Development of Technology for the Bioleaching of Uranium in a Solution of Bacterial Immobilization

Abstract

This study presents findings regarding the kinetics of ferrous iron oxidation in solution mediated by Acidithiobacillus ferrooxidans bacteria within a continuous-flow bioreactor employing diverse types of immobilizers. The objective is to augment the rate of ferrous iron oxidation in solutions utilizing an immobilizer for Acidithiobacillus ferrooxidans strains. Immobilization represents a promising avenue for enhancing the efficiency of Fe²⁺ oxidation via acidophilic ferrooxidizing bacteria, leading to a several-fold increase in oxidation rate. A comparative analysis was conducted to evaluate the efficacy of different types of immobilizer in facilitating iron oxidation within a continuous-flow bioreactor, including the application of wood chips coated with Fe(OH)₃. The results indicate that wood chips coated with iron hydroxide serve as effective type of immobilizer, facilitating the robust attachment of Acidithiobacillus ferrooxidans via electrostatic interactions between negatively charged bacteria and positively charged surfaces. Experimental investigations were conducted using novel immobilization matrices in pilot-scale tests simulating the underground borehole leaching (UBL) of uranium. The bioactivation of leaching solutions enhances the efficiency and environmental compatibility of UBL compared to conventional chemical oxidation methods. The relationships between redox potential and ferric iron content in bioactivated solutions during the UBL of uranium were delineated. The significance of this study lies in its elucidating the pivotal role of Fe²⁺ oxidation in uranium extraction processes, particularly in the context of UBL. By employing bioactivation mediated by Acidithiobacillus ferrooxidans, the study demonstrates not only enhanced uranium extraction efficiency, but also markedly improved environmental sustainability compared to traditional chemical oxidation methods. The findings reveal crucial correlations between redox potential and ferric iron concentration in bioactivated solutions.

1. Introduction

The oxidation process of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) holds significance. For instance, Fe³⁺ finds utility in the metallurgical extraction of metals from ores and serves as a pivotal constituent of numerous wastewater treatment methodologies. The oxidation of Fe²⁺ can be achieved through diverse methods, encompassing chemical oxidation, electrochemical oxidation, and biological oxidation.

The utilization of an immobilizer leads to an increase in the concentration of the active biomass of iron-oxidizing bacteria in the bioreactor. This process involves the oxidation of ferrous iron to ferric iron. The resulting biocomplex, in the form of ferric iron, when introduced into the ore body, oxidizes the poorly soluble tetravalent uranium to its soluble hexavalent form, which is then carried to the surface in a productive solution.

The utilization of ferric iron as a biogenic oxidizer yields economically and environmentally efficient outcomes compared to chemically obtained ferric iron. Indigenous strains of A. Ferrooxydans, obtained from the ore deposit, readily adapt to the solution in the bioreactor, subsequently becoming activated and immobilized on the immobilizer.

Kazakhstan’s uranium mines annually expend significant sums on the procurement of chemical oxidizers (hydrogen peroxide, etc.). Utilizing iron biooxidation increases the uranium content in the productive solution by 40%. Investigating and determining the use of a bacterial immobilizer to enhance the bacterial concentration in the bioreactor is a pertinent task.

The literature on the utilization of bacterial strains A. Ferrooxydans via an immobilizer in uranium production remains limited. Existing studies primarily focus on the use of polymetallic ores and gold deposits in the disruption of the crystalline lattice of refractory gold-bearing sulfide minerals.

Chemical oxidation [1] involves the loss and acquisition of electrons by substances in aerobic and anaerobic conditions and is used in the creation of chemical compounds. Electrochemical oxidation, accompanied by electron transfer between substances through electrodes in an electrolyte, is employed for energy generation and compound synthesis. Biological oxidation [2] plays a significant role in the energy exchange of living organisms, facilitating the processing of nutrients.

The biological oxidation of Fe²⁺ plays a pivotal role in the technology used in the bioleaching of various metals from ores, including uranium. In this process, iron-oxidizing bacteria, such as Acidithiobacillus, are involved. Ferrooxidants convert ferrous iron to ferric iron (from Fe²⁺ to Fe³⁺). During underground in situ uranium leaching, ferric iron interacts with tetravalent uranium (insoluble) in the ore body, transforming it into soluble compounds. This method provides an efficient and, in some cases, economically advantageous alternative to traditional approaches to uranium extraction using expensive chemical oxidizers (hydrogen peroxide and others).

