As consequence of the dual demands for pollution control and carbon (C) fixation in soils, Fe(II)-catalyzed mineral transformation may be a promising method to simultaneously immobilize heavy metals or organic matter (OM), but the underlying mechanisms remain unclear. Here, the synchronous sequestration mechanism of cadmium (Cd) and fulvic acid (FA) during Fe(II)-catalyzed the transformation ferrihydrite with C/Fe molar ratio of 0.21 were examined. Mineral phase analysis revealed that increasing the Fe(II) concentration (1–5 mM) favored the transformation of lepidocrocite and goethite to magnetite, and ferrihydrite transformation rate increased with increasing Fe(II) concentration. Color overlays and line profiles of elements depicted that Cd was dominantly adsorbed on the lepidocrocite and goethite surfaces. A positive correlation between the quantity of nonextractable Cd and magnetite further indicated that Cd may be sequestered by magnetite. Meanwhile, FA molecules were adsorbed on goethite surfaces and magnetite aggregates, and incomplete structure of lepidocrocite provide spaces for immobilizing C. Newly formed iron (Fe) (oxyhydr)oxides may immobilize Cd through surface binding, structural substitution, and physical encapsulation. The OM bound to the newly formed Fe (oxyhydr)oxides was rich in aromatic and carboxyl functional groups, which was beneficial for binding Cd, whereas the presence of Cd promoted the generation of nano pore spaces or defects and consequently enhanced FA sequestration. Therefore, Cd immobilization and FA sequestration can be synchronously achieved during the phase transformation. The findings provide a profound insight into various nanoscale mechanisms accounting for the fate of Cd and FA coupled with mineral transformation. The findings also are very helpful for developing strategies for simultaneously immobilizing heavy metals and C in soils.

由于对土壤中污染控制和碳 (C) 固定的双重需求，Fe(II) 催化的矿物转化可能是一种同时固定重金属或有机物 (OM) 的有前景的方法，但其潜在机制仍不清楚。在这里，研究了在 Fe(II) 催化转化 C/Fe 摩尔比为 0.21 的水铁矿过程中镉 (Cd) 和富里酸 (FA) 的同步螯合机制。矿物相分析表明，增加 Fe(II) 浓度 (1-5 mM) 有利于纤铁矿和针铁矿向磁铁矿的转变，水铁矿转变率随着 Fe(II) 浓度的增加而增加。元素的颜色叠加和线条轮廓描绘了 Cd 主要吸附在纤铁矿和针铁矿表面。不可萃取的镉含量与磁铁矿呈正相关，进一步表明镉可能被磁铁矿螯合。同时，FA分子吸附在针铁矿表面和磁铁矿聚集体上，纤铁矿的不完整结构为固定C提供了空间。新形成的铁（Fe）（氢氧化物）氧化物可以通过表面结合、结构取代和物理封装等方式固定Cd。与新形成的Fe（羟基）氧化物结合的OM富含芳香族和羧基官能团，有利于结合Cd，而Cd的存在促进了纳米孔空间或缺陷的产生，从而增强了FA的螯合。因此，在相变过程中可以同时实现 Cd 的固定和 FA 的封存。这些发现为解释 Cd 和 FA 的命运以及矿物转化的各种纳米级机制提供了深刻的见解。这些发现对于制定同时将重金属和碳固定在土壤中的策略也非常有帮助。