Separation of Trace Radium from Thorium-Rich Systems via BaSO₄ Co-Precipitation

Abstract

²¹²Pb is an important medical radionuclide for targeted alpha therapy, and its reliable supply depends on the efficient production of parent nuclides such as ²²⁸Ra, ²²⁸Th, and ²²⁴Ra. Natural thorium resources are abundant and represent a potential source of these radionuclides. However, the separation and enrichment of trace radium from thorium-rich high-salinity systems remain challenging due to extremely low radium concentrations and Th/Ra mass ratios on the order of 10⁹. In this work, a radium separation strategy based on BaSO₄ co-precipitation was developed. The precipitation behavior of BaSO₄, precipitation kinetics, radium co-precipitation efficiency, and thorium recovery in concentrated thorium nitrate solutions were systematically investigated. The results show that elevated ionic strength and competitive interactions between Th⁴⁺ and SO₄²⁻ reduce the effective sulfate activity under high-thorium conditions, making excess sulfate necessary to achieve efficient BaSO₄ precipitation. Under optimized conditions, the radium co-precipitation recovery exceeded 80% at a Ba²⁺ concentration of 3 mM. Meanwhile, thorium exhibited negligible incorporation into the BaSO₄ phase and could be almost completely recovered via subsequent hydroxide precipitation. The proposed method features operational simplicity, use of common reagents, low cost, and compatibility with high-salinity matrices. It provides a feasible technical pathway for the subsequent production of high-purity ²²⁸Th or ²²⁴Ra and the preparation of ²²⁸Th/²¹²Pb or ²²⁴Ra/²¹²Pb generator systems.

1. Introduction

Targeted radionuclide therapy (TRT) has emerged as an important modality in nuclear medicine [1,2]. Compared with conventional β-emitting radionuclides, α-emitting radionuclides exhibit high linear energy transfer (LET) and a short tissue penetration range, enabling highly efficient induction of DNA double-strand breaks in tumor cells [3]. These characteristics make them particularly promising for the treatment of micrometastatic and refractory malignancies [4,5].

Among α-emitting systems, ²¹²Pb has attracted considerable attention due to its decay chain. Its daughter nuclides, ²¹²Bi and ²¹²Po, emit high-energy α particles, while its half-life (10.64 h) is well matched to the pharmacokinetics of many targeting vectors [6]. As a result, ²¹²Pb is regarded as a promising in vivo generator for α therapy. At present, ²¹²Pb is mainly produced via generator systems based on ²²⁸Th/²¹²Pb or ²²⁴Ra/²¹²Pb [7,8]. Therefore, the availability of parent radionuclides is a key factor limiting their broader application.

Natural thorium (²³²Th) is an abundant resource, and its decay series contains several medically relevant radionuclides, including ²²⁸Ra, ²²⁸Th, and ²²⁴Ra, which can potentially serve as precursors for ²¹²Pb production systems (Figure 1) [9]. However, direct separation of ²²⁸Th from natural thorium matrices is extremely difficult. In contrast, separation of ²²⁸Ra followed by ingrowth of its daughter nuclides provides a more practical route for generator construction.

A major challenge arises from the extremely low concentration of radium in thorium ores and processed solutions. The Th/Ra mass ratio can reach up to 10⁹, resulting in a system dominated by high concentrations of Th⁴⁺ and associated nitrate salts. Such conditions lead to high ionic strength, complex speciation, and strong matrix effects, which significantly hinder selective radium separation. Conventional methods, including solvent extraction, ion exchange, and chromatographic techniques [10,11,12,13,14], often suffer from reduced selectivity, limited loading capacity, complex operation, and high reagent consumption under these conditions.

Sulfate co-precipitation is a well-established and widely used approach for radium separation in environmental and industrial systems due to its simplicity and strong enrichment capability [15,16,17,18,19]. Owing to the chemical similarity between Ra²⁺ and Ba²⁺, radium can be effectively incorporated into BaSO₄ crystals via isomorphic substitution during precipitation. However, systematic studies on BaSO₄ precipitation behavior, radium co-precipitation efficiency, and thorium distribution in high-concentration thorium nitrate systems remain limited.

