## 1 Introduction
The test had been carried out for 18 years and has been dismantled, providing a large set of chemical data that can be used for studying chemical state in the FEBEX bentonite. The other bentonite are being investigated as well. For example, the Wyoming-type bentonite and the Czech bentonite of type BCV in the HotBENT test started in 2021 (Grimsel Test Site, Switzerland), but the data from dismantling will be available after 5 years [8]. In line with experimental studies and knowledge from various disciplines, numerical models are being developed for complex analysis and long-term predictions. Numerical models differ as they were developed with varying states of interest, i.e., some for the bentonite hydration state and some for dry density distribution [9; 10]. Other studies focused on the reactive transport of dissolved species under non-isothermal conditions [11; 12; 13; 14], taking into account the effects of microstructure evolution during hydration and dehydration [15]. [PERSON] et al. [11; 14] developed a THMC model for the FEBEX in situ test, and their numerical analysis showed that water content and the concentrations of dissolved species were strongly dependent on the changes in intrinsic permeability, thermal osmotic permeability, and initial dissolved concentration. [PERSON] et al. [13] revisited the THMC model developed by [PERSON] et al. [11] by updating boundary conditions near the heater, refining the finite element grid, and increasing the vapour tortuosity factor and analysed species concentrations sensitivity to the bentonite retention curve, smectite dissolution, vapour tortuosity, and diffusion coefficients. However, these studies did not consider the impact of reactive gases such as CO\({}_{2}\)[11; 14] on the bentonite performance. The groundwater coming into contact with the bentonite barrier contains a number of dissolved chemical species and dissolved CO\({}_{2}\), which will affect calcite dissolution/precipitation in the bentonite. Thus, it is important to assess the CO2 potential transport. The CO2 transport in the gas phase was considered in [13; 16], but the study did not take into account the CO2 degassing due to elevated temperature. CO2 released near the heater could then be transported in the gas phase and redissolve in the condensation zone, and it could affect the pH in the bentonite. The impact of separate processes under isothermal geological repository conditions--such as variable flow conditions, CO2 gas diffusion, mineral dissolution/precipitation, and cation exchange--was analysed in [17], where six cases of increasing complexity were benchmarked with different numerical simulators. The sensitivity of pH and CO2 profiles to chemical activity calculation and mesh discretisation were reported. In the study, different codes were used and reasonable agreement between results was obtained. This study considers non isothermal conditions and additional phenomena (temperature-dependent CO2 solubility in water, surface complexation) for the evaluation of the thermo-hydro-chemical (THC) state of the bentonite. The reactive transport modelling of bentonite under non-isothermal conditions was performed with COMSOL Multiphysics, coupled with the geochemical solver Phreeqc via the iCP interface [18]. This required performance of the modelling of the two-phase flow of a miscible fluid (water and air) considering important phenomena, such as heat transfer; gas dissolution; advective-diffusive transport in the aqueous and gaseous phase, including CO2 transport; and chemical interactions (aqueous complexation, mineral dissolution/precipitation, ion exchange, and surface complexation). In order to evaluate the effect of the CO2 gas diffusion on the bentonite barrier performance, additional cases of different complexity were simulated, and the results between the cases were compared. In addition, this paper presents some insights about possible CO2 degassing and flux of different magnitude imposed by surrounding rock.