Scorodite precipitation in the presence of antimony

Abstract

The effects of Sb on the precipitation of synthetic scorodite, and the resultant phases formed, were investigated. Nine synthetic precipitates with varying concentrations of Sb, together with As-only and Sb-only end members, were prepared using a scorodite synthesis method, and these were characterised using XRD, SEM, chemical digestion and μXRF mapping. XRD analysis shows that the end members are scorodite (FeAsO<inf>4</inf>·2H<inf>2</inf>O) and tripuhyite (FeSbO<inf>4</inf>), and that the intermediate members are not Sb-substituted scorodite, but instead are physical mixtures of scorodite and tripuhyite, with tripuhyite becoming more prominent with increasing amounts of Sb in the synthesis. Electron microprobe analysis on natural scorodites confirms that they contain negligible concentrations of Sb. With increasing Sb in the synthesis, the morphology of the scorodite changes from rosettes of intergrown crystals to anhedral masses of smaller crystallites. Chemical digestion of the series also became increasingly difficult with increasing Sb content. We conclude that Sb is not taken up in scorodite (perhaps due to its larger ionic radius and different co-ordination with O compared to As), that increasing amounts of Sb in the system affect scorodite morphology, and that tripuhyite is a highly stable and perhaps underestimated Sb-sink.