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International audience; 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.

International audience; Sorptive fractionation of Suwannee River Fulvic Acid (SRFA) and Purified Aldrich Humic Acid (PAHA) on $\alpha$-Al$_2$O$_3$ at pH 6 was probed in the supernatant using different spectroscopic techniques. Comparison of dissolved organic carbon (DOC) analysis with UV/Vis spectrophotometric measurements at 254 nm, including Specific UV absorbance (SUVA) calculation, revealed a decrease in chromophoric compounds for the non sorbed extracts after a 24 h contact time. This fractionation, only observable below a certain ratio between initial number of sites of humic substances and of $\alpha$-Al$_2$O$_3$, seems to indicate a higher fractionation for PAHA. C(1s) near-edge X-ray absorption fine structure spectroscopy (NEXAFS) confirmed this trend and points to a decrease in phenolic moieties in the supernatant and to an eventual increase in phenolic moieties on the surface. Time-resolved luminescence spectroscopy (TRLS) of Eu(III) as luminescent probe showed a decrease in the ratio between the $^5$D0$\rightarrow$$^7$F$_2$ and $^5$D0$\rightarrow$$^7$F$_1$ transitions for the fractionated organic matter (OM) that is thought to be associated to a lower energy transfer from the OM to Eu(III) due to the loss of polar aromatics. These modifications in the supernatant are a hint for the modification of sorbed humic extracts on the surface.