The geochemical behavior of Pu strongly depends on its redox speciation. In this study, we investigated Pu sorption onto Na-illite, a relevant component of potential host rocks for high-level nuclear waste repositories, under anaerobic conditions. When contacting Pu (85% Pu(IV), 11% Pu(V), and 4% Pu(III); 8 × 10(-11) < [Pu]tot/M < 10(-8)) with illite in 0.1 M NaCl at pH between 3 and 10, Pu uptake was characterized by log Rd > 4 (Rd: distribution coefficient in L kg(-1)). Small amounts of aqueous Pu(V) were detected in solution on contact with illite after 1 week, which is not expected to be stable at the measured redox potentials (Eh) in our experiments. This observation suggests time-dependent reduction of Pu(V) to Pu(IV). After one year, log Rd values had increased compared to those after 1 week due to the reduction of weakly adsorbing Pu(V). For pH < 5, Pu(IV) and Pu(III) coexisted in solution under our experimental conditions, showing that Pu(IV) reduction to Pu(III) occurred in the illite suspension. Taking (i) surface complexation constants determined for Eu(III)-illite interaction (with redox-insensitive Eu(III) as a chemical analogue to Pu(III)), (ii) the known constant for Pu(III)-Pu(IV) redox transition, and (iii) measured Eh and pH, overall Pu uptake was well-predicted.

The long-term safety assessment for nuclear waste repositories requires a detailed understanding of actinide (geo)chemistry. Advanced analytical tools are required to gain insight into actinide speciation in a given system. The geochemical conditions in the vicinity of a nuclear repository control the redox state of radionuclides, which in turn has a strong impact on their mobility. Besides the long-lived radionuclides plutonium (Pu) and neptunium (Np), which are key elements in high level nuclear waste, iron (Fe) represents a main component in natural systems controlling redox-related geochemical processes. Measuring the oxidation state distribution for redox sensitive radionuclides and other metal ions is challenging at trace concentrations below the detection limit of most available spectroscopic methods (≥10(-6) M). Consequently, ultrasensitive new analytical techniques are required. Capillary electrophoresis (CE) is a suitable separation method for metal cations. CE hyphenated to inductively coupled plasma sector field mass spectrometry (CE-ICP-SF-MS) was used to measure the redox speciation of Pu (III, IV, V, VI), Np (IV, V, VI), and Fe (II, III) at concentrations lower than 10(-7) M. CE coupling and separation parameters such as sample gas pressure, make up flow rate, capillary position, auxiliary gas flow, as well as the electrolyte system were optimized to obtain the maximum sensitivity. We obtain detection limits of 10(-12) M for Np and Pu. The various oxidation state species of Pu and Np in different samples were separated by application of an acetate-based electrolyte system. The separation of Fe (II) and Fe (III) was investigated using different organic complexing ligands, EDTA, and o-phenanthroline. For the Fe redox system, a limit of detection of 10(-8) M was calculated. By applying this analytical system to sorption studies, we were able to underline previously published results for the sorption behavior of Np in highly diluted concentrations, and we monitored the time-dependent reduction of Pu(VI) by Fe(II). This study clearly shows that CE-ICP-SF-MS is a suitable separation method for the redox states of Pu, Np, and Fe.

International audienceNeptunium (Np) uptake on illite is investigated in 1 and 3.2 molal (m) NaCl solutions under inert (Ar) atmosphere for 4 < pHm < 10 (pHm = −log ) and 5 × 10−8 < [Np(V)]tot < 3 × 10−4 M. In agreement with a previous study in 0.1 m NaCl solutions (Marsac et al., 2015a), Np(V) is the prevailing oxidation state in the aqueous solution, but Np uptake by illite is affected by surface induced reduction. The extent of Np(V) reduction to Np(IV) follows the measured redox potential (or the pe = −log ae−), which is influenced by the introduced Np(V) amount, because of the low redox capacity of the illite. The presence of Np(IV) on the solid phase is verified by X-ray Absorption Near Edge Spectroscopy (XANES). We can conclude that Np uptake by illite is not significantly affected by the variation of from 0.1 to 3.2 m and thus is in agreement with reports on tetravalent actinide and Np(V) sorption to clays at high ionic strength. The combination of (i) the two site protolysis non-electrostatic surface complexation and cation exchange model, (ii) the specific ion interaction theory to calculate activity coefficients for dissolved species and (iii) by accounting for redox equilibria and the stability of surface Np species, the overall Np uptake by illite can be simulated as a function of pHm, pe and using a single set of parameters. The present experimental and modeling results are particularly important in the context of deep geological nuclear waste disposal since many sedimentary rocks or clay formations that are deemed suitable for this purpose exhibit highly saline porewaters

Abstract. 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 3 < pH m (= -log ) < 10 and = 1.0 and 3.2 molal (m). Results are compared with previous data for = 0.1 m.According to redox potential measurements and based on Eu(III)-illite sorption data (taken as analogue of Pu(III)), the strong effect of 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 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 can be very well reproduced.Keywords: Plutonium, illite, clay, saline, speciation, redox, sorption, surface complexation model, ion exchange. Graphical Abstract