Influence of solution parameters on europium(III), $\alpha$-Al2O3, and humic acid interactions: Macroscopic and time-resolved laser-induced luminescence data | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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

Journal Article

### Auteurs:

Janot, Noemie; Marc F. Benedetti; Reiller, Pascal E

### Source:

Geochimica Et Cosmochimica Acta, Volume 123, p.35–54 (2013)

### URL:

http://www.sciencedirect.com/science/article/pii/S0016703713004900

### Résumé:

Abstract 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{&}{\#}xa0;⩾{&}{\#}xa0;7. In the ternary system, adsorption of both Eu(III) and \PAHA\ onto the surface decreases with ionic strength. At I{&}{\#}xa0;={&}{\#}xa0;0.01{&}{\#}xa0;M NaClO4, Eu(III) luminescence decay is much faster than at I{&}{\#}xa0;={&}{\#}xa0;0.1{&}{\#}xa0;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{&}{\#}xa0;$μ$M Eu(III) is reached for 16{&}{\#}xa0;mgPAHA{&}{\#}xa0;l{\textminus}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.