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

Abstract

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 PtCl 6 2and square-planar PdCl 4 2-), 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 PtCl 6 2compared to that with PdCl 4 2-. 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. File list (2) download file view on ChemRxiv Origins_of_clustering_acsami_revised.pdf (1.94 MiB) download file view on ChemRxiv SI_Origins of clustering_acsami_revised.pdf (1.82 MiB)

Effects of Ion Adsorption on Graphene Oxide Films and Interfacial Water Structure: A Molecular-Scale Description

“…SFG experiments were carried out using an EKSPLA laser system, described in detail elsewhere. [56][57][58][59] Briefly, an amplified Nd:YAG laser system produces 29 ps pulses with 28 mJ energy centered at 1064 nm at a repetition rate of 50 Hz. A harmonic unit and a second harmonic crystal split the 1064 nm laser into two beams of 532 nm.…”

Thiocyanate Ions Form Antiparallel Populations at the Concentrated Electrolyte / Charged Surfactant Interface

“…We have previously demonstrated that a sub-ensemble analysis of MD simulations can identify the populations leading to a certain VSFG signal in experiments. 12,22 However, a similar analysis is beyond the limits of this Letter and will be explained in another study.…”

“…For instance, in liquid-liquid extraction (LLE), metal ions are transferred from an aqueous phase into an organic phase with the help of the amphiphilic extractant molecules. 12 In many cases extractants are neutral or positively charged, requiring the transfer of anions with the target metal ions in order to form neutral or anionic complexes in the organic phase. [13][14][15] Interestingly, in most cases these metal-anion complexes are not stable in the aqueous phase, raising the question where and how these complexes form.…”

Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

“…The VSFG measurements are acquired using an EKSPLA system, which has been described previously. 14,[16][17]41 Briefly, an amplified Nd:YAG laser system produces 29 ps pulses having 28 mJ energy centered at 1064 nm with a repetition rate of 50 Hz. The harmonic unit splits the 1064 nm laser and generates two beams of 532 nm.…”