To convert poorly soluble tetravalent uranium from ores into a soluble form in leaching solutions, various methods are employed, including chemical oxidizers and iron-oxidizing bacteria. The effectiveness of iron-oxidizing bacteria is associated with their increasing concentration in the bioreactor and their access to oxygen using immobilizers. Immobilizers contribute to the increase in bacterial concentration in the bioreactor by attaching bacteria to them and increasing their contact with oxygen. The surface modification of immobilizers, such as with iron hydroxide, enhances bacterial attachment, leading to the more efficient oxidation of ferrous iron. Thus, the hypothesis was proposed that modifying the surface of the immobilizer would allow bacteria to attach more effectively. Iron hydroxide was applied as a surface modification to the immobilizer. This substantial effect is due to the fact that the trivalent iron was formed biologically and will exhibit a higher affinity for bacteria.

On the other hand, the influence of trivalent iron obtained through iron-oxidizing bacteria requires detailed investigation. Therefore, the task was to examine the effect of trivalent iron obtained in the bioreactor on the uranium content in the productive solution during UBL. It is known that tetravalent uranium is sparingly soluble in sulfuric acid solutions. Trivalent iron serves as an effective oxidizer of tetravalent uranium, converting it into the soluble hexavalent form, which is easily soluble in leaching solutions. Pilot trials were conducted to confirm the scientific hypothesis regarding the significant impact of biologically obtained trivalent iron on increasing the uranium content in the productive solution at the geotechnological field of a uranium mine. Extracting tetravalent, sparingly soluble uranium from the ore body may increase the recovery of uranium from the uranium deposit, leading to a more comprehensive development of the deposit.

The main objectives of our research are as follows: (a) increase the rate of ferrous iron (Fe²⁺) oxidation in solutions using modified surface immobilizers for strains of Acidithiobacillus ferrooxidans bacteria; (b) study the effect of the bioactivation of leaching solutions in a flow-through bioreactor with an immobilizer on the uranium content in the productive solution during the UBL of uranium.

To achieve these goals, various types of immobilizers in a flow-through bioreactor were studied and compared to determine the most effective method for increasing the rate of Fe²⁺ biooxidation. Pilot trials were also conducted to determine the effect of ferrous iron biooxidation on the uranium content in the productive solution.

Although previous studies have focused on various aspects of these processes, ta critical gap trmsind in the understanding of the efficiency and stability of these biological systems, especially in dynamic environments such as flow-through bioreactors used for uranium extraction. This study aims to fill this gap by examining the effectiveness of different immobilizers in enhancing the stability and activity of Acidithiobacillus ferrooxidans in flow-through bioreactors.

Previous studies have established that in the presence of solid carriers [3] (immobilizers), a large portion of Acidithiobacillus ferrooxidans attaches to the surface of the solid carrier. When using iron biooxidation in continuous bioreactors, there is also the issue of bacteria dissociating from the surface of the immobilizing agent at high flow rates of rust through the bioreactor. To stabilize their position and ensure long-term operation, it is necessary to use a robust holder that will act as an effective immobilizer. To stabilize their position and ensure their prolonged operation, a robust holder is required, which will serve as an effective immobilizer. The use of an effective immobilizer ensures the reliable fixation of bacteria on the surface, which is important for optimizing the process and increasing its productivity. Another primary goal of the research is to increase the rate of ferrous iron (Fe²⁺) oxidation in solutions using immobilizers for strains of Acidithiobacillus ferrooxidans bacteria. This is achieved by studying and comparing various types of immobilizers in a flow-through bioreactor to determine the most effective method of increasing the rate of Fe²⁺ biooxidation.

The presented analytical review of publications on the use of bacterial leaching in the hydrometallurgical processing of uranium ores indicates that this technology is underexplored, yet promising and applicable to ores with few valuable components and high-cost chemical reagents. As an alternative to the use of chemical oxidizers in the uranium extraction process, bacterial oxidation is characterized by its low operating costs and environmentally friendly process. However, its drawback lies in its low oxidation rate, limitations regarding the living conditions of microorganisms, and its dependence on bioreactor parameters and operating modes. Based on a critical analysis of the literature data and our own research, conducted at a uranium mine, the necessary steps to develop an economically efficient uranium extraction technology using bacterial–chemical processes were formulated.

