Precipitation and adsorption during fluoride removal from water by calcite in the presence of acetic acid

ABSTRACT

Defluoridation of groundwater is of major concern in several places around the world, such as North and East Africa, India and China. In this paper, removal of fluoride from model solutions and a Tunisian groundwater sample was performed using calcite particles in the presence of acetic acid in batch experiments. After 1 h decantation, dead-end microfiltration was further used to retain the fluorite (CaF₂) precipitates present in the supernatant. At 5 mg L⁻¹ fluoride concentration, removal efficiency was found to increase from 17.4% without acid to 30.4% with 0.1 M acetic acid. The increase in fluoride removal with acetic acid was mainly attributed to the renewal of the area available for adsorption on the calcite particles. At the fluoride concentration of 50 mg L⁻¹, the removal efficiency was equal to 9.5% without acid and 94.3% with the addition of 0.1 M acetic acid. The removal of fluoride was attributed to precipitation as a result of higher supersaturation in the solution. Optimum parameters were selected for the defluoridation of a Tunisian groundwater sample with initial fluoride concentration around 2.7 mg L⁻¹. The final F⁻ concentration after treatment with calcite in the presence of 0.1 M acetic acid was found equal to 1.2 mg L⁻¹, which was below the standard of the World Health Organisation.

Keywords:

• Acetic acid
• Calcite
• Defluoridation
• Fluoride
• Microfiltration
• Precipitation
• Adsorption

Acknowledgements

We thank M. Ruben Vera (Centre de Diffractométrie Henri Longchambon, Université Claude Bernard Lyon 1, Villeurbanne, France) for providing the X-Ray diffraction pattern. We thank Audrey Minost (Laboratoire d’Automatique et de Génie des Procédés, Villeurbanne, France) and Xavier Jaurand (Centre des Microstructures, Villeurbanne, France) for the SEM-EDS measurements.