Fluoride removal from waters by Donnan dialysis

Excess or lack of levels of fluoride in drinking water being harmful to human 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 permitted value. Two synthetic waters, which are models of waters from countries of Africa (Maghreb, Senegal), were prepared and treated with a DD pre-industrial 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 studied under two circulating modes of the receiver solution, single pass and batch, while the feed solution flowed continuously as a single pass. To maintain 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 aluminum-fluoride compounds was studied in order to define the optimized conditions of pH and concentration. Despite the different anions (Cl-, HCO3-, SO42-) 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 be reached whatever the water treated. However, the mineralization of water was increased by about 25% resulting from the electrolyte diffusion which occurs from the receiver to the feed solution. (C) 2000 Elsevier Science B.V. All rights reserved.