Feasibility and Challenges of In Situ Uranium Leaching Using Ozone Bubbles: A Review

Abstract

Utilization of ozone micro-nano bubbles during uranium leaching process has attracted attention in recent years because of ozone’s potent oxidizing capacity, high efficiency in mass transfer, and environmental compatibility. This review systematically presents the properties, generation methods and characterization approaches pertaining to ozone micro-nano bubbles (OMNBs) for the application of uranium leaching. In addition, the potentials and challenges of using ozone micro-nano bubbles to enhance uranium resources recovery are summarized. A lack of comprehensive understanding regarding uranium oxidation mechanism by ozone micro-nano bubbles under different pH conditions, along with the gaps in field experiments, has hindered the exploration and development of uranium leaching by OMNBs. In summary, further research endeavors on uranium oxidation mechanism by OMNBs and field trials are needed to facilitate the implementation of uranium leaching by OMNBs.

1. Introduction

Nuclear power, produced through nuclear fission reactions, yields considerably lower carbon emissions compared to energy from fossil fuels [1,2]. Carbon emissions only arise during the mining, uranium ore refining, and reactor fuel manufacturing processes. Nuclear energy plays an important role in mitigating the reliance on traditional fossil fuels and ensuring energy security. Nuclear energy plays a critical role in achieving net-zero greenhouse gas emissions by the mid-21st century, particularly for the decarbonization of electrical energy generation [3,4].

In recent years, with increasing energy consumption and exacerbating climate change, there has been an increasing demand for uranium production (Figure 1a). At the current stage, the limited supply of uranium imparts limitations to the advancement of nuclear energy. As shown in Figure 1b,c, the annual demand of China for uranium is more than 7500 tons, but China’s domestic uranium production is less than 2500 tons, which is unable to meet the production needs, necessitating substantial imports [5]. At the same time, high-quality, easily accessible uranium deposits are depleting, leading to extensive mining of low-permeability, low-grade uranium deposits, thereby escalating the cost of uranium mining. Under such circumstances, it is imperative to improve mining methods and augment the recovery of uranium resources.

According to the International Atomic Energy Agency (IAEA) Red Book in 2018, sandstone-type uranium deposits stand as the foremost source of global uranium production, taking the lead in all types of uranium mining [6]. Notably, sandstone-type uranium reservoirs constitute approximately 37.5% of the world’s uranium deposits, accounting for about 28% global uranium resources [7]. In situ leaching (ISL, also commonly referred to as in situ recovery (ISR)) technology, characterized by its cost-effectiveness, low intensity and reliance on favorable hydrogeological conditions, is really suitable for sandstone-type uranium reservoirs. ISL is a mining method that selectively dissolves uranium as a useful component of ore by chemical reaction between leaching agents and minerals under natural conditions, and extracts uranium solutions in subsequent chemical reactions without displacing the ore or surrounding rocks [8,9]. The production of uranium extracted through the ISL method is expected to account for a major portion of total uranium production in the future [10]. Meanwhile, compared with traditional methods of uranium recovery, ISL has several advantages, including lower infrastructure investment, shorter construction period, reduced production cost and minimized environmental impact. Given its unique superiority and substantial economic benefits, the development and implementation of the ISL method is imperatively critical to address energy scarcity and climate change. However, ISL also presents challenges, including potential groundwater contamination, high chemical consumption, and dependence on favorable hydrogeological conditions, which may limit its applicability in certain geological settings.

ISL techniques in uranium recovery can be classified into acid, alkaline and neutral leaching methods, depending on the employed leaching reagents. During the uranium recovery process, oxidation and complexation reactions occur between the uranium-bearing ore and the leach solution. Sulfuric acid is commonly used as the leaching agent in acid leaching, and acid leaching approaches are commonly utilized by countries such as Kazakhstan, Ukraine and Uzbekistan [9]. The acid leaching method offers the advantages of high uranium ore solubility, high reaction rates and high leaching efficiency. However, it also presents challenges, including high impurity concentrations in the leaching solution, strong corrosion effects on equipment and pipelines, and the need to neutralize the acidic waste solutions. These limitations make acid leaching unsuitable for carbonate-rich deposits with high acid consumption. As an alternative, alkaline leaching methods have been developed, initially derived from North America. Most alkali leaching methods use bicarbonate, carbonate, or a combination of both as leaching agents, supplemented by oxygen or hydrogen peroxide as oxidizing agents, with a pH value ranging from 9 to 10 in the leaching agent. This method is suitable for host rock containing carbonates, causing less damage to deposits. Consequently, alkaline leaching methods exhibit reduced erosion and less contamination compared to the acid leaching method. Moreover, CO₂ and O₂ leaching, as a relatively new uranium recovery method, has drawn considerable attention from the industry and academia due to its environmentally friendly characteristics, economic value and distinctiveness [11,12,13,14,15]. According to the IAEA report, the CO₂-O₂ ISL method has promising prospects for application in China, where sandstone-type uranium deposits are characterized by high carbonate content, high mineralization, low permeability and low grade [5]. Nevertheless, this method also faces some challenges, including limited oxygen solubility, slow oxidation rates, and the tendency of large particles to detach and clog the ore bed, thus impeding the application and development of this technology [16]. Therefore, given that conventional leaching methods may not be the best way to extract uranium, there is a need to improve the ISL techniques to enhance uranium recovery.

Ozone micro-nano bubbles (OMNBs), as an advanced technique for wastewater treatment and oxidation, have garnered significant attention from researchers in recent years. The unique properties of OMNBs, including tiny size, large specific surface area, potent oxidizing ability, long-term stability in solution and high efficiency in mass transfer, make them desirable for multiple purposes. Notably, OMNBs have been successfully applied in industrial wastewater treatment, especially in degradation of toxic heavy metals and microbial pollutants [17,18,19]. In addition, OMNBs have been applied in groundwater remediation projects and hydrometallurgical industries, showcasing their high practical value [20,21,22,23]. These industrial applications indicate that OMNBs are innovative and have great potential in the context of in situ leaching, but studies on this topic are insufficient and dispersed. Consequently, a comprehensive review of the application of OMNBs in ISL is conducted. The objectives of this review are the following:

• Summarize the characteristics of OMNBs and their preparation approaches, characterization techniques, and examine the factors influencing the properties of OMNBs, with the aim of enhancing the understanding pertaining to this type of aeration;
• Analyze the mechanism behind the application of OMNBs in uranium recovery and explore their potential uses;
• Discuss the difficulties in the implementation of OMNBs in ISL methods and propose relevant suggestions to overcome the difficulties.

2. The Characteristics of MNBs and Ozone

Ozone micro and nanobubbles (OMNBs) consist of microbubbles, nanobubbles, and ozone. Understanding the distinct characteristics of these components is essential for comprehending the nature of OMNBs. Of particular significance are microbubbles and nanobubbles, which find extensive application in mineral processing, environmental remediation, and various other fields, garnering increasing attention annually.

2.1. The Size of MNBs

The bubble size is one of the most important properties, influencing its ability. Bubbles are classified based on their sizes as macro bubble, micro bubble, and nano bubble. The characteristics of these categories are demonstrated in Table 1. MNBs are very tiny bubbles with diameters ranging from a dozen nanometers to microns [24]. Compared to macro bubbles, MNBs are favored by academia and industry for their unique physicochemical properties. With the same total amount of gas in the bubbles, MNBs are smaller in size, larger in number, and exhibit a larger total specific surface area, which significantly enhances the contact area between the bubble and the liquid. At the same time, the MNBs have a longer residence time in the liquid, compared with macro bubbles. These features make MNBs have higher mass transfer efficiency than ordinary millimeter bubbles, as shown in Figure 2a. Furthermore, several studies have shown that MNBs have the ability to generate free radicals and localized high energy upon rupturing [25,26]. With these characteristics, MNBs are widely used in wastewater treatment [27,28], groundwater and river remediation [29,30], aquaculture [31] and mineral extraction [32].

Figure 2. (a) CO₂ MBs observed by microscope [33]. (b) Size and distribution of NBs imaged by AFM [34]. (c) Nanobubble size distribution and radius detected by NTA and DLS [35].

Figure 2. (a) CO₂ MBs observed by microscope [33]. (b) Size and distribution of NBs imaged by AFM [34]. (c) Nanobubble size distribution and radius detected by NTA and DLS [35].

Table 1. The properties of bubbles with different sizes [36,37].

Properties	Bubble Type
	Macro-Bubble	Microbubble	Nanobubble
Size range	100~10,000 mm	1~100 mm	0.001~1 mm
Lasting time 1	Seconds	Minutes	Hours-month
Rising velocity	Fast	Slow	Brownian motion
Mass transfer Coefficient (m/s)	~10−4	10−4~10−3	>10−3
Surface area to volume ratio	Low	Large	Superior
Internal pressure	Low	High	High
Zeta potential (mV)	~0	−50~−20	−70~−30
Gas dissolution rate	Low	High	High
Collapse type	Burst at surface	Burst in bulk liquid	Shrink and burst in bulk liquid

Note: ¹ The lasting time refers to the duration that the bubbles remain separated, without merging together to form larger bubbles or breaking. The values correspond to the condition of DI water, room temperature and atmospheric pressure.

