, the application of Raman spectroscopy is a highly promising tool for deciphering the isotope exchange reaction kinetics in water–oxo-anion systems.