Adsorption of substituted benzoic acids onto α-Al2O3 surface in mixed-adsorbate mode: 2,4-Dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid and 1,2,4-benzenetricarboxylic acid (trimellitic acid)

Abstract

Influence of position and type of functional groups (COOH and phenolic OH groups) in a benzene ring on the adsorption of three natural organic matter (NOM) analogues e.g. 2,4-dihydroxybenzoic acid (2,4-DHBA), 2,6-dihydroxybenzoic acid (2,6-DHBA) and trimellitic acid (1,2,4-benzenetricarboxylic acid) onto α-alumina surface are explored in terms of adsorption kinetics and isotherms in single- and mixed-adsorbate mode. Focusing at the positional functional groups, adsorption profile of 2,4-DHBA and 2,6-DHBA is compared in single- and mixed-adsorbate mode. 2,4-DHBA produces higher adsorption density than 2,6-DHBA in both single- and mixed-adsorbate mode, governing factor is the ortho-para position of phenoilc OH group in 2,4-DHBA for higher adsorption density. Concerning type of functional groups, adsorption profile of 2,4-DHBA is also compared with trimellitic acid. Here again 2,4-DHBA produces higher adsorption density than trimellitic acid in single-adsorbate mode. In contrast, in mixed-adsorbate mode both adsorbates exhibit roughly equal adsorption densities indicating competition of co-operative nature, which is also supported by their activation energy values in mixed-adsorbate mode. Nevertheless, there exists a competition for the same surface sites of α-alumina that results in reduction of adsorption densities in mixed-adsorbate mode than that of the single-adsorbate mode.