Fluoride removal from brackish groundwater by directcontact membrane distillation

The direct contact membrane distillation applied for fluoride removal from brackish groundwater was investigated. The self-preparedpolyvinylidene fluoride membrane exhibited high rejection of inorganic salt solutes. The maximum permeate flux 35.6 kg/(m2 hr) wasobtained with the feed solution at 80°C and the cold distillate water at 20°C. The feed concentration had no significant impact on thepermeate flux and the rejection in fluoride. The precipitation of CaCO3 would clog the hollow fiber inlets and foul the membrane surfacewith increasing concentration factor when natural groundwater was used directly as the feed, which resulted in a rapid decline in themodule e ciency. This phenomenon was diminished by acidification of the feed. The experimental results showed that the permeateflux and the quality of obtained distillate kept stable before concentration factor reached 5.0 with the acidified groundwater as feed. Themembrane module e ciency began to decline gradually when the feed continued to be concentrated, which can be mainly attributedto the formation of CaF2 deposits on the membrane surface. In addition, a 300 hr continuous fluoride removal experiment of acidifiedgroundwater was carried out with concentration factor at 4.0, the permeate flux kept stable and the permeate fluoride was not detected.