Studies on electrochemical dissolution of sintered molybdenum discs as a potential method for targets dissolution in 99mTc production

Electrochemical dissolution of pressed into discs and sintered metallic molybdenum powder with the mass of 712 ± 10 mg (n = 15) in potassium hydroxide solution was studied in detail. The technique was considered to apply for dissolution of irradiated 100Mo target in the 99mTc production. The effect of various parameters, e.g., the concentration of the electrolyte solution, temperature, current density, and surface area of the platinum cathode, was investigated. The shortest time for total dissolution of molybdenum target was 70 min. This result was achieved using an electrolyte solution of 5 M KOH, temperature 55 °C and the current density of 365 mA/cm2.

It's worth noting that targets made of pressed molybdenum oxide or molybdenum carbide were also used in the production of 99m Tc [11,18]. Nevertheless, metallic molybdenum is preferred due to its high melting point, resistance to high temperatures generated by the bombardment of charged particles, and high thermal conductivity. Molybdenum enriched in 100 Mo in the form of metallic powder is available commercially (99.815%, ISOFLEX, USA). However, the appropriate technique must be employed to prepare it in a durable target resistant to a high current beam. The approaches to prepare natural and enriched molybdenum targets included the use of foils [30], pressed and sintered powder [15,16,31], physical vapor deposition (PVD) [32], and electrophoretic deposition [33]. In our previous work, we demonstrated the manufacturing of a high-density 100 Mo metallic target using pressing of molybdenum powder into pellets and their subsequent sintering in a reductive atmosphere [31].
For the processing of 100 Mo, the irradiated target needs to be dissolved. Both the dissolution and the chemical separation of 99m Tc from the irradiated target material should be fast because of the relatively short half-life of 99m Tc (~ 6 h). Also, the chemical composition of solvent used for the target dissolution should be rather simple, preferably mono component, thus making further separation of 99m Tc from the excess of molybdenum feasible. There are few approaches to the dissolution of metal molybdenum. The most common and well-defined is alkaline dissolution by the oxidation of metallic molybdenum to molybdenum trioxide or molybdic 1 3 acid (VI) with 30% or 50% hot hydrogen peroxide and then dissolving the resulting precipitate in either NaOH, KOH, or (NH 4 ) 2 CO 3 solutions [16,[34][35][36][37][38][39][40]. That process led to the formation of sodium/potassium/ammonium molybdate, according to the following reactions (1)-(4) given with NaOH as an example: or The time of molybdenum dissolution in 30% hydrogen peroxide depended on the physical form of Mo [34]. The 500-530 mg metal molybdenum foil and 720-760 mg metallic molybdenum powder pressed into disc dissolved within 3-5 min at 80 °C and within 2 min when the temperature was 90 °C. The pressed and sintered molybdenum disc with a mass of 720 mg dissolved under these conditions within 60 and 45 min, respectively. A significantly longer time of dissolution of sintered molybdenum results from less porous structure and hence increasing hardness and mechanical strength after a high-temperature process carried out in a hydrogen atmosphere [31]. There are reported results of a higher dissolution rate of molybdenum sintered discs [35]. However, hydrogen peroxide of a higher concentration of 50% was used in such a case. The average processing times of 600 g molybdenum discs depended on the brand of hydrogen peroxide used, ranging from 140 to 255 min.
The dissolution of molybdenum in inorganic acids was also reported [22,41,42]. However, molybdenum is insoluble in HCl, H 2 SO 4 , HF, H 3 PO 4, and most other mineral acids, but it can be dissolved in HNO 3 [43]. The dissolution is fast in diluted HNO 3 and much slower in concentrated HNO 3 due to the formation of inert oxide film on the metal surface, making it difficult to dissolve. Guro et al. [41] studied in detail the hydrogen peroxide oxidation of molybdenum and its subsequent dissolution in concentrated nitric and sulfuric acids. They demonstrated that the dissolution of metallic molybdenum in hydrogen peroxide is based on the phenomenon of corrosion and corrosion inhibition, and the oxidation