Divergent repartitioning of copper, antimony and phosphorus following thermal transformation of schwertmannite and ferrihydrite

The consequences of fire-induced thermal transformation of poorly crystalline iron minerals on the availability of coprecipitated trace-metals, metalloids and nutrients in fire-prone acid sulfate soils (ASS) landscapes are not well understood. Here, we heat schwertmannite and ferrihydrite containing either coprecipitated copper (Cu), antimony (Sb) or phosphorus (P) at a range of temperatures from 200 °C to 800 °C. Changes in Fe mineralogy and the partitioning of Cu, Sb and P in thermally transformed products were investigated using X-ray diffraction, 57Fe Mössbauer spectroscopy and selective extracts (e.g. water, 1 M MgCl2/0.5 M NaH2PO4/0.5 M NaHCO3, 1 M HCl and aqua regia). The mainly structural incorporation of trace amounts (0.01-0.2%) of Cu, Sb and P into schwertmannite and ferrihydrite negligibly affected initial mineralogy. Irrespective of the presence of coprecipitated Cu, Sb and P, all three types of schwertmannite and ferrihydrite (i.e. Cu, Sb and P-bearing) were transformed to hematite upon heating, although transformation temperatures varied marginally between treatments. All schwertmannites transformed to hematite via a surface adsorbed intermediate iron sulfate oxide phase formed between 400 °C to 700 °C, whereas all ferrihydrites transformed to hematite directly. During heating, the majority of initially structurally incorporated Cu in schwertmannite repartitioned to surface-exchangeable sites, particularly at intermediate temperatures (~300-700 °C). In contrast, Cu in ferrihydrite became stabilized within the structure of hematite. Upon heating both schwertmannite and ferrihydrite, Sb became increasingly stabilized and strongly retained within neoformed hematite. Heating also caused some of the initially structurally incorporated P in schwertmannite and ferrihydrite to repartition to readily exchangeable sites. Our study reveals that thermal transformation of schwertmannite and ferrihydrite can alter and enhance the bioavailability of both Cu (if incorporated in schwertmannite) and P in ASS landscapes, with potential consequences for soil and water quality. In contrast, Sb can be effectively immobilized, likely via isomorphic substitution for Fe(III) in neoformed hematite. The divergent repartitioning behaviour of Cu, P and Sb can be largely explained by interactions between fundamental properties of the trace elements (i.e. atomic radii and valence) and the host Fe mineral(s) structure. Further studies of trace metal(loid) mobility during thermal transformation of common iron minerals would be useful to refine our understanding of the broader consequences of fire for bioavailability of trace metal(loid)s in acid sulfate landscapes.