Modelling the adsorption of phenolic acids onto $\alpha$,$\gamma$-alumina particles
Résumé
Adsorption of three phenolic acids, namely parahydroxybenzoic acid (4-hydroxybenzoic acid, H$_2$Phb), protocatechuic acid (3,4-dihydroxybenzoic acid, H$_2$Proto) and gallic acid (3,4,5-trihydroxybenzoic acid, H$_2$Gal) onto $\alpha$,$\gamma$-Al$_2$O$_3$ particles was studied vs. ligand concentration at pH 5.0, and vs. pH. The oxide surface was characterized with both potentiometric titrations and electrophoretic measurements; a difference in the point of zero salt effect (PZSE) and the isoelectric point (IEP) was evidenced, which could be attributed to the presence of impurities or to the heterogeneity of the oxide. The potentiometric titration experiments lead to the determination of a PZSE of 8.5. Moreover, the particular shape of the curves were fitted in the framework of the constant capacitance model (CCM), using FITEQL 4.0 software, to determine the oxide parameters (protolytic properties and site density). The electrophoretic measurements were fitted in the framework of the double diffuse layer model (DLM) and an IEP of 9.5 was determined. The constant-pH isotherms of the acids were fitted using the CCM. Constant-pH isotherms of H$_2$Gal and H$_2$Proto onto the Al$_2$O$_3$ surface sites at pH 5 were similar. Two adsorption sites of different affinities were clearly evidenced for H$_2$Gal and can also be proposed for H$_2$Proto. H$_2$Phb showed a lower affinity for the surface than the two other acids, as the logK$_{sorb}$ for H$_2$Phb is one and a half time lower than the one for H$_2$Proto when adsorption is described with one adsorption site. As expected for a carboxylic acid, adsorption of H$_2$Phb decreased with pH and experimental data were well fitted using three adsorbed species
$\equiv$MOH$_2$H$_2$Phb$^+$, $\equiv$MHPhb$^−$, and $\equiv$MPhb$^−$). Adsorption of H$_2$Proto and H$_2$Gal did not change significantly upon increasing pH, meaning that the different functional groups on the aromatic ring (carboxylate and phenolate) were involved in adsorption as pH increases. Dissolution of the oxide was also estimated by measuring the amount of soluble aluminum at pH 5. Increasing acid concentration promoted dissolution, especially for the low concentration range ([acid] < 3 mmol L$^{−1}$), but higher acid concentration lowers the increase of the solubility increase, likely due to adsorption on surface of an aluminum-organic complex.
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