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1. Introduction
With the rapidly increasing energy consumption worldwide, numerous green energy developments have been researched to achieve carbon neutrality; however, these developments are insufficient to replace fossil fuels in a rapidly changing energy world [1, 2]. During this transitional period, nuclear power is anticipated to increase its global share and emerge as the most viable means of achieving carbon neutrality [3]. This trend will result in increased uranium resource development and the expansion of nuclear power facilities, including uranium fuel-processing plants. However, these nuclear power facilities have a high probability of generating massive quantities of uranium-contaminated soil waste [4]. This waste results in significant disposal costs if handled through permanent disposal and severely limits the utilization of radioactive waste disposal sites [5]. Moreover, the inadequate management of this waste can potentially affect the efficiency of nuclear power generation. To address this challenge, uranium-contaminated soil can be treated using acid washing method to reduce the uranium content to levels below the clearance criteria [6]. Following treatment, the purified soil can be disposed of as a non-radioactive waste; however, this process generates secondary uranium-contaminated acidic wastewater that must be properly treated. If left untreated, the volume of radioactive waste could significantly increase. Treating radioactive wastewater primarily involves methods that reduce radioactive nuclides to the discharge criteria level. Methods such as precipitation, adsorption, and ion exchange can be used to remove radioactive nuclides. The precipitation method exhibits a high removal efficiency for radioactive nuclides, which are difficult to selectively separate [7, 8]. While the adsorption and ion exchange methods enable the selective separation of radioactive nuclides, their applicability is limited by the high concentrations of co-dissolved metal ions [9, 10]. Therefore, the precipitation method was selected for this study owing to the high concentration of codissolved metal ions. Moreover, precipitation is easy to control and highly cost-effective.
In this study, we evaluated the applicability of different precipitation methods for the removal of uranium from radioactive soil-washed wastewater. The evaluation focused on three precipitation methods: metal hydroxide(M–OHx) co-precipitation, uranium peroxide (UO4) precipitation, and potassium uranium phosphate (KUO2PO4) precipitation. When uranium-containing wastewater is treated, the generation of secondary radioactive waste is minimized, thereby improving the efficiency and stability of uraniumcontaminated soil treatment processes.
2. Experimental
2.1 Simulated Soil-Washed (SSW) Wastewater
The SSW wastewater was prepared using soil near the uranium conversion facility at the Korea Atomic Energy Research Institute (KAERI) and 0.5 M sulfuric acid (97% H2SO4, Showa Chemical Industry) diluted with deionized water (DQ-3,18.2 MΩ, Millipore). We simulated the leaching conditions of uranium-free wastewater as closely as possible, and the results are presented in Table 1 [11]. The solid-to-liquid ratio in the soil-washing step used five times more liquid than solid within 2 h, and each phase was separated via vacuum filtration using filter paper (pore size: 8 μm, Whatman 2). Because the soil samples did not contain uranium isotopes, uranium nitrate hexahydrate (UO2(NO3)2∙6H2O; UNH, 99.99%, Alfa Aesar) was dissolved to approximately 500 mg∙L−1 in the SSW wastewater.
Table 1
Concentration of major dissolved elements in the washing solution
| Element | Ca | Fe | Si | Al |
|---|---|---|---|---|
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|
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| Concentration (mg∙L−1) | 660 | 830 | 380 | 510 |
2.2 Precipitation Experiments
Precipitation experiments were performed using 50 mL of SSW wastewater with a multi-position magnetic stirrer (S136035Q, Thermo Scientific) and a magnetic stirring bar at 200 rpm for a maximum of 3 days. Varying the pH was necessary to precipitate the dissolved elements in solution, and different precipitation characteristics were observed depending on the elements and pH range. The pH of the SSW wastewater was adjusted using 5 M sodium hydroxide (NaOH, 99.0%, Merck) or potassium hydroxide (KOH, 99.0%, Merck) solutions prepared with deionized water. A micropipette (0.02–10 mL) was used to inject the chemical agents throughout the experiments, and the pH of the SSW wastewater was measured using a pH meter (A221, Thermo Fisher).
