Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes

Too many or too few fluoride ions in drinking water are harmful to the consumer's health. The acceptable fluoride concentration is generally in the range of 0.5 to 1.5 mg.L-1. In the present study, Donnan dialysis (DD) with an anion-exchange membrane (AEM) was applied for the defluoridation of diluted NaF solutions. The initial concentration of the feed solution was maintained at 10(-3) mol.L-1, corresponding to a 19 mg.L-1 fluoride concentration. Five kinds of AEMs (DSV, AFX, AFN, AMX, ACS) were tested. First, membrane properties were studied at equilibrium. The values of the exchange capacity of the membranes in Cl- and F- form, water content, selectivity coefficient for the Cl-/F- exchange, diffusion coefficient of Cl- and F- ions in the membrane, were determined for each membrane. DD experiments, performed using a laboratory cell, showed that the DSV membrane is the most effective AEM, despite its electrolyte leakage. Subsequently, a pre-industrial pilot with a total membrane area of 1760 cm(2) was used to study the different physico-chemical and hydrodynamic parameters of the process. As the driving ion, the chloride ion is more efficient than the sulfate ion. At flow rates lower than 0.6 L.h(-1), the fluoride concentration remains lower than the permitted values despite the presence of others anions generally present in ground water such as chloride, sulfate and bicarbonate ions.