Thorium in Energy and Ecology: Prospects for Clean Fuel Sources and Protection of Water and Soil Systems from Radiation Risks

Abstract

Thorium occupies a unique position in the global energy agenda, being simultaneously considered a promising nuclear fuel and an ecological risk factor. Its fuel cycle (Th-232 → U-233) offers significant advantages over uranium, including reduced waste, improved resistance to burnup, and lower proliferation risks, while molten salt reactor designs demonstrate potential to reduce electricity costs and consume transuranic elements from spent nuclear fuel. At the same time, the geochemical mobility of Th⁴⁺ ions, prone to forming soluble and colloidal species, increases the likelihood of their migration into soils and waters, with subsequent accumulation in biota and induction of radiotoxic effects. This study applied a comprehensive review of thorium’s energy potential and environmental risks, analyzing advances in reactor technology alongside mitigation methods such as coagulation, membrane separation, ion exchange, and adsorption with natural and modified sorbents. The findings emphasize that thorium’s strategic role in sustainable nuclear power is inseparable from the development of reliable safeguards to protect ecosystems. We conclude that a dual approach—integrating innovative reactor engineering with effective environmental countermeasures—will be essential for safe deployment of thorium technologies, ensuring their contribution to clean energy generation while minimizing ecological impacts.

1. Introduction

In recent years, growing attention has been drawn to the convergence of two major trends: the rising interest in small modular reactors (SMRs) and the global need to phase out fossil-fuel-based power plants—particularly coal-fired facilities that remain among the largest sources of carbon emissions and air pollution. The study [1] highlights that SMRs can be deployed at sites previously occupied by coal-fired power plants, utilizing existing infrastructure and creating an attractive “coal-to-nuclear” (C2N) pathway.

In this context, thorium as a nuclear fuel deserves particular attention, especially in combination with molten salt reactors (MSRs) or other advanced SMR designs. Thorium offers several theoretical advantages, including wider availability, potentially higher fuel utilization, and lower volumes of long-lived transuranic waste [2]. Linking the urgent replacement of coal-fired plants with the deployment of SMRs using thorium thus presents a logically justified research direction.

The renewed interest in thorium over the past decades is driven by its wide availability, chemical stability, and high potential as fuel for next-generation reactors. The Th-232-based fuel cycle is increasingly recognized as a safer alternative to uranium, offering benefits in terms of waste management, non-proliferation, and environmental performance [2,3].

The authors of the reviews [4,5,6] are pursuing thorium-related research in Kazakhstan, focusing on the energy, economic, and ecological aspects. The group has analyzed mathematical models of a thorium reactor core. Their analysis demonstrated that optimizing both the geometry and the fuel composition enhances the efficiency and safety of Th-232-based systems [6]. Another study, devoted to the nuclear-chemical characteristics of subcritical reactors driven by an external neutron source, showed that such systems extend the operational range and lower the risks of producing long-lived radioactive waste. Molten-salt systems, in particular, have emerged as the most promising and inherently safe option [5].

Comparative assessments of thorium reactor designs in India [7], China [8], the United States [9], and Europe [10,11] further highlight the cycle’s advantages, such as resistance to fuel burnup and minimization of waste.

Momentum has grown following the launch of China’s first operational molten salt reactor, renewing international interest in thorium-based nuclear fuels and emphasizing the importance of careful assessment of associated environmental and safety implications. The 2 MWt TMSR-LF1, located in Wuwei, Gansu, is part of a $3.3 billion program. The reactor is designed to operate with fuel enriched to below 20% U-235, a thorium inventory of approximately 50 kg, and a conversion ratio of about 0.1. It uses FLiBe containing 99.95% Li-7 as the primary coolant and UF4 as fuel. Initial operation is planned on a batch basis, with some online refueling and removal of gaseous fission products. After 5–8 years, all fuel salt is expected to be discharged for reprocessing, including separation of fission products and minor actinides for storage. Subsequently, the reactor is intended to operate on a continuous recycling process, involving uranium and thorium recovery and online separation of fission products and minor actinides. The system is projected to progress from approximately 20% thorium fission toward 80% over time [12].

In April 2025, a significant operational milestone was announced: the reactor was reported to have reloaded its fuel while remaining active, suggesting that continuous operation may be feasible. During a 10-day full-power run using thorium fuel, protactinium-233 was detected, which developers interpret as a sign of successful nuclear breeding. According to Chinese authorities, a larger 10 MW version is planned for development, with potential commercial deployment targeted by 2030.

European efforts in molten salt reactor development are especially noteworthy. In these projects, refueling relies exclusively on fertile Th-232, an approach that significantly reduces fuel costs and ensures a stable energy balance through the gradual in situ accumulation of U-233 in the surrounding blanket. Economic projections suggest that such systems could deliver electricity at around $20/MWh by 2035, making them competitive even with renewable sources. Technological features also play a crucial role. The compact design of these MSRs (with an active zone diameter of only 2.4 m) and their flexible load-following capability facilitate seamless integration into modern energy systems with a high share of renewable energy sources (RES).The environmental benefits are equally significant: beyond minimizing new radioactive waste, these reactors can consume transuranics from spent fuel as seeding material, thereby contributing both to waste reduction and to the broader goals of clean energy [11].

Nonetheless, thorium presents a dual identity—as a strategic fuel and as a potential environmental contaminant. Under natural conditions it is relatively immobile, but specific hydrochemical settings can make it soluble, raising ecological concerns [13,14]. Studies of thorium distribution have been reported from diverse regions of the world, although systematic evaluations at the scale of watersheds remain rare. Anthropogenic sources further complicate the picture, with mining and processing of rare-earth ores, metallurgical activities, and the use of phosphate fertilizers serving as the main contributors, alongside experimental research designed to substitute uranium with thorium [3,15].

For this reason, the deployment of thorium in energy systems must be analyzed in close connection with its geochemical behavior and associated ecological risks. Systematic investigations into thorium migration and the advancement of effective removal techniques for wastewater and soils are essential tasks not only for nuclear energy but also for environmental protection.

