Effects of common dissolved anions on the efficiency of Fe⁰-based remediation systems

Quiescent batch experiments were conducted to evaluate the influences of Cl^-, F^-, HCO₃^-, HPO₄^2-, and SO₄^2- on the reactivity of metallic iron (Fe⁰) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe⁰ and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe⁰, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO₃^- enhanced the extent of MB discoloration, while Cl^-, F^-, HPO₄^2-, and SO₄^2- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO₃^- inhibited O-II discoloration. The increased MB discoloration in the HCO₃^- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe⁰-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.