Several related studies have been conducted: A. Ahmadi et al. studied fuzzy logic models for predicting the efficiency of the biological leaching of copper concentrates in stirred tank reactors [4]; M. Ranjbar et al. described the kinetic modeling processes of the biological leaching of sulfide copper concentrates in traditional and electrochemically controlled systems [5]; P. Botane et al. discussed material distribution by size during simultaneous biological leaching and precipitation in experimental methodology and modeling [6]; HR Watling et al. reviewed the bio-hydrometallurgical extraction of metals from polymetallic ores [7].

Several studies address various aspects of this field: I.V. Algunova et al. assessed the effectiveness of using burnt rocks as a medium for water deironing [8]; A.G. Bulaev et al. evaluated the impact of organic substrates on the activity of Ferroplasmaceae archaea [9]; S.N. Gladenov et al. analyzed filtering materials in practical applications [10]; HH Gubeydullin and I.I. Shigapov compared the use of flat and tubular textile filter screens [11]; M.D. Konev et al. analyzed iron content in water and the methods of its removal [12]; B.E. Ryabchikov et al. used water treatment for juice production [13]; V.I. Fedorenko et al. deironized technical water using multilayer filtration [14]; E.K. Filipov et al. used dual-layer loading in contact filters [15]; T.S. Khainasova reviewed kinetic models of ferrous iron oxidation by acidophilic chemolithotrophic microorganisms [16]; A.A. Balykov et al. studied microbe immobilization and iron oxidation by an immobilized cell biomass [17].

The use of a bioreagent derived from a concentrated bacterial biomass fixed on solid supports increases the rate of biooxidation several times, raising it from 1–2 to 10–15 g/L of iron per hour [18]. The efficiency of biosynthesis directly impacts the costs of carrier materials, reagents, electricity, bioreactor sizes, etc., which, in turn, determines the economic viability of the heap bioleaching process. Microorganism-mediated bioleaching demonstrates high metal extraction efficiency from ores. Research conducted by Baybatsha A. et al. [19] explored the geological and mineralogical characteristics of ore tailings, revealing their potential in biotechnology. A molecular characterization of Acidithiobacillus ferrooxidans, conducted by Wu X. et al. [20], aids in selecting optimal strains for processing specific ores. Mukherjee S. et al. [21] studied the interaction between Acidithiobacillus ferrooxidans and minerals, paying special attention to argentorrozite synthesis. Edwards B.A. et al. [22] analyzed the bacterial oxidation of Fe(II) and subsequent precipitation of Fe(III), highlighting the role of microorganisms in these reactions. Bryce S. et al. [23] investigated anaerobic Fe(II) oxidation by microorganisms, delving into the ecological and mechanical aspects of this process. Tian Y. et al. [24] confirmed the possibility of bioleaching rare earth elements from phosphate ores, expanding the application horizons of this technology.

The aim of this research is to determine the optimal parameters for the biooxidation of ferrous iron in the bioreactor with an immobilizer, including the bacterial biomass concentration, access to atmospheric oxygen, bacterial interaction effects with the immobilizer surface, and optimal bioreactor environment.

The oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) is crucial for uranium extraction, as Fe³⁺ actively participates in the leaching processes of uranium from ore, making it more accessible for extraction.

This process allows for more efficient and environmentally friendly uranium mining, reducing dependence on chemical oxidants. The enhanced oxidative activity of the bioreagent synthesized by iron-oxidizing bacteria, used for metal leaching, is likely determined by the most energetically favorable conformation of molecules and consists of a higher partial charge on the iron atom and lower energy of the free molecular orbital of the bioreagent molecule, facilitating a greater charge transfer when interacting with quadrivalent uranium.

The biological oxidation method utilizing bacteria, such as Acidithiobacillus ferrooxidans, provides an environmentally friendly and economically viable alternative to traditional chemical processes. This approach not only increases the uranium content in the productive solution but also reduces the costs and environmental damage associated with the use of chemical oxidants, as practiced in uranium mines in Kazakhstan.

Despite the proven effectiveness of bioleaching in other fields, research on its application in uranium extraction through in situ leaching is limited. Our study aims to optimize the biooxidation process in a bioreactor using specialized immobilizers to enhance the oxidation rate of Fe²⁺ and improve uranium extraction efficiency. This includes examining the biomass concentration, oxygen availability, and bacterial interaction with immobilizers.