A regional focus on Kazakhstan further underscores the relevance of this approach. Approximately 66% of Kazakhstan’s electricity is produced from coal, supporting tens of thousands of jobs [16]. SMRs, and potentially thorium fuel cycles, could offer a more sustainable, low-carbon alternative while supporting a “just transition” for workers in the coal sector. Nevertheless, current assessments indicate that Kazakhstan is proceeding cautiously: SMRs are under consideration, but commercial deployment has not yet been achieved [17].

It is important to acknowledge limitations. The C2N transition does not occur automatically; significant investments, regulatory maturity, and well-designed business models are required [1]. Additionally, while thorium and MSR technologies offer theoretical benefits, practical and economic challenges remain, including material requirements, salt chemistry, and complex fuel cycles [2].

Kazakhstan’s extensive thorium reserves, which are relatively accessible and contain low levels of uranium, could potentially be utilized over the next one to two decades [17]. Modern thorium-based SMR designs with thermal capacities between 5 and 100 MW align well with the output profiles of existing coal-fired power plants and offer enhanced safety compared to conventional water-cooled reactors [2]. Beyond environmental advantages, the deployment of thorium SMRs could provide socioeconomic benefits, including high-tech job creation and the development of a competitive energy market without requiring government subsidies [2,16]. Overall, thorium-based SMRs present a technically, economically, and socially sustainable solution for replacing inefficient coal-fired power generation while supporting national energy and environmental goals [1,2,16,17].

2. Global Distribution and Geochemistry of Thorium

Thorium, a low radioactive element with atomic number 90, belongs to the actinide family. First identified in 1828 by the Swedish chemist J. J. Berzelius, thorium occurs widely in the Earth’s crust, with an average concentration of 6–10 mg/kg—making it more abundant than uranium. Despite this prevalence, research on the geochemistry, migration, and biological effects of thorium remains relatively scarce compared to the extensive body of work devoted to uranium [18]. The global distribution of thorium reserves is shown in Figure 1, which highlights the major resource-holding countries as of 2023.

The distribution of thorium reserves in the world is in the thousands of metric tons (2023).

The geochemical behavior of both thorium and uranium is of central importance to radioecology. Understanding their migration pathways is essential not only for reconstructing natural geochemical cycles but also for addressing applied challenges in energy production, water management, agriculture, wastewater treatment, and ecosystem restoration. While both elements have been investigated, uranium has drawn far more scientific attention, largely because of its direct role in nuclear power and nuclear weapons, which currently accounts for about 16% of global electricity production, with annual uranium consumption reaching roughly 65,000 tons [20,21]. According to the World Nuclear Association, global thorium resources are estimated at approximately 6.2 million tons, distributed across more than 35 countries, with India holding the largest share [19].

In recent years, interest in thorium has grown considerably, supported by evidence of its involvement in biogeochemical cycles and its assimilation by living organisms [22]. Thorium occurs in trace amounts in water, soils, rocks, plants, and animals. Being lithophilic, its geochemical behavior is closely aligned with that of rare-earth elements (especially cerium), as well as zirconium, hafnium, and uranium [23].

The primary thorium-bearing minerals are monazite (Ce, La, Nd, Th) PO₄ and bastnaesite, which also serve as significant sources of rare-earth elements. During their extraction and processing, thorium can be released into the environment through mechanical, hydrometallurgical, and thermal pathways [24].

Monazite, a phosphate mineral of rare-earth elements and thorium with the general formula (Ce, La, Nd, Th) PO₄, is dense, chemically stable, and monoclinic in structure. It has a hardness of 5–5.5 on the Mohs scale and a specific gravity ranging from 4.6 to 5.7 g/cm³. The ThO₂ content in monazite can vary from 1–2% up to 20–30%, with an average of about 6–12%. The largest monazite deposits are located in India (holding nearly two-thirds of the world’s reserves), but significant deposits are also found in Brazil, Australia, the USA, China, Madagascar, and Thailand [25,26,27].

Estimates by the USGS place global monazite resources at roughly 12 million tons. Although only moderately radioactive due to its thorium and uranium content, monazite is highly valued for its concentration of rare-earth elements, which are essential for advanced technologies such as Nd-Fe-B magnets used in renewable energy systems. At the same time, monazite is increasingly regarded as a potential thorium feedstock for nuclear fuel cycles. Its processing, however, requires enrichment and acid leaching to separate the rare earth and actinide components [24].

3. Isotopic Composition and Ecological Significance of Thorium

For a long time, natural thorium was considered a mono-isotopic element, represented entirely by ²³²Th. However, revised IUPAC data (2013) indicate that its share is 99.98% ± 0.02% [28]. A total of twelve thorium radionuclides are known, with mass numbers ranging from 223 to 234, but only six occur in nature. The principal isotopes of thorium and their environmental relevance are summarized in

Table 1. The main isotopes of thorium and their environmental significance.

Isotope	Half-Life	Decay Type	Ecological Significance
232Th	1.40 × 1010 years	α	The main natural isotope; determines long-term radiation background; potential source of 233U
230Th	~75,600 years	α	Member of the uranium series; accumulates in sediments; used in geochronology
228Th	698.6 days	α	Daughter product of 232Th; increases local radiation levels in water and soils
234Th	24.1 days	β−	Decay product of 238U; indicator of migration processes in the hydrosphere
231Th	25.5 h	β−	Member of the 235U series; relevant near uranium deposits
227Th	18.7 days	α	Short-lived but highly active; can contribute to contamination of surface waters

[29].

These isotopes belong to different radioactive decay series:

• ²³⁸U series: ²³⁴Th (T½ = 24.1 days), ²³⁰Th (T½ ≈ 75,600 years);
• ²³²Th series: ²³²Th (T½ = 1.40 × 10¹⁰ years) and its daughter ²²⁸Th (T½ = 698.6 days);
• ²³⁵U series: ²³¹Th (T½ = 25.5 h), ²²⁷Th (T½ = 18.7 days).

Among them, ²³²Th is the most important, with its exceptionally long half-life (~14 billion years) ensuring geological stability and a persistent presence in the lithosphere and hydrosphere [29]. At the same time, it acts as a source of short-lived isotopes that shape localized radiation impacts. Of particular significance is the fact that ²³²Th can capture a thermal neutron and transform into fissile ²³³U, which does not occur naturally but is of great interest for nuclear energy [30].

