Geochemical Characteristics and Uranium Neutral Leaching through a CO 2 + O 2 System—An Example from Uranium Ore of the ELZPA Ore Deposit in Pakistan

: Geochemical characterization studies and batch leaching experiments were conducted to explore the e ﬀ ects of a CO 2 + O 2 leaching system on uranium (U) recovery from ores obtained from an eastern limb of Zinda Pir Anticline ore deposit in Pakistan. The mineralogy of the ore was identiﬁed by Electron Probe Micro-analyzer (EPMA) and Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS), showing that pitchblende is the main ore mineral. XRD was also used along with EPMA and SEM characterization data. Experimental results indicate that U mobility was readily facilitated in the CO 2 + O 2 system with Eh 284 mV and pH 6.24, and an 86% recovery rate of U 3 O 8 was obtained. U speciation analysis implied the formation of UO 2 (CO 3 ) 22 − in the pregnant solution. The plausible mechanism may be attributed to the dissolved CO 2 gas that forms carbonate / bicarbonate ion releasing oxidized U from the ore mineral. However, U recovery in the liquid phase was shown to decrease by higher U(VI) initial concentration, which may be due to the saturation of Fe adsorption capacity, as suggested by an increase in Fe concentration with increasing initial U(VI) concentration in the solid phase. However, further studies are needed to reveal the inﬂuencing mechanism of U(VI) initial concentration on U recovery in the solid phase. This study provides new insights on the feasibility and validity of the site application of U neutral in situ leaching. associated (probably as biotite). the geochemistry of U-containing indicates a relatively pre-complexed environment of the investigated ore deposit, where the occurrence of U(IV) was favored as compared to U(VI). It in turn indicates that the injection of oxidant was needed for the mobilization of the reduced form of U during in situ leaching. A similar case has been reviewed and reported by Shamim Akhtar [43], showing that U deposits in Pakistan were formed in a reduced environment. Various studies have reported that U solubility is affected by its speciation and aqueous chemistry including organic matter (OM) concentrations Low OM concentration was shown to promote U solubility, whereas high OM concentration may immobilize U Of all the inorganic ligands, carbonates active to complex U, as compared to as SiO 24+ and PO 43 −


Introduction
Uranium (U) ore deposits are important to society as a primary material for the generation of nuclear power [1][2][3]. Interest in in situ leach (ISL) mining has grown considerably over the last 30 years [4] because of its significant advantages over conventional methods for mining, especially low-grade sandstone-type ore deposits. Research on U mobilization has focused on its migration and mineralization under natural conditions, as well as its impact on the environment [5][6][7][8][9][10][11][12][13][14][15][16][17]. The incorporation or release of U to or from mineral structures is determined primarily by its redox state and mineral solubility [18]. The change in oxidation state from U(IV) to U(VI) usually occurs in nature under oxidizing conditions at the geosphere/atmosphere interface or within the lithosphere from contact

Sample Collection
The material was sourced from the sandstone-type U deposit that existed in the ELZPA, Sulaiman Range, Dera Ghazi Khan, Pakistan. At each sampling location, three replicate samples were collected and the unrepresentative materials were removed. The depth of the investigated boreholes spanned from 90 to 165 m. Samples were coated with wax during the core recovery process to avoid contact with atmospheric oxygen. All samples were transported to the laboratory for further treatment (discussed below).

Sample Preparation
Representative (in composite) ore samples were prepared before being analyzed and for laboratory leaching experiments. Briefly, the natural grain size of the crushed ore sample was used for the leaching batch test. For this study, natural grain size means that the ore sample was first gently loosened without reducing the sand grain size. The corresponding particle size distribution ranges were <0.5 mm, 0.5 mm to 1.00 mm, and >1.00 mm. A total of 36 samples were dried, homogenized, and sieved. All the ore samples were grouped as mixed, high, medium, and low grade on the basis of % U 3 O 8 concentration, as shown in Table 2. The coning and quartering method was used to prepare representative samples. A portion of the homogenized ore material was then milled to 200 mesh size for the determination of % U 3 O 8 in each sample group.

Reagents
All chemicals used in this work were of analytical reagent grade and used without further purification. The solutions and dilutions used for calibration were prepared using deionized water. Lixiviant solutions used for batch leaching experiments were prepared with laboratory tap water (LTW), of which the composition is shown in Table 3. The representative ore sample was first grounded into a final 200 mesh size and pulverized before being analyzed (discussed below).  Table 3. Chemical composition of laboratory tap water (g/L).

