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1. Introduction
As technology advances and industries grow, the demand for energy increases, and the need for highly efficient energy generation has a stronger impact on industries around the world. Nuclear power is employed extensively around the world due to its high energy efficiency, thereby making a substantial contribution to modern society’s energy requirements. However, uranium, which is used as an energy source in nuclear power plants, is a typical radioactive metal that has harmful effects on the environment [1-5]. Due to both its utility and potential risks, uranium must be safely separated from humanity and the environment. This issue has attracted global attention.
Prolonged operation of nuclear facilities can cause the surrounding soil to become contaminated with uranium, while during decommissioning processes, large amounts of uranium-contaminated soil can be generated [6-8]. Various techniques have been studied for the safe treatment of soil contaminated with radioactive isotopes, including chemical methods involving acid, chelates, and reducing agents, as well as physical methods like foam flotation, electromagnetic separation, and thermal desorption [9-18]. Among these techniques, soil decontamination methods using acid have the advantage of a high decontamination factor [19-26]. However, most existing techniques rely on high concentrations of acid, which can generate significant amounts of secondary waste [12, 27]. The amount of waste generated varies depending on the application of the decontamination technology, and generating low amounts of waste is as important as ensuring high decontamination performance. In other words, it is crucial for decontamination technologies to consider both high performance and low waste generation [28, 29]. Therefore, a technology employing low-concentration acid washing that meets these requirements is necessary. If uranium can be selectively dissolved during decontamination using this approach, the excessive generation of waste due to overdissolution could be dramatically reduced. Additionally, reducing the concentration of impurities in the wastewater could facilitate the recovery of uranium.
The Korea Atomic Energy Research Institute (KAERI) possesses a large quantity of uranium-contaminated soil that was generated from the uranium conversion facility. The development of decontamination technologies is necessary to dispose of this contaminated soil, taking into account both their decontamination efficiency and the volume of waste produced. In this study, low-concentration acid washing of uranium-contaminated soil stored at KAERI was conducted to evaluate the decontamination performance of this method and to interpret the dissolution behavior of uranium ions during the process. An analysis of the soil and solution from each acid-washing cycle was conducted in order to ascertain the behavior of ions in the uranium-contaminated soil. Furthermore, the pH of the acid-washing solution was monitored, thereby confirming that the excessive dissolution of ions could be prevented and that the optimal washing time could be derived.
2. Experimental and Method
2.1 Acid Washing of Uranium-Contaminated Soil
In this study, 0.05 M sulfuric acid was selected as the acid washing solition, a decision informed by the findings of prior experiments that evaluated its efficacy [30]. The target of these experiments is uranium-contaminated soil that has been generated from the decommissioning of uranium conversion facility in KAERI. The acid washing solution was prepared using 95.0% H2SO4 from DUKSAN and DI water from Younglin’s AquaMAX Basic 360 model.
For each gram of uranium-contaminated soil, 5 ml of the sulfuric acid solution were used for washing. The uranium- contaminated soil was dispersed in the acid washing solution at a speed of between 200 and 300 rpm. Subsequent to acid washing, a filtration process was conducted in which soil and washing solution were separated under reduced pressure using a 0.22 micron filter. The separated soil was washed again using a fresh acid solution. The process of acid washing in uranium-contaminated soil was repeated on three times, followed by a final wash with distilled water. All experiments were conducted at ambient temperature, and the pH of the washing solution was monitored in real time during all washing processes.
2.2 Analysis Method
In order to interpret the phenomenon of uranium dissolution in the soil by the acid-washing solution, the concentrations of ions in the soil and washing effluent obtained at each washing step were analyzed. The analysis of the ions in the soil was done via scanning electron microscopy (SU8010, Hitachi, Japan) and energy dispersive X-ray fluorescence (XEPOS, Spectro, Germany). The pH of the solution was measured using a pH meter (ORION STAR A211, 8302BNUMD, Thermo Scientific, USA). Calibration of the pH meter was performed at three pH points: 1.68, 4.0, 7.0. An elemental analysis of the acidwashing solution by ICP-OES was carried out at the accredited Radiochemical Analysis Laboratory at KAERI.
