Desalination and Defluoridation of Tap Water by Electrodialysis

Abstract

The water resources in Tunisia are very limited and not fairly distributed: 80 % of them are located in the north and only 20 % in the south. In addition, 50 % of them are characterized by a bad quality since they have salinity greater than 1.5 g L-1. The bad quality is not only due to the high salinity, but also to the presence of natural pollutants such as fluoride. In fact, according to physicochemical analyses of samples of tap waters collected by the Regional Service of Environmental Health from different monitoring sites in mining area (south of Tunisia), in February 2014, it was shown that waters were contaminated by fluoride and contained high concentrations of sulphate and chloride anions. Fluoride concentrations varied between 0.8 and 4 mg L-1, and then, greatly exceeded the World Health Organization (WHO) standards in some monitoring sites. In this study, the electrodialysis (ED) technique was applied to remove fluoride from these waters. Experiments were carried out using a pilot unit as a conventional ED in batch recirculation mode. It was shown that ED is an efficient technique to remove fluoride and to reduce salinity of water. Removal rate of fluoride tended to 92 % and the concentrations of different species in the treated water were below the amounts recommended by WHO for drinking water. Considering the effect of chloride and sulphate on fluoride removal through the anion exchange membrane, the performance of two types of ion exchange membrane (PC Cell and Neosepta) was compared. Experiments were carried out with synthetic solutions containing 0.0357 mol L-1 of NaHCO3 doped with 15 mg L-1 of fluoride. Sodium hydrogenocarbonate was replaced by sodium chloride or sodium sulphate at different values of molar ratio in order to maintain the initial ionic strength of the solution constant at 0.0357 mol L-1. It was shown that fluoride removal was influenced by chloride ions but not by sulphate ones.