Adsorption comparison at the α-alumina/water interface: 3,4-Dihydroxybenzoic acid vs. catechol

Adsorption kinetics and isotherms and the surface complexation of 3,4-dihydroxybenzoic acid (3,4-DHBA) and catechol at the α-alumina/electrolyte interface were investigated. The state of equilibrium for adsorption of 3,4-DHBA onto α-alumina surface at pH 5 was attained at 120 min, whereas it was 90 min for catechol, but at pH 10 the state of equilibrium for the both the systems was same (∼60 min). The pseudo-second-order kinetic equation of nonlinear form (Eq. (3)) fits the experimental kinetic data significantly better than the linear form (Eq. (2)) in the entire time duration. The adsorption density of 3,4-DHBA onto the α-alumina surfaces at pH 10 and at similar experimental conditions is equivalent to catechol. DRIFT spectra indicate that 3,4-DHBA forms both outer- and inner-sphere complexes and catechol forms bidentate mononuclear complex with the α-alumina surface.

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► Higher adsorption density of 3,4-DHBA at pH 10 is due to adjacent –OH groups.
► Deprotonation of –OH groups of 3,4-HDBA at pH > 9 produces maximum adsorption density.
► 3,4-DHBA forms both outer- and inner-sphere surface complexes with α-alumina surface.
► Catechol forms bidentate mononuclear surface complexes with α-alumina surface.