Kinetic, isothermal and thermodynamic modeling of fluoride ion adsorption by raw and activated clays

The objective of this study is to identify the different mechanisms involved in the phenomenon of adsorption of fluoride ions (F-) on raw and activated clays through some empirical models. The methodology is based on the application of a few kinetic (pseudo first order and pseudo second order), isothermal (Langmuir and Freundlich) and thermodynamic models on the experimental data of the adsorption of F- ions by plotting linear regression lines for determine the different characteristic parameters of each model. The results obtained show that the pseudo second order kinetic model better describes the adsorption kinetics of F- ions on raw and activated clays with correlation coefficients close to unity (range from 0.9994 to 0.9999) and similar values between experimental (0.37 to 0.676 mg.g^-1) and theoretical (0.387 to 0.684 mg.g^-1) capacities. The Freundlich model better describes the adsorption isotherm of F- ions on raw and activated clays with constants (K_F) less than unity (0.175 to 0.953) and value of n (heterogeneity factor) which tend towards unity (0.399 to 0.921) with the exception of Arg1-2C and Arg3-2C. The thermodynamic model shows an endothermic, spontaneous process and an adsorption of physical and electrostatic type. The models used therefore apply well to the experimental data on the adsorption of F- ions on raw and activated clays because they have made it possible to identify the kinetics and the isotherm of this adsorption.