Antimony and arsenic partitioning during Fe²⁺-induced transformation of jarosite under acidic conditions

Jarosite [KFe₃(SO₄)₂(OH)₆] is considered a potent scavenger for arsenic (As) and antimony (Sb) under oxidizing conditions. Fluctuations in water levels in re-flooded acid sulfate soils (ASS) can lead to high Fe²⁺_(aq) concentrations (∼10-20 mM) in the soil solution under acidic to circumneutral pH conditions. This may create favorable conditions for the Fe²⁺-induced transformation of jarosite. In this study, synthetic arsenate [As(V)]/antimonate [Sb(V)]-bearing jarosite was subjected to Fe²⁺_(aq) (20 mM) at pH 4.0 and 5.5 for 24 h to simulate the pH and Fe²⁺_(aq) conditions of re-flooded freshwater ASS/acid mine drainage (AMD)-affected environments at early and mid-stages of remediation, respectively. The addition of Fe²⁺ at pH 5.5 resulted in the formation of a metastable green rust sulfate (GR- SO₄) phase within ∼60 min, which was replaced by goethite within 24 h. In contrast, at pH 4.0, jarosite underwent no significant mineralogical transformation. Although the addition of Fe²⁺_(aq) induced the dissolution/transformation of jarosite at pH 5.5 and increased the mobility of Sb during the initial stages of the experiment (Sb_(aq) = ∼0.05 μmol L^-1), formation of metastable green rust (GR-SO₄) and subsequent transformation to goethite effectively sequestered dissolved Sb. Aqueous concentrations of As remained negligible in both pH treatments, with As being mostly repartitioned to the labile (∼10%) and poorly crystalline Fe(III)-associated phases (∼10-30%). The results imply that, under moderately acidic conditions (i.e. pH 5.5), reaction of Fe²⁺_(aq) with jarosite can drive the dissolution of jarosite and increase Sb mobility prior to the formation of GR-SO₄ and goethite. In addition, repartitioning of As to the labile fractions at pH 5.5 may enhance the risk of its mobilisation during future mineral transformation processes in Fe²⁺-rich systems.