Adsorption of Eu(III) on a Heterogeneous Surface Studied by Time-Resolved Laser Fluorescence Microscopy (TRLFM)

Time-resolved laser fluorescence microscopy (TRLFM) is a useful tool to simultaneously investigate the intensity, location, type, and surrounding chemical environment of a fluorophore. In this study, we demonstrated the applicability of TRLFM for the adsorption of Eu(III) on a natural heterogeneous surface. Different adsorption species of Eu(III) were observed on the Makabe granite surface and its constituents (biotite, plagioclase, potassium feldspar, and quartz). Eu(III) heterogeneously adsorbed on biotite, plagioclase, and quartz and homogeneously on potassium feldspar. The histograms of the fluorescence decay rates of adsorbed Eu(III) indicated efficient quenching of Eu(III) fluorescence probably due to Eu(III)-surface interaction or the formation polynuclear hydoxo Eu(III) species on the surfaces. It was also revealed that single species of Eu(III) was observed on biotite and two species on plagioclase and potassium feldspar. The adsorption of Eu(III) on the granite surface was highly heterogeneous. The TRLFM measurements of different regions of the granite surface turned into the finding of Eu(III) with different fluorescence decay rates. Comparing with the fluorescence decay histograms of the mineral constituents, Eu(III) clearly adsorbed on the feldspar family. It was also found that Eu(III) adsorbed as an outer-sphere complex and on an altered mineral of the granite.