Schwertmannite: A review of its occurrence, formation, structure, stability and interactions with oxyanions

In this article, we provide a critical review on the occurrence, formation, structure and stability of schwertmannite – an enigmatic Fe(III) oxyhydroxy-sulfate mineral that is of widespread interest in the soil science, environmental geochemistry, hydrometallurgy and wastewater fields. We also review interactions between schwertmannite and selected, environmentally relevant oxyanions (arsenate, phosphate, antimonate, selenate, chromate, and molybdate). Schwertmannite is one of the most common minerals to form via direct precipitation of iron in acid sulfate waters at pH 2 – 4. As such, it has been found to occur in systems that are affected by acid mine drainage, acid sulfate soils, natural acid rock drainage, and hydrometallurgical settings involving oxidation of iron-sulfide minerals. Despite its official recognition as a new mineral almost 30 years ago, schwertmannite’s poor crystallinity, fine particle size, variable composition and metastability have constrained our ability to conclusively resolve its crystal structure, possibly polyphasic nature, and solubility/stability. Nevertheless, since the early 1990’s, there has been a steadily-growing body of literature showing that, in acid-sulfate systems, schwertmannite strongly impacts iron and sulfur cycling, acidity dynamics, and electron flow. In addition, interactions with schwertmannite also strongly influence the mobility and fate of co-associated oxyanionic species. In parallel, many oxyanions have themselves been found to also exert a powerful control on schwertmannite’s geochemical behavior and mineralogical stability. Schwertmannite-oxyanion interactions occur via adsorption to the schwertmannite surface, anionic exchange with sulfate in the schwertmannite tunnel structure, and co-precipitation by substitution for either structural sulfate or Fe(III) during schwertmannite formation. This array of distinct sorption mechanisms results in oxyanions being able to both stabilize and conversely destabilize schwertmannite. This distinction appears to reflect an interplay of kinetic and thermodynamic constraints, which depend heavily on the specific oxyanion, its concentration (i.e. level of loading on/in schwertmannite), and other background geochemical conditions (e.g. pH and presence of other ions). Resolving these complex interactions and their importance in controlling schwertmannite occurrence and stability represents a challenge for future research.