The ecological impact of thorium is determined not only by its principal isotope ²³²Th but also by its daughter products. Long-lived isotopes (²³²Th, ²³⁰Th) exhibit low radioactivity but ensure prolonged presence in geological systems. ²³²Th establishes the overall radiation background and acts as the progenitor of short-lived isotopes, while ²³⁰Th accumulates in sediments, making it valuable for geochronology but also potentially integrated into biogeochemical cycles.

Short-lived isotopes (²²⁸Th, ²³⁴Th, ²³¹Th, ²²⁷Th) are highly active and capable of migrating in soils and aquatic systems. Their higher bioavailability compared to long-lived isotopes increases the risk of radio ecological effects. For example, ²²⁸Th and ²³⁴Th can elevate radiation levels in surface waters and enter food chains. Thus, while long-lived ²³²Th guarantees the constant presence of thorium in the biosphere, its short-lived daughters create localized zones of radiation exposure, heightening risks for ecosystems and human health [31,32,33].

4. Thorium in Soils

Natural ionizing radiation is a significant component of environmental exposure, with important contributions from radionuclides naturally present in the Earth’s crust and redistributed through the atmosphere, water, and soils. Levels of soil radioactivity are shaped not only by geological and climatic processes but also by human activities such as mining, ore processing, energy production, agriculture, and industry [34].

The geochemical behavior of thorium is governed mainly by its stable oxidation state (+4) and by its ionic radius, which is close to that of U⁴⁺, Ce³⁺, and Zr⁴⁺. These features favor isomorphic substitution in minerals and explain its widespread occurrence in magmatic and silicate rocks. In soils, thorium typically occurs in the form of insoluble oxides and hydroxides, such as ThO₂, which accounts for its low mobility. Nevertheless, acidic or reducing conditions, as well as organic acids released by plants and microorganisms, can promote its remobilization [35]. Thorium (IV) also forms stable complexes with carbonates, fluorides, phosphates, sulfates, and organic ligands, which increases its solubility and elevates ecological risks. The main factors that control thorium mobility in soils are pH, organic matter, and the presence of Fe-Mn oxides. At low pH, solubility and phytoavailability rise, while humic substances often contribute to immobilization by forming stable thorium-organic complexes. Conversely, dissolved organic matter at higher pH values can enhance transport [36]. The effects of humic acids also vary with mineralogy: retention is strong on hematite but weaker on bentonite. In soils contaminated by rare-earth industries, the distribution of thorium fractions is driven by acid dissolution, hydrolysis, and complexation, as demonstrated by spectroscopic analyses [37].

Field studies across diverse regions provide concrete evidence of how thorium accumulates under both geological and industrial conditions. In the Southern Urals (Russia), background concentrations ranged from <3 to >15 mg/kg, with localized hotspots reflecting the influence of parent rocks [20]. In India, investigations near the Lambapur uranium mining site revealed surface soil activities of 32–311 Bq/kg (≈8–77 mg/kg), indicating enrichment within mineralized zones [38]. Along the Odisha coast, high background radiation areas with sandy soils and monazite deposits yielded average thorium concentrations of about 186 mg/kg, underlining the geogenic contribution of heavy mineral assemblages [39]. Industrial impacts have also been documented. In the Irkutsk–Angarsk industrial zone of Siberia, thorium levels in soils ranged from 1.8 to 30.8 mg/kg, while slags and ash from industrial processes contained 29–44 mg/kg [40]. In tropical settings such as the Ipojuca River basin in Brazil, soils and sediments contained on average 28.6 mg/kg of Th, with suspended particles acting as the principal carriers [41]. Together, these examples highlight both natural and anthropogenic enrichment of thorium in soils and emphasize the need for site-specific monitoring and for assessing the long-term ecological consequences of remobilization and bioavailability.

Anthropogenic activities intensify thorium accumulation, raising the risks of migration and possible entry into food chains. Although thorium is generally of low solubility, transitions into mobile forms can occur under acidic or sulfate-rich conditions, such as in sulfide oxidation zones or at tailings repositories [37]. Unlike uranium, thorium usually remains immobilized under natural conditions, but even modest environmental changes can transform it into a significant source of radiological exposure. For example, thorium has been detected in 81 of 1854 hazardous waste storage sites in the United States. However, the number of sites where thorium content has been assessed is unknown. The main factors determining thorium mobility and its environmental behavior in soils are summarized in

Table 2. Main factors influencing the mobility and environmental behavior of thorium in soils.

Factor	Conditions	Ecological Significance
Soil acidity (pH)	Low pH increases solubility and phytoavailability of thorium	Enhanced radiological risks, transition to mobile forms [36]
Organic matter	Humus and humic acids immobilize thorium; dissolved organic matter at high pH enhances migration	Formation of stable organo-mineral complexes or, conversely, promotion of transport [42]
Fe–Mn oxides	Act as active sorbents for Th(IV)	Reduced mobility, long-term fixation in soils [43]
Mineral composition	Isomorphic substitution in silicates, apatites, carbonates	Controls thorium distribution in geological environments [44]
Anthropogenic activity	Mining and ore processing, lignite combustion, phosphate fertilizers	Increased concentrations of Th, co-release of Ra and Po into soils [45]
Natural transport processes	Erosion and sedimentation in catchment basins	Redistribution of thorium from soils to rivers and oceans [14]

.

The environmental behavior of thorium is largely determined by its low solubility and strong sorption capacity, yet mobile forms may arise under unfavorable conditions, intensifying risks for ecosystems and human health. Its presence in soils reflects the interplay of natural processes and anthropogenic inputs. Significant releases occur during ore processing, lignite combustion at power plants, and the application of phosphate fertilizers, often accompanied by co-contaminants such as radium and polonium.

