Acid mine drainage (AMD) raises a global environmental concern impacting the iron cycle. Although the formation of Fe(III) minerals in AMD-impacted waters has previously been reported to be regulated by biological processes, the role of abiotic processes remains largely unknown. This study first reported that a photochemical reaction coupled with O₂ significantly accelerated the formation of Fe(III) flocculates (i.e., schwertmannite) in the AMD, as evidenced by the comparison of samples from contaminated sites across different natural conditions at latitudes 24–29° N. Combined with experimental and modeling results, it is further discovered that the intramolecular oxidation of photogenerated Fe(II) with a five-coordinative pyramidal configuration (i.e., [(H₂O)₅Fe]²⁺) by O₂ was the key in enhancing the photooxidation of Fe(II) in the simulated AMD. The in situ attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FTIR), UV–vis spectroscopy, solvent substitution, and quantum yield analyses indicated that, acting as a precursor for flocculation, [(H₂O)₅Fe]²⁺ likely originated from both the dissolved and colloidal forms of Fe(III) through homogeneous and surface ligand-to-metal charge transfers. Density functional theory calculations and X-ray absorption spectroscopy results further suggested that the specific oxidation pathways of Fe(II) produced the highly reactive iron species and triggered the hydrolysis and formation of transient dihydroxo dimers. The proposed new pathways of Fe cycle are crucial in controlling the mobility of heavy metal anions in acidic waters and enhance the understanding of complicated iron biochemistry that is related to the fate of contaminants and nutrients.

中文翻译：

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酸性矿山排水中铁絮凝的光化学起源


酸性矿山排水 (AMD) 引起了全球环境问题，影响了铁循环。尽管先前报道受 AMD 影响的水中 Fe(III) 矿物质的形成受到生物过程的调节，但非生物过程的作用仍然很大程度上未知。本研究首次报道了与 O ₂结合的光化学反应显着加速了 AMD 中 Fe(III) 絮凝物（即施威特曼石）的形成，通过对纬度 24-29 不同自然条件下污染地点的样品进行比较证明了这一点。结合实验和模型结果，进一步发现五配位金字塔构型的光生Fe(II)（即[(H ₂ O) ₅ Fe] ²⁺ ）被O _{2 的}分子内氧化反应为增强模拟 AMD 中 Fe(II) 光氧化的关键。原位衰减全反射-傅里叶变换红外光谱（ATR-FTIR）、紫外-可见光谱、溶剂替代和量子产率分析表明，作为絮凝的前体，[(H ₂ O) ₅ Fe] ²⁺可能源自溶解形式和胶体形式的 Fe(III) 通过均匀和表面配体到金属的电荷转移。密度泛函理论计算和X射线吸收光谱结果进一步表明，Fe(II)的特定氧化途径产生了高活性铁物种，并引发了水解和瞬时二羟基二聚体的形成。 所提出的铁循环新途径对于控制酸性水中重金属阴离子的迁移率至关重要，并增强了对与污染物和营养物命运相关的复杂铁生物化学的理解。