Efficient and selective extraction of uranium from seawater based on a novel pulsed liquid chromatography radionuclide separation method

Efficient and selective extraction of uranium from seawater based on a novel pulsed liquid chromatography radionuclide separation method 1 2 3 first-author https://orcid.org/0000-0002-1578-1992 1 2 3 corresp * xiaoshan770@163.com * xiaoshan770@163.com xiaoshan770@163.com https://orcid.org/0000-0002-0622-5986 State Key Laboratory of Nuclear Resources and Environment (East China University of Technology), Nanchang 330013, China State Key Laboratory of Nuclear Resources and Environment (East China University of Technology), Nanchang 330013, China Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices (East China University of Technology), Nanchang 330013, China Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices (East China University of Technology), Nanchang 330013, China School of Nuclear Science and Engineering, East China University of Technology, East China University of Technology, Nanchang 330013, China School of Nuclear Science and Engineering, East China University of Technology, East China University of Technology, Nanchang 330013, China The novel pulsed liquid chromatography radionuclide separation method presented here provides a new and promising strategy for the extraction of uranium from seawater. In this study, a new chromatographic separation method was proposed, and a pulsed nuclide automated separation device was developed, alongside a new chromatographic column. The length of this chromatographic column was 10 m, with an internal warp of 3 mm and a packing size of 1 mm, whilst the total separation units of the column reached 12,250. The most favorable conditions for the separation of nuclides were then obtained through optimizing the separation conditions of the device: Sample pH in the column = 2, sample injection flow rate = 5.698 mL/min, chromatographic column heating temperature = 60 ℃. Separation experiments were also carried out for uranium, europium, and sodium ions in mixed solutions; Uranium and sodium ions in water samples from the Ganjiang River; and uranium, sodium, and magnesium ions from seawater samples. The separation factors between the different nuclei were then calculated based on the experimental data, and a formula for the separation level was derived. The experimental results showed that the separation factor in the mixed solution of uranium and europium (1:1) was 1.088, whilst achieving the initial separation of uranium and europium theoretically required a 47-stage separation. Considering the separation factor of 1.50 for the uranium and sodium ions in water samples from the Ganjiang River, achieving the initial separation of uranium and sodium ions would have theoretically required at least a 21-stage separation. Furthermore, for the seawater sample separation experiments, the separation factor of uranium and sodium ions was 1.2885, therefore more than 28 stages of sample separation would be required to achieve uranium extraction from seawater. The novel pulsed liquid chromatography method proposed in this study was innovative in terms of uranium separation and enrichment, whilst expanding the possibilities of extracting uranium from seawater through chromatography. Pulsed liquid chromatography Nuclide separation Seawater uranium extraction Uranium enrichment Introduction 1 Uranium extraction from seawater is strategically important in guaranteeing the sustainable development of nuclear energy and ensuring successful completion of China's carbon peak and carbon-neutral goals. As an efficient and clean energy source, nuclear power is gradually becoming a more important factor in China adjusting its energy structure, achieving coordinated economic and environmental development, and promoting industrial structure upgrades [ 1 1 , 2 2 ]. Uranium is a key resource and forms the basis for the development of nuclear energy, although the existing terrestrial uranium reserves are relatively poor and are only expected to be sufficient for human use for approximately 100 more years. In contrast, seawater contains more than 4.5 billion tons of uranium, almost 1000 times greater than the amount of terrestrial uranium reserves; thus, seawater uranium extraction is expected to be a feasible solution for ensuring the sustainable development of nuclear energy [ 3 3 , 4 4 ]. In addition, to satisfy China's commitment to achieve "peak carbon" by 2030 and to be "carbon neutral" by 2060, the proportion of nuclear power in China's total electricity needs to be increased further. This will require a higher demand for a diversified and sustainable supply of uranium resources. Research on uranium extraction from seawater has been conducted for several decades, and this was primarily focused on two aspects. Firstly, in methodological studies, mainly adsorption [ 5 5 – 10 10 5–10 ], extraction [ 11 11 , 12 12 ], precipitation [ 13 13 , 14 14 ], ion exchange [ 15 15 ], electrochemical extraction [ 16 16 , 17 17 ], photocatalysis, etc. [ 18 18 , 19 19 ], and bioconversion [ 20 20 , 21 21 ]; secondly, in materials research, mainly focused on the preparation and study of adsorbent materials and extractants. For example, Das, Sadananda et al. synthesized polymeric fiber-based adsorbents, aminooxane macroporous membranes, and other materials through radiation-induced graft polymerization and subsequently used these for uranium extraction from seawater, considering that these materials had a high adsorption capacity and faster adsorption kinetics for uranium [ 22 22 , 23 23 ]. Furthermore, Salam et al. synthesized a new cationic surfactant (GCS) for the extraction and separation of sulfate solutions from U(VI) ions [ 24 24 ]. Adsorption is the most widely used method for uranium extraction from seawater owing to its simplicity of operation. Inorganic, polymeric, and nanostructured materials, as well as biomaterials, have been previously used to prepare seawater uranium adsorbents. Among these, the amidoxime group (AO) adsorbent has advantages such as simple preparation, high physical and chemical stability, and strong coordination binding with uranium at the pH of seawater (8.3). Moreover, amidoxime group adsorbents have already been used in large-scale engineering tests around