Industrial Cellolignin Wastes as Adsorbent for Removal of Methylene Blue Dye from Aqueous Solutions

Cellolignin, a by-product from the wood processing industry, was studied as a new, eco-friendly adsorbent for the removal of methylene blue cationic dye from aqueous solutions, using a batch adsorption procedure. Experimental data were processed in order to study the equilibrium, thermodynamics, and kinetics of methylene blue adsorption onto cellolignin. Between the two studied isotherm models (Freundlich and Langmuir) the Langmuir model better described the equilibrium adsorption data at temperatures higher than 25 degrees C; the mean free energy (E) values obtained from the Dubinin-Radushkevich isotherm model show that the sorption of dye occurs via surface electrostatic interactions with the active sites of the cellolignin. The equilibrium data were used to calculate the free energy, enthalpy and entropy changes, and isosteric heat of adsorption(Delta H-X). Results confirm the feasibility and the endothermic nature of the adsorption process, suggesting that adsorption is a physico-chemical process. The isosteric heats of adsorption indicated energetic heterogeneity of adsorption sites and possible interactions between the adsorbed dye molecules. Kinetic assessment suggests that the adsorption process followed a pseudo-second order model and the rate-limiting step may be the binding of dye onto the adsorbent surface. The diffusion models show that intraparticle diffusion is not the sole rate-limiting step; the external mass transfer also influences the adsorption process in its initial period.