PNNL

Abstract

Studies have shown interfacial electron transfer (IET) between sorbed Fe(II) and structural Fe(III) in clays, but how such process alters their reactivity has not been investigated. Here we studied nitrate reduction by Fe(II)-sorbed nontronite (NAu-2) and montmorillonite (SWy-2) in the presence of an Fe(II)-oxidizer, Pseudogulbenkiania sp. At pH 6 a similar amount of Fe(II) (~1.4% wt) sorbed onto the basal surface of both clays, but with different extents of IET. For NAu-2, because a large fraction of sorbed Fe(II) remained on unreactive basal surface, its reactivity towards nitrate reduction was low. For SWy-2, IET changed majority of sorbed Fe(II) to structural Fe(II), resulting in a higher reactivity. At pH 8 the amount of edge-sorbed Fe(II) to NAu-2 (2.93%) was lower than that on SWy-2 (4.66%), and IET oxidized the majority of NAu-2 sorbed Fe(II) to Fe(III). The resulting IET-induced structural Fe(II) and the remaining fraction of sorbed-Fe(II) as a “magnetite-like” phase were highly reactive towards nitrate. In contrast, because of isolated Fe(III) atoms within the SWy-2 structure, the extent of IET was limited and the majority of sorbed Fe(II) co-precipitated or was associated with oxidized Fe(III) as mixed Fe(II)-Fe(III) nanoparticles, which was responsible for its highest reactivity towards nitrate reduction.