of molybdenum and associated reduction of the hydrogen peroxide is accompanied by heterogeneous catalytic hydrogen peroxide decomposition and oxygen release. Gumiela et al. [42] dissolved metallic molybdenum powder and irradiated with protons pressed and sintered natural molybdenum disc in 3.5 M HNO 3 . Azarov et al. [22] proposed dissolving gram amounts of metallic molybdenum subjected to irradiation in the linear accelerator in a mixture of various inorganic acids resulting in obtaining soluble molybdenile salts. In such a way, they dissolved 30 g of molybdenum in the mixture of 53 ml, 56% nitric acid, and 51 ml, 40% hydrofluoric acid. Furthermore, researchers optimized the dissolution of 28.28 g of molybdenum in the mixture containing 54 ml of nitric acid and 123 ml of hydrochloric acid. The process lasted 2.5 h.
Regardless whether the acidic or alkaline dissolution is used, it should be adapted to the subsequent separation of 99m Tc from the excess of molybdate. In order to selectively separate 99m Tc from the dissolved target material using the solid phase extraction chromatography, we wanted to use the resin selective for 99m Tc in the alkaline solution. In contrast to earlier published studies, in this work, we focused on the alkaline dissolution of pressed and sintered molybdenum target by electrochemical oxidation. The process was investigated and optimized towards complete and fast dissolution of molybdenum. To reach that goal, we explored whether external electromotive force (EMF) will shorten molybdenum dissolution time compared to chemical dissolution and if so, propose this method for dissolution of 100 Mo irradiated target. The dissolution by electrochemical oxidation was compared with the conventional dissolution method.
Several possibilities of electrochemical dissolution of molybdenum were reported [44][45][46][47]. However, described techniques are not adapted to our purpose. The vast majority of scientific papers refer to anodizing molybdenum to obtain a molybdenum oxide as a functional coating protecting against corrosion [46][47][48][49][50][51][52][53][54]. The literature on the electrochemical, quantitative dissolution of molybdenum is scarce and limited to non-aqueous media [48,55]. In view of the above, the rapid, complete electrochemical dissolution of molybdenum is needed.
Molybdenum can be present in oxidation states from − 2 to + 6. However, its most common oxidation state in aqueous solutions at room temperature is + 6 [56,57]. The molybdate species in aqueous solutions depend on the molybdenum concentration and the pH. In neutral to mildly alkaline solutions, the monomeric MoO 4 2− predominates. As the pH decreases, the anion becomes protonated, and the following molybdate species are formed: Molybdates tend to polymerize at higher molybdenum concentrations. At molybdenum concentrations above 10 -3 mol Mo/L and pH 5-6, the heptamolybdate ion is formed, and at pH 3-5, octamolybdate predominates [56,57].
Anodic dissolution of molybdenum depends mainly on the pH of the electrolyte [49]. The corrosion and passivation behavior of molybdenum in aqueous solutions at various pH was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization. These studies revealed that molybdenum metal surface was always covered with a passive film, which was more stable in acidic solutions [50]. The activation energy of dissolution of the molybdenum oxide film was lower in alkaline solutions (128.3 kJ/mol) than in neutral (152.5 kJ/mol) or acidic solutions (162.4 kJ/mol). This reflects the relative instability of the passive film in alkaline solutions, tending to metal dissolution.
The process of anodic dissolution of molybdenum in alkaline solution can be described by the following reactions (5)-(7) [46]: Molybdenum oxides layer formed on its surface during molybdenum anodic dissolution is continuously dissolved in alkaline solution. The efficiency of the process depends on the concentration of the solution [46]. The electrochemical potential and pH of the solution affect the exact ratio between different valence states. Therefore, in our experiments, we investigated the influence of parameters such as the concentration of the electrolyte solution, the addition of 30% hydrogen peroxide, temperature, current density, molybdenum mass and surface area of the platinum cathode on the dissolution rate of pressed and sintered molybdenum discs.

