Kinetics Studies of Adsorption and Desorption of South African Fly Ash for Some Phenolic Compounds

The extent of adsorbability and desorbability of an unclassified South African coal fly ash (SACFA) from the Lethabo power plant was evaluated in the treatment of phenol, 2-nitrophenol, and 4-nitrophenol from aqueous solutions. Batch studies were performed to evaluate the effects of various experimental parameters such as adsorbent dosage, initial pH (pHo), and contact time on the removal of these three adsorbates. The adsorption isotherm indicates that the Freundlich model correctly fits the experimental data for the adsorbates better than the Langmuir model, with fly ash having the highest adsorption capacity of 6.51 x 10-2 mg/g for 4-nitrophenol, 2-nitrophenol 6.00 x 10-2 mg/g, and phenol 6.31 x 10-2 mg/g. SACFA was found to adsorb 90.2% of phenol, 88.9% of 2-nitrophenol, and 92.6% of 4-nitrophenol at an initial concentration of 20 mg/l. The desorption of the adsorbates from the surface of the fly ash was achieved using 30 ml of distilled water at 307 K. The analyses of the results obtained revealed that the desorption of 4-nitrophenol was the most difficult of the three adsorbates to be desorbed. The desorption efficiencies obtained were 17.9% for phenol, 18.8% for 2-nitrophenol, and 10.2% for 4-nitrophenol.