- Email: [email protected]
Characterizations of electrodeposited uranium layer on stainless steel disc
Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Characterizations of electrodeposited uranium layer on stainless steel disc Young Gun Ko ∗ , Jong-Myoung Lim, Geun-Sik Choi, Kun Ho Chung, Mun Ja Kang Environmental Radioactivity Assessment Team, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Uranium is chemically separated with a bauxite ore with high chemical yield. • The oxidation state of the uranium changes with the depth of electrodeposited layer. • Thickness of the uranium layer is measured with the XPS depth profiling.
a r t i c l e
i n f o
Article history: Received 29 July 2015 Received in revised form 11 September 2015 Accepted 17 September 2015 Available online 21 September 2015 Keywords: Electrodeposition Uranium Surface characterization Chemical separation Bauxite
a b s t r a c t Various techniques of the uranium layer were carried out to investigate its surface and crystal structure using ATR FT-IR, TOF-SIMS, XPS, XRD, optical microscopy, and AFM. Uranium was chemically separated from a bauxite ore using a chromatographic column, and electrodeposited on a stainless steel disc. The prepared layer on the disc was nearly flat, but not homogeneous. Although other alpha-particle emitting radionuclides were clearly separated using a chromatographic column, the layer consisted of unseparated iron and oxygen atoms. The oxidation state of the uranium was different depending on the depth of the layer because oxygen atoms on the surface of the layer penetrated into the layer and bonded with neighboring uranium atoms in the layer. This phenomenon causes the concentration of the oxygen atoms bonded with the uranium atoms on the surface to be higher than in the uranium layer. Our study might be a promising tool to examine the homogeneity, the thickness and the flatness of the electrodeposited layer as an alpha source for the measurement of the low-level activity concentration of radionuclides with a high level of accuracy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction ∗ Corresponding author. Fax: +82 42 863 1289. E-mail address: [email protected] (Y.G. Ko). http://dx.doi.org/10.1016/j.colsurfa.2015.09.053 0927-7757/© 2015 Elsevier B.V. All rights reserved.
Bauxite is the world’s main source of aluminum ore [1]. The extractable aluminum in the ore is in the form of
122
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Fig. 1. Schematic diagram of procedure for determination of uranium isotopes in bauxite.
Fig. 2. Schematic of the electrodeposition cell used in the experiment.
gibbsite Al(OH)3 , diaspora ␣-AlO(OH) and boehmite ␥-AlO(OH). The ore also contains various minerals such as hematite ␣-Fe2 O3 , goethite ␣-FeO(OH), anatase TiO2 , rutile TiO2 , ilmenite TiFeO3 , kaolinite Si4 Al4 O10 (OH)8 , and quartz SiO2 [2]. However, some bauxite may contain alpha-emitting radionuclides above the natural background rates owing to the presence of naturally occurring radioactive materials (NORMs) such as 238 U and/or 232 Th and their decay products [3]. Although uranium is the fuel for the nuclear power plant, its binding affinity toward biomolecules can cause acute and/or chronic harmful effects by inhalation and ingestion of uranium [4]. Therefore, the monitoring of uranium isotopes in ores such as bauxite is important from the standpoint of health physics and environmental protection although the concentration of uranium is low. Alpha-particle spectrometry is a powerful analytical tool for the identification and assay of the alpha-particle emitting sources primarily owing to its high counting efficiency, high sensitivity and low price [5]. To analyze alpha-particle emitting nuclides in a sample using an alpha spectrometer, numerous methods for the preparation of the alpha-radioactive source have been studied such as micro-precipitation [6], electrodeposition [7], spontaneous deposition [8], vacuum sublimation [9], drying of a liquid drop on a substrate [10], direct evaporation [11], electrostatic deposition [12], etc. The first three mentioned methods are extensively used for the determination of low level environmental activities of alpha-particle emitting radionuclides. Among these methods, electrodeposition is the most common technique to prepare the
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Fig. 3. Alpha-particle spectrum of uranium isotopes separated from bauxite with UTEVA resin.
123
source owing to the advantage of high deposition yield and high qualities of the sources obtained with simple and inexpensive equipment [13]. In the electrodeposition process, the source is prepared through electrodeposition of an alpha-particle emitting nuclide such as actinide elements from an organic or aqueous electrolyte solution onto a metallic substrate (such as stainless steel, silver, copper, nickel, platinum, etc.) which act as a cathode of the electrodeposition cell. The actinide elements (such as Ac, Th, U, Np, Pu, Am, etc.) are electropositive to be reduced on the metal cathode surface and the deposit consists of an insoluble compound of the element. The alpha-particle determination of the prepared source is carried out using an alpha-particle spectrometer. The activity concentration of alpha-particle emitting radionuclides in an environmental sample such as bauxite is low. Therefore, to measure the low-level activity concentration of radionuclides with a high level of accuracy, precise and careful work is required in all steps (from the sample dissolution to the quantitative analysis using an alpha spectrometer) of the procedure for the sample analysis, and each step of the procedure should also be analyzed
Fig. 4. (a) Photo image of uranium-electrodeposited disc. (b) 3D surface structure image of the red rectangular region in (a), and (b) height profile of dotted line in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
124
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
in detail. While various experimental parameters affecting the electrodeposition have been studied from many diverse perspectives by many research groups [14–20], only few researches for the nuclide-electrodeposited disc have been reported [21–23]. Herein, we report the various techniques of the uraniumelectrodeposited disc. The surface-chemical structure of the electrodeposited layer was confirmed using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR). The crystal structure of the layer was investigated using X-ray diffraction (XRD). The thickness of the electrodeposited layer was measured with an optical profiler and the XPS depth profiling. The surface morphology was observed using an atomic force microscope (AFM) and an optical microscope. The XPS depth profiling is also used to obtain the chemical-composition information on the depth of the layer. We expect that our data and analyses in this study are significantly meaningful to develop and improve the preparation method of a thin and pure uranium source.
2. Experimental
Fig. 5. ATR FT-IR spectra of (a) stainless steel and (b) uranium-electrodeposited discs.
the test was positive, a small amount of L-ascorbic acid was added into the supernatant to reduce Fe(III) to Fe(II).
