The surface speciation of the ternary sorption system U(VI)/phosphate/silica

The impact of inorganic ligands on the sorption behavior of actinide ions is commonly known. However, detailed knowledge of the molecular events occurring during the sorption processes is often lacking. In particular, the presence of inorganic anions forming actinide complexes of low solubility hampers the application of many spectroscopic approaches due to the formation of binary complexes precipitating from the aqueous solutions.
This study sustains our preliminary results on the ternary sorption system U(VI)/phosphate/silica introduced at ATAS 2012 [1]. The advanced results obtained from a combined approach of in situ vibrational and luminescence spectroscopy provide a more detailed insight into the surface speciation of this ternary sorption system.
From in situ vibrational spectroscopic sorption experiments of the binary system U(VI)/silica, infrared data exhibit the formation of a uranyl inner sphere complex at the silica surface, whereas from the ternary sorption system, spectra showing great homologies to spectra of solid U(VI)phosphate phases are obtained. The results obtained from the in situ IR experiments strongly suggest the formation of a solid U(VI)phosphate as a surface precipitate on the silica phase.
Laser fluorescence spectroscopy reveals the presence of U(VI) phosphate species in aqueous solution most probably solid or colloidal (UO2)(PO4)2∙4H2O. For the U(VI) sorption samples, two different surface species were derived from luminescence spectra irrespective of the absence or presence of phosphate [2]. However, the spectral differences became more apparent after prolonged equilibration of the solid phase with a stable U(VI) phosphate solution suggesting chemical rearrangements of the sorbed U(VI) ion towards a ternary surface species.
In summary, IR and luminescence data suggest the formation of a ternary surface species where the U(VI) acts as a bridging ion to the SiO2 surface with subsequent formation of the ternary surface species SiO2–U(VI)-phosphate. This ternary species most likely constitutes a precursor of the formation of a surface precipitate showing spectral properties similar to U(VI) phosphate minerals.