Kinetic and Thermodynamic Parameters for the Adsorption of Methylene Blue Using Fly Ash under Batch, Column, and Heap Leaching Configurations

In this article, the use of fly ash, a low-cost adsorbent, for the removal of a toxic cationic dye, methylene blue (MB), from aqueous solution has been investigated. Fly ash is a waste by-product generated from coal combustion. Contrary to many other reported investigations, the fly ash was not treated prior to the experiments for this investigation. The study was done in order to ascertain the viability of using the waste material to remove cationic dyes from wastewater. While MB adsorption has been described often in batch studies, batch, column, and heap leaching studies were carried out under the same conditions (where pH was kept neutral) in this work. Kinetic and thermodynamic studies were carried out during batch operation where the temperature was varied and the adsorbent dosage was kept constant. Adsorption isotherm studies were carried out during batch and column operation where the temperature was kept constant and the adsorbent doses were varied. The experimental f indings showed that the removal of MB was best in column and heap leaching operations, removing more than 98% of the MB, with the highest adsorption capacity found in the column operation. Batch adsorption favoured Lagergren pseudosecond-order kinetics. Langmuir, Freundlich, and Temkin adsorption isotherm models were investigated. Both the batch and column studies best fit the Freundlich adsorption isotherm. Thermodynamic parameters G°, H°, and S° were determined, and it was shown that the adsorption of MB on fly ash is a spontaneous, endothermic, and dissociative process on a laboratory scale. The activation energy for the process indicates a chemisorption process. Preliminary economic evaluation and experimental findings suggest that column operation would be the most feasible for practical implementation.