Influence of Water Salinity on the Efficiency of Fe⁰-Based Systems for Water Treatment

Metallic iron (Fe-0) is a reactive material for treating polluted water. The effect of water salinity on the efficiency of Fe-0-based remediation systems is not yet established. This work aims to clarify the reasons why Cl- ions are often reported to improve the efficiency of Fe-0/H2O remediation systems. Quiescent batch experiments were carried out to characterize the effect of chloride (Cl-) ions on the efficiency of methylene blue (MB) discoloration in the presence of Fe-0. Cl- was used in the form of NaCl at concentrations ranging from 0 to 40 g L-1. The MB concentration was 10 mg L-1, the Fe-0 loading was 5 g L-1, and the duration of the experiment varied from 2 to 46 days. Four different Fe-0 materials were tested in parallel experiments. Tests with different NaCl levels were performed in parallel with three other organic dyes: Methyl orange (MO), orange II (OII), and reactive red 120 (RR 120). The results clearly show that the presence of Cl- reduces the extent of dye discoloration in all systems investigated. The efficiency of the dyes increased in the order MB < MO < RR 120 < OII. In systems with varying NaCl concentrations, dye discoloration initially decreases with increasing NaCl and slightly increases for [NaCl] > 30 g L-1. However, the extent of dye discoloration for [NaCl] = 40 g L-1 remains much lower than for the system with [NaCl] = 0 g L-1. The results clearly demonstrate that the presence of Cl- fundamentally delays the process of contaminant removal in Fe-0/H2O systems, thus improving the understanding of the contaminant interactions in Fe-0-based remediation systems. These results also suggest that the effects of other inorganic anions on the efficiency of Fe-0/H2O systems should be revisited for the design of field applications.