Equilibrium study of selected ion exchange reactions
Date
2016
Authors
Khanyile, Sabelo Mlungisi
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Abstract
The thermodynamic and equilibrium data for Na+/RH+ exchange reactions were generated using the batch method. The experimental data is unique in that the reactions were carried out at elevated temperatures (50oC–90oC), using low level counter ion solutions (0.001100–1.258 mg/l, Na+). The equilibrium data was modeled against the non-linear Langmuir and Freundlich isotherm models. The model parameters were determined by an iterative approach, using the ‗SOLVER‘ function of Microsoft Excel. The coefficient of determination, R2 was used to evaluate the goodness of fit. The R2 values for both models were satisfactory (R2 > 0.9) to a certain extent. From the experimental data, the equilibrium parameters; Selectivity coefficient, , and Corrected selectivity coefficient KI were computed. Irrespective of the initial solution concentration, the equilibrium parameters were found to decrease with increasing reaction temperature, indicating a poor uptake of Na+ from solution by candidate ion exchange resins. Thermodynamic parameters; enthalpy of reaction, ΔH (J/mol.K), and entropy, ΔS (J/mol.K) were calculated from the slope and intercept of the Van‘t Hoff plot respectively. The reaction‘s Gibbs free energy change, ΔG was calculated from the values of ΔH, and ΔS, and reaction temperature. Generally, ΔG was positive and increased with increasing reaction temperature, which proved that an elevated reaction temperature favours the reverse reaction. The reactions were found to be exothermic process with negative entropy. The effect of pH on the exchange reactions was also studied. There was a decrease in the equilibrium Na+ concentration in solution after pH conditioning, with a considerable difference when compared with that obtained from reactions without pH conditioning. The equilibrium quotient was used to define the selectivity coefficient for reactions carried out at varying pH. The equilibrium quotient increased across the initial solution concentration and was highest for reactions carried out at 1.258 mg/L. The equilibrium quotient decreased across the pH range, per initial solution concentration, indicating the pH effect.
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. May 2016.