Modeling of fluoride retention in nanofiltration and reverse osmosis membranes for single and binary salt mixtures

High levels of fluoride in groundwater cause a major problem for drinking water quality in many countries around the world. In addition to reverse osmosis (RO), recent publications indicate that nanofiltration (NF) is a promising technology to deal with fluoride removal from aqueous solutions. First of all, a characterization of commercial NF and RO membranes was performed in terms of hydraulic permeability (L p and L p ′) and contact angle (θ). Then, performances of the selected membranes on Cl – , NO 3 – , HCO 3 – , SO 4 2– as well as F – rejection were determined within a laboratory study in simple salt. At 10 mg/L of concentration, pH = 6.7 and 10 bar, the rejections of fluoride with NF270, NF90 and BW30 were 63%, 91% and 97%, respectively. The influence of sulfate, chloride, nitrate and bicarbonate on fluoride rejection was also studied in binary mixtures with different concentration ratios (1/1, 1/2 and 2/1). Unlike the divalent anions, the results have shown that higher concentrations of monovalent anions affect positively fluoride rejection. Experimental data of fluoride rejection were modeled by using Spiegler–Kedem–Katchalsky model, first to confirm the experiment by calculating the non-linear parameter (χ 2 ), and second to evaluate transfer mechanisms (convection and diffusion) of fluoride ions by estimating the phenomenological parameters of mass transfer (σ and P s ) for the selected NF and RO membranes in simple and binary mixture. For NF membranes, a duality between both transfer mechanisms was recorded which was strongly affected by the feed composition.