Removal of Tannin from Aqueous Solution by Adsorption onto Treated Coal Fly Ash: Kinetic, Equilibrium, and Thermodynamic Studies

The study investigated the performance of tannin adsorption on treated coal fly ash. The adsorption capacity of coal fly ash could be enhanced through appropriate pretreatment by acid. There was an apparent increase of tannin adsorption when fly ash dosage changed from 0 to 20 g/L. The adsorption isotherm data fitted to Freundlich and Langmuir models. The adsorption process followed pseudo-second-order kinetics. The adsorption processes presented a two-stage pattern, a rapid adsorption process followed by a slow adsorption process. The adsorbed amount of tannin was higher at 20 °C than other temperature levels. The thermodynamic studies indicated the adsorption of tannin onto fly ash was exothermic in nature and the entropy of the system decreased during adsorption process. The removal efficiency of tannin adsorption onto fly ash decreased when NaCl concentration ranged from 0 to 0.4 mol/L.