Uranium Electrodeposition Using Direct Potential Technique in Less Acidic Aqueous Media

Electrodeposition of uranium is a common practice to create samples for alpha spectrometry [1] and this methodology could be an alternative way to produce irradiation LEU targets [3, 4] to fabricate radiopharmaceuticals as ⁹⁹Mo used for cancer diagnosis [5]. Many workers [6] studied the uranium deposition at high temperature (>200ºC) using salt baths and producing metallic uranium and alloys. The low temperature electrodeposition in ionic solutions is an ongoing investigation, mainly using RTIL way with moderate success as accounted in recent papers in the literature [7].

The aqueous route to produce metallic uranium appeared to have a marginal success, using high acidity (pH <1) [8, 9]. At pH>2,  it is possible to deposit the oxidized uranium compound UO₂(OH)₂.H₂O as shown in the equilibrium diagram of molar fraction against pH presented in Figure 1.

Figure 1 - Equilibrium fraction diagram of UO₂(NO₃)₂ .6H₂O solution with 50mM [U] against pH at 25°C, calculated by Medusa [2].

Experimental.  Aqueous solution of UO₂(NO₃)₂ .6H₂O with 50mM [U] (pH = 2.6);  temperature at levels: 30°C and 60°C; electrodeposition times: 600, 1000, 1500 and 2000 s; cathodic potential of -1.80 V_Ag/AgCl; reference electrode: Ag/AgCl; substrate: electrodeposited Ni over an AA6061 plate.

Results and Discussion. After using an aqueous solution with pH > 3, we got a precipitate which was a loose yellowish oxidized substance. Below this pH level, using the range of 2.5<pH<3, we succeeded to get a solid grayish deposit, adherent to the substrate, with . In this work, we aimed to achieve higher uranium deposition amount in a shorter time. This is crucial for technological reasons to produce commercial targets.

Figure 2 – Curves representing the experimental uranium electrodeposition at two levels of temperature 30 and 60°C, along process duration.

Figure 2 displays the experimental results of the uranium deposition experimentation. An equation, following an exponential decay, consistently modeled the electrodeposition rate (R_adj >0.85). The alpha-emission is directed related to ²³⁸U+²³⁵U amount. The gradient of alpha activity in DPS (Bq/cm²) by time was found to be represented by:

dDPS/dt=A+B.exp(-t/Q)

The fitted parameters were: A-parameter varying from 38.7 (30°C) to 75.8 (60°C); B from -52.2 (30°C)  to -41.7 (60°C) and Q from 723.4 (30°C) to 2640.8 (60°C). B and Q, with proper units, represent the driving force parameters given by the temperature increase. An apparent deceleration of the process seems to occur, probably due to the properties of deposited uranium compound as insulator.

Acknowledgement. Thanks are due to FAPESP for granting this research by the project PJ2013/08614-3.

References:

1.         Crespo, M.T., A review of electrodeposition methods for the preparation of alpha-radiation sources. Appl Radiat Isot, 2012. 70(1): p. 210-5.

2.         Puigdomenech, I., MEDUSA. 2009, Royal Institute of Technology: Stockholm, Sweden. p. Software to calculate equilibrium diagrams.

3.         Saliba-Silva, A.M., et al., Uranium Electrodeposition for Irradiation Targets. ECS Meeting Abstracts, 2012(25): p. 999-999.

4.         Saliba-Silva, A.M., et al., Uranium Briquettes for Irradiation Target 2011 Int. Nuclear Atlantic Conference, 2011.

5.         National Research Council, C., Medical isotope production without highly enriched uranium, C. National Research Council, Editor. 2009, National Academies Press.

6.         Thied, R.C., et al., Process for separating metals - US 6,911,135, in United States Patent, U.S. Patent, Editor. 2005: US.

7.         Giridhar, P., et al., Extraction of uranium (VI) by 1.1M tri-n-butylphosphate/ionic liquid and the feasibility of recovery by direct electrodeposition from organic phase. Journal of Alloys and Compounds, 2008. 448(1-2): p. 104-108.

8.         Saliba-Silva, A., et al., Electrochemical Studies on 99Mo Target Materials: Acidic Deposition of Uranium Compounds., in RERTR 2012 - 34th Int. Meeting on Reduced Enrichment for Research and Test Reactors. 2012, DOE/Argonne National Lab/RERTR.

9.         Saliba-Silva, A.M., et al., Pulse Electrodeposition of Natural Uranium in 2-Propanol Acidic Ionic Solution. ECS Meeting Abstracts, 2013(25): p. 1002-1002.