Influence of ionic strength and humic acid concentration on europium(III), α-Al2O3 and humic acid interactions: macroscopic and time-resolved laser-induced luminescence data

International audienceSpeciation of Eu(III) in the presence of purified Aldrich humic acid (PAHA) and/or a-Al2O3 has been studied by time-resolved luminescence spectroscopy as a function of pH, ionic strength and PAHA concentration. The comparisons of macroscopicand spectroscopic data (adsorption, spectra, and decay times analyses) between the ternary system, i.e., Eu(III)/PAHA/a-Al2O3, and the corresponding binary systems are comprehensively presented. As expected, results show almostno influence of ionic strength on Eu(III) adsorption onto a-Al2O3. However, in the binary Eu(III)/PAHA system, it is clearlyshown that variations of electrolyte concentration, which modify PAHA conformation, influence the symmetry of the humic-boundEu(III) at pH P7. In the ternary system, adsorption of both Eu(III) and PAHA onto the surface decreases with ionicstrength. At I = 0.01M NaClO4, Eu(III) luminescence decay is much faster than at I = 0.1 M NaClO4. This is most likely dueto the lower surface concentration of PAHA at lower ionic strength, leading to a less constrained environment for Eu(III)ions. At high pH, luminescence spectra are different at the two ionic strengths studied. Concerning the influence of PAHAconcentration, spectroscopic results show that in the binary Eu(III)/PAHA system complete complexation of 1 lM Eu(III)is reached for 16 mgPAHA l1 at pH 4, and for lower PAHA concentrations at higher pH. At the same PAHA concentration,asymmetry ratios are comparable between the binary Eu(III)/PAHA system and the ternary system between pH 4 and 7.7.This means that the presence of mineral surface has almost no influence on Eu(III) environment symmetry below pH 8; hence,under these acid to neutral pH conditions, the occurrence of Eu(III)-bridged humic surface complexes is not likely. In theternary system, at different pH, luminescence decay times of Eu(III) increase with PAHA concentration. They are much higherin the ternary system than in the binary Eu(III)/PAHA system, which would mean that the Eu(III) environment in PAHA-boundmolecules is more constrained, as proposed recently (Janot et al., 2011; Eita 2011). Macroscopic results show that theinfluence of PAHA on Eu(III) adsorption onto a-Al2O3 depends on pH: at low pH, Eu(III) retention is always higher in theternary system, whereas at higher pH it depends on PAHA concentration