Abstract
To contribute to the comprehension of Eu(III) sorption properties in oxide-organic material systems, the influence of humic acid (HA) on Eu(III) sorption on bare and HA-bounded gamma-Al2O3 is studied using batch experiments and time resolved laser fluorescence spectroscopy (TRLFS). The effect of pH and addition sequences of Eu(111)/HA/Al2O3 on Eu(III) sorption is carried out at different experimental conditions, and the results show that the sorption of Eu(III) is strongly dependent on pH and ionic strength. The sorption of Eu(III) is mainly dominated by ion exchange or outer-sphere surface complexation at low pH, and by inner-sphere surface complexation at high pH values. The presence of HA enhances Eu(III) sorption at low pH, but reduces Eu(III) sorption at high pH values. The addition sequences of Eu(III)/HA to alumina suspensions do not affect Eu(III) sorption, which is also evidenced by the TRLFS measurements. The surface complexes are characterized by their emission spectra [the ratio of emission intensities of D-5(0) -> F-7(1) (lambda = 594 nm) and D-5(0) -> F-7(2) (lambda = 619 nm) transitions] and their fluorescence lifetime. The TRLFS results show clearly that no effect of addition sequences on the sorption and speciation of Eu(III) in HA-Al2O3 hybrids after equilibration. These findings on the effect of humic substances in the complexation properties of Eu(III) indicate that the clarification of the environmental behavior of humic substances is necessary to understand the physicochemical properties of Eu(III), or its analog trivalent lanthanide and actinide ions in the natural environment. (C) 2015 Elsevier B.V. All rights reserved.