Batch Leaching Process
Batch extraction tests typically involve mixing a sample with a specific amount of leaching solution without renewal of the leaching solution [39]. Leaching experiments were carried out in batches with a liquid-to-solid (L/S) ratio of 2 in the autoclave reactor [40]. A Karl Kolb Scientific Technical Supplies autoclave (25 bar, BTR temperature of 300 • C, volume of 0.5 L, Dreieich, Germany) was used. An amount of 150 g of ore sample (TCS-04) was mixed in the prepared 300 mL of lixiviant (liquid) of varying conditions (variable bicarbonate solution concentration), while the ore samples (TCS-03 and TCS-02) were processed under only the best result of HCO 3 − concentration. Experiments were performed to investigate the long-term release of the recalcitrant uranium from the ore under varying lixiviant solution conditions (variable bicarbonate solution concentration), and replicate tests were conducted under the same conditions. The leached solution was sampled at regular time intervals, i.e., from 0.5, 2.5, 7.5, 17.5, 41.5 h to days, and so on, until the equilibrium condition was acquired (i.e., no further change in U 3 O 8 recovery in the leached solution). Leaching tests performed by the researchers indicated that the L/S ratio and grain size were important factors for determining the metal release; however, 24 h was not sufficient for a thorough assessment of leaching [41]. The leaching process was kept free from mechanical agitation at room temperature. The batch sampling process consisted of removing 10 mL of leached solution and filtering it through a 0.45 µm filter (Millipore, Merck, Germany). The pH and Eh were monitored throughout the batch process. The filtered sample was analyzed for U 3 O 8 and HCO 3 − concentration. Immediately after sampling, the sample was kept in a refrigerator until analysis. The HCO 3 − concentration was controlled by adding ammonium bicarbonate (NH 4 HCO 3 ) in the lixiviant solution. The pH of the leaching regulation system was set in the range of 6 to 6.6. The industrial oxygen gas (O 2 ) was injected as oxidant to oxidize U(IV) into U(VI

Characterization Techniques
Alkalinity of the leachate was measured by titration with HCl. The endpoint of the titration was determined by a color change. Briefly, methyl orange and phenolphthalein were used to detect the bicarbonates and carbonates, respectively. The liquid phase was analyzed for U 3 O 8 through fluorimetry and spectrophotometrically using dibenzoyl methan as coloring agent, depending on the concentration of U 3 O 8 in the leached solution. The cations Ca 2+ , Mg 2+ , Fe 2+ , Fe 3+ , Na + , and K + were measured by acid-base titration and flame photometry, respectively. The anions (SO 4 2− , Cl − ) were analyzed spectrophotometrically. All liquid samples were filtered using a syringe. The elemental composition and SiO 2 were analyzed through atomic absorption spectrometer and spectrophotometrically, respectively, while the whole rock was analyzed through XRF. For solid material, U assays were carried out by taking 5 g of pulverized sample of the finely milled to 200 mesh size. The 5 g sample was moistened with (1.00 N) HNO 3 for the removal of carbonates with effervescence, then 2 mL HClO 4 and 10 mL HF were added; the sample was then heated to dry in a microwave oven-digester. Again, 10 mL HNO 3 and 10 mL HF were added and the sample was heated to dry again. Followed by addition of 50 mL of (1.00 N) HNO 3 , and the mixture was leached for one hour, and finally it was diluted to a volume of 100 mL. Inorganic carbon was analyzed through acidification of the finely grounded sample using concentrated HCl followed by back titration with NaOH. Organic carbon (C org.) was analyzed by oxidizing the sample with a K 2 Cr 2 O 7 and H 2 SO 4 mixture followed by back titration using freshly prepared ferrous ammonium sulphates. Total carbons were analyzed in this manner and cross-checked using a sulfur analyzer (CS-800 Eltra, Germany). XRD patterns of the U ore sample were recorded using a diffractometer (Bruker D8 ADVANCE, Karlsruhe, Germany) with CuKα radiation (40 kV, 40 mA) in a continuous scanning mode, and the 2θ scanning ranged from 3 • to 80 • . EVA software (V4.2.1, developed by Bruker Corporation) was used to analyze the mineral composition with XRD data using the default crystallography open database COD 2013. An EPMA (JXA-8230) was used to identify the U ore mineral. A small sample (<0.5 g) of material was drawn from the fraction that was expected to be enriched with U mineral. The feed ore and leached residue sediment samples were characterized using a scanning electron microscope (SEM) (NovaNanao 450, Fei Czech, Co., Ltd.), while the distribution of elements was detected by energy-dispersive X-ray spectrometry (EDS) (Oxford X-Max N ).

