A model for the adsorption of weak electrolytes on solids as a function of pH: I. Carboxylic acid-charcoal systems

Abstract

The adsorption from aqueous solutions on charcoal at 25°C as a function of pH has been obtained for four carboxylic acids: 3,6-dichloro-2-methoxybenzoic acid, 2,4-dichlorobenzoic acid, 2,4-dichlorophenoxy acetic acid, and 2,4-dichlorophenyl acetic acid. The adsorption measurements were treated semiempirically in terms of a general Langmuir-type adsorption process by considering the presence of both ionic and molecular species in the bulk solutions. This semiempirical treatment furnished specific Langmuir adsorption equation parameters which could be interpreted in terms of the surface concentrations at saturation and the desorption energies of the adsorbed species. For the weak electrolytes studied, all the Langmuir parameters except the surface saturation concentration for the uncharged molecules were found to vary in a consistent way with pH. Synergistic adsorption effects were shown to occur at low concentrations in the pH range where ion and molecule bulk concentrations were comparable. Good agreement with observed adsorption measurements was obtained for calculated values obtained from the Langmuir adsorption equation parameters over the entire range of pH used in the measurements. The concentrations of ions and molecules in the bulk solutions were determined from the dissociation constants and mass balance. Similarly, from dissociation constants and mass balance for the adsorbed particles, surface concentrations of ions and molecules were determined at each pH. The ratio of ions to molecules adsorbed onto the surface of the charcoal showed that ions were adsorbed in excess of the molecules throughout much of the pH range for all four weak electrolytes studied.