PNNL

Abstract

The biotic or abiotic reduction of nano-crystalline 2-line Ferrihydrite (2-line FH) into more thermodynamically stable phases (TSP) such as Lepidocrocite, Goethite, Magnetite and Hematite plays an important role in the geochemical cycling of elements (e.g. Fe, As, U, Se) and nutrients (e.g. C, N, P) in aqueous systems. In our study, we employed the use of in-situ liquid cell (LC) and semi in-situ analysis in an environmental TEM to gain further insights into the reaction mechanisms by which Fe(II)(aq) catalyzes 2-line FH into more TSP. In our work, we visually observed for the first time the following intermediate steps: (1) formation of round and wire-shaped nano-particles arising only from the Fe(II)(aq) solution, (2) two distinct dissolution mechanisms (reduction of size as well as density and breakage through smaller nano-particles) for 2 line-FH, (3) lack of complete dissolution of 2-line FH (“induction-period”), (4) amorphous phase growth (“reactive-FH”) on 2 line-FH, (5) deposition of amorphous nano-particles on the surface of 2 line-FH and (6) assemblage of elongated crystalline lamellae to form tabular Lepidocrocite crystals. Furthermore, we observed phenomena consistent with the movement of adsorbate ions from solution onto the surface of a Fe(III)-oxy/hydroxide crystal. Thus our work here reveals that the catalytic transformation of 2-line FH by Fe(II)(aq) into more TSP does not simply occur via the adsorption of Fe(II)(aq) onto 2-line FH and a dissolution-reprecipitation or surface nucleation-solid state conversion mechanism. Rather it is an intricate chemical process that goes through a series of intermediate steps that may affect the environmental fate, bioavailability, and transport of elements in aqueous environments.