Fluoride removal from natural waters by polymer inclusion membranes

A polymer inclusion membrane (PIM) containing commercial ionic liquid methyltrioctylammonium chloride (Aliquat 336) as a carrier and cellulose triacetate (CTA) as a base polymer has been used for the first time to remove fluoride from natural waters. Due to the anion-exchange characteristics of the carrier, a 1 M NaCl solution has been used as a receiving phase. It has been found that the addition of tributyl phosphate (TBP) in the formulation of the PIM increased the membrane's performance. Several parameters have been investigated using a PIM made of 30% CTA, 50% Aliquat 336, and 20% TBP (% in mass) including the effect of other ions such as chloride and sulphate and the effect of pH. Only when the feed phase contains high amounts of chloride (concentration 10 times higher than fluoride) a slight decrease on PIM's efficiency is observed (85% defluoridation). The separation system developed here has been used satisfactorily for fluoride removal from different naturally occurring waters. Moreover, the membrane's reusability has also been investigated, showing that the membrane can be used for four cycles without loss of efficiency in removing fluoride.