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1. Introduction
Global technology trends currently show significant increases in AI (Artificial Intelligence) and robotization, both of which are expected to increase global power consumption [1]. The need to find alternative energy sources that are environmentally friendly and free from carbon dioxide emissions is imperative considering the reduced availability of resources and global warming [2,3]. Uranium is a key resource to supply such global power [4,5]. There are 442 commercial nuclear power reactors in operation, and 42 more reactors are under construction [6]. This status has led to the expansion of nuclear facility operation sites. For this reason, uranium contamination has been a major concern [7].
Uranium contamination in soils is generated from nuclear facilities that use uranium. For example, uraniumcontaminated soils were discovered in Korea following the decommissioning of a uranium conversion facility and a research reactor [8]. Such contamination poses significant environmental and health risks due to uranium’s radioactive and chemical toxicity [7]. In addition, uranium is a nuclide with a long half life, so special management is required from the perspective of disposal facilities. It is thus necessary to remove uranium from the soil. These contaminated soils can be decontaminated by various methods such as washing, electro-kinetic extraction, and phytoremediation [9-13]. Among these methods, soil washing using an acidic reagent is widely used to remove uranium from soil and generates large amounts of wastewater containing uranium and impurities. This wastewater can be purified by a precipitation method using an alkali material. However, this precipitation method has a problem of generating a considerable amount of radioactive waste containing alkali material. Acid-washed uranium-bearing solution can also be treated to extract uranium selectively through multiple processes via solvent extraction, ion exchange, and precipitation [14]. These conventional extraction processes are however associated with environmental risks as they generate considerable amounts of radioactive organic waste [14]. It is therefore necessary to develop a method capable of extracting uranium selectively from the wastewater so as to minimize the generation of radioactive waste.
In this study, thermodynamic equilibrium calculations for a selective precipitation reaction of uranium from an acid- washed uranium-bearing solution were conducted to derive a more environmentally acceptable uranium extraction method. These calculation results were confirmed through an experimental test using an acid-washed uranium-bearing solution. Based on the results, a simple process for the selective extraction of uranium from acid-washed uraniumbearing wastewater was designed.
2. Experimental and Method
2.1 Equilibrium Calculation
Thermodynamic equilibrium calculations for a selective precipitation reaction of uranium from an acid-washed uranium-bearing solution were conducted using software (HSC-Chemistry 9.3). The Gibbs free energy (ΔG) of the precipitation reaction was evaluated using the ‘Reaction Equation’ module in the software. The composition of each chemical constituent in the solution was derived using the ‘Equilibrium Compositions’ module [15]. The composition of the acid-washed uranium-bearing solution used in this study is shown in Table 1, as determined based on the composition of uranium-containing soil washing solutions.
Table 1
Composition of the uranium-containing soil leaching solution
| Element | U | Ca | Fe | Mg | Al | Na | K | Si |
|---|---|---|---|---|---|---|---|---|
|
|
||||||||
| Concentration (mM) | 1.61 | 15.18 | 0.73 | 0.72 | 1.74 | 4.37 | 0.85 | 2.32 |
2.2 Experimental
Uranium-contaminated soil used in this study was sampled from soil that had accumulated in a waste storage facility at KAERI (Korea Atomic Energy Research Institute) [16]. An acid-washed uranium-bearing solution was prepared by washing this soil using 0.01 M H2SO4 (Duksan, 95%). This solution composition is shown in Table 1. Based on the equilibrium calculation results, an experimental test of the selective precipitation of uranium from the solution was conducted in a 0.2 L beaker as follows. First, a constant amount of H2O2 (Daejung, 30%) was injected into the uranium-containing soil washing solution. NH4OH (Duksan, 25−29%) was then injected stepwise in a form diluted five-fold at a regular intervals until precipitation occurred. At this time, the solution was mixed at 200 rpm using a magnetic stirrer. The precipitation products were separated from the solution and observed using XRD (D8 Advance A25, Bruker, Conditions: Cu target, 40 kV, 40 mA, 0.2 ∘/ sec). The concentrations of uranium in the solution were measured by means of ICP-OES (PQ 9000, Analytikjena).
3. Results and Discussion
3.1 Thermodynamic Evaluation
Uranium in an acid-washed uranium-bearing solution exists in the form of uranyl ions (UO22+). Uranyl ions can be converted into insoluble uranium peroxide (UO4·4H2O) through a reaction with hydrogen peroxide (H2O2). This is described in Eq. 1 [17,18].
The Gibbs free energy of this reaction was computed using HSC-Chemistry 9.3, and the result is expressed in Table 2. Table 2 shows that Eq. 1 is not an active reaction. It is reported that the reaction can be conducted effectively at pH 3−4 [17]. The pH of the uranium-bearing solution can vary depending on the concentration of the acid, and it is generally below 3−4. Thus, the pH must be increased to carry out the reaction, and an alkali additive can be applied to promote it [18]. Conversion reactions of uranyl ions into uranium peroxide by adding hydrogen peroxide and an alkali additive were established, and their Gibbs free energies were calculated as shown in Table 2. Table 2 shows that Eq. 1 can proceed actively by adding an alkali material such as NH4OH or NaOH. This means that the alkali material promotes the conversion of UO22+ into UO4·4H2O. It is expected that the promotion can be implemented more effectively when using NH4OH. A previous study reported that the addition of NaOH is effective only when the uranium concentration is less than 100 ppm [18]. The use of NaOH can generate an impurity such as Na+ which increases the amount of waste generated. For these reasons, NH4OH is determined to be a proper alkali additive for promoting the conversion.