In this work, we propose a BaSO₄ co-precipitation-based strategy for the separation of trace radium from thorium-rich nitrate solutions. The precipitation behavior of BaSO₄, precipitation kinetics, radium co-precipitation efficiency, and thorium recovery characteristics were systematically investigated. This study provides experimental evidence for radium recovery from natural thorium systems and supports the development of ²²⁸Th/²¹²Pb or ²²⁴Ra/²¹²Pb generator systems.

2. Materials and Methods

2.1. Chemicals and Reagents

Thorium nitrate (Th(NO₃)₄·xH₂O), disodium ethylenediaminetetraacetate dihydrate (EDTA-2Na·2H₂O), sodium hydroxide (NaOH), sodium sulfate (Na₂SO₄), nitric acid (HNO₃), and barium chloride dihydrate (BaCl₂·2H₂O) of analytical grade were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China) or Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and were used as received without further purification. A ²²⁶Ra standard solution was supplied by Isotope and Radiation Corporation (Beijing, China). Ultrapure water with a resistivity of 18.2 MΩ (Hitech Master-S, Shanghai, China) was used in all experiments.

2.2. Preparation of Working Solutions

A 100 mM BaCl₂ stock solution was prepared by dissolving 1.2274 g of BaCl₂·2H₂O in 50 mL of ultrapure water. A thorium nitrate solution with a thorium concentration of 20.0 g/L was prepared by dissolving 24.2775 g of Th(NO₃)₄·xH₂O in 500 mL of ultrapure water. An alkaline EDTA solution was prepared by dissolving 15.0 g of EDTA-2Na·2H₂O and 4.5 g of NaOH in 100 mL of ultrapure water and was subsequently used to dissolve the precipitate prior to radionuclide analysis.

The concentrations of metal ions in all solutions were calibrated using inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima Expert, Kyoto, Japan).

All experiments were performed at least in duplicate. The relative standard deviation (RSD) was within ±5% for non-radioactive measurements and within ±10% for radiotracer experiments.

2.3. Barium Precipitation Experiments

Barium precipitation experiments were performed in 100 mL glass beakers. A 12.5 mL aliquot of thorium nitrate solution (20.0 g/L Th) was first added, and the total volume was adjusted to 50 mL using ultrapure water. The solution was stirred at 600 rpm to ensure homogeneity. Sodium sulfate (Na₂SO₄) was then added in stoichiometric amounts according to the designed sulfate concentrations. After complete dissolution, 0.5 mL of 100 mM BaCl₂ solution was added dropwise under continuous stirring. The reaction was maintained under constant stirring for 6 h to ensure precipitation equilibrium. The resulting suspension was filtered through a 0.45 μm membrane filter. The residual Ba²⁺ concentration in the filtrate was determined by ICP-AES to evaluate the precipitation efficiency.

To investigate the effect of thorium matrix concentration on BaSO₄ precipitation behavior, the volume ratios of Th(NO₃)₄ solution, BaCl₂ solution, and ultrapure water were systematically adjusted while maintaining a constant total volume of 50 mL. The dependence of Ba²⁺ precipitation efficiency on sulfate concentration was evaluated under different thorium concentrations.

2.4. Kinetics of BaSO₄ Precipitation

The precipitation kinetics of BaSO₄ were investigated in thorium nitrate solutions with a thorium concentration of 15 g/L. Na₂SO₄ was first dissolved under magnetic stirring at 600 rpm to ensure complete homogenization. Subsequently, a predetermined volume of 100 mM BaCl₂ solution was added dropwise while maintaining the total reaction volume at 50 mL. The suspension was continuously stirred throughout the reaction to ensure uniform mixing. At selected time intervals, 2 mL aliquots were withdrawn from the reaction mixture. Each sample was immediately filtered through a 0.45 μm membrane filter to remove solid phases, followed by appropriate dilution prior to ICP-AES analysis of residual Ba²⁺ concentration.

