Mechanisms of contaminant removal with metallic iron (Fe0): A comprehensive and critical review

Abstract

Metallic iron (Fe⁰) has attracted great interest as a reactive material for water remediation. Several mechanisms of pollutant removal by Fe⁰ were released over the last 160+ years. Redox-sensitive contaminant removal by Fe⁰ is now generally agreed to occur primarily via direct reductive processes, as stated in the early 1990s by the North American pioneers of this technology. This has been challenged since 2006 by a German research group advocating for adsorption, coprecipitation, size-exclusion, and indirect reduction as main removal pathways. Since then, several reviews have been devoted to assessing the validity of the two concepts. The present work has demonstrated that the two concepts should be considered complementary. Unfortunately, while this topic was the subject of active debate, much less attention has been given to other mechanisms of contaminant removal with Fe⁰. In addition, the available information is also largely fragmented in the literature. Therefore, this work aims to contribute to the field by addressing current knowledge gaps in mechanisms. For the first time, a systematic, historical, and critical overview of the mechanistic pathways of Fe⁰-based water remediation technology was provided. Additionally, a chronology of Fe⁰ remediation mechanisms is presented, from the mid-1800s to this day. Finally, some conclusions are drawn, and recommendations are proposed.