Thorium is thus a dual-natured element. On the one hand, it is a promising clean fuel source for the future of nuclear energy; on the other, its extraction and utilization pose ecological and radiological challenges. A deeper understanding of thorium biogeochemistry and migration pathways is crucial for accurate ecological risk assessment, the design of wastewater treatment technologies, more effective mining regulations, and the safe advancement of nuclear energy [46].

5. Ecology of Thorium in Aquatic Systems

The geochemistry of thorium in aquatic environments is relatively straightforward, since under natural conditions it remains stably in the tetravalent state (Th⁴⁺), acting as a highly charged cation that readily interacts with water molecules and anions [47]. These interactions lead to the formation of a wide range of complex species [48,49], which in turn define its solubility, stability, and migration potential in ecosystems. As shown in Figure 2, Th(IV) occurs in various species forms under aquatic conditions.

Figure 2 illustrates the main hydrolysis pathways and the formation of Th(IV) hydroxo-complexes in water. Depending on precursor compounds (ThO, ThO₂, ThO₃), both simple hydroxide species such as Th(OH)₄ and more complex oxo- and peroxo-hydroxo species may form. At acidic pH values, mobile aquo-complexes dominate, whereas in neutral and alkaline media insoluble hydroxides and polymeric species prevail. Hydrolysis thus creates a balance between migration and fixation: soluble forms increase the risk of dispersion in aquatic environments, while insoluble ones promote sedimentation and accumulation in bottom deposits [49].

Thorium in aquatic systems occurs mainly as colloidal particles and complex compounds, while soluble species are extremely rare (usually in the ng·L⁻¹ range) [47]. This very low solubility limits direct bioavailability, but thorium’s strong sorptive capacity ensures its rapid adsorption onto suspended particles, which then settle into bottom sediments. There, thorium may remain immobilized for long periods but can act as a secondary source of contamination upon desorption and remobilization [50]. The input of thorium into aquatic systems is associated with both natural processes -hydrothermal activity, acidic drainage, colloidal transport—and anthropogenic impacts such as mining, ore processing, tailings, and industrial discharges [47].

The chemical forms of thorium strongly depend on pH and the composition of the medium. At low pH and in the presence of organic ligands such as humic and fulvic acids, thorium may exist in mobile aquo-complexes; in neutral waters it is typically present as simple hydroxides like Th(OH)₄; and in alkaline conditions insoluble hydroxides dominate, leading to precipitation and long-term fixation in sediments [51]. In addition, Th(IV) forms carbonate, phosphate, sulfate, and fluoride complexes, which may either adsorb onto mineral surfaces or persist as mobile species depending on geochemical conditions [52]. Colloidal fractions play a particularly important role in thorium migration, as they are easily transported with water flow and deposited downstream into sediments. To summarize the main chemical forms of thorium, their conditions of formation, and ecological significance, the key data are provided in

Table 3. Forms of thorium in aquatic systems, conditions of their formation and environmental significance.

Form of Th in Water	Conditions of Formation	Ecological Behavior
Th(IV) aquo-complexes (soluble)	Acidic pH, presence of organic ligands, wastewater inputs	High mobility, risk of contamination of water bodies, bioavailability to aquatic organisms
Simple hydroxides (Th(OH)4)	Neutral media, hydrolysis of Th(IV)	Limited solubility, partial fixation in bottom sediments
Oxo- and peroxo-hydroxo complexes	Mixed redox conditions, presence of organic matter	Potential migration under changing conditions, possible bioaccumulation
Insoluble hydroxides	Alkaline media (pH > 7)	Precipitation and long-term fixation in sediments, reduced mobility
Colloidal forms	Suspended particles, minerals, organo-mineral associations	Transport with water flows, accumulation in silts and sediments, long-term ecological impact

.

Colloidal and soluble species of thorium represent the greatest ecological risk. They are the most mobile and can enter aquatic food chains. Mobile Th isotopes are alpha emitters capable of producing genotoxic effects, disrupting plankton and fish reproduction, altering microbial community structures, and reducing biodiversity, thus contributing to long-term ecosystem degradation [50]. Interactions with organic matter complicate this behavior further: solid-phase humic substances tend to immobilize thorium, while dissolved organic compounds promote its mobilization [53]. These processes are well documented for uranium and partly extrapolated to thorium, but their role in real aquatic systems still requires further study. Particularly sensitive environments include lacustrine, coastal, and wetland systems, where high organic matter and strong redox gradients drive radionuclide accumulation and remobilization [54,55].

Quantitative case studies confirm thorium’s presence in aquatic systems from both natural and anthropogenic sources. In Rajasthan (India), ²³²Th concentrations in drinking water reached 0.57–1.46 µg/L, exceeding some guideline values [56]. In Brazil and Ghana, levels remained below regulatory thresholds but reflected mining influences [57,58]. In the Ipojuca River basin (Brazil), thorium accumulated mainly in suspended sediments (2.8–32.9 mg/kg), with almost no thorium detected in the dissolved phase [41]. Similar patterns were found in rivers of Azerbaijan, where concentrations were <0.01 µg/L but increased locally due to lithological and industrial factors [59]. Stable isotopes and trace elements are increasingly applied as tracers to distinguish natural from anthropogenic sources [60]. Statistical methods such as correlation, factor, and cluster analyses also help reveal associations between thorium and other elements; regional studies show that U–Th correlations are often weak, reflecting differences in migration pathways and strong local geochemical influences [61].

Uranium (U) in natural environments mainly occurs as the uranyl ion (UO₂²⁺), forming stable complexes with carbonates, sulfates, and organic ligands, which enhances its mobility [13,62]. Thorium (Th(IV)), by contrast, is poorly soluble, hydrolyzes readily, and mostly exists in colloidal or adsorbed forms. In freshwater, U concentrations are typically below ~4 μg/L, while Th is at sub-μg/L levels. Acidic or metal-rich environments, such as acid mine drainage, can elevate U concentrations to several thousand μg/L, and Th may also become enriched due to disturbed chemical equilibria [13].

Both U and Th can be mobilized under acidic and oxidizing conditions, but Th strongly adsorbs onto iron (oxy)hydroxides such as goethite and schwertmannite, with precipitate concentrations reaching hundreds of μg/g. Uranium remains largely dissolved and mobile, especially in acidic or organic-rich waters, reflecting the contrasting geochemical behaviors of U and Th, controlled, respectively, by complexation/redox processes and surface adsorption/precipitation [62].

