Photochemical behavior between oxalate and Sb(V)/As(V)-bearing jarosite: Reaction mechanism, mineral transformation, and elemental fate

Jarosite is a common metastable iron mineral in acid sulfate-rich environments, which is known to retain antimony (Sb) and arsenic (As). Sunlight has emerged as a critical driver for the dissolution of iron minerals, especially in the presence of dissolved organic matter (DOM). However, the mechanisms governing jarosite dissolution and associated Sb/As migration under the coexistence of sunlight and DOM remain poorly understood. Here, we investigated the photochemical behavior of Sb(Ⅴ)/As(Ⅴ)-bearing jarosite with the representative DOM-oxalate (OA) under simulated sunlight. In situ attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculations revealed that oxalate was predominantly adsorbed onto the jarosite surface via mononuclear bidentate (MB) binding geometry. Under light irradiation, oxalate underwent photolysis on the jarosite surface, thereby inducing reductive dissolution of the mineral. The dissolution rate of jarosite was faster under anoxic conditions than under oxic conditions, and both rates increased with increasing oxalate dosage. Notably, high OA dosage (2.5 mM) induced jarosite transformation to humboldtine under anoxic conditions and to lepidocrocite under oxic conditions. Scanning transmission electron microscopy combined with energy-dispersive spectroscopy (STEM-EDS) showed that the formed lepidocrocite exhibited a superior capability to fix Sb and As compared to humboldtine. Chemical extraction further demonstrated that photoaged Sb preferentially partitioned into the poorly crystalline phase, whereas As tended to adsorb onto the mineral surface. These findings provide new insights into the photochemical behavior between iron minerals and DOM, and offer theoretical guidance for predicting the migration of Sb and As in sunlit environments.
Keywords: Jarosite; Oxalate; Antimony; Arsenic; Photochemical behavior.