Mineral Association and Geochemical Characteristics
Mineral association in the U ore shown by sample TCS-04 is listed in Table 4 and the XRD pattern is shown in Figure 1. It shows that quartz and mica are the predominant phases with percentages accounting for 30.40% and 20.30%, respectively. The percentages of albite and labradorite are 14.40% and 10.80%, respectively. The quartz is angular with a 0.05 to 0.4 cm size suggesting a nearby source. The oligoclase and pyroxene have percentages of 8.0% and 7.7%, respectively, along with orthoclase accounting for 3.40%. Mostly weathered oligoclase shows polycrystalline orthoclase aggregates. The distribution of calcite is very uneven and it can be localized as calcareous cemented masses or calcareous cemented thin layers. The content of calcium carbonate in the non-calcareous uncemented core samples is about 1.50%. The minor minerals including amphibole, kyanite, staurolite, epidote, zircon, magnetite, and hematite possess percentages below unity. The content of magnetite and hematite is 0.4% and 0.10%, respectively. The minor minerals including amphibole, kyanite, staurolite, epidote, zircon, magnetite, and hematite possess percentages below unity. The content of magnetite and hematite is 0.4% and 0.10%, respectively.
The geochemical composition for the four groups' ore samples is shown in Table 5. SiO2 is the major composition with a percentage range accounting for 57.36-68.87%, followed by Al2O3, CaO, and CO2 with percentage ranges of 10.09-14.89%, 4.75-11.85%, and 4.10-10.35%, respectively. The percentages of oxides of Na, K, Mg and Fe are in the range of 1.00-2.00%, and TiO2 is less than unity.  The U mineral in the investigated sample (TCS-04) of ore is mainly silica-bearing pitchblende and its composition analysis through EPMA is listed in Table 6. An electronic photograph is shown in Figure 2. The geochemical composition for the four groups' ore samples is shown in Table 5. SiO 2 is the major composition with a percentage range accounting for 57.36-68.87%, followed by Al 2 O 3 , CaO, and CO 2 with percentage ranges of 10.09-14.89%, 4.75-11.85%, and 4.10-10.35%, respectively. The percentages of oxides of Na, K, Mg and Fe are in the range of 1.00-2.00%, and TiO 2 is less than unity. The U mineral in the investigated sample (TCS-04) of ore is mainly silica-bearing pitchblende and its composition analysis through EPMA is listed in Table 6. An electronic photograph is shown in Figure 2.  The U oxide content results of the examined mixed ore sample (TCS-04) and of the ore samples (TCS-03, TCs-02, and TCs-01) with three different grades (high, medium, and low) are shown in Table  7. The geochemical information about sample TCS-04 reflects an overview of the ore deposit with respect to presence of special element characteristics. This sample has an average of 363 ppm of U3O8 with a U(IV)/U(VI) ratio of 21 (Table 7) and shows the mobility of U in terms of the U oxidation state. This sample analysis shows that 4.55% of U3O8 in the ore deposit may exist in the oxidized form (UO2 2+ ). The oxidized form may be leachable without injecting an oxidant during the leaching process. However, 95.19% of U3O8 in the ore deposit exists in the reduced form (U(IV)), and might be made leachable by injecting an oxidant during the leaching process. A similar composition was observed in the other three groups (TCS-03, TCS-02, and TCS-01), where the oxidized form of U accounted for 12.23%, 9.50%, and 7.08%, respectively. The presence of the U species in ore samples was attributed to the evolution of redox conditions. This is indicated by the Fe species (Fe 2+ and/or Fe 3+ ) being positively correlated to U(VI), as shown in Figure 3. The general reactivity of Fe significantly influences U geochemistry, and Fe (Fe 2+ and/or Fe 3+ ) forms mostly oxide, oxyhydroxide, and S-mineral compounds with its substitution in U-minerals commonly [42]. Moreover, there is no well-defined correlation between the U species and organic carbon. It