The characteristics of in situ leaching, related to the mechanisms of the solution’s passage deep underground in the ore body, necessitate further research to enhance understanding and develop technologies that would contribute to increasing the economic efficiency and environmental safety of uranium mining processes using biological oxidation.

2. Materials and Methods

2.1. Materials and Reagents

The study employed aerobic iron-oxidizing microorganisms, Acidithiobacillus ferrooxidans, which utilize th oxidative reactions of inorganic compounds as an energy source. Bacterial strains were pre-acclimatized to elevated levels of iron and non-ferrous metals in solutions typical for uranium leaching conditions. The bacterial strains used in this study were Acidithiobacillus ferrooxidans strains obtained from the Republican State Enterprise on the Right of Economic Management “Republican Collection of Microorganisms” B-RKM 0767 of the Republic of Kazakhstan (Astana, Kazakhstan). Iron-oxidizing bacteria were cultivated using a thermostat and 9K medium (Silverman and Lundgren’s medium), which is commonly used in this methodology. The physicochemical and physiological properties of iron solutions from oxidized bacteria were established in comparison with solutions without their involvement. An increase in the redox potential and a decrease in the surface tension of the solution due to the binding of iron in bacterial solutions into high-molecular-weight complexes were noted. An increase in oxidative activity and resistance to changes in the external factors of the iron-oxidizing biomass were established as a result of the concentration of bacterial cells via adsorptive immobilization on porous materials. In underground well leaching, the solution is delivered through injection wells. The solution passes through the ore-bearing layer underground and rises to the surface through a solution pumping system. This solution, saturated with dissolved uranium, is referred to as the productive solution.

2.2. Types of Immobilizers

In the study, the following immobilizing agents were used: pyrite, activated carbon, ion-exchange resin, zeolite, wood chips, and wood chips modified with iron hydroxides Fe(OH)₃. The flow rate of the solution through the bioreactor was determined using a laboratory flowmeter. In field conditions, the flow rate through the bioreactor was assessed using an electromagnetic flowmeter, Optima (Glencoe, IL, USA). The residence time of the solution in the bioreactor was determined by calculating the effective volume of the bioreactor (the volume of the bioreactor minus the volume occupied by the immobilizer). The residence time was calculated based on the ratio of the flow rate of the solution to the effective volume of the bioreactor. For the laboratory bioreactor, this was 4 h, and for the pilot-scale industrial bioreactor, it was 2 h. Solution samples were collected at the inlet and outlet of the bioreactors under laboratory conditions and at the pilot plant. In field conditions, the analysis of the solutions entering the bioreactor was conducted in the laboratories of the uranium mine. In laboratory conditions, distilled water and the derived 9K medium were used. The composition of the Silverman and Lundgren 9K medium for Acidithiobacillus ferrooxidans (g/L) includes the following:

• (NH₄)SO₄—3;
• KCl—0.1;
• K₂HPO₄—0.5;
• MgSO₄·7H₂O—0.5;
• Ca(NO₃)₂·4H₂O—0.01;
• FeSO₄·7H₂O—44.2.

2.2.1. Measuring Instruments

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The HI 8314 pH meter, a portable instrument with a pH electrode (pH/mV/T), from Hanna Instruments (Woonsocket, RI, USA), was used for pH measurement.

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The RP meter HM Digital HM Digital RP-200 (HM Digital, Inc., Seoul, Republic of Korea) was utilized for measuring the redox potential of the solution.

2.2.2. Analytical Measurements

Uranium and sulfuric acid contents in solutions were measured in the physicochemical laboratory of the uranium mine using spectroscopy. This method is based on the measurement of light absorption by the solution at specific wavelengths. Uranium was detected via spectroscopy due to the formation of colored complexes with certain reagents, which absorb light at specific wavelengths. The accuracy and sensitivity of the method depend on the selected reagent and measurement conditions.

Advanced analytical techniques and methods were employed in the research, including microscopic and X-ray structural analyses and mass spectrometry of a spark plasma source (performed on a dual-focus instrument JMS-BM2, manufactured by JEOL, Tokyo, Japan). Statistical data processing was conducted using IBM SPSS Statistics software (version: 29.0.0.0 (241)).

