Microscale analysis of metal uptake by argillaceous rocks using positive matrix factorization of microscopic X-ray fluorescence elemental maps

Abstract

Argillaceous rocks are considered in most European countries as suitable host rock formations for the deep geological disposal of high-level radioactive waste (HLW). The most important chemical characteristic in this respect is their generally strong radionuclide retention property due to the high sorption capacity. Consequently, the physico-chemical parameters of these processes have to be studied in great detail. Synchrotron radiation microscopic X-ray fluorescence (SR µ-XRF) has sufficient sensitivity to study these processes on the microscale without the necessity of the application of radioactive substances. The present study focuses on the interaction between the escaped ions and the host-rock surrounding the planned HLW repository. SR µ-XRF measurements were performed on thin sections subjected to sorption experiments using 5 µm spatial resolution. Inactive Cs(I), Ni(II), Nd(III) and natural U(VI) were selected for the experiments chemically representing key radionuclides. The thin sections were prepared on high-purity silicon wafers from geochemically characterized cores of Boda Claystone Formation, Hungary. Samples were subjected to 72-hour sorption experiments with one ion of interest added. The µ-XRF elemental maps taken usually on several thousand pixels indicate a correlation of Cs and Ni with Fe-and K-rich regions suggesting that these elements are predominantly taken up by clay-rich phases. U and Nd was found to be bound not only to the clayey matrix, but the cavity filling minerals also played important role in the uptake. Multivariate methods were found to be efficient tools for extracting information from the elemental distribution maps even when the clayey matrix and fracture infilling regions were examined in the same measured area. By using positive matrix factorization as a new approach the factors with higher sorption capacity could be identified and with additional mineralogical information the uptake capacity of the different mineral phases could be quantified. The results were compared with cluster analysis when the regions dominated by different mineral phases are segmented. The multivariate approach based on µ-XRF to identify the minerals was validated using microscopic X-ray diffraction. Keywords: micro-XRF; sorption; positive matrix factorization; radioactive waste; argillaceous rock IntroductionIn most European countries, clay-rich rocks are considered as suitable host rock formations for isolation of the radionuclides generated by the nuclear industry from the biosphere for thousands of years. During this time period the radionuclides might migrate from the engineered barriers and the safety is determined by the retention capability of the host rock. Following water intrusion into the repository, radionuclide contaminants can be released. Clay rocks have very good retention capability due to their high sorption capacity. Sorption depends on the water/solid compositions, porewater properties such as pH and Eh, as well as the presence of complexing anions as they determine t...

Combined X‐ray microanalytical study of the Nd uptake capability of argillaceous rocks

“…In previous studies, the differences in the ion uptake of G Block and WMA Block have been examined . The results pointed out that only in WMA Block, a new mineral phase was formed with high ion binding capability for the case of uranium and neodymium treated samples . Therefore, hereinafter, only samples from WMA Block will be discussed …”

“…The ionic strength of this latter solution was similar to the porewater, while the pH was set to 7.0 to avoid precipitation of Nd phases. Hereinafter, only samples treated with NaCl solution will be discussed, because this electrolyte ensured a sufficient loading of the element of interest to match the sensitivity of micro‐XRF while avoiding precipitation in the solution itself . The concentrations of the key element to be applied for the sorption experiments based on the sorption isotherms were measured and modelled for Eu(III) on illite, taking into account the clay content of the BCF samples (around 40%) as well as the solid‐to‐liquid ratio .…”

“…The chemical conditions present at the depth of the planned repository as the concentrations of the elements of interest in the solutions involved should be sufficiently low in order to avoid precipitation. Therefore, the element of interest will be present in minor or trace concentration level heterogeneously distributed in the sample after the sorption experiment . Nd(III) was used as a chemical analogue for trivalent transuranium elements of the radioactive waste to examine the ion uptake capability of the micrometre size mineral phases occurring in the rock.…”

“…Na and Mg); therefore, it is inadequate to ascertain the mineral phases containing elements only with low atomic number (e.g. dolomite and albite) . The missing additional information can be obtained with the elemental mapping of the same sample area using micro‐XRF in vacuum mode or with a helium flush around the sample to avoid its damage.…”

“…This problem can be overcome by application of PMF, commonly used in air pollution studies, where the factors should contain only non‐negative values . PMF was successfully applied to process three‐dimensional data sets answering questions related to other topics . However, no applications of PMF on combined micro‐XRF and SEM/EDX data sets have been reported to date.…”