Computational investigation on the molecular driving forces for extraction of Co2+, Ni2+, and Fe3+in poly(ethylene oxide)/thiocyanate salt aqueous two-phase systems

Aqueous two-phase systems are environmentally friendlier alternatives for hydrometallurgical processing and purification of metals. However, the viability of application of those systems in such processes is still under investigation, in which the comprehension of the fundamental aspects that govern the metal extraction is essential and can be further constructed using computational approaches. This research paper presents a comprehensive computational study on the fundamental nature of metal extraction in polymer-salt aqueous twophase systems (ATPS), comparing the results of modeling and simulation work to the theoretical and experimental data available in the literature. Specifically, the mechanisms underlying the extraction of Co2+, Ni2+, and Fe3+, in ATPS formed by poly(ethylene oxide) (PEO) and thiocyanate salt are investigated, using quantum chemical calculations and molecular dynamics simulations. The formation of pseudo-polycations (PPCs) and thiocyanate metalates (complexes formed by a metal cation bonded to one or more anions) and the interaction of PEO fragments, thiocyanate, and aquo ligands with metal cations are explored. The binding affinity of these species is evaluated from various perspectives, including structural, electronic, and thermodynamic considerations. The solvation behavior of the species in solvents with different polarities is also examined. The findings elucidate the mechanisms governing metal cations partitioning behavior in PEO/thiocyanate ATPS, confirming the mechanisms proposed previously in the literature, involving charge transfer between metal-thiocyanate complexes and polyethylene oxide ether groups as well as interactions between metalate and PPCs. This study represents the first-ever computational investigation into the molecular driving forces behind metal extraction in polymer-salt ATPS, providing valuable insights for optimizing extraction processes and paving the way for future research in this field.