Sorption mechanism of Fe(II) on illite: Sorption and modelling

Different countries consider clay formations as potential host rocks for a deep geological disposal of radioactive waste. Minor amounts of Fe(II), which is an important reducing agent in geochemical processes, are typically present in illite, which is a one of the major clay minerals present in different argillaceous rocks. On other hand, the corrosion of nuclear waste casks may act as sources of Fe(II) over time. The presence of Fe(II) in clay minerals may control the behavior of redox sensitive radionuclides such as ⁹⁹Tc, ⁷⁵Se and U, whose sorption, solubility and migration are largely affected by their redox state, which is affected by the redox potential in the environment. Thus, a quantitative description of Fe(II) sorption on clay minerals such as illite, is important to understand subsequent reactions with redox sensitive radionuclides. In this study, the sorption of Fe(II) on illite was investigated in batch experiments, including sorption edge and sorption isotherms measurements under anoxic condition. A 2 SPNE SC/CE (2 sites protolysis non-electrostatic surface complex/cation exchange) sorption model extended to account for the surface oxidation of sorbed Fe(II) was used to model the sorption data with geochemical modelling code PHREEQC. Three models (A, B, C) with increasing level of complexity were tested. The model A, in which only Fe(II) interact with illite, fit the experimental data not satisfactorily. This model was modified to include oxidation of surface complexes (Model B) and precipitation of iron bearing phases (Model C). The modified models showed that most of the sorbed Fe(II) was oxidized at pH below 6.5. At pH above 6.5, the oxidized surface complexes either react with water forming surface iron hydroxides or precipitate as hematite, although presence of these could not be confirmed by X-ray diffraction measurements. These sorption processes could be used to couple with redox sensitive radionuclides in future reactive transport research.