probably suggests that the oxidized U associated with inorganically dominated carbon as compared to organics. Recently, a similar study was documented from the eastern Suliman range of Dear Ghazi Khan, Pakistan [37,38] showing that U ore was mostly associated with organic matter (probably petroleum) as well as other phases of the ore (e.g., biotite). Consequently, the geochemistry of U-containing minerals indicates a relatively pre-complexed environment of the investigated ore deposit, where the occurrence of U(IV) was favored as compared to U(VI). It in turn indicates that the injection of oxidant was needed for the mobilization of the reduced form of U during in situ leaching. A similar case has been reviewed and reported by Shamim Akhtar [43], showing that U deposits in Pakistan were formed in a reduced environment. Various studies have reported that U solubility is affected by its speciation and aqueous chemistry including organic matter (OM) concentrations [44][45][46]. Low OM concentration was shown to promote U solubility, whereas high OM concentration may immobilize U [47]. Of all the inorganic ligands, carbonates are more active ones to complex U, as compared to others such as SiO2 4+ and PO4 3− [48,49].  The U oxide content results of the examined mixed ore sample (TCS-04) and of the ore samples (TCS-03, TCs-02, and TCs-01) with three different grades (high, medium, and low) are shown in Table 7. The geochemical information about sample TCS-04 reflects an overview of the ore deposit with respect to presence of special element characteristics. This sample has an average of 363 ppm of U 3 O 8 with a U(IV)/U(VI) ratio of 21 (Table 7) and shows the mobility of U in terms of the U oxidation state. This sample analysis shows that 4.55% of U 3 O 8 in the ore deposit may exist in the oxidized form (UO 2 2+ ). The oxidized form may be leachable without injecting an oxidant during the leaching process. However, 95.19% of U 3 O 8 in the ore deposit exists in the reduced form (U(IV)), and might be made leachable by injecting an oxidant during the leaching process. A similar composition was observed in the other three groups (TCS-03, TCS-02, and TCS-01), where the oxidized form of U accounted for 12.23%, 9.50%, and 7.08%, respectively. The presence of the U species in ore samples was attributed to the evolution of redox conditions. This is indicated by the Fe species (Fe 2+ and/or Fe 3+ ) being positively correlated to U(VI), as shown in Figure 3. The general reactivity of Fe significantly influences U geochemistry, and Fe (Fe 2+ and/or Fe 3+ ) forms mostly oxide, oxyhydroxide, and S-mineral compounds with its substitution in U-minerals commonly [42]. Moreover, there is no well-defined correlation between the U species and organic carbon. It probably suggests that the oxidized U associated with inorganically dominated carbon as compared to organics. Recently, a similar study was documented from the eastern Suliman range of Dear Ghazi Khan, Pakistan [37,38] showing that U ore was mostly associated with organic matter (probably petroleum) as well as other phases of the ore (e.g., biotite). Consequently, the geochemistry of U-containing minerals indicates a relatively pre-complexed environment of the investigated ore deposit, where the occurrence of U(IV) was favored as compared to U(VI). It in turn indicates that the injection of oxidant was needed for the mobilization of the reduced form of U during in situ leaching. A similar case has been reviewed and reported by Shamim Akhtar [43], showing that U deposits in Pakistan were formed in a reduced environment.
Various studies have reported that U solubility is affected by its speciation and aqueous chemistry including organic matter (OM) concentrations [44][45][46]. Low OM concentration was shown to promote U solubility, whereas high OM concentration may immobilize U [47]. Of all the inorganic ligands, carbonates are more active ones to complex U, as compared to others such as SiO 2 4+ and PO 4 3− [48,49].

