Molecular-level speciation of Eu(III) adsorbed on a migmatized gneiss as determined using µTRLFS

The interaction of Eu(III) with thin sections of migmatised gneiss from the Bukov Underground Research Facility (URF), CZ 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 Eu-152. 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. Especially 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 metal distribution in heterogeneous systems.