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Design of Microchannel Suitable for Packing with Anion Exchange Resins: Uranium Separation from Seawater Containing a Large Amount of Cesium

We present a resin-packed microchannel that can reduce the radiation exposure risk and secondary radioactive wastes during uranium (U) separation by downscaling the separation using a microchip. Two types of microchips were designed to densely pack the microchannels with resins. The microchannels had almost the same cross-sectional area but different outer circumferences. A satisfactory separation performance could be obtained by arranging more than ca. 10 resins along the depth and width of the microchannels. The resin-packed microchannel is an effective separation technique for determining U concentration via inductively coupled plasma mass spectrometry owing to its ability to avoid the contamination of the equipment by cesium and reduce the matrix effect. The size of the separation site was scaled down to <1/5000 compared to commonly used counterparts. The radiation exposure risk and secondary radioactive wastes can be reduced by 10- and 800-fold, respectively, using the resin-packed microchannel.


Introduction
Uranium (U) has a long half-live and is one of the important nuclides for the evaluation of radioactive wastes containing various elements. The concentrated solution of a low-level radioactive liquid waste generated at a research facility often contains high concentrations of matrix elements because a large amount of sodium (Na) is used for neutralization. The accident at the Fukushima Daiichi Nuclear Power Plant (referred as to 1F) and its decommissioning have been generated radioactive wastes (1F samples) containing high concentrations of matrix samples comprising contaminated seawater, and soil. For example, the retained water in a nuclear reactor contains U, other actinides, cesium (Cs-137), and elements contained in seawater. 1 It contains a large amount of -ray-emitting nuclide Cs-137 (1.4 × 10 9 Bq•L -1 ) compared to the target U-238 (5.7 Bq •L -1 ).
The concentration of U can be determined via atomic spectrometric techniques, such as inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). 2 The direct injection of a sample containing high matrix elements (e.g., Na and K) that are easily ionized into ICP apparatuses suppresses and destabilizes the target signal due to the matrix effect.
Additionally, injection of samples containing high concentrations of Cs-137 causes the radioactive contamination of the equipment. Therefore, separation of U from various matrix elements is necessary to avoid the matrix effect and the radioactive contamination. Ion exchange and extraction chromatography were reported 3-8 as conventional bulk-scale methods of U separation from interfered Cs-137 and matrix metal ions. However, these methods are disadvantageous as they are manually operated, have long separation periods, and generate high volumes of waste solutions, increasing 4 the risk of radiation exposure for analysis workers while generating a large amount of secondary radioactive wastes (e.g., consumable supplies and solutions).
To address the limitations of conventional bulk-scale methods, scaled-down analysis using a microchip has been proposed as an effective solution. A microchip can offer several advantages, such as small volume (l) of sample and waste solution, accelerated operation, and suitability for automation. Various microchip-based analytical methods have been developed [9][10][11][12][13] . G.E. Collins et al. 14 reported electrophoresis using a microchip for U separation. Since UV-visible absorption-based detection was adopted, the limit of detection was limited to approximately 100 ppb in this method. G. Hellé et al. 15 reported U analysis combined with solvent extraction using a microchip and an online ICP-MS determination. Solvent extraction with a microchip is a versatile separation method but offers poor control over the flow rates of water and organic solvents, resulting in a relatively low recovery ratio of U (extraction, 72 %; stripping, 42 %; overall, 30 %). Moreover, resin-packed microchannels can accurately separate small amounts of target samples and be coupled with analytical detectors. On-chip chromatographic separation of biomolecules and organic dyes has been performed using the particle-packed column. 16,17 Applications for separation of hazardous metal ions are limited due to the lack of suitable experimental tools.
Therefore, the method of U separation using a microchip is in the basic research stage. 18 In this study, we focus on a resin-packed microchannel, which facilitates control over the solution and sensitive U determination via ICP-MS. We integrated an anion-exchange resin to a microchannel based on the results of the study on the separation of U from 1F fuel debris via anion exchange. 19 A commercially available anion exchange resin (average particle size, 11 µm) that can be packed into a microchannel was used.

