Neptunium redox speciation at the illite surface

作者：Marsac Remi; Banik Nidhu lal^*; Luetzenkirchen Johannes; Marquardt Christian Michael; Dardenne Kathy; Schild Dieter; Rothe Joerg; Diascorn Alexandre; Kupcik Tomas; Schaefer Thorsten; Geckeis Horst

来源：Geochimica et Cosmochimica Acta, 2015, 152: 39-51.

DOI：10.1016/j.gca.2014.12.021

Neptunium (Np(V)) sorption onto a purified illite is investigated as a function of pH (3-10) and [(NpO2+)-O-V] tot(3 x 10(-8)-3 x 10(-4) M) in 0.1 M NaCl under Ar atmosphere. After about one week reaction time, only insignificant variation of Np sorption is observed and the establishment of reaction equilibrium can be assumed. Surprisingly, solid-liquid distribution ratios (R-d) are clearly higher than those measured for Np(V) sorption onto illite under aerobic conditions. The observation that Rd increases with decreasing pe (pe = -log a(e-)) suggests partial reduction to Np(IV), although measured redox potentials (pe values) at a first glance suggest the predominance of Np(V). Reduction to Np(IV) at the illite surface could indeed be confirmed by X-ray absorption near-edge spectroscopy (XANES). Np speciation in presence of the purified Na-illite under given conditions is consistently described by applying the 2 sites protolysis non-electrostatic surface complexation and cation exchange model. Measured pe data are taken to calculate Np redox state and surface complexation constants for Np(IV) are derived by applying a data fitting procedure. Constants are very consistent with results obtained by applying an existing linear free energy relationship (LFER). Taking Np(IV) surface complexation constants into account shifts the calculated Np(V)/Np(IV) redox borderline in presence of illite surfaces by 3-5 pe units (0.2-0.3 V) towards redox neutral conditions. Our study suggests that Np(V) reduction in presence of a sorbing mineral phase is thermodynamically favored.

• 出版日期2015-3-1