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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