the beginning of the century, thus proving their practicality [ 25 25 – 27 27 25–27 ]. Similarly to inorganic adsorbents, they also have a low cost, large specific surface area, adjustable pore structure, and easy preparation. However, they are mostly in a powdered form, are difficult to recover, and presently are still in the laboratory research stage. Nanosorbents have advantages such as a high specific surface area, high adsorption site density, and rapid mass transfer, although their technology is still developing and they are difficult to prepare in bulk. Polymer adsorbents have high strength, good flexibility, an adjustable molecular structure, and are easy to mass produce; however, this material is costly and vulnerable to the marine environment. The main advantages of microbial adsorbents include their fast adsorption rate, good selectivity, and low cost, yet they are difficult to control and unsuitable for high-concentration UO 2 2+ wastewater[ 28 28 , 29 29 ]. The extraction method used for uranium separation plays an important role in hydrometallurgy, reprocessing, and the treatment of uranium in uranium-containing wastewater. The extractant holds the most significant role in the extraction process, and currently, the main uranium extractants are tributyl phosphate (TRPO) and trialkylphosphine oxide [ 30 30 ]. Recent research has been mainly focused on new extractants, such as phthalamides, aluminescents, macrocyclic compounds, and ionic liquids [ 31 31 , 32 32 ]. Membrane extraction, also known as fixed-membrane interface extraction or membrane-based solvent extraction is a novel method that has unique advantages. These include the fact that the membrane extraction mass transfer process occurs on the microporous membrane surface or in the pore channel, which can reduce the entrapment loss of extractant in the feed-liquid phase, reducing the physical requirements of extractant, broadening the range of extractant selection, and avoiding the influence of "re-mixing" phenomenon in the extraction process. The extraction process also breaks the limitation of the "liquid flooding" condition and can use a very high water-oil ratio [ 33 33 ]. Solid phase extraction (SPE) plays an increasingly important role in the separation and analysis of key nuclides such as uranium, especially in the treatment of uranium contamination in various environmental waters, owing to its high enrichment factor, relatively high operational flexibility and irradiation stability, fast sorption kinetics, reusable extractants, low consumption of organic reagents, low waste generation, and easy solidification of failed materials [ 34 34 ]. Photocatalysis is another hot topic in research on seawater uranium extraction. Photocatalytic reactions have been widely used for CO2 reduction, water decomposition, heavy metal treatment, organic pollutant degradation, and photosynthesis, and can adjust the valence state of metal ions over a wide range without requiring other reducing agents or electric fields. During photocatalytic reduction, U(VI) in water was effectively reduced and removed[ 35 35 , 36 36 ]. In seawater, the photocatalytic method can achieve an extraction of 3960 mg/g in just a few hours with a simple and efficient elution of the extracted uranium [ 35 35 ]. In addition, photocatalysis has high selectivity for U(VI) in the presence of multiple coexisting ions [ 36 36 , 37 37 ]. The photocatalytic extraction of uranium from aqueous solutions has advantages such as a high extraction capacity, high selectivity, and fast kinetics, all of which are superior to those of oth er methods, as shown in Table 1 Table 1 . 1 1 Materials 1 1 pH top value 1 1 Efficiency 1 1 Light reaction time 1 1 Uranium content 1 1 References 1 1 Bi 2 WO 6 1 1 6.5 1 1 53% 1 1 600 min 1 1 30 mg/L 1 1 [ 38 38 ] 1 1 ZnO/TiO2-SiO2 1 1 5.5 1 1 97% 1 1 240 min 1 1 47 mg/L 1 1 [ 39 39 ] 1 1 Carboxylated g-C 3 N 4 1 1 8.2 1 1 100% 1 1 50 min 1 1 27 mg/L 1 1 [ 40 40 ] 1 1 TiO 2 1 1 5.0 1 1 50–100% 1 1 180 min 1 1 27 mg/L 1 1 [ 41 41 ] 1 1 Te@O-SnS 2 1 1 4.8 1 1 97.3% 1 1 60 min 1 1 8 mg/L 1 1 [ 42 42 ] 1 1 ZnFe 2 O 4 1 1 5.0 1 1 95% 1 1 40 min 1 1 50 mg/L 1 1 [ 43 43 ] Presently, the use of chromatography for uranium extraction from seawater is relatively rare [ 44 44 ]. In the past, the performance of chromatographic separation has been largely dependent on the type and nature of the stationary phase. However, the differences in the properties of the solution components themselves and the mechanisms of their interaction with the stationary phase were neglected, and thus, have been greatly limited in dealing with the separation of complex systems such as seawater. Research focused on chromatographic stationary phases with high selectivity and separation performance is the main driver for the development of chromatographic techniques. Therefore, this study is based the mechanisms and methods of uranium extraction from seawater, with the context of national strategic requirements. Herein, the problems and shortcomings of traditional methods are addressed, including adsorption and extraction, which are costly, complicated, and time-consuming, the separation requirements to reach the equilibrium of adsorption and extraction, and the inability to achieve dynamic separation. A dynamic chromatographic separation method was used here to achieve rapid separation of uranium in a non-equilibrium state. The key aims of this study were (1) to establish a pulsed liquid chromatographic nuclide separation device and to construct a chromatographic separation system consisting of six components to achieve pulsed automatic sample feeding, automatic separation, and online automatic detection. (2) to determine the separation factors and steps of this method by testing optimal separation conditions and conducting research on the separation conditions and mechanisms of nuclide chromatography. The effects of varying column fillings, sample flow rates, pH levels, heating and cooling temperatures, and types and concentrations of coexisting ions on the separation effect were all investigated. Furthermore, the kinetic and thermodynamic mechanisms under these various conditions were explored. (3) Laboratory studies on seawater uranium extraction were then carried out under the optimal experimental conditions