Electron transfer between sorbed Fe(II) and structural Fe(III) in smectites and its effect on nitrate-dependent iron oxidation by Pseudogulbenkiania sp. strain 2002

Abstract

Iron redox cycling in clay minerals plays important roles in nutrient cycling and contamination migration in soils and sediments. Studies have shown interfacial electron transfer (IET) between sorbed Fe(II) and structural Fe(III) in clays, but the impact of IET on biological redox reactions has not been investigated. Here we studied the impact of such IET process on an example redox reaction, i.e. coupled Fe(II) oxidation and nitrate reduction, in the presence of the nitrate-reducing bacterium Pseudogulbenkiania sp. strain 2002. Aqueous Fe2+ was sorbed to basal surface (pH 6) and edge sites (pH 8) of nontronite (NAu-2) and montmorillonite (SWy-2). The amount of Fe(II) sorption was lower at pH 6 than at pH 8. At pH 6, the extent of IET from basal Fe(II) to structural Fe(III) was higher in SWy-2 than in NAu-2, resulting in a higher proportion of structural Fe(II) in SWy-2. Because structural Fe(II) is more reactive than basal Fe(II), such IET resulted in a higher reactivity of SWy-2-associated Fe(II) than that of NAu-2-associated Fe(II) towards biologically-mediated nitrate reduction. At pH 8, extensive IET from highly reactive edge-Fe(II) to structural Fe(III) in NAu-2 resulted in formation of structural Fe(II) and Fe oxides, which lowered the reactivity of NAu-2-associated Fe(II). In contrast, due to limited IET in SWy-2 at pH 8, a large fraction of sorbed Fe(II) remained and was associated with SWy-2 and/or goethite/mixed Fe(II)-Fe(III) nanoparticles, which were highly reactive. As a result, SWy-2-associated Fe(II) is more reactive than NAu-2-associated Fe(II) at pH 8. The results of this study have important implications for understanding clay redox reactions in such environments where clay minerals and aqueous Fe2+ are in contact and IET occurs.