These species pose ecological threats due to their potential bioavailability and capacity to enter trophic networks. Conversely, insoluble hydroxides and mineral associations restrict its mobility, fixing thorium in sediments where it can persist for centuries. Sediments, however, are dynamic reservoirs, and changes in redox conditions, pH, or organic matter may release previously immobilized thorium back into the aquatic system [63].

To mitigate such risks, various remediation technologies are employed, including adsorption using zeolites, activated carbon, or ion-exchange resins, membrane filtration, electrochemical precipitation, and phytoremediation [64]. New approaches emphasize integrated strategies combining chemical and biological treatments, which are more effective across diverse hydrochemical settings [65]. Rehabilitation requires not only cleaning the water column but also stabilizing sediments, restoring trophic chains, and implementing long-term monitoring. Practical measures involve controlled tailings management, safe disposal sites such as endorheic lakes in arid regions, and preventive environmental technologies [66].

Thus, the aquatic geochemistry of thorium is governed by its strong tendency to hydrolyze and form complex and colloidal species. Its environmental mobility and ecological significance are determined by chemical speciation, pH, redox gradients, and the presence of organic and inorganic ligands. While thorium is often less mobile than uranium, anthropogenic impacts and disruptions in chemical equilibrium can make it a significant aquatic contaminant. Effective management therefore requires a combination of speciation studies, isotopic and statistical analyses, predictive modeling, remediation technologies, and site-specific environmental monitoring to address the long-term ecological hazards of thorium in aquatic systems.

6. Meta-Analytic Framework for Thorium Concentrations and Hydrogeochemical Mobility in Environmental Compartments

Thorium in its tetravalent state, Th(IV), is generally regarded as sparingly soluble and strongly particle-reactive and is therefore often treated as “immobile” in near-surface environments. However, global compilations across soils, sediments, suspended particulate matter and natural waters reveal that this immobility is conditional: Th remains strongly bound under many background conditions, yet can become measurably mobile in specific hydrogeochemical and land-use contexts [14]. Table 4 summarizes typical thorium concentration ranges and central tendencies across major environmental compartments, while Figure 3 illustrates the corresponding “mobility windows” in Eh–pH–composition space.

In soils and sediments, Th concentrations are primarily controlled by parent lithology and the abundance of accessory minerals such as monazite, xenotime and zircon, with felsic rocks generally yielding higher values than mafic ones [14,67]. Global upper-crust averages of about 8–12 mg kg⁻¹ Th [14] are consistent with large-scale European data (GEMAS), which report medians of 2.5–2.9 mg kg⁻¹ and a broad range from <0.1 to >60 mg kg⁻¹ depending on soil type and geology [68]. Typical background soils contain ~0.5–20 mg kg⁻¹ Th (median 5–10 mg kg⁻¹; IQR 3–15 mg kg⁻¹) [14,67,68], while mineralized terrains and mine-affected areas frequently exceed 50–100 mg kg⁻¹ [14,68,69,70,71]. In river and stream sediments, Th is efficiently scavenged by clays, Fe–Mn (oxyhydr)oxides and heavy-mineral phases, with background medians around 10 mg kg⁻¹ and maxima above 200 mg kg⁻¹ in felsic or mineralized catchments [14,41,72]. Suspended particulate matter (SPM) is often slightly enriched relative to bed load because of preferential sorption onto fine and colloidal fractions [14,41].

In natural waters, hydrolysis and surface sorption maintain extremely low dissolved Th(IV) concentrations. Uncontaminated rivers and lakes typically contain 10⁻³–10⁻¹ µg L⁻¹ Th, with medians around 10⁻² µg L⁻¹, whereas coastal and open seawaters exhibit slightly higher levels (0.1–1.0 µg L⁻¹; median 0.3–0.5 µg L⁻¹) [63,67]. By contrast, mine drainage, tailings seepage, geothermal fluids and mineralized groundwaters can reach µg L⁻¹–mg L⁻¹ levels, with much of the Th occurring in colloidal or particle-bound forms, particularly in DOC- and carbonate-rich systems [14,41,73].

Table 4. Indicative ranges, median values and interquartile ranges (IQR) for thorium concentrations in major environmental compartments.

Compartment/Setting	Units	Typical Range (P5–P95)	Indicative Median	Approx. IQR (P25–P75)	Notes/Dominant Controls
Soils, global background	mg kg−1	0.5–20	5–10	3–15	Controlled by parent lithology and accessory Th minerals; higher in felsic terrains [14,67,68,69]
Soils, high-Th/mineralized (REE, phosphate, etc.)	mg kg−1	20–100+	40–70	30–80	Monazite/xenotime-rich horizons, U–Th–REE mineralization, mine-affected or phosphatic soils [14,68,69,70]
Stream/river sediments, background	mg kg−1	3–40	~10	~5–20	Enrichment in clays, Fe–Mn (oxyhydr)oxides and heavy minerals; lithology-controlled [14,41,72]
Stream/river sediments, mineralized	mg kg−1	20–250+	50–100	30–150	Drainage of U–Th–REE deposits, monazite placers, mine tailings [14,41,72]
Suspended river particulates (SPM)	mg kg−1	5–40 (background)	10–20	8–30	Fine fraction; strong sorption to clays and Fe–Mn colloids; often enriched [14,41]
Fresh surface waters (rivers, lakes)	µg L−1	10−3–10−1	~10−2	~3 × 10−3–3 × 10−2	Very low solubility; mainly colloidal Th; enhanced in DOC- and carbonate-rich systems [14,41]
Coastal/open seawater	µg L−1	0.1–1.0	0.3–0.5	0.2–0.7	Particle–water exchange; resuspension and river inputs [14,73]
Impacted waters (mine drainage, geothermal, tailings seepage)	µg L−1–mg L−1	10−2 µg L−1–103 µg L−1 (site-specific)	highly variable	—	Strongly site-specific; up to mg L−1 near mining or mineralized groundwaters [14,41]

Data compiled from global and regional datasets; P₅–P₉₅ represent approximate percentile envelopes inferred from reported ranges.

