Influence of chloride, nitrate, and sulphate on the removal of fluoride ions by using nanofiltration membranes

This study investigates the removal of fluoride from various solutions(NaF, NaCl, NaNO3, and Na-2 SO4) by using three commercial nanofiltration membranes denoted NF70(Filmtec), DESAL5 DL(Osmonics), and MT08(PCI) under 8 bars, 293 K in batch recirculation mode. The membranes were tested for their rejection potential of fluorides in the presence of chlorides, nitrates and sulphates. To identify the mechanisms of selectivity, we successively analyzed the rejections in turn of different combinations of NaF, NaCl, NaNO3, and Na2SO4 in order to simulate the behavior of a real brackish water from Senegal. Fluorides rejection efficiency ranges from 78% to 95%. The efficiency of nanofiltration membranes improved is closely linked to the nature of the solution. The high rejection level(above 98%) of the divalent sulphate ions(50 or 200 mg/L) induces a Donnan effect establishment. In brackish water conditions, for all the membranes, there was a noticeable influence of anion size and hydration energy on fluoride rejection.