2.2.1 M–OHx co-precipitation
M-OHx co-precipitation is used to remove target metal ions in wastewater below permissible release levels, particularly when initial concentrations are low [12, 13]. Target metal ions coprecipitate with a carrier metal ion such as Fe and Al by adsorbing onto or becoming incorporated into a lattice for the saturated solid phase of carrier [14, 15, 16]. Among the major soil elements, Fe and Al dissolved with uranium can be removed via physiochemical binding to colloidal particles, fine particles, and dissolved substances during floc formation and coagulation within a neutral pH range [17]. Several studies have been conducted on the removal of radioactive nuclides from solutions using the flocculation–coagulation method [8, 18, 19, 20]. In this study, two types of experimental conditions were investigated: (1) SSW wastewater pH of 8 (using NaOH), and (2) SSW wastewater pH of 8 (using NaOH) with addition of 200 mg∙L−1 Fe ions. The objective of the second experiment was to confirm the enhancement of uranium removal by adding Fe ions to the solution.
2.2.2 UO4 precipitation
The UO4 precipitation method converts uranyl ions in solution into a studtite mineral phase, which has a very low solubility in the water [21]. The formation of studtite occurs through a reaction between uranyl ions and an excess of hydrogen peroxide under acidic pH conditions, as described in Equation 1 [22, 23]. Moreover, it selectively precipitates uranium from acidic solutions, resulting in high-purity uranium. However, the presence of impurities, such as decomposed peroxide ions in the solution, can significantly reduce its effectiveness [24, 25]. In this study, experiments were conducted using 1 M hydrogen peroxide (H2O2, 30%, Merck) at a pH of 3–3.5, which was controlled using 5 M NaOH.
2.2.3 KUO2PO4 precipitation
The formation of uranium phosphate minerals has emerged as an effective treatment method for uranium recovery [21, 25, 26]. These minerals exhibit low solubility and high stability, facilitating the long-term sequestration and immobilization of uranium. The phosphate precipitation method transforms uranyl ions into MUO2PO4 (M: cation) minerals that are determined by cations, such as Na+, K+, and NH4+ [21, 28]. This is related to the solubility of the precipitated solid, and KUO2PO4 exhibiting the lowest solubility among the cations [29]. Therefore, we decided to precipitate KUO2PO4 by adding potassium phosphate (KH2PO4, Sigma-Aldrich), which is known as meta-ankoleite, as shown in Equation 2. For KUO2PO4 precipitation, 1 M KH2PO4 diluted with deionized water was directly injected at a pH of 1. The target PO43− concentrations in the solution were 5, 10, and 20 mM, and the pH was adjusted to approximately 6.25 by adding 5 M KOH.
2.3 Analysis Method
The concentration of uranium in the wastewater was measured five times using inductively coupled plasma optical emission spectroscopy (PQ-9000 Elite, ICP-OES, Analytik- jena). The detection was performed at a wavelength of 385.957 nm, and the quantitation limit was 0.0267 mg∙L−1. The analytical samples were collected and filtered using a syringe and syringe filter (pore size: 0.2 μm, Advantec). Subsequently, they were diluted 100 times with 5% nitric acid (HNO3, Merck), which was diluted from 65% HNO3 with deionized water.
3. Results and Discussion
3.1 Uranium Removal Efficiency
Fig. 1 shows the uranium removal efficiency achieved using the different precipitation methods after 2 h, 1 day, and 3 days. The M–OHx and KUO2PO4 precipitation methods effectively removed uranium from the wastewater, achieving residue levels ˂1 mg∙L−1 within 2 h. Although the UO4 precipitation method achieved 78%, 92%, and 98% uranium removal within precipitation times, respectively. It demonstrated the lowest removal efficiency compared to the other methods. This occurred because the injected H₂O₂ for UO4 precipitation was consumed by co-dissolved elements from the acid soil washing process. These elements like Si and Fe changed their oxidation states, becoming more soluble or re-dissolving as saturated particles in the solution [24, 25, 30, 31]. The results indicated that the UO4 precipitation method was ineffective for removing uranium from solutions containing excessive amounts of co-dissolved ions. Depending on the treatment strategy selected for uranium separation, the M–OHx and KUO2PO4 methods were suitable. One approach co-precipitates uranium along with impurities as M–OHx foams, and the other method involves selectively separating uranium as the KUO2PO4 mineral form by injecting PO4³⁻.
