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

The environmental mobility and bioavailability of antimony (Sb) are strongly influenced by sorption to Fe(III) oxide minerals, such as goethite (alpha FeOOH). Exposure to aqueous Fe(II), as occurs, for example, under reducing conditions in soils and sediments, catalyzes the recrystallization of goethite. Herein, we examine, for the first time, the effect of Fe(II)-catalyzed goethite recrystallization on the sorption of co-associated Sb(V). The use of an enriched aqueous Fe-57(II) tracer provided direct evidence of Fe(II)-catalyzed goethite recrystallization and revealed lower levels of recrystallization at higher Sb(V) loadings. Goethite recrystallization caused increases in the total amount of sorbed Sb along with decreases in the percentage of sorbed Sb that was surface-bound (based on PO(4)3extractions). Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Sb K-edge shows that goethite recrystallization led to increases in the coordination number (CN) for both edge-sharing and double-corner-sharing linkages between Sb(V)-O and Fe(III)-O octahedra. The CN increases are consistent with partial structural Sb(V) incorporation into goethite, with the degree of incorporation increasing with the extent of recrystallization. Overall, the results indicate that Fe(II)-catalyzed recrystallization of goethite caused a shift from Sb(V) adsorption at the goethite surface to partial Sb(V) incorporation into the goethite structure [via heterovalent substitution for up to similar to 1.4 mol % Fe(III)]. These results necessitate a re-evaluation of current concepts regarding the sorption of Sb to goethite. In particular, goethite should be considered as a dynamic phase whose solid structure (not just the reactive surface) may be available for Sb(V) sorption via structural incorporation during Fe(II)-catalyzed recrystallization.