and at the most suitable separation levels to determine the effectiveness of the separation devices, verify the correctness of the separation principles and the economy of the methods, and to provide new feasible methods and ideas for seawater uranium extraction. Experimental section 1 Instruments and reagents 2 The instruments required for this study included a new self-developed chromatographic column and switching pulse injection device, modified 721 online spectrophotometric detector, peristaltic pump with model BQ80S + FZ10-CE, 85-2 digital display temperature-controlled magnetic stirrer, inductively coupled plasma mass spectrometer (ICP-MS) with model Agilent 7500cx, and inductively coupled plasma emission spectrometer (ICP-OES) with model iCAP7400. The experimental reagents and samples used in this study were pH = 1 HCl as the elution liquid, pH = 2 HCl as the mobile phase, treated Ganjiang River water samples, seawater samples, varying concentrations of europium and uranium solutions, and varying concentrations of cation solutions. Preparation of a new chromatographic column 2 The column used for the experiments in this study was constructed to be 10 m long rubber tube, with a 5 mm outer diameter, and 3 mm inner diameter as the column body and 1 mm diameter adsorbent beads as fillers, with glass tube joints connected to each end of the rubber tube and sand cores inside the joints, which were used with characteristics such as high strength, acid resistance, and reusability. It was calculated that 0.82 mm in the column was equivalent to one separation unit, whilst the total separation units could reach 12,250. Before preparation, the filling and tube body were cleaned using dilute hydrochloric acid, followed by ethanol, and then distilled water. The column was then loaded using the wet method after drying. The separation principle of this column used the differences in the masses of elements to achieve separation: quantum separation. As shown in Figure 1 Figure 1 : Switching pulse injection device design 2 The switching pulse injection device consisted of two parts: Pulse generation and control of the solution feed and separation, which is illustrated in Figs. 2(a) and 2(b) Figs. 2(a) and 2(b) . Pulse action is the decisive factor in achieving the sustainable separation of nuclides, with each pulse in the sample being separated. The pulses were generated in such a way that the column first entered the air at T1 time, the solution at T2 time, and then the air at T1 time at the end of T2 time, and so on, to generate a pulse injection. When the device automatically recognized that the incoming solution was coming out of the chromatographic column, T3 began timing. T3 was the optimal shunt time, and at the end of T3, the device collected the samples coming out of the column individually to achieve separation. The control of sample separation in the column was key in achieving nuclide separation, whilst the time setting of separation action was directly related to the effect of nuclide separation. Establishment and application of pulsed liquid chromatography system for nuclide separation 2 A pulsed liquid chromatography nuclide separation device was established to achieve nuclide separation based on precise spatial and temporal control. There were six components of this nuclide separation device: The chromatographic cycle separation system, mainly composed of chromatographic columns for chromatographic separation, the switching pulse injection system, a device to generate injection pulses, the time-controlled automatic shunting system, mainly used to monitor and control sample shunt, and was a control device to realize automatic nuclide separation, and the online detection and analysis system, a system for real-time detection of product composition and verification of separation effect. Unlike the physical nuclide detection device principle [ 45 45 , 46 46 ], this device used ICP-MS to detect and analyze the nuclides coming out of the column in real time, and controlled the nuclide separation by sending analysis results to the time-controlled automatic shunting system. There was also the adjustable flow system, which was the power source of the entire system and the automatic control system, an auxiliary device to control the interaction between nuclides and fillers. Among these, the chromatographic cycle separation system was the core of the device, whilst the other five systems responded to it and were linked by connecting devices. This is shown below in Fig. 3 Fig. 3 . The sample solution to be separated and concentrated was added to the cuvettes and a switching pulse feed system was then used to drive the connecting pipeline between the inlet end of the column and one of the cuvettes. The sample solution was delivered to the column in pulsed mode using the timed automatic switching inlet as the source of the pulsed flow. The sample entered the column, and after separate blocking action, the nuclide was separated from other interfering particles, and the sample subsequently exits the column. At the exit end of the column, the fluid flow direction was controlled by a time-controlled automatic shunt system; the first part of the sample containing less uranium flowed back to the primary cuvette through the reflux tube, whilst the latter part of the sample containing more uranium flowed to the second sample/sub-cuvette to wait for the next level of separation and concentration. The separation and concentration of nuclides could then be achieved by repeating this several times. Results and Discussion 1 Determination of optimum conditions for the separation of uranium solutions through a chromatographic column 2 To determine the optimal conditions for chromatographic separation, experimental studies on the over-column acidity, over-column flow rate, heating and cooling bath temperatures, pulse feed method (amount of sample per pass and pulse interval time), coexisting ions, and shunting time were required. Graph of uranium separation variation within the chromatographic column 3 The experiment required 30 mL of uranium solution in the column, with the uranium-containing solution exiting through the column, which developed color with an azoarsine III chromogenic agent being placed into the detector (online spectrophotometer). A set