Neutral Leaching
Different leaching methods including neutral, acidic, and alkaline have been reported in U mining sites [50][51][52][53][54][55]. The present study investigated the processing of neutral leaching with the pH value being monitored in the experimental scope. These operational conditions were suggested to favor the application of the neutral leaching since it avoids excessive consumption of reagents (discussed below).

Neutral Leaching
Different leaching methods including neutral, acidic, and alkaline have been reported in U mining sites [50][51][52][53][54][55]. The present study investigated the processing of neutral leaching with the pH value being monitored in the experimental scope. These operational conditions were suggested to favor the application of the neutral leaching since it avoids excessive consumption of reagents (discussed below). was used for the preparation of the lixiviant solution, the U 3 O 8 was determined to be 157 mg/L after equilibrium was obtained, while its initial concentration was of 12 mg/L before starting the batch processing ( Figure S1). For the two tests (#2 and #3), 0.80 g/L and 1.00 g/L of HCO 3 − concentrations were used, respectively, and the U 3 O 8 was determined to be 25 mg/L and 64 mg/L, with an initial concentration of 12 and 5 mg/L before starting the batch processing, respectively. Similarly, in the fourth test (#4), 1.20 g/L of HCO 3 − concentration was used, and the final concentration of U 3 O 8 was determined to be 124 mg/L, with its initial concentration of 3 mg/L. The highest U recovery (86%) was found using a 0.60 g/L of HCO 3 − concentration. The U recovery with a concentration of 1.00 g/L of HCO 3 − was 45%, followed by a 0.8 g/L batch where recovery was 13%. The U recovery with 1.20 g/L of HCO 3 − was 68%. The changes in uranium recovery for the tests are shown in Figure 5.
Consequently, the dissolution of the U was favored more easily with 0.6 g/L of HCO 3 − , and only 8.70 days was needed to achieve high recovery leaching time. The lower limit of HCO 3 − concentrations in the aforementioned tests shows a slightly gentle process fluctuation as compared to those on the higher side, and a general improvement in the recovery was observed as the HCO 3 − concentrations increased from 0.8 to 1.2 g/L during the process. The effect of varying concentrations of the leaching agent (HCO 3 − ) on U recovery was studied by Elizângela and Ana [56]. Results obtained from the authors showed that 1.00 mol/L of HCO 3 − was the best concentration, under which >95% recovery was obtained in 24 h via alkaline leaching, while a general improvement in the recovery was observed as the HCO 3 − concentrations increased. However, the highest recovery of 86%, as a special result, was obtained from test #1 with 0.6 g/L of HCO 3 − . The lowest recovery of the 0.8 g/L HCO 3 − test may be due to lower bicarbonates and the constantly stable concentration of HCO 3 − as U recovery may highly correlated with it [57]. The geochemical analysis results of ore and residuals are shown in Table 8. It can be seen that the CaO concentration in all test residues was decreased significantly. This may be due to the injection of CO 2 gas causing the dissolution of calcite (Figure 6), as shown by Equation (1): as shown by Equation (1): The maximum dissolution of calcium with a concentration of 0.80 g/L of HCO3 − was 2.33%, followed by the 1.0 g/L batch where the dissolution was 1.56%, while U recoveries were 13% and 45%, respectively. In contrast, the calcium dissolution with 0.60 g/L of HCO3 − was 1.14%, and in the 1.20 g/L batch, 0.56% calcite was dissolved, with U recoveries being 86% and 68%, respectively. The relationship between U recovery and calcite dissolution is not totally positively correlated. However, 0.60 g/L concentrations of HCO3 − test have comparatively higher U3O8 recovery than that of 1.20 g/L HCO3 − test with a calcite dissolution range accounting for 1.14% and 0.56%, respectively.     The maximum dissolution of calcium with a concentration of 0.80 g/L of HCO 3 − was 2.33%, followed by the 1.0 g/L batch where the dissolution was 1.56%, while U recoveries were 13% and 45%, respectively. In contrast, the calcium dissolution with 0.60 g/L of HCO 3 − was 1.14%, and in the 1.20 g/L batch, 0.56% calcite was dissolved, with U recoveries being 86% and 68%, respectively. The relationship between U recovery and calcite dissolution is not totally positively correlated. However, 0.60 g/L concentrations of HCO 3 − test have comparatively higher U 3 O 8 recovery than that of 1.20 g/L HCO 3 − test with a calcite dissolution range accounting for 1.14% and 0.56%, respectively.