2.2.3. Bioreactor

The laboratory flow bioreactor FB-1/3 (Labfirst Scientific, Laguna Hills, CA, USA), designed for microbiological fermentation and cell culture cultivation, was used for laboratory studies. The bioreactor has a working volume of 3 L and is used for developing technological regimes and selecting microorganisms. It is equipped with an automated control that allows for the adjustment of parameters such as the temperature, pH, dissolved oxygen content, and flow rate of the solution. An integrated monitoring and data logging system enables the real-time tracking of key process indicators. Bioreactor materials included stainless steel and borosilicate glass. The change in the solution’s composition in the presence of bacteria was confirmed by studying solutions using infrared spectroscopy methods.

In bacterial solutions, intense stretching vibration bands of OH groups were observed in the range of 3500–3300 cm⁻¹, and deformation vibration bands of OH groups were observed in the range of 1600–1500 cm⁻¹. The presence of organic compounds produced by microorganisms leads to changes in the rheological properties of bacterial iron solutions compared to those obtained through chemical oxidation, resulting in increased viscosity, density, decreased surface tension, and a tendency to form gels upon drying. The impact of the complexes formed on the oxidative activity of the bacterial solutions was established, demonstrating that the redox potential (ORP) of bacterial iron solutions differs by more than 100 mV compared to chemical solutions.

Experimental and industrial tests were conducted at the “Semizbai” uranium mine, a hydrogeological-type uranium deposit located in loose sedimentary rocks at the junction of the northeastern part of the Kazakh Shield and the West Siberian part of the Ural-Siberian Epipaleozoic platform.

2.3. Test Equipment

The BOI-1 installation consisted of three containers. In two of them, there were 10 bioreactors, each with a volume of 1 m3, equipped with special air and BP pipes, as well as regulating valves. These pipes were connected to aerators that evenly distribute the air. The bioreactors contained immobilizers to which bacteria were attached; the attachment mechanism was related to the surface properties of the bacteria and their lipid membranes. Aerators converted the air into bubbles of 1–2 mm in diameter, which helps increase the contact area with the bacteria. The placement of bacterial immobilizers helps to prevent their loss from reactors in a continuous flow mode. All bioreactors were located on a single horizontal plane, ensuring even air distribution. A STALKER ASD-24/260 (manufactured by Alteco, Wuxi, China) compressor with a capacity of 300 m³/h was located outside the containers to supply air to the bioreactors. The air flow volume was regulated by a SIEMENS Micromaster 440 (Siemens, Berlin, Germany) frequency converter. An air extraction system from the aerators, consisting of 150 mm diameter pipes, is installed above them. Air was expelled outdoors using a fan. Drainage pipes of 63 mm diameter were installed, which merged into a common pipe of 160 mm diameter for the continuous flow of active leaching solution (ALS) to storage. The third container, designated for reservoirs, was located below the others, ensuring a natural flow of ALS from the bioreactors. The volumes of the supplied solution were measured using an electromagnetic flow meter. The effectiveness of immobilizers was tested on one of the uranium blocks during the development stage in the geotechnological field of the uranium deposit using the BOI-1 bioreactor (BMC Kazakhstan LLP., Almaty, Kazakhstan) (Figure 1 and Figure 2) [25].

3. Results

3.1. Oxidation Rate of Immobilizers

The results of the laboratory investigations are presented in

Table 1. Results of laboratory tests.

Parameters and Modes	Materials for Immobilization of Acidithiobacillus ferrooxidans
	Pyrite	Activated Carbon	Ion Exchange Resin	Zeolite	Wood Chips	Wood Chips with Fe(OH)3
Material weight, g	500	500	500	500	100	100
The size of the material, mm	2–5 mm	2–5 mm	1.0 mm	2–5 mm	5–10 mm	5–10 mm
Dimensions of the bioreactor, H—height D—diameter	H = 0.7 m D = 76 mm	H = 0.7 m D = 76 mm	H = 0.7 m D = 76 mm	H = 0.7 m D = 76 mm	H = 0.7 m D = 76 mm	H = 0.7 m D = 76 mm
The working volume of the bioreactor, L	3	3	3	3	3	3
Free volume in the bioreactor, %	20	20	20	20	75	75

Method of aeration during immobilization into a bioreactor (dispersant 1–2 mm diameter of bubbles).