To minimize disturbance of the system, sampling was performed rapidly and consistently under identical conditions. The temporal evolution of Ba²⁺ concentration was used to evaluate the precipitation kinetics, including the initial precipitation rate, approach to equilibrium, and the influence of the thorium matrix on precipitation behavior.

2.5. Recovery of Thorium from the Supernatant

After completion of BaSO₄ precipitation, the suspension was allowed to stand overnight to ensure complete solid–liquid separation. Subsequently, 20 mL of the supernatant was collected by siphoning. Under continuous magnetic stirring at 1000 rpm, 10 M NaOH solution was slowly added dropwise to the supernatant while the pH was continuously monitored. The addition of NaOH induced the hydrolysis of thorium, resulting in the formation of thorium precipitate. At predetermined pH values, 2 mL aliquots were withdrawn, filtered, and appropriately diluted for ICP-AES analysis to determine the residual thorium concentration. The precipitation efficiency of thorium was evaluated based on its concentration decrease in solution. The evolution of thorium precipitation as a function of pH was used to assess its distribution behavior in the BaSO₄ co-precipitation system. This also provided insight into the feasibility of thorium recovery from high-salinity nitrate media.

2.6. Radiotracer Experiments for Radium Co-Precipitation

The co-precipitation behavior of radium was investigated using a standard ²²⁶Ra solution as a radiotracer, owing to its chemical similarity to ²²⁸Ra. Na₂SO₄ was dissolved in 37.5 mL of thorium nitrate solution (20 g/L), followed by the addition of predetermined volumes of ultrapure water and a ²²⁶Ra standard solution. The mixture was stirred at 600 rpm using a magnetic stirrer. Subsequently, a specified volume of BaCl₂ solution was slowly added dropwise under continuous stirring. After 3 h of reaction, stirring was stopped, and the suspension was allowed to stand overnight to ensure complete phase separation. The supernatant was removed by siphoning, leaving approximately 10 mL of residual suspension, which was then vacuum-filtered through a 0.45 μm membrane filter. Both the filter membrane and the collected precipitate were dissolved in an alkaline EDTA solution.

The radioactivity of ²²⁶Ra in the dissolved solution was determined using the radon emanation method with an FD-125B radon/thoron analyzer (CNCS, Beijing, China). The radium co-precipitation recovery was calculated based on the measured radioactivity, thereby evaluating the enrichment efficiency of the BaSO₄ co-precipitation system.

3. Results and Discussion

3.1. Precipitation Behavior of Barium in High-Concentration Thorium Nitrate Solutions

The precipitation efficiency of barium was evaluated based on the residual Ba²⁺ concentration in the filtrate. A lower residual Ba²⁺ concentration indicates a higher extent of BaSO₄ precipitation. Figure 2a presents the effect of sulfate concentration on Ba²⁺ precipitation efficiency at different thorium concentrations.

As shown in Figure 2a, relatively high residual Ba²⁺ concentrations were observed at low sulfate concentrations, indicating incomplete precipitation of BaSO₄ under sulfate-deficient conditions. With increasing SO₄²⁻ concentration, the residual Ba²⁺ concentration decreased sharply, demonstrating a progressive enhancement in precipitation efficiency. Even in highly concentrated thorium nitrate solutions, nearly complete precipitation of Ba²⁺ was achieved when sulfate was present in excess, suggesting that BaSO₄ precipitation remained effective at elevated thorium concentrations.

A comparison of different thorium concentrations further revealed that higher sulfate concentrations were required to achieve comparable Ba²⁺ removal efficiencies as thorium concentration increased. This trend can be attributed primarily to the elevated ionic strength induced by Th⁴⁺, which reduces the effective activity of sulfate ions and weakens their interaction with Ba²⁺. In addition, competitive complexation between Th⁴⁺ and SO₄²⁻ may further decrease the availability of free sulfate ions, thereby inhibiting BaSO₄ nucleation and crystal growth. Figure 2b shows the influence of initial Ba²⁺ concentration on precipitation behavior at a fixed thorium concentration of 15 g/L. When the sulfate concentration exceeded 80 mM, the residual Ba²⁺ concentrations became nearly identical across different initial Ba²⁺ concentrations. Upon further increasing sulfate concentration to 150–200 mM, the precipitation efficiency reached a maximum, indicating that sulfate was present in significant excess and sufficient to drive complete BaSO₄ formation.

