PNNL

Abstract

Iron oxides regulate the flux of electrons in the environment, yet little is known about the effect of structurally incorporated impurities on their redox activity. Here, we present the first evidence of a direct link between impurity-induced long-range lattice strain and the redox properties of their host phase goethite (a-FeOOH), the most abundant stable Fe(III) (oxyhydr)oxide at Earth’s surface. EXAFS and PDF analysis confirmed that the common impurities Ni(II) and Zn(II) substituted for structural Fe(III). Redox measurements in aqueous solutions show that E_H^0 increases in the order: Ni-goethite >> Zn-goethite = pure goethite. Counterintuitively, the local coordination environment of the smaller impurity, Ni, causes more bulk strain energy than Zn, which nearly accounts for the difference in E_H^0 between Ni- and Zn-goethite. This study reveals how two common impurities affect the stability of goethite with implications for critical element mobility during reductive recrystallization of Fe(III) oxyhydroxides.