Aqueous biphasic separation of ⁹⁷Ru and ^95,96Tc from yttrium

Abstract

An aqueous biphasic separation technique has been developed for the separation of ⁹⁷Ru, a potential candidate radionuclide in nuclear medicine, from its target matrix, yttrium. The extraction of ruthenium and technetium from bulk yttrium has been carried out with 50% (w/v) PEG-4000 and PEG-6000 against 2 M solution of various salts, such as Na-citrate, Na-tartarate, Na-malonate, Na₂CO₃, NaHSO₃, Na₂SO₄, Na₂S₂O₃, K₂HPO₄, K₃PO₄, (NH₄)₂SO₄ and 4 M KOH, at room temperature. The influence of the pH of some salt rich phases (e.g., Na-tartarate and (NH₄)₂SO₄) on the extraction behavior of ⁹⁷Ru and ^95,96Tc into the PEG rich phase was also studied. In the presence of Na-tartarate, Na-citrate, K₂HPO₄, K₃PO₄, KOH, Na₂CO₃, Na₂SO₄, Na₂SO₃ and (NH₄)₂SO₄ salt solutions, ⁹⁷Ru and ^95,96Tc were preferentially extracted into the PEG rich phase. In 50% (w/v) PEG-4000-2 M (NH₄)₂SO₄ ABS system, 83% of ⁹⁷Ru along with 96% of ^95,96Tc were extracted into the PEG rich phase without any contamination of the yttrium target. Back extraction of ⁹⁷Ru into the salt rich phases from the PEG rich phase was also carried out using 2 M salt solutions of K₂CO₃, Na₂S₂O₃ and 4 M KOH. About 90% back extraction of ⁹⁷Ru into the salt rich phases without any contamination of ^95,96Tc was obtained with 2 M Na₂S₂O₃ salt solution. Dialysis of the PEG rich phase containing ⁹⁷Ru along with ^95,96Tc was also carried out against deionised water to obtain pure ⁹⁷Ru.

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Introduction

Polyethylene glycol (PEG) based aqueous biphasic systems (ABS) are greener analytical techniques compared to traditional solvent extraction systems, because both the phases in ABS are aqueous in nature.¹ ABS can be formulated with the water-soluble polymer PEG and other polymers or PEG with inorganic salts, such as sulphate, phosphate, and carbonate, in particular concentrations.² The properties of ABS generally depend on the phase components of the ABS system. PEG can be chosen as one of the phase forming components because of its nontoxic, non-flammable, and inexpensive nature.^3–5 In addition, the PEG-rich phase in a PEG-ABS system is tunable and, therefore, the phase characteristics of such systems can be modified by fine tuning the PEG-rich phase for better partitioning behavior of the solute into the PEG-rich phase.

Various applications of ABS systems in the fields of separation and purification of organic molecules and metal ions have been reported in the literature. Roger et al. reported the partitioning behavior of pertechnetate using a PEG-ABS system.^3,5,6 Over the last ten years, our laboratory has made continuous endeavors to develop new green separation methodologies, using aqueous biphasic systems^7–16 or other environmentally benign reagents such as polyvinylpyrrolidone^17,18 and ionic liquids.¹⁹

