Defluoridation of drinking water using Al³⁺-modified bentonite clay: optimization of fluoride adsorption conditions

Abstract

Al³⁺-bentonite clay (Alum-bent) was prepared by ion exchange of base cations on the matrices of bentonite clay. Intercalation of bentonite clay with Al³⁺ was performed in batch experiments. Parameters optimized include time, dosage, and Al³⁺ concentration. Physicochemical characterization of raw and modified bentonite clay was done by X-ray fluorescence, X-ray diffraction, energy dispersive X-ray spectrometry attached to scanning electron microscopy, Brunauer–Emmett–Teller analysis, cation exchange capacity (CEC) by ammonium acetate method, and pH_pzc by solid addition method. Chemical constituents of water were determined by atomic absorption spectrometry (AAS), ion selective electrode (Crison 6955 Fluoride selective electrode) and a Crison multimeter probe. For fluoride removal, the effect of contact time, adsorbent dosage, adsorbate concentration, and pH were evaluated in batch procedures. The adsorption capacity of fluoride by modified bentonite clay was observed to be 5.7 mg g⁻¹ at (26 ± 2) °C room temperature. Maximum adsorption of fluoride was optimum at 30 min, 1 g of dosage, 60 mg L⁻¹ of adsorbate concentration, pH 2–12, and 1:100 solid/liquid (S/L) ratios. Kinetic studies revealed that fluoride adsorption fitted well to pseudo-second-order model than pseudo first order. Adsorption data fitted well to both the Langmuir and Freundlich adsorption isotherms, hence, confirming monolayer and multilayer adsorption. Alum-bent showed good stability in removing fluoride from ground water to below the prescribed limit as stipulated by World Health Organization. As such, it can be concluded that Alum-bent is a potential defluoridation adsorbent which can be applied in fabrication of point of use devices for defluoridation of fluoride-rich water in rural areas of South Africa and other developing countries. Based on that, this comparative study proves that Alum-bent is a promising adsorbent with a high adsorption capacity for fluoride and can be a substitute for conventional defluoridation methods.

Keywords:

• adsorption
• fluoride
• bentonite clay
• defluoridation
• cation exchange capacity
• kinetics
• isotherms

Acknowledgments

The authors wish to convey their sincere gratitude to Research and Innovation Directorate, Department of Ecology and Resource Management, School of Environmental Sciences, University of Venda, Council of Scientific and Industrial Research (CSIR), ESKOM-TESP, SASOL-INZALO, National Research Foundation (NRF) and Department of Science and Technology (DST) (DST/NRF) for funding this project.