Thallium transformation and partitioning during Pb-Zn smelting and environmental implications

Thallium in Technosols from Allchar (North Macedonia): Isotopic and speciation insights

Allchar (North Macedonia) mining area is known for anomalous background Tl concentrations. In this study, we combine accurate detection of Tl stable isotope ratios with data on mineralogy/speciation and chemical extraction of Tl in Tl-contaminated Technosol profiles. We demonstrate that Tl in the studied soils varies significantly in both concentration (500 mg/kg-18 g/kg) and isotopic composition (-1.6 and +3.2 of ε205Tl, a ∼0.5‰ spread), which is due to changes in the phase chemistry and/or mineralogy of Tl. Moreover, the observed 205Tl/203Tl ratios do not reflect the extent to which individual soils undergo Tl isotopic fractionation during mineral weathering and soil formation. Clearly, they reflect the initial isotopic signal(s) of the primary ore or ore minerals, and thus, the general history or type of their genesis. As the Tl carriers, various types of Tl-Me-arsenates, mixtures of jarosite and dorallcharite and minor Mn-oxides predominated. We revealed intense adsorption of Tl by the identified Mn-oxides (≤6.7 at.%). It is hypothesized that these phases are of key importance in the fractionation of Tl isotopes, meaning at this type of secondary oxide-soil solution interface. However, model studies involving primary/secondary components (sulfides, sulfates, oxides and arsenates) are required to understand the mechanisms that may lead to post-depositional Tl isotopic redistribution in soils, as well as Tl isotope systematics in mining wastes in general.

Assessing thallium phycoremediation by applying Anabaena laxa and Nostoc muscorum and exploring its effect on cellular growth, antioxidant, and metabolic profile

Thallium (Tl), a key element in high-tech industries, is recognized as a priority pollutant by the US EPA and EC. Tl accumulation threatens aquatic ecosystems. Despite its toxicity, little is known about its impact on cyanobacteria. This study explores the biochemical mechanisms of Tl(I) toxicity in cyanobacteria, focusing on physiology, metabolism, oxidative damage, and antioxidant responses. To this end, Anabaena and Nostoc were exposed to 400 µg/L, and 800 µg/L of Tl(I) over seven days. Anabaena showed superior Tl(I) accumulation with 7.8% removal at 400 µg/L and 9.5% at 800 µg/L, while Nostoc removed 2.2% and 7.4%, respectively. Tl(I) exposure significantly reduced the photosynthesis rate and function, more than in Nostoc. It also altered primary metabolism, increasing sugar levels and led to higher amino and fatty acids levels. While Tl(I) induced cellular damage in both species, Anabaena was less affected. Both species enhanced their antioxidant defense systems, with Anabaena showing a 175.6% increase in SOD levels under a high Tl(I) dose. This suggests that Anabaena's robust biosorption and antioxidant systems could be effective for Tl(I) removal. The study improves our understanding of Tl(I) toxicity, tolerance, and phycoremediation in cyanobacteria, aiding future bioremediation strategies.

The determination of thallium in the environment: A review of conventional and advanced techniques and applications

Thallium (Tl) is a potential toxicity element that poses significant ecological and environmental risks. Recently, a substantial amount of Tl has been released into the environment through natural and human activities, which attracts increasing attention. The determination of this hazardous and trace element is crucial for controlling its pollution. This article summarizes the advancement and progress in optimizing Tl detection techniques, including atomic absorption spectroscopy (AAS), voltammetry, inductively coupled plasma (ICP)-based methods, spectrophotometry, and X-ray-based methods. Additionally, it introduces sampling and pretreatment methods such as diffusive gradients in thin films (DGT), liquid-liquid extraction, solid phase extraction, and cloud point extraction. Among these techniques, ICP-mass spectrometry (MS) is the preferred choice for Tl detection due to its high precision in determining Tl as well as its species and isotopic composition. Meanwhile, some new materials and agents are employed in detection. The application of novel work electrode materials and chromogenic agents is discussed. Emphasis is placed on reducing solvent consumption and utilizing pretreatment techniques such as ultrasound-assisted processes and functionalized magnetic particles. Most detection is performed in aqueous matrices, while X-ray-based methods applied to solid phases are summarized which provide non-destructive analysis. This work improves the understanding of Tl determination technology while serving as a valuable resource for researchers seeking appropriate analytical techniques.