of images were taken after color development on the detector, and the change in the curve was equal to the change in uranium concentration after the reaction exited the column, which was reflected by the absorbance value. As shown in Fig. 4 Fig. 4 , point O was the time at which the sample entered the column, point F was the time at which the sample exited the column (150 s), the absorbance of the OF section was the value after the chromogenic agent was zeroed, and point G was the time at which the uranyl ion in the sample exited (270 s). After the sample exited the FG section, the chromogenic agent was diluted, the absorbance decreased, and there were almost no uranyl ions in this section of the solution. The absorbance increased after point G, and this change in absorbance allowed monitoring of the changes in uranium within the column. Point H was the time at which uranium was at its highest concentration, that is, the peak time. The fluctuation after the peak was caused by pulse injection, which resulted in a better separation between the nuclides that had reached the peak and those that had not yet reached the peak during the separation of multiple nuclides. Effect of pH on the separation of uranium and europium 3 The sample acidity across the column was one of the most important factors in determining ion movement and separation status. In solution, with a decrease in hydrogen ions (pH increase), the filler surface underwent protonation and deprotonation, which was reflected in the change in the surface potential of the filler. Considering that the purpose of this study was to slow down the movement of uranyl ions through the action of the filler and its liquid film, rather than adsorbing uranyl ions on the filler surface, the separation of nuclides was typically selected at a low pH (pH < 3) stage. The separation of uranium and europium mixed solutions in the column at different pH values was then measured by adding 15 mL each of the same concentrations of uranium and europium into the column with a sample injection flow rate of 3.802 mL/min. In the 30 mL sample solution, the first 7 mL sample solution in the column was discarded because of the low ion concentration, as shown in the FG segment of Fig. 4 Fig. 4 . Therefore, the last 21 mL of the sample solution was collected, with every 3 mL of solution comprising one sample. The test results are shown in Fig. 5 Fig. 5 . Figure 5(a) Figure 5(a) shows the concentration variation of uranyl ions at different pH levels, whilst Fig. 5(b) Fig. 5(b) shows the concentration variation of europium ions at different pH values, which were detected and analyzed by ICP-OES. The results of this showed that the concentration of uranyl ions exiting chromatography was highest at pH = 2 for uranium solutions, i.e., the highest separation concentration; separation concentration was highest for europium solutions at pH = 0.1. Figure 6(a) Figure 6(a) shows the separation of uranium and europium mixed solution at pH = 0.1, whilst Fig. 6(b) Fig. 6(b) shows the separation of uranium and europium mixed solution at pH = 1, and Fig. 6(c) Fig. 6(c) shows the separation of uranium and europium mixed solution at pH = 2. Comparing these three cases, at pH = 0.1 and pH = 1, the column had a lesser effect on the separation hindrance effect of uranium and europium, which led to a uniform change in the concentration of uranium and europium exiting the column, and the separation here was poor. When pH = 2, the concentration difference between uranium and europium exiting the column was the largest, that is, the separation was greatest. Therefore, pH 2 was chosen as the acidity of the experimental sample passing through the column. Effect of sample injection flow rate on the separation of uranium and europium 3 The sample flow rate could control the ion separation time and improved the maneuverability of nuclide ion separation. Therefore, the choice of sample flow rate was highly important; a speed too slow or too fast could lead to difficulty in the separation of nuclide ions. Experimentally, 30 mL of a mixed solution of uranium and europium at pH = 2 was used for separation sampling; one sample was taken every 3 mL and subsequently detected by ICP-OES. The results of this are shown for different sample flow rates in Fig. 7 Fig. 7 . The sample flow rate shown in Fig. 7(a) Fig. 7(a) was 1.908 mL/min, at which, the separation of the column for the mixed nuclides was the least effective, and separation only began at the seventh set of samples. The sample flow rate shown in Fig. 7(b) Fig. 7(b) was 3.802 mL/min; at this flow rate, the separation of the mixed solution began from the 6th set of samples. The sample flow rate shown in Fig. 7(c) Fig. 7(c) was 5.698 mL/min; at this sample flow rate, the separation of uranium and europium was optimal, with separation of uranium and europium starting at the 4th set of samples, and the best separation time point being approximately 150 s. Achieving a larger separation factor and fewer separation stages were the objectives of the experimental study. The separation factor could be obtained theoretically through preliminary experiments, and the separation stages were obtained by the following equation: <math display="block" id="M1"><mrow><mtext>a</mtext><msup><mi>b</mi><mi>n</mi></msup><mtext>=c,</mtext></mrow></math> where a is the ratio of the concentration of uranium in the initial solution to the concentration of the major coexisting ions (e.g., europium or sodium ions); b is the separation factor, which can be calculated experimentally; n is the number of stages of separation, and c is the ratio of the concentration of uranium in the solution after separation to the concentration of the major coexisting ions. As shown in Fig. 7(c) Fig. 7(c) , the separation factor of uranium and europium was 1.088 at a sample flow rate of 5.698 mL/min. According to Eq. (1), the initial separation of uranium and europium from the solution was calculated at more than 47 times. Therefore, the sample flow rate of the column during the experiment was set at 5.698 mL/min. Effect of column heating temperature on the separation of uranium, europium, and sodium ions 3 Heating and cooling during the chromatographic column path can have the effect of "slowing down" and "speeding up" the ions, which