Photochemical Origins\nof Iron Flocculation in Acid\nMine Drainage

Acid mine drainage (AMD) raises a global environmental\nconcern\nimpacting the iron cycle. Although the formation of Fe(III) minerals\nin AMD-impacted waters has previously been reported to be regulated\nby biological processes, the role of abiotic processes remains largely\nunknown. This study first reported that a photochemical reaction coupled\nwith O<sub>2</sub> significantly accelerated the formation of Fe(III)\nflocculates (i.e., schwertmannite) in the AMD, as evidenced by the\ncomparison of samples from contaminated sites across different natural\nconditions at latitudes 24–29° N. Combined with experimental\nand modeling results, it is further discovered that the intramolecular\noxidation of photogenerated Fe(II) with a five-coordinative pyramidal\nconfiguration (i.e., [(H<sub>2</sub>O)<sub>5</sub>Fe]<sup>2+</sup>) by O<sub>2</sub> was the key in enhancing the photooxidation of\nFe(II) in the simulated AMD. The <i>in situ</i> attenuated\ntotal reflectance-Fourier transform infrared spectrometry (ATR-FTIR),\nUV–vis spectroscopy, solvent substitution, and quantum yield\nanalyses indicated that, acting as a precursor for flocculation, [(H<sub>2</sub>O)<sub>5</sub>Fe]<sup>2+</sup> likely originated from both\nthe dissolved and colloidal forms of Fe(III) through homogeneous and\nsurface ligand-to-metal charge transfers. Density functional theory\ncalculations and X-ray absorption spectroscopy results further suggested\nthat the specific oxidation pathways of Fe(II) produced the highly\nreactive iron species and triggered the hydrolysis and formation of\ntransient dihydroxo dimers. The proposed new pathways of Fe cycle\nare crucial in controlling the mobility of heavy metal anions in acidic\nwaters and enhance the understanding of complicated iron biochemistry\nthat is related to the fate of contaminants and nutrients.