Sorption of Eu(III) by amorphous titania, anatase and rutile: Denticity difference in surface complexes

To study the batch sorption of Eu(III) by different phases of titania viz. amorphous, anatase and rutile, amorphous titania was synthesized by organic route, followed by annealation to prepare anatase and rutile. Characterization of these phases was done by zeta potential measurement and potentiometric titration. Eu(III) sorption by titania phases increases with pH, pH50 (pH corresponding to 50% sorption) being lower in case of rutile than other two. Sorption edges for anatase and amorphous titania overlap on normalizing the data with specific surface area of particles. Surface complexation modelling of sorption data reveals the formation of higher hydrolysed Eu(III) surface species (TiEu(OH)2) on rutile compared to other two phases (TiOEu(OH)+). Consideration of bidentate surface complex for anatase and rutile gave equally good fit. Time resolved fluorescence spectroscopy showed higher lifetime for Eu sorbed on rutile, suggesting higher binding for Eu(III) in the surface complex. Keeping in view the regular arrangement of Ti octahedron on rutile surface, rutile surface can have more bidentate surface complex in comparison to other two phases. These results indicate the simultaneous existence of monodentate and bidentate surface complexes of Eu(III) on titania surfaces, with rutile having higher proportion of bidentate complex.