. The collected data revealed that the rate of Fe²⁺ oxidation by A. ferrooxidans was augmented with the utilization of an immobilizer. The most elevated oxidation rate was observed for wood chips coated with iron hydroxides (Fe(OH)₃).

The rate of oxidation of divalent iron when using various immobilizers is shown in Figure 3.

It has been determined that the strength of bacterial adhesion depends on the absorbent material. The degree of bacterial attachment to the surface of the immobilizing agent was evaluated based on alterations in the rate of oxidation by biomass after washing the carrier material with five volumes of iron-free nutrient medium.

Following washing, the rate of iron oxidation by the immobilized bacteria on coal decreased by 15%, whereas, on other adsorbents, the reduction in the oxidation rate was approximately 40%.

The observed enhancement in the rate of Fe²⁺ oxidation with the use of an immobilizing agent can be attributed to several factors. The immobilizer offers a substantial surface area for bacterial attachment, thereby increasing the availability of bacteria for Fe²⁺ oxidation. Figure 4 presents images depicting the deposition of iron hydroxide at various magnifications, captured using the scanning electron microscope “JEOL” JSM-6490LA (Tokyo, Japan).

A comparative analysis demonstrated that wood chips coated with iron hydroxide Fe(OH)₃) exhibited the highest effectiveness as an immobilizing agent in this investigation. The superior efficiency of this immobilizer was further validated during pilot tests conducted on the block of the geotechnological field of the uranium deposit using the “bioreactor system” BOI-1 (Figure 5). These studies were conducted during the operational phase of the uranium block during the development stage (passive leaching).

3.2. Data Analysis

A statistical analysis was conducted of the dataset, comprising 323 observations over a period of 9 months on one of the uranium block leaching (UBL) blocks. The analysis revealed a strong correlation between the redox potential (RP) of the solution and the concentration of trivalent iron.

The presented data indicate that the increase in RP in the flow mode reached 430–440 mV, aligning with the predefined objectives of the pilot tests. To further examine the obtained data, an extensive statistical analysis of the interrelationship among the parameters was performed.

From

Table 2. Pearson correlation of indicators of the ALS solution from the bioreactor.

	pH	RP	Fe2⁺	Fe3⁺	H2SO4
pH	1	0.476 1	−0.490 1	0.518 1	−0.829 1
	323	323	323	323	323
RP	0.476 1	1	−0.906 1	0.911 1	−0.237 1
	323	323	323	323	323
Fe2⁺	−0.490 1	−0.906 1	1	−0.952 1	0.240 1
	323	323	323	323	323
Fe3⁺	0.518 1	0.911 1	−0.952 1	1	−0.248 1
	323	323	323	323	323
H2SO4	−0.829 1	−0.237 1	0.240 1	−0.248 1	1
	323	323	323	323	323

¹ The correlation is significant at 0.01 (two-sided).

, it is evident that there is a high correlation coefficient of −0.89 between pH and sulfuric acid content.

Furthermore, the data indicate a high correlation coefficient between RP and trivalent iron content. This suggests that the oxidation of divalent iron to the trivalent state primarily contributes to the elevated RP levels in the activated leaching solution.

From a process perspective, the primary interest lies in the impact of the bioactivation of the leaching solution on the uranium content in the productive solution. A long-term monitoring spanning 9 months on the uranium block confirmed the central hypothesis of this study, revealing that bioactivation of the leaching solution leads to an augmentation in the uranium content in the productive solution. Pilot-industrial tests conducted using the developed flow bioreactors, utilizing the selected immobilizer, on one of the uranium blocks at the UBL of uranium demonstrated that the RP of the activated solution increased from 360 mV to 420–450 mV due to the biooxidation of divalent iron in the leaching solution. A strong correlation between the RP of the solution and the content of trivalent iron was established, with a correlation coefficient of 0.91.

Throughout the research, the hypothesis regarding the effect of the bioactivation of the leaching solution on the uranium content in the productive solution was substantiated. Notably, the uranium content increased by 20% without the addition of extra sulfuric acid.