3. Results and Discussion
3.1 Characteristics of Uranium-Contaminated Soil
Before acid washing the uranium-contaminated soil, we analyzed the composition of the soil using energy dispersive x-ray fluorescence (ED-XRF) as noted above. These results are presented in Table 1. The composition of the uranium-contaminated soil is dominated by substantial amounts of silica and aluminum, with the remaining components being potassium, iron, calcium, and other elements. The concentration of uranium was found to be 3,965 ppm. Conversion of this value to the radiation concentration, based on 238U, yields a result of 49 Bq∙g−1.
Table 1
Major ion concentrations in uranium-contaminated soil (ED-XRF)
| Element | Concentration (ppm) |
|---|---|
|
|
|
| Si | 190,525 |
| Al | 105,750 |
| K | 24,215 |
| Fe | 21,268 |
| Ca | 15,315 |
| Mg | 10,056 |
| U | 3,966 |
| Others | 17,082 |
The morphology and composition of uranium particles within uranium-contaminated soil was analyzed using SEM-EDS and are presented in Fig. 1. The SEM analysis results show that particles form clusters at the center. Through elemental mapping, it was confirmed that uranium exists only at this location. In contrast, other major ions are distributed uniformly across multiple locations. Based on these findings, it appears that uranium does not combine with other metal ions and is present only at specific exterior positions of soil particles. Due to its exposed state, low-concentration acid washing may be sufficient for cleaning the uranium in uranium-contaminated soil. Therefore, it is possible for uranium to dissolve before the other ions present in the uranium-contaminated soil. In addition, the mapping results revealed trace amounts of potassium in areas where uranium was detected. The peak position of uranium at 3.16 keV is very close to that of potassium at 3.31 keV [31-33]. Therefore, if there are trace amounts of potassium present, it can be affected by overlapping with the uranium peak.
3.2 Analysis of Uranium-Contaminated Soil and Effluent After Acid Washing
After acid washing of the uranium-contaminated soil, the uranium elemental concentration was analyzed in ppm using ED-XRF. Then, the uranium elemental concentrations (in ppm) were converted to radioactivity concentrations (in Bq∙g−1) and are presented in Table 2. This conversion was performed based on the conversion factor provided in IAEA TECDOC 1363 (1 ppm U = 12.35 Bq∙kg−1 of 238U) [34]. Table 2 shows that the uranium concentration in the soil decreases continuously with each washing step. According to IAEA recommendations, the radioactivity concentration of uranium for clearance must not exceed 1.0 Bq∙g−1 [5]. The results of this study indicate that the concentration of uranium in the soil was reduced to the clearance level by washing the uranium-contaminated soil three times. This suggests that a 0.05 M sulfuric acid solution may be an effective decontamination agent for uranium-contaminated soil.
Table 2
Specific activity of Uranium in Soil (Bq∙g−1)
| Raw soil | 1st wash | 2nd wash | 3rd wash | DI water wash |
|---|---|---|---|---|
|
|
||||
| 48.96 | 4.94 | 1.31 | 0.43 | 0.33 |
We also analyzed the concentrations of ions that were leached into the solution during the washing process of the uranium-contaminated soil and present these results in Table 3. As the number of washes increased, the concentration of ions dissolved into the solution decreased, with the majority of the dissolution occurring in the first and second acid washes. In the initial and secondary acid-washing steps, uranium and calcium ions were predominant in the dissolved ion composition of the wastewater. Notably, uranium, the primary target of the treatment, was dissolved at a rate of 99.2% of the total amount removed during the acidwashing process. Furthermore, uranium demonstrated the most substantial decline in concentration among all ions at each wash stage. This suggests that uranium is preferentially leached before other ions are dissolved, thereby minimizing the generation of secondary waste. The analysis of uranium in the wastewater revealed concentrations of 5 ppm and 0.55 ppm in the third acid-washed wastewater and DI water-washed wastewater, respectively. The presence of uranium in the DI water-washing wastewater is presumed to be due to the presence of dissolved uranium ions in the previous acid-washing process remaining in the soil and then draining into the DI water. This interpretation also applies to the results of the DI water washing presented in Table 2. As the number of washes increases, the acid cleaning efficiency decreases; therefore, it appears effective to wash only a specific number of times. Consequently, the identification of process conditions that minimize the number of washes is regarded as an effective strategy to enhance the efficiency of the acid-washing process.