In this study, we made an attempt to separate Ru and Tc radionuclides from bulk yttrium target. Radiometric methods were employed for detection. Corresponding radioisotopes, such as ⁹⁷Ru, ^95,96Tc and ⁸⁸Y, were used as the precursors of Ru, Tc and Y, respectively. The motivation of the experiment lies in the fact that ⁹⁷Ru is a candidate radionuclide in nuclear medicine, which may have potential applications in diagnostic imaging as well as for therapeutic purposes because of its suitable chemical and nuclear properties, such as a moderate half-life (T_1/2: 2.83 d) and high intensity low energy γ rays (216 keV, 86% and 324.5 keV, 10.25%). Due to the presence of multiple oxidation states such as Ru(II), Ru(III), Ru(IV) and Ru(VIII) and various coordination numbers (4, 5 and 6), Ru can form a series of complexes that have useful properties for tuning various metal-ligand combinations for radiopharmaceutical chemistry.²⁰ Generally, the reported production routes of ⁹⁷Ru are neutron, proton and alpha particle activation on suitable targets.^21–31 Recently, we reported two new production routes of ⁹⁷Ru by heavy ion activation such as activation through ^natNb(⁷Li, 3n)⁹⁷Ru³² and ^natY(¹²C, p3n)⁹⁷Ru reactions.³³ In the latter reaction, i.e., by bombarding a yttrium target with 75 MeV ¹²C, ⁹⁷Ru and ^95,96Tc are produced in the target matrix.

The separation of ⁹⁷Ru from the corresponding targets were reported using various analytical techniques such as solvent extraction, dry distillation, co-precipitation, wet distillation, liquid–liquid extraction and solid–liquid extraction methods. The separation of ⁹⁷Ru from Tc and Rh targets using a distillation technique based on the distillation of ⁹⁷RuO₄ in concentrated HNO₃ or H₂SO₄ medium at 90 °C has been reported in the literature with a total separation time of 6–7 h.^21,22,25 Comar et al. developed and described a solvent extraction process that is simple and rapid compared to distillation processes for the separation of Ru and co-produced Tc radionuclides from a molybdenum target.²⁸ A tin dioxide column followed by an anion exchange column was employed for the separation of ⁹⁷Ru from bulk molybdenum target.³¹ Liquid–liquid extraction (LLX) using a liquid anion exchanger, trioctylamine (TOA), or a liquid cation exchanger, di-(2-ethylhexyl)phosphoric acid (HDEHP) along with tri-butyl phosphate (TBP) was used for the separation of ⁹⁷Ru from coproduced Tc and Nb radionuclides and bulk Mo target by Lahiri et al.^34,35 The radiochemical separation of NCA ⁹⁷Ru from bulk Nb and coproduced Tc by LLX using both HDEHP and SLX with a cation exchanger resin, DOWEX-50, was exploited by Maiti et al.³² Maiti et al. also reported the separation of ⁹⁷Ru and coproduced ⁹⁵Tc from bulk yttrium target by LLX using TOA.³³ Recently, we developed a PEG-based aqueous biphasic system for the separation of ⁹⁷Ru from bulk niobium target.¹² We also developed a method of separation of ⁹⁷Ru from ¹²C induced natural yttrium target by ion exchange resins.³⁶ In this study, we have made an attempt to develop another green method for the separation of ⁹⁷Ru from ¹²C-induced bulk Y target and co-produced ^95,96Tc using a PEG based ABS system.

Materials and methods

Irradiation

Dictated by theoretical calculations,³³ we bombarded natural Y foil (99.9% purity, Alfa Aesar) with 75 MeV of ¹²C⁶⁺ beam for 14 h at the BARC-TIFR Pelletron facility, Mumbai for the production of ⁹⁷Ru and ^95,96Tc.^33,37 The bulk Y target was monitored radiometrically using ⁸⁸Y. Gamma-spectroscopic measurements were carried out using an HPGe (CANNBERA) detector with 2.7 keV resolution at 1332 keV. After the end of bombardment (EOB), the ¹²C irradiated ^natY target was cooled for 50 h to allow the decay of all short-lived products. The carbon irradiated ^natY foil was dissolved in a minimum volume of 0.1 M HCl and was spiked with ⁸⁸Y, evaporated to dryness, and re-dissolved in 0.01 M HCl to prepare the stock solution containing ⁹⁷Ru, ^95,96Tc and ⁸⁸Y along with bulk Y.

