Abstract

Dental and skeletal fluorosis are known to be caused by excessive fluoride ingestion particularly from drinking water sources. Dental fluorosis is common in some parts of Malawi but studies on fluoride levels of drinking water sources have not been adequately done. This paper presents our findings in fluoride levels of some drinking water sources in selected rural areas of Southern Malawi and studies on the potential of locally sourced bauxite in water defluoridation at laboratory scale. The work has revealed high levels of fluorides (>1.5 mg/L) in some parts of the study area. A positive correlation was observed between the pH of the water and fluoride concentrations. No correlation existed between fluoride concentration and electroconductivity. Experiments on water defluoridation with bauxite showed that the raw bauxite has a capacity of 93.8 % at a dose of 2.5g/200 ml of 8 mg/L F^- solution. Powder X-ray diffraction characterization of the raw bauxite showed gibbsite (Al(OH)₃) and kaolinite (Al₂Si₂O₅(OH)₄) as the major components. The high defluoridation capacity of the bauxite is thus attributable to gibbsite and kaolinite minerals. Precipitation of AlF₃ is predicted to be the fluoride removal mechanism with the gibbsite while exchange of OH^- groups in the gibbsite layers of the kaolinite with F^- ions occurs in the kaolinite component of the defluoridation material. Evaluated by the residual fluoride concentration in solution the fluoride uptake reaction kinetics of the system was found to be consistent with pseudo-first-order kinetics. 

Key words: Fluoride, bauxite, Southern Malawi, gibbsite, kaolinite.