Table 1. The properties of bubbles with different sizes [36,37].

Properties	Bubble Type
	Macro-Bubble	Microbubble	Nanobubble
Size range	100~10,000 mm	1~100 mm	0.001~1 mm
Lasting time 1	Seconds	Minutes	Hours-month
Rising velocity	Fast	Slow	Brownian motion
Mass transfer Coefficient (m/s)	~10−4	10−4~10−3	>10−3
Surface area to volume ratio	Low	Large	Superior
Internal pressure	Low	High	High
Zeta potential (mV)	~0	−50~−20	−70~−30
Gas dissolution rate	Low	High	High
Collapse type	Burst at surface	Burst in bulk liquid	Shrink and burst in bulk liquid

Note: ¹ The lasting time refers to the duration that the bubbles remain separated, without merging together to form larger bubbles or breaking. The values correspond to the condition of DI water, room temperature and atmospheric pressure.

2.2. The Rise Velocity of MNBs

The rise velocity of a bubble is an important factor affecting its duration of existence in water or solution, and it depends on the relationship between the buoyancy and drag forces acting on the bubble. The rising velocity of the bubble can be calculated from the Stokes equation, as shown in Equation (1).

$$u={\displaystyle \frac{g{d_b^2(\mathit{\rho }}_l-{\mathit{\rho }}_g)}{18\mathit{\mu }}}$$

(1)

where $d_b$ denotes the diameter of the bubble, ${\mathit{\rho }}_l$ and ${\mathit{\rho }}_g$ denote the densities of the liquid and gas, respectively, g represents the acceleration of gravity, and $\mathit{\mu }$ represents the viscosity of the liquid. Notably, the rising velocity of a bubble is proportional to the square of its diameter. According to Equation (1), a bubble with a diameter of 50 mm rises in water at a rate of 1.53 × 10⁻³ m/s, while the rising velocity of a bubble with a diameter of 10 mm is 6.12 × 10⁻⁵ m/s. Comparing these two rise velocities, a decrease in the size of the bubble greatly reduces its rate of ascent. In contrast with MNBs, millimeter bubbles rise rapidly due to their large size and buoyancy, causing them to quickly float to the liquid surface where they burst. However, the motion of nanobubbles in water is mainly affected by Brownian motion since they are considerably smaller in size and are subjected to negligible buoyancy in water [38]. As a result, MNBs rise at a much smaller speed than large bubbles, existing in water for a longer period of time and dissolving gases more efficiently. In addition, the speed of bubbles is affected by various factors such as solution concentration, surfactant and gas composition. Surfactants alter the surface tension of the bubbles, thus influencing their ascent rate. During micrometer bubble rise experiments, the rising speed of bubble was reduced by the addition of surfactant [39].

2.3. The Mass Transfer Efficiency of MNBs

Mass transfer involves the movement of mass from one system to another. In the context of gas–liquid phase, the efficiency of gas mass transfer is closely linked to the contact area between the two phases. MNBs exhibit significantly higher mass transfer efficiencies than large bubbles due to their small size and large specific surface area. For the same total amount of gas, a millimeter-sized bubble is equivalent to one million MNBs. The presence of numerous MNBs dispersed in the liquid increases the contact area between the gas and liquid phases, resulting in improved mass transfer efficiency. Some theoretical models have been developed to describe the mass transfer process of oxygen macro bubbles and ozone MNBs, and the results show that the life cycle of MNBs improves and outperforms that of large bubbles since the half-life of dissolved gases increases [40]. In addition, the efficiency of mass transfer is positively correlated with the bubble internal pressure. The greater the bubble internal pressure, the higher the gas mass transfer efficiency. The high internal pressure within MNBs, as described by the Young-Laplace equation, results in a significantly elevated concentration of ozone gas at the gas–liquid interface compared to conventional bubbles. In addition, the collision and collapse of MNBs generate localized zones of high temperature and pressure, favoring the thermal decomposition of both ozone (O₃ → O₂ + O) and water vapor (H₂O → H• + •OH). The oxygen atom (O) can further react with water to produce additional •OH radicals. Haapala et al. [39] explored the mass transfer process between carbon dioxide and water in different phases under reservoir conditions through microfluidic experiments. The experimental results demonstrated that the mass transfer coefficient improved with the increase in pressure under supercritical conditions. When the diameter of carbon dioxide bubble was reduced to 50 μm, the mass transfer efficiency of carbon dioxide was significantly increased by 2 to 3 orders of magnitude. Xie et al. [41] explored the effectiveness of MNBs in assisting ozone removal of organic contaminants from groundwater. The experimental results indicated that the MNBs improve the mass transfer efficiency at the ozone interface and promote the generation of free hydroxyl radicals, while increasing dissolved ozone concentration in solution, leading to stable and efficient degradation of organic pollutants. Moreover, nanobubbles exhibit higher mass transfer efficiency compared to large bubbles, so the use of nanobubbles can provide more oxygen to biofilm systems at a relatively low cost [42]. The low rising velocity of MNBs and their prolonged residence time in water are features that contribute to the mass transfer efficiency.

2.4. The Gas Dissolution of MNBs

Gas bubbles in water are affected by surface tension due to the existence of a two-phase interface between the gas and liquid phases in water. Surface tension plays an important role in gas solubility, in which the gas within the bubble becomes compressed and dissolved into the water. According to the Laplace equation, it can be seen that the bubble radius is inversely proportional to the difference between internal and external pressure, as shown in Equation (2). Consequently, the smaller bubble radius corresponds to a higher internal pressure [43]:

$$∆p={\displaystyle \frac{2\mathit{\sigma }}{r}}$$

(2)

where $\mathit{\sigma }$ is the surface tension, r is the bubble radius, and $∆p$ is the difference between bubble’s internal and external pressure. For nanobubbles with a radius of 50 nanometers, the pressure difference is about 5.4 MPa, which is much higher than that of ordinary bubbles. Influenced by the surface tension, the size of the bubbles is gradually decreasing in the liquid, thus increasing their internal pressure. The increased internal pressure further accelerates the bubble contraction, compresses the internal gas, and ultimately leads to the dissolution of the MNBs in the liquid, which improves the rate of gas dissolution. Compared with the ozone macro bubble aeration method, the saturated gas content of OMNBs after 10 min was 15.9%, while that of ozone macro bubbles was only 3.1%. The saturated gas content of the former was about five times greater than the latter due to the stronger dissolution ability and longer residence time characteristics of MNBs [44].

2.5. The Zeta Potential of MNBs

MNBs have a high zeta potential due to the preferential adsorption of hydroxide ions on the surface [26,45]. In gas-water two-phase systems, the potential of MNB is induced by the potential difference between the sliding surface near the gas–liquid interface and the liquid, as shown in Figure 3 [46]. Surrounding counter ions (OH⁻) are attracted due to the surface charge of MNBs, leading to the formation of a diffusive double-electric layer at the interface [47]. Zeta potential is an important indicator for evaluating the stability of MNBs, which are considered stable when the zeta potential is >30 mv or <−30 mv. Since the MNBs have the identical charge at their surface, electrostatic repulsion occurs between the bubbles, thus preventing bubble aggregation and enhancing their stability. The zeta potential of MNBs can be calculated by the Smoluchowski equation. In addition, various test methods, including laser Doppler testing and electrophoretic scattering testing, can be employed to test the zeta potential of MNBs by capturing the velocity of MNBs during electrophoresis. The zeta potential of MNBs is influenced by many factors, such as the nature of the electrolyte, surfactant dosage, and pH value [48]. In general, the zeta potential of MNBs is between −20 and −50 in the pH range of 2 to 12. When MNBs contract, charges accumulate at their interfaces, thus elevating the zeta potential [49]. Furthermore, the zeta potential of the bubbles rises when the electrolyte concentration in the solution increases, which is due to the fact that the two-layer diffusion of electrons within MNBs is affected. In contrast, the zeta potential of bubbles decreases with rising temperature, because of enhanced ion mobility and reduced adsorption of charged ions on the bubble’s surface under elevated temperature conditions [50]. When MNBs are destroyed, a sudden release of a large number of charges at the bubble interface results in the generation of hydroxyl radicals with high oxidizing properties [36].

