Np M₅-edge HR-XANES and L₃-edge EXAFS spectroscopy on illite samples with 20 - 140 ppm Np

A deep geological disposal with a multi-barrier system is proposed, to safely isolate the
high-level radioactive waste from the biosphere. Clays are considered as potential host rock
and as potential backfill material. This study is focused on understanding the interaction
between illite, a clay mineral present in several clay host rocks, and Np-237, a radionuclide
presenting a considerable fraction in high-level waste. Np is a redox-sensitive element with
pentavalent Np being stable in oxidizing to redox-neutral environment and tetravalent Np
prevailing under reducing conditions. To understand the mechanism of Np retention, it is
necessary to understand the sorption but also the migration behaviour.
This investigation probes the redox-species and the atomic environment of Np sorbed onto
illite for samples from batch sorption and diffusion experiments applying different
spectroscopic and microscopic techniques (Np M5-edge high-energy resolution X-ray
absorption near edge structure (HR-XANES) [1], Np L3-edge extended X-ray absorption fine
structure (EXAFS), Fe K-edge XANES, and Mössbauer spectroscopy as well as scanning
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electron (SEM), transmission electron (TEM), and fluorescence microscopy).[2]
This is the first Np M5-edge HR-XANES study where it was possible to record conclusively
spectra at very low Np loadings of (18 ± 6) ppm due to an optimised set-up and sample
preparation. It was conducted at the CAT-ACT beamline for catalysis and actinide science at
the Karlsruhe research accelerator (KARA) at KIT, Karlsruhe [3]. Np L3-edge XANES and
Np M5-edge HR-XANES spectra strongly suggest the presence of Np(IV) and Np(V) in both
the dispersed illite sorption samples kept under anaerobic conditions but also in segments
taken from the diffusion experiments carried out under an aerobic atmosphere. These results
are corroborated by the results of solvent extraction according to the method of Bertrand and
Chopin (1982) [4]. EXAFS analyses of illite sorption samples prepared at pH 9 confirm the
presence of solely sorbed mononuclear Np species and a Np-O-Fe coordination at
(3.44 ± 0.02) Å. Fe K-edge XANES and Mössbauer spectroscopy found Fe(III) and Fe(II) in
the structure of illite. Results obtained by SEM, TEM, and fluorescence microscopy studies
exclude the presence of amorphous Fe phases and microorganisms as possible reductants.
The observation of a partial reduction of Np(V) and the finding that Np is coordinated to FeOH groups points to a partial reduction through electron transfer from structural Fe(II) in the
illite.
The present study demonstrates that by the complementary application of advanced
spectroscopic methods, mechanistic insight into complex geochemical actinide reactions can
be achieved at even low concentrations.
References
[1] - T. Vitova, I. Pidchenko, D. Fellhauer et al., Nat. Commun. 8, (2017).
[2] - R. Marsac, N. Banik, J. Lützenkirchen et al., Geochim. Cosmochim. Acta 152, 39 (2015).
[3] - A. Zimina, K. Dardenne, M.A. Denecke et al., Rev. Sci. Instrum. 88, 1 (2017).
[4] - P.A. Bertrand and G.R. Choppin, Radiochim. Acta 31, 135 (1982).