While widely recognized as a promoter of dental health, fluoride can also exceed safe levels in drinking water sources. The U.S. Maximum Contaminant Level (MCL) is currently 4.0 mg/L, but this limit is under reconsideration by the EPA due to concerns associated with fluorosis. In the likely event of a lower MCL, impacted small water systems may find it difficult to comply with the new standards. Alum coagulation, conventionally used for natural organic matter (NOM) and particle removal, can be adapted to remove fluoride through adsorption and co-precipitation. However, interactions between fluoride, NOM, and other inorganic ligands such as silica are not well understood. In addition to fluoride health concerns, diminished NOM removal could increase production of disinfection-by-products. A series of jar tests were conducted with varied combinations of sulfate, chloride, fluoride, silica, and organic acid (NOM surrogate) ligands. The effects of pH were also examined to explore the effects on aluminum solubility and ligand removal. Each jar test was repeated weekly over a month and precipitates were aged until analysis. Jar supernatant was tested for residual aluminum, fluoride, organics and silica. Aluminum hydroxide precipitates were analyzed using attenuated total reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) to elucidate the types of ligand-surface interactions that were formed. Jar test results indicate that fluoride removal is not significantly hindered by NOM presence, but NOM removal is hindered by fluoride, suggesting a preferential removal of fluoride via complexation with aluminum. IR spectra of the pyromellitic acid jars exhibited patterns of asymmetric and symmetric stretching vibrations for COO- that indicated outer-sphere complexation of the NOM surrogate. This result is consistent with previous findings that aromatic carboxylate adsorption to aluminum hydroxides is dominated by outer-sphere complexes at natural pH ranges. Weakened absorbance intensities were observed in the COO- stretch for pyromellitic acid in the presence of fluoride which was consistent with NOM removal patterns under these conditions. Sulfate, silicate, and carbonate absorbances were also diminished in the presence of fluoride. Future investigations will include the interaction of other competing ions of concern such as chromium, manganese, and arsenic as well as the use of X-ray photoelectron spectroscopy for further analysis of the precipitate surfaces.

Aluminum hydroxide, fluoride, and NOM: Insights using infrared spectroscopy

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.