Investigating the suitability of Fe⁰ packed-beds for water defluoridation

A commercial granular metallic iron (Fe⁰) specimen was used to evaluate the suitability of Fe⁰ materials for removing aqueous fluoride (F^-) (water defluoridation). Experiments were performed to characterize the defluoridation potential of the tested Fe⁰ as influenced by the presence of chloride (Cl^-) and bicarbonate (HCO₃^-) ions using tap water (H₂O) as operational reference system. Duplicate column studies were conducted for 120 days (4 months) using an initial F^- concentration of 22.5 mg L^-1, columns flow rates were about 17 mL h^-1. Each column contained a reactive layer (11 cm) made up of 100 g of Fe⁰ in a 1:1 volumetric Fe⁰:sand mixture. The reactive layer was sandwiched between two layers of the same sand. A pure sand column was used as control system. After the F^- removal experiments, the columns were flushed by methylene blue (MB) and Orange II for 21 days. Removal studies revealed (i) no F^- removal in the control system, (ii) no F^- significant removal on the Cl^- system, (iii) limited F^- removal in the HCO₃^- system, and (iv) the best F^- removal efficiency in tap water (H₂O). Dye flushing studies confirmed the ion-selective nature of the Fe⁰/H₂O system and demonstrated the relatively low efficiency of the same for F^- removal. The overall results challenge the prevailing perception that water defluoridation using granular Fe⁰ is not possible and suggest that effective water defluoridation in Fe⁰ packed-beds is pure a site-specific design issue.