Global efforts to build a net-zero economy and the irreplaceable roles of rare-earth elements (REEs) in low-carbon technologies urge the understanding of REE occurrence in natural deposits, discovery of alternative REE resources, and development of green extraction technologies. Advancement in these directions requires comprehensive knowledge on geochemical behaviors of REEs in the presence of naturally prevalent organic ligands, yet much remains unknown about organic ligand-mediated REE mobilization/fractionation and related mechanisms. Herein, we investigated REE mobilization from representative host minerals induced by three representative organic ligands: oxalate, citrate, and the siderophore desferrioxamine B (DFOB). Reaction pH conditions were selected to isolate the ligand-complexation effect versus proton dissolution. The presence of these organic ligands displayed varied impacts, with REE dissolution remarkably enhanced by citrate, mildly promoted by DFOB, and showing divergent effects in the presence of oxalate, depending on the mineral type and reaction pH. Thermodynamic modeling indicates the dominant presence of REE–ligand complexes under studied conditions and suggests ligand-promoted REE dissolution to be the dominant mechanism, consistent with experimental data. In addition, REE dissolution mediated by these ligands exhibited a distinct fractionation toward heavy REE (HREE) enrichment in the solution phase, which can be mainly attributed to the formation of thermodynamically predicted more stable HREE–ligand complexes. The combined thermodynamic modeling and experimental approach provides a framework for the systematic investigation of REE mobilization, distribution, and fractionation in the presence of organic ligands in natural systems and for the design of green extraction technologies.

中文翻译：

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有机配体介导的碳酸盐和磷酸盐矿物中稀土元素 (REE) 的溶解和分馏

全球构建净零经济的努力以及稀土元素 (REE) 在低碳技术中不可替代的作用，促使人们了解稀土元素在天然矿床中的赋存情况、发现替代稀土资源以及开发绿色提取技术。这些方向的进展需要对天然存在的有机配体存在下 REE 的地球化学行为有全面的了解，但对于有机配体介导的 REE 动员/分馏和相关机制仍知之甚少。在此，我们研究了三种代表性有机配体（草酸盐、柠檬酸盐和铁载体去铁胺 B (DFOB)）诱导的代表性宿主矿物的 REE 动员。选择反应 pH 条件来隔离配体络合效应与质子溶解的关系。这些有机配体的存在表现出不同的影响，柠檬酸盐显着增强稀土元素的溶解，DFOB 轻微促进稀土元素的溶解，并且在草酸盐存在的情况下表现出不同的影响，具体取决于矿物类型和反应 pH 值。热力学模型表明，在研究条件下，稀土元素-配体复合物占主导地位，并表明配体促进的稀土元素溶解是主要机制，与实验数据一致。此外，这些配体介导的 REE 溶解在溶液相中表现出明显的重 REE (HREE) 富集分馏，这主要归因于热力学预测的更稳定的 HREE-配体复合物的形成。热力学建模和实验相结合的方法为系统研究自然系统中有机配体存在下的稀土元素动员、分布和分馏以及绿色提取技术的设计提供了框架。