2.1. Sample preparation The overall process from the sample preparation to the quantitative analysis using an alpha-particle spectrometer is depicted in Fig. 1. All reagents and solvents were of AR grade and used without further purification unless otherwise noted. HNO3 , HCl, HF and NH4 OH were purchased from Merck. NH4 SCN, Na2 SO4 , and NaHSO4 were bought from Aldrich. l-ascorbic acid and ammonium oxalate monohydrate are purchased from Showa Chemical (Japan) and Kanto Chemical (Japan), respectively. KOH, Al(NO3 )3 ·9H2 O and oxalic acid dehydrate were bought from Junsei Chemical (Japan). Ethanol was purchased from Samchun Chemicals (Korea). Deionized water (DI water) was obtained using a Milli-Q Direct 8 water system (18.2 M cm). The bauxite sample was taken from Ulsan in Korea. After the sample was put (ca. 20 g) into a porcelain dish and ashed in a muffle furnace with a gradual heating program up to 600 ◦ C to eliminate organic matter, a known amount of sample (ca. 1 g) and a spike of 232 U tracer solution (0.1073 Bq/g, ca. 1 g) were transferred to a Teflon bomb. The 232 U tracer solution was manufactured through the dilution of a concentrated 232 U solution (National Physical Laboratory, R20-15) with 3 M HNO3 . The sample was then dissolved in 20 ml of concentrated HNO3 and 5 ml of concentrated HCl on a 180 ◦ C hot plate for ca. 6 h. After cooling down until room temperature, 50 ml of deionized water was added into the sample dissolved solution, then the solution was centrifuged at 3000 rpm for 20 min. The supernatant was immediately transferred to a Teflon beaker and dried at 180 ◦ C. The precipitate was dissolved in 20 ml of concentrated HNO3 and 15 ml of concentrated HF on a 180 ◦ C hot plate for ca. 6 h for acid decomposition of silicates. After cooling down to room temperature, the solution was centrifuged at 3000 rpm for 20 min. The supernatant was immediately added to the Teflon beaker and dried again on a 180 ◦ C hot plate for ca. 6 h. The acid leaching procedure with the concentrated HNO3 and the concentrated HF was repeated twice. 5 ml of concentrated HNO3 was added into the residue in a Teflon beaker and the residue was dried on a 180 ◦ C hot plate for ca. 3 h. The dried residue was dissolved in 20 ml of 3 M HNO3 —0.5 M Al(NO3 )3 and centrifuged at 3000 rpm for 20 min. The supernatant was then transferred to a new Teflon beaker. The presence of Fe(III) was tested with the addition of one droplet of 0.1 M NH4 SCN into the supernatant. If
2.2. Separation of uranium isotopes The chromatographic column was prepared, containing approximately 3.8 ml of Eichrom UTEVA resin (100–150 m). The column was pre-conditioned with 30 ml of 3 M HNO3 . The sample that had been digested and dissolved in 3 M HNO3 —0.5 M Al(NO3 )3 was then passed through the column using the gravity flow rate, followed by a 20 ml wash of 3 M HNO3 . 10 ml of 9 M HCl was added to convert to resin for a chloride system. Then, 30 ml of 5 M HCl—0.05 M oxalic acid was added to remove thorium, neptunium, and plutonium from the UTEVA resin. Uranium was eluted with 25 ml of 0.01 M HCl and evaporated to dryness. 2.3. Electrodeposition and quantitative analysis using alpha-particle spectrometry The dry residue was dissolved in 2.5 ml of 5 wt% NaHSO4 , 5 ml of deionized water, and 5 ml of 15 wt% Na2 SO4 . The solution was transferred to an electrodeposition cell (Fig. 2). 1 ml of 0.02 wt% ammonium oxalate was added into a cell as an electrolyte solution. The uranium isotopes were electrodeposited on a stainless steel disc for 2 h at 880–890 mA (average current density 330 mA/cm2 ). One minute prior to the end of this stage, 2 ml of 25 wt% KOH was added into the cell before switching off the current to prevent a re-dissolution of the electrodeposited uranium layer into the electrolyte solution. The cell was disassembled and the disc was then rinsed with 3 × 2 ml of 5 wt% NH4 OH solution and ethanol. In addition, the back of the disc was heated with a gas torch to bind the deposit to the disc surface and remove any organic impurities. Alpha-particle spectrometry measurement was conducted using a Canberra Alpha Analyst system with a passivated implanted planar silicon (PIPS) detector (450 mm2 active area, Canberra) and the data were analyzed using Canberra Genie 2000 software. The count time was 90,000 s. 2.4. Characterizations of the electrodeposited disc To analyze the surface-chemical structure of the uraniumelectrodeposited stainless disc, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) was performed using
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
125
Fig. 6. (a) Positive and (b) negative TOF-SIMS spectra with elemental mappings of uranium-electrodeposited disc.
a Frontier spectrometer (PerkinElmer) equipped with a diamond coated KRS-5 crystal (PerkinElmer, Universial Diamond ATR). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was carried out in a mass range of 0.5–400 m/z using an IONTOF TOF-SIMS 5 system. Mass spectra were obtained using a Bi+ liquid metal ion source (1 pA pulsed ion current) from an area of 500 m × 500 m. The mass resolution measured on the Si± signal of a silicon wafer was m/m = 4000 in negative and positive modes. X-ray photoelectron spectroscopy (XPS) analysis of the disc was carried out using a PHI 5000 VersaProbe (Ulvac-PHI) with an Al K␣ X-ray source (1486.6 eV of photons). The X-ray source was run at a reduced power of 250 W (15 kV), and the pressure in the analysis chamber was maintained at less than 6.7 × 10−8 Pa during each measurement. High-resolution spectra of the C 1s core
level were recorded using pass energies of 160 eV at a takeoff angle of 90◦ . All binding energies were referenced to the neutral C 1s peak at 285.0 eV to compensate for the surface-charging effects. The intensity ratios were converted into atomic ratios using sensitivity factors provided by the manufacturer. The peaks were deconvoluted using a curve-fitting method with a series of Gauss–Lorentzian curves that allowed for an adjustment of full width at half-maximum. The disc was ion sputtered using argon operating at 3 keV to conduct in a depth profile analysis and to determine the thickness of the uranium layer on the disc. The argon ion beam was rastered over a 2 mm × 2 mm area yielding a sputter rate of 20 nm/min. The CasaXPS (version 2.3.12) software was employed to process all survey and high-resolution spectra.
126
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
X-ray diffraction (XRD) patterns were recorded on a Rigaku ATXG diffractometer (Tokyo, Japan) with a high-power Cu K␣ source operating at 60 kV and 300 mA. The height profile and 3D surface structure of the disc were obtained using a WYKO NT1100 optical profiler (VEECO). Atomic force microscopy (AFM) images were taken using a commercial force microscope (XE-100, Park Systems). Topographic images were recorded in non-contact mode. AFM images were obtained under ambient laboratory conditions, and scan areas were 40 m × 40 m and 5 m × 5 m. The surface of the uranium electrodeposited disc was observed using a digital optical microscope (Reichert Metaplan 2, Leica) equipped with software (Nex Measure Pro 5, Bestecvision, Korea).
3. Results and discussion 3.1. Overview of the prepared disc source
Fig. 7. XRD patterns of (a) stainless steel and (b) uranium-electrodeposited discs.
To measure the radioactivity concentrations of uranium isotopes, other radionuclides should be separated completely in the step of the radiochemical separation. If the separation is not carried out completely, peaks of unseparated nuclides appear in the alpha spectrum [24]. The unwanted peaks appear near the peaks of interesting uranium isotopes in the alpha spectrum and overlap the shoulder of the interesting peaks slightly or heavily. This overlap influences the peak area determination for the specific peak and makes it difficult to calculate the radioactivity concentration of uranium isotopes. In our experimental procedure, uninteresting nuclides were clearly separated and only interesting uranium isotopes (238 U, 235 U, 234 U and 232 U) appeared in the alpha-particle spectrum (Fig. 3). 232 U was added to the bauxite sample as a tracer to calculate the radioactivity concentration. The radioactivity concentration was calculated based on the ratio of peak areas belonging to the bauxite and tracer. This method is called isotope dilution alpha-particle spectrometry [25], and has the advantage that the calculated activity is inherently corrected for the chemical losses, because the losses of the analyte are taken to be equal to those of the tracer. The peak area determination in the alpha-particle spectrum for the uranium-electrodeposited disc shows that the chemical yield (tracer recovery) is 98.9%, and the radioactivity concentrations of 238 U, 235 U and 234 U are 214.28 ± 6.03, 10.35 ± 0.80 and 182.85 ± 5.29 Bq/kg, respectively. The thickness of the electrodeposited layer was measured using an optical profiler at the edge of the layer (Fig. 4a). Fig. 4b shows a three-dimensional view of a selected area of the layer (marked by a red rectangle in Fig. 4a). The height profile of the dotted line in Fig. 4b shows that the thickness of the layer is ca. 450 nm (Fig. 4c). We measured six points around the electrodeposited layer with a rotation of ca. 60◦ , and obtained the same values. However, the center of the thickness can differ with the edge because the surface profile of the electrodeposition layer is influenced by the electrode types or other electrodeposition conditions [20]. The optical profiler is very convenient equipment to obtain a 3D image, height profile, and other useful morphological information. However, the optical profiler has a limit with a narrow measurement area. Therefore, the whole electrodeposited area was not represented in the 3D image and height profile. To obtain the layer thickness of its center point, a depth analysis was carried out using XPS with ion sputtering. The measured thickness was ca. 460 nm, which is similar with the measured value at the edge of the layer using the optical profiler. In this result, we can conclude that the electrdeposited layer is nearly flat. The detailed results for the XPS depth analysis of the layer were discussed in Section 3.3 of this article.