Overall, these results demonstrate that excess sulfate conditions are essential to overcome matrix effects in concentrated thorium nitrate systems, thereby ensuring efficient BaSO₄ precipitation and providing a suitable chemical environment for subsequent radium co-precipitation.

3.2. Kinetic Characteristics of BaSO₄ Precipitation

The precipitation kinetics of BaSO₄ were investigated at a thorium concentration of 15 g/L and a sulfate concentration of 200 mM, using initial Ba²⁺ concentrations ranging from 1 to 3 mM. The corresponding kinetic profiles are shown in Figure 3. The purpose of this experiment was to determine the minimum reaction time required for essentially complete BaSO₄ precipitation under high-thorium conditions, thereby providing an appropriate reaction time for subsequent radium co-precipitation experiments.

As illustrated in Figure 3, BaSO₄ precipitation proceeded rapidly upon the introduction of sulfate ions. Within the first 5 min of reaction, the residual Ba²⁺ concentration decreased to below 1 mg/L, indicating that BaSO₄ nucleation occurred almost instantaneously under the experimental conditions. With increasing reaction time, the residual Ba²⁺ concentration continued to decrease gradually, indicating continued BaSO₄ crystal growth and the progressive establishment of precipitation equilibrium. After approximately 3 h, the Ba²⁺ concentration stabilized at values below 0.2 mg/L, suggesting that the system had effectively reached precipitation equilibrium.

The rapid initial decrease in Ba²⁺ concentration indicates that high supersaturation, induced by excess sulfate, promotes instantaneous nucleation of BaSO₄. In contrast, the slower decrease observed at later stages is attributed to the gradual approach of the precipitation system toward equilibrium, resulting in a further decrease in dissolved barium concentration.

Overall, these results demonstrate that BaSO₄ precipitation in high-concentration thorium nitrate solutions exhibits favorable and reproducible kinetics. A reaction time of 3 h was therefore selected for subsequent radium co-precipitation experiments to ensure complete attainment of equilibrium conditions.

3.3. Behavior and Recovery of Thorium During BaSO₄ Precipitation

The variation of thorium concentration during the BaSO₄ precipitation process was systematically monitored to evaluate its distribution behavior in the co-precipitation system. The experimental results showed that the concentration of Th⁴⁺ in the solution remained essentially unchanged throughout BaSO₄ formation, indicating that thorium did not undergo detectable co-precipitation or incorporation into the BaSO₄ solid phase.

This negligible interaction between thorium and the BaSO₄ precipitate suggests that BaSO₄ precipitation may be advantageous for thorium-radium separation systems. On one hand, it is expected to minimize the loss of thorium during the radium enrichment process, thereby facilitating subsequent thorium recovery. On the other hand, it may reduce the contribution of thorium to the radioactive background in the BaSO₄ precipitate, which is beneficial for subsequent radium purification and handling [20].

To enable thorium recovery and minimize radioactive liquid waste, the thorium remaining in the supernatant was further precipitated by controlled addition of NaOH solution under continuous stirring. The precipitation behavior of thorium as a function of pH is shown in Figure 4.

As shown in Figure 4, a significant decrease in thorium concentration was observed when the pH reached approximately 5.0 ± 0.05, at which point the thorium concentration decreased from 15 g/L to approximately 150 mg/L. Upon further increase in pH toward near-neutral conditions, thorium precipitation became nearly complete. The recovered thorium precipitate can be readily re-dissolved and recycled into the separation process, enabling a closed-loop utilization of thorium resources. This recycling strategy not only improves process sustainability but also reduces the environmental burden associated with thorium-containing waste streams.

Overall, these results demonstrate that the BaSO₄ co-precipitation process enables highly selective radium enrichment while effectively retaining thorium in the liquid phase for subsequent recovery, thereby establishing a practical and sustainable separation-recycle framework for thorium-based systems.

