Adsorptivities of Substituted Benzoic Acids on Activated Carbons in Aqueous Solution

Abstract

The equlibrium and rate of adsorption of thirteen mono-substituted benzoic acids (Table 2) on granular activated carbons (Table 1) in aqueous solution were examined. The adsorption isotherms were conformed to the Freundlich equation (Figs.1'-'4, Table 3). Independently of the kinds of the carbon, Traube's rule was applicable to the adsorption of the m-substituted benzoic acids except isophthalic acid. The adsorption capacity regularly increased with decrease in the solubilities (Figs.1, 3). In the case of the o-substituted benzoic acid, Traube's rule was not applicable, and the adsorption 'capacity decreased in the following order of the substituents (Figs.2, 4). NH2H Cl-18Cl OHNO2 COOH at CR40.01-0.1 ClCH84HNH2OHNO2C0OH at CR40.1-4where CR is the ratio of the equilibrium concentration to the saturated concentration. It is likely that the adsorptivities of the o-substituted compounds are affected complicatedly by the ortho effects. In all cases, it seems that the carbon surface does not directly interact with the functional groups of the adsorbates, but with the aromatic ring. The adsorption rates were estimated by the intraparticle diffusivities, which little varied with the kinds of adsorbates at low concentrations. But in high concentrations, the diffusivities of the o-substituted compounds exceeded those of the m-substituted ones. It is suggested that the effective molecular radii of the o-substituted benzoic acids are smaller than those of m-substituted ones in aqueous solution.