Table 4. Indicative ranges, median values and interquartile ranges (IQR) for thorium concentrations in major environmental compartments.

Compartment/Setting	Units	Typical Range (P5–P95)	Indicative Median	Approx. IQR (P25–P75)	Notes/Dominant Controls
Soils, global background	mg kg−1	0.5–20	5–10	3–15	Controlled by parent lithology and accessory Th minerals; higher in felsic terrains [14,67,68,69]
Soils, high-Th/mineralized (REE, phosphate, etc.)	mg kg−1	20–100+	40–70	30–80	Monazite/xenotime-rich horizons, U–Th–REE mineralization, mine-affected or phosphatic soils [14,68,69,70]
Stream/river sediments, background	mg kg−1	3–40	~10	~5–20	Enrichment in clays, Fe–Mn (oxyhydr)oxides and heavy minerals; lithology-controlled [14,41,72]
Stream/river sediments, mineralized	mg kg−1	20–250+	50–100	30–150	Drainage of U–Th–REE deposits, monazite placers, mine tailings [14,41,72]
Suspended river particulates (SPM)	mg kg−1	5–40 (background)	10–20	8–30	Fine fraction; strong sorption to clays and Fe–Mn colloids; often enriched [14,41]
Fresh surface waters (rivers, lakes)	µg L−1	10−3–10−1	~10−2	~3 × 10−3–3 × 10−2	Very low solubility; mainly colloidal Th; enhanced in DOC- and carbonate-rich systems [14,41]
Coastal/open seawater	µg L−1	0.1–1.0	0.3–0.5	0.2–0.7	Particle–water exchange; resuspension and river inputs [14,73]
Impacted waters (mine drainage, geothermal, tailings seepage)	µg L−1–mg L−1	10−2 µg L−1–103 µg L−1 (site-specific)	highly variable	—	Strongly site-specific; up to mg L−1 near mining or mineralized groundwaters [14,41]

Data compiled from global and regional datasets; P₅–P₉₅ represent approximate percentile envelopes inferred from reported ranges.

6.1. Hydrogeochemical Context and Eh–pH Mobility Framework

To interpret these concentration patterns mechanistically, Figure 3 presents an Eh–pH diagram of thorium stability and mobility in natural waters and pore solutions. The diagram delineates the dominant redox–pH fields controlling Th speciation and solubility. The abscissa represents pH (2–12) and the ordinate the redox potential Eh (V vs. SHE). Together, these parameters define the chemical regimes that govern Th behavior across the litho-, pedo-, and hydrosphere.

The term pedo- (from Greek pedon, “soil”) refers to the pedosphere, where Th participates in hydrolysis, sorption, and complexation reactions. Pore waters, occupying intergranular spaces in soils and sediments, constitute the principal medium for Th transport; their Eh, pH, and ligand activities (carbonate, fluoride, organic matter) determine the effective mobility conditions.

Eh–pH stability and mobility fields of thorium in natural waters and pore solutions. Black dots indicate representative environmental Eh–pH ranges (acid-mine drainage, river water, peatland). The diagram shows dominant thorium species—Th⁴⁺, Th(OH)₂²⁺, Th³⁺, Th⁺, ThO₂(s), Th(CO₃)₆⁶⁻, and organo-Th complexes—and corresponding low-, moderate-, and high-mobility regimes governed by sorption/hydrolysis, organic complexation, and carbonate complexation.

Five principal domains can be distinguished:

Low-mobility field (sorption and hydrolysis).

Neutral to slightly acidic, oxidizing conditions (pH 5–8; Eh 0–+0.8 V) stabilize Th⁴⁺ and Th(OH)₂²⁺ species, promoting hydrolysis and precipitation as ThO₂(s). This regime characterizes background soils and fresh surface waters where Th is particle-reactive and largely immobile.

High-mobility window (reductive Fe–Mn dissolution).

At lower Eh (0 to –0.5 V) and pH 3–8, reductive dissolution of Fe–Mn (oxyhydr)oxides releases sorbed Th, occasionally producing transient Th⁺ species. Such conditions typify acid-mine drainage and Fe–Mn-rich pore waters.

Transitional zone (Th³⁺).

A narrow redox boundary (Eh ≈ 0 V; pH 7–9) allows metastable Th³⁺ to persist locally in suboxic sediments and peatlands.

Carbonate-complexation field (high mobility).

Under alkaline, oxidizing conditions (pH > 8–9; Eh > 0 V), Th forms stable anionic complexes—primarily Th(CO₃)₆⁶⁻—that markedly enhance solubility. This field corresponds to carbonate-rich rivers and alkaline lakes.

Organic-complexation field (moderate mobility).

At high pH (9–11) and mildly reducing potentials, dissolved organic carbon and fluoride promote strong organo-Th complexes, sustaining moderate mobility. Fe-oxyhydroxide minerals such as akaganeite (β-FeOOH) mark associated redox transitions.

Black dots on Figure 3 represent measured Eh–pH values for acid-mine drainage, river water, and peatland systems, confirming close correspondence between theoretical speciation fields and environmental observations.

6.2. Geochemical Significance

The integrated evidence from Table 4 and Figure 3 demonstrates that thorium mobility is governed by the coupled effects of redox potential, pH, and ligand activity:

-

In oxidizing, near-neutral environments, Th is immobilized as ThO₂(s) or strongly adsorbed phases.

-

Under reducing or acidic conditions, dissolution of Fe–Mn phases and proton-promoted desorption release Th into solution.

-

In alkaline, carbonate-rich waters, Th forms highly soluble anionic complexes.

-

In organic-rich reducing settings, Th occurs as stable organo-complexes of intermediate mobility.

This conceptual and quantitative framework links empirical concentration data with thermodynamic speciation models, providing a coherent basis for assessing thorium transport, radiological risk, and natural attenuation processes in soils, sediments, and waters [14,41,67,68,69,70,71,72,73,74].