Materials and apparatus
Molybdenum foil (0.1 mm thick, 99.9% pure) used in preliminary experiments on electrochemical dissolution was purchased from Goodfellow Cambridge Ltd., UK. Natural metal molybdenum powder (≥ 99.9% chemical purity) with a particle size of 1-2 µm in diameter used to prepare pressed and sintered discs was obtained from Sigma-Aldrich, Poland. Molybdenum discs with a density of 9.4 g/cm 3 , a diameter of 12 mm, and a thickness of 0.6 mm were manufactured by pressing molybdenum powder, followed by their sintering in hydrogen flow at a temperature of 1600 °C [31]. The mass of each molybdenum disc was 712 ± 10 mg (n = 15) and corresponded to the mass of 100 Mo target irradiated in cyclotron [34]. Platinum foil (0.1 mm thick, 99.9% pure) serving as a counter electrode (CE) and platinum mesh (99.9% purity, nominal aperture: 0.12 mm, wire diameter: 0.04 mm, open

Electrochemical dissolution
The electrochemical dissolution of molybdenum was performed by the galvanostatic method. To determine the current-voltage parameters of the investigated electrochemical systems, open-circuit voltage (OCV) measurements were performed. For this purpose, three electrodes system was used with molybdenum as the working electrode (WE), platinum foil as the counter electrode (CE), and mercury/mercury oxide (Hg/HgO 6 M KOH) reference electrode (RE).
The electrochemical setup (Fig. 1) consisted of two electrodes: small pieces (1-5 cm 2 ) of molybdenum foil or molybdenum disc with a surface area of about 3 cm 2 were used as the anode. The dissolving Mo disc was wrapped in Pt mesh, forming a pocket for the disc. Electrical contact was ensured through this mesh hanging on Pt wire. Platinum foil (3-21 cm 2 ) or mesh served as a cathode. Before each electrochemical process, electrodes in the form of foils were cleaned with a surfactant, rinsed with distilled water, degreased with ethanol, re-rinsed in distilled water, and finally dried for 30 min at 100 °C. Due to the deposition of dissolved molybdenum on the opposite electrode, an electrochemical process was conducted in the vessel divided into two chambers by the separator made of polyethylene foil doped with silica. The catholyte and the anolyte solutions had the same composition and contained 1-5 M KOH. The selection of potassium hydroxide as electrolyte solution instead of more commonly used sodium hydroxide was justified by the greater solubility of molybdenum in the former one [58]. The volume of both catholyte and anolyte solutions was 13-20 ml. The anolyte solution was mixed during electrolysis with an overhead mixer at ~ 500 rpm. The process was carried out at constant current density in the range of 14-280 mA/cm 2 for molybdenum foil and of 80-755 mA/ cm 2 for molybdenum sintered discs at ambient temperature and elevated to 55 °C. The effect of 30% hydrogen peroxide addition on the rate and the efficiency of electrochemical dissolution of molybdenum was investigated. To ensure the reliable effect of investigated parameters, all experiments were performed within 60 min. This time limit was associated with the time needed for the chemical dissolution of pressed and sintered molybdenum [34]. Since this method allowed molybdenum to be completely dissolved in less than 60 min, we assumed that our approach should provide no worse results. The efficiency of molybdenum dissolution was determined gravimetrically after drying the electrode.
Each experiment was repeated at least three times, and the presented curve and value describe the average result. Experiment errors were calculated using the commonly known standard deviation rule.

Open circuit voltage (OCV) measurements
In order to assess the usefulness of the electrochemical cell for the dissolution of molybdenum, the voltage between two electrodes at an open-circuit voltage (OCV) was measured. The graph of voltage between molybdenum electrode (2 cm 2 , WE) and platinum electrode (3cm 2 , CE) immersed in KOH solutions (15 ml) of different concentrations is shown in Fig. 2. The negative values of registered voltage result from the way of electrodes connection: molybdenum electrode has lower standard potential (-0.20 V) than platinum electrode (+ 1.2 V), but since it is subjected for dissolution, it is connected as the working electrode. As depicted in Fig. 2, the voltage between these two electrodes increases with the increasing concentration of electrolyte solution. This is due to the increase of ionic conductivity in more concentrated KOH solutions. To verify a dominant oxidation state and type of chemical species formed by molybdenum in the investigated solution, molybdenum anode versus Hg/ HgO 6 M KOH reference electrode potentials were measured. Recorded values were then recalculated to the potentials versus normal hydrogen electrode (NHE). The measurements were performed for the OCV system comprised of molybdenum foil (1 cm 2 ) as a working electrode (WE) and Hg/HgO 6 M KOH counter electrode (CE). The electrolyte was divided into two solutions: catholyte and anolyte solutions containing 13 ml of 3 M KOH each. The measurements were conducted through a salt bridge saturated with 3 M KOH. The exact measurements were repeated for the platinum electrode (3 cm 2 ). The obtained results indicate that the potential of molybdenum and platinum electrodes versus Hg/HgO 6 M KOH electrodes is accordingly -0.443 V and -0.045 V, which after recalculation for NHE gives -0.399 V and 0.089 V, respectively. According to the Pourbaix diagram [56], for calculated potential and pH of 14, the molybdenum half-cell exists in the area of MoO 4 2− , whereas platinum half-cell in the area of Pt cryst .