2.6. The Characteristics of Ozone

Ozone, a blue-color gas with a sharp scent, is a remarkably reactive and unstable allotrope of oxygen. With a standard redox potential of 2.07 v, ozone stands as one of the strongest oxidizing agents and can be used in the field of oxidative leaching [51,52]. Guo et al. [53] studied the mineralogical characterization of concentrates and the extraction of antimony by ozone in hydrochloric acid solution. The effects of liquid–solid ratio, temperature and hydrochloric acid concentration on the antimony extraction rate were investigated. The results showed that under the optimized conditions, the antimony extraction rate of ozone pretreatment could reach 93.75%, which greatly improved the metal extraction rate. Meanwhile, Liu et al. [54] investigated the effect of ozone ice as an oxidizing agent in heap leaching of gold ore at 5 °C. In the ore heap leach test, the addition of 300 mg/L ozone ice increased the gold extraction rate by 4.1% compared to the heap leach test without ozone ice. However, the continuous augmentation in ozone ice content failed to yield a significant effect, as the solution was already saturated with dissolved ozone.

Ozone, an unstable gas prone to decomposition in water, exhibits varied decomposition pathways under different environmental conditions. Under acidic conditions, ozone decomposes into oxygen and water, as shown in Equation (3) [55]. However, under alkaline conditions, ozone undergoes decomposition into oxygen and hydroxyl groups, such as Equation (4) [55].

$$O_3\left(\mathrm{g}\right)+{2\mathrm{e}}^{-}\left(\mathrm{a}\mathrm{q}\right)+2H^{+}\left(\mathrm{a}\mathrm{q}\right)\to O_2\left(\mathrm{g}\right)+H_{2}O\left(\mathrm{l}\right) {\mathit{\Delta }}_r{G}^{\circ }=-98.3 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(3)

$$3O_3\left(\mathrm{g}\right)+{OH}^{-}\left(\mathrm{a}\mathrm{q}\right)+H^{+}\left(\mathrm{a}\mathrm{q}\right)\to 2·OH\left(\mathrm{a}\mathrm{q}\right)+4O_2\left(\mathrm{g}\right){\mathit{\Delta }}_r{G}^{\circ }=-6.4 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(4)

The solubility of ozone in water greatly exceeds that of oxygen, being 13 times higher [30]. While the solubility of ozone in water decreases rapidly according to Henry’s law, wherein the solubility of gas in water is affected by its partial pressure. Given the low atmospheric concentration and correspondingly small partial pressure of ozone, the dissolved ozone continually volatilizes into the atmosphere. Consequently, the low ozone partial pressure in the air results in diminished mass transfer, thereby constraining the ability of ozone treatment.

The physical characteristics of the OMNBs are central to their effectiveness, as they directly govern the mechanical and chemical processes that enhance uranium leaching. The small size of the ozone bubbles, which results in high internal pressure, serves as a primary mechanism for enhancing permeability by propagating micro-fractures within the ore body, thereby creating new pathways for the lixiviant to access trapped uranium minerals. Furthermore, the gas–liquid interfacial properties and stability of the ozone bubbles are critical for determining their residence time and interaction with the rock surface. Stable small-sized ozone bubbles with a longer lifespan can travel deeper into the formation than unstable large-sized ozone bubbles with a shorter lifespan, sustaining the mechanical action while also influencing local wettability and solution dynamics to improve the efficiency of oxidant delivery and the removal of dissolved uranium.

Previous studies on ozone-based leaching of uranium ores have been conducted by researchers. For example, Ren et al. demonstrated the generation of ozone nanobubbles with remarkably extended stability at ambient pressure [56]. However, the stability and generation efficiency of such nanobubbles under the elevated pressure conditions (representing typical ISR operating conditions) remain an important area for future investigation. Compared with conventional ozonation leaching, OMNBs significantly increased the oxidation rate of U (IV) [57]. Zhang et al. conducted batch and continuous-flow oxidation experiments on sandstone uranium ore and UO₂ to evaluate the effectiveness of OMNBs in enhancing oxidation efficiency and leaching rates. The results demonstrate that OMNBs significantly accelerate the leaching kinetics, primarily through the direct oxidative action of ozone. This technique proved feasible for processing sandstone uranium ore, showing potential to address current challenges in production. Furthermore, the continuous-flow experiment validated its scalability, suggesting OMNBs as a promising new method for in situ leaching (ISL) of uranium using CO₂ and O₂ at ambient pressure [58].

3. The Selection and Generation of Ozone MNBs

The selection of ozone MNBs should be comprehensively understood to assess the feasibility and efficiency during the ISL process. Moreover, the generation methods of MNBs and their limitations are explained as follows.

3.1. The Selection of MNB Generating Methods

It is worth noting that different MNB generating methods are suited to different application scenarios, and factors such as bubble size, density and stability should be considered when selecting an appropriate method. When selecting MNBs generators, the following factors need to be considered. First, the operational environment of the bubble generator, whether indoors or on-site, bears significance. For field experiments, the portability of the bubble generator assumes an important factor, and ensures that its functions align with the needs of field operations. There are also some other problems that need to be overcome when conducting field operations, such as gas clogging and environmental pollution. Secondly, the selection of a suitable bubble generator depends upon the specific objectives of the research or application. For example, some studies focusing on the oxidizing efficiency of high-concentration bubbles demand a high-power and high-efficiency bubble generator. When examining the characteristics of bubbles with varied sizes, minimizing the number of produced bubbles holds primacy. In particular, the separation of individual bubbles is facilitated when the settings of the bubble generator are adjusted to vary the size of the bubbles. However, when the existing bubble generator fails to meet the experimental requirements, refinement and augmentation of the current generator in line with the experimental purpose become necessary. Lastly, the cost consideration assumes significance. The choice of an apt bubble generator should align with both the experimental requirements and the budget at hand.

3.2. MNB Preparation Techniques

The preparation methods of MNBs can be classified into physical and chemical methods based on the generation patterns [59].

Table 2. Generation methods for MNBs.

Methods	Mechanism	Applications	Advantages and Disadvantages
Electrolysis	Through electrochemical reaction on the electrodes	Generation of H2 NB [60], O2 NB [61] and CO2 NB [62]	Easy process, high purity, mature technique [63]; MNB collapse affects the surface of erosion; the accumulated hydrogen may cause a safety problem [36].
Chemical reaction	By chemical reaction	Function material preparation [64,65]	Low energy consumption and high efficiency; Employed under certain conditions, limited bubble type.
Ultrasonic	Local pressure changes induced by ultrasonic waves lead to formation of MNBs	Drug delivery, surface cleaning [63]	Mature technology, simple process; Limited in large-scale industrial applications [66].
Gaseous diffusion	Utilize various devices to break the gas phase into tiny size [67]	Food process, function, material preparation [63]	Mature technology, high flexibility; Relatively large bubble size, high energy consumption [68].

demonstrates some common generation methods of MNBs and their mechanism, applications, advantages and disadvantages. Although MNBs can be prepared by utilizing some methods by adjusting the working parameters, there are differences in the properties of the bubbles produced by different methods, including their number, size, and zeta potential. The following section outlines several prevalent methods for producing MNBs.

3.2.1. Electrolysis and Chemical Reaction Methods

Chemical methods mainly include electrolysis and chemical reaction methods. Electrolysis methods involve generating MNBs by electrolysis of electrodes in solution, with the addition of surfactants (e.g., sodium dodecyl sulfate (SDS) and Triton X-100) to the solution to maintain their stability [69]. In a short period of time, accompanied by high saturation, the high current generates nanobubbles uniformly. However, over time, these generated nanobubbles may polymerize into micron bubbles, especially under high temperature conditions [69]. The electrolytic generation of MNBs is influenced by various factors such as solution concentration, pH value and distance between the ends of the electrodes, with higher current density leading to faster bubble generation rate. MNBs can be easily prepared using chemical reaction methods. For instance, Bai et al. [64,65] produced CO₂ MNBs by reaction between sodium carbonate and hydrochloric acid, which can be utilized to create multi-vacuum adsorbents for selective uranium extraction. It is important to note that chemical reaction methods can only be employed under specific conditions due to the restricted reaction conditions and reactants, which limit the types of bubbles generated.

Ozone can be generated by electrolysis. During the process of electrolysis, ozone is produced electrolytically by oxidation of water. The generation of ozone by electrolysis competes with the oxygen production, with a lower redox potential. Therefore, to promote the formation of ozone, electrolytic ozone generators with diamond electrodes (boron-doped diamond, BDD) need to be adopted, because they are highly efficient at ozone production and resistant to corrosion. Also, large current densities should be applied during electrolysis [70].