Cryptic footprint of thallium in soil-plant systems; A review

The current review highlights the complex behavior of thallium (Tl) in soil and plant systems, offering insight into its hazardous characteristics and far-reaching implications. The research investigates the many sources of Tl, from its natural existence in the earth crust to its increased release through anthropogenic activities such as industrial operations and mining. Soil emerges as a significant reservoir of Tl, with diverse physicochemical variables influencing bioavailability and entrance into the food chain, notably in Brassicaceae family members. Additionally, the study highlights a critical knowledge gap concerning Tl influence on legumes (e.g., soybean), underlining the pressing demand for additional studies in this crucial sector. Despite the importance of leguminous crops in the world food supply and soil fertility, the possible impacts of Tl on these crops have received little attention. As we traverse the ecological complexity of Tl, this review advocates the collaborative research efforts to eliminate crucial gaps and provide solutions for reducing Tl detrimental impacts on soil and plant systems. This effort intends to pave the path for sustainable agricultural practices by emphasizing the creation of Tl-tolerant legume varieties and revealing the complicated dynamics of Tl-plant interactions, assuring the long-term durability of our food systems against the danger of Tl toxicity.

High geogenic soil thallium shows limited impact on bacterial community

Thallium (Tl) is a highly toxic trace metal, included in the US EPA list of priority pollutants. Even though its toxicity is potentially higher or comparable to Cd or Hg, its environmental impact is largely unknown. Despite its toxicity, only a few recent studies are mapping the impact of recently introduced Tl on soil microbial communities, namely in agricultural systems but no studies focus on its long term effect. To complement the understanding of the impact of Tl on soil, this study aims to describe the influence of extremely high naturally occurring Tl concentration (50 mg/kg of potentially bioavailable Tl) on soil microbial communities. Our investigation concentrated on samples collected at Buus (Erzmatt, Swiss Jura, Switzerland), encompassing forest and meadow soil profiles of the local soil formed on hydrothermally mineralized dolomite rock, which is naturally rich in Tl. The soil profiles showed a significant proportion of potentially bioavailable Tl. Yet, even this high concentration of Tl has a limited impact on the richness of the soil bacterial community. Only the meadow soil samples show a reduced richness compared to control samples. Furthermore, our analysis of geogenic Tl contamination in the region unveiled a surprising finding: compared to other soils of Switzerland and in stark contrast to soils affected by recent mining activities, the structure of the bacterial community in Buus remained relatively unaffected. This observation highlights the unique ability of soil microbial communities to withstand extreme Tl contamination. Our study advances the understanding of Tl's environmental impact and underscores the resilience of soil microbes in the face of severe long-term contamination.

A novel application of thallium isotopes in tracing metal(loid)s migration and related sources in contaminated paddy soils

Thallium (Tl) is a highly toxic heavy metal, which is harmful to plants and animals even in trace amounts. Migration behaviors of Tl in paddy soils system remain largely unknown. Herein, Tl isotopic compositions have been employed for the first time to explore Tl transfer and pathway in paddy soil system. The results showed considerably large Tl isotopic variations (ε²⁰⁵Tl = -0.99 ± 0.45 ~ 24.57 ± 0.27), which may result from interconversion between Tl(I) and Tl(III) under alternative redox conditions in the paddy system. Overall higher ε²⁰⁵Tl values of paddy soils in the deeper layers were probably attributed to abundant presence of Fe/Mn (hydr)oxides and occasionally extreme redox conditions during alternative dry-wet process which oxidized Tl(