Effect of current density
The preliminary experiments of the electrochemical dissolution of molybdenum were performed for molybdenum foil with a lower mass of 60 mg. The effect of current density on voltage was investigated. The results obtained for dissolution Fig. 2 Open circuit voltage (OCV) curves recorded for an electrochemical system containing molybdenum anode (2 cm 2 ) and platinum cathode (3 cm 2 ) immersed in KOH solutions (15 ml) of various concentrations. The measurements performed at ambient temperature of molybdenum anode (60 mg, 1 cm 2 ) in 13 ml 3 M KOH are presented in Fig. 3. As was expected, the higher the current density, the greater the working electrochemical cell voltage. Figure 4 illustrates the influence of current density on the dissolution of 60 mg metal molybdenum in 3 M KOH. The efficiency of molybdenum dissolution increases linearly with current density. It can be seen that at the current density of 15 mA/cm 2 , only 15 ± 2% (n = 5) of molybdenum was dissolved. Increasing this parameter to 60 mA/cm 2 improved the yield to 56 ± 3% (n = 5), whereas almost all molybdenum, 99 ± 1% (n = 5), was dissolved when applying a current density of 100 mA/cm 2 .

Effect of initial molybdenum mass
As expected, the rate and effectiveness of electrochemical dissolution were mass-dependent. When using the constant current of 100 mA and room temperature conditions, molybdenum anode with the mass of 60 mg (Mo foil with a thickness of 0.1 mm) entirely dissolved in 13 ml 3 M KOH after 60 min. Under the same conditions, the dissolving efficiency of pressed and sintered molybdenum disc with a mass of 712 ± 10 mg and thickness of 0.6 mm (n = 15) reached only 23 ± 1% (n = 3). It is worth mentioning that not only molybdenum mass but also thickness of molybdenum anode can significantly affect dissolution rate. Considering two molybdenum discs of the same mass but different thicknesses, the surface of the one with the smaller thickness will be greater. The larger the metal surface, the greater the contact area of the metal with the solution, and thus the faster dissolution. This applies not only to chemical dissolution but also to electrochemical dissolution.

Effect of platinum cathode surface area
The attempts to improve the electrochemical dissolution of 712 ± 10 mg (n = 15) pressed and sintered molybdenum disc by increasing platinum cathode surface area gave no satisfying results (Fig. 5). After increasing the platinum cathode surface area from 3 to 21 cm 2 , the yield of molybdenum dissolution remained at the same level of 23 ± 2% (n = 4). Simultaneously, it was observed that the increase of the platinum surface area from 3 cm 2 to 21 cm 2 reduced voltage between the electrodes from 9.4 ± 0.4 V (n = 4) to 5.2 ± 0.6 V (n = 4), respectively. Replacement of platinum foil with a platinum mesh of the same size, hence increasing the surface of the platinum electrode, resulted in a further decrease of voltage to 3.3 ± 0.4 V (n = 4). This is an essential observation since while the increase in current density increases voltage (Fig. 3), the reduction of  The experiments were performed at a current density of 100 mA/cm 2 at ambient temperature the voltage with the extension of platinum cathode dimensions allows to use higher current densities, resulting in increased rate and efficiency of molybdenum dissolution (Fig. 4). Figure 6 illustrates the effect of potassium hydroxide concentration of 1, 3, and 5 mol/dm 3 on the electrochemical dissolution of molybdenum at room temperature and current density of 285 mA/cm 2 . In 1 M KOH, molybdenum did not dissolve. The yield of dissolution increased with the concentration of KOH and reached 23 ± 2% (n = 4) for 3 M KOH and 37 ± 2% (n = 4) for 5 M KOH. However, particulates dark brown-colored of a near colloidal nature came off the anode.