3.2. Surface analyses The surface-chemical structure of the electrodepostied layer is analyzed using ATR FT-IR spectroscopy (Fig. 5). No peaks related with organic groups appeared in Fig. 5b because the residue of organic compounds on the stainless steel disc was removed by heating the back of the disc with a gas torch after the electrodeposition of uranium isotopes on it. Three strong peaks arised at 638, 524, and 474 cm−1 in Fig. 5b. Other specific peaks were also observed at the shoulder of the three strong peaks (735, 708, 519, 486, 468, 452, and 437 cm−1 ). These peaks indicate that the electrodeposited layer is a uranium oxide composite (UOx ), which consists of UO2 , U4 O9 , UO2.12 , etc [26,27]. Oxygen in either air or an organic compound, or both, might be combined with uranium during the experimental procedure of heating the back of the disc. In positive and negative TOF-SIMS spectra, the uranium oxide composite was confirmed (Fig. 6). In addition, no peaks for other uranium complexes/compounds such as UO2 CO3 or UO2 (NO3 )2 appeared except peaks for the uranium oxide and hydrogen combined uranium oxides. The peaks in the TOF-SIMS spectra indicate that oxygen and hydrogen atoms were located near the uranium atoms in the chemical structure of the electrodeposited layer. The elemental mapping of TOF-SIMS is a reasonable method to confirm the distribution of the chemical structure or element of the material surface. The images obtained by the elemental mapping of the electrodeposited layer on the disc exhibited an uniform distribution of the uranium oxides. This result shows that the elctrodeposition was well conducted without any local side-reaction on the disc in the electrodeposition cell. The composite of the electrodeposited layer was investigated using XRD patterns (Fig. 7). In thin flims, if the thickness of the film is smaller than the peneration depth of the X-rays, the substrate under the film is also detected by an XRD analysis [28]. The XRD pattern of the substrate can cause confusion in analyzing the film. Therefore, the XRD pattern of the stainless steel disc, which is used as a substrate for the electrodeposition, was shown in Fig. 7a. The peaks in the pattern were assigned to iron oxides such as FeO and Fe2 O3 . After electrodeposition of the extracted uranium from the bauxite ore, various peaks appeared which match those of U (PDF# 72-0659), UO2 (PDF# 75-0421), U4 O9 (PDF# 72-0125), UO2.12 (PDF# 71-0258), UO1.96 (PDF# 75-0413) and UO4 · H2 O (PDF# 491821) (Fig. 7b). Aside from these peaks, peaks related with other UOx were also observed. However their intensities were too weak to assign the crystal structure and appeared at the shoulder of the above peaks. Interestingly, peaks for face-centered cubic carbon
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
(nanodiamond, PDF# 79-1467) and primitive hexagonal carbon (PDF# 74-1602) were observed with peaks of the uranium composites. It is thought that carbon materials formed on the surface of the electrodeposited layer during the removal of organic residues and ethanol by heating the back of the disc with a gas torch. These carbon materials on the electrodeposited layer were investigated and are discussed in Section 3.3 of this article.
3.3. Depth analysis and surface morphology of electrodeposited uranium layer The center point of the electrodeposited layer was analyzed using XPS (Fig. 8a). The XPS survey spectrum of the layer showed that the layer surface is composed of only U, O, C, and Fe (Fig. 8b). The noteworthy result is that C and Fe were detected on the surface of the layer. Atomic concentrations on the depth of the electrodeposited layer were obtained through argon ion sputtering in the center point (red rectangular region in Fig. 8a, area: 2 mm × 2 mm) of the layer (Fig. 8c). The uranium concentration was nearly zero at a depth of ca. 460 nm (red line in Fig. 8c). From this result, the film thickness of the center point of the layer is estimated as ca. 460 nm. The value is similar with a value of ca. 450 nm, which is measured using an optical profiler (Fig. 4c). This result indicates that the prepared electrodeposited layer might be nearly flat. The atomic concentration of carbon was very high on the surface of the layer and suddenly decreased with an increase of the depth (black line in Fig. 8c). We can estimate that some carbon materials exist on the surface of the layer. The electrodeposited layer was composed of large quantities of Fe (green line in Fig. 8c) and O (blue line in Fig. 8c) together with U. The Fe might be unseparated from the bauxite matrix, although all elements except uranium isotopes
127
were removed well in the chemical separation step (extraction chromatography) with the separation column. The O was combined with U and Fe because of the oxidation while heating the back of the disc in the experimental procedure, and therefore appears in large quantity, as shown in the depth profile. The carbon on the layer surface was assessed with the high-resolution C1s spectrum (Fig. 8d). Two peaks appeared at 283.7 and 285.0 eV corresponding to sp2 - and sp3 -hybridized carbon, respectively. The sp2 - and sp3 -hybridized carbon peaks arise from the presence of graphitic and diamond phases, respectively [29,30]. In the XPS spectrum, the peak intensity for the diamond phase (sp3 -hybridized carbon) is significantly higher than the peak intensity for the graphic phase (sp2 -hybridized carbon). This result matches well with the XRD analysis that the peak intensity of face-centered cubic nanodiamond is significantly higher than that of primitive hexagonal carbon (Fig. 7b). Fig. 9a shows U 4f XPS spectra on the depth of the electrodeposited layer. The intensities of U 4f5/2 and U 4f7/2 decreased with the increase of the depth, and reached nearly zero at a depth of ca. 460 nm. This result indicates that the electrodeposition rate of Fe atoms is somewhat higher than that of U atom on the stainless steel disc in our experimental conditions for the electrodeposition. The oxidation states of U on the depth of the electrodeposited layer were obtained by fitting the high-resolution U 4f7/2 XPS spectra obtained at a depth of 0 nm (layer surface), 100 nm, 200 nm, 300 nm, and 400 nm on a Shirley background (Fig. 9b–f). The peaks corresponding to U6+ and U5+ decreased significantly with the increase of the depth. The peak for U4+ increased until a depth of 200 nm, and decreased after that. The peak for U metal (U0 ) appeared after a depth of 300 nm and increased with the depth. From the result of this XPS study, oxygen atoms in either air or
Fig. 8. (a) Photo image of uranium-electrodeposited disc. (b) XPS survey spectrum, (c) XPS depth profile and (d) high-resolution C 1s XPS spectrum of the red rectangular region in (a).
128
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Fig. 9. (a) High-resolution U 4f XPS spectra on the depth of uranium-electrodeposited disc, and peak-fitting of U 4f7/2 XPS spectra at various depths: (b) surface (0 nm), (c) 100 nm, (d) 200 nm, (e) 300 nm and (f) 400 nm.
chemicals, or both, bonded with the uranium-electrodeposited layer on the stainless steel disc while heating the back of the disc. The oxygen atoms on the surface of the layer penetrated into the layer and bonded with the neighboring uranium atoms in the layer. Therefore, the concentration of oxygen atoms bonded with the uranium atoms on the surface is higher than that on the near surface of the stainless steel disc. The surface of the electrodeposited layer was observed using an optical microscope (Fig. 10). Irregular-shaped crack lines covered the surface of the layer (red arrows in Fig. 10b and d). Irregularshaped particles also formed on the surface (white arrows in Fig. 10d). In AFM images of the electrodeposited layer, grooves
were not observed (Fig. 11b). Only flat pieces of ca. 1 m thick were observed in the AFM image. From this result, assumed crack lines in the optical microscope images are not crack lines. The lines might be grain boundaries at the interfaces between the crystals of the electrodeposited layer [31]. To summarize, flat irregularshaped pieces are on the surface of the electrodeposited layer, and many grain boundaries are in the layer. This result is in agreement with the XRD and XPS studies showing that carbon materials, such as face-centered cubic carbon and primitive hexagonal carbon, form on the surface of the electrodeposited layer, and the layer consists of many crystal pieces of various uranium oxides.
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
129
Fig. 10. Zoom-out and zoom-in optical microscopy images of (a, c) stainless steel and (b, d) uranium-electrodeposited discs.
Fig. 11. Zoom-out and zoom-in AFM images of (a, c) stainless steel and (b, d) uranium-electrodeposited discs. Scan area: 40 × 40 m2 and 5 × 5 m2 .