3.2.2. Ultrasonic Method

Compared to electrolysis and chemical reaction methods, the ultrasonic method produces MNBs without impurities while allowing for precise control over bubble size and concentration. This method involves placing a liquid in an ultrasonic irradiator, where bubbles are generated under the influence of ultrasonic waves. The localized pressure changes induced by the ultrasonic waves lead to the formation of MNBs in the liquid. The frequency, power and time of the ultrasound can be adjusted to control the size and density of the bubbles [71]. With respect to hydrodynamic method, the MNBs prepared by ultrasonic method exhibit a smaller size and higher gas content [72]. In order to investigate the effect of ultrasonic radiation on the production and reduction of MNBs, Yasuda et al. [73] conducted ultrasonic experiments and numerical simulations in ultrapure water. And the experimental results showed that bubbles with diameters of about 100 nm were produced in ultrapure water, and their size and concentration were correlated with the power and frequency of the ultrasound. An increase in ultrasonic power and decrease in frequency led to a higher concentration of bubbles. Furthermore, a decrease in ultrasonic frequency enhanced the cavitation collapse effect, thereby increasing the rate coefficient of bubble production. Currently, the ultrasonic method is mainly used within the laboratory. Because its widespread application in large-scale industrial treatments is hindered by its operational costs and interdisciplinary technical limitations, it necessitates expertise in the fields of physics, chemistry, and materials.

3.2.3. Gaseous Diffusion Method

The gaseous diffusion method involves passing a liquid containing a gas through a pressure release device, which causes rapid gas expansion to form bubbles. Typically used to produce tiny bubbles, this method relies on the employment of specialized bubble devices to generate MNBs. Bubble generators are usually based on special mechanical structures or physical principles that form MNBs by controlling the flow of water and the vibration of particles. For instance, the Venturi tube is an effective bubble generator, in which the gas–liquid two-phase fluid is accelerated due to the sudden pressure change, converting large bubbles into a multitude of micro and nano bubbles through cavitation. This method offers advantages such as low energy consumption, high bubble concentration, and small bubble size [74,75]. Some other bubble generators produce MNBs by initially increasing and then decreasing the pressure. Increased pressure causes more gas dissolution in the liquid, forming a supersaturated liquid. Subsequent decrease in pressure through a pressure-reducing valve causes flashing of the supersaturated liquid, leading to formation of MNBs. The size and concentration of the bubbles are affected by the pressure change process [76].

3.2.4. UV-C and Vacuum-UV (VUV) Ozone Generation

Ultraviolet-based ozone generation relies on the photolytic excitation and dissociation of oxygen molecules, and it operates primarily in two spectral regions: UV-C (around 185–254 nm) and VUV (vacuum ultraviolet, typically below 200 nm). In photolytic ozone generation, high-energy photons interact with molecular oxygen (O₂), breaking the O=O bond to form reactive atomic oxygen (O·). These oxygen atoms rapidly recombine with intact O₂ to form ozone (O₃). UV-C systems commonly use the 185 nm wavelength to drive this process; at this wavelength, oxygen absorbs strongly enough to dissociate efficiently while still allowing photons to penetrate the gas flow, enabling continuous O₃ formation. Simultaneously, many UV lamps also emit at 254 nm, which selectively photolyzes ozone back to O₂ and O·; depending on lamp design, ozone-producing systems suppress 254 nm output to maximize net yield. VUV-based ozone generation uses deeper-UV photons—often around 172 nm—from excimer sources, where absorption is even stronger. This enhances dissociation efficiency and can produce higher ozone concentrations in shorter path lengths, although it also restricts penetration depth and requires materials compatible with high-energy radiation. Both UV-C and VUV methods are “cold” processes, meaning they generate ozone without substantial heating or electrical discharge, offering clean operation with minimal nitrogen oxides. The governing principles are rooted in photon–molecule interaction: photon absorption must exceed the dissociation energy of O₂; the gas must provide sufficient collision frequency for O·+ O₂ recombination; and the optical environment—lamp intensity, geometry, and gas transparency—must allow enough photons to reach the reaction zone. Photolytic generators, therefore, balance wavelength selection, lamp power, and flow dynamics to optimize the interplay between O₂ dissociation and O₃ destruction. This makes photolysis-based systems especially useful in applications requiring low-temperature, chemically pure ozone and precise control over production rates. UV-C lamps have been applied in leaching processes, as UV-C irradiation can enhance oxidizing conditions, promote radical formation, and improve metal dissolution efficiency.

3.2.5. Ozone Storage and Pressurized Use

Ozone is unstable and decomposes back to O₂, so it is usually generated on-site rather than stored. At very low temperatures (<−112 °C), ozone can be stored as a liquid, but this is rare due to explosion risks. Liquid ozone is highly explosive, especially if organic contaminants or sudden temperature changes trigger decomposition. Also, maintaining cryogenic conditions is energy-intensive and impractical for most applications. Storing gaseous ozone in vessels is also hazardous due to its explosive nature. High-pressure ozone systems are risky due to potential decomposition into O₂, which can cause explosions. In summary, ozone’s instability and explosive potential make real-time generation and low-pressure application the safest approach. In practice, ozone is commonly delivered under low pressure in gas form for water and air treatment.

4. Characterization Methods of MNBs

It is well known that MNBs have a number of unique properties, such as tiny size, high zeta potential, and good stability. This section will focus on measurement methods for characterizing these properties of MNBs.

4.1. Size Measurement

The size and distribution range of MNBs are key factors affecting their properties. Various advanced microimaging techniques, including optical microscopy, atomic force microscopy (AFM), electron microscopy, dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and image characterization techniques (Figure 2), can be used to measure the size of MNBs [77]. The advantages and limitations of different imaging techniques for measuring the dimensions of MNBs will be compared below.

For electron microscopy and atomic force microscopy, both can measure the particle size of bubbles at the nanoscale. While electron microscopy can also determine the composition of matter, it necessitates a vacuum environment for operation. On the other hand, atomic force microscopy exhibits flexibility in its environmental requirements, being able to function in natural as well as liquid environments. Therefore, atomic force microscopy is commonly preferred for dimensional characterization of MNBs, particularly in the observation of their surface properties [78].

Microscopy-derived images of MNBs necessitate processing through image analysis techniques. This combination enables non-destructive visualization and assessment of bubble sizes and their distribution [79]. The image analysis process includes noise reduction, threshold segmentation, bubble separation, and subsequent treatment of the bubbles as regular spheres to facilitate diameter and size distribution calculations, as shown in Figure 4. Juwana et al. [80] utilized a high-speed camera and shadow imaging technique to observe bubbles and obtained the average diameter and size distribution of the bubbles. Although the image analysis technique can effectively analyze the bubble size, it requires high resolution and shooting speed of the image. For example, some optical microscopes offer resolutions ranging between 200–500 nm, allowing observation of micrometer-sized bubbles but not those at the nanoscale [81]. Azevedo et al. [82] used an advanced optical microscope and an ultra-high-resolution camera to capture nanosized bubbles, enhancing their visibility by coloring the bubbles with dye.

Dynamic light scattering serves as a valuable technique for studying MNB systems, as it captures the variation in the scattered light intensity and calculates information about bubble size [84]. However, this method has some limitations. It can only obtain the average radius and distribution of bubbles, lacking the capability to assess their concentration. In addition, it requires a high intensity of scattered light and exhibits low measurement accuracy for MNBs. To address these issues, nanoparticle tracking analysis techniques have been developed to analyze bubble concentration and particle information. The nanoparticle tracking analysis technique can track the trajectory of a single bubble to obtain information about its size. Moreover, the technique is suitable for analyzing samples with a tiny size and little heat dissipation, especially MNBs [85].

4.2. Measurement of Zeta Potential

Zeta potential is an important factor affecting the stability of MNBs. When the zeta potential is close to 0, the bubbles tend to aggregate and condense under the influence of van der Waals interparticle attraction. Therefore, it is necessary to describe the methods employed to measure the zeta potential of MNBs, as well as the advantages and limitations of each method.

The acoustic method and the single particle method are commonly utilized to measure the zeta potential of MNBs, but both methods have their flaws. In the single particle method, the accuracy of measurements can be compromised by the influence of the rate of electroosmosis, particularly in the context of tiny bubbles. This can lead to less precise measurements due to the size-dependent effects on electroosmosis. Similarly, the acoustic method faces challenges due to the sensitivity and variability of measurement results. Factors such as charge density, bubble size, and the interaction between electricity and sound contribute to the complexity of the relationship, impacting the reliability of the measurements. To tackle these problems above, the electrophoretic light scattering method has been developed. This technique is based on measuring the motion speed of charged particles under an applied electric field and utilizes a theoretical model to calculate the zeta potential [86].