can help to further achieve separation of nuclide ions. The heating and cooling paths of the column were divided into five stages according to column length, whilst cooling bath temperature was fixed at 15 ℃. The separation of a mixture of uranium, europium, and sodium ions from the column in a heating bath at 20, 40, and 60 ℃ was investigated at pH = 2 and at a sample flow rate of 5.698 mL/min. A mixture of 25 mL of uranium, europium, and sodium ions was added to the column for separation, in which the concentrations of uranium and europium were the same. The last 18 mL of sample exiting the column, one sample for every 3 mL of solution, was collected and analyzed, as shown in Fig. 8 Fig. 8 . Figure 8(a) Figure 8(a) shows the separation of uranyl ions in the mixed solution at different temperatures, and it can be seen here that uranyl ions had the highest concentration after exiting the column at the same time (t > 180 s) at 40 ℃ i.e., the best separation concentration, whilst 60 ℃ was the lowest. Figure 8(b) Figure 8(b) shows the separation of europium ions in the mixed solution at different temperatures, and the concentration of europium ions exiting the column increased with an increase in temperature. Figure 8(c) Figure 8(c) shows the separation of sodium ions in the mixed solution at different temperatures. The separation of sodium ions also increased with increasing temperature. Figure 8(d) Figure 8(d) shows the separation of the three mixed elements at a column temperature of 20 ℃. The separation curves for uranium, europium, and sodium ions were essentially the same at 20 ℃. The separation of uranium and sodium ions was relatively poor in the separation solution exiting the column before 300 s, although the separation was good after 300 s, when the concentrations of uranium and sodium ions exiting the column showed high variation. Figure 8(e) Figure 8(e) shows the separation of the three mixed elements at a column temperature of 40 ℃. The separation of uranium and europium and uranium and sodium ions was good at 40 ℃, particularly after 300 s, when the difference in concentration between uranium and sodium ions was the largest. Furthermore, the separation factor of uranium and europium under this condition was 1.088. Figure 8(f) Figure 8(f) shows the separation of the three mixed elements at a column temperature of 60 ℃. The separation of europium and uranium at 60 ℃ was poor, whilst the difference in concentration between the two elements after separation was not significant, although the separation of uranium and sodium was greatest after 300 s. In Figs. 8 (d), (e), and (f) Figs. 8 (d), (e), and (f) , the separation factors of uranium and sodium were 1.037 at 20 ℃, 1.0567 at 40 ℃, and 1.0809 at 60 ℃. These figures showed that the separation of uranium and europium was best under low-temperature conditions, whilst separation of uranium and sodium ions was the worst. The separation of uranium and europium was the worst under high-temperature conditions, although this was when the separation factor of uranium and sodium ions was the highest. The main reason for this was that the ionic motion states of light and heavy ions were slower under low-temperature conditions, and after the light and heavy ions were blocked by the separation unit inside the column, the motion rate did not change significantly. The concentration of sodium ions in the back-end solution was therefore higher, so the separation effect of sodium ions and uranium was poor. At higher temperatures, the light ions were more active than the heavy ions, so the sodium ions could pass through the separation unit layer more easily whilst the sodium ions moved at a faster rate and exited the column first. Therefore, the latter part of the separation solution had a lower sodium ion content with a higher uranium content, which could better achieve the separation of uranium and sodium ions. However, as a result of the small mass difference between uranium and europium ions, the temperature increased, and the ions were therefore more likely to collide and intermingle when passing through the separation unit, leading to a lower separation effect. In summary, the heating temperature of the column for the seawater uranium extraction experiment was chosen to be 60 ℃. Effect of sodium ions on the separation of uranium and europium 3 The cations in seawater are more complex, and the separation of uranium in the column under different cation environments was experimentally studied, as shown in Fig. 9 Fig. 9 , whereby 1 mL of varying concentrations of cations was added to 20 ml of uranium and europium mixed solution. Figure 9(a) Figure 9(a) shows the separation of uranium and europium after adding 1 mL of 1 mol/L sodium ions. The separation time of the mixed solution coming out from the column was between 100 and 200 s, with the concentration difference between uranium and europium being the largest, and having the greatest separation. In addition, the concentration of europium in the separation solution exiting the column was constantly higher than that of uranium. Figure 9(b) Figure 9(b) shows the separation of uranium and europium after adding 1 mL of 0.5 mol/L sodium ions, which was higher than that of uranium before 125 s, whilst the concentration of europium was higher than that of uranium after 125 s. Furthermore, the best separation time of uranium and europium was after 200 s. Figure 9(c) Figure 9(c) shows the separation of uranium and europium after adding 1 mL of 0.1 mol/L sodium ions, and the concentration of uranium in the separation solution from the column was clearly higher than that of europium. The concentration difference between uranium and europium was larger and the separation effect was better in this case. Comparing the three plots in Figs. 9 (a), (b), and (c) Figs. 9 (a), (b), and (c) , the separation of uranium and europium on the column was greatest between 125 and 200 s when 1 mol/L sodium ions was added. After 200 s, the concentration of sodium ions in the mixed solution had little effect on the separation of uranium and europium. Separation of uranium and sodium ions in water samples from the Ganjiang River 2 By studying the performance of the chromatographic column, the separation of uranium and other nuclide ions could be achieved after a