Adsorption isotope effects for carbon dioxide from illite- and quartz-packed column experiments

• Carbon dioxide adsorption isotope effects are measured on illite substrate.
• A coupled isotope advection–diffusion–adsorption model is developed.
• Isotope effects are applicable to respired soil carbon dioxide studies.
• Results may better inform carbon capture and storage studies.

The transport of gas through porous media can be affected by diffusion and advection, reaction with solid and liquid phases, and adsorption onto solid substrates. Stable isotope substitution within a gas affects its adsorption affinity, solubility and diffusion coefficients and therefore isotope fractionation occurs with these processes. Adsorption of CO2 onto dry solid substrates has been shown to have an apparent inverse isotope effect whereby 12CO2 is preferentially retained during chromatography compared to 13CO2. Here, carbon isotope compositions are reported for CO2 eluted from 1-dimensional flow-through column experiments on dry geologic substrate including quartz sand and crushed illite. An analytical solution to a coupled advection–diffusion–adsorption equation is developed to fit the carbon isotope data and interpret the results from each experiment and a Rayleigh fractionation model is used to interpret adsorption results. Adsorption results from the illite-packed column are shown to closely follow a Rayleigh fractionation model with an alpha value (αadsorbed-gas) of 0.9909 whereas only diffusion and advection are required to describe the empty and quartz-packed columns with a diffusivity (D) ratio of 0.9991 between 13CO2 and 12CO2. These results demonstrate that adsorption isotope effects may significantly change the δ13C value of CO2 that is transported across dry geologic media and these effects should be considered in carbon capture and storage (CCS) leakage detection studies and studies attempting to accurately source CO2 respired in soils under transient state conditions. Isotope effects associated with CO2 adsorption in partially or fully water-saturated geologic media should be investigated in the future.