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 °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 (ΔH 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 pseudosecond 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.
In this work, the use of an agro-industrial waste, i.e., sunflower seed
shells, was investigated as a sorbent for the removal of Orange 16 reactive
dye from aqueous environments. Batch experiments were performed as a function
of pH, sorbent dose, dye concentration, temperature and contact time. The
percent dye removal increased with increasing sorbent dose and temperature of
the aqueous solution, and decreased with increasing dye concentration; the
required contact time was five hours. The Freundlich, Langmuir,
Dubinin-Radushkevich and Tempkin adsorption isotherms were used to describe
the equilibrium sorption data and to determine the corresponding isotherm
constants. The thermodynamic parameters ?G, ?H and ?S were also determined.
These parameters indicated that the sorption of reactive dye onto sunflower
seed shells was a spontaneous, endothermic and entropy-driven process. The
kinetic data were evaluated by pseudo-first order, pseudo-second order and
intra-particle diffusion kinetic models. The results of the kinetic study
indicated that the sorption of Orange 16 reactive dye onto sunflower seed
shells is a complex process and both chemical surface sorption and
intraparticle diffusion contribute to the rate-limiting step. Therefore, the
sunflower seed shell showed itself to be a promising cheap sorbent for the
decolourization of aqueous coloured solutions or effluents.
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