A Closer Look at Fe(II) Passivation of Goethite

Abstract

Our understanding of how Fe(II) reacts with Fe(III) oxides has evolved based on evidence for electron transfer at the oxide−water interface and Fe(II)-catalyzed recrystallization. There is, however, some evidence that these, and other processes, such as microbial reduction, cease after continued contact with Fe(II) as the Fe oxide becomes "passivated". Here, we explore the mechanism of oxide passivation by measuring whether exposure to Fe(II) inhibits Fe(II)−goethite electron transfer, and whether this inhibition is reversible. To quantify the extent of electron transfer, we used selective isotope labeling with 57 Fe Mossbauer spectroscopy. We provide experimental evidence that pre-exposure to Fe(II) alters the products formed and inhibits the extent of electron transfer between goethite and Fe(II). We demonstrate that the goethite surface can accumulate a passivation layer of sorbed Fe(II) and that further electron transfer between Fe(II) and goethite is inhibited. Importantly, however, electron transfer can be partially restored upon removal of the layer of Fe(II) by extraction or oxidation. Our results suggest that in environments that are commonly subjected to transient geochemical fluctuations, electron transfer between Fe(II) and Fe oxides, and processes linked to it are likely to be relevant beyond just short time scales.

Antimony Sorption to Goethite: Effects of Fe(II)-Catalyzed Recrystallization

“…Exposure of goethite to aqueous Fe­(II) triggers a cascade of processes involving (i) Fe­(II) sorption, (ii) electron transfer between sorbed Fe­(II) and lattice Fe­(III), and (iii) conduction of transferred electrons to different Fe­(III) lattice sites, which then (iv) undergo reductive release of Fe­(II). − During this process, there is no net change in the overall Fe redox state, yet significant numbers of Fe atoms are exchanged between the aqueous and solid phases and, thus, goethite undergoes recrystallization. Recent research suggests that the thermodynamic driving force for Fe­(II)-catalyzed goethite recrystallization is enhanced stability due to the “healing” of defects or increases in the crystallite size. ,,− …”

“…However, it is clear that aqueous Fe­(II) can catalyze rapid goethite recrystallization, a reaction in which a stable mineral extensively exchanges atoms with ions in solution with no overt changes in mineralogy . Evidence for goethite recrystallization comes from Fe isotope tracer studies showing that Fe­(II) catalyzes Fe atom exchange between the aqueous and solid phases on the time scale of days to weeks. − The emerging concept is that, contrary to long-held views, Fe atoms within goethite can interact dynamically with the aqueous phase, potentially facilitating the uptake or release of co-associated metal­(loids). − …”

Further insights into the Fe(ii) reduction of 2-line ferrihydrite: a semi in situ and in situ TEM study

“…The exact reasons for such an "induction period" as observed here are unknown but may be a result of some type of surface Fe(II) passivation that occurs on the surface of the 2-line FH particles as recently noted for GT. 55 Moreover, the differences in dissolution times observed ( Fig. 1-short vs. Fig.…”

Oxalate-Induced Photoreduction Dissolution and Transformation of Schwertmannite: Change of Mineral Phase and Elemental Fate

“…However, a minor reductive dissolution also occurred in the process (Figure a and eq ). The aqueous Fe­(II) could be readsorbed on the Sch surface, and it can stimulate mineral dissolution and transformation to goethite by means of electron transfer–atomic exchange (ETAE). − Through overlapping d-orbitals, electron transfer occurs between the adsorbed Fe­(II) and the structural Fe­(III) (eq ). The unstable structural Fe­(II)–O bonds are formed on the surface of Sch, causing the collapse of the mineral structure, which can promote mineral dissolution .…”