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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钙对有机碳螯合至水铁矿的协同作用。

隔离环境系统中的有机碳（OC）对于缓解气候变化至关重要。有机矿物质的结合，尤其是与铁（Fe）氧化物的结合，驱动了土壤中OC螯合和稳定性的化学反应。短程铁氧化物，例如三水铁矿，在二元体系中表现出对OC的高度亲和力。钙通常与土壤中的OC和Fe氧化物共缔合，尽管其结合机理（例如阳离子桥联）和共缔合对OC螯合的影响仍未解决。我们探讨了钙（Ca2 +）对溶解的OC吸附到2线亚铁水合物上的影响。在pH为4到9的具有不同初始C / Fe摩尔比和Ca2 +浓度的叶凋落物可萃取OC和亚铁酸盐之间进行了吸附实验。在所有测试的pH值下，特别是在pH≥7的情况下，在Ca2 +存在下OC对水铁矿的吸附程度均增加了。pH9的吸附OC浓度从8.72±0.16增至13.3±0.20 mmol。添加Ca2 +和添加30 mM Ca2 +。批处理实验与光谱学研究相结合，以探测所吸收的OC的形态并阐明其吸收机理。ATR-FTIR光谱分析表明，羧基官能团是主要吸附在三水铝石上的主要吸附的OC种类，并表明在pH为9的Ca存在下，Fe-羧酸盐配体交换增加。从分批到光谱实验的结果为增加溶解的OC螯合成2线水铁矿提供了重要的证据，并表明了Fe-Ca-OC三元络合物的形成。这项研究的结果将为环境碳循环建模提供参考，并有可能影响土地管理策略以最大程度地降低OM稳定性。