mixed solution of uranium, europium, and sodium ions was separated by the column. Therefore, to further verify the feasibility of this device, experiments were conducted on water samples from the Ganjiang River to determine the concentration ratio of separated uranium and sodium ions in the water samples. A 20 L water sample from the Ganjiang River was evaporated and then concentrated to 2 L. The pH of this 2 L water sample was subsequently adjusted to 2. The sample feed flow rate was set at 5.698 mL/min. The chromatographic column was heated to 60 ℃ and then cooled to 15 ℃, for five cycles between hot and cold in this way. The parameters of the pulse injection device were set as follows: T 1 = 70 s, T 2 = 200 s, T 3 = 220 s, where T 1 is the air pulse time, T 2 is the pulse injection time (Injection volume is 20 ml), and T 3 is the optimum separation time after the sample was removed from the column. According to Fig. (8 Fig. (8 ), the optimal separation point for uranium and sodium ions was at around 220 s (15 mL) for each 20 mL sample injection, therefore the first 15 mL of separation was collected experimentally. Pulsed injection separation was then performed according to these optimum separation conditions, and the sample solution collected from the separation was subjected to the next stage of separation for four stages. The uranium to sodium ion concentration ratio was then determined for each stage of the sample, with the experimental results for this being shown in Fig. 10 Fig. 10 . These results showed that the uranium-sodium ion concentration ratio in the front-end liquid of the first separation stage was 0.0022, whilst the uranium-sodium ion concentration ratio after the fourth separation was 0.00056. Therefore, the uranium-sodium ion concentration ratio after the first separation was 3.93 times higher than that after the fourth separation, indicating that the sodium ion concentration in the front-end liquid of the separation increased continuously whilst the uranium concentration decreased continuously with the increase in separation stages. In other words, the separation front-end solution was enriched with sodium ions. Conversely, the separation back-end solution is the separation enrichment of uranium. The experimental results showed that the water sample in the Ganjiang River could separate uranium and sodium ions after entering the column using pulsed feed. According to Equation (1), the separation factor of uranium and sodium was calculated to be 1.50, and more than 21 stages were theoretically required to achieve the initial separation of uranium and sodium. Study on uranium extraction from seawater 2 The separation of water samples from the Ganjiang River further validated the feasibility and scientific validity of the pulsed liquid chromatography separation method, which was designed to allow for the separation and extraction of uranium from seawater in complex environments. Therefore, 20 L of seawater was taken (from Xiamen, Fujian Province) for the experiment and the samples were pretreated according to the optimum separation conditions, followed by a pulsed injection separation. The experimental conditions were the same as those used for the separation of the water samples from the Ganjiang River. The parameters of the pulsed injection device were set, where the parameters are the same as in section 3.2 of the text, and 20 mL of the sample was then pulsed into the column each time. The first 15 mL of the solution from the column was used as the front-end solution and was refluxed into the primary cuvette. The final 5 mL of the solution was subsequently collected as the back-end enrichment solution. All 20 L of seawater were separated once for one stage, and the collected back-end solution was then separated for the next stage, with a total of four stages being separated in the experiment. The variations in uranium, sodium, magnesium, and potassium ion concentrations in the seawater samples were determined using ICP-MS, with the results of this being shown in Fig. 11 Fig. 11 . Table 2 Table 2 presents an analysis of the corresponding standard substances for each element. 1 1 Elements 1 1 Linear equation 1 1 R 2 1 1 LOD (ppb) 1 1 BEC (ppb) 1 1 U 1 1 f ( x ) = 170484.2193 x + 107.3543 1 1 0.9999 1 1 0.0018 1 1 0.001 1 1 Na 1 1 f ( x ) = 1048.2703 x +7591.6695 1 1 0.9998 1 1 0.3925 1 1 7.242 1 1 Mg 1 1 f ( x ) = 488.2858 x + 139.3347 1 1 0.9997 1 1 0.0741 1 1 0.285 1 1 K 1 1 f ( x ) = 396.1091 x + 7906.5578 1 1 0.9998 1 1 1.1170 1 1 19.961 Figure 11(a) Figure 11(a) shows the variation in uranium concentration in the back-end enrichment solution for the four stages of seawater separation, whilst Fig. 11(b) Fig. 11(b) shows the variation in sodium ion concentration in the back-end solution for the four stages of seawater separation. These results showed that the concentration of uranium continued to increase whilst the concentration of sodium ions continued to decrease as the number of separation stages increases. The initial concentration of uranium in seawater measured in Fig. 11(a) Fig. 11(a) was 6㎍/L, higher than that of normal seawater (3.3㎍/L). This was mainly because the experiment first used nitric acid to adjust the pH of the 20 L seawater samples to 2 during the pretreatment of seawater to prevent deterioration A vacuum pump was then used to filter the seawater to remove solid impurities. However, the pores of the Brinell funnel used for filtration contained residues from the previous filtration, which could not be cleaned off when the funnel was cleaned usinbg low acid or distilled water. The seawater sample to be removed was adjusted to pH = 2 using strong nitric acid, so that the seawater sample passed through the funnel at low pH and washed the uranium from the residue into the sample, resulting in an increase in uranium concentration. Figure 11 (c) Figure 11 (c) shows the ratios of uranium and sodium ion concentrations for the four separation stages. The ratio of uranium and sodium ion concentrations in the original seawater sample was 4.883 × 10 -7 , whilst the ratio of uranium and sodium ion concentrations after the separation of the 4th stage was 1.346 × 10 -6 . According to the calculation formula (1) for the number of separation stages, the separation factor of uranium and sodium ions in