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Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction

revised on 17.03.2021, 15:57 and posted on 18.03.2021, 07:55 by Srikanth Nayak, Raju R. Kumal, Zhu Liu, Baofu Qiao, Aurora Clark, Ahmet Uysal
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 organization; 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 PtCl62- and square-planar PdCl42-), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit more long-range order (clustering) with PtCl62- compared to that with PdCl42-. Vibrational sum frequency generation spectroscopy, and complimentary atomistic molecular dynamics simulations on model Langmuir monolayers, indicate that the two metalates affect the interfacial hydration structures differently. Further, 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 different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.


The work presented here is supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences, Geosciences, and Biosciences, under contract DE-AC02-06CH11357. Z.L. and A.E.C. are supported by the same program, under contract DE-SC0001815. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. NSF’s ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under Grant NSF/CHE-1834750.


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Argonne National Laboratory


United States

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No conflicts of interest