Sorption and redox speciation of plutonium at the illite surface under highly saline conditions

Natural groundwater may contain high salt concentrations, such as those occurring at several potential deep geological nuclear waste repository sites. Actinide sorption to clays (e.g. illite) under saline conditions has, however, been rarely studied. Furthermore, both illite surface and ionic strength may affect redox speciation of actinides like plutonium. In the present study, Pu sorption to illite is investigated under anaerobic conditions for 3m (=-log [Formula: see text] )<10andm_NaCl=1.0and3.2 molal (m). Results are compared with previous data for m_NaCl=0.1m. According to redox potential measurements and based on Eu(III)-illite sorption data (taken as analogue of Pu(III)), the strong effect of m_NaCl on overall Pu uptake observed for pH_m<6 is mainly attributed to the presence of Pu(III) and its competition with Na⁺ for ion exchange sites. For pH_m>6, overall Pu uptake is largely insensitive to m_NaCl due to the prevalence of strongly adsorbed Pu(IV). By applying appropriate corrections to the activity coefficients of dissolved ions and using the 2-site protolysis non-electrostatic surface complexation and cation exchange (2 SPNE SC/CE) model, experimental data on Pu sorption to illite as a function of pH, Eh and m_NaCl can be very well reproduced.