Removal of fluoride from multicomponent water solutions with the use of monovalent selective ion-exchange membranes

Abstract

Fluorine is a chemical element which is often present in natural environment. According to the World Health Organization (WHO) standards fluoride content in drinking water cannot be higher than 1.5 mg F−/L. Nitrate was also found in many regions in the world. According to guidelines its content in drinking water cannot exceed 50 mg NO3−/L. Ingestion of fluoride and nitrate in excess leads to various health issues. There are many methods of fluoride removal, e.g. reverse osmosis, ion exchange or electrodialysis (ED). The aim of the research was to evaluate the influence of nitrate on fluoride separation by electrodialysis with monovalent selective ion-exchange membranes (PC-MVA and PC-MVK). During experiments the laboratory installation PCCell BED-1-System was applied. The ED tests were conducted at constant current density (0.78, 1.72, and 2.34 mA/cm2). Model solutions containing fluoride (5, 10, 15 mg F−/L), nitrate (15, 30, 45 mg NO3−/L) and sodium chloride (0.5 g NaCl/L) were applied. The obtained results showed that the ED process with monovalent selective ion-exchange membranes allowed to decrease fluoride content in product water below the WHO guidelines. It was observed that competition between nitrate and fluoride ions occurred during their transport through the anion-exchange membranes. The energy demand was dependent on the current density as well as on the salt concentration in treated solutions.