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

Significance Statement

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 (HNO3(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 (HNO3(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 (HNO3(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.

About the author

Prof. Koichiro Takao was born in Hiroshima, Japan, in 1980. He studied (and is still working for) actinide coordination chemistry, and received Ph.D. in 2006 from Tokyo Tech. After that, he joined a project to develop a simple reprocessing process for spent nuclear fuels. His main tasks were synthesis and evaluation of N-substituted 2-pyrrolidone derivatives precisely designed for selective precipitation of hexavalent actinides (U and Pu) from a nitric acid solution dissolving the spent nuclear fuels which are complicated mixture of U, Pu and various fission products and minor actinides.

In 2008, he moved to Forschungszentrum Dresden-Rossendorf (Germany) to explore fundamental coordination and solution chemistry of actinides. The main focus was structure determination of various actinide complexes occurring in the solution by using X-ray absorption fine structure (XAFS). After 2 and half years including 6-month child-care leave, he was appointed as an assistant professor in Seikei University (Japan). He was involved in the coordination chemistry group, and started to investigate photophysical and photochemical behaviour of transition metal complexes like Cu, Ag, Pd and Pt for photocatalysts or OLEDs. Because any nuclear fuel materials were not accessible at that time, He shifted his activity in the actinide coordination chemistry from experiment to DFT calculation.

He moved to the current affiliation in Tokyo Tech as an assistant professor in 2013, and then promoted to an associate professor in 2015. Through the whole period of his research, he is also strongly interested in ionic liquids (ILs) as alternative solvents in coordination and solution chemistry of metal ions, especially for the separation of actinides and relevant elements in the nuclear fuel cycle. One of his successful cases is rapid and efficient extraction of platinum group metals like Pd, Ru and Rh to a thermomorphic ionic liquid under microwave irradiation. 

                                                                                                                  

Journal Reference

Saki Ikeda, Takahiro Mori, Yasuhisa Ikeda, Koichiro Takao*. Microwave-Assisted Solvent Extraction of Inert Platinum Group Metals from HNO3(aq) to Betainium-Based Thermomorphic Ionic Liquid ACS Sustainable Chem. Eng., 2016, 4 (5), pp 2459–2463.

Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 N1-32, O-okayama, Meguro-ku, Tokyo 152-8550, Japan.

 

 

 

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