Complexation of radionuclide ^152+154Eu(III) with alumina-bound fulvic acid studied by batch and time-resolved laser fluorescence spectroscopy

To contribute to the understanding of Eu(III) interaction preperties on hydrous alumina particles in the absence and presence of fulvic acid (FA), the complexation properties of Eu(III) with hydrous alumina, FA and FA-alumina hybrids are studied by batch and time-resolved laser fluorescence spectroscopy (TRLFS) techniques. The continuous increase in the fluorescence lifetime of Eu-alumina and Eu-FA with increasing pH indicates that the complexation is accompanied by decreasing number of hydration water in the first coordination sphere of Eu(III). Eu(III) is adsorbed onto alumina particles as outer-sphere surface complexes of ≡(Al−O)−Eu· (OH)· 7H₂O and ≡(Al−O)−Eu· 6H₂O at low pH values, and as inner-sphere surface complexes as ≡(Al−O)₂−Eu⁺· 4H₂O at high pH. In FA solution, Eu(III) forms complexes with FA as (COO)₂Eu⁺(H₂O)_x and the hydration water number in the first coordination sphere decreases with pH increasing. The formation of ≡COO−Eu−(O−Al≡)· 4H₂O is observed on FA-alumina hybrids, suggesting the formation of strong inner-sphere surface complexes in the presence of FA. The surface complexes are also characterized by their emission spectra [the ratio of emission intensities of ⁵D₀→⁷F₁ (λ=594 nm) and ⁵D₀→⁷F₂ (λ=619 nm) transitions] and their fluorescence lifetime. The findings is important to understand the contribution of FA in the complexation properties of Eu(III) on FA-alumina hybrids that the clarification of the environmental behavior of humic substances is necessary to understand fully the behavior of Eu(III), or its analogue trivalent lanthanide and actinide ions in natural environment.