Antimony Fate during Transformation of Schwertmannite to Jarosite in Acid Mine Drainage

Schwertmannite is an important host-phase for Sb(V) in acid mine drainage (AMD) systems. However, schwertmannite is metastable and transforms to jarosite under strongly acidic conditions. Little is currently known about how this mineralogical transformation affects associated Sb(V). We address this knowledge gap by presenting the first systematic examination of Sb(V) mobility and fate during the transformation of schwertmannite to jarosite. This was achieved by allowing schwertmannite, with zero, low, or high levels (Sb:Fe = 0, 0.005, or 0.04, respectively) of sorbed or coprecipitated Sb(V), to age under strongly acidic conditions (pH 1.1-2.5). Our results demonstrate that the initial stage of schwertmannite transformation released up to 1.8 mg/L Sb(V) into solution. However, the release of Sb(V) was mitigated by subsequent mineral-water interactions that resulted in Sb(V) being sequestered by newly formed jarosite. Antimony K-edge EXAFS spectroscopy indicates that these interactions included Sb(V) incorporation into the jarosite structure via partial Sb(V)-for-Fe(III) substitution as well as surface precipitation of an antimonic acid-like phase. Overall, the results shed new light on the stability of schwertmannite-bound Sb(V), while highlighting the role that jarosite plays in controlling the Sb(V) mobility and fate at low pH in AMD systems.