5
The microchannel must be densely packed with resins to efficiently separate traces of the target from highly concentrated matrix elements. As a strategy of microchip design, changing the outer circumference while retaining almost the same cross-sectional area of the microchannels controlled the resin-packed density. In this study, we designed two types of microchips: In Microchip 1, the outer circumference was larger, and the resin-packed density was small by increasing the aspect ratio (depth, 30 µm; width, 1000 µm) of the microchannel. The resin was retained by connecting the microchannels on the upper and under glass plates so that the joint was about 10 µm due to the limitation in fabricating the dam for packing the resin in the shallow depth of the microchannel. Two-three and ca. 90 resins along the depth and width of the microchannel, respectively. In Microchip 2, the outer circumference was smaller, and the resin-packed density was large. Although the shape of the cross-section with the smallest outer circumference was a circle, it had a semicircular shape (inner diameter, 300 µm) due to the difficulty of fabricating a circular microchannel. The dam for packing the resin was fabricated in the microchannel. In the depth and width of the microchannel, ca. 10 and 30 resins were arranged, respectively. The U separation performance of these microchannels packed with the anion-exchange resin was evaluated using a standard solution of U mixed with Lanthanides (Lns). Furthermore, reduce of the matrix effect and the removal ratio of Cs were demonstrated using a seawater sample containing Cs as reference of a high matrix sample.

Reagents, chemicals, and sample preparation
The strong base anion-exchange resin (CA08S, average particle size, 11  The nearshore seawater certificated reference material NASS-7 (National Research Council of Canada, Ottawa, Canada) was used as a reference sample containing high concentrations of Na and K. The certification of trace metals in this seawater was described in previous studies. 20 The seawater test sample was prepared by adding 11.3 M HCl (final concentration, 9 M) and filtering it with a 0.2 µm filter (hydrophilic PTFE).

Microchip fabrication
The microchips used in this study are shown in Figs. 1 (A) and (B). Microchip 1 having a microchannel (depth, 30 ± 1.5 m; width, 1000 ± 50 m; outer circumference, 2060 ± 70 m; cross-sectional area, 3.0 ± 0.2 × 10 4 m 2 ) was fabricated via photolithography and dry etching, as described in previous studies. 21,22 A quartz glass substrate (size, 30 × 70 mm; thickness, 0.7 mm; VIOSIL-SX, Shin-Etsu Quartz Co., Ltd., Tokyo, Japan) was used. The glass substrate was sputtered with Cr to a thickness of 50 µm. The Cr-sputtered substrate was spin-coated using a negative photoresist. After the UV lithographic micropattern was developed, a plasma dry etching was performed under an atmosphere containing mixture of C3F8/CHF3/Ar gases. The processed substrate was washed with a sulfuric acid-hydrogen peroxide (3:1) mixture and ultrapure water. Then, it was heat-sealed in a vacuum furnace at 1080 ℃ for 10 h.

Resin-packed microchannel and column preparation
A dispersion solution of the anion exchange resin in ultrapure water was loaded into the microchannels of Microchips 1 and 2 to prepare 8 mm and 11 mm long resin-packed microchannels 1 and 2, respectively. A dispersion solution of the resin was loaded into 9 and 16.5 mm long Teflon tube columns (inner diameter, 300 µm) connected to a two-way cock filled with quartz wool ( Figure S1 (SI)).

Ion adsorption and elution
All solutions were loaded into resin-packed microchannels or a tube column using a microsyringe pump (model.

Performance evaluations of resin-packed microchannels and column
The adsorption, elution and recovery ratios of the U (or other elements) ion were determined using Eqs. 1-3.
Elution ratio Recovery ratio [%] = we / wi × 100 where wi is the amount of U (or other elements) injected into the microchannel, wa is the amount of U (or other elements) in the injection and washing fraction which is the effluent of sample and washing solutions, and we is the amount of U (or other elements) in the elution fraction.