3.4. Co-Precipitation Behavior of Radium in the BaSO₄ System

The co-precipitation behavior of radium in the BaSO₄ system was investigated using a standard ²²⁶Ra tracer solution, and the results are summarized in

Table 1. Radium co-precipitation recovery in the BaSO₄ system at different initial Ba²⁺ concentrations.

Initial Ba2+ Concentration (mM)	Radioactivity of Dissolved Solution (Bq)	Radioactivity of Original Solution (Bq)	Recovery Rate of Ra (%)
1 mM	138.50 Bq	191.26 Bq	72.41%
2 mM	141.70 Bq	191.26 Bq	74.09%
3 mM	154.92 Bq	191.26 Bq	81.00%

.

At an initial Ba²⁺ concentration of 1 mM, the radium co-precipitation recovery exceeded 70%, demonstrating that efficient radium incorporation into the BaSO₄ phase can be achieved even at relatively low barium levels. With increasing Ba²⁺ concentration, the radium recovery gradually increased. When the Ba²⁺ concentration reached 3 mM, the recovery reached approximately 81%. This trend can be attributed to the increased amount of BaSO₄ solid phase formed at higher Ba²⁺ concentrations, which provides a greater number of available lattice sites for radium incorporation via isomorphic substitution. Due to the close chemical similarity between Ra²⁺ and Ba²⁺, radium ions can be effectively incorporated into the growing BaSO₄ crystal lattice during precipitation, rather than remaining in the aqueous phase [21].

The results indicate that radium enrichment can be efficiently achieved under millimolar Ba²⁺ concentrations without requiring excessive reagent consumption, highlighting the practical economic advantage of the proposed approach. Although the single-stage recovery was limited to approximately 81%, the reproducibility of the precipitation process suggests that overall radium recovery could be further improved through multistage co-precipitation or repeated enrichment cycles. Importantly, given the analogous chemical behavior of ²²⁶Ra and ²²⁸Ra, these results provide strong experimental support for extending this method to natural thorium-derived systems for ²²⁸Ra separation. This, in turn, offers a feasible pathway for the subsequent production of ²²⁴Ra and its application as a parent nuclide in ²¹²Pb generator systems.

4. Conclusions

This study systematically demonstrated the feasibility of separating trace ²²⁸Ra from high-salinity thorium nitrate systems using a BaSO₄ co-precipitation strategy, with emphasis on precipitation behavior, kinetic characteristics, radium incorporation, and thorium recovery. After the co-precipitation of radium with BaSO₄, the resulting Ba(Ra)SO₄ precipitate is retained for the ingrowth of ²²⁸Th. The accumulated ²²⁸Th is subsequently separated and used as the parent radionuclide for the preparation of a ²¹²Pb generator. Therefore, efficient recovery of radium by BaSO₄ co-precipitation is a crucial step in the overall production route of ²¹²Pb.

The results showed that BaSO₄ precipitation proceeds efficiently under excess sulfate conditions even in concentrated thorium matrices. Elevated ionic strength and competitive interactions involving Th⁴⁺ require sufficient sulfate excess to ensure complete Ba²⁺ removal. Under optimized conditions, radium co-precipitation efficiency exceeded 80% at a Ba²⁺ concentration of 3 mM, demonstrating effective enrichment of trace radium species at relatively low reagent dosage. Thorium exhibited negligible co-precipitation with BaSO₄, indicating excellent selectivity of the proposed process. Furthermore, thorium in the supernatant could be efficiently recovered by precipitation and recycled, enabling a closed-loop separation strategy and reducing radioactive waste generation. Compared with conventional solvent extraction and chromatographic approaches, the proposed method offers advantages including operational simplicity, low cost, and high applicability to high-ionic-strength systems.

Overall, the BaSO₄ co-precipitation process provides an effective and scalable route for radium isotope separation from thorium-containing systems. This approach offers important experimental support for the preparation of ²²⁸Th/²¹²Pb or ²²⁴Ra/²¹²Pb generator systems and contributes to the development of reliable supply pathways for medical alpha-emitting radionuclides.