Thermodynamics and kinetic study of metal adsorption on coal fly ash based on a binary mixture of copper and nickel

Abstract

The main purpose of this investigation was to study the potential application of Morupule fly ash for the removal of copper and nickel in wastewater. Batch and column study experiments were carried out by varying parameters such as pH, adsorbent dosage, contact time, temperature, bed height and flow rate. The data was fitted into the kinetic models, kinetic isotherms and the thermodynamic parameters to determine the nature of the process. Batch experiments showed a more energetically favourable adsorption process with a negative ΔGº, and positive values of ΔHº and ΔSº, indicating that the process was endothermic with a random solid – liquid interface. The pseudo first order and pseudo second order models were compared and the single metal system showed a better fit for the pseudo first order at 298 K and pseudo second order at 338K while for the binary metal system, the pseudo second order was predominant at both 293 and 313K. The results obtained were also fitted into the modified Langmuir and Freundlich kinetic isotherms and the modified Langmuir isotherm gave the best fit indicating a homogeneous and monolayer approach without interaction between the adsorbed molecules. Breakthrough curves were obtained for the fixed bed column study and the breakthrough time increased with an increase in bed height for copper while for nickel it decreased. As for the flowrate, the breakthrough time was similar at 10 and 20 ml/min for both metal ions but was slightly higher at 15 ml/min for copper and lower for nickel. The data from the column studies followed the Thomas model indicating that the process fits the Langmuir isotherm and is second order, controlled by mass transfer at the interface, which is in agreement with the models in the batch study. This generally indicates that the main mechanism of adsorption was chemisorption.