Sustainable extraction and separation of precious metals from hydrochloric media using novel ionic liquid-in-water microemulsion

A novel ionic liquid-in-water microemulsion (IL101/TX100/water) was developed for sustainable extraction and separation of precious metals (i.e., Au, Pd, and Pt) in acidic chloride media. Five-step protocol is composed of (i) mixing the feed solution containing precious metals with the ionic liquid based microemulsion; (ii) heating for phase splitting; (iii) phase separation; (iv) stripping; (v) regeneration. The aqueous microemulsion combines green properties of ionic liquid and advantages of microemulsion, and thus affords significant environmental benefits. Phase behaviour and structure of the microemulsion were determined by measuring the conductivity. Effect of various parameters was investigated for the complete extraction and selective recovery of highly pure precious metals ( > 99.9% w/w). Based on anion exchange mechanism and micellar structure of the aqueous microemulsion phase, the extraction of Au(III), Pt(IV) and Pd(II) from chloride media characterized by P-31 NMR spectroscopies was proposed as{(([P-66614](+))(m)([A](n-))(q)(Cl-)(m-nq)(TX-100)(p)} where [A](n-) refers to the anionic metal species to be extracted, p denotes the average aggregation number of the TX-100 micelles, m the average number of [P-66614]Cl-+(-) molecules in each (TX-100)(p) micelle and q the average number of [A](n-) species extracted per micelle. Two-step stripping strategy was successfully adapted to treat the loaded ionic liquid-surfactant-rich (ILSR) phases with low aggressive media for the selective recovery of Au(III) with Na2S2O3 and Pd (II) and Pt(IV) with thiourea/HCl. The possible recirculation of regenerated ILSR phase without loss of performance was examined within five extraction-stripping cycles. Finally, a flow-sheet towards "zero-waste" concept was proposed for the complete extraction and selective recovery of individual precious metals using environmentally friendly aqueous microemulsion.