Nayak Srikanth, Kumal Raju R, Liu Zhu, Qiao Baofu, Clark Aurora E, Uysal Ahmet
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States.
Department of Chemistry, Washington State University, Pullman, Washington 99164, United States.
ACS Appl Mater Interfaces. 2021 May 26;13(20):24194-24206. doi: 10.1021/acsami.0c23158. Epub 2021 Apr 13.
Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl and square-planar PdCl), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl compared to that with PdCl. Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.
有效且节能地分离贵金属和稀有金属对于各种先进技术而言非常重要。液 - 液萃取（LLE）是一种相对能耗较低的分离技术，广泛应用于镧系元素、锕系元素和铂族金属（PGM）的分离。在液 - 液萃取中，离子在水相和有机相之间的分布由焓（配位相互作用）和熵（流体重组）贡献决定。这些贡献的分子尺度细节尚未得到很好的理解。从水相中优先萃取离子通常与有机相中形成的流体组织相关，因为随着金属负载量的增加，远程组织会增加。然而，很难确定有机相流体组织导致金属负载或由金属负载引起的程度。在本研究中，我们证明了具有相同金属负载量的两个系统可能赋予非常不同的有机相组织，并研究了潜在的分子尺度机制。小角X射线散射表明，尽管两种金属酸盐（八面体PtCl和平面正方形PdCl）都完全转移到有机相中，但甲苯中季铵萃取剂溶液的结构受它们萃取的影响不同。与PdCl相比，金属酸盐 - 萃取剂络合物形成的聚集体（近似为反胶束）与PtCl形成的聚集体表现出更长程的有序（聚集）。对模型朗缪尔单分子层的振动和频产生光谱及互补的原子分子动力学模拟表明，两种金属酸盐对界面水合结构的影响不同。此外，界面水合与水萃取到有机相中相关。这些结果支持了有机相组织结构与金属酸盐 - 萃取剂络合物聚集体内存在的不同局部水合之间的强关系，这与金属酸盐浓度无关。
