Overcoming Fe(III) precipitation barrier in acid mine drainage via a visible light-assisted photo-electrochemical system.

Acid mine drainage (AMD) is characterized by high concentrations of Fe(II) and Fe(III), which can be harnessed for the in-situ formation of schwertmannite, enabling the efficient immobilization of toxic heavy metals. However, existing biological and chemical methods for schwertmannite synthesis face significant challenges, including low Fe(II) oxidation rates and particularly limited Fe(III) precipitation efficiency in acidic environments. In this study, we develop a visible light-assisted photo-electrochemical (PEC) system that effectively overcomes these barriers. By leveraging anodically evolved O2 and cathodically generated OH-, we achieved facile Fe(II) oxidation at pH 3.0, and an impressive Fe(III) precipitation efficiency of 82.8 %, significantly exceeding the < 30 % efficiency reported by other methods. Mössbauer spectroscopy and X-ray diffraction confirmed that the generated minerals are high-purity schwertmannite. Experimental and theoretical analyses revealed that in the presence of cathodic alkalinity, Fe(III) undergoes further hydrolysis to form [(H₂O)3Fe(OH)2(SO4)]- species, which are thermodynamically capable of spontaneous polymerization and mineralization. Furthermore, the photoreduction of [(H₂O)4Fe(SO4)2]- within the PEC system, followed by subsequent oxidation, plays a crucial role in facilitating Fe(III) mineralization. The PEC system also effectively transformed As(III) to As(V) and Cr(VI) to Cr(III) in AMD, promoting their immobilization in the resultant schwertmannite.