Precipitative Removal of Fluoride from Electronics Wastewater

Synthetic wastewater was used for the investigation of main factors affecting F⁻ removal performance. The existence of ${\mathrm{SiF}}^{\mathit{2}{\mathit{-}}_{\mathit{6}}}$ or ${\mathrm{PO}}^{\mathit{3}{\mathit{-}}_{\mathit{4}}}$, and a low initial F⁻ concentration were the main causes of low and unstable F⁻ removal. Lowering the pH reduced the adverse effects of ${\mathrm{SiF}}^{\mathit{2}{\mathit{-}}_{\mathit{6}}}$ and ${\mathrm{PO}}^{\mathit{3}{\mathit{-}}_{\mathit{4}}}$, 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 ${\mathrm{PO}}^{\mathit{3}{\mathit{-}}_{\mathit{4}}}$ concentration. The seed-generation method was also found to be effective for such wastewater with high concentrations of ${\mathrm{PO}}^{\mathit{3}{\mathit{-}}_{\mathit{4}}}$.