Results and Discussion
Performance evaluation of resin-packed microchannels 1 and 2 The adsorption and elution performance of the U ion on resin-packed  Fig. S3 (B) in SI). The adsorption, elution, and recovery ratios of the Fe ion obtained using ICP-OES were >95 % (Table. S1 in SI). Therefore, we succeeded in preparing a resin-packed microchannel for the U separation.

Factors for obtaining satisfactory separation performance
We considered the factors for obtaining satisfactory separation performance by The adsorption and elution ratios at various linear velocities were investigated (Fig. 3 (a)). The adsorption and elution ratios were >95 % for a linear velocity range from 1.7 to 32 m•h -1 . This indicates that the range of the investigated linear velocity did not affect the separation performance. Second, the injected sample volume in resin-packed microchannel 2 (ratio of the sample injection volume (Vi) to the column volume (Vc), Vi/Vc = 290) was smaller than that injected in resin-packed microchannel 1 (Vi/Vc = 417).
This verification was also conducted using a Teflon tube column. The adsorption and elution ratios at various sample injection volumes were investigated ( Fig. 3 (b)). The adsorption and elution ratios at Vi/Vc = 64-943 were >95 %. Sample injection volume also did not affect the separation performance. Third, the microchannel of Microchip 2 with the high U separation performance had ca. 10 and ca. 30 resins (average particle size, 11 μm) along the depth and width of the microchannel, respectively, whereas only 2-3 resins existed in the depth direction of the microchannel of Microchip 1, whose separation performance was relatively poor. Additionally, the cross-sectional shape of Microchip 1 was rectangular, and that of Microchip 2 was semicircular. Since even rectangular microchannels with more than ca. 10 anion-exchange resins can separate metal ions, 18 there is almost no effect of the shape of the cross-section. These results indicate that resins need to be properly arranged in the microchannel, and the size of the depth and width of the microchannel should be large enough to arrange >10 resins. The lack of arranged resins may form channeling in a microchannel and limit contact with the loaded solution.

Optimization of ion adsorption and elution
The recovery ratios of U and Ln ions from 0-120 μL of the washing solution and 30-130 μL of the elution solution were investigated to optimize the washing and elution volume. In the washing fraction, the recovery ratios of Lns were approximately 85 % when the washing volume was 0 μL (Fig. 4 (A)). This is because the injected sample remained in the microchannel and connected tube. The recovery ratios of Lns were >95 % upon the injection of the washing solution >14 μL, and the Lns were completely washed. The recovery ratio of U was <1 % up to 85 l of washing solution. When the washing solution was 130 μL, the recovery ratio of U was 1.1 %. Although the detected U was negligible for the injection amount, it indicates that the adsorbed U on the column may elute with an increase in the amount of washing volume. Based on these results, the washing volume was set to 50 μL. In elution fraction, the adsorbed U was completely eluted at >30 μL, and the elution ratios of Lns were <0.1 % (Fig. 4 (B)). The elution volume was also set to 50 μL.

Demonstration using seawater
The applicability of resin-packed microchannel 2 to a high matrix sample was also confirmed using a standard seawater sample (Scheme S2 in SI). The adsorption, elution, and recovery ratios of U in the seawater were ~100 % (Table S2 in  ppb) 1 (contained in the retained water in a nuclear reactor). As shown in Table 2, the adsorption, elution, and recovery ratios of U were ~100 % and those of Cs were <3 %.
The measured value of U in the seawater standard was 2.85 ± 0.07 ppb, which was in good agreement with the certified value (2.81 ± 0.16 ppb). The concentration of Cs in 13 the elution fraction was <1/1000 of the initial concentration. The value of U obtained without separation was smaller than the certified value due to the matrix effect of seawater elements. Additionally, the standard deviation of the value obtained by separating U was smaller than that obtained without separating U (Fig. 5). These results indicate that the prepared resin-packed microchannel 2 can remove Cs and reduce the effect of matrix elements. Added value ---431 a: n = 3.