Effect of hydrogen peroxide addition
It was found that the addition of 1 ml of 30% hydrogen peroxide per 20 ml of alkaline electrolyte solution increased the yield of molybdenum dissolution. This effect improves with the increase of KOH concentration. As shown in Fig. 7, the efficiency of molybdenum dissolution in electrolytes containing 1 M, 3 M, and 5 M KOH doped with 1 ml of 30% hydrogen peroxide achieved 22 ± 3% (n = 4), 62 ± 3% (n = 4) and 72 ± 3% (n = 4), respectively. However, when the volume of 30% hydrogen peroxide was increased to 2 ml, the electrochemical dissolution yield decreased to 38 ± 2% (n = 4). This effect is illustrated in Fig. 8.

Effect of temperature
At an elevated temperature of 55 °C, the molybdenum electrochemical dissolution yield in 5 M KOH increased from 37 ± 2% (n = 4) to 80 ± 3% (n = 4). Such results were obtained when attempting to dissolve 712 ± 10 mg (n = 15) molybdenum sintered discs at the current density of 100 mA/cm 2 . However, at temperatures higher than 55 °C, foaming of the electrolyte and leaps of the voltage between electrodes were observed.

Determination of complete dissolution rate
In summary, the electrochemical dissolution of molybdenum was most efficient in 5 M KOH. The yield of 80 ± 3% (n = 4) was achieved in 5 M KOH solution at an elevated temperature of 55 °C, whereas at ambient temperature, the highest yield of 72 ± 3% (n = 4) was reached for the mixture of 5 M KOH and 30% hydrogen peroxide. In order to reduce the time needed for the complete dissolution of molybdenum anode, the influence of current density on dissolution rate was studied. Results are given in Table 1. It can be seen that the rate of dissolution of pressed and sintered molybdenum disc raised with increasing current density from 283 to 367 mA/cm 2 . Time required for complete dissolution in this range of current density decreased from 95 ± 2 min (n = 4) to 69 ± 1 min (n = 4). Further increase of current density led to a slower dissolution rate. The shortest time required for complete electrochemical dissolution of sintered molybdenum disc with a mass of 712 ± 10 mg (n = 15) was about 70 min. As a result, a yellowish, clear solution was obtained. This color is most likely from the presence of molybdic acid (hydrated forms of molybdenum trioxide and related species) forming monohydrates MoO 3 ·H 2 O and dihydrates MoO 3 ·2H 2 O. For further dissolution, after the electrochemical process, 2 ml of 30% hydrogen peroxide were added, and the yellow color disappeared.

Conclusions
This work presents the results of the electrochemical dissolution of pressed and sintered molybdenum discs, typically serving as targets for cyclotron production of 99m Tc [31]. The shortest time for total dissolution of molybdenum target 712 ± 10 mg (n = 15) in mass was 70 min. This result was obtained using an electrolyte solution of 5 M KOH, temperature 55 °C, and the current density of 365 mA/cm 2 . For complete dissolution of molybdenum, resulting in the formation of a colorless solution, the addition of 30% hydrogen peroxide was needed. These results are comparable to the chemical dissolution method. The lower temperature used in electrochemical dissolution (55 °C vs. 90 °C in the chemical dissolution method) is easier to control. In the chemical process, high concentrations of reagents are used (oxidation of molybdenum by 30% or even 50% hydrogen peroxide as the first step of dissolution), the reaction is turbulent, exothermic, and the oxygen is released. For these reasons, the process of chemical dissolution of molybdenum must be monitored, and reagents should be added in portions. From that point of view, the electrochemical dissolution of molybdenum allows remote control by adjusting the current-voltage conditions. The method can be easily adopted for hot-cell operation and for automation. Nevertheless, the final choice of the method used for the dissolution of the molybdenum target will depend on a particular user. This work may help in making a choice giving closer insight into the electrochemical process.
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