4.3. Other Measurements for MNBs

The characterization of MNBs extends beyond measuring their size and zeta potential. Several methods can be employed to measure other properties of MNBs. The Tyndall effect is a phenomenon where light is scattered by particles in a colloidal solution, making the beam visible. This effect can be utilized to ascertain the existence or absence of nanobubbles in a solution, as it is sensitive to the presence of colloidal suspensions [56]. This method provides a qualitative means of recognizing solutions containing nanobubbles. Michailidi et al. [87] used laser to irradiate untreated pure water, water containing air nanobubbles, and water containing oxygen nanobubbles. And the results of their experiments showed that only water containing nanobubbles scattered the color of the laser light, which proved presence of nanobubbles in solution with a size range similar to the laser wavelength. Li et al. [88] used a molecular dynamics approach to investigate how gas species and concentration affect the aggregation and motion of nanobubbles. When the gas concentration increased, more bulk nanobubbles were generated, exhibiting greater stability and aggregation ability. The order of merging between nanobubbles was found to be controlled by the distance between the bubbles. A high-speed camera and magnifying lens were used to obtain the positions of MNBs at different moments in solution, enabling the calculation of their rising velocity [89]. These methods collectively contribute to a comprehensive understanding of the properties and behavior of MNBs, encompassing their presence, motion, stability, and interactions within a solution.

5. The Impact Factors of OMNBs

As mentioned above, ozone possesses strong oxidizing capabilities and fast reaction speeds. Under ambient conditions, it can achieve the oxidizing effect of oxygen with high temperature and high pressure. Moreover, the oxygen produced by ozone decomposition will not cause environmental pollution, which is in line with the environmental protection requirements of clean and green, and sustainable development. In addition, with the increase in demand for ozone use and the upgrading of production processes, the cost of ozone use has decreased. However, as an oxidizer, ozone presents the following disadvantages. Firstly, it decomposes quickly in water due to the low partial pressure. Secondly, its mass transfer efficiency is limited. In contrast, MNBs are characterized by high internal pressure, small bubble size, large specific surface area, extended residence time in water, and high mass transfer efficiency. Therefore, the combination of ozone and MNBs improves the oxidation ability and mass transfer efficiency. It is important to note that the oxidizing capacity of OMNBs is influenced by pH, gas flux, and ozone production rate, which will be analyzed in detail below.

5.1. pH Value

The pH value in aqueous solution has an important influence on the oxidation mechanism and oxidizing capacity of OMNBs. When the solution pH is alkaline or nearly neutral, the OH⁻ in the solution promotes the decomposition of ozone to produce oxygen and hydroxyl radicals. The oxidizing capacity of the solution depends on the ozone, the hydroxyl radicals and oxygen produced by ozone decomposition, and the hydroxyl radicals resulting from the rupture of nanobubbles. Nevertheless, in acidic solutions, the decomposition of ozone does not yield hydroxyl radicals. The oxidizing power of the solution depends only on the ozone, the oxygen produced by ozone decomposition, and the hydroxyl radicals produced by the rupture of the nanobubbles. Hu et al. [90] conducted experiments on the degradation of nitrobenzene by OMNBs and ozone macro-bubbles at different pH values ranging from 3 to 11. Their findings demonstrated that OMNBs have a wide pH reaction range and can effectively remove nitrobenzene across different pH values. Meanwhile, the reaction rate constants of OMNBs at different pH values were more than two times those of ozone macro-bubbles, suggesting that the OMNBs exhibit less sensitivity to pH variations with respect to ozone macro-bubbles. However, when the solution’s pH is equal or greater than 9, the concentration of dissolved ozone in the solution tends to approach zero due to rapid ozone activation in the presence of OH⁻, decomposing to generate a large number of hydroxyl radicals (which can also be generated from concentrated hydrogen peroxide (H₂O₂) under the influence of strong ultra violet light). In addition, given a certain amount of ozone, more nitrobenzene can be removed by aeration with MNBs at the same pH, thereby reducing the dosage of ozone and the cost of pollutant degradation. Additionally, the stability of MNBs is enhanced when the solution pH is increased because the absolute value of the zeta potential rises [91].

5.2. The Concentration of Ozone

Increasing production rate or the gas flow rate of ozone is a key factor in enhancing the oxidation rate of OMNBs. This is because the concentration of ozone in the gas phase drives the conversion of ozone from the gas phase to the liquid phase, ultimately elevating the available ozone concentration in solution for the reaction. Rodríguez-Rodríguez et al. [92] used ozone as an oxidation agent for the extraction of silver from pyrargyrite under acidic conditions. The experimental results showed that the rate of dissolution of silver was linear with the amount of added ozone, and the dissolution rate of silver improved with increasing ozone content. Zhang et al. [93] investigated the effect of gas flow rate on the oxidation of pyrite by OMNBs. The oxidation rate of iron in pyrite correlated positively with gas flow rate. This was attributed to the higher influx of ozone gas into the solution, facilitating enhanced participation in the reaction, alongside an increased contact area, thus improving the oxidation efficiency. However, beyond a certain gas flow rate threshold, the contact area between reactants and ozone becomes saturated, leading to less noticeable improvements in oxidation efficiency with further increases in gas flux [18]. Therefore, the optimal gas flow rate should be determined on the basis of comprehensive consideration of economy, oxidation efficiency, environmental protection and other factors.

5.3. Other Impact Factors

In addition to ozone concentration and pH, the properties of OMNBs are affected by many other factors, including temperature, pressure, gas concentration, and solution composition. Temperature is one of the most important factors affecting gas solubility. When the temperature rises, the size of OMNBs and content of dissolved ozone in solution increase, and the mobility of charged particles is enhanced [35]. Li et al. [94] conducted experiments to investigate the effect of temperature on the nanobubble nucleation. The findings indicated that as temperature decreased, the nucleation rate of nanobubbles decreased, but the stability increased. Pressure is an important parameter that affects bubble stability. An increase in pressure leads to a higher amount of dissolved gas and increased gas saturation, thereby enhancing the stability of nanobubbles. Additionally, higher gas solubility results in a greater number of bubbles being produced and increased bubble stability due to the decrease in surface tension [95]. These factors collectively demonstrate the complex interplay of various physical parameters in influencing the properties and behavior of OMNBs.

The stability of OMNBs is also affected by the electrolyte concentration and ionic valence in solution. Normally, OMNBs are negatively charged due to the adsorption of OH⁻ ions in solution on their surface. However, when trivalent aluminum and iron ions are added to the solution, the OMNBs become positively charged [96]. As the electrolyte concentration in solution increases, the bubble size increases while the zeta potential decreases. Moreover, nanoparticles and surfactants also have an effect on the nature of OMNBs. Surfactant adsorption plays an important role in the formation and stabilization of bubbles, and different types of surfactants exert varying effects on bubbles. When the charge of the added surfactant aligns with the surface charge of the OMNBs, electrostatic repulsion occurs, increasing the absolute value of the zeta potential of the bubbles. On the contrary, when the charge of the added surfactant is opposite to that of the bubbles, the charge on the surface of the bubbles is neutralized, leading to decreased electrostatic repulsion between bubbles, aggregation, and reduced stability. Furthermore, nonionic surfactants can improve the contraction and rupture of bubbles, increasing their stability [97]. The addition of nanoparticles to OMNB solutions improves bubble size, surface charge, and stability. Nanoparticles have different mechanisms of action with micron bubbles and nanobubbles. Nanoparticles adsorb on the surface of micrometer bubbles and prevent gas diffusion, thus improving the stability of the bubbles [98]. However, nanobubbles adsorb to the surface of nanoparticles due to electrostatic attraction, and nanoparticles promote gas nucleation due to their wettability and roughness [99].

6. Potential and Challenge of Applying OMNBs During In Situ Uranium Leaching

OMNBs have attracted much attention in the field of hydrometallurgy and resource recovery due to their high oxidizing capacity, fast reaction rate, and high mass-transfer efficiency, all of which hold enormous economic and engineering values (

Table 3. Overview of OMNB applications.

Reactants	Operating Condition-Initial Solution pH	Operating Condition-Gas Flow Rate	Operating Condition- Reaction Time	Operating Condition-Other Key Parameters	Oxidation Extent (%)	References
Sandstone uranium ore	1	1 L/min	2 h	Ozone production rate: 1.67 mg/s; Stirring speed: 200 r/min	96.3 ± 3.5	[58]
Pyrite	1	Not reported	1 h	Ozone production rate: 1.67 mg/s; Stirring speed: 500 r/min; Initial concentration of pyrite: 500 mg/L,	57.6	[93]
As(III)	7	Not reported	1 h	Ozone production rate: 0.56 mg/s; As(III) concentration range: 50 to 200 μg/L,	>90	[100]
Nitrobenzene	3, 5, 7, 9, 11	0.5 L/min	15 min	Measured ozone concentration: 20 mg/L; Initial nitrobenzene concentration: 0.1 mmol/L	>80	[90]
Methyl orange	3, 5, 7, 9, 11	Not reported	50 min	Initial methyl orange concentration: 50 mg/L; temperature: 20 °C	100	[101]
Ammonia nitrogen (NH3-N)	9.	Not reported	90 min	Initial ammonia nitrogen concentration: 50 mg/L	97.6	[32]

Note: Most experiments reported in Table 3 were conducted under atmospheric pressure, which may not fully represent high-pressure ISR conditions.

