Eu(III) sorption to TiO2 (anatase and rutile): batch, XPS, and EXAFS studies.

The sorption of Eu(III) on anatase and rutile was studied as a function of ionic strength, humic acid (HA, 7.5 mg/L), and electrolyte anions over a large range of pH (2-12). The presence of HA significantly affected Eu(III) sorption to anatase and rutile. The sorption of Eu(III) on anatase and rutile was independent of ionic strength. Results of an X-ray photoelectron spectroscopy (XPS) analysis showed that Eu(III) was chemically present within the near-surface of TiO2 due to the formation of triple bond SOEu and triple bond SOHAEu complexes. An extended X-ray absorption fine structure (EXAFS) technique was applied to characterize the local structural environment of the adsorbed Eu(III), and the results indicated that Eu(III) was bound to about seven or eight O atoms at a distance of about 2.40 A. The functional groups of surface-bound HA were expected to be involved in the sorption process. The measured Eu-Ti distance confirmed the formation of inner-sphere sorption complexes on a TiO2 surface.