Effect of pH
The pH decreases quickly and then it remains stable ( Figure 7); in addition, its relationship to Eh shows regular patterns (Figure 8). Different patterns of leaching as a function of pH were also observed ( Figure 9). The leaching recovery of U increased with changing pH in the early stages of the process and then there was no further change in U recovery. For example, in the test using 0.6 g/L of HCO3 − , high U recovery was found, while the pH value reduced from 8 to 6.57. Mean and final values recorded for pH for each of the leaching protocols are shown in Table 9. Probably, the plausible mechanism may be due to the injection of CO2 gas. CO2 dissolved in water results in reducing pH of the system. The carbonate/bicarbonate ion in turn promotes the release of adsorbed U(VI) from the U-bearing mineral. The released U(VI) dissolved into the liquid phase depends upon contact time and mineral charge density. In addition, increasing H + results in the dissolution of gangue minerals like calcite (Figure 6). It can be seen (Figure 9) that the lowest pH value and highest Eh value (most oxidizing conditions) were observed for the best addition of HCO3 − (0.6 g/L). It should also be noted that pH showed almost a negative correlation with recovery (Figure 9a), i.e., more recovery resulted

Effect of pH
The pH decreases quickly and then it remains stable ( Figure 7); in addition, its relationship to Eh shows regular patterns (Figure 8). Different patterns of leaching as a function of pH were also observed ( Figure 9). The leaching recovery of U increased with changing pH in the early stages of the process and then there was no further change in U recovery. For example, in the test using 0.6 g/L of HCO 3 − , high U recovery was found, while the pH value reduced from 8 to 6.57. Mean and final values recorded for pH for each of the leaching protocols are shown in Table 9. Probably, the plausible mechanism may be due to the injection of CO 2 gas. CO 2 dissolved in water results in reducing pH of the system. The carbonate/bicarbonate ion in turn promotes the release of adsorbed U(VI) from the U-bearing mineral. The released U(VI) dissolved into the liquid phase depends upon contact time and mineral charge density. In addition, increasing H + results in the dissolution of gangue minerals like calcite ( Figure 6). It can be seen (Figure 9) that the lowest pH value and highest Eh value (most oxidizing conditions) were observed for the best addition of HCO 3 − (0.6 g/L). It should also be noted that pH showed almost a negative correlation with recovery (Figure 9a), i.e., more recovery resulted towards the lower pH of the neutral range, resulting in the formation of more H + in the solution.
Briganti et al. [28] conducted a study using chemical leaching and adsorption tests with a simple modeling using PHREEQC. They found that the geochemical behavior of U during ignimbrite-water interaction was controlled mainly by temperature, pH, and solution chemistry (especially alkalinity).
The main results of their work indicated that U was more easily mobilized by a slightly basic solution (pH 7.5). However, the results obtained from the research revealed that U was more easily mobilized by a slightly acidic solution (pH range 6 to 6.6) during nearly neutral leaching conditions. In many respects, leaching behavior as reflected by pH leaching tests and related characterization provide a better means of assessing environmental impact than an analysis of total composition, such as how the solubility changes if in situ pH changes occur. The pH is one of the key parameters that determines heavy metal mobility depending upon soil and sediment properties. The aqueous U(VI) carbonate system has been thoroughly studied by many researchers [58][59][60][61][62][63][64]. It is well accepted that the three monomeric complexes of general formula UO 2 (CO 3 ), UO 2 (CO 3 ) 2 2− , and UO 2 (CO 3 ) 3 4− present under the appropriate conditions [65].
In neutral to carbonate media, U is converted into a series of carbonate complexes (UO 2 CO 3 (maximum fraction at pH 5), UO 2 (CO 3 ) 2 2− (maximum fraction at pH 6.5), and UO 2 (CO 3 ) 3 4− (maximum fraction at pH 10 to 11)) when the pH of the solution is increased. Nevertheless, in all the tests performed, the concentration of CO 3 2− was not detected in the leached solution during analysis because of the pH range condition was 6 to 6.6 (which indicates that UO 2 (CO 3 ) 2 2− is the main carbonate complex in the condition studied in this work). In such cases, the recovery of U from a nearly neutral medium is possibly due to the formation of stable U carbonate complex-like UO 2 (CO 3 ) 2 2− [66] via Equation (2): In neutral to carbonate media, U is converted into a series of carbonate complexes (UO2CO3 (maximum fraction at pH 5), UO2(CO3)2 2− (maximum fraction at pH 6.5), and UO2(CO3)3 4− (maximum fraction at pH 10 to 11)) when the pH of the solution is increased.
Nevertheless, in all the tests performed, the concentration of CO3 2− was not detected in the leached solution during analysis because of the pH range condition was 6 to 6.6 (which indicates that UO2(CO3)2 2− is the main carbonate complex in the condition studied in this work). In such cases, the  This is a stable U compound in the determined pH range. Many researchers have reported that when pH > 6, U(VI) is dominant in the presence of more complexes such as uranyl hydroxyl complexes and polynuclear uranyl hydroxyl complexes, and the reactivity of Fe inhibits U(VI) [67,68]. It may be the advantage of relatively less Fe (Table 4) in the investigated ores in terms of the experimental scope (pH 6 to 6.6). Under near-neutral conditions, U forms soluble complexes with carbonate and phosphates. Ma et al. [31] conducted an adsorption study of the U species and reported that these species are also influenced by the charge of the mineral surface, depending on the pH of the sorbate solution. Recently, Zhou et al. [46] reported from ISL research from mining sites that, as the pH value decreases, the adsorbed UO 2 2+ can be replaced by protons and U concentration increases significantly in the leached solution.

