KINETICS AND EQUILIBRIUM STUDIES OF FLUORIDE ADSORPTION FROM DRINKING WATER ONTO ALUMINIUM MODIFIED ZEOLITE NA-LSX

Abstract

Fluoride related health issues such as dental and skeletal fluorosis have become a global concern in recent times. Adsorption is one of the affordable defluoridation techniques. This study reports the potentiometric determination of fluoride adsorbed onto Aluminium modified zeolite Na-LSX (AMZL) from aqueous solution at 25oC and various experimental conditions: pH (4.5-9), initial fluoride concentrations (3-15 mg/L), adsorbent dosage (2.5-12.5 g/L) and contact time (4-6 days) in a batch system. The obtained adsorption data was fitted to Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms. Error functions, χ2, SSE, SAE were used in conjunction with R2 to ascertain the best fitted isotherm. The various kinetic models Lagergren’s pseudo-first-order, Ho’s pseudo-second-order, Intra-particle Diffusion and Elovich models were used to evaluate the kinetics of fluoride adsorption. The maximum fluoride uptake unto AMZL occurred at pH 5.5 over a period of 360 min. An increase in adsorbent dosage and pH affected fluoride removal. The presence of bicarbonate ions had a negative effect on the adsorption of fluoride however, chloride and nitrate did not affect the adsorption of fluoride. By the use of error functions, χ2, SSE, SAE and the coefficient of regression, R2 values, Langmuir isotherm model gave the best description of fluoride adsorption onto AMZL. The maximum fluoride adsorption capacity of AMZL based on Langmuir model was found to be 4.122 mg/g. The Ho’s pseudo-second order kinetics best fitted the fluoride adsorption data obtained. The adsorption mechanism was found to be predominantly physisorption than chemisorption.