From the data presented in

Table 3. Pearson correlation of technological indicators of the productive solution on BOI-1.

	pH	U	RP	Fe2⁺	Fe3⁺
pH	1	0.892 *	−0.591	0.355	−0.581
U	0.892 *	1	−0.704	0.437	−0.597
RP	−0.591	−0.704	1	−0.946 *	0.925 *
Fe2⁺	0.355	0.437	−0.946 *	1	−0.899 *
Fe3⁺	−0.581	−0.597	0.925 *	−0.899 *	1

* The correlation is significant at 0.05 (two-sided).

, it is notable that there is a robust relationship between the uranium content and pH in the productive solution, with a correlation coefficient of 0.892. Additionally, the uranium content is substantially dependent on the redox potential (RP), indicated by a correlation coefficient of 0.7, and shows a relatively high correlation with the concentration of trivalent iron in the productive solution. As anticipated, there is a remarkably high correlation between the concentration of trivalent iron and the RP of the solution, with a correlation coefficient of 0.925, thereby confirming the influence of a potent oxidizer (Fe³⁺) on the RP of the solution.

4. Discussion

The use of wood chips coated with iron hydroxide as an immobilizing agent demonstrates remarkable efficiency, enhancing the oxidation rate of ferrous iron to 10–15 g/L of iron per hour, which opens new possibilities for industrial applications. This approach offers advantages in terms of economic efficiency and raw material accessibility, potentially positioning it as a competitive alternative to more expensive and technologically complex solid carriers. In the context of prevailing trends emphasizing sustainable development and environmental protection, incorporating such immobilizing agents into production cycles is a strategy that provides a balance between economic benefits and ecological integrity. The use of immobilizers ensures the increased efficiency of continuous bioreactors, allowing for bacteria to be retained on the surface of the solid carrier even at high flow rates of the solution, reaching 360–450 mV through the biooxidation of ferrous iron. The deployment of continuous bioreactors with an immobilized oxidation system of ferrous iron is promising for various applications. It finds use in metal extraction from ores and wastewater treatment processes. Moreover, the use of an immobilized system based on an immobilizer coated with Fe(OH)₃ provides an additional electrostatic effect, facilitating the attachment of bacteria to surfaces with different surface charges. The immobilizer provides a larger surface area for bacterial attachment, thereby increasing the pool of bacteria available for the oxidation of ferrous iron. This technology, utilizing such immobilizing agents for the bioactivation of the solution, demonstrates promising efficiency, providing a high rate of oxidation of ferrous iron. In biological leaching processes, for instance, as described in studies such as [26] Abnilash et al.’s “Bioreactor Leaching of Uranium from Low-Grade Indian Silicate Ore,” the optimal density of the “pulp” plays a crucial role in enhancing the efficiency of uranium extraction. Maintaining the pulp density at 20–40% ensures optimal contact between microorganisms (such as the enriched culture of A. Ferrooxydans) and the ore, which enhances the rate and efficiency of leaching. Thus, the effective management of pulp density is a critical aspect of maximizing the process efficiency. Additionally, in the study “The Role of Acidithiobacillus ferrooxidans in the Oxidation of Fe(II) Pyrite in Bioleaching Processes” by [27] Kang, Jin-xing et al., it was found that Acidithiobacillus ferrooxidans significantly accelerate the oxidation of Fe(II) in pyrite, thereby enhancing the efficiency of the biological leaching of this sulfide ore.

5. Conclusions

• The use of continuous bioreactors with an immobilizer demonstrates high efficiency, achieving ferrous iron (Fe²⁺) oxidation rates of up to 10–15 g/L per hour.
• Modifying the surface of wood chips with iron hydroxides Fe(OH)₃) improved the adsorption of Acidithiobacillus ferrooxidans bacteria, increasing the oxidation rate of ferrous iron by 37.5% compared to unmodified immobilizers. These results were confirmed during pilot-scale industrial trials, where a 20% increase in uranium recovery was observed in the leachate at one of the uranium sites.
• Pilot tests showed that the redox potential (RP) of the activated solution increased from 360 mV to 420–450 mV, indicating the high efficiency of biooxidation. The correlation between the RP of the solution and the content of trivalent iron (Fe³⁺) was confirmed with a correlation coefficient of 0.91, indicating the direct dependence of the RP level on the concentration of Fe³⁺ in the solution.
• The effect of bioactivation on increasing uranium content in the productive solution was confirmed during tests: the uranium content increased by 20% without additional sulfuric acid, demonstrating an improvement in leaching efficiency through the use of biological oxidation methods.