Precipitative Removal of Fluoride from Electronics Wastewater

Synthetic wastewater was used for the investigation of main factors affecting F- removal performance. The existence of SiF2-6 or PO3-4, and a low initial F- concentration were the main causes of low and unstable F- removal. Lowering the pH reduced the adverse effects of SiF2-6 and PO3-4, and adding over 4 mg/L-1 of freshly formed CaF2 precipitates as seed greatly enhanced F- removal during treatment of low concentration wastewater (50 mg/L-1 F-). The seed-generation method, in which a small portion of wastewater was first mixed with the whole dose of Ca2+ and then mixed with the remaining portion of wastewater, greatly improved F- removal performance and could be used instead of the method of adding seed. The conclusions obtained from the treatment of synthetic wastewater were verified by using several kinds of actual electronics wastewater. The pH should be lower than 9 for efficiently removing F- from wastewater having a high ratio of SiO2 to F-, and an even lower pH was needed for treating wastewater with a relatively high PO3-4 concentration. The seed-generation method was also found to be effective for such wastewater with high concentrations of PO3-4.