Electrochemical removal of fluoride ions from industrial wastewater

Fluoride ions were removed electrochemically from a solution using a combined electrocoagulation and electroflotation process. For an influent fluoride concentration of 15 mg/l, a value after lime precipitation, the effluent fluoride concentration can be lower than 2 mg/l when the pH in the coagulation cell is around 6, charge loading is at 4.97 F/m^3 water, and the residence time is 20 min. Even lower effluent concentration can be achieved if 50 mg/l of Fe^3+ or Mg^2+ are added into the coagulation unit. The anions generally reduce the fluoride removal efficiency except Cl- whose corrosion pitting of the electrode can result in 130% current efficiency. The composition of the sludge produced from the operation was analyzed using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscope system (ToF-SIMS). The characterization results show that the defluorination is a chemical adsorption process with F- replacing the –OH group from the Aln(OH)3n flocs.