The persistence of organic carbon (OC) in natural environments is widely attributed to OC associations with minerals such as iron (Fe) minerals. Carboxyl, a critical structure of OC, can form strong complexes with Fe minerals, but it is still largely unknown about the effects of carboxyl groups on the transformation of Fe oxides and the stabilization mechanisms of OC on Fe oxides at nanoscales. In this study, four carboxylic acids with varying numbers of carboxyl groups (2, 3, 4, and 27 carboxyls) and molecular weight (ranging from 100 to 2000 Da) were added during the coprecipitation of Fe oxides and OC. This approach was taken to systematically elucidate the nanoscale distribution and sequestration mechanisms of carboxyl-containing OC on Fe oxides over different aging periods. Results suggested that ferrihydrite transformed quickly into lepidocrocite, goethite, and magnetite with the presence of Fe(II). The transformation rate of lepidocrocite to goethite slowed with increasing carboxyl number in OC molecules for the low molecular weight carboxylic acids (100–300 Da), but the high molecular weight carboxylic acid (∼2000 Da) promoted ferrihydrite transformation into goethite. Meanwhile, the particle sizes of produced magnetite decreased with increasing carboxyl number in OC molecules. During the mineral transformation, the release of OC from Fe oxides to aqueous solution decreased with increasing numbers of carboxyl groups. Spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) coupled with electron-energy-loss spectroscopy (EELS) suggested that carboxyl-rich OC promoted the formation of abundant defective or porous structures in goethite, resulting in OC accumulation in the bulk areas of goethite, which provided an effective way to sequester OC. The aggregation of high molecular weight OC containing more carboxyl groups with magnetite nanoparticles also played an important role in the preservation of OC. Furthermore, Fourier-transform infrared spectroscopy (FTIR) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) indicated that OC may form bidentate binding with Fe oxides and the binding strength of Fe(III) and OC may change with varying numbers of carboxyl groups. Results in this study provided a comprehensive understanding of the dynamic interactions between OC with different numbers of carboxyl groups and Fe oxides, and shed insights into the nanoscale mechanisms of OC stabilization during Fe oxide transformation process.

中文翻译：

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Fe(II)诱导的水铁矿转化过程中羧基碳保存的纳米机制

有机碳 (OC) 在自然环境中的持久存在广泛归因于 OC 与铁 (Fe) 矿物等矿物质的结合。羧基是OC的关键结构，可以与Fe矿物形成牢固的络合物，但羧基对Fe氧化物转化的影响以及OC对纳米尺度Fe氧化物的稳定机制仍知之甚少。在本研究中，在 Fe 氧化物和 OC 共沉淀过程中添加了四种具有不同羧基数量（2、3、4 和 27 个羧基）和分子量（范围从 100 至 2000 Da）的羧酸。该方法旨在系统地阐明不同老化时期铁氧化物上含羧基OC的纳米级分布和螯合机制。结果表明，在 Fe(II) 的存在下，水铁矿迅速转化为纤铁矿、针铁矿和磁铁矿。对于低分子量羧酸（100-300 Da），纤铁矿向针铁矿的转化速率随着OC分子中羧基数的增加而减慢，但高分子量羧酸（~2000 Da）促进水铁矿向针铁矿的转化。同时，随着OC分子中羧基数的增加，生成的磁铁矿的粒径减小。在矿物转化过程中，随着羧基数量的增加，从 Fe 氧化物到水溶液中释放的 OC 减少。球差校正扫描透射电子显微镜（Cs-STEM）与电子能量损失光谱（EELS）相结合表明，富含羧基的OC促进了针铁矿中丰富的缺陷或多孔结构的形成，导致OC在针铁矿的大部分区域中积累。针铁矿，它提供了一种有效的方法来隔离 OC。含有更多羧基的高分子量OC与磁铁矿纳米粒子的聚集对于OC的保存也发挥了重要作用。此外，傅里叶变换红外光谱（FTIR）和近边X射线吸收精细结构光谱（NEXAFS）表明OC可能与Fe氧化物形成双齿结合，并且Fe（III）和OC的结合强度可能随着不同数量的Fe（III）和OC的结合强度而变化。羧基。这项研究的结果提供了对不同数量羧基的OC与Fe氧化物之间的动态相互作用的全面理解，并深入了解Fe氧化物转化过程中OC稳定的纳米级机制。