Identification of Np(V) sorption complexes at the hematite-water interface studied by in-situ ATR FT-IR spectroscopy


Identification of Np(V) sorption complexes at the hematite-water interface studied by in-situ ATR FT-IR spectroscopy

Müller, K.; Gröschel, A.

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 en-richment, the long half-life and the high toxicity of Np-237. Hence, great attention is attract-ed to its geochemistry [1]. Among the various geochemical reactions, the molecular pro-cesses 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 hematite is studied under a variety of environmentally rele-vant sorption conditions by in-situ ATR FT-IR spectroscopy [5]. The IR spectra obtained from the subsequent steps of the experiment, that is (1) conditioning, (2) sorption, and (3) flushing, are shown in Fig. 1.
The absence of significant bands below 1000 cm−1 in the conditioning spectrum demon-strates the stability of the stationary hematite film directly prepared on the ATR crystal’s surface. The bands at 1491 and 1356 cm−1 represent the removal of carbonate by rinsing the hematite film, prepared in air with the CO2 free solution. Upon Np(V) sorption, the band observed at 790 cm−1 is assigned to the antisymmetric stretching vibrational mode (ν3) of the neptunyl ion. The IR spectrum obtained from an aqueous solution at 50 µM Np(V), 0.1 M, pH 6 shows the absorption of ν3(NpVO2) at 818 cm−1. The red shift of ν3 to 790 cm−1 upon sorption can be assigned to an inner-sphere monomeric sorption complex, as previ-ously reported for TiO2, SiO2 and ZnO [5]. The band at 1042 cm−1 is most probably due to surface modes of the mineral oxide provoked by the sorption processes and were already observed for interactions with U(VI), Cs(I) and CO32 [5]. In the flushing stage, a weakly bound species is released from the stationary phase, reflected by a negative band at 795 cm−1 in the respective spectra.
Additional experiments were performed at varied values of pH (5.6 – 12) and ionic strength (0.001 – 0.1) (Fig. 2). Upon increasing the pH from 5.6 to 8.6, no shifts of the bands at 1041 and 790 cm−1 are observed. But the intensities of these spectral features are consid-erably increased at higher pH values indicating an enhanced sorption capacity close to the IEP at pH 9.2. At pH > 10, the aqueous Np(V) speciation changes and NpO2OHaq is formed and distinctly changes the sorption behavior. The band of ν3(NpVO2) is shifted to 773 cm−1. The variation of ionic strength between 0.1 and 0.01 does not change the spectral characteristics. The higher intensities observed at 0.0001 M NaCl can be attributed to contributions of an outer-sphere complex which has to be verified by future experiments.
In summary, the IR spectra evidence the formation of Np surface complexes on hematite which can be easily removed to a considerable extent by flushing with blank solution. From this behavior, the simultaneous formation of an inner-sphere species with con-tributions of an outer-sphere complex is suggested.

[1] Kaszuba, J. P. et al. (1999) Environ. Sci. Technol. 33, 4427-4433.
[2] O'Day, P. A. (1999) Reviews of Geophysics 37, 249-274.
[3] Tochiyama, O. et al. (1996) Radiochim. Acta 73, 191-198.
[4] Brendler, V. et al. (2003) Journal of Contaminant Hydrology 61, 281-291.
[5] Müller, K. et al. (2009) Environ. Sci. Technol. 43, 7665-7670.

  • Contribution to proceedings
    Actinides 2013 - 9th International Conference on the Chemistry and Physics of the Actinide Elements, 21.-26.07.2013, Karlsruhe, Deutschland
    Proceedings of Actinides 2013
  • Lecture (Conference)
    Actinides 2013 - 9th International Conference on the Chemistry and Physics of the Actinide Elements, 21.-26.07.2013, Karlsruhe, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-18663
Publ.-Id: 18663