). There have been some applications of OMNBs for uranium recovery. Based on previous applications, the feasibility of applying OMNBs during ISL to achieve uranium recovery is described in detail in the following sections.

6.1. Theoretical and Applicable Basis for OMNBs During In Situ Leaching

In the introductory part of this paper, the proportion of sandstone-type uranium ores in China’s uranium resources and the advantages of ISL method were introduced [102,103,104]. However, the existing ISL methods have many drawbacks, including insufficient understanding of the oxidation mechanism, low oxygen solubility, poor oxidizing capacity, and a large volume of pressurized oxygen bubbles. These shortcomings collectively contribute to low recovery of uranium resources and high mining costs, constraining the development of uranium extraction by the ISL method [105,106,107]. Given these challenges, the potential of uranium resource recovery by OMNBs, as a novel and efficient oxidation method, needs to be evaluated in detail.

The reaction between OMNBs and uranium ore is very complex, involving three media—solid, liquid, and gas—along with oxidation and coordination reactions. The oxidizing agents include ozone, oxygen, and hydroxyls dissolved in solution, where the hydroxyl radicals are produced by nanobubble rupture and ozone decomposition. Through these oxidizing agents, tetravalent uranium is converted to hexavalent uranium. Subsequently, the hexavalent uranium undergoes a coordination reaction with sulfate ions or bicarbonate ions in solution, ultimately leading to the formation of uranyl sulfate or uranyl bicarbonate complex in solution, such as Equations (5)–(14).

$$2{UO}_2\left(\mathrm{s}\right)+O_2\left(\mathrm{g}\right)+{2H}_2{SO}_4\left(\mathrm{a}\mathrm{q}\right)\to {2UO}_2{SO}_4\left(\mathrm{a}\mathrm{q}\right)+2H_{2}O\left(\mathrm{l}\right) {\Delta }_r{G}^{\circ }=-267.5 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(5)

$$3{UO}_2\left(\mathrm{s}\right)+O_3\left(\mathrm{g}\right)+{3H}_2{SO}_4\left(\mathrm{a}\mathrm{q}\right)\to {3UO}_2{SO}_4\left(\mathrm{a}\mathrm{q}\right)+3H_{2}O\left(\mathrm{l}\right) {\Delta }_r{G}^{\circ }=-564.4 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(6)

$${UO}_2\left(\mathrm{s}\right)+2·OH\left(\mathrm{a}\mathrm{q}\right)+H_2{SO}_4\left(\mathrm{a}\mathrm{q}\right)\to {UO}_2{SO}_4\left(\mathrm{a}\mathrm{q}\right)+2H_{2}O\left(\mathrm{l}\right) {\Delta }_r{G}^{\circ }=-696.8 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(7)

$$C{O}_2\left(\mathrm{g}\right)\to C{O}_2 \left(\mathrm{a}\mathrm{q}\right) {\Delta }_r{G}^{\circ }=8.4 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(8)

$$C{O}_2\left(\mathrm{a}\mathrm{q}\right)+H_{2}O\left(l\right)\to {H_{2}CO}_3 \left(\mathrm{a}\mathrm{q}\right) {\Delta }_r{G}^{\circ }=0.2 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(9)

$${H_{2}CO}_3 \left(\mathrm{a}\mathrm{q}\right)\to H^{+}\left(\mathrm{a}\mathrm{q}\right)+{HCO}_3^{-} \left(\mathrm{a}\mathrm{q}\right) {\Delta }_r{G}^{\circ }=36.3 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(10)

$${HCO}_3^{-}\left(\mathrm{a}\mathrm{q}\right)\to H^{+}\left(\mathrm{a}\mathrm{q}\right)+{CO}_3^{2-} \left(\mathrm{a}\mathrm{q}\right) {\Delta }_r{G}^{\circ }=59.0 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(11)

$$2{UO}_2\left(\mathrm{s}\right)+O_2\left(\mathrm{g}\right)+4{HCO}_3^{-}\left(\mathrm{a}\mathrm{q}\right)\to 2{{{{[UO}_2(CO}_3)}_2]}^{2-}\left(\mathrm{a}\mathrm{q}\right)+2H_{2}O \left(\mathrm{l}\right) {\Delta }_r{G}^{\circ }=-498.2 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(12)

$$3{UO}_2\left(\mathrm{s}\right)+O_3\left(\mathrm{g}\right)+6{HCO}_3^{-}\left(\mathrm{a}\mathrm{q}\right)\to {{{{3[UO}_2(CO}_3)}_2]}^{2-}\left(\mathrm{a}\mathrm{q}\right)+3H_{2}O\left(\mathrm{l}\right) {\Delta }_r{G}^{\circ }=-910.5 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(13)

$${UO}_2\left(\mathrm{s}\right)+2·OH\left(\mathrm{a}\mathrm{q}\right)+2{HCO}_3^{-}\left(\mathrm{a}\mathrm{q}\right)\to {{{{[UO}_2(CO}_3)}_2]}^{2-}\left(\mathrm{a}\mathrm{q}\right)+2H_{2}O\left(\mathrm{l}\right) {\Delta }_r{G}^{\circ }=-812.2 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(14)

In terms of the oxidizing ability of OMNBs, one of the strongest oxidizing agents, their oxidizing ability and oxidation rate are much higher than that of oxygen [108,109]. Meanwhile, OMNBs have been applied in the fields of hydrometallurgy [110,111,112] and drinking water treatment [113,114,115], which provide potential applications in uranium leaching. Zhang et al. [58] designed a batch test for the oxidative leaching of sandstone uranium ore. The experimental results showed that under the same conditions, the use of OMNBs increased the oxidation rate of sandstone uranium ore by a factor of 12 compared to the use of oxygen microbubbles. It is worth noting that, as with oxygen, OMNBs do not produce new impurities during the oxidation of tetravalent uranium process. These characteristic underscores the potential of OMNBs as a clean and efficient oxidation method, further solidifying their appeal for applications in the ISL method.

OMNBs have a wide range of applicable pH values and have the ability to oxidize substances under both acidic and alkaline conditions. Notably, OMNBs exhibit a higher oxidizing capacity than oxygen, positioning them as novel and potent oxidizers in CO₂-rich leaching conditions, thereby offering potential advantages for ISL processes. Zhang et al. [93] used different gas sources of MNB to compare their effects on pyrite oxidation. The experiments showed that the OMNBs exhibited the highest oxidation ability, resulting in a 57.6% oxidation of pyrite, surpassing the oxidation achieved by oxygen alone by 35.2%. The acid leaching method is not suitable when the reservoir contains significant amounts of acid-consuming minerals, such as carbonate minerals and clay minerals. In this case, the ISL method involving CO₂ and O₂ is typically employed, particularly in reservoirs with high carbonate content. OMNB has a higher oxidizing capacity, making it more suitable for use as a replacement for oxygen. In alkaline environments, the reaction of ozone with hydroxide ions to form hydroxyl groups is more favorable [116]. Compared with ozone, the hydroxyl radical exhibits a stronger oxidizing capacity due to its higher redox potential (i.e., 2.8 V), which is favorable for reacting with numerous compounds that are typically resistant to oxidation by ozone. However, OMNB solution typically contains both of these two oxidants, and monitoring the hydroxyl radical content presents challenges due to its instability [117]. Zhang et al. [58] designed an experiment to investigate the effect of hydroxyl radicals on the leaching rate of sandstone uranium ore. Tert-butanol (TBA) serves as a hydroxyl radical inhibitor due to its capacity to capture these radicals. Its reaction rate with hydroxyl radical is much higher than that of ozone, thereby responding preferentially with hydroxyl radicals. After adding TBA reagent, the oxidation reaction rate of uranium ore was slightly reduced compared to the OMNBs solution without TBA, resulting in a 9.19% reduction in uranium recovery during the leaching test. The experimental results indicated that the oxidation reaction of uranium ore with OMNBs was the joint action of ozone and free hydroxyl groups.

