Colloidal $\alpha$-Al$_2$O$_3$, Europium(III) and humic substances interactions: A macroscopic and spectroscopic study
Résumé
Eu(III) sorption onto $\alpha$-Al$_2$O$_3$ in the presence of purified Aldrich humic acid (PAHA) is studied by batch experiments and time-resolved laser-induced luminescence spectroscopy of Eu(III). Experiments are conducted at varying pH, at 0.1 mol/L NaClO$_4$, 10$^{-6}$ mol/L Eu(III), 1 g/L $\alpha$-Al$_2$O$_3$ and 28 mg/L PAHA, which assured a complete Eu(III)-PAHA complexation. Adsorption of Eu(III) presents the expected pH-edge at 7, which is modified by addition of PAHA. Presence of Eu(III) slightly increases PAHA sorption throughout the pH range. The evolutions of luminescence spectra and decay times of the binary systems, that is, Eu(III)/$\alpha$-Al$_2$O$_3$ and Eu(III)/PAHA, indicate a progressive surface- and humic-complexation with increasing pH. The typical biexponential luminescence decay in Eu(III)/PAHA system is also recorded; the fastest deactivation depending barely on pH. In ternary Eu(III)/PAHA/$\alpha$-Al$_2$O$_3$ system, the existence of a luminescence biexponential decay for all pH means that Eu(III) is always in the direct neighborhood of the humic substance. Below pH 7, the spectra of the ternary system (Eu(III)/PAHA/$\alpha$-Al$_2$O$_3$) are not different from the ones of Eu(III)/PAHA system, implying the same complex symmetry. Nevertheless, the increase of luminescence decay time points to a change in PAHA conformation onto the surface
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