Cr(III)-bearing iron minerals are widely present in acid mine drainage (AMD)-contaminated areas. In the light-transmitting layer of natural systems, dissolved organic matter (DOM) can synergize with solar irradiation to transform Fe-bearing minerals, but the behavior of Cr(III) and its interfacial reaction with schwertmannite (Sch) under the combined effect of DOM and solar radiation remain unclear. Oxalic acid (OA) was selected as a form of DOM to investigate its combined effect with solar radiation on substituted-Cr(III) schwertmannite (Cr-Sch) under acidic conditions (pH = 3). Batch photoaging experiments in combination with in situ attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FTIR), aberration-corrected scanning transmission electron microscopy (Cs-STEM), ultra-performance liquid chromatography-quadrupole-orbitrap mass spectrometry (UPLC-Q-Orbitrap MS), were employed to investigate Cr-Sch photoreductive dissolution process and elemental distribution. With increasing OA concentrations, Sch transformed to different second minerals: goethite and humboldtine under 1 mM and 5 mM oxalate condition, respectively. Photoreductive dissolution processes occur both on the surface of the minerals and in solution. Ligand-to-metal charge transfer (LMCT) is the dominant pathway of Sch photoreduction in solution when oxygen is absent, and it also occurs with superoxide mediated iron reduction (SMIR) under the aerobic conditions. With increasing OA concentrations, the conversion rate of Sch increases (the ratio of humboldtine to goethite increases), and the proportion of extractable Cr and solid-associated C increases. The findings reveal that OA plays a significant role in controlling the cycling of Fe/C and heavy metal elements under sunlight in the AMD-contaminated environment.

含 Cr(III) 的铁矿物广泛存在于酸性矿山排水 (AMD) 污染区域。在自然系统的透光层中，溶解有机物（DOM）可以与太阳辐照协同转化含铁矿物，但Cr（III）的行为及其与施韦特曼石（Sch）的界面反应在共同作用下DOM 和太阳辐射仍不清楚。选择草酸 (OA) 作为 DOM 的一种形式，以研究其在酸性条件下 (pH = 3) 与太阳辐射对取代-Cr(III) schwertmannite (Cr-Sch) 的联合影响。与原位相结合的批量光老化实验衰减全反射-傅里叶变换红外光谱 (ATR-FTIR)、像差校正扫描透射电子显微镜 (Cs-STEM)、超高效液相色谱-四极杆-轨道阱质谱 (UPLC-Q-Orbitrap MS)研究Cr-Sch光还原溶解过程和元素分布。随着 OA 浓度的增加，Sch 分别在 1 mM 和 5 mM 草酸盐条件下转化为不同的第二种矿物质：针铁矿和洪堡碱。光还原溶解过程发生在矿物表面和溶液中。配体到金属的电荷转移 (LMCT) 是缺氧时溶液中 Sch 光还原的主要途径，并且在有氧条件下也发生在超氧化物介导的铁还原 (SMIR) 中。随着 OA 浓度的增加，Sch的转化率增加（洪堡碱与针铁矿的比例增加），可萃取Cr和固相C的比例增加。研究结果表明，在 AMD 污染的环境中，OA 在阳光下控制 Fe/C 和重金属元素的循环方面发挥着重要作用。