Application of coal fly ash for treatment of wastewater containing a binary mixture of copper and nickel

The adsorptive properties of coal fly ash in removing copper and nickel from waste water was investigated using batch and fixed bed column studies. Various process parameters such as solution pH, solid loading, residence time, temperature, bed height and flow rate were tested to determine the effectiveness of fly ash. The study revealed that fly ash is an effective adsorbent in the selective separation of copper and nickel from binary mixtures. Copper showed a higher competitive strength and greater affinity to active sites as it was easily adsorbed than nickel due to its smaller atomic radius and higher electronegativity. Analysis of the batch studies results indicated that the removal process followed the Pseudo Second Order kinetic model and the modified Langmuir isotherm (q_m of 12.18 and 1.99 mg/g for copper and nickel respectively). Values of ΔGº, ΔHº and ΔSº showed a more energetically favourable sorption process which is endothermic in nature (ΔH^o of 50 and 62 kJ/mol for copper and nickel respectively), a random solid – liquid interface and a low adsorbate – adsorbent interaction. The Thomas model better described the mechanism for adsorption of copper and nickel in the fixed bed column study and it agreed with the models in batch studies because the model predicts that the sorption process follows the Langmuir isotherm and was derived based on the second order kinetics. Generally, the study found that the predominant mechanism of adsorption was chemisorption which is defined by higher heating values, a unimolecular layer and a homogeneous approach.