Influence of hydroxybenzoic acids on the adsorption of Eu(III) onto $\alpha$,$\gamma$-Al$_2$O$_3$ particles in mildly acidic conditions: a macroscopic and spectroscopic study
Résumé
The influence of hydroxybenzoic acids (HAH$_n$), namely $p$-hydroxybenzoic acid (4 hydroxybenzoic acid, HPhbH) and protocatechuic acid (3,4-dihydroxybenzoic acid, HProtoH2), on the adsorption of europium(III) onto $\alpha$,$\gamma$ Al$_2$O$_3$ particles is studied as a function of acid concentration. After measuring the adsorption edge of the Eu(III)/ $\alpha$,$\gamma$ Al$_2$O$_3$ binary system, and using the previously studied binary component system Eu(III)/HAH$_n$—Moreau et al. (2015) Inorg. Chim. Acta 432, 81—, and HAH$_n$/ $\alpha$,$\gamma$ Al$_2$O$_3$—Moreau et al. (2013) Colloids Surf. A 435, 97—, it is evidenced that HPhbH does not enhance Eu(III) adsorption onto $\alpha$,$\gamma$ Al$_2$O$_3$ in the Eu(III)/HPhbH/ $\alpha$,$\gamma$ Al$_2$O$_3$ ternary system. Conversely, HProtoH$_2$ enhances Eu(III) adsorption onto $\alpha$,$\gamma$ Al$_2$O$_3$ in the Eu(III)/HProtoH$_2$/ $\alpha$,$\gamma$ Al$_2$O$_3$ ternary system. Adsorption of the acids are also found higher in the Eu(III)/acid/ $\alpha$,$\gamma$ Al$_2$O$_3$ ternary systems as compared with the corresponding binary systems assessing synergetic effects. For high HPhbH concentrations a ternary surface species involving $\equiv$AlOH surface sites, Eu(III), and PhbH$^–$ is evidenced by time-resolved luminescence spectroscopy (TRLS). However, in the Eu(III)/HProtoH$_2$/ $\alpha$,$\gamma$ Al$_2$O$_3$ ternary system, chemical environment of Eu(III) is found to be very close to that in the Eu(III)/HProtoH2 binary system. Ternary surface species could not be evidenced in the Eu(III)/HProtoH$_2$/ $\alpha$,$\gamma$ Al$_2$O$_3$ ternary system with TRLS because of the very short decay time of Eu(III) in the presence of protocatechuic acid.
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