X-Ray Microspectroscopic Investigations of Ni(II) Uptake by Argillaceous Rocks of the Boda Siltstone Formation in Hungary

NEA/RWM/CLAYCLUB(2013)1

X-ray Microspectroscopic Investigations of Ni(II) Uptake by Argillaceous Rocks of the Boda Siltstone Formation IN Hungary

Dániel Breitner1*, János Osán1, Szabina Török1, István Sajó2, Rainer Dähn3, Zoltán Máthé4, Csaba Szabó5

1KFKI Atomic Energy Research Institute (HU) 2Chemical Research Centre of the Hungarian Academy of Sciences (HU) 3Laboratory for Waste Management, Paul Scherrer Institute (CH) 4MECSEKÉRC Plc. (HU) 5Lithosphere Fluid Research Lab, Eötvös Loránd University (HU)

* Corresponding author: [email protected]

Abstract

Synchrotron-based µ-focused X-ray fluorescence, X-ray diffraction and X-ray absorption spectroscopy were applied to identify the potential minerals responsible for the uptake of Ni in microscopically het- erogeneous clay-rich rock samples, originating from the Boda Siltstone Formation (BSF, Hungary). The X-ray fluorescence measurements indicate a correlation of Ni with Fe- and K-rich regions sug- gesting that Ni is predominantly taken up by these phases. X-ray diffraction identified the Fe- and K- rich regions as illite and iron oxides, respectively. X-ray absorption spectroscopy demonstrated that under the experimental conditions employed inner-sphere complexation of Ni(II) to clay minerals prevail, indicating that illite and iron oxides are an effective sink for Ni(II) in BSF.

Introduction

One of the main aspects for evaluating the safety case of a potential radioactive waste repository in a deep geological formation is to understand and quantify the geochemical and physical processes that influence the mobility of the radionuclides in the geochemical environment imposed by the host rock. This information is needed to make reliable predictions of the long-term retardation behaviour of radionuclides. The present study focuses on the interaction of a key radionuclide with the host-rock of the planned high level radioactive waste (HLW) repository in Hungary (BSF). The aim is to investi- gate the uptake mechanisms of selected metals representing fission products on clays and other minerals present in the host rock.

Materials and methods

Small pieces of selected Boda core samples from a depth of 540 m were ground down to ca. 30 µm and mounted onto high purity Si holder and polished by 0.25 µm diamond paste. Based on the XRD study of the bulk sample, its mineral composition is 34 wt.% clay minerals, 7 wt.% albite, 16 wt.%

58 NEA/RWM/CLAYCLUB(2013)1

analcime, 28 wt.% quartz, 10 wt.% calcite and 5 wt.% hematite. The samples were equilibrated for 72- hour with a 1 mM NiCl2 solution at pH = 7.05 in a 0.1 M NaCl background electrolyte. The structure and mineral heterogeneity of the rock matrix of the BSF samples and the Ni distribution on the sample surface after the sorption experiment was studied by low-vacuum FEI Quanta 3D scanning electron microscope using 15 kV acceleration voltage. The analyses were performed at Eötvös Loránd University, Budapest. Combined -XRF/XRD measurements were performed at HASYLAB Beamline L (Hamburg, Germany) and at the ANKA FLUO Beamline (Karlsruhe, Germany) using a multilayer monochromator at a fixed energy of 17.5 keV. The single-bounce capillary at HASYLAB resulted in a beam diameter of 20 µm, whereas the compound refractive lens employed at ANKA resulted in a beam footprint of 3 µm (V) × 8 µm (H) on the sample. -XRF maps were recorded from pre-selected areas of the samples, using a step size comparable to the beam size (20 µm at HASYLAB and 5 µm at ANKA) and 1 s counting time per pixel. The -XRF maps were analysed using the AXIL software. The elemental maps served as a basis for selection of small areas of interest where µ-XRD images were collected by a CCD detector. The µ-EXAFS measurements were measured in fluorescence mode at HASYLAB Beamline L, using a Si(111) monochromator and a polycapillary half-lens, resulting in a beam diameter of 20 µm for certain points of interest (POI).

Results and discussion

The scanning electron microscopic (SEM) investigations showed that the average size of the mineral grains in the rock matrix is ca. 5 microns surrounded by finer clay rich materials. The identified minerals in the matrix are illite, hematite, rutile, quartz, K-feldspar and calcite/dolomite (Figure 1). The X- ray mapping demonstrated that Ni is present mainly in iron oxide grains and the clay rich regions sur- rounding larger grains such as K-feldspar and quartz (Figure 1), and practically no Ni was identified on quartz and K-feldspar.