Materials and procedure

Chemicals, such as PEG-4000, PEG-6000, HNO₃, HCl, and HF, and salts, such as Na-citrate, Na-tartarate, Na-malonate, (NH₄)₂SO₄, NaHSO₃, Na₂SO₄, Na₂SO₃, Na₂S₂O₃, K₂HPO₄, K₃PO₄, Na₂CO₃, and KOH, were obtained from Merck, India. All the reagents were of analytical grade. The dialysis sack was procured from Spectrum Laboratory Inc.

The molecular weight and concentration of PEG, the salt concentration and the type of salts employed are important parameters to obtain maximum phase separation in ABS systems. PEG-4000 with 50% (w/v) concentration is reported as an optimum condition to minimize the solubility of any salt rich phase in the polymer rich phase.^9,38 Moreover, it has been found that 2–4 M salt concentration is ideal to obtain maximum phase separation. In the present study, therefore, 50% (w/v) PEG-4000 solution and 2 M salt solutions of Na-citrate, Na-tartarate, Na-malonate, (NH₄)₂SO₄, NaHSO₃, Na₂SO₄, Na₂SO₃, Na₂S₂O₃, K₂HPO₄, K₃PO₄, Na₂CO₃, and 4 M KOH were prepared by dissolving appropriate quantities of these compounds in deionized water. In the case of PEG-6000, the optimum concentration of the PEG-rich phase was also observed to be 50% (w/v) because the solubility of any salt rich phase in the PEG rich phase was minimal at this concentration. Therefore, throughout the experiment, 50% (w/v) of PEG-4000 and PEG-6000 was employed. In many of our earlier experiments, it has been found that lower molecular weight PEGs, such as PEG-400 or PEG-600, are not suitable for metal separation studies. This is because these polymers are like wool balls whose complexing-end becomes difficult to identify.¹⁰ Similarly, we have seen that PEG-20000 is also not very effective in separating metal ions.¹² Therefore, the extraction studies were performed with 3 mL of various 2 M salt solutions with equal volumes of 50% (w/v) of PEG-4000 as well as PEG-6000 solutions. 0.2 mL of the stock solution containing ⁹⁷Ru, ^95,96Tc and bulk yttrium spiked with ⁸⁸Y was added to this system and was shaken for 10 min. Then, the system was maintained for 10 min to achieve phase separation before collecting 2 mL of each phase for the γ-spectroscopic studies. Chemical separations were carried out at room temperature. The effect of pH and the efficiency of PEG-6000 over PEG-4000 were also studied.

Back extraction of ⁹⁷Ru and ^95,96Tc into the salt rich phases from the PEG rich phase was carried out using 2 M salt solutions of K₂CO₃, Na₂S₂O₃ and 4 M KOH. Dialysis was performed using a dialysis membrane sack of suitable length (molecular weight cut-off 1000 Dalton, wet in 0.1% Na-azide) against deionized water on a low speed mechanical shaker to obtain pure NCA ⁹⁷Ru in an aqueous medium.

