Ferrihydrite (Fh) is an important scavenger for sequestering cadmium (Cd). However, Fh is poorly crystallized and tends to undergo phase transformation under the impact of dissolved organic matter (DOM). The roles of DOM in Fh transformation pathway and the consequent Cd2+ remobilization behavior remain debate due to its structural heterogeneity and diversity. Herein, formic acid (FA), oxalic acid (Ox), and citric acid (CA) were selected as model DOM, which have one, two, and three carboxyl ligands, respectively. Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra revealed an organic-specific correlation between Fh transformation and Cd2+ transport. FA with monodentate carboxyl ligand, lacking binding with Fh particles, had negligible influence on transformation dynamics and Cd2+ transport. The complexation of bidentate Ox with Fh particles impaired the stability of Fe(O,OH)6 octahedra, accelerating Fh phase transformation, thereby increasing Cd2+ release. During the subsequent recrystallization, Ox guided crystal growth via oriented attachment, which possibly occluded the remaining Cd2+ within structural defects of the secondary minerals. Conversely, tridentate CA exhibited a strong affinity with Fh, resulting in the formation of ferric citrate surface complexes that stabilized Fh against transformation and thus enhanced the long-term immobilization of Cd2+. These findings highlight the significant role of organic-ion/mineral interactions in Fh transformation, which are fundamentally linked to predicting Cd2+ transport behavior in environmental systems.

中文翻译：

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受溶解有机物影响的水铁矿相变过程中镉的命运：来自有机矿物相互作用的见解


水铁酸盐 （Fh） 是封存镉 （Cd） 的重要清除剂。然而，Fh 的结晶度很差，并且在溶解有机物 （DOM） 的影响下往往会发生相变。由于其结构异质性和多样性，DOM 在 Fh 转化途径中的作用和随之而来的 Cd2 + 再动员行为仍然存在争议。本文选择甲酸 （FA） 、草酸 （Ox） 和柠檬酸 （CA） 作为模型 DOM，它们分别具有 1 个、2 个和 3 个羧基配体。Fe K 边缘扩展 X 射线吸收精细结构 （EXAFS） 光谱揭示了 Fh 转变与 Cd2+ 传输之间的有机特异性相关性。具有单齿羧基配体的 FA 与 Fh 颗粒缺乏结合，对转化动力学和 Cd2+ 转运的影响可以忽略不计。双齿 Ox 与 Fh 颗粒的络合损害了 Fe（O，OH）6 八面体的稳定性，加速了 Fh 相变，从而增加了 Cd2+ 的释放。在随后的再结晶过程中，Ox 通过定向附着引导晶体生长，这可能遮挡了次生矿物结构缺陷内剩余的 Cd2+。相反，三齿 CA 与 Fh 表现出很强的亲和力，导致形成柠檬酸铁表面复合物，稳定 Fh 抵抗转化，从而增强 Cd2+ 的长期固定。这些发现强调了有机离子/矿物相互作用在 Fh 转化中的重要作用，这与预测环境系统中 Cd2+ 的传输行为从根本上有关。