During the ISL process, the addition of other agents, in addition to oxidizing agents, is essential. For instance, in the CO₂ + O₂ ISL method, CO₂ is added to adjust the pH value of the solution, replenish the bicarbonate ions in the formation water, and increase the rate of uranyl carbonate generation. Moreover, the carbonate ions in solution react with the hydroxyl groups produced by OMNBs to form reactive carbonate ion radicals. As shown in Equation (15) [55], this radical is a powerful single-electron oxidizer. Khuntia et al. [100] explored the impact of carbonate ion radicals on the oxidation of As(III) by ozone. The experimental results demonstrated that the existence of carbonate ion radicals enhanced the oxidation rate of trivalent arsenic to pentavalent arsenic, surpassing the oxidation of trivalent arsenic by hydroxyl radicals and ozone. Similarly, the activated carbonate ion radicals may accelerate the oxidation rate of tetravalent uranium to hexavalent uranium, although this requires confirmation in future studies to investigate the mechanism of uranium oxidation by activated carbonate ion radicals.

$$OH\left(aq\right)+{CO}_3^{2-}\left(aq\right)\to {CO}_3^{-}·\left(aq\right)+{ OH}^{-}\left(aq\right) { \Delta }_r{G}^{\circ }=457.6 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(15)

The characteristics of MNBs, including tiny particle size, large specific surface area, and high gas dissolution ability, significantly extend the residence time of ozone in the liquid and increase the gas–liquid mass transfer, thereby facilitating the efficient oxidation of uranium ore. Despite ozone’s higher solubility in water compared to oxygen (~1.2 × 10⁻² mol/(L.atm) of the Henry’s Law constant for ozone vs. ~1.3 × 10⁻³ mol/(L.atm) of the Henry’s Law constant for oxygen at atmospheric pressure and 25 °C [118]), the ultimate dissolved ozone content in water is very low. According to Henry’s law, the partial pressure of ozone in air is very small, and the dissolved ozone will continuously escape from the contact interface between water and air, so the content of ozone in water is continuously decreasing. Therefore, the combination of ozone with MNBs can solve this problem by increasing the concentration of dissolved ozone in water. Studies have shown that dissolved ozone content in the solution after microbubble and nanobubble aeration is approximately several times higher than that after ozone macro bubble aeration [119]. The dramatic increase in the ozone content in the solution after micro and nano bubble aeration creates a favorable oxidizing environment, promoting the oxidation of tetravalent uranium into hexavalent uranium and thereby enhancing the recovery rate of uranium resources. In addition, OMNBs have a high internal pressure, and their volume decreases due to effect of surface tension. As these bubbles contract, the temperature and pressure within them increase, ultimately leading to their rupture. This rupture leads to the release of some hydroxyl groups at nanobubble interface, which further enhances the oxidizing capacity of OMNBs [120]. Meanwhile, microbubbles and nanobubbles have the ability to persist in water for long periods, especially the extended existence of nanobubbles for days or even months [121]. These bubbles have the capability to move with the water flow, thereby offering continuous gas supply to the dissolved phase [122]. Moreover, OMNBs have the ability to increase ozone solubility in water and improve oxidizing capacity, which holds potential practical applications for the oxidation of uranium. 

Ozone has several advantages over conventional oxidants like oxygen and hydrogen peroxide. While oxygen is a low-cost option, its slow reaction kinetics and poor solubility often lead to prolonged leaching cycles. Hydrogen peroxide is a fast-reacting oxidizing agent. However, its commercial application is primarily limited by cost and by observed efficiency challenges in some alkaline leaching environments. In this context, ozone MNBs present a compelling alternative: the microbubble form drastically increases the gas–liquid surface area and residence time, while ozone itself is a potent and rapid oxidant. This combination addresses the core limitations of its predecessors by potentially accelerating the oxidation of insoluble U(IV) to soluble U(VI), improving oxidant utilization efficiency, and enabling effective operation in less permeable formations where conventional oxidant delivery is challenging.

Though the focus of this review is the application of ozone in in situ leaching, ozone may also be applied to extract uranium in ex situ leaching. Conventional alkaline leaching that involves compressed air injection for the oxidation of insoluble U (IV) is often carried out in the multistage autoclave continuous stirred tank reactors (CSTRs). It is possible to use ozone to replace compressed air to increase the uranium oxidation efficiency. However, an upgrade of the existing conventional alkaline leaching system to adopt ozone for uranium oxidation may cause an increase in cost and a decrease in system robustness, and the feasibility of such an upgrade needs to be carefully investigated.

In conclusion, OMNBs have a theoretical and practical engineering basis for uranium recovery during the ISL process. This technology amalgamates the advantages of ozone and MNBs, including increased ozone solubility, prolonged existence in solution, and enhanced oxidizing capacity, which collectively contribute to its potential as a superior alternative to the traditional oxidation of uranium ores.

6.2. Difficulties of Applying OMNBs During ISL

In the last decade, with the deepening understanding of the nature and characteristics of OMNBs, OMNBs have been widely used in the fields of industrial wastewater treatment, groundwater remediation and hydrometallurgy. These characteristics and applications provide potential possibilities and a basis for the application of OMNBs in the ISL method. Still, the oxidation of uranium by OMNBs faces some problems and challenges during the ISL process.

6.2.1. Oxidation Mechanism of OMNBs and Impact Factors

Only a limited number of scholars have investigated the research and application of oxidation of uranium ores by OMNBs. Consequently, issues persist, including a lack of comprehension regarding the oxidation mechanism of uranium by OMNBs and an experimental design that fails to consider the impact of external factors on the oxidizing capacity of OMNBs.

OMNBs can produce hydroxyl groups without the use of additives, a result of ozone decomposition and nanobubble rupture. These hydroxyl radicals possess potent oxidizing properties, facilitating the conversion of tetravalent uranium to hexavalent uranium in ores and enhancing uranium resource recovery. However, there remains a dearth of understanding regarding the generation of hydroxyl radicals, their roles in uranium ore oxidation, and the oxidation mechanism. The existing time of hydroxyl radicals in solution is transient, and the ores wrapped around uranium limit their oxidizing effect to a certain extent. Specifically, the redox-sensitive ore minerals may consume oxygen and ozone at a higher rate than that of the uranium-bearing minerals. In particular, pyrite may show faster oxidation kinetics than uraninite, coffinite, or brannerite. Preferential flow pathways could also limit the efficacy of OMNB/MNB. Additionally, studies investigating the oxidation of hydroxyl radicals in OMNBs have yielded disparate results. Hydroxyl radicals produced by OMNBs are active oxidants for the removal of butylated hydroxytoluene [123,124]. Zhang et al. [58] observed minimal reduction in the oxidized leaching rate of uraninite when a hydroxyl inhibitor was introduced during OMNBs’ reaction with sandstone uraninite under acidic conditions. This suggests that ozone serves as the primary oxidizing agent in the reaction, with the oxidizing effect of hydroxyl groups being less pronounced. Nevertheless, acidic conditions do not support the decomposition of ozone into hydroxyl radicals, resulting in a limited presence of hydroxyl radicals, primarily stemming from nanobubble rupture, and a correspondingly low oxidizing capacity. Instead, ozone decomposes to form more hydroxyl radicals under alkaline conditions, improving the oxidation rate of hydroxyl radicals. Furthermore, under alkaline conditions, MNBs carry a more negative charge, increasing the repulsive force between bubbles and extending their lifetime in solution, necessitating an exploration of hydroxyl radical oxidation of uranium ore under alkaline conditions. But there is another issue: during the process of uranium extraction by the CO₂ + O₂ ISL method, carbonate ions in the leach solution consume a substantial quantity of hydroxyl groups, diminishing their oxidizing effect on uranium ore. Some studies have shown that reactive carbonate ion radicals generated by the reaction of hydroxyl groups with carbonate ions exhibit oxidizing properties; their reaction rates are considerably lower than those of hydroxyl groups [21,125,126,127]. Nevertheless, other studies have presented conflicting findings, with some indicating the oxidation rate of carbonate ion radicals to be greater than that of hydroxyl radicals [100,128]. Consequently, further exploration of the oxidation rates of activated carbonate ion radicals and hydroxyl radicals on uraninite is warranted in future studies.

The pH range within which OMNBs can work is broad, yet the oxidation mechanism of OMNBs under varying pH conditions remains unclear. Zhang et al. [58] explored the oxidizing ability of OMNBs on uranium dioxide under different pH conditions by a continuous column leaching test, as shown in

Table 4. Effect of different pH values on the uranium extraction, dissolved ozone and Eh in leaching solution after 2 h [58].

pH	U Extraction (%)	Dissolved Ozone (mg/L)	Eh (mV)
1	93.02	12.84	1303
3	74.58	10.95	1210
5	11.76	10.66	1085
7	82.89	9.84	989

. The experimental results demonstrated that the extraction rate of uranium was higher at pH values of 1, 3 and 7. Compared with the solutions at pH 3 and 5, the concentration and potential of dissolved ozone in the solution at pH 7 were relatively lower, but the extraction rate of uranium was higher, indicating that the oxidation mechanism of OMNBs under neutral/alkaline conditions was different from that under acidic conditions. This may be due to the fact that ozone decomposes to form substantial hydroxyl groups with stronger oxidizing ability under alkaline conditions, which requires further study of the oxidation mechanism of OMNBs under alkaline conditions. On the other hand, hydrogen ions can dissolve partial minerals on the surface of uranium-bearing ores and increase the contact area of the uranium reaction under acidic conditions, thus accelerating the reaction rate between uranium and OMNBs. The structure of uraninite undergoes minimal change under alkaline conditions, posing challenges for a comprehensive and accurate comparison of OMNBs’ oxidizing capabilities on uraninite in acidic and alkaline environments with the removal of other interfering factors [129]. Furthermore, the rupture of nanobubbles in solution produces hydroxyl groups, reducing the solution pH value. Minamikawa et al. [130] irrigated rice with oxygen nanobubble water, and at the end of the experiment, the pH value of the 0 to 5 cm soil layer decreased significantly, probably due to an increase in redox potential. The effect of nanobubbles on pH and its impact on oxidizing capacity of OMNBs needs to be further investigated.