Results and discussion

The extraction patterns of ⁹⁷Ru, ^95,96Tc and bulk Y in the PEG-rich phase against different salt-rich phases [Fig. 1] show preferential extraction of ⁹⁷Ru and ^95,96Tc into the PEG phase in all 12 salts-PEG combinations. Ru, along with Tc radionuclides, was extracted to the PEG rich phase without any contamination of bulk yttrium when Na-tartarate, Na-citrate, K₂HPO₄, K₃PO₄, KOH, Na₂CO₃, Na₂SO₄, Na₂SO₃ and (NH₄)₂SO₄ were used as the salt solutions. This could be due to the strong complexation of yttrium with citrate, tartarate, HPO₄⁻², PO₄⁻³, CO₃⁻², SO₄⁻² and SO₃⁻²; moreover, bulk Y prefers to stay in the salt-rich phase, whereas the complexing ability of Ru and Tc with these salts was less due to their larger size, and thus they were extracted into the PEG phase. Generally, Ru is present as ruthenate (RuO₄⁻²; calculated ionic radius = ∼180 pm,³⁹) and Tc is present as pertechnate (TcO₄⁻, ionic radius: 206 pm), whereas yttrium is present as free Y³⁺ (ionic radius: 89 pm,⁴⁰). Therefore, the preferential extraction of ⁹⁷Ru and ^95,96Tc into the PEG phase could be due to the larger size of Ru and Tc compared to Y. Usually, ions with smaller ionic radii are more solvated and prefer to stay in the salt rich phase, whereas cations with a larger size cations act as hydrophobic molecules, because the entropies of hydration of these ions are positive; thus, they prefer to stay in the PEG-rich phase. However, in three cases, e.g., when Na-malonate, NaHSO₄ and Na₂S₂O₃ were used as the salt-rich phase, slight extraction of Y was also observed. The maximum extractions of ⁹⁷Ru and ^95,96Tc in the PEG rich phase were 83% and 96%, respectively, when (NH₄)₂SO₄ was used as the salt-rich phase, without any contamination of bulk yttrium.

Fig. 1 Extraction profiles of ⁹⁷Ru, ⁹⁵Tc and bulk Y in the PEG-rich phase against different salt-rich phases at the natural pH of the salts at room temperature.

To study the influence of the molecular weight of PEG on the extraction system, the same experiment was carried out with PEG-6000 against 2 M Na-tartarate, Na-citrate, Na-malonate and K₂HPO₄ as the salt-rich phases. The results are shown in Fig. 2. It was observed that PEG-6000 has a marginal impact on the extraction patterns of ⁹⁷Ru, ^95,96Tc and bulk Y over PEG-4000. Therefore, all other experiments were carried out with PEG-4000 only.

Fig. 2 Extraction profiles of ⁹⁷Ru and ⁹⁵Tc in PEG-6000 against different salt-rich phases at the natural pH of the salts at room temperature (bulk Y was not extracted in any condition).

The effect of pH on the extraction of ⁹⁷Ru, ^95,96Tc and bulk Y into the PEG-rich phase was investigated by varying the pH of (NH₄)₂SO₄ and Na-tartarate salt solutions as the salt rich phases (Fig. 3 and 4). The pH of the (NH₄)₂SO₄ and Na-tartarate salt solutions was adjusted using dilute HCl or ammonia solution before mixing with the PEG-rich phase. It has been observed that the extraction patterns of the radionuclides under investigation are almost invariant with changing pH. However, the best separation was obtained at pH 5 when Na-tartarate or (NH₄)₂SO₄ were used as the salt-rich phase. To further improve the chemical yield of ⁹⁷Ru and ^95,96Tc, the relative volumes of PEG-4000 and the salt-rich phase (2 M Na-tartarate or (NH₄)₂SO₄) were varied. In the case of 2 M Na-tartarate, when the volume of PEG phase was doubled compared to the salt rich phase, about 78% ⁹⁷Ru and 100% ⁹⁵Tc were extracted into the PEG rich phase. The higher volume of the PEG-rich phase offers more sites for the salting out of ⁹⁷Ru and ⁹⁵Tc into the PEG rich phase. In the case of (NH₄)₂SO₄ as the salt-rich phase, the volume of PEG was also increased to improve the chemical yield of ⁹⁷Ru along with ^95,96Tc. However, when the volume of PEG was increased, the extraction of bulk yttrium along with ⁹⁷Ru and ^95,96Tc was observed. The distribution ratios (D) and separation factors (S) of ⁹⁷Ru, ⁹⁵Tc and yttrium under various experimental conditions were calculated, and the results are shown in Table 1. Under typical experimental conditions (PEG-4000, 2 M (NH₄)₂SO₄), separation factors (S_Ru/Y) and (S_Tc/Y) were as high as 4.0 × 10³ and 2.0 × 10⁴, respectively.