Effect of U(VI) Initial Concentration
The effect of different U(VI) initial concentrations (in the solid phase) on U 3 O 8 recovery was investigated. The results are listed in Table 10 and shown in Figure 10, showing that recovery was 86% at the lower initial U(VI) concentration (sample TCS-04). A higher U(VI) initial concentration in the solid phase leads to a lower recovery of U 3 O 8 ( Figure 10). This may be due to the saturation of Fe 2+ + Fe 3+ adsorption capacity as its concentration increases with increasing initial U(VI) concentration in the solid phase (Table 10). As the concentration of U(VI) increases in the liquid phase, UO 2 2+ ions compete for adsorption sites or available functional groups [69]. It is reported that adsorption capacity continues to increase with the rise of initial U(VI) concentration, which may be due to the presence of more U(VI) ions around Fe [70]. Laboratory experiments focusing on U chemical species have been previously done as well for the purpose of researching in situ leaching of U ore, and have reported that the recovery of leaching U is decided jointly by both U content and its activity, according to Ma et al. [31]. Probably, it seems that another mechanism may be due to the presence of different U(VI) initial concentrations in the solid phase, as it is negatively correlated with U 3 O 8 recovery ( Figure 10). A similar nature of correlation between U(VI) initial concentration (in the solid phase) and leaching recovery was examined under alkaline leaching. A total of five samples with varying U(VI) initial concentrations were tested. These were the five ore samples (FR1T-860C-28, FR1T-859C-24, FR1T-860C-26, FR1T-860C-20, and FR1T-860C-24, Table 11) out of a total of 36 collected ore samples, which were used in the neutral experimental study after sample grouping. The objective was to confirm the effect of this special influential parameter on leaching recovery comparison as a trial through other leaching media. The results are listed in Table 11 and shown in Figure 10, showing that recovery was 58% at the lower initial U(VI) concentration, while the recovery decreased by increasing U(VI) initial concentration in the solid phase. The parallel leaching experimental study showed a similar negative correlation of U(VI) initial concentration with U 3 O 8 recovery in the leached solution.

Effect of Redox Conditions
Trends of redox potential (Eh) values are shown as a function of time ( Figure 11). Different patterns of oxidation as a function of Eh were observed. The test with 0.6 g/L of HCO3 − has a high leaching rate, and the Eh value was enhanced more than three times (from 90 to 284 mV), while the test with 1.2 g/L of HCO3 − enhanced the Eh value less than three times and a similar Eh increment was observed in the tests with 0.80 and 1.0 g/L of HCO3 − concentrations, as shown in Figure 12. For the most part, the leaching of U increased with increasing Eh, although there was then no further change in U recovery at slightly increased Eh. Mean and final values recorded for Eh for each of the leaching protocols are shown in Table 9. It should also be noted that Eh showed almost a positive correlation with recovery, i.e., a higher recovery rate in higher oxidizing conditions at the later stage of the process (Figure 12), in the test with 0.6 g/L of HCO3 − . The slope of the figure from different curves was 0.409, 0.050, 0.204, and 0.410 against HCO3 − concentrations of 0.6, 0.8, 1.0, and 1.2 g/L, respectively. The Eh was almost directly affecting U mobilization. It is well accepted that Eh of the solution is the key factor affecting the dissolution of tetravalent uranium during the leaching of U, which is related to the composition and content of the variable valence ions in the solution. Zhou et al. [46] reported that change in the uranium concentration in the leached solution is time-lagged and its peak occurred synchronously with that of high Eh.
. Figure 10. Trend of uranium recovery (in leaching solution) with initial U 6+ concentration (conc.) in rock samples.

