Abstract
Water streams and groundwater containing fluoride ions above permissible levels present considerable environmental and health hazards. Consequently, a study has been undertaken to assess the applicability of Al- and La- loaded zeolite in removing fluoride from water under varying process conditions such as agitation rate, sorbent mass and initial fluoride concentration.Using a fixed sorbent mass, equilibrium data were generated. These data were correlated with the two-site Langmuir(L), Freundlich(F), Langmuir-Freundlich(LF), Redlich-Peterson(RP), Tóth(T) and Dubinin-Radushkevitch(DR) isotherm models . Error analyses showed that all the isotherms correlated well the experimental data suggesting that the different active sites on Al- and La-loaded zeolites had varying affinities for fluoride. From the DR model, the calculated mean free energies of sorption were 11.78 kJ/mol for Al-loaded zeolite and 7.67 kJ/mol for La-loaded zeolite suggesting that fluoride removal from the solution by Al-loaded zeolite proceeded through ion exchange while physical adsorption was the dominant mechanism when using La-loaded zeolite. The transient sorption curves obtained from the dynamic experiments using Al-loaded zeolite showed a two-stage phenomenon. The first stage, where the bulk of fluoride was removed, lasted a few minutes while the second stage lagged over the entire sorption period. Simple dynamic models such as the pseudo-second order, Elovich, film and empirical pore diffusion model were used to determine the reaction and mass transfer parameters. The reaction kinetic was consistent with the Elovichian mechanism and is supported from the interactions between fluoride and protonated and non-protonated active sites. The film and pore diffusion coefficients were slightly influenced by the process variable. The Al-loaded zeolite showed promising results and is proposed for further tests to evaluate its robustness.
