Validating the efficiency of the MB discoloration method for the characterization of Fe0/H2O systems using accelerated corrosion by chloride ions

•Granular metallic iron (Fe0) is a powerful material for water treatment.•Methylene blue (MB) discoloration eases Fe0 testing at lab scale.•Early MB breakthrough is documented is systems with large Cl− levels.•In the long term, MB discoloration is enhanced in the presence of chloride ions (Cl−).•MB discoloration is recommended for characterizing the effects of PO43−, SO42−, etc.

The effect of the chloride ions (Cl−) on the efficiency of Fe0/sand systems for methylene blue (MB) discoloration was characterized in column studies for 4 months. Tested systems were: pure sand (0% Fe0 – reference) and Fe0/sand (50% Fe0 – vol/vol). Tested Cl− concentrations were 0.0, 2.1, 21.1 and 42.2 mM. The used MB concentration was 16 μM (5 mg L−1) and used Fe0 mass was 100 g. Discoloration experiments lasted for 89 days and was followed by a 26 days desorption experiment. Each system was characterized by the time-dependent changes of the pH value, the iron breakthrough, the MB breakthrough, and the hydraulic conductivity (permeability). During the discoloration experiments, a total of 17.4 L (184 pore volumes) of the MB solution flowed through each column (279.0 μM dye or 87.2 mg). No significance changes in pH value and permeability could be documented. Discoloration and desorption results confirmed that sand is an excellent MB adsorbent (only 11% desorption in the reference system). In the absence of Cl−, 15 mg MB was discolored. Up to 18 mg of MB was discolored in the presence of Cl−. The same trend was observed in changes of the extent of iron release which was 7.3 mg at 0.0 mM Cl− and 16.2 mg at 42.2 mM Cl−. These results confirm that chloride ions enhance the kinetics of Fe0 corrosion and thus the production of contaminant collectors (for MB co-precipitation). Results confirmed the suitability of MB as a powerful operative indicator for the characterization of processes in the Fe0/H2O system.

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