Effect of some physical and chemical parameters on fluoride removal by nanofiltration

In natural waters, fluoride ions are necessary and beneficial for the human being. At higher level of F⁻ in water, it is toxic and detrimental to human health, leading to serious problems such as dental and skeleton fluorosis. According to the World Health Organization, the acceptable concentrations of fluoride in potable water are in the range of 0.7–1.5 mg L⁻¹. Various treatment technologies for fluoride removal from water have been used such as ion exchange, adsorption and membrane processes. In the present study, removal of fluoride ions from aqueous solutions was investigated using a polyamide thin film composite nanofiltration membrane denoted as HL 2514 T from Osmonics Company. Through this membrane, the mechanism of transport was investigated. The Kedem–Katchelsky model was applied in order to determine phenomenological parameters σ and P _s, respectively, the reflection coefficient of the membrane and the solute permeability of ions. The convective and diffusive parts of the mass transfer were quantified. The retention of monovalent and bivalent salts by this membrane shows that it is negatively charged. In the second part, retention of fluoride anions was investigated. Results show that the retention of fluoride by HL membrane exceeds 80%. The influence of the chemical parameters (feed concentration and ionic strength) and the physical parameters (applied pressure and recovery) on the elimination of fluoride was studied.