4. Conclusions In this work, uranium isotopes were chemically separated from a bauxite ore using a chromatographic column, and electrodeposited on a stainless steel disc to measure the radioactivity concentration of uranium isotopes with an alpha-particle spectrometer. The concentration of the separated uranium isotopes were 214.28 ± 6.03, 10.35 ± 0.80 and 182.85 ± 5.29 Bq/kg for 238 U, 235 U and 234 U, respectively. The thickness of an electrodeposited
layer of ca. 450–460 nm was measured using an optical profiler and XPS depth profiles. The images obtained by the TOF-SIMS elemental mapping exhibited a uniform distribution of the uranium oxide on the disc. This result showed that the elctrodeposition was well conducted without any local side-reaction on the disc in the electrodeposition cell. The XRD study showed that the electrodeposited layer consists of uranium oxides and iron oxides. In addition, carbon materials were also observed on the surface of the layer. An XPS study on the depth profiles showed that oxygen atoms on the
130
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
surface of the layer penetrated into the layer and bonded with neighboring uranium atoms in the layer while heating a back of the disc. Although unnecessary elements and radionuclides are chemically separated for determining the concentration of the specific alpha-particle emitting nuclide, a small amount of those elements can remain until the procedure step of electrodeposition because the separation efficiency depends on the type of matrix of the ores or the environmental samples. These remaining elements can significantly influence the electrodeposition of alpha-particle emitting nuclides on a stainless steel disc and the measurement of the radioactivity concentration. Therefore, to measure the lowlevel activity concentration of a radionuclides with a high level of accuracy, the information from the electrodeposited layer is important. To the best of our knowledge, this is the first report on the detailed analysis of the uranium-electrodeposited layer with various techniques. We expect that our various techniques of the uranium-electrodeposited disc can motivate the improvements in the procedure steps for an alpha-particle emitting sample analysis to measure the low-level activity concentration of radionuclides with a high level of accuracy.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20] [21]
References [22] [1] A.M. Donoghue, N. Frisch, D. Olney, Bauxite mining and alumina refining, J. Occup. Environ. Med. 56 (2014) S12–S17. [2] M. Gräfe, G. Power, C. Klauber, Bauxite residue issues: III. Alkalinity and associated chemistry, Hydrometallurgy 108 (2011) 60–79. [3] B.H. O’Connor, A.M. Donoghue, T.J.H. Manning, B.J. Chesson, Radiological assessment for bauxite mining and alumina refining, Ann. Occup. Hyg. 57 (2013) 63–76. [4] G.A. Bird, Uranium in the environment: behavior and toxicity, in: R.A. Meyers (Ed.), Encyclopedia of Sustainability Science and Technology, Springer, New York, 2012, pp. 11220–11262. [5] N. Vajda, P. Martin, C.-K. Kim, Alpha spectrometry, in: M.F. L’Annunziata (Ed.), Handbook of Radioactivity Analysis, third ed., Academic Press, San Diego, 2012, pp. 363–422. [6] X. Dai, Isotopic uranium analysis in urine samples by alpha spectrometry, J. Radioanal. Nucl. Chem. 289 (2011) 595–600. [7] M.T. Crespo, A review of electrodeposition methods for the preparation of alpha-radiation sources, Appl. Radiat. Isot. 70 (2012) 210–215. [8] P.L. Fernández, J. Gómez, C. Ródenas, Evaluation of uncertainty detection limits in 210 Pb and 210 Po measurement in water by alpha spectrometry using 210 Po spontaneous deposition onto a silver disk, Appl. Radiat. Isot. 70 (2012) 758–764. ˜ M.T. Crespo, M. Roteta, G. Sibbens, S. Pommé, A.M. Sánchez, [9] E. García-Torano, M.P.R. Montero, S. Woods, A. Pearce, ␣-particle emission probabilities in the decay of 235 U, Nucl. Instrum. Methods Phys. Res. A 550 (2005) 581–592. [10] S.K. Hanson, A.H. Mueller, W.J. Oldham Jr., Kläui ligand thin films for rapid plutonium analysis by alpha spectrometry, Anal. Chem. 86 (2014) 1153–1159. [11] O. Kurihara, F. Saito, J. Koarashi, E. Kim, K. Shiraishi, Y. Yamada, M. Akashi, Emission rates of alpha particles from supports with different surface conditions in direct deposition sources, Radiat. Prot. Dosim. 145 (2011) 13–20. [12] V. Goliáˇs, D. Pittauerová, R. Procházka, Z. Tˇrískala, Field alpha-spectroscopy of
[23]
[24]
[25]
[26] [27] [28]
[29]
[30]
[31]
radon (222 Rn) and action (219 Rn) progeny in soil gas: locating a radon source, J. Radioanal. Nucl. Chem. 266 (2005) 461–470. Y.M. Panta, D.E. Farmer, P. Johnson, M.A. Cheney, S. Qian, Preparation of alpha sources using magnetohydrodynamic electrodeposition for radionuclide metrology, J. Colloid Interface Sci. 342 (2010) 128–134. J.-S. Oh, P.E. Warwick, I.W. Croudace, S.-H. Lee, Evaluation of three electrodeposition procedures for uranium, plutonium and americium, Appl. Radiat. Isot. 87 (2014) 233–237. M. Trdin, L. Benedik, Z. Samardˇzija, B. Pihlar, Investigation of factors affecting the quality of americium electroplating, Appl. Radiat. Isot. 70 (2012) 2002–2005. E.M. Bond, W.A. Moody, D.E. Dry, M.W. Rabin, Factors affecting the quality of plutonium deposits by electrodeposition, J. Radioanal. Nucl. Chem. 296 (2013) 793–798. O.A. Dumitru, R. Cs Begy, D.C. Nita, L.D. Bobos, C. Cosma, Uranium electrodeposition for alpha spectrometric source preparation, J. Radioanal. Nucl. Chem. 298 (2013) 1335–1339. V. Jobbágy, M.T. Crespo, R.V. Ammel, M. Marouli, A. Moens, S. Pommé, E. ˜ Preparation of high-resolution 238 U ␣-sources by García-Torano, electrodeposition: a comprehensive study, J. Radioanal. Nucl. Chem. 298 (2013) 345–352. R. Zarki, A. Elyahyaoui, A. Chiadli, Preparation of ␣-sources of U(VI) and Th(IV) by the electrodeposition technique in the presence of Ca2+ and some trivalent metals, Appl. Radiat. Isot. 55 (2001) 167–174. H. Klemenˇciˇc, L. Benedik, Alpha-spectrometric thin source preparation with emphasis on homogeneity, Appl. Radiat. Isot. 68 (2010) 1247–1251. J.L. Ferrero Calabuig, A. Martín Sánchez, C. Roldán García, J. Roselló ferrando, M.F. da Silva, J.C. Soares, F. Vera Tomé, Characterization of alpha sources by Rutherford backscattering spectrometry, Nucl. Instrum. Method A 369 (1996) 603–607. A.M. Beesley, M.T. Crespo, N. Weiher, N. Tsapatsaris, J.S. Cózar, H. Esparza, C.G. Méndez, P. Hill, S.L.M. Schroeder, M.E. Montero-Cabrera, Evolution of chemical species during electrodeposition of uranium for alpha spectrometry by the Hallstadius method, Appl. Radiat. Isot. 67 (2009) 1559–1569. D.C. Burciaga-Valencia, C.G. Méndez, H. Esparza-Ponce, A.M. Beesley, M.T. Crespo, L. Fuentes-Cobas, L. Fuentes-Montero, M.E. Montero-Cabrera, Synchrotron radiation study of the uranium chemical species electrodeposited for alpha spectrometry sources, Rev. Mex. Fís. S 57 (2011) 21–29. N. Casacuberta, M. Lehritani, J. Mantero, P. Masqué, J. Garcia-Orellana, R. Garcia-Tenorio, Determination of U and Th ␣-emitters in NORM samples through extraction chromatography by using new and recycled UTEVA resins, Appl. Radiat. Isot. 70 (2012) 568–573. A.W. Knight, E.S. Eitrheim, A.W. Nelson, S. Nelson, A simple-rapid method to separate uranium, thorium, and protactinium for U-series age-dating of materials, J. Environ. Radioact. 134 (2014) 66–74. G.C. Allen, N.R. Holmes, Characterization of binary uranium oxides by infrared spectroscopy, Appl. Spectrosc. 48 (1994) 525–530. J.-G. Kim, Y.-S. Park, Y.-K. Ha, K. Song, Infrared spectra of uranium oxides measured by ATR-FTIR, J. Nucl. Sci. Technol. 46 (2009) 1188–1192. M. Klaus, C. Genzel, X-ray residual stress analysis on multilayer systems: an approach for depth-resolved data evaluation, J. Appl. Cryst. 46 (2013) 1266–1276. A. Kumar, P.A. Lin, A. Xue, B. Hao, Y.K. Yap, R.M. Sankaran, Formation of nanodiamonds at near-ambient conditions via microplasma dissociation of ethanol vapour, Nat. Commun. 4 (2013) 2618. T. Petit, J.-C. Arnault, H.A. Girard, M. Sennour, P. Bergonzo, Early stages of surface graphitization on nanodiamond probed by x-ray photoelectron spectroscopy, Phys. Rev. B 84 (2011) 233407. G.S. Rohrer, Grain boundary energy anisotropy: a review, J. Mater. Sci. 46 (2011) 5881–5895.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Characterizations of electrodeposited uranium layer on stainless steel disc Young Gun Ko ∗ , Jong-Myoung Lim, Geun-Sik Choi, Kun Ho Chung, Mun Ja Kang Environmental Radioactivity Assessment Team, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Uranium is chemically separated with a bauxite ore with high chemical yield. • The oxidation state of the uranium changes with the depth of electrodeposited layer. • Thickness of the uranium layer is measured with the XPS depth profiling.
