Experimental study of methylene blue adsorption on granular activated carbon

It is hard to remove dye pollutants from wastewater. Adsorption is one of the effective methodologies for dye pollutants removal. The big challenge for adsorption is the determination of the equilibrium point. The equilibrium point may be misidentified when the change in concentration is in the acceptable experimental error range and/or when fitted isotherm parameters do not change over time. This study provides an estimation method based on integrated adsorption kinetics and isotherms to predict the aqueous adsorbate concentration at equilibrium and give a sense of the approach to equilibrium. In addition, the method provides information on how plots of absorbed concentration versus aqueous concentration change over time. To accomplish this, a synthetic model test was initiated in the laboratory using methylene blue as the adsorbate and granular activated carbon as the adsorbent. Two kinetic models (pseudo-first order and pseudo-second order) and three isotherms (linear, Langmuir, and Freundlich) were employed, yielding six integrated models. The pseudo-second order kinetic model with either Langmuir or Freundlich isotherm provides a good estimation of the aqueous adsorbate concentration at equilibrium. While the data collected were not well by any isotherm model, the pseudo-second order kinetic model with the Freundlich isotherm reasonably fit plots of absorbed concentration versus aqueous concentration over time.