Adsorption of fluoride by waste iron oxide: The effects of solution pH, major coexisting anions, and adsorbent calcination temperature

In this study, a waste iron oxide material (BT3), which is a by-product of the fluidized-bed Fenton reaction (FBR-Fenton), was thermally treated between 200 and 900°C and was used as an efficient adsorbent for the removal of fluoride ions in an aqueous system. The highest fluoride adsorption capacity occurred at the termination of the BT3 goethite dehydroxylation phase at about 300°C calcination where both the volume of nanopores formed by dehydroxylation and the specific surface area reached their maximum values. Above 300°C, BT3 transformed to the hematite phase in which fluoride adsorption capacity decreased as calcination temperature increased. On the other hand, the effect of pH on the fluoride adsorption capacity of BT3 for various initial fluoride concentrations was examined. The optimum pH value was found to be about 4. After that efficiency decreased as pH became more alkaline. Finally, coexisting anions affected the fluoride adsorption capacity of BT3 at pH 3.9±0.2 in this order: PO₄^3->SO₄^2->Cl^->NO₃^-.