Although this review focuses primarily on the geochemical behavior of thorium, it is worth noting that the concentration ranges summarized in Table 4 are generally well below levels associated with radiological concern. According to the World Health Organization (WHO, 2017) and the International Atomic Energy Agency (IAEA, 2014) safety guidance, typical background thorium concentrations in soils and waters correspond to sub-threshold ingestion and inhalation doses for both adults and children. The ATSDR Toxicological Profile for Thorium (2019) likewise indicates that natural background exposures are orders of magnitude lower than levels linked to measurable health risk. Only in highly mineralized or mining-impacted systems can thorium activities approach values that warrant site-specific risk evaluation. This contextual linkage helps place environmental concentrations within a screening-level perspective while preserving the geochemical focus of this review [67,75,76].

7. Methods for Remediation of Thorium Contamination in Water and Soils

The removal of thorium from water and soils is a critical task for ensuring both ecological and radiological safety. In soils and plants, thorium exhibits low mobility, predominantly accumulating in the root zone, which enables its stabilization and effective extraction by sorbents. Similar processes govern the behavior of uranium, though its mobility is generally higher and better documented [74,77]. Investigations into the natural distribution of radionuclides confirm these tendencies: uranium tends to accumulate in plant roots, while thorium remains immobile in soil, a factor of importance for migration modeling and ecological risk assessment [78].

A wide variety of methods exist for the removal of actinides such as Th(IV) and U(VI), including membrane separation, chemical precipitation, ion exchange, complexation, adsorption, and biosorption. Among these, adsorption has attracted particular attention because it offers an accessible, effective, and cost-efficient technique applicable to both water and soil systems. It is considered the most practical method due to its operational simplicity, high selectivity, and significant sorption capacity. Considerable progress has been achieved through the use of nanomaterials and modified natural adsorbents, which provide enhanced efficiency and adaptability to real-world conditions.

The diversity of available sorbents is promising and lays the foundation for developing reliable thorium-remediation strategies. These include natural minerals such as bentonite [79], diatomite [80], calcined diatomite [81], and clinoptilolite modified with phosphate groups [82]; carbon-based materials such as activated carbon [83], biochar from rice husk [84], and protonated orange peel [85]; and biological sorbents such as algae (Sargassum spp.) [86], fungal mycelium [87], and Aspergillus fumigatus biomass capable of adsorbing up to 370 mg Th/g at pH 4 with equilibrium reached within two hours [88]. Each sorbent interacts with Th(IV) differently—via ion exchange, complexation, or surface adsorption—and their efficiencies vary with pH, ionic strength, and competing ions.

Nanomaterials further enhance sorption efficiency and selectivity due to their large surface area and abundance of functional groups. Functionalized graphene oxide (AMPA-GO), for example, demonstrated a sorption capacity of 138.84 mg/g for thorium, while graphene oxide in other studies has shown capacities up to 180 mg/g [89]. Copper-based nanoparticles (Cu-NPs) are also gaining attention due to their redox flexibility, antimicrobial approval by the US EPA, and demonstrated effectiveness: Cu-NPs coated with polyethylene glycol derivatives show improved colloidal stability and sorption performance for both uranium and thorium [90,91]. Similarly, copper nanoparticles bound with organic ligands prevent aggregation, accelerate sorption, and improve resistance to matrix effects.

Nanoscale zero-valent iron (nZVI) represents another highly promising sorbent because of its multiphase removal mechanisms that combine reduction, adsorption, and co-precipitation. Its efficiency increases dramatically with surface modification: phosphate-treated nZVI achieved 98.8% Th(IV) removal under acidic conditions [92], phosphate-modified biochar/nZVI composites reached 967.53 mg/g for U(VI) [93], and nZVI/UiO-66 hybrids exhibited a maximum uptake of 404.86 mg/g [94]. These results highlight the versatility of nano-iron and the importance of surface engineering in radionuclide removal.

Other modern synthetic sorbents also demonstrate remarkable performance. Covalent organic frameworks (COFs) bind thorium with high selectivity through nitrogen or carboxyl groups [95]. Chelating sorbents based on EDTA exhibit strong affinity for Th(IV) [96], while polymer carriers modified with ligands further improve sorption selectivity [97]. Modified activated carbon with amino groups also displays enhanced chemisorption [98]. Cellulose-based iron composites combine high efficiency with recyclability [99], while N-phosphorylureas demonstrate stable coordination with Th(IV) [100].

Natural sorbents remain equally important for large-scale applications due to their cost-effectiveness and availability. Diatomite etched with hydrofluoric acid (HF) shows increased porosity and surface area, improving Th(IV) adsorption [80]. Bentonite retains its efficiency in ion exchange [79], while phosphate-modified clinoptilolite achieved near-complete Th removal from industrial residues [82]. Biochar derived from different biomass sources is increasingly optimized through surface modifications, regeneration, and composite formation, enhancing both sustainability and performance [101].

Table 5. Types of sorbents for thorium and their characteristics.

Sorbent	Sorption Mechanism	Optimal pH	Capacity (mg/g)
Bentonite	Ion exchange [89]	4.0–6.0	20–35
Diatomite	Physical adsorption [90]	5.0–6.0	15–25
Biochar (from rice husk)	Chemical adsorption [94]	4.5–6.5	45–65
Algae (Sargassum spp.)	Carboxyl and sulfate groups [96]	4.0–5.5	75–85
Aspergillus niger mycelium	Ion exchange [98]	3.0–4.5	40–50
Ion exchange resins (Dowex 50WX8)	Ion exchange [102]	2.0–3.5	100–130
Graphene oxide	Surface adsorption [99]	3.5–5.5	150–180
Fe3O4@SiO2-NH2	Chemical coordination [103]	3.5–4.5	120–140
Modified activated carbon (amino groups)	Chemisorption [104]	4.0–5.5	60–80
Chelating sorbent based on EDTA	Complexation [103]	2.0–4.0	100–160

provides an overview of the sorbents applied for thorium removal along with their key properties.

