13C/ 12C partitioning and kinetics of CO 2 absorption by hydroxide buffer solutions

Abstract

Absorption of CO 2 gas into a borate buffer solution is a linear function of time. The measured absorption rate log r = −10.33(mol cm −2 s −1) compares well to a value calculated from the diffusion constant of CO 2(aq) and the rate constant for the reaction of CO 2(aq) with OH −. The stable carbon-isotope composition of the dissolved carbonate is constant as function of time, and is depleted in 13C by 19.5‰ with respect to the gas phase. The kinetic fractionation due to diffusion is [( 13D/ 12D) 0.5 − 1] × 10 3 = − 0.7/% , while that caused by the chemical reaction is [( 13k/ 12k) 0.5 − 1] × 10 3 = − 18.8/% . These values have to be taken into account if a borate buffer is used for sample concentration techniques in order to deduce the carbon-isotope composition of CO 2(g). Absorption of CO 2 into a Ba(OH) 2 solution is a linear function of time, too. But the absorption rate varies with the experimental boundary conditions. The precipitated BaCO 3 is depleted in 13C with respect to the gas phase. However, δ 13C values are not constant and decrease as a function of time.