Factors Influencing the Removal of Fluoride from Groundwater by Nanofiltration

The influence of co-existing chloride, sulfate, pH, and temperature to defluoridation of groundwater using nanofiltration membrane and membrane fouling were studied. The results show that nanofiltration has retention effect to all co-existing ions. When feed fluoride concentration was less than 6 mg/L, the fluoride concentration in permeated water was less than 1.2 mg/L which could reach the drinking water standard in China rural area. The fluoride retention rate decreases with the increase of feed fluoride, co-existing ions concentration or pH. The decreasing percentage of fluoride retention rate is the same when Cl^- concentration increased 1 mol or SO₄ ^2- concentration increased 0.5 mol. The fluoride retention rate increase when feed chloride concentration exceeded 220 mg/L. But the variation had no significant change. The fluoride retention rate increases with the increasing of pH, while there is a milestone on pH 4.4. Environmental scanning electron microscope (ESEM) and its energetic chart analysis apparatus, X-ray Diffration(XRD) analysis show that the main fouling component was CaCO₃. The fouling membrane could be recovered flux using citric acid and ammonia cleaning method.