Influence of functional groups on the adsorption behaviour of substituted benzoic acids at the α-alumina/water interface

Adsorption kinetics of hemimellitic and trimellitic acids at a fixed initial concentration of 0.30 mM onto α-alumina surfaces were carried out in batch method in the presence of 0.05 mM NaCl(aq) at pH 5 and 288.15, 298.15, 303.15, and 313.15 K. The experimental data were significantly better fitted to pseudo-second-order kinetic equation of non-linear form in the entire time duration. The state of equilibration time of benzenetricarboxylic acids at 298.15 K follows the sequence: hemimellitic acid < trimellitic acid < trimesic acid. Adsorption isotherms of hemimellitic and trimellitic acids were carried out at 298.15 K, pH 5–10, and 0.05 mM NaCl(aq) by varying acid concentration. The adsorption data were fitted to Langmuir adsorption model. Adsorption density of benzenetricarboxylic acids follows the sequence: trimesic acid < trimellitic acid < hemimellitic acid. Comparison of the adsorption densities of benzenetricarboxylic acids with dihydroxybenzoic and monohydroxybenzoic acids indicates that (i) in benzenetricarboxylic acids the position and equally the number of –COOH groups, (ii) the two phenolic –OH groups at any two position in the benzene ring of dihydroxybenzoic acids, and (iii) the phenolic –OH groups at ortho position in the benzene ring of monohydroxybenzoic acids are the governing parameters for yielding higher adsorption density onto α-alumina surface. Exception is the 2,4-dihydroxybenzoic acid amongst dihydroxybenzoic acids series, which yields highest adsorption density due to ortho and para position of the phenolic –OH groups. The thermodynamic data indicate that the adsorption process is spontaneous for both the systems. The magnitude of shifting of νs(–COO−) and νas(–COO−) after adsorption of trimellitic and hemimellitic acids onto α-alumina surface indicate that both acids form outer-sphere surface complexes.