Competing Effects of Chloride, Nitrate, and Sulfate Ions on the Removal of Fluoride by a Modified Zeolitic Tuff

Abstract

Natural zeolitic tuff was modified with FeCl3 solution for the removal of fluoride, and the effect of chloride, nitrate, and sulfate ions was examined on fluoride sorption from solutions and drinking water. The unmodified zeolitic tuff (Z) and the iron-modified zeolitic tuff (Fe(III)-Z) were characterized by scanning electron microscopy and X-ray diffraction analysis. The elemental composition, the specific surface area, and the point of zero charge of the zeolitic material were also determined. The fluoride adsorption was carried out in a batch system considering the effect of contact time, the initial concentration of fluoride ions, and the effect of other anions naturally present in the drinking water. The kinetic and isotherm results were adjusted to the pseudo-second-order and Freundlich models, respectively, which indicated that the sorption mechanism was chemisorption on a heterogeneous material. The fluoride sorption capacity was higher in solutions (2.7 mg/g) than in drinking water (0.41 mg/g), and this could be attributed to the presence of other anions. Overall, the presence of chloride ions significantly diminished the fluoride adsorption capacity, while the presence of nitrate and sulfate ions did not show any significant effect; the anion removal efficiency by Fe(III)-Z followed the order F− > > Cl− > NO3 − > SO4 2−.