Note. Adsorption capacities listed in Table 5 are compiled from studies performed under different solution chemistries, including variations in pH, ionic strength, and co-ion composition. Reported values are therefore indicative and intended for comparative orientation only. Detailed Langmuir/Freundlich parameters, kinetic constants, and column breakthrough data can be found in the respective source publications. Performance under mixed-ion or organic-rich conditions may vary depending on sorbent stability and surface functionality.

Biosorbents show particularly strong promise. Aspergillus fumigatus can capture up to 370 mg Th/g under acidic conditions, with adsorption following a pseudo-second-order kinetic model indicative of mixed physical and chemical sorption [98]. Similarly, activated carbon exhibits pseudo-second-order kinetics and fits both Langmuir and Dubinin–Radushkevich isotherms [104]. These results underline the importance of kinetic and isotherm modeling for predicting sorption efficiency under varying environmental conditions.

Modern remediation strategies thus represent a synthesis of natural and synthetic sorbents, nanotechnology, and biosorption. They must also account for environmental parameters such as pH, temperature, concentration, and competing ions. Integrated approaches that combine different classes of sorbents-minerals, nanomaterials, polymers, and biosorbents-offer are the best prospects for efficient thorium removal. At the same time, literature reviews reveal a clear imbalance: uranium remediation is well characterized, whereas thorium adsorption mechanisms and scalable technologies remain underexplored [91,105]. Addressing this gap will require further research into Th(IV) sorption, the development of advanced functionalized sorbents, and the evaluation of their environmental safety under real-world conditions.

Ultimately, thorium remediation practices involve not only efficient removal from water and soils but also stabilization in the upper soil layers to prevent migration into groundwater. Efficient systems must integrate advanced sorbent technologies with environmental monitoring, ensuring sustainable protection of aquatic and terrestrial ecosystems from radiological contamination.

8. Conclusions

Thorium stands at the intersection of energy innovation and environmental responsibility. On the one hand, it offers a realistic path toward a new generation of nuclear power systems characterized by safety, efficiency, and sustainability. On the other, its environmental behavior presents risks that demand close scientific attention. Although thorium is widely distributed in the Earth’s crust and, due to its low solubility, often remains immobile and relatively safe, changes in geochemical conditions—such as soil acidification, the presence of organic ligands, or the consequences of mining and ore processing—can increase its mobility. Under such circumstances, Th⁴⁺ may form soluble or colloidal species, accumulate in soils and aquatic systems, infiltrate food chains, and contribute to localized radiation exposure. These processes underscore the need for systematic monitoring and proactive risk management even for elements traditionally considered geologically stable.

From an energy perspective, thorium possesses unique advantages over uranium-based technologies. Its abundance, low probability of sustaining uncontrolled chain reactions, and minimal production of long-lived transuranic isotopes position it as a strategic fuel for building a cleaner and more secure nuclear future [1]. Current developments in molten salt reactor (MSR) technology further emphasize these prospects: modularity, flexible load-following, reduced waste generation, and the ability to recycle spent nuclear fuel all point to an economically viable and environmentally friendly option for the global energy sector [106]. Nonetheless, translating these advantages into practice requires overcoming significant challenges, including the construction of new reactor infrastructure, the establishment of reliable fuel reprocessing technologies, and the creation of robust waste management systems aligned with non-proliferation safeguards [107].

Equally critical are the ecological dimensions of thorium use. The release of Th⁴⁺ during extraction and processing integrates this element into biogeochemical cycles, where it may alter soil chemistry, migrate through aquatic systems, and affect biodiversity. Advances in remediation science—particularly the development of efficient sorbents, biosorbents, and nanomaterials—demonstrate that it is possible to control mobile thorium species, thereby reducing ecological risks and strengthening environmental protection frameworks.

Mitigating the potential radiological effects of thorium, particularly its gamma-emissions and long-term biological accumulation, remains a key challenge in the safe development of thorium-based nuclear systems. To ensure environmental and human health protection, future research should prioritize several directions:

• Industrial thorium mining: Design selective and efficient solid-phase extraction methods for recovering thorium from minerals such as monazite while minimizing wastewater generation and ecological disturbance.
• Thorium biogeochemistry: Advance geochemical modeling and targeted experimental studies on the speciation, stability, and mobility of thorium in natural waters and during waste disposal or site remediation.
• Radiobiological effects: Deepen understanding of thorium’s cellular and systemic impacts in humans and experimental organisms to refine radiological safety standards and risk assessments.
• Decorporation strategies: Develop next-generation chelating agents and medical countermeasures to mitigate internal contamination in case of occupational or accidental exposure.

Looking ahead, future research should move toward integrated, interdisciplinary approaches. A promising avenue is the coupling of multiphysics transport simulations, such as those performed in COMSOL Multiphysics 6.3, with advanced geochemical modeling tools like PHREEQC. This combined methodology would enable detailed assessments of Th(IV) migration under realistic field conditions, accounting for hydrolysis, sorption, redox transformations, organic complexation, and transport in porous media. Such hybrid models could predict long-term ecological risks, identify the boundary conditions that control thorium immobilization versus mobilization, and evaluate the effectiveness of remediation measures in situ. Beyond scientific insights, this approach would foster practical tools for policymakers and engineers, bridging nuclear fuel cycle innovation with hydrogeology, water management, and ecological restoration [108].

In summary, thorium’s future lies in a balanced vision—one that integrates technological innovation in reactor and fuel cycle design with rigorous environmental safeguards and forward-looking risk assessment strategies. With sustained collaboration between governments, the scientific community, and industry, thorium has the potential to become a cornerstone of safe, sustainable, and ecologically responsible nuclear energy in the decades to come.

9. Patents

1. Patent of the Republic of Kazakhstan No. 10064 “Method for obtaining a sorbent for thorium extraction from natural materials.”

Alsar Zh., Gajimuradova A., Insepov Z.

2. Patent of the Republic of Kazakhstan No. 10073 “Sorbent for purification of soil and wastewater from radionuclides.”

Mansurov Z., Tanirbergenova S., Lesbaev B., Insepov Z., Alsar Zh.

3. Patent of the Republic of Kazakhstan No. 11235 “Method for decomposition of monazite concentrate.”

Mansurov Z., Insepov Z., Tanirbergenova S., Lesbaev B., Serik N.