Anion Transport Through Bentonite Under Various Geochemical Conditions

The use of bentonite clay in industrial applications is widespread: it is used as an engineered barrier for long-term management of radioactive wastes, CO₂ storage, landfill liners, and contaminant containment. These applications have diverse environmental conditions ranging from various temperatures, pHs, saline contents, and ionic concentrations. Since bentonite is a low permeability clay, anion transport is diffusion dominated but geochemical reactions can also play a significant role and transport will be affected by environmental conditions. In this study, anion transport (bisulfide) under various conditions was examined using experimental and numerical techniques to understand the various geochemical and surface mediated reactions that are occurring in the bentonite. The case study presented is for the use of bentonite in long term storage of nuclear waste but can be extended to other applications.

First, diffusion experiments were performed to examine the transport and reactive nature of bisulfide (HS^-) through bentonite compacted at dry density of 1090-1330 kg m^-3. Experimental data of bisulfide transport were fitted using the inverse solution technique of Hydrus-1D model and different fitting parameters (e.g., diffusion, sorption, and reaction sink). Simulation results suggest that the HS^- sorption/reaction affecting itsdiffusive transport through bentonite can be modeled using a simple nonlinear adsorption process.

Second, batch experiments were performed to understand the maximum allowable sorption that could take place under key geochemical conditions, including temperature, pH, and ionic strength. The results of batch sorption experiments performed suggest that HS^- sorption increases with increasing temperature but decreases with increasing pH and ionic strength.

Lastly, since transport and reactive processes are interconnected, the results of these experiments were incorporated into a 1D transport COMSOL model to understand which geochemical process governs bisulfide transport through bentonite. Various processes were examined including linear and non-linear sorption, reactive transport, and anion exclusion. The model was validated using the experiments and showed that HS^- was retained in the bentonite due to reactive processes and anion exclusion effects.