Rapid and efficient solvent extraction of inert platinum group metals agitated under microwave irradiation

Despite various applications of platinum group metals (PGMs) in various kinds of electronic devices and catalysts, the radioactive waste containing platinum group metals in nuclear energy system remains an issue to be solved.

Ionic liquids ILs have been used as solvent extraction of metal ions Mⁿ⁺ as it has proven that ionic liquids consisting betainium ([Hbet]⁺) and bis(trifluoromethylsulfonyl)amide (Tf₂N^–) is enough hydrophobic to form an organic phase immiscible with water at room temperature which shows complete miscibility at a temperature above 55⁰C (upper critical solution temperature).

Researchers from Tokyo Institute of Technology made expanding contributions in application of microwave-assisted extraction technologies. Their study published an article in journal, ACS Sustainable Chemistry and Engineering demonstrated the effectiveness of microwave irradiation to extraction of inert platinum group metals  in current aqueous nitric acid (HNO_3(aq)) and ionic liquids consisting of [Hbet][Tf₂N^–].

Previous application of [Hbet][Tf₂N^–] to recover PGMs like Ru(III), Rh(III) and Pd(II) from aqueous nitric acid under vigorous shaking at 25⁰C instead of thermomorphic behavior of extraction system has shown high extractability for Pd(II) but low extractability for both Ru(III) and Rh(III). Further extraction of Ru(III) and Rh(III) was seen to slowly proceed after period of time which could be ascribed to their inertness in ligand substitution reactions.

Raising the temperature may accelerate chemical reaction of interest. Hence, a microwave irradiation may be useful because it’s been previously used in organic and inorganic synthesis.

Before proceeding into the results, mixture was found to be heated efficiently reaching temperature greater than 70⁰C within 40 s and aqueous/[Hbet][Tf₂N] layers got to be miscible with each other above its upper critical solution temperature (55⁰C) to form a homogenous phase.

Results obtained using a 0.30 M (HNO_3(aq))/[Hbet][Tf₂N] system showed distribution of Pd(II) rapidly reached its equilibrium within 40 s of each microwave irradiation while extraction of Rh(III) was also completed with 80 s. Ru(III) showed the slowest extraction kinetics but it was much faster than the ordinary shaking method previously used.

Up to 2.00 M of HNO₃ in aqueous layer, distribution of Ru(III), Rh(III) and Pd(II) was equilibrated within 360, 160 and 40 s respectively. The effectiveness of microwave treatment in extraction of metal ions in liquid-liquid system reported for the first time showed effectiveness not only in extraction system above upper critical solution temperature but also promotion of significant extraction kinetics of inert platinum group metals .

Distribution ratio D of Rh(III) was 1.0 at [HNO₃] = 2.00 M, and attained 21.6 at [HNO₃] = 0.30 M which gave the highest distribution ratio in Rh (III) till date. This results confirmed effectiveness of the current technique utilizing a combination of the thermomorphic ionic liquid and microwave-assisted extraction.

Further results studied from microwave-assisted extraction of the PGMs mixed together in 0.30 M (HNO_3(aq))/ [Hbet][Tf₂N] within 80 s under 200 W microwave irradiation when compared under vigorous shaking which showed higher difference with time as effectiveness of microwave-assisted extraction for inert platinum group metals.

This study prove that the kinetic aspect of microwave-assisted extraction could be employable as one of solutions to isolate each metal ions efficiently.