seawater was b = 1.2885, and the separation of uranium and sodium ions in seawater was successful. Theoretically, 28 stages were required for this. Figure 11 (d) Figure 11 (d) shows the ratio of uranium and magnesium ion concentrations for the four stages of separation. The ratio of uranium and magnesium ion concentrations in the original seawater sample was 4.206 × 10 -6 , whilst the ratio of uranium and magnesium ion concentrations after the fourth stage of separation was 1.144 × 10 -5 , 2.72 times that of the initial sample. Furthermore, the separation factor of uranium and magnesium ions in seawater was b = 1.2847. Figure 11(e) Figure 11(e) then shows the ratios of the uranium and potassium ion concentrations for the four stages of separation. The ratio of uranium and potassium ion concentrations in the original seawater sample was 1.276 × 10 -5 , and the ratio of uranium and potassium ion concentrations after the fourth stage of separation was 3.398 × 10 -5 . Additionally, the separation factor of uranium and potassium ions in seawater was b = 1.2773. The main cations were sodium, magnesium, and potassium ions. The separation factors of these three ions and uranium were all similar, whilst also being relatively large. Furthermore, a lower number of stages was required to separate uranium from these three cations, therefore, theoretically, after 28 stages of seawater sample separation, uranium solution could be extracted from the seawater. After experimental verification, this new pulsed liquid chromatography radionuclide separation method could be applied to seawater uranium extraction, and was more effective for the separation of uranium and sodium ions in seawater with high selectivity. Study on separation mechanism 2 The retention time of uranyl ions in the column is defined as the time from when the aqueous solution flows to the "time-controlled automatic shunt system" to when the uranyl ions start to appear here. The duration of this depends on the flow rate, pH, filling particle size, coexisting ion strength, tube diameter, temperature, and other factors. Under the same conditions, the retention times of different ions were different. The average retention time of the ions is calculated from Eq. (2). <math display="block" id="M2"><mtable><mtr><mtd><mtext>Average</mtext><mtext> </mtext><mtext>retention</mtext><mtext> </mtext><mtext>time</mtext><mtext> </mtext><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext>Retention</mtext><mtext> </mtext><mtext>time</mtext><mrow><mo>(</mo><mi>T</mi><mo>)</mo></mrow><mo>/</mo></mtd></mtr><mtr><mtd><mtext>Total</mtext><mtext> </mtext><mtext>number</mtext><mtext> </mtext><mtext>of</mtext><mtext> </mtext><mtext>separation</mtext><mtext> </mtext><mtext>units</mtext><mrow><mo>(</mo><mrow><msub><mi>n</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow><mo>,</mo></mtd></mtr></mtable></math> where t is the average retention time, T is the retention time, and n 0 is the total number of separated units. Retained liquid film: When a solid is removed from the liquid that can wet it, the liquid film attached to the surface is called the retained liquid film. The most important physical quantity of this is the thickness, d. If the liquid film thickness of the column filling is the same as the liquid film thickness of the tube wall, the retained liquid film thickness of each filling in the column is given by Eqs. (3) and (4). <math display="block" id="M3"><mrow><mfrac><mrow><msub><mi>V</mi><mn>1</mn></msub></mrow><mrow><mtext>separation units</mtext><mo>×</mo><mi>Z</mi></mrow></mfrac><mo>+</mo><msub><mi>V</mi><mi>x</mi></msub><mo>=</mo><mfrac><mrow><mn>4</mn><mtext>π</mtext></mrow><mn>3</mn></mfrac><msup><mrow><mrow><mo>(</mo><mrow><mi>d</mi><mo>+</mo><msub><mi>r</mi><mi>x</mi></msub></mrow><mo>)</mo></mrow></mrow><mn>3</mn></msup></mrow></math> <math display="block" id="M4"><mrow><msub><mi>V</mi><mn>2</mn></msub><mo>−</mo><mrow><mo>(</mo><mrow><msub><mi>V</mi><mn>0</mn></msub><mo>−</mo><msub><mi>V</mi><mn>1</mn></msub></mrow><mo>)</mo></mrow><mo>=</mo><mtext>π</mtext><mo>×</mo><mi>L</mi><mo>×</mo><msup><mrow><mrow><mo>(</mo><mrow><msub><mi>r</mi><mn>2</mn></msub><mo>−</mo><mi>d</mi></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msup></mrow></math> where d is the thickness of the retained liquid film per filling, V 0 is the total volume of the retained liquid film inside the column (mm 3 ), V 1 is the volume of the liquid film on the filling (mm 3 ), V x is the volume of each filling (mm 3 ), V 2 is the volume of the internal space of the column (mm 3 ), Z is the number of particles of the filling on each separation unit, r x is the radius of each filling (0.5 mm), r 2 is the column inner cavity half-warp (1.5 cm), and L is the column length (10 m). V 0 can be measured experimentally , whilst V x , r x , and Z are related to the selected filling particle size; and V 2 , r 2 , and L are related to the required chromatographic column tube. The liquid film thickness d per filler was 38.02㎛ in this study. Non-equilibrium kinetics: Considering that the filling particle size was much larger than the nuclide ions and that its concentration change was negligible, this interaction could be regarded as a first-order reaction and the first order kinetic equation for the interaction rate of nucleophile ions with the packing is derived as in Eq. (5). <math display="block" id="M5"><mrow><mi>r</mi><mo>=</mo><mfrac><mrow><mtext>d</mtext><mi>x</mi></mrow><mrow><mtext>d</mtext><mi>t</mi></mrow></mfrac><mo>=</mo><mi>k</mi><mrow><mo>(</mo><mrow><mi>a</mi><mo>−</mo><mi>x</mi></mrow><mo>)</mo></mrow></mrow></math> where r is the rate of ion-filler interaction (mg/L), a is the concentration of ions in the packed liquid film at the initial time (mg/L), x is the concentration of ions in the filling liquid film at time t (mg/L), t is the interaction time (average retention time), and k is the reaction rate constant. Since the rate of interaction r and average retention time t can be obtained through macroscopic experiments, the change in ion motion of the liquid membrane of each separation unit could be calculated. From the previous experimental data, it can be observed that when pH > 6, most of the uranyl ions were adsorbed on the surface of the filling, both within the surface liquid film of the