Molecular-Level Speciation of Eu(III) Adsorbed on a Migmatized Gneiss As Determined Using μTRLFS

Abstract

The interaction of Eu­(III) with thin sections of migmatized gneiss from the Bukov Underground Research Facility (URF), Czech Republic, was characterized by microfocus time-resolved laser-induced luminescence spectroscopy (μTRLFS) with a spatial resolution of ∼20 μm, well below typical grain sizes of the material. By this approach, sorption processes can be characterized on the molecular level while maintaining the relationship of the speciation with mineralogy and topography. The sample mineralogy was characterized by powder X-ray diffraction and Raman microscopy, and the sorption was independently quantified by autoradiography using 152Eu. Representative μTRLFS studies over large areas of multiple mm2 reveal that sorption on the heterogeneous material is not dominated by any of the typical major constituent minerals (quartz, feldspar, and mica). Instead, minor phases such as chlorite and prehnite control the Eu­(III) distribution, despite their low contribution to the overall composition of the material, as well as common but less studied phases like Mg–hornblende. In particular, prehnite shows high a sorption uptake as well as strong binding of Eu to the mineral surface. Sorption on prehnite and hornblende happens at the expense of feldspar, which showed the highest sorption uptake in a previous spatially resolved study on granitic rock. Similarly, sorption on quartz is reduced, even though only low quantities of strongly bound Eu­(III) were found here previously. Our results illustrate how competition of mineral surfaces for adsorbing cations drives the metal distribution in heterogeneous systems.

Heterogeneous Sorption of Radionuclides Predicted by Crystal Surface Nanoroughness

“…Typically, SCMs include a description of the electric double layer (e.g., the Gouy–Chapman model) and conventional mass action laws for intrinsic chemical reactions at the surface, but adsorption is ultimately treated as an interaction of metal ions with functional groups uniformly distributed on the surface. Consequently, heterogeneous sorption variabilities as observed in our experiment and previously reported in our previous work , cannot be modeled by this standard approach. The variabilities are based on heterogeneously distributed affinities of cations for distinct positions on the calcite surface, which are interpreted to be related to the variable amount of coordinated water molecules and Ca–O bond lengths on the different positions of the crystal surface.…”

“…The experimental conditions are identical to those described in the literature for studying dissolution kinetics of calcite surfaces at far-from-equilibrium conditions . Second, a batch europium sorption experiment was conducted by immersing the sample surface upside down into a solution of 10 –5 mol/L Eu­(NO 3 ) 3 and 0.1 mol/L NaCl with a pH value of 8.5 for 24 h. In this step, the larger concentration and longer sorption period were made necessary to enable the μTRLFS studies. , The sample was then rinsed with the background solution and dried. This experimental procedure minimized the possible effect of the precipitation of europium solids.…”

“…Analytically, the recent development of the μTRLFS technique allows us to identify interfacial speciation with μm-spatial resolution and the very high sensitivity required to investigate surface complexes. , This technique revealed heterogeneous sorption behavior of the trivalent actinide analog Eu­(III) on a granite rock surface that cannot be quantitatively explained by mineral type and composition alone . Consequently, insight into the intrinsic variability of surface reactivity is required to improve our predictive capabilities of reactive transport processes .…”

Impact of Ion–Ion Correlations on the Adsorption of M(III) (M = Am, Eu, Y) onto Muscovite (001) in the Presence of Sulfate

“…One group of relevant contaminants is the rare earth elements (REEs), which are released into the environment due to their increasing usage, e.g., in high-technology electronic devices . Main anthropogenic sources of REEs to natural waters are mining, , medical applications, , and electronic wastes and their recycling. , The total REE concentrations at contaminated mining sites are reported to reach up to 80 μM. − Ecotoxicological effects of REEs, such as stress reactions in plants as well as health effects on animals and humans, have been demonstrated previously. ,, In addition, REEs have a typical oxidation state of +III and serve as analogues for trivalent actinides (Pu, Am, Cm) in various studies. − Here as well, the actinides’ interaction with mineral surfaces is of particular interest, , which needs to be understood for making critical decisions in selecting deep geological repository sites for radioactive waste …”