Partitioning and transformation behavior of arsenic during Fe(III)-As(III)-As(V)-SO4 2− coprecipitation and subsequent aging process in acidic solutions: Implication for arsenic mobility and fixation

Highlights: • The Fe(III)-As(III)-As(V)-${\mathrm{SO}}_4^{2-}$ coprecipitation and aging process was studied. • The Fe(AsO₄)_x(AsO₃)_y(SO₄)_z solid solution was formed. • The coexisting As(III) inhibited the transformation of ferric arsenate to scorodite. • Coprecipitation and aging process reduced the mobility of As. The coprecipitation and subsequent aging of Fe(III)-As(III)-As(V)-SO₄²⁻ play an important role in controlling As behavior in acidic systems, such as acid mine drainage and hydrometallurgical acid waste. In this study, we investigated the redistribution and transformation of As in the Fe(III)-As(III)-As(V)-SO₄²⁻ system (As(III)/As(V) ≈ 1) at different Fe/As molar ratios (i.e., 0.25, 0.5, and 1) and pH (1.2 and 1.8) at 60 °C. The results showed that As(III) and SO₄²⁻ can be incorporated into the amorphous ferric arsenate and scorodite host phases by forming a Fe(AsO₄)_x(AsO₃)_y(SO₄)_z solid solution. As(III) contents in the freshly coprecipitated solids increased with pH and initial As(III) concentrations. During aging process, As(III) contents in the solid products with Fe/As molar ratios of 0.5 and 1 increased with aging time at pH 1.8. In contrast, As(III) was gradually expelled from aging products with aging time at pH 1.2, regardless of Fe/As molar ratio. X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy characterization results showed that an As(III)-SO₄²⁻-doped scorodite was formed at Fe/As molar ratio ≤0.5 during the aging process. It was also found that As(III) had an inhibitory effect on the transformation of poorly crystalline ferric arsenate to scorodite. The present study may have important implications for understanding the geochemical cycle of As, Fe, and SO₄²⁻ in acidic solutions and give further understanding on the mechanisms involved in As removal and fixation in hydrometallurgical unit operations.