a r t i c l e
i n f o
Article history: Received 29 July 2015 Received in revised form 11 September 2015 Accepted 17 September 2015 Available online 21 September 2015 Keywords: Electrodeposition Uranium Surface characterization Chemical separation Bauxite
a b s t r a c t Various techniques of the uranium layer were carried out to investigate its surface and crystal structure using ATR FT-IR, TOF-SIMS, XPS, XRD, optical microscopy, and AFM. Uranium was chemically separated from a bauxite ore using a chromatographic column, and electrodeposited on a stainless steel disc. The prepared layer on the disc was nearly flat, but not homogeneous. Although other alpha-particle emitting radionuclides were clearly separated using a chromatographic column, the layer consisted of unseparated iron and oxygen atoms. The oxidation state of the uranium was different depending on the depth of the layer because oxygen atoms on the surface of the layer penetrated into the layer and bonded with neighboring uranium atoms in the layer. This phenomenon causes the concentration of the oxygen atoms bonded with the uranium atoms on the surface to be higher than in the uranium layer. Our study might be a promising tool to examine the homogeneity, the thickness and the flatness of the electrodeposited layer as an alpha source for the measurement of the low-level activity concentration of radionuclides with a high level of accuracy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction ∗ Corresponding author. Fax: +82 42 863 1289. E-mail address: [email protected] (Y.G. Ko). http://dx.doi.org/10.1016/j.colsurfa.2015.09.053 0927-7757/© 2015 Elsevier B.V. All rights reserved.
Bauxite is the world’s main source of aluminum ore [1]. The extractable aluminum in the ore is in the form of
122
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Fig. 1. Schematic diagram of procedure for determination of uranium isotopes in bauxite.
Fig. 2. Schematic of the electrodeposition cell used in the experiment.
gibbsite Al(OH)3 , diaspora ␣-AlO(OH) and boehmite ␥-AlO(OH). The ore also contains various minerals such as hematite ␣-Fe2 O3 , goethite ␣-FeO(OH), anatase TiO2 , rutile TiO2 , ilmenite TiFeO3 , kaolinite Si4 Al4 O10 (OH)8 , and quartz SiO2 [2]. However, some bauxite may contain alpha-emitting radionuclides above the natural background rates owing to the presence of naturally occurring radioactive materials (NORMs) such as 238 U and/or 232 Th and their decay products [3]. Although uranium is the fuel for the nuclear power plant, its binding affinity toward biomolecules can cause acute and/or chronic harmful effects by inhalation and ingestion of uranium [4]. Therefore, the monitoring of uranium isotopes in ores such as bauxite is important from the standpoint of health physics and environmental protection although the concentration of uranium is low. Alpha-particle spectrometry is a powerful analytical tool for the identification and assay of the alpha-particle emitting sources primarily owing to its high counting efficiency, high sensitivity and low price [5]. To analyze alpha-particle emitting nuclides in a sample using an alpha spectrometer, numerous methods for the preparation of the alpha-radioactive source have been studied such as micro-precipitation [6], electrodeposition [7], spontaneous deposition [8], vacuum sublimation [9], drying of a liquid drop on a substrate [10], direct evaporation [11], electrostatic deposition [12], etc. The first three mentioned methods are extensively used for the determination of low level environmental activities of alpha-particle emitting radionuclides. Among these methods, electrodeposition is the most common technique to prepare the
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Fig. 3. Alpha-particle spectrum of uranium isotopes separated from bauxite with UTEVA resin.
123
source owing to the advantage of high deposition yield and high qualities of the sources obtained with simple and inexpensive equipment [13]. In the electrodeposition process, the source is prepared through electrodeposition of an alpha-particle emitting nuclide such as actinide elements from an organic or aqueous electrolyte solution onto a metallic substrate (such as stainless steel, silver, copper, nickel, platinum, etc.) which act as a cathode of the electrodeposition cell. The actinide elements (such as Ac, Th, U, Np, Pu, Am, etc.) are electropositive to be reduced on the metal cathode surface and the deposit consists of an insoluble compound of the element. The alpha-particle determination of the prepared source is carried out using an alpha-particle spectrometer. The activity concentration of alpha-particle emitting radionuclides in an environmental sample such as bauxite is low. Therefore, to measure the low-level activity concentration of radionuclides with a high level of accuracy, precise and careful work is required in all steps (from the sample dissolution to the quantitative analysis using an alpha spectrometer) of the procedure for the sample analysis, and each step of the procedure should also be analyzed
Fig. 4. (a) Photo image of uranium-electrodeposited disc. (b) 3D surface structure image of the red rectangular region in (a), and (b) height profile of dotted line in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
124
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
in detail. While various experimental parameters affecting the electrodeposition have been studied from many diverse perspectives by many research groups [14–20], only few researches for the nuclide-electrodeposited disc have been reported [21–23]. Herein, we report the various techniques of the uraniumelectrodeposited disc. The surface-chemical structure of the electrodeposited layer was confirmed using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR). The crystal structure of the layer was investigated using X-ray diffraction (XRD). The thickness of the electrodeposited layer was measured with an optical profiler and the XPS depth profiling. The surface morphology was observed using an atomic force microscope (AFM) and an optical microscope. The XPS depth profiling is also used to obtain the chemical-composition information on the depth of the layer. We expect that our data and analyses in this study are significantly meaningful to develop and improve the preparation method of a thin and pure uranium source.
2. Experimental
Fig. 5. ATR FT-IR spectra of (a) stainless steel and (b) uranium-electrodeposited discs.
the test was positive, a small amount of L-ascorbic acid was added into the supernatant to reduce Fe(III) to Fe(II).
