Association of organic matter (OM) with mineral phases via co-precipitation is expected to be a widespread process in environments with high OM input and frequent mineral dissolution and re-precipitation. In contrast to surface area-limited adsorption processes, co-precipitation may allow for greater carbon (C) accumulation. However, the potential sub-micrometer scale structural and compositional differences that affect the bioavailability of co-precipitated C are largely unknown. In this study, we used a combination of high-resolution analytical electron microscopy and bulk spectroscopy to probe interactions between a mineral phase (ferrihydrite, nominally Fe₂O₃•0.5H₂O) and organic soil-derived water-extractable OM (WEOM). In co-precipitated WEOM-Fe, nanometer-scale scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) revealed increased Fe(II) and less Fe aggregation relative to adsorbed WEOM-Fe. Spatially distinct lower- and higher-energy C regions were detected in both adsorbed and co-precipitated WEOM-Fe. In co-precipitates, lower-energy aromatic and/or substituted aromatic C was spatially associated with reduced Fe(II), but higher-energy oxidized C was enriched at the oxidized Fe(III) interface. Therefore, we show that co-precipitation does not constitute a non-specific physical encapsulation of C that only affects Fe chemistry and spatial distribution, but may cause a bi-directional set of reactions that lead to spatial separation and transformation of both Fe and C forms. In particular, we propose that abiotic redox reactions between Fe and C via substituted aromatic groups (e.g., hydroquinones) play a role in creating distinct co-precipitate composition, with potential implications for its mineralization.

有机质 (OM) 通过共沉淀与矿物相结合预计将成为高 OM 输入和频繁矿物溶解和再沉淀的环境中的普遍过程。与表面积受限的吸附过程相反，共沉淀可能允许更多的碳 (C) 积累。然而，影响共沉淀 C 的生物利用度的潜在亚微米级结构和组成差异在很大程度上是未知的。在这项研究中，我们结合使用高分辨率分析电子显微镜和体光谱来探测矿物相（水铁矿，名义上是 Fe ₂ O ₃ •0.5H ₂O) 和有机土壤衍生的可提取水的 OM (WEOM)。在共沉淀的 WEOM-Fe 中，具有电子能量损失谱 (STEM-EELS) 的纳米级扫描透射电子显微镜显示，相对于吸附的 WEOM-Fe，Fe(II) 增加，Fe 聚集减少。在吸附和共沉淀的 WEOM-Fe 中都检测到了空间上不同的低能和高能 C 区域。在共沉淀物中，低能量的芳烃和/或取代的芳烃 C 在空间上与还原的 Fe(II) 相关，但高能量的氧化 C 在氧化的 Fe(III) 界面富集。因此，我们表明共沉淀不构成仅影响 Fe 化学和空间分布的 C 的非特异性物理包封，但可能会引起一系列双向反应，导致 Fe 和 C 形式的空间分离和转化。特别是，我们提出 Fe 和 C 之间通过取代芳族基团（例如对苯二酚）的非生物氧化还原反应在产生独特的共沉淀物组成方面发挥作用，对其矿化具有潜在影响。