The interactions between iron oxides and organic matter (OM) play vital roles in the geochemical cycle of cadmium (Cd). However, the effects of the formation modes of ferrihydrite (Fh)-low molecular weight OM (e.g., fulvic acid (FA)) composites on Cd(II) adsorption remain poorly understood. The immobilization mechanisms of Cd(II) on Fe-OM composites formed by adsorption and coprecipitation at varying C/Fe molar ratios were investigated by means of adsorption batch experiments, two-dimensional correlation spectroscopy, and surface complexation models (SCMs). The composites formed by adsorption or coprecipitation exhibited a crystal structure similar to that of Fh. Ligand exchange and hydrogen bonding were identified as the primary mechanisms between components in adsorption composites and coprecipitates, respectively. Compared to coprecipitates, the adsorption composites showed a higher adsorption capacity and formed ternary complexes (Fh-FA-Cd). In coprecipitates, Cd(II) primarily interacted with the carboxyl and hydroxyl groups of FA and the hydroxyl groups of Fh. With increasing C/Fe molar ratios, the FA functional group (R-COOH) in adsorption composites responded more quickly to Cd(II). However, the order of functional group reactions in coprecipitates was unaffected by C/Fe molar ratios, which is due to the irregular distribution of C and Fe elements. SCM calculation results indicated that Cd(II) distribution on Fh in adsorption composites was higher than that in coprecipitates. The molar ratios of C/Fe and Cd(II) concentrations influenced the distribution of Cd(II) on the composites, with the highest proportion of Cd(II) on Fh reaching about 70%. These findings contribute to understanding Cd behavior in environments with periodically fluctuating redox conditions.

中文翻译：

---

水铁-低分子量有机物复合材料的形成模式对 Cd（II） 吸附的影响。


氧化铁和有机物 （OM） 之间的相互作用在镉 （Cd） 的地球化学循环中起着至关重要的作用。然而，水铁酸盐 （Fh）-低分子量 OM （例如黄腐酸 （FA）） 复合材料的形成模式对 Cd（II） 吸附的影响仍然知之甚少。通过吸附批量实验、二维相关光谱和表面络合模型 （SCM） 研究了不同 C/Fe 摩尔比吸附和共沉淀形成的 Cd（II） 在 Fe-OM 复合材料上的固定机制。吸附或共沉淀形成的复合材料表现出与 Fh 相似的晶体结构，配体交换和氢键分别被确定为吸附复合材料和共沉淀物组分之间的主要机制。与共沉淀物相比，吸附复合材料表现出更高的吸附能力，并形成了三元络合物 （Fh-FA-Cd）。在共沉淀物中，Cd（II） 主要与 FA 的羧基和羟基以及 Fh 的羟基相互作用。随着 C/Fe 摩尔比的增加，吸附复合材料中的 FA 官能团 （R-COOH） 对 Cd（II） 的响应更快。然而，共沉淀物中官能团反应的顺序不受 C/Fe 摩尔比的影响，这是由于 C 和 Fe 元素分布不规则。SCM 计算结果表明，吸附复合材料中 Cd（II） 在 Fh 上的分布高于共沉淀物。C/Fe 和 Cd（II） 浓度的摩尔比影响了 Cd（II） 在复合材料上的分布，其中 Cd（II） 在 Fh 上的最高比例达到约 70%。这些发现有助于理解氧化还原条件周期性波动的环境中的 Cd 行为。