2.1. Sample preparation The overall process from the sample preparation to the quantitative analysis using an alpha-particle spectrometer is depicted in Fig. 1. All reagents and solvents were of AR grade and used without further purification unless otherwise noted. HNO3 , HCl, HF and NH4 OH were purchased from Merck. NH4 SCN, Na2 SO4 , and NaHSO4 were bought from Aldrich. l-ascorbic acid and ammonium oxalate monohydrate are purchased from Showa Chemical (Japan) and Kanto Chemical (Japan), respectively. KOH, Al(NO3 )3 ·9H2 O and oxalic acid dehydrate were bought from Junsei Chemical (Japan). Ethanol was purchased from Samchun Chemicals (Korea). Deionized water (DI water) was obtained using a Milli-Q Direct 8 water system (18.2 M cm). The bauxite sample was taken from Ulsan in Korea. After the sample was put (ca. 20 g) into a porcelain dish and ashed in a muffle furnace with a gradual heating program up to 600 ◦ C to eliminate organic matter, a known amount of sample (ca. 1 g) and a spike of 232 U tracer solution (0.1073 Bq/g, ca. 1 g) were transferred to a Teflon bomb. The 232 U tracer solution was manufactured through the dilution of a concentrated 232 U solution (National Physical Laboratory, R20-15) with 3 M HNO3 . The sample was then dissolved in 20 ml of concentrated HNO3 and 5 ml of concentrated HCl on a 180 ◦ C hot plate for ca. 6 h. After cooling down until room temperature, 50 ml of deionized water was added into the sample dissolved solution, then the solution was centrifuged at 3000 rpm for 20 min. The supernatant was immediately transferred to a Teflon beaker and dried at 180 ◦ C. The precipitate was dissolved in 20 ml of concentrated HNO3 and 15 ml of concentrated HF on a 180 ◦ C hot plate for ca. 6 h for acid decomposition of silicates. After cooling down to room temperature, the solution was centrifuged at 3000 rpm for 20 min. The supernatant was immediately added to the Teflon beaker and dried again on a 180 ◦ C hot plate for ca. 6 h. The acid leaching procedure with the concentrated HNO3 and the concentrated HF was repeated twice. 5 ml of concentrated HNO3 was added into the residue in a Teflon beaker and the residue was dried on a 180 ◦ C hot plate for ca. 3 h. The dried residue was dissolved in 20 ml of 3 M HNO3 —0.5 M Al(NO3 )3 and centrifuged at 3000 rpm for 20 min. The supernatant was then transferred to a new Teflon beaker. The presence of Fe(III) was tested with the addition of one droplet of 0.1 M NH4 SCN into the supernatant. If
2.2. Separation of uranium isotopes The chromatographic column was prepared, containing approximately 3.8 ml of Eichrom UTEVA resin (100–150 m). The column was pre-conditioned with 30 ml of 3 M HNO3 . The sample that had been digested and dissolved in 3 M HNO3 —0.5 M Al(NO3 )3 was then passed through the column using the gravity flow rate, followed by a 20 ml wash of 3 M HNO3 . 10 ml of 9 M HCl was added to convert to resin for a chloride system. Then, 30 ml of 5 M HCl—0.05 M oxalic acid was added to remove thorium, neptunium, and plutonium from the UTEVA resin. Uranium was eluted with 25 ml of 0.01 M HCl and evaporated to dryness. 2.3. Electrodeposition and quantitative analysis using alpha-particle spectrometry The dry residue was dissolved in 2.5 ml of 5 wt% NaHSO4 , 5 ml of deionized water, and 5 ml of 15 wt% Na2 SO4 . The solution was transferred to an electrodeposition cell (Fig. 2). 1 ml of 0.02 wt% ammonium oxalate was added into a cell as an electrolyte solution. The uranium isotopes were electrodeposited on a stainless steel disc for 2 h at 880–890 mA (average current density 330 mA/cm2 ). One minute prior to the end of this stage, 2 ml of 25 wt% KOH was added into the cell before switching off the current to prevent a re-dissolution of the electrodeposited uranium layer into the electrolyte solution. The cell was disassembled and the disc was then rinsed with 3 × 2 ml of 5 wt% NH4 OH solution and ethanol. In addition, the back of the disc was heated with a gas torch to bind the deposit to the disc surface and remove any organic impurities. Alpha-particle spectrometry measurement was conducted using a Canberra Alpha Analyst system with a passivated implanted planar silicon (PIPS) detector (450 mm2 active area, Canberra) and the data were analyzed using Canberra Genie 2000 software. The count time was 90,000 s. 2.4. Characterizations of the electrodeposited disc To analyze the surface-chemical structure of the uraniumelectrodeposited stainless disc, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) was performed using
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
125
Fig. 6. (a) Positive and (b) negative TOF-SIMS spectra with elemental mappings of uranium-electrodeposited disc.
a Frontier spectrometer (PerkinElmer) equipped with a diamond coated KRS-5 crystal (PerkinElmer, Universial Diamond ATR). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was carried out in a mass range of 0.5–400 m/z using an IONTOF TOF-SIMS 5 system. Mass spectra were obtained using a Bi+ liquid metal ion source (1 pA pulsed ion current) from an area of 500 m × 500 m. The mass resolution measured on the Si± signal of a silicon wafer was m/m = 4000 in negative and positive modes. X-ray photoelectron spectroscopy (XPS) analysis of the disc was carried out using a PHI 5000 VersaProbe (Ulvac-PHI) with an Al K␣ X-ray source (1486.6 eV of photons). The X-ray source was run at a reduced power of 250 W (15 kV), and the pressure in the analysis chamber was maintained at less than 6.7 × 10−8 Pa during each measurement. High-resolution spectra of the C 1s core
level were recorded using pass energies of 160 eV at a takeoff angle of 90◦ . All binding energies were referenced to the neutral C 1s peak at 285.0 eV to compensate for the surface-charging effects. The intensity ratios were converted into atomic ratios using sensitivity factors provided by the manufacturer. The peaks were deconvoluted using a curve-fitting method with a series of Gauss–Lorentzian curves that allowed for an adjustment of full width at half-maximum. The disc was ion sputtered using argon operating at 3 keV to conduct in a depth profile analysis and to determine the thickness of the uranium layer on the disc. The argon ion beam was rastered over a 2 mm × 2 mm area yielding a sputter rate of 20 nm/min. The CasaXPS (version 2.3.12) software was employed to process all survey and high-resolution spectra.
126
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
X-ray diffraction (XRD) patterns were recorded on a Rigaku ATXG diffractometer (Tokyo, Japan) with a high-power Cu K␣ source operating at 60 kV and 300 mA. The height profile and 3D surface structure of the disc were obtained using a WYKO NT1100 optical profiler (VEECO). Atomic force microscopy (AFM) images were taken using a commercial force microscope (XE-100, Park Systems). Topographic images were recorded in non-contact mode. AFM images were obtained under ambient laboratory conditions, and scan areas were 40 m × 40 m and 5 m × 5 m. The surface of the uranium electrodeposited disc was observed using a digital optical microscope (Reichert Metaplan 2, Leica) equipped with software (Nex Measure Pro 5, Bestecvision, Korea).
3. Results and discussion 3.1. Overview of the prepared disc source
Fig. 7. XRD patterns of (a) stainless steel and (b) uranium-electrodeposited discs.
To measure the radioactivity concentrations of uranium isotopes, other radionuclides should be separated completely in the step of the radiochemical separation. If the separation is not carried out completely, peaks of unseparated nuclides appear in the alpha spectrum [24]. The unwanted peaks appear near the peaks of interesting uranium isotopes in the alpha spectrum and overlap the shoulder of the interesting peaks slightly or heavily. This overlap influences the peak area determination for the specific peak and makes it difficult to calculate the radioactivity concentration of uranium isotopes. In our experimental procedure, uninteresting nuclides were clearly separated and only interesting uranium isotopes (238 U, 235 U, 234 U and 232 U) appeared in the alpha-particle spectrum (Fig. 3). 232 U was added to the bauxite sample as a tracer to calculate the radioactivity concentration. The radioactivity concentration was calculated based on the ratio of peak areas belonging to the bauxite and tracer. This method is called isotope dilution alpha-particle spectrometry [25], and has the advantage that the calculated activity is inherently corrected for the chemical losses, because the losses of the analyte are taken to be equal to those of the tracer. The peak area determination in the alpha-particle spectrum for the uranium-electrodeposited disc shows that the chemical yield (tracer recovery) is 98.9%, and the radioactivity concentrations of 238 U, 235 U and 234 U are 214.28 ± 6.03, 10.35 ± 0.80 and 182.85 ± 5.29 Bq/kg, respectively. The thickness of the electrodeposited layer was measured using an optical profiler at the edge of the layer (Fig. 4a). Fig. 4b shows a three-dimensional view of a selected area of the layer (marked by a red rectangle in Fig. 4a). The height profile of the dotted line in Fig. 4b shows that the thickness of the layer is ca. 450 nm (Fig. 4c). We measured six points around the electrodeposited layer with a rotation of ca. 60◦ , and obtained the same values. However, the center of the thickness can differ with the edge because the surface profile of the electrodeposition layer is influenced by the electrode types or other electrodeposition conditions [20]. The optical profiler is very convenient equipment to obtain a 3D image, height profile, and other useful morphological information. However, the optical profiler has a limit with a narrow measurement area. Therefore, the whole electrodeposited area was not represented in the 3D image and height profile. To obtain the layer thickness of its center point, a depth analysis was carried out using XPS with ion sputtering. The measured thickness was ca. 460 nm, which is similar with the measured value at the edge of the layer using the optical profiler. In this result, we can conclude that the electrdeposited layer is nearly flat. The detailed results for the XPS depth analysis of the layer were discussed in Section 3.3 of this article.
