Adsorption from aqueous phenol and aniline solutions on activated carbons with different surface chemistry

Abstract

Microporous carbons of similar surface area (1200–1500 m 2/g) and porosity but different surface composition were prepared from poly(ethyleneterephthalate) (PET) based activated carbon by chemical (cc HNO 3) and thermal (700 °C) post-treatment. pH, pH PZC measurements and Boehm titration proved that the concentration and distribution of the surface functional groups is different. The waste removal capacity was studied by adsorption from buffered aqueous phenol and aniline solutions. Adsorption isotherms were satisfactorily fitted by both the Langmuir and Freundlich model. Since the parameters of the former approach can be converted to a physical model, however, discussion is based on this fit. The adsorption of phenol and aniline from their aqueous solution is a complex process governed by the pH of the medium, due to the acid/base character of both these molecules and the carbon surface. The surface concentration of the aromatic pollutant molecules was found to be always greater than that of functional groups. Even when electrostatic repulsion is present the graphene layer adsorbs 70–80 molecules/100 nm 2. This means that the major contribution to the adsorption comes from the dispersion effect and enhancement of the interactions is obtained only through attractive electrostatic forces. Adsorbates decorating the graphene edge, however, can limit access of further molecules to the interlayer region. Aromatic monolayers are never completed as the surface layer also contains a considerable amount of water. The weak acid and base studied do not behave symmetrically: the uptake of phenol exhibits a maximum, while a monotonic increase with pH was found with aniline. The pH dependence of the adsorption capacity and the interaction parameter is stronger for aniline than for phenol. The lower water solubility of the aniline does not result in higher adsorption capacity. Its maximum uptake and interaction parameter are lower than those of phenol.