In summary, in ozone-assisted uranium leaching, the oxidative mechanism is highly dependent on pH, dictating the primary reactive species and their efficacy. Under very acidic conditions, molecular ozone (O₃) acts directly as a potent and relatively selective oxidant, effectively attacking uranium-bearing minerals like UO₂. Under neutral and alkaline conditions, ozone rapidly decomposes in water to generate highly reactive but non-selective hydroxyl radicals (•OH). The primary knowledge gaps center on the •OH oxidation pathway: the quantitative yield of •OH in realistic ore matrices, its kinetics for oxidizing solid-phase UO₂ versus competing reactions with other mineral phases, and the overall efficiency of this indirect oxidation process compared to the direct action of ozone at very low pH. Understanding these radical-driven pathways is essential for optimizing oxidant delivery and predicting potential detrimental side reactions, such as the oxidation of gangue minerals that can consume oxidants and harm permeability.

6.2.2. Issues That Limit the Field Applications of OMNBs During ISL Process

Although laboratory-scale tests have demonstrated the strong oxidizing properties of OMNBs and their potential to enhance uranium oxidation efficiency, there has been little exploration and utilization of OMNBs at an industrial scale. Field test conditions are complicated and experimental costs are high, and a pilot experiment is necessary to evaluate the practical effectiveness and accumulate operational experience. Compared with macro bubbles, OMNBs are smaller in size and exist in solution for a longer period of time. As a result, their migration in the medium needs to be taken into account. However, few scholars have studied flow of discrete and dispersed MNBs [131]. Meanwhile, during the ISL process, OMNBs mainly move through the pores within reservoir rock, which possesses a complex pore size distribution, thus necessitating study of the migration characteristics of OMNBs in multiple porous media [132]. In addition, the transport of OMNBs in saturated porous media is affected by numerous factors, such as pH, ionic species, gas species, injection rate, etc. Consequently, future research should focus on investigating the migration patterns of MNBs in porous media to establish a coupled model that accounts for migration, reaction, and mass transfer, thereby advancing the application of OMNBs in ISL technology.

One challenge that limits field application of OMNBs for uranium leaching is to understand the complex multiphase flow dynamics governing ozone bubble transport through heterogeneous porous media, addressing how factors like pore throat size distribution, capillary forces, and inherent permeability variations can lead to channeling and influence the radial reach of the bubbles from the injection well. Furthermore, formation water chemistry can affect the efficiency of uranium leaching by ozone bubbles. For example, Ionic strength, the presence of organic matter, etc., can critically impact bubble stability. High ionic strength can accelerate coalescence, thereby affecting their longevity and effectiveness of in situ leaching. The presence of natural organic matter (NOM) in the formation water presents a dual and competing effect on the efficiency of ozone microbubbles in enhancing uranium mineral dissolution. On one hand, NOM can act as a natural surfactant, adsorbing at the gas–liquid interface. This adsorption can enhance microbubble stability by reducing coalescence and increasing residence time of MNBs in the reservoir, which is potentially beneficial for delivering ozone deeper into the ore body. On the other hand, NOM constitutes a large pool of ozone-demanding substances. Ozone will preferentially and rapidly react with NOM rather than targeting the desired oxidation of insoluble U(IV) to soluble U(VI). This side reaction leads to a rapid and inefficient consumption of ozone, drastically reducing the oxidant available for leaching.

Ozone micro and nanobubbles may create a challenge in restoring the aquifer to baseline conditions after mining by oxidizing other redox-sensitive contaminants. The application of ozone in in situ U leaching introduces a significant complication: its non-selective oxidation power can adversely affect the reservoir geochemistry by reacting with other reduced minerals commonly associated with uranium ores. When injected, ozone will not only target the desired uranium (U(IV)) oxidation but will also readily oxidize species such as pyrite (FeS₂), which generates sulfuric acid and releases ferric iron (Fe³⁺), potentially causing unwanted aquifer acidification and precipitation of iron hydroxides that can coat mineral grains and clog pores. This competitive oxidation consumes large quantities of ozone, drastically reducing the oxidant efficiency for the target uranium reaction and increasing operational costs. Moreover, the oxidation of other minerals can mobilize undesirable trace metals like arsenic and selenium, complicating groundwater management and post-leaching restoration efforts.

Another challenge of using ozone bubbles for in situ uranium leaching lies in achieving effective pore-fluid mixing within the complex, water-saturated porous medium of the ore body. While ozone is a powerful oxidant, its delivery as a gaseous phase creates fundamental inefficiencies; the significant disparity in density and viscosity between the gas bubbles and the liquid lixiviant causes the ozone to channel preferentially through the most permeable pathways, bypassing uranium trapped in lower-permeability zones and leading to poor recovery. Furthermore, capillary forces can trap bubbles, potentially blocking fluid flow, while the overall process is severely limited by mass transfer—the rate at which ozone dissolves from the gas phase into the liquid and diffuses to the mineral surfaces is often the limiting step, not the oxidation reaction itself. Consequently, despite its chemical advantages, the practical application of ozone is hampered by these inherent fluid dynamics issues, such as channeling, bypassing, and inefficient mass transfer.

Clay swelling may also be a problem in in situ U leaching by ozone. Clay swelling is heavily influenced by the chemistry of the surrounding water. When the concentration of ions in the water (the ionic strength) is high, clays remain stable. When the water is diluted (low ionic strength), clays absorb water and swell. Ozone itself is not a primary cause of clay swelling. However, if injection of ozone causes ionic strength change in formation water due to oxidation reactions of minerals by ozone, clay swelling may be triggered.

Advancements in material synthesis technology have led to the development of diverse methods for preparing MNBs. However, controlling the concentration and size of the bubbles remains challenging, thereby constraining the utility of OMNBs. Additionally, the majority of MNB generators are either homemade or assembled, with their core components obtained according to the principles of bubble generation. High-quality MNB generators are costly, and acquiring suitable bubble generators directly for specific purposes and large-scale applications is challenging [36]. Utilizing ozone bubbles in ISL process also faces cost challenge due to the difficulty in generating large amounts of ozone. The ISL process is a large-scale mineralogical process, in which the amount of the injected leaching solution is often tremendous. Ozone is commonly produced on-site using energy-intensive methods such as corona discharge or ultraviolet (UV) light systems. These technologies require substantial electrical input. In large-scale ISL operations, where continuous ozone injection is necessary to enhance mineral oxidation and recovery, the cumulative energy demand can lead to super high operational costs. Additionally, ozone has a short half-life, decomposing rapidly into oxygen, which necessitates constant regeneration to maintain effective concentrations in the leaching solution. This further escalates costs, as the system must compensate for ozone losses in real time. Therefore, there is a pressing need for a cost–benefit assessment of OMNB applications in the field.

7. Conclusions and Future Perspectives

The need for technologies to economically extract uranium resources in an environmentally friendly manner has drawn substantial attention from both the academic and industrial spheres. Enhanced uranium extraction through the use of OMNBs represents an innovative approach owing to their potent oxidizing properties, high mass transfer efficiency, and prolonged presence in the liquid. During ISL process, uranium extraction requires not only oxidizing and complexing agents, but is also affected by various other factors (pH, oxidant concentration, catalyst, etc.). The application of OMNBs in oxidization leaching of uranium fulfills these conditions and provides a novel, efficient and profitable route for uranium resource recovery.

However, there has been insufficient research on the reaction mechanism between OMNBs and uranium ore, and the cost of OMNBs during ISL method, especially the lack of field practice and life cycle assessment. The process of uranium extraction via the ISL method is complex, involving a significant interplay of physics and chemistry. But the lack of understanding may impede the development and application of this advanced technology.

Therefore, future research should focus on the following:

(1)

Studying the reaction mechanism: Investigating the reaction mechanism of OMNBs with uranium ore under varying pH conditions, particularly focusing on the primary oxidizing components. This will provide valuable insights for optimizing test conditions and will help in understanding the underlying chemical processes, and aid in refining the application of OMNBs in uranium extraction.

(2)

Feasibility, economics, and risk assessment: Conducting a comprehensive evaluation of the feasibility and risks associated with the ISL method using OMNBs from economic cost and environmental perspectives is essential. This assessment will guide engineering practices and provide a clear understanding of the potential benefits and challenges of implementing OMNB-based uranium extraction on a larger scale.

(3)

Updating MNB generating devices: Improving the design of bubble generating devices to produce MNBs with controllable sizes and concentrations is crucial for reducing operational costs and enhancing overall efficiency.