Researchers on metallic iron (Fe⁰) for environmental remediation and water treatment are walking in a valley of confusion for 25 years. This valley is characterized by the propagation of different beliefs that have resulted from a partial analysis of the Fe⁰/H₂O system as (1) a reductive chemical reaction was considered an electrochemical one and (2) the mass balance of iron has not been really addressed. The partial analysis in turn has been undermining the scientific method while discouraging any real critical argumentation. This communication re-establishes the complex nature of the Fe⁰/H₂O system while recalling that, finally, proper system analysis and chemical thermodynamics are the most confident ways to solve any conflicting situation in Fe⁰ environmental remediation.

Continuous gravity-fed column experiments using the methylene blue (MB) discoloration method were performed to characterize the suitability of a pozzolan (PZ) specimen as alternative admixing aggregate for metallic iron filters (Fe⁰-filters). Investigated systems were: (i) pure sand, (ii) pure PZ, (iii) pure Fe⁰, (iv) Fe⁰/sand, (v) Fe⁰/PZ, and (vi) Fe⁰/sand/PZ. The volumetric proportion of Fe⁰ was 25%. The volumetric proportions of the Fe⁰/sand/PZ system was 25/45/30. The initial MB concentration was 2.0 mg·L⁻¹, 6.0 g of Fe⁰ was used, and the experiments lasted for 46 days. The individual systems were fed with 3.9 to 8.4 L (7.80 to 16.69 mg of MB) and were characterized by the time-dependent changes of: (i) the pH value, (ii) the iron breakthrough, (iii) the MB breakthrough, and (iv) the hydraulic conductivity. Results showed that the Fe⁰/sand/PZ system was the most efficient. This ternary system was also the most permeable and therefore the most sustainable. The suitability of MB as a powerful operative indicator for the characterization of processes in the Fe⁰/H₂O system was confirmed. The tested PZ is recommended as an alternative material for efficient but sustainable Fe⁰ filters.