Adsorption of weak and non-electrolytes by activated carbon

Abstract

Four activated carbons, one of them commercially available Darco active carbon and the remaining three from a series of coconut charcoals steam activated to varying degrees, are used to study the adsorption of weak and non-electrolytes. Introduction of —CI into the CH 3COOH molecule increases adsorption onto activated carbon while —OH and —NH 2 have the opposite effect. Substitution in the benzene ring shows that adsorption from aqueous solutions is in the order —NH 2, >—OH, >CO 2, >(—OH + —COOH). Effect of polarity of solvent on adsorption capacities is studied in H 2O, CHCl 3 and C 6H 6. Substitution of —Cl into the CH 3COOH molecule invariably increases the adsorption irrespective of the polarity of the solvent. On introducing a specific group in the benzene ring the adsorption is in the order: aniline ⩾ phenol > benzoic acid > salicylic acid in H 2O medium but in CHCl 3 and C 6H 6 media aniline ⩾ phenol > salicylic acid > benzoic acid. However, on comparing the results of individual adsorbates in three media, generally the magnitude of adsorption is H 2O > C 6H 6 ⩾ CHCl 3. Adsorption of lower aliphatic acids (formic to caproic) from aqueous solutions increases regularly as one ascends the homologous series—a behaviour known as Traube's rule; however, of all the surface area available for N 2 adsorption, only a fraction of it is available for adsorption of the aliphatic acids.