Table 3
Analysis of major ion concentration in uranium-contaminated soil washing wastewater
| Washing step | Concentration (ppm) | ||||||
|---|---|---|---|---|---|---|---|
|
|
|||||||
| U | Ca | Mg | Fe | Al | Si | K | |
|
|
|||||||
| 1st wash | 733 | 1,064 | 28 | 42 | 70 | 94 | 20 |
| 2nd wash | 51 | 205 | 7.2 | 38 | 26 | 30 | 7.9 |
| 3rd wash | 5.75 | 34.4 | 6.9 | 34.75 | 19.35 | 17 | 3.9 |
| DI water wash | 0.555 | 6.2 | 4.55 | 0.935 | 3.265 | 6.85 | 0.562 |
3.3 Correlation Between pH and Acid Washing
Analysis of uranium-contaminated soil revealed the presence of various metallic ions within the soil. These are expected to exist in the form of oxides or hydroxides [13, 23, 36, 37]. When the acid solution is applied to the uranium- contaminated soil, metal oxides or hydroxides react with H+ ions and dissolve into an ionic form according to the following reaction:
As a result of this reaction, H+ ions in the solution are consumed, and the O2− ion generated at this time is combined with the H+ ions in the solution to produce H2O. Consequently, the pH of the solution increases. Based on this mechanism, it can be interpreted that metal oxides dissolve as the pH of the solution is increased. As the pH of the solution can be monitored in real time, the progress of the acid wash can also be checked simultaneously. By applying these concepts to actual industrial processes, the optimum decontamination time for uranium removal can be determined. Moreover, this approach allows for a reduction in unnecessary waste generated by acid washing. However, pH changes can be influenced not only by uranium oxides but also by other metal oxides present in the uranium-contaminated soil. Therefore, utilizing pH monitoring to determine the optimal washing time is only valid when there is a clear association between pH fluctuations and uranium levels. To confirm this relationship, the pH of the solution was monitored in real time during the cleaning process of the uranium-contaminated soil and the relationship between the pH of the solution and uranium dissolution was confirmed and evaluated.
The pH of the solution was monitored during the acid washing of the uranium-contaminated soil, and these results are shown in Fig. 2. In each acid-washing stage, the pH of the solution exhibited an upward trend over time. This tendency was especially pronounced during the initial acid wash. However, with an increase in the number of washes, the rate at which the pH rose became less pronounced. During the third acid washing cycle, no increase in pH was observed. This finding indicated that the increase in pH decreased in proportion to the number of acid washes of the solution. These pH monitoring results are correlated with the analyzed data on decontaminated soil.
Fig. 3 presents the pH changes during acid washing and the uranium removal efficiency from the soil. During each acid washing stage, the pH of the washing solution decreased while the uranium removal efficiency increased. Although these parameters change in opposite directions, the magnitude of their variations followed a similar trend. Additionally, among the ions dissolved during acid washing, only uranium exhibited a consistently decreasing trend with each successive washing stage. Consequently, within the conditions of this study, uranium is identified as the most sensitive element in the acid-dissolution reaction. This result suggests that monitoring the pH during acid washing for uranium-contaminated soil is an effective means of predicting the dissolution of uranium in soil in real time. Therefore, in this study, the procedure of acid washing was continued until the pH level was stabilized. This ensured that the uranium concentrations reached the target levels.
Fig. 3
Plot of the uranium removal efficiency from soil by acid washing and changes in the solution pH.
4. Conclusion
The objective of this study was to evaluate the effectiveness of acid washing using a 0.05 M sulfuric acid solution in removing uranium from contaminated soil. Experimental results showed that the uranium concentration in the soil was reduced to the clearance level after three acid washings. Furthermore, an evaluation of the relationship between the pH of the solution and the uranium behavior in the soil confirmed that pH monitoring could determine the current status of uranium removal in the soil in real time and allow the determination of the optimal washing process time. In conclusion, this study demonstrated that acid washing with a 0.05 M sulfuric acid solution is a highly effective method for the removal of uranium from uranium-contaminated soil. This approach has two major benefits: it reduces waste treatment costs and enables an environmentally friendly treatment. These results are considered to provide an important basis for the development of an efficient disposal process for uranium-contaminated soil to be carried out in the future. The findings of this study are anticipated to have a significant impact on the promotion of the development of sustainable technologies capable of addressing the uranium- contaminated soil issue.