Table 2
Gibbs free energy of uranium precipitation reactions using HSC-Chemistry 9.3
| Reactions | ΔG (kcal) |
|---|---|
|
|
|
| UO22+ + H2O2 + 4H2O = UO4·4H2O + 2H+ | 3.05 |
| UO22+ + H2O2 + 4NaOH = UO4·4H2O + 4Na+ | −116.88 |
| UO22+ + 7H2O2 + 4NH4OH = UO4·4H2O + 2N2 + 12H2O + 2H+ | −494.16 |
Thermodynamic equilibrium calculations of the reactions between UO22+, H+ , SO42−, H2O2, and NH4OH in an acid-washed uranium-bearing solution were conducted to determine the injection conditions of H2O2 and NH4OH. The injection amount of H2O2 was varied to 1, 50, 100, 200, 300, 400 and 500 times of the theoretical equivalent based on Eq. 1. These calculation results are shown in Fig. 1. According to Fig. 1, the conversion of UO22+ into UO4·4H2O dose not proceed at the theoretical equivalent condition of H2O2 based on Eq. 1. The conversion increases as the H2O2 injection amount increases. The conversion product (UO4·4H2O) is however converted back to its original form (UO22+) when all the injected H2O2 is decomposed in the solution. It is thus necessary to inject a sufficient amount of H2O2 into the solution to progress the conversion effectively. This means that a significant excess amount of H2O2 can be used for the conversion. Fig. 1 shows that almost all UO22+ can be effectively converted into UO4·4H2O when the H2O2 injection is at more than 400 times of the theoretical equivalent for Eq. 1. It is expected that the proper NH4OH injection level is from 1.6 M to 2.4 M (U: 1.61 mM, H2O2 injection: 400−500 times of the theoretical equivalent).
Fig. 1
Thermodynamic equilibrium calculations for a uranium selective precipitation reaction of uranium with increasing H2O2 injections (1, 50, 200, 300, 400, and 500 times of the theoretical equivalent based on Eq. 1).
From the above calculation results, the behaviors of elements other than uranium in the solution are as follows. Ca, which has the highest concentration in the solution, is generally converted into CaSO4, and its trace remains in an ionic form. It has been reported that 20 mM CaSO4·2H2O can be dissolved in water at 25°C [19]. It is thus expected that the conversion product, CaSO4, is dissolved in the solution. In contrast, Mg, K, Na, and Fe appear to remain in an ionic form, and a very small fraction of them is converted into an sulfate form or an hydroxide form. Their conversions also increase with increasing H2O2 injection amount. These conversion amounts are however very minor. It is considered that the sulfate forms of Mg, K, and Na are also dissolved in the solution because they are soluble in water. Al and Si are not converted into any form and remain in an ionic form.
The above calculation results indicate that uranium peroxide (UO4·4H2O) containing a very small amount of iron hydroxide (Fe(OH)3) can be generated in the form of a precipitate through the precipitation reaction using H2O2 and NH4OH, as shown in Table 1. It is thus expected that the uranium can be extracted at a high purity level from an acid-washed uranium-bearing solution.
3.2 Experimental Evaluation
Based on the calculation results, an experimental test of selective uranium extraction from an acid-washed uranium- bearing solution with the composition in shown Table 1 was conducted. The precipitation reaction for selective uranium extraction from the solution lasted for 2 hr. After the precipitation, it was confirmed that the concentration of residual uranium in the solution was about 0.017 mM. The extraction efficiency of uranium from the solution was about 99%. The precipitation products were almost fully identified as UO4·4H2O by means of the XRD patterns, as shown in Fig. 2. These results mean that almost all uranium in an acid-washed uranium-bearing solution containing various metal ions can be recovered at a high purity level through only a single-step precipitation process using H2O2 and NH4OH. This agrees well with the calculation results.
A selective extraction of uranium from an acid-washed solution can be a very simple process, as shown in Fig. 3. This process consists of injecting H2O2 and NH4OH, stirring, and filtering. The process allows high purity uranium extraction from an acid-washed uranium-bearing solution containing various metal ions in a single step. Thus, It is considered that the process can become a much more acceptable method both economically and environmentally when compared with conventional methods such as precipitation using an alkali material and uranium extraction through multiple processes via solvent extraction, ion exchange, and precipitation.
4. Conclusion
A single-step precipitation process for selective uranium extraction from an acid-washed uranium-bearing solution was derived through thermodynamic equilibrium calculations. The high-purity uranium extraction through the proposed process was experimentally verified. It is expected that this process can contribute to solving certain problems associated with the conventional uranium removal or extraction process and can improve the economic feasibility of the extraction of uranium.
