Cationic dyes removal by multilayer adsorption on activated carbons from lignin

The adsorption of two cationic dyes: fuchsine (FS) and malachite green (MG), from aqueous solutions has been studied using activated carbons (ACs) prepared from eucalyptus kraft lignin. The great specific surfaces of these activated carbons have led to high capacities of adsorption in the equilibrium. They were in the ranges from 1.8 to 3 mmol/g for FS, and 1.7 to 4 mmol/g for MG. The Guggenheim–Anderson–de Boer isotherm model has been applied to explain the experimental data. We have developed the thermodynamic studies from the equilibrium data. Both the first and the second adsorption layers involved apparent endothermic physical adsorption processes. The kinetic tests ranged the main variables. The ACs showed high adsorption rates and yields for the first 10 min of contact time suggesting interesting practical possibilities to be used at a field scale. A hyperbolic equation has been used to reproduce the experimental decay curves. Because of the controlling mass transfer inside the pores, the effective diffusivities were very low, in the range from 10−13 to 10−11 cm2/s. Their estimated values have been reported for various selected cases.