Ionic liquid as adjuvant in an aqueous biphasic system composed of polyethylene glycol for green separation of Pd(II) from hydrochloric solution

An aqueous biphasic system (ABS) is considered to be an environmentally friendly technology for the extraction of diverse substances because the main component of the two phases is water. In this study, extraction behaviors of palladium(II) from hydrochloric solution using a typical polymer/salt ABS with ionic liquid (IL) as an adjuvant were studied. Three imidazolium-based ionic liquids ([Cnmim]Br, n = 4, 6 and 8) were used to tune the properties of the ABS to enhance the extraction performance of Pd(II). The parameters that influence palladium extraction including IL species, IL concentration, salinity, original Pd(II) concentration, equilibrium pH, temperature, and contact time, were systematically studied and discussed. These results demonstrate that incorporating ILs into the PEG2000/salt ABS markedly improved the extraction efficiency, and 96–99% of Pd(II) was distributed into the PEG-rich phase. Among the three imidazolium-based ILs, the maximum extraction efficiency was achieved by the IL that had the shortest alkyl chain attached to the imidazole ring, and it can be interpreted that the electrostatic attraction was the main driving force during Pd(II) extraction. The results of FT-IR and TEM characterization show that the anionic PdCl42− transferred into the PEG-rich phase via anion exchange with the anionic Br− in [C4mim]Br, and the mechanism was confirmed by the stoichiometry of the extracted species of [2C4mim+·PdCl42−]. Hydrazine hydrate (80 wt%) completely reduced Pd(II) in the PEG-rich phase, and EDX and SEM were used to analyze the sediments. The results established that the developed polymer/salt ABS with IL as an adjuvant is a potential environmentally friendly extraction system for palladium recovery from an acidic media.