Adsorbent ability of lignin-based activated carbons for the removal of p-nitrophenol from aqueous solutions

This paper reports the p-nitrophenol (PNP) removal from aqueous solutions by adsorption onto active carbons (ACs). Nine ACs were prepared from acid-precipitated eucalyptus kraft lignin following a two-step process consisting in CO2 partial gasification (750-850 degrees C) after carbonization (350-800 degrees C) in N-2 atmosphere. The amount adsorbed ranged from 1 to 4.4 mmol/g, and it is related to the initial concentration of adsorbate, temperature, pH. burn off of the activated carbons, and contact time. The equilibrium results were fitted by the Temkin, Dubinin-Radushkevich, and Guggenheim-Anderson-de Boer equations. The main thermodynamic magnitudes were estimated as well, and their values indicated that the adsorption processes were spontaneous and exothermic. The kinetic study showed that the processes are of apparent second order related to the concentrations of the empty active sites on the ACs surface. The values of the effective diffusivities have been calculated and they have suggested that the internal diffusion controls the net mass transfer. The results obtained in the present work can be for the benefit of the preparation of new adsorbents, as well as the primary design of the adsorption equipment with separation or environmental purposes. (C) 2012 Elsevier B.V. All rights reserved.