Testing the suitability of metallic iron for environmental remediation: Discoloration of methylene blue in column studies

A new method to correlate intrinsic reactivity and treatability efficiency of metallic iron (Fe0) was evaluated. A 2.0 mg L-1 methylene blue (MB) solution was used in gravity fed column experiments. The intrinsic reactivity of nine Fe0 materials (ten samples) was characterized using the EDTA test. Three commercial Fe0 materials ZVI1 (0.40 - 0.80 mm), ZVI9 (0.50 mm) and ZVI10 (0.45 - 0.55 mm) were tested in column experiments. A layer containing 100 g of Fe0 was sandwiched between 19.0 to 20.0 cm upper coarse sand (1.6 - 2.0 mm) and 8.0 cm lower fine sand (0.25 - 0.30 mm). 500 mL of the MB solution was daily filtered through each column for one month. Effluent solutions were characterized for MB and Fe concentrations. The columns were also characterized by the evolution of the hydraulic conductivity (k values). Results showed (i) quantitative MB removal (> 88 %) and (ii) limited Fe release for all three columns. After about 25 days, the Fe levels were constantly less than 1.0 mg L-1. The most significant difference was observed in the evolution of the k value and was attributed to the different material sorting. Less sorted ZVI1 exhibited the lowest initial k value (8.0 vs 43.0 mm min-1 for ZVI9 and ZVI10) and most significant permeability loss. Results confirmed the usefulness of the tested protocol as a reliable method to assess the efficiency of Fe0 materials in short term column experiments. Well-sorted Fe0 materials are recommended for long term efficient Fe0 filtration systems