filling; when 2 < pH < 6, some of the uranyl ions were adsorbed on the surface of the filling and reached relative equilibrium as a result. Furthermore, when pH < 2, the ions were in non-equilibrium with the filling, as shown in Fig. 12 Fig. 12 . The non-equilibrium separation mechanism of pulsed liquid chromatography nuclide separation method was as follows: The ions to be separated (aqueous phase containing uranium and other impurity ions at pH = 2) interacted with the column filling (stationary phase), and the dynamic separation of ions could be carried out because of the different forces between different ions and the column, the different average retention times of the ions, and the different accelerations of the forces on ions. The light and low-valence ions were subjected to the small blocking effect of the column and flowed out from the column first, whilst the heavy particles and high-valence ions were subjected to the large blocking effect of the column and therefore flowed out from the column later, so as to achieve the separation of light and heavy particles. This separation mechanism of nuclides is shown in Fig. 13 Fig. 13 . Using the separation mechanism of the new column, the average retention time of the nuclides in the column was studied, and the separation between different nuclide ions could be improved by changing the average retention time of different nuclides in the column. The separation enrichment of the target nuclides and other nuclide ions at different time periods was also analyzed and determined. A pulsed feed separation device was used to selectively separate and collect the enrichment of target nuclides at different time periods. This pulsed injection method was key to the separation method, which used a "one-by-one" injection method to achieve segmented and graded separation and extraction of uranium in a pulsed discontinuous manner. This segmented method intercepted the target nuclide according to the separation of different nuclides in the column; the graded method re-entered the column for the segmented separation of the intercepted target nuclide. Using this method to separate and extract uranium from seawater, sodium ions belonged to light ions and low valence ions, therefore the sample solution exited out the front end with a high sodium ion content after separation from the column, whilst uranium ions in seawater belonged to heavy particles and high valence ions. The latter part of the sample solution coming out of the column contained a high uranium concentration, and each time the pulse feed was separated, the latter part of the sample solution was collected, and the enriched solution was then repeatedly separated by pulse feed to realize uranium extraction from seawater. Conclusion 1 The new pulsed liquid chromatography method proposed here was able to solve the problem of continuous injection in traditional chromatographic separation, and realized the rapid, efficient, and low-cost separation of uranium and other elements in water, whilst also achieving the stepwise and graded separation and extraction of uranium in a pulsed discontinuous manner. The separation of uranium was investigated at different pH over-column acidities, sample injection flow rates, column heating temperatures, and cation systems. The experimental study for each condition tentatively determined that this method could be used for the separation of uranium and other ions (europium and sodium ions), whilst the optimal conditions for the separation of uranium and sodium ions were also obtained here. Based on analysis of the experimental data for each condition, the formulae for the number of separation stages required to achieve separation by the method were derived. The separation of uranium and sodium ions was performed using samples from the Ganjiang River under these optimal separation conditions, with a separation factor of 1.50 given for uranium and sodium ions, which would theoretically require 21 stages to achieve the separation of uranium and sodium. Overall, these experiments demonstrated the feasibility of this pulsed liquid chromatography separation method. Finally, preliminary separation experiments were also conducted for uranium, sodium, magnesium, and potassium ions in seawater samples. Subsequently, their separation factors were obtained, in which the separation factor of uranium and sodium ions was 1.2885, suggesting that the complete separation of uranium and sodium ions in seawater would require more than 28 stages. As the most abundant ions in seawater, sodium ions can theoretically separate uranium from other impurity ions if the separation of uranium and sodium ions is achieved, meaning that, after 28 stages of seawater separation, uranium can be extracted from seawater. Through a preliminary separation study on seawater samples, it was determined that our pulsed liquid chromatography separation method could be applied in seawater uranium extraction studies. The pulsed liquid chromatography method presented in this study is innovative and feasible regarding the separation and enrichment of nuclides whilst providing new ideas for the study of uranium extraction from seawater. Reference R.R. Alnuaimi , B. Khuwaileh , M. 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Tech.  32 , 33 ( 2021 ). doi: 10.1007/s41365-021-00867-1 http://doi.org/10.1007/s41365-021-00867-1 . Study on the production characteristics of 131I and 90Sr isotopes in a molten salt reactor All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Jian-Hua Ye and Tao Yu. The first draft of the manuscript was written by Jian-Hua Ye and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. 10.1007/s41365-023-01180-9 This work was supported by the Natural Science Foundation of Jiangxi Province, China (No. 20202BABL203004), the Opening Project of the State Key Laboratory of Nuclear Resources and Environment (East China University of Technology) (No. 2022NRE23), and the Opening Project of Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices (No. PMND202101). 2022-09-08 2022-11-08 2022-11-14 2023-02-20 2023-03-20 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2023 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-023-01180-9 http://dx.doi.org/10.1007/s41365-023-01180-9 . 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