Effect of Redox Conditions
Trends of redox potential (Eh) values are shown as a function of time ( Figure 11). Different patterns of oxidation as a function of Eh were observed. The test with 0.6 g/L of HCO 3 − has a high leaching rate, and the Eh value was enhanced more than three times (from 90 to 284 mV), while the test with 1.2 g/L of HCO 3 − enhanced the Eh value less than three times and a similar Eh increment was observed in the tests with 0.80 and 1.0 g/L of HCO 3 − concentrations, as shown in Figure 12. For the most part, the leaching of U increased with increasing Eh, although there was then no further change in U recovery at slightly increased Eh. Mean and final values recorded for Eh for each of the leaching protocols are shown in Table 9. It should also be noted that Eh showed almost a positive correlation with recovery, i.e., a higher recovery rate in higher oxidizing conditions at the later stage of the process (Figure 12), in the test with 0.6 g/L of HCO 3 − . The slope of the figure from different curves was 0.409, 0.050, 0.204, and 0.410 against HCO 3 − concentrations of 0.6, 0.8, 1.0, and 1.2 g/L, respectively. The Eh was almost directly affecting U mobilization. It is well accepted that Eh of the solution is the key factor affecting the dissolution of tetravalent uranium during the leaching of U, which is related to the composition and content of the variable valence ions in the solution. Zhou et al. [46] reported that change in the uranium concentration in the leached solution is time-lagged and its peak occurred synchronously with that of high Eh.

Conclusions
This investigation included a geochemical characterization study and nearly neutral (pH: 6-6.6) leaching mechanisms of uranium (U) ore from the ELZPA deposit. The ore mineral was determined to be dominated by pitchblende, as revealed by EPMA and SEM-EDS analytical results. Leaching experiments with a CO2 + O2 system were performed to investigate U mobility and transformation. Parameters of pH, Eh, alkalinity, and major ion(s) were shown to influence U mobilization. U3O8 recovery was achieved to a high level (86%), mainly in the form of a stable UO2(CO3)2 2− complex in the leached solution. The plausible U migration mechanism may be due to the oxidizing conditions, and the dissolved CO2 gas via a formation of protons releasing oxidized U form the uranium-bearing minerals. However, the injection of CO2 may cause a dissolution of gangue minerals (i.e., calcite).

Conclusions
This investigation included a geochemical characterization study and nearly neutral (pH: 6-6.6) leaching mechanisms of uranium (U) ore from the ELZPA deposit. The ore mineral was determined to be dominated by pitchblende, as revealed by EPMA and SEM-EDS analytical results. Leaching experiments with a CO2 + O2 system were performed to investigate U mobility and transformation. Parameters of pH, Eh, alkalinity, and major ion(s) were shown to influence U mobilization. U3O8 recovery was achieved to a high level (86%), mainly in the form of a stable UO2(CO3)2 2− complex in

Conclusions
This investigation included a geochemical characterization study and nearly neutral (pH: 6-6.6) leaching mechanisms of uranium (U) ore from the ELZPA deposit. The ore mineral was determined to be dominated by pitchblende, as revealed by EPMA and SEM-EDS analytical results. Leaching experiments with a CO 2 + O 2 system were performed to investigate U mobility and transformation. Parameters of pH, Eh, alkalinity, and major ion(s) were shown to influence U mobilization. U 3 O 8 recovery was achieved to a high level (86%), mainly in the form of a stable UO 2 (CO 3 ) 2 2− complex in the leached solution. The plausible U migration mechanism may be due to the oxidizing conditions, and the dissolved CO 2 gas via a formation of protons releasing oxidized U form the uranium-bearing minerals. However, the injection of CO 2 may cause a dissolution of gangue minerals (i.e., calcite). Comparatively, the overall uranium recovery in different ore group tests was significantly negatively correlated with prevalent organic complexed oxidized U in the solid phase under nearly neutral and alkaline leaching environments. Therefore, further studies are needed to explore such influential parameters within the research scope.