In-situ vibrational spectroscopic identification of Np(V) sorption complexes at the mineral oxide-water interface

Neptunium (Np) is one of the most important components of nuclear waste to consider for the long-term safety assessment of nuclear waste repositories, due to the increasing enrichment, the long half-life and the high toxicity of Np-237. Hence, great attention is attracted to its geochemistry [1]. Among the various geochemical reactions, the molecular processes occurring at the solid-water interface, e.g. sorption onto mineral phases, surface precipitation, and colloid formation strongly affect the migration behavior of the radioactive contaminant in the environment [2]. Thus, various components of geological materials, such as iron oxides and hydroxides play an important role in regulating the mobility of actinides in aquifers, due to their widespread environmental presence, high sorption capacity and tendency to form coatings on mineral surfaces [3]. In recent years, the sorption behavior of Np(V), the most relevant oxidation state under ambient conditions, onto iron oxides was mainly studied by macroscopic experiments [4]. For a better understanding of the molecular events occurring at the mineral’s surfaces, ATR FT-IR spectroscopy is a useful tool for the in-situ identification of surface species [5]. In addition, time-resolved measurements provide kinetic information on the surface reactions.
In this work, Np(V) sorption on the oxyhydroxides of Fe, Mn, Si and Ti is investigated by in-situ ATR FT-IR spectroscopy under a variety of environmentally relevant sorption conditions. Upon sorption of micromolar Np(V) on Fe2O3, a band observed at 789 cm−1 is assigned to the antisymmetric stretching vibrational mode (ν3) of the neptunyl ion (Fig.1). The IR spectrum obtained at equal conditions in an aqueous solution shows the absorption of ν3(NpVO2) at 818 cm−1 [5]. The red shift of ν3 to 789 cm−1 upon sorption can be assigned to an inner-sphere sorption complex. Kinetic experiments have shown that only one sorption complex was formed independent from Np(V) loading. Furthermore, no impact of ionic strength (1- 10-4 M NaCl) and pH (≤ 10) on the sorbed species was found. From a comparison of Np(V) surface complexation on different mineral oxides, namely Si, Mn, Fe and Ti oxides, a very similar sorption behavior was elucidated.

Fig. 1: ATR FT-IR spectra of the sorption complexes formed onto several mineral oxides (50 µM Np(V), 0.1 M NaCl, pH 7, 60 min sorption, 0.1 mg mineral oxide/cm2, N2).

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[5] Müller, K. et al. (2009) Environ. Sci. Techn. 43, 7665-7670.