3.2. Surface analyses The surface-chemical structure of the electrodepostied layer is analyzed using ATR FT-IR spectroscopy (Fig. 5). No peaks related with organic groups appeared in Fig. 5b because the residue of organic compounds on the stainless steel disc was removed by heating the back of the disc with a gas torch after the electrodeposition of uranium isotopes on it. Three strong peaks arised at 638, 524, and 474 cm−1 in Fig. 5b. Other specific peaks were also observed at the shoulder of the three strong peaks (735, 708, 519, 486, 468, 452, and 437 cm−1 ). These peaks indicate that the electrodeposited layer is a uranium oxide composite (UOx ), which consists of UO2 , U4 O9 , UO2.12 , etc [26,27]. Oxygen in either air or an organic compound, or both, might be combined with uranium during the experimental procedure of heating the back of the disc. In positive and negative TOF-SIMS spectra, the uranium oxide composite was confirmed (Fig. 6). In addition, no peaks for other uranium complexes/compounds such as UO2 CO3 or UO2 (NO3 )2 appeared except peaks for the uranium oxide and hydrogen combined uranium oxides. The peaks in the TOF-SIMS spectra indicate that oxygen and hydrogen atoms were located near the uranium atoms in the chemical structure of the electrodeposited layer. The elemental mapping of TOF-SIMS is a reasonable method to confirm the distribution of the chemical structure or element of the material surface. The images obtained by the elemental mapping of the electrodeposited layer on the disc exhibited an uniform distribution of the uranium oxides. This result shows that the elctrodeposition was well conducted without any local side-reaction on the disc in the electrodeposition cell. The composite of the electrodeposited layer was investigated using XRD patterns (Fig. 7). In thin flims, if the thickness of the film is smaller than the peneration depth of the X-rays, the substrate under the film is also detected by an XRD analysis [28]. The XRD pattern of the substrate can cause confusion in analyzing the film. Therefore, the XRD pattern of the stainless steel disc, which is used as a substrate for the electrodeposition, was shown in Fig. 7a. The peaks in the pattern were assigned to iron oxides such as FeO and Fe2 O3 . After electrodeposition of the extracted uranium from the bauxite ore, various peaks appeared which match those of U (PDF# 72-0659), UO2 (PDF# 75-0421), U4 O9 (PDF# 72-0125), UO2.12 (PDF# 71-0258), UO1.96 (PDF# 75-0413) and UO4 · H2 O (PDF# 491821) (Fig. 7b). Aside from these peaks, peaks related with other UOx were also observed. However their intensities were too weak to assign the crystal structure and appeared at the shoulder of the above peaks. Interestingly, peaks for face-centered cubic carbon
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
(nanodiamond, PDF# 79-1467) and primitive hexagonal carbon (PDF# 74-1602) were observed with peaks of the uranium composites. It is thought that carbon materials formed on the surface of the electrodeposited layer during the removal of organic residues and ethanol by heating the back of the disc with a gas torch. These carbon materials on the electrodeposited layer were investigated and are discussed in Section 3.3 of this article.
3.3. Depth analysis and surface morphology of electrodeposited uranium layer The center point of the electrodeposited layer was analyzed using XPS (Fig. 8a). The XPS survey spectrum of the layer showed that the layer surface is composed of only U, O, C, and Fe (Fig. 8b). The noteworthy result is that C and Fe were detected on the surface of the layer. Atomic concentrations on the depth of the electrodeposited layer were obtained through argon ion sputtering in the center point (red rectangular region in Fig. 8a, area: 2 mm × 2 mm) of the layer (Fig. 8c). The uranium concentration was nearly zero at a depth of ca. 460 nm (red line in Fig. 8c). From this result, the film thickness of the center point of the layer is estimated as ca. 460 nm. The value is similar with a value of ca. 450 nm, which is measured using an optical profiler (Fig. 4c). This result indicates that the prepared electrodeposited layer might be nearly flat. The atomic concentration of carbon was very high on the surface of the layer and suddenly decreased with an increase of the depth (black line in Fig. 8c). We can estimate that some carbon materials exist on the surface of the layer. The electrodeposited layer was composed of large quantities of Fe (green line in Fig. 8c) and O (blue line in Fig. 8c) together with U. The Fe might be unseparated from the bauxite matrix, although all elements except uranium isotopes
127
were removed well in the chemical separation step (extraction chromatography) with the separation column. The O was combined with U and Fe because of the oxidation while heating the back of the disc in the experimental procedure, and therefore appears in large quantity, as shown in the depth profile. The carbon on the layer surface was assessed with the high-resolution C1s spectrum (Fig. 8d). Two peaks appeared at 283.7 and 285.0 eV corresponding to sp2 - and sp3 -hybridized carbon, respectively. The sp2 - and sp3 -hybridized carbon peaks arise from the presence of graphitic and diamond phases, respectively [29,30]. In the XPS spectrum, the peak intensity for the diamond phase (sp3 -hybridized carbon) is significantly higher than the peak intensity for the graphic phase (sp2 -hybridized carbon). This result matches well with the XRD analysis that the peak intensity of face-centered cubic nanodiamond is significantly higher than that of primitive hexagonal carbon (Fig. 7b). Fig. 9a shows U 4f XPS spectra on the depth of the electrodeposited layer. The intensities of U 4f5/2 and U 4f7/2 decreased with the increase of the depth, and reached nearly zero at a depth of ca. 460 nm. This result indicates that the electrodeposition rate of Fe atoms is somewhat higher than that of U atom on the stainless steel disc in our experimental conditions for the electrodeposition. The oxidation states of U on the depth of the electrodeposited layer were obtained by fitting the high-resolution U 4f7/2 XPS spectra obtained at a depth of 0 nm (layer surface), 100 nm, 200 nm, 300 nm, and 400 nm on a Shirley background (Fig. 9b–f). The peaks corresponding to U6+ and U5+ decreased significantly with the increase of the depth. The peak for U4+ increased until a depth of 200 nm, and decreased after that. The peak for U metal (U0 ) appeared after a depth of 300 nm and increased with the depth. From the result of this XPS study, oxygen atoms in either air or
Fig. 8. (a) Photo image of uranium-electrodeposited disc. (b) XPS survey spectrum, (c) XPS depth profile and (d) high-resolution C 1s XPS spectrum of the red rectangular region in (a).
128
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
Fig. 9. (a) High-resolution U 4f XPS spectra on the depth of uranium-electrodeposited disc, and peak-fitting of U 4f7/2 XPS spectra at various depths: (b) surface (0 nm), (c) 100 nm, (d) 200 nm, (e) 300 nm and (f) 400 nm.
chemicals, or both, bonded with the uranium-electrodeposited layer on the stainless steel disc while heating the back of the disc. The oxygen atoms on the surface of the layer penetrated into the layer and bonded with the neighboring uranium atoms in the layer. Therefore, the concentration of oxygen atoms bonded with the uranium atoms on the surface is higher than that on the near surface of the stainless steel disc. The surface of the electrodeposited layer was observed using an optical microscope (Fig. 10). Irregular-shaped crack lines covered the surface of the layer (red arrows in Fig. 10b and d). Irregularshaped particles also formed on the surface (white arrows in Fig. 10d). In AFM images of the electrodeposited layer, grooves
were not observed (Fig. 11b). Only flat pieces of ca. 1 m thick were observed in the AFM image. From this result, assumed crack lines in the optical microscope images are not crack lines. The lines might be grain boundaries at the interfaces between the crystals of the electrodeposited layer [31]. To summarize, flat irregularshaped pieces are on the surface of the electrodeposited layer, and many grain boundaries are in the layer. This result is in agreement with the XRD and XPS studies showing that carbon materials, such as face-centered cubic carbon and primitive hexagonal carbon, form on the surface of the electrodeposited layer, and the layer consists of many crystal pieces of various uranium oxides.
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
129
Fig. 10. Zoom-out and zoom-in optical microscopy images of (a, c) stainless steel and (b, d) uranium-electrodeposited discs.
Fig. 11. Zoom-out and zoom-in AFM images of (a, c) stainless steel and (b, d) uranium-electrodeposited discs. Scan area: 40 × 40 m2 and 5 × 5 m2 .