To identify the characteristics of the two methods with high uranium-removal efficiency, we investigated the effects of Fe and PO4³⁻ concentrations on the M–OHx coprecipitation and KUO2PO4 precipitation methods, respectively.
3.2 Characterization of M–OHx Co-precipitation With Different Fe Concentrations
Based on preliminary evaluations, experiments were conducted to assess the uranium-removal characteristics of the M–OHx co-precipitation method. The Fe ion concentration was increased in these experiments, and we measured the residual uranium concentration in the wastewater over 360 min (Fig. 2). When using metal ions dissolved in soil-washed wastewater for co-precipitation, the residual uranium concentration was ˂1 mg∙L−1 within 10 min, and injecting Fe into the wastewater further reduced this concentration to ˃0.5 mg∙L−1. However, the residual uranium concentration increased with precipitation time. This phenomenon was caused by weakly bound uranium ions on the surface of the M–OHx precipitates, which were desorbed over time depending on the solution conditions [32]. Notably, uranium could be leached from the solids for permanent disposal.
Fig. 2
Variation of residual uranium concentrations using M–OHx co-precipitation with different Fe concentrations.
To prevent uranium leaching from the M–OHx precipitate, additional Fe ions were required to increase the amount of M–OHx. However, this approach increased the volume of secondary radioactive waste, thereby requiring solidification for permanent disposal, which is disadvantageous in terms of disposal costs and site efficiency.
3.3 Evaluation of KUO2PO4 Precipitation With Different PO43− Concentrations
The KUO2PO4 precipitation method, which selectively separates uranium, was applied to evaluate the uranium removal performance according to the injected PO43− concentration (Fig. 3). In this study, the PO43− concentration was approximately 2.5–10 times higher than the molar concentration of dissolved uranium (approximately 1.9 mM). When injecting 5 mM PO43⁻, which was 2.5 times higher than the initial uranium concentration, the residual uranium concentration was approximately 1.5 mg∙L−1. When using ˃5 times the initial concentration of PO43−, we achieved a residual uranium concentration of ˂0.3 mg∙L−1. Notably, the residual uranium concentration did not increase with precipitation time, suggesting that the KUO2PO4 method was highly effective in preventing uranium leaching. The reason excess PO43− was required compared to the uranium concentration was because commonly dissolved metal ions precipitated as metal hydroxides, consuming PO43− from the solution and hindering the complexation of KUO2PO4 [33, 34]. Thus, to effectively remove uranium from wastewater using the KUO2PO4 precipitation method, an excess amount of PO43− (at least five times the molar concentration of dissolved uranium) must be added.
Fig. 3
Residual uranium concentrations following KUO2PO4 precipitation with different PO43⁻ concentrations.
Because PO43− can have harmful effects on the environment, stringent concentration regulations and pollution load management are in place for final discharge wastewater, discouraging its excessive use. Additional equipment or treatment processes are required to satisfy these environmental regulatory standards.
4. Conclusions
This study evaluated three uranium-removal methods (UO4 precipitation, M–OHx co-precipitation, and KUO2PO4 precipitation) for effectively treating acidic wastewater generated from a uranium-contaminated soil washing process. The soil contained uranium and various metal ions derived from the soil. The results indicated that M–OHx co-precipitation and KUO2PO4 precipitation exhibited high applicability compared to UO4 precipitation. This difference in effectiveness was attributed to the presence of codissolved metal ions, emphasizing the need to select an appropriate precipitation method based on the composition of the initial wastewater. The M–OHx co-precipitation and KUO2PO4 precipitation methods were suitable for specific waste management strategies. However, further studies are required to determine the optimal conditions and advance the decontamination technique by considering environmental impact and waste minimization. This study contributes to the sustainable management of uranium-contaminated wastewater and a more efficient and environmentally friendly nuclear fuel cycle.
