Deep-sea mud shows tremendous resource potential for rare earth elements (REEs) and its formation is closely associated with sea-floor hydrothermal activities. Iron (oxyhydr)oxides link the sources and sinks of REEs in sea-floor hydrothermal systems. However, the complexation mechanisms of REEs on iron (oxyhydr)oxides have not been well understood yet. In this study, by using the first principles molecular dynamics technique, we first calculated the pKa’s of surface groups on goethite (110) surface at elevated temperatures relevant to sea-floor hydrothermal systems and then evaluated the complexation structures and free energies of REEs on goethite (110) and (010) surfaces using the method of constraint by taking Sc3+, Y3+, and La3+ as model REE cations. The results show that REE complexation occurs in mildly acidic to neutral conditions. The most thermodynamically stable complexes of REEs are bidentate complexes on two neighboring FeOH sites on goethite (110) surface and tridentate complexes on two neighboring FeOH sites plus one Fe2OH site on goethite (010) surface. Sc3+ complexes match the goethite lattice and can be incorporated into the lattice. The stabilities of REE complexes increase with the distance from hydrothermal vents. Complexation of Y3+ is less favored on goethite compared to other REEs whereas Sc3+ prefers complexation on goethite (010) surface and La3+ exhibits similar stabilities on both (010) and (110) surfaces. The derived atomic level complexation mechanisms would be helpful for the interpretation of experimental data and the prediction of REEs’ behavior in the sea-floor. The findings presented here provide valuable insights into REEs fractionation and enrichment in deep-sea muds.

中文翻译：

---

海底热液环境中针铁矿上稀土元素的表面络合：来自第一性原理模拟的见解


深海泥浆显示出稀土元素 （REE） 的巨大资源潜力，其形成与海底热液活动密切相关。铁（羟基）氧化物连接海底热液系统中 REE 的源头和汇。然而，稀土元素对铁（氢氧）氧化物的络合机制尚未得到很好的了解。在本研究中，通过使用第一性原理分子动力学技术，我们首先计算了与海底热液系统相关的高温下针铁矿 （110） 表面基团的 pKa，然后利用 Sc3+ 的约束方法评估了针铁矿 （110） 和 （010） 表面上稀土元素的络合结构和自由能， Y3+ 和 La3+ 作为模型 REE 阳离子。结果表明，REE 络合发生在弱酸性至中性条件下。REE 热力学最稳定的配合物是针铁矿 （110） 表面两个相邻 FeOH 位点上的双齿络合物和针铁矿 （010） 表面上两个相邻 FeOH 位点上的三齿络合物和一个 Fe2OH 位点。Sc3+ 复合物与针铁矿晶格匹配，可以掺入晶格中。REE 复合物的稳定性随着与热液喷口的距离而增加。与其他稀土元素相比，Y3+ 的络合在针铁矿上不太受欢迎，而 Sc3+ 更喜欢在针铁矿 （010） 表面的络合，而 La3+ 在针铁矿 （010） 和 （110） 表面上表现出相似的稳定性。由此得出的原子级络合机制将有助于解释实验数据和预测 REE 在海底的行为。这里介绍的发现为深海泥中的 REE 分馏和富集提供了有价值的见解。