4. Conclusions In this work, uranium isotopes were chemically separated from a bauxite ore using a chromatographic column, and electrodeposited on a stainless steel disc to measure the radioactivity concentration of uranium isotopes with an alpha-particle spectrometer. The concentration of the separated uranium isotopes were 214.28 ± 6.03, 10.35 ± 0.80 and 182.85 ± 5.29 Bq/kg for 238 U, 235 U and 234 U, respectively. The thickness of an electrodeposited
layer of ca. 450–460 nm was measured using an optical profiler and XPS depth profiles. The images obtained by the TOF-SIMS elemental mapping exhibited a uniform distribution of the uranium oxide on the disc. This result showed that the elctrodeposition was well conducted without any local side-reaction on the disc in the electrodeposition cell. The XRD study showed that the electrodeposited layer consists of uranium oxides and iron oxides. In addition, carbon materials were also observed on the surface of the layer. An XPS study on the depth profiles showed that oxygen atoms on the
130
Y.G. Ko et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 487 (2015) 121–130
surface of the layer penetrated into the layer and bonded with neighboring uranium atoms in the layer while heating a back of the disc. Although unnecessary elements and radionuclides are chemically separated for determining the concentration of the specific alpha-particle emitting nuclide, a small amount of those elements can remain until the procedure step of electrodeposition because the separation efficiency depends on the type of matrix of the ores or the environmental samples. These remaining elements can significantly influence the electrodeposition of alpha-particle emitting nuclides on a stainless steel disc and the measurement of the radioactivity concentration. Therefore, to measure the lowlevel activity concentration of a radionuclides with a high level of accuracy, the information from the electrodeposited layer is important. To the best of our knowledge, this is the first report on the detailed analysis of the uranium-electrodeposited layer with various techniques. We expect that our various techniques of the uranium-electrodeposited disc can motivate the improvements in the procedure steps for an alpha-particle emitting sample analysis to measure the low-level activity concentration of radionuclides with a high level of accuracy.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20] [21]
References [22] [1] A.M. Donoghue, N. Frisch, D. Olney, Bauxite mining and alumina refining, J. Occup. Environ. Med. 56 (2014) S12–S17. [2] M. Gräfe, G. Power, C. Klauber, Bauxite residue issues: III. Alkalinity and associated chemistry, Hydrometallurgy 108 (2011) 60–79. [3] B.H. O’Connor, A.M. Donoghue, T.J.H. Manning, B.J. Chesson, Radiological assessment for bauxite mining and alumina refining, Ann. Occup. Hyg. 57 (2013) 63–76. [4] G.A. Bird, Uranium in the environment: behavior and toxicity, in: R.A. Meyers (Ed.), Encyclopedia of Sustainability Science and Technology, Springer, New York, 2012, pp. 11220–11262. [5] N. Vajda, P. Martin, C.-K. Kim, Alpha spectrometry, in: M.F. L’Annunziata (Ed.), Handbook of Radioactivity Analysis, third ed., Academic Press, San Diego, 2012, pp. 363–422. [6] X. Dai, Isotopic uranium analysis in urine samples by alpha spectrometry, J. Radioanal. Nucl. Chem. 289 (2011) 595–600. [7] M.T. Crespo, A review of electrodeposition methods for the preparation of alpha-radiation sources, Appl. Radiat. Isot. 70 (2012) 210–215. [8] P.L. Fernández, J. Gómez, C. Ródenas, Evaluation of uncertainty detection limits in 210 Pb and 210 Po measurement in water by alpha spectrometry using 210 Po spontaneous deposition onto a silver disk, Appl. Radiat. Isot. 70 (2012) 758–764. ˜ M.T. Crespo, M. Roteta, G. Sibbens, S. Pommé, A.M. Sánchez, [9] E. García-Torano, M.P.R. Montero, S. Woods, A. Pearce, ␣-particle emission probabilities in the decay of 235 U, Nucl. Instrum. Methods Phys. Res. A 550 (2005) 581–592. [10] S.K. Hanson, A.H. Mueller, W.J. Oldham Jr., Kläui ligand thin films for rapid plutonium analysis by alpha spectrometry, Anal. Chem. 86 (2014) 1153–1159. [11] O. Kurihara, F. Saito, J. Koarashi, E. Kim, K. Shiraishi, Y. Yamada, M. Akashi, Emission rates of alpha particles from supports with different surface conditions in direct deposition sources, Radiat. Prot. Dosim. 145 (2011) 13–20. [12] V. Goliáˇs, D. Pittauerová, R. Procházka, Z. Tˇrískala, Field alpha-spectroscopy of
[23]
[24]
[25]
[26] [27] [28]
[29]
[30]
[31]
radon (222 Rn) and action (219 Rn) progeny in soil gas: locating a radon source, J. Radioanal. Nucl. Chem. 266 (2005) 461–470. Y.M. Panta, D.E. Farmer, P. Johnson, M.A. Cheney, S. Qian, Preparation of alpha sources using magnetohydrodynamic electrodeposition for radionuclide metrology, J. Colloid Interface Sci. 342 (2010) 128–134. J.-S. Oh, P.E. Warwick, I.W. Croudace, S.-H. Lee, Evaluation of three electrodeposition procedures for uranium, plutonium and americium, Appl. Radiat. Isot. 87 (2014) 233–237. M. Trdin, L. Benedik, Z. Samardˇzija, B. Pihlar, Investigation of factors affecting the quality of americium electroplating, Appl. Radiat. Isot. 70 (2012) 2002–2005. E.M. Bond, W.A. Moody, D.E. Dry, M.W. Rabin, Factors affecting the quality of plutonium deposits by electrodeposition, J. Radioanal. Nucl. Chem. 296 (2013) 793–798. O.A. Dumitru, R. Cs Begy, D.C. Nita, L.D. Bobos, C. Cosma, Uranium electrodeposition for alpha spectrometric source preparation, J. Radioanal. Nucl. Chem. 298 (2013) 1335–1339. V. Jobbágy, M.T. Crespo, R.V. Ammel, M. Marouli, A. Moens, S. Pommé, E. ˜ Preparation of high-resolution 238 U ␣-sources by García-Torano, electrodeposition: a comprehensive study, J. Radioanal. Nucl. Chem. 298 (2013) 345–352. R. Zarki, A. Elyahyaoui, A. Chiadli, Preparation of ␣-sources of U(VI) and Th(IV) by the electrodeposition technique in the presence of Ca2+ and some trivalent metals, Appl. Radiat. Isot. 55 (2001) 167–174. H. Klemenˇciˇc, L. Benedik, Alpha-spectrometric thin source preparation with emphasis on homogeneity, Appl. Radiat. Isot. 68 (2010) 1247–1251. J.L. Ferrero Calabuig, A. Martín Sánchez, C. Roldán García, J. Roselló ferrando, M.F. da Silva, J.C. Soares, F. Vera Tomé, Characterization of alpha sources by Rutherford backscattering spectrometry, Nucl. Instrum. Method A 369 (1996) 603–607. A.M. Beesley, M.T. Crespo, N. Weiher, N. Tsapatsaris, J.S. Cózar, H. Esparza, C.G. Méndez, P. Hill, S.L.M. Schroeder, M.E. Montero-Cabrera, Evolution of chemical species during electrodeposition of uranium for alpha spectrometry by the Hallstadius method, Appl. Radiat. Isot. 67 (2009) 1559–1569. D.C. Burciaga-Valencia, C.G. Méndez, H. Esparza-Ponce, A.M. Beesley, M.T. Crespo, L. Fuentes-Cobas, L. Fuentes-Montero, M.E. Montero-Cabrera, Synchrotron radiation study of the uranium chemical species electrodeposited for alpha spectrometry sources, Rev. Mex. Fís. S 57 (2011) 21–29. N. Casacuberta, M. Lehritani, J. Mantero, P. Masqué, J. Garcia-Orellana, R. Garcia-Tenorio, Determination of U and Th ␣-emitters in NORM samples through extraction chromatography by using new and recycled UTEVA resins, Appl. Radiat. Isot. 70 (2012) 568–573. A.W. Knight, E.S. Eitrheim, A.W. Nelson, S. Nelson, A simple-rapid method to separate uranium, thorium, and protactinium for U-series age-dating of materials, J. Environ. Radioact. 134 (2014) 66–74. G.C. Allen, N.R. Holmes, Characterization of binary uranium oxides by infrared spectroscopy, Appl. Spectrosc. 48 (1994) 525–530. J.-G. Kim, Y.-S. Park, Y.-K. Ha, K. Song, Infrared spectra of uranium oxides measured by ATR-FTIR, J. Nucl. Sci. Technol. 46 (2009) 1188–1192. M. Klaus, C. Genzel, X-ray residual stress analysis on multilayer systems: an approach for depth-resolved data evaluation, J. Appl. Cryst. 46 (2013) 1266–1276. A. Kumar, P.A. Lin, A. Xue, B. Hao, Y.K. Yap, R.M. Sankaran, Formation of nanodiamonds at near-ambient conditions via microplasma dissociation of ethanol vapour, Nat. Commun. 4 (2013) 2618. T. Petit, J.-C. Arnault, H.A. Girard, M. Sennour, P. Bergonzo, Early stages of surface graphitization on nanodiamond probed by x-ray photoelectron spectroscopy, Phys. Rev. B 84 (2011) 233407. G.S. Rohrer, Grain boundary energy anisotropy: a review, J. Mater. Sci. 46 (2011) 5881–5895.