Precipitative removal of fluoride from electronics wastewater

Synthetic wastewater was used for the investigation of main factors affecti ng F- removal performance. The existence of SiF62- or PO43-, and a low init ial F- concentration were the main causes of low and ri g p 2 - unstable F- removal. Lowering the pH reduced the adverse effects of SiF62- and PO43- a nd adding over 4 mg/L-1 of freshly formed CaF2 precipitates as seed greatly enhanced F- removal during treatment of low concentration wastewater (50 m g/L-1 F-). The seed-generation method, in which a small portion of wastewat er was first mixed with the whole dose of Ca2+ and then mixed with the rema ining portion of wastewater, greatly improved F- removal performance and co uld be used instead of the method of adding seed. The conclusions obtained from the treatment of synthetic wastewater were verified by using several k inds of actual electronics wastewater. The pH should be lower than 9 for ef ficiently removing F- from wastewater having a high ratio of SiO2 to F-, an d an even lower pH was needed for treating wastewater with a relatively hig h PO43- concentration. The seed-generation method was also found to be effe ctive for such wastewater with high concentrations of PO43-.