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.