Fluoride removal from waters by Donnan dialysis

Excess or lack of levels of fluoride in drinking water being harmful to hum an health, the concentration of F- ions must be maintained in the range 0.5 to 1.5 mg l(-1). The purpose of this study is to apply Donnan dialysis (DD ) for fluoride removal from waters with a concentration exceeding the permi tted value. Two synthetic waters, which are models of waters from countries of Africa (Maghreb, Senegal), were prepared and treated with a DD pre-indu strial pilot. The anion exchange membrane was the DSV membrane (Asahi Glass ), with a total area of 0.176 mt. The initial fluoride concentration was 9. 5 and 6.1 mg l(-1) in each model water, respectively. The DD process was st udied under two circulating modes of the receiver solution, single pass and batch, while the feed solution flowed continuously as a single pass. To ma intain the fluoride concentration below the acceptable values at the outlet of the feed compartment, the extracted fluoride ions are complexed by Al3 ions which were added in the receiver solution. Chemical speciation of alu minum-fluoride compounds was studied in order to define the optimized condi tions of pH and concentration. Despite the different anions (Cl-, HCO3-, SO 42-) and cations (Na+, K+, Ca2+, Mg2+) generally present in ground waters, a fluoride concentration in agreement with the norm (<1.5 mg l(-1)) could b e reached whatever the water treated. However, the mineralization of water was increased by about 25% resulting from the electrolyte diffusion which o ccurs from the receiver to the feed solution. (C) 2000 Elsevier Science B.V . All rights reserved.