Fluoride naturally occurs in some ground and surface waters at high concentrations all around the world. Due to health concerns, the USEPA  is reviewing fluoride as a drinking water contaminant. If  the fluoride maximum contaminant limit (MCL) is lowered, many water systems will require defluoridation as part of their drinking water treatment. Alum coagulation was investigated as a defluoridation treatment strategy in this research, with particular attention paid to the possible interaction with natural organic matter.<br/>Defluoridation of synthetic and natural waters using jar tests elucidated interactions of fluoride, NOM, and aluminum during alum coagulation. Alum coagulation was able to remove 80% of fluoride from natural waters with a 500 mg/L alum dose; however, 50% fluoride removal was possible with an alum dose of 150 mg/L. The optimum pH for fluoride removal in synthetic and natural waters was observed to be approximately 6.5. The presence of fluoride during alum coagulation was found to reduce the removal of three low molecular weight (LMW) organics, acting as surrogates for NOM, to different extents depending on their functionality and adsorption mechanism. The presence of LMW organic acids in synthetic waters did not impact the removal of fluoride; however, increasing NOM concentrations in the natural waters likely accounted for decreasing fluoride removals.<br/>Additional jar tests with natural waters revealed that pH adjustment was unnecessary for defluoridation of high pH and high alkalinity waters and that an enhanced precipitation effect occurred at low alum doses when no pH adjustment was made during alum coagulation. The enhanced precipitation effect caused comparable or enhanced removals of fluoride and NOM to be observed despite system pH values being higher than the optimal defluoridation pH of 6.5.<br/> <br/>

Mechanisms of fluoride removal: Adsorption and co-precipitation with aluminum hydroxide in the presence and absence of NOM

249th National Meeting (2015)

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