On the molecular diffusion coefficients of dissolved CO2,HCO3-, and CO32- and their dependence on isotopic mass

The molecular diffusion coefficients of dissolved carbon dioxide (CO2)(CO2), bicarbonate ion (HCO3-), and carbonate ion (CO32-) are fundamental physico-chemical constants and are of practical significance in various disciplines including geochemistry, biology, and medicine. Yet, very little experimental data is available, for instance, on the bicarbonate and carbonate ion diffusion coefficient. Furthermore, it appears that no information was hitherto available on the mass-dependence of the diffusion coefficients of the ionic carbonate species in water. Here I use molecular dynamics simulations to study the diffusion of the dissolved carbonate species in water, including their dependence on temperature and isotopic mass. Based on the simulations, I provide equations to calculate the diffusion coefficients of dissolved CO2,HCO3-, and CO32- over the temperature range from 0° to 100 °C. The results indicate a mass-dependence of CO2CO2 diffusion that is consistent with the observed 12CO2/13CO212CO2/13CO2 diffusion ratio at 25 °C. No significant isotope fractionation appears to be associated with the diffusion of the naturally occurring isotopologues of HCO3- and CO32- at 25 °C.