Sequestration of organic carbon (OC) in environmental systems is critical to mitigating climate change. Organo-mineral associations, especially those with iron (Fe) oxides, drive the chemistry of OC sequestration and stability in soils. Short-range-ordered Fe oxides, such as ferrihydrite, demonstrate a high affinity for OC in binary systems. Calcium commonly co-associates with OC and Fe oxides in soils, though the bonding mechanism (e.g., cation bridging) and implications of the co-association for OC sequestration remain unresolved. We explored the effect of calcium (Ca2+) on the sorption of dissolved OC to 2-line ferrihydrite. Sorption experiments were conducted between leaf litter-extractable OC and ferrihydrite at pH 4 to 9 with different initial C/Fe molar ratios and Ca2+ concentrations. The extent of OC sorption to ferrihydrite in the presence of Ca2+ increased across all tested pH values, especially at pH ≥ 7. Sorbed OC concentration at pH 9 increased from 8.72 ± 0.16 to 13.3 ± 0.20 mmol OC g-1 ferrihydrite between treatments of no added Ca2+ and 30 mM Ca2+ addition. Batch experiments were paired with spectroscopic studies to probe the speciation of sorbed OC and elucidate the sorption mechanism. ATR-FTIR spectroscopy analysis revealed that carboxylic functional moieties were the primary sorbed OC species that were preferentially bound to ferrihydrite and suggested an increase in Fe-carboxylate ligand exchange in the presence of Ca at pH 9. Results from batch to spectroscopic experiments provide significant evidence for the enhancement of dissolved OC sequestration to 2-line ferrihydrite and suggest the formation of Fe-Ca-OC ternary complexes. Findings of this research will inform modeling of environmental C cycling and have the potential to influence strategies for managing land to minimize OM stabilization.

中文翻译：

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钙对水铁矿有机碳封存的协同作用。


环境系统中有机碳（OC）的封存对于缓解气候变化至关重要。有机矿物组合，尤其是与铁 (Fe) 氧化物的组合，推动土壤中 OC 封存和稳定性的化学反应。短程有序的 Fe 氧化物，例如水铁矿，在二元系统中表现出对 OC 的高亲和力。钙通常与土壤中的 OC 和 Fe 氧化物共缔合，尽管键合机制（例如阳离子桥联）和共缔合对 OC 封存的影响仍未解决。我们探讨了钙 (Ca2+) 对溶解的 OC 吸附到 2 线水铁矿上的影响。在pH 4至9、不同的初始C/Fe摩尔比和Ca2+浓度下，在落叶可提取的OC和水铁矿之间进行吸附实验。在 Ca2+ 存在的情况下，水铁矿对 OC 的吸附程度在所有测试的 pH 值范围内都会增加，特别是在 pH ≥ 7 时。在 pH 9 时，吸附的 OC 浓度从 8.72 ± 0.16 增加到 13.3 ± 0.20 mmol OC g-1 水铁矿。添加 Ca2+ 和添加 30 mM Ca2+。批量实验与光谱研究相结合，探索吸附 OC 的形态并阐明吸附机制。 ATR-FTIR 光谱分析表明，羧基功能部分是主要吸附的 OC 物质，优先与水铁矿结合，并表明在 pH 9 的 Ca 存在下，Fe-羧酸盐配体交换增加。从批次到光谱实验的结果提供了重要的证据增强了溶解的 OC 对 2 系水铁矿的封存，并表明形成了 Fe-Ca-OC 三元复合物。 这项研究的结果将为环境碳循环建模提供信息，并有可能影响土地管理策略，以最大限度地减少有机质稳定性。