Fig. 3 Extraction profiles of ⁹⁷Ru, ⁹⁵Tc and bulk Y against different pH values of Na-tartarate.

Fig. 4 Extraction profiles of ⁹⁷Ru, ⁹⁵Tc and bulk Y against different pH values of (NH₄)₂SO₄.

Table 1 Distribution ratios (D) and separation factors (S) of ⁹⁷Ru, ⁹⁵Tc and yttrium at room temperature

Salt-rich phase (2 M)	pH	PEG rich phase	Distribution ratios (D)			Separation factors (S)
			DRu	DY	DTc	SRu/Y	STc/Ru	STc/Y
Na-citrate	7	4000	2.1	1.8 × 10^−3	12.7	1.1 × 10^3	6.1	7.0 × 10^3
Na-citrate	7	6000	1.4	1.2 × 10^−3	9.8	1.2 × 10^3	7.0	8.2 × 10^3
Na-tartarate	5	4000	1.9	1.1 × 10^−3	18.8	1.7 × 10^3	9.8	1.7 × 10^4
Na-tartarate (3 mL)	5	4000 (6 mL)	3.5	1.4 × 10^−3	1.0	2.5 × 10^3	0.3	7.1 × 10^2
Na-tartarate	3	4000	1.6	1.4 × 10^−3	2.6	1.1 × 10^3	1.6	1.8 × 10^3
Na-tartarate	4	4000	3.0	1.4 × 10^−3	8.3	2.1 × 10^3	2.7	5.9 × 10^3
Na-tartarate	5	4000	3.9	1.4 × 10^−3	1	2.8 × 10^3	0.2	0.7 × 10^3
Na-tartarate	8	4000	1.1	1.4 × 10^−3	32.5	7.8 × 10^2	29.5	23.2 × 10^3
Na-tartarate	5	6000	2.0	1.1 × 10^−3	9.1	8.3 × 10^3	4.5	8.3 × 10^3
Na-malonate	7	4000	1.5	6.5 × 10^−2	9.8	2.3 × 10^1	6.5	1.5 × 10^2
Na-malonate	7	6000	1.6	1.3 × 10^−3	5.6	2.1 × 10^3	3.5	4.3 × 10^3
Na2CO3	11	4000	0.7	1.3 × 10^−3	10.9	5.5 × 10^2	14.3	7.9 × 10^3
K2HPO4	8	4000	1.3	9.7 × 10^−4	5.5	1.3 × 10^3	4.4	5.7 × 10^3
K2HPO4	8	6000	1.3	9.8 × 10^−4	5.0	1.3 × 10^3	3.8	5.1 × 10^3
K3PO4	10	4000	1.1	1.1 × 10^−3	6.7	9.8 × 10^2	9.4	5.8 × 10^3
KOH	10	4000	0.1	1.3 × 10^−3	4.3	8.9 × 10^1	3.1	3.2 × 10^3
NaHSO4	5	4000	2.1	2.1 × 10^−1	6.8	1.0 × 10^1	3.4	3.2 × 10^1
Na2SO4	6	4000	2.9	1.3 × 10^−3	10.1	2.1 × 10^3	3.5	7.3 × 10^3
Na2S2O3	5	4000	4.3	1.1 × 10^−1	15.2	3.6 × 10^1	20.9	1.3 × 10^2
Na2SO3	9	4000	0.4	1.3 × 10^−3	8.9	3.3 × 10^2	4.9	6.9 × 10^4
(NH4)2SO4	5	4000	4.9	1.3 × 10^−3	24.8	3.9 × 10^3	6.1	1.9 × 10^4
(NH4)2SO4	3	4000	0.9	3.7 × 10^−2	5.9	2.4 × 10^1	6.5	1.6 × 10^2
(NH4)2SO4	4	4000	0.9	1.0 × 10^−3	8.08	9.0 × 10^2	8.9	8.1 × 10^3
(NH4)2SO4	7	4000	0.9	1.1 × 10^−3	8.08	8.2 × 10^2	8.9	7.3 × 10^3
(NH4)2SO4 (3 mL)	5	4000 (6 mL)	5.4	1.4 × 10^−3	1.0	3.8 × 10^3	0.2	7.1 × 10^2