Figure 1. Back scattered electron image of Boda sample and the identified minerals and the X- ray maps of the selected elements at the selected sample area.

Based on the SEM measurements two areas for -XRF analysis were selected (Ib4cr1 and Ib4cr4, Figure 2 and 3, respectively). The correlations of elemental intensities in the area Ib4cr1 indicate that Ni is well correlated with K and Fe, with correlation coefficients of ~0.8and anti-correlated with Ca (Figure 2).

59 NEA/RWM/CLAYCLUB(2013)1

Figure 2. Optical image of the thin section ib4-540c; elemental maps of the selected area ib4cr1 (1x1 mm, indicated as black square on the optical image) and scatter plots of characteristic X- ray intensities of elements measured at beamline L/HASYLAB.

The mineralogical characterisations were complemented by -XRF/XRD measurements at the FLUO (ANKA) beamline with a much smaller spatial resolution (Figure 3). The correlation coefficients be- tween K and Ni, as well as Fe and Ni are below 0.5, which are significantly lower than for the larger (20 m) beam. The -XRF map exhibits only a few Ni hotspots. The distribution maps indicate that Ni is mainly associated to K- and Fe-rich phases (Figure 3). With the smaller beam the K-Fe correlation coefficient is also lower (0.73) compared to the area ib4cr1 (0.92, as plotted in Figure 2), due to the lower counting statistics. -XRD analyses with a 5 m beam diameter indicate that the composition of the argillaceous rock matrix on the micro-scale agree well with the mineral composition of the clay rich areas obtained by bulk-XRD measurements. The good correlation of Fe with K is caused by the high K content in illite (a K-rich 2:1 clay mineral) and the coexistence of small (ca. 10 nm) hematite particles. The -EXAFS spectra for various POI‘s can be attributed to two different species (Figure 4). One of the species exhibited a strong second peak in the radial structure function (RSF) at a R + R position of 2.75 Å, which is characteristic for Ni-Ni backscattering pairs, and is indicative for a Ni-precipitate. For other POI this Ni-Ni backscattering pair is absent. Instead, these spectra can be fitted well, with Ni-O, Ni-Al, and Ni-Si backscattering pairs. The resulting coordination numbers and distances are consistent with the formation of inner sphere complexes at clay edge sites (Dähn et al., 2003).

Figure 3. Optical image of the thin section ib4-540c; elemental maps of the selected area ib4cr4 (0.5x0.5 mm, indicated as yellow square on the optical image) and scatter plots of characteristic X-ray intensities of elements measured at beamline FLUO/ANKA.

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10 Ni precipitate Ni inner-sphere complex 8 6 Ni precipitate Ni inner-sphere complex

4 )

2 (k)

(k)

 

0 3

k ( k -2 -4 FT -6 -8 -10 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 -1 k (Å ) R + R (Å)

Figure 4. k3-weighted Ni K-edge micro-EXAFS spectra representative for the formation of Ni- precipitates and inner-sphere complexes, and the corresponding RSF‟s, obtained by Fourier transforming the micro-EXAFS spectra in the range from 2 to 10 Å-1.

Summary and Conclusions

The formation of Ni precipitated phases such as Ni-phyllosilicates has important geochemical implica- tions because layered silicates are stable minerals in mildly acidic to basic pH conditions and can irre- versibly bind metals in waste and soil matrices. The uptake of contaminants on mineral surfaces form- ing inner-sphere complexes strongly reduces the mobility of metals in the geosphere. The results of the analyses demonstrated that for Ni(II) the clay mineral illite and iron oxide are the most effective sinks in the BSF sample.

Acknowledgement

The research greatfully acknowledges funding from the Swiss-Hungarian Cooperation Programme through Project n° SH/7/2/11 and from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 226716. The authors would like to thank Karen Appel and Manuela Borchert (HASYLAB), and Rolf Simon (ANKA) for the support during the measurements.

References

Dähn, R., Scheidegger, A.M., Manceau, A., Schlegel, M.L., Baeyens, B., Bradbury, M.H., and Chateigner, D. (2003). Geochim. Cosmochim. Acta 67, 1.