Influence of solution parameters on europium(III), α-Al2O3, and humic acid interactions: Macroscopic and time-resolved laser-induced luminescence data

Speciation of Eu(III) in the presence of purified Aldrich humic acid (PAHA) and/or alpha-Al2O3 has been studied by time-resolved luminescence spectroscopy as a function of pH, ionic strength and PAHA concentration. The comparisons of macroscopic and spectroscopic data (adsorption, spectra, and decay times analyses) between the ternary system, i.e., Eu(III)/PAHA/alpha-Al2O3, and the corresponding binary systems are comprehensively presented. As expected, results show almost no influence of ionic strength on Eu(III) adsorption onto alpha-Al2O3. However, in the binary Eu(III)/PAHA system, it is clearly shown that variations of electrolyte concentration, which modify PAHA conformation, influence the symmetry of the humic-bound Eu(III) at pH >= 7. In the ternary system, adsorption of both Eu(III) and PAHA onto the surface decreases with ionic strength. At I = 0.01 M NaClO4, Eu(III) luminescence decay is much faster than at I = 0.1 M NaClO4. This is most likely due to 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 PAHA concentration, spectroscopic results show that in the binary Eu(III)/PAHA system complete complexation of 1 mu M Eu(III) is reached for 16 mg(PAHA) l(-1) 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 the ternary system, at different pH, luminescence decay times of Eu(III) increase with PAHA concentration. They are much higher in the ternary system than in the binary Eu(III)/PAHA system, which would mean that the Eu(III) environment in PAHA-bound molecules is more constrained, as proposed recently (Janot et al., 2011; Eita 2011). Macroscopic results show that the influence of PAHA on Eu(III) adsorption onto alpha-Al2O3 depends on pH: at low pH, Eu(III) retention is always higher in the ternary system, whereas at higher pH it depends on PAHA concentration. (C) 2013 Elsevier Ltd. All rights reserved.