ADSORPTION OF FLUORIDE FROM AQUEOUS SOLUTIONS BY CARBON NANOTUBES: DETERMINATION OF EQUILIBRIUM, KINETIC, AND THERMODYNAMIC PARAMETERS

The adsorption of fluoride (F-) onto single-walled carbon nanotubes (SWCNTs) from aqueous solutions was studied by investigating the parameters of contact time, dosage, temperature, pH, agitation speed, and fluoride concentration in the solution. The equilibrium adsorption data were analyzed using four common adsorption models: Langmuir, Freundlich, Temkin, and Doubinin-Radushevich. The results revealed that the Langmuir isotherm fitted the experimental results well. The regression results showed that the adsorption kinetics were more accurately represented by the pseudo-second-order model. Values of activation parameters such as standard free energy changes (Delta G(0)), standard enthalpy change (Delta H-0), and standard entropy change (Delta S-0) were calculated using adsorption equilibrium constants obtained from the Langmuir isotherm at different temperatures. All Delta G(0) values were negative. The (Delta H-0) values and (Delta S-0) values of SWCNTs were 17.39 kJ/mol and 0.1215 kJ/mol K, respectively, indicating that the adsorption was feasible, spontaneous, and an endothermic process in nature.