Distribution of oxides on iron materials used for remediation of organic groundwater contaminants — Implications for hydrogen evolution reactions

Abstract

The distribution of oxides on commercial iron materials used for remediation of organic groundwater contaminants was studied using confocal Raman microspectroscopy. Raman microprobe mapping experiments revealed that commercial iron materials used for the construction of permeable reactive barriers are covered with an oxide film consisting of an inner film of magnetite Fe3O4 and an outer film of Fe2O3. The calculated probability of the charge transfer process (i.e., electron tunnelling probability) at commercial Fe | Fe2O3 (semiconductor) | solution interfaces approaches zero, indicating that "as received" commercial iron materials should not be reactive towards both organic contaminants and water. The combination of OCP (corrosion potential) – time measurements with in situ Raman spectroscopy demonstrated that the breakdown of the protective Fe2O3 oxide film followed by autoreduction of Fe2O3 is a requirement for the commercial materials to be chemically active towards both water and organic contaminants. Hydrogen evolution reactions on metals and oxide-covered iron were also investigated. In particular, spectroscopic evidence for adsorption of atomic hydrogen on polycrystalline Pt is presented. The possibility of adsorption of H on magnetite-covered iron was considered using thermodynamic calculations after Marcus and Protopopoff. The evolution of hydrogen on commercial iron materials is described. It is suggested that the separation of the anodic and cathodic sites because of opening of pores in the bulk metallic Fe and the involvement of spill-over hydrogen may play a crucial role in the catalytic hydrogenation of groundwater contaminants.Key words: Raman microprobe confocal spectroscopy, iron oxides, hydrogen evolution, remediation of organic groundwater contaminants.