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Back extraction of ⁹⁷Ru from the PEG rich phase

After removing bulk yttrium, back extraction of ⁹⁷Ru and ^95,96Tc into the salt-rich phase from the PEG rich phase was carried out using 2 M of K₂CO₃, Na₂S₂O₃ and 4 M of KOH as salt rich phases. With 2 M K₂CO₃ and 4 M KOH, 5% and 14% of ⁹⁵Tc were also stripped back along with 68% and 81% of ⁹⁷Ru, respectively, into the salt rich phase. However, with 2 M Na₂S₂O₃, about 93% stripping of ⁹⁷Ru into the salt-rich phase without any contamination of ^95,96Tc was observed [Fig. 5]. Tc may have preferred to stay in the PEG phase due to the larger negative value of the Gibbs free energy of hydration for pertechnate (ΔG_hyd: −637 kJ mol⁻¹) compared to ruthenate (ΔG_hyd: −307 kJ mol⁻¹).⁴¹

Fig. 5 Back extraction profiles of NCA ⁹⁷Ru and co-produced Tc from the PEG rich phase to the salt rich phase.

Dialysis studies of ⁹⁷Ru containing PEG rich phase

An attempt was made to extract ⁹⁷Ru into deionized water only. This experiment was carried out with the back-extracted ⁹⁷Ru fraction in the Na₂S₂O₃ phase (Fig. 5). All the ruthenium was back extracted to the PEG-4000 phase by shaking the salt solution for 10 min with 3 mL PEG-4000. Then, dialysis of the PEG rich phase was carried out again in deionized water. During dialysis, the dissociated ⁹⁷Ru from the ⁹⁷Ru-PEG association was continuously removed from the dialysis sack. The percentage of retention of ⁹⁷Ru in the dialysis sack is shown in Fig. 6. It was observed that after 4 h, about 74% ⁹⁷Ru was removed from the dialysis sack. ⁹⁷Ru-PEG association was also plotted against time to measure the half-life of the ⁹⁷Ru-PEG association [Fig. 7]. The half-life of the association was found to be 5.4 h, which is sufficient for the equilibration process when this complex is present inside the body for therapeutic or diagnostic purposes.

Fig. 6 Retention of ⁹⁷Ru in the dialysis sack with respect to time.

Fig. 7 Plot of ln (counts of ⁹⁷Ru-PEG in the dialysis sack) vs. time.

Conclusions

An environmental friendly greener method was developed for the separation of Ru and Tc radionuclides from bulk yttrium using a PEG-based aqueous biphasic system. The method is rapid and cost-effective. The synthetic polymer PEG is water-soluble and is not considered to be toxic. However, some applications, e.g., in vivo use of ⁹⁷Ru, may require these radionuclides to be in an aqueous solution only. Therefore, ⁹⁷Ru may be obtained in an aqueous medium by dialysis of the PEG phase containing ⁹⁷Ru. The favorable half-life of ⁹⁷Ru, its complexing ability, and the simplicity of the separation method we developed without using any toxic chemicals may be useful in future in the field of radiopharmaceuticals and in the clinical application of this radionuclide.

Acknowledgements

Authors acknowledge the co-operation of the staff of BARC-TIFR Pelletron facility, Mumbai and TIFR target laboratory, Mumbai for preparing targets and the financial support of SINP-DAE 12^th five year plan project TULIP. AD is also thankful to the Council of Scientific and Industrial Research for providing CSIR-RA fellowship.

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Footnote

† Present address: Amity Institute of Nuclear Science and Technology, Amity University, Noida-201303, India.

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