Comparative adsorption involving ortho- and para-hydroxybenzoic acids in mixed-adsorbate mode onto α-alumina surface: Effect of molecular structure

•Adsorption density in single-adsorbate mode is higher than mixed-adsorbate mode.•OH group in ortho position in salicylic acid causes higher adsorption density.•Negative Gibbs free energy and positive enthalpy support competitive adsorption.•Mode of surface complexation is responsible for the dissolution of minerals.

Adsorption kinetics of o-hydroxybenzoic acid (salicylic acid) and p-hydroxybenzoic acid, in single-adsorbate and mixed-adsorbate mode, in the pH range 5–8 are reported using α-alumina as an adsorbent. Adsorption isotherms for both the acids are compared in mixed-adsorbate mode. Adsorption densities of both the acids onto α-alumina surface from the mixed-adsorbate mode are lower in comparison to single-adsorbate mode since a competition is generated between two acids for the surface sites. o-Hydroxybenzoic acid is more active for the surface sites in comparison to p-hydroxybenzoic acid due to the presence of adjacent phenolic OH group. Diffuse reflectance infrared Fourier transform (DRIFT) spectra indicate that p-hydroxybenzoic acid forms inner-sphere surface complex onto α-alumina in single- and mixed-adsorbate mode at pH 5 and 8 resulting in dissolution of α-alumina. In contrast coexistence of both outer- and inner-sphere surface complexes of o-hydroxybenzoic acid onto α-alumina at pH 5 and 8 is proposed under single-adsorbate mode that results in lower magnitude of dissolution whereas it is outer-sphere surface complex under mixed-adsorbate mode. In the scope of competitive adsorption we conclude that the dissolution of α-alumina primarily takes place by the type of surface complex formed by p-hydroxybenzoic acid.

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