Pyrite oxidation impacts radionuclide migration in clay rock - thermodynamically, kinetically or not at all?

Pyrite is ubiquitous in the Earth’s crust, redox-sensitive and prone to oxidation. This applies as well to clay rock formations targeted for the final disposal of radioactive waste. Those rocks are tested in experiments in regard to their suitability for the long-term containment of radionuclides (RN). In the laboratory, samples are often handled under atmospheric conditions, whereas the subsurface provides mainly reducing environments. This means that pyrite can potentially oxidise during experimental procedures, which in turn influences the conditions for the migration of redox-sensitive RN. Different modelling approaches exist to account for pyrite oxidation in geochemical simulations of experiments. The most simple form is the assumption of thermodynamic equilibrium. The application of kinetic rate laws is more complex and computationally intensive.

The electrochemical reaction of pyrite oxidation can be separated into the anodic and cathodic reaction part. They are linked to each other through electron transfer taking place at the interface between mineral surface and pore water. The reductant within the anodic reaction is pyrite itself. Oxidants for the cathodic reaction could be oxygen or ferrous iron. In addition, the direct reaction of other reactants with pyrite, such as oxidised RN, is in some cases thermodynamically feasible for experimental conditions with no or low iron and minor oxygen concentrations in the pore water.

Reactive transport simulations of RN migration in clay rock are compared against experimental data sets. RN occur in different oxidation states, if redox-sensitive. Their mobility and subsequent migration length is governed by the ratio between the most stable oxidised and reduced states under environmental conditions. This is controlled by the inherent redox conditions in the core samples as well as imposed by the introduced pore water chemistry. We test three approaches to model the underlying redox reactions coupled to diffusion and sorption. Firstly, pyrite oxidation in thermodynamic equilibrium. Secondly, different well known kinetic reaction rates for pyrite oxidation. Thirdly, reduction of RN triggered via an iron source associated with the clay minerals. Hence, pyrite oxidation is modelled thermodynamically, kinetically and not at all.