Influence of hydroxybenzoic acids on the adsorption of Eu(III) onto α,γ-Al2O3 particles in mildly acidic conditions: A macroscopic and spectroscopic study

•Adsorption in the Eu(III) – hydroxybenzoic acid – alumina system is studied.•Adsorption is quantified and luminescence parameters are obtained independently.•Synergistic effect of organic acids adsorption are evidenced in the ternary systems.•A ternary species is clearly shown for the p-hydrobenzoic acid.

The influence of hydroxybenzoic acids (HAHn), namely p-hydroxybenzoic acid (4-hydroxybenzoic acid, HPhbH) and protocatechuic acid (3,4-dihydroxybenzoic acid, HProtoH2), on the adsorption of europium(III) onto α,γ-Al2O3 particles is studied as a function of acid concentration. After measuring the adsorption edge of the Eu(III)/α,γ-Al2O3 binary system, and using the previously studied binary component system Eu(III)/HAHn—Moreau et al. (2015) Inorg. Chim. Acta432, 81—, and HAHn/α,γ-Al2O3—Moreau et al. (2013) Colloids Surf. A435, 97—, it is evidenced that HPhbH does not enhance Eu(III) adsorption onto α,γ-Al2O3 in the Eu(III)/HPhbH/α,γ-Al2O3 ternary system. Conversely, HProtoH2 enhances Eu(III) adsorption onto α,γ-Al2O3 in the Eu(III)/HProtoH2/α,γ-Al2O3 ternary system. Adsorption of the acids are also found higher in the Eu(III)/acid/α,γ-Al2O3 ternary systems as compared with the corresponding binary systems assessing synergetic effects. For high HPhbH concentrations, a ternary surface species involving ≡AlOH surface sites, Eu(III), and PhbH– is evidenced by time-resolved luminescence spectroscopy (TRLS). However, in the Eu(III)/HProtoH2/α,γ-Al2O3 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)/HProtoH2/α,γ-Al2O3 ternary system with TRLS because of the very short decay time of Eu(III) in the presence of protocatechuic acid.