Interfacial structure in the liquid–liquid extraction of rare earth elements by phosphoric acid ligands: a molecular dynamics study. Issue 7 (10th February 2020)

• Record Type: Journal Article
• Authors: Dwadasi, Balarama Sridhar; Goverapet Srinivasan, Sriram; Rai, Beena
• Is Part Of: Physical chemistry chemical physics. Volume 22:Issue 7(2020)
• Page Start: 4177
• Page End: 4192
• Publication Date: 2020-02-10
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c9cp05719f
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : MD simulations reveal the chemical and physical heterogeneity at the liquid–liquid interface, nature of complexes formed by phosphoric acid ligands with lanthanides, and the sequence of events in the extraction of these ions. Abstract : Solvent extraction (SX), wherein two immiscible liquids, one containing the extractant molecules and the other containing the solute to be extracted are brought in contact to effect the phase transfer of the solute, underpins metal extraction and recovery processes. The interfacial region is of utmost importance in the SX process, since besides thermodynamics, the physical and chemical heterogeneity at the interface governs the kinetics of the process. Yet, a fundamental understanding of this heterogeneity and its implications for the extraction mechanism are currently lacking. We use molecular dynamics (MD) simulations to study the liquid–liquid interface under conditions relevant to the SX of Rare Earth Elements (REEs) by a phosphoric acid ligand. Simulations revealed that the extractant molecules and varying amounts of acid and metal ions partitioned to the interface. The presence of these species had a significant effect on the interfacial thickness, hydrogen bond life times and orientations of the water molecules at the interface. Deprotonation of the ligands was essential for the adsorption of the metal ions at the interface, with these ions forming a number of different complexes at the interface involving one to three extractant molecules and four to eight water molecules. Although the interface itself was rough, no obvious 'finger-like' water protrusions penetrating the organic phase were seen in our simulations. While the results of our work help us gain fundamental insights into the sequence of events leading to the formation of a variety of interfacial complexes, they also emphasize the need to carry out a more detailed atomic level study to understand the full mechanism of extraction of REEs from the aqueous to organic phases by phosphoric acid ligands.