Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), also known as the remediation Fe0/H2O system, a consensus on electro-chemical contaminant reduction was established during the 1990s and still prevails. Arguments against the reductive transformation concept have been regarded for more than a decade as ‘science denial’. However, is it the prevailing concept that denies the science of aqueous iron corrosion? This article retraces the path taken by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of the arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass-transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) the volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as an electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are inter-related, as the Fe0 surface is permanently shielded by a non-conductive oxide scale acting as a diffusion barrier to dissolved species and a barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and calls for alternative theories.
Metallic iron (Fe0) corrosion under immersed conditions (Fe0/H2O system) has been used for water treatment for the past 170 years. Fe0 generates solid iron corrosion products (FeCPs) which are known to in situ coat the surface of aggregates, including granular activated carbon (GAC), gravel, lapillus, manganese oxide (MnO2), pyrite (FeS2), and sand. While admixing Fe0 and reactive aggregates to build hybrid systems (e.g., Fe0/FeS2, Fe0/MnO2, Fe0/sand) for water treatment, it has been largely overlooked that these materials would experience reactivity loss upon coating. This communication clarifies the relationships between aggregate addition and the sustainability of Fe0/H2O filtration systems. It is shown that any enhanced contaminant removal efficiency in Fe0/aggregate/H2O systems relative to the Fe0/H2O system is related to the avoidance/delay of particle cementation by virtue of the non-expansive nature of the aggregates. The argument that aggregate addition sustains any reductive transformation of contaminants mediated by electrons from Fe0 is disproved by the evidence that Fe0/sand systems are equally more efficient than pure Fe0 systems. This demonstration corroborates the concept that aqueous contaminant removal in iron/water systems is not a process mediated by electrons from Fe0. This communication reiterates that only hybrid Fe0/H2O filtration systems are sustainable.
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