Speciation of Np(V) uptake by Opalinus Clay using synchrotron microbeam techniques

Abstract

Synchrotron-based X-ray absorption spectroscopy has been used to determine the chemical speciation of Np sorbed on Opalinus Clay (OPA, Mont Terri, Switzerland), a natural argillaceous rock revealing a micro-scale heterogeneity. Different sorption and diffusion samples with Np(V) were prepared for spatially resolved molecular-level investigations. Thin sections of OPA contacted with Np(V) solution under aerobic and anaerobic conditions as well as a diffusion sample were analysed spatially resolved. Micro-X-ray fluorescence (μ-XRF) mapping has been used to determine the elemental distributions of Np, Fe and Ca. Regions of high Np concentration were subsequently investigated by micro-X-ray absorption fine structure spectroscopy to determine the oxidation state of Np. Further, micro-X-ray diffraction (μ-XRD) was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of Np enrichments. One thin section was also analysed by electron microprobe to determine the elemental distributions of the lighter elements (especially Si and Al), which represent the main elements of OPA. The results show that in most samples, Np spots with considerable amounts of Np(IV) could be found even when the experiments were carried out in air. In some cases, almost pure Np(IV) L(III)-edge X-ray absorption near-edge structure spectra were recorded. In the case of the anaerobic sample, the μ-XRF mapping showed a clear correlation between Np and Fe, indicating that the reduction of Np(V) is caused by an iron(II)-containing mineral which could be identified by μ-XRD as pyrite. These spatially resolved investigations were complemented by extended X-ray absorption fine structure measurements of powder samples from batch experiments under aerobic and anaerobic conditions to determine the structural parameters of the near-neighbour environment of sorbed Np.

Exploring Actinide Materials Through Synchrotron Radiation Techniques

“…Catalano et al found out certain regions of high uranium concentration from SR‐μ‐XRF mapping of uranium distribution in contaminated sediments and collected μ‐XAF, μ‐XANES and μ‐XRD spectra at these hotspots to explore the speciation of uranium. The elemental distributions of Np, Fe and Ca were determined by Fröhlich et al using SR‐μ‐XRF mapping to work out the regions of high Np concentration. Further investigation on them via micro‐XAFS was performed to determine the oxidation state of Np.…”

Characteristics of uranium uptake of Boda Claystone Formation as the candidate host rock of high level radioactive waste repository in Hungary

“…Although information concerning spatial heterogeneity in uptake mechanism would be essential, studies involving petrographic thin sections after sorption experiments are very limited and dealing only with other elements such as Np (e.g. Fröhlich et al 2012). Investigations with high sensitivity analytical methods such as micro-XRF are missing for BCF.…”

Influence of humic acid on neptunium(V) sorption and diffusion in Opalinus Clay

“…This difference was addressed to a possible reduction of Np(V) to Np(IV) within the clay. This effect has already Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/31/15 2:06 PM been observed in sorption experiments under anaerobic conditions [22] and in XAFS spectroscopic investigations [31]. Taking into account the findings of the present work, the more visible differences between sorption and diffusion experiments found by Wu et al [24] may also be partly a result of differences in properties (e.g., mineralogical composition) of the OPA samples as different batches of OPA were used.…”

“…The intact OPA bore cores used in the diffusion experiments as well as the powder for batch experiments under same experimental conditions were taken from the bore core BLT 14. The elemental composition of this OPA batch has been analyzed by XRF and published in [31]. There were only slight differences in the elemental composition of BHE-24/1 and BLT 14.…”