Activated carbons from lignin: Their application in liquid phase adsorption

The extent of the methylene blue (MB) adsorption from an aqueous solution is a convenient indicator in the evaluation of activated carbons. The adsorption of MB ( cationic dye) from aqueous solution has been studied using twenty activated carbons. The activated carbons were prepared from acid-precipitated eucalyptus kraft lignin following a two-step process consisting of CO2 partial gasification after carbonization in N-2 atmosphere. The adsorbed amount was studied as a function of the contact time, temperature, pH, concentration of adsorbate, and burn-off of the activated carbons. The equilibria results obtained in a batch contactor were fitted by the Langmuir equation. The calculated values of Delta G demonstrate that the adsorption of the dye onto these activated carbons occurs by physical adsorption. Both the apparent values of Delta H and Delta S are positive, indicating that the adsorption process is endothermic and can produce spontaneously in our experimental conditions. The kinetic study was developed using a second-order exponential decay equation and the results were correlated using the Lagergren first-order equation relative to the concentration on the solid phase. The intraparticle diffusion coefficients have been estimated on the basis of an internal diffusion controlling mechanism for the net adsorption rate.