Time resolved fluorescence spectroscopy and modeling of Eu(III) sorption by silica in presence and absence of alpha hydroxy isobutyric acid

Sorption of Eu(III) by silica from aqueous solutions of varying pH (4.0–8.0), fixed ionic strength of 0.1 M (NaClO4) and at room temperature has been studied by batch sorption experiments as well as time resolved fluorescence spectroscopy (TRFS). The percentage sorption of Eu(III) (5.0 × 10−5 M) by silica was found to increase with the pH of the suspension and remaining close to 100% beyond pH 7.0. In the presence of alpha hydroxy isobutyric acid (HIBA) (2.5 × 10−3 M) the sorption edge was shifted to higher pH indicating decreased sorption, in the pH range of 5.5–7.0, with respect to that in absence of HIBA. The supernatant solutions of Eu(III) equilibrated silica suspensions were found to have significant concentration of silicon, indicating dissolution of silica. The complexation of Eu(III) by silicic acid dominated the speciation of Eu(III) even in the presence of HIBA. TRFS measurements of the Eu(III) sorbed on silica showed two components, with life time close to 150 and 400 s. This information has been used in the surface complexation modeling (SCM), which explains the sorption data in terms of two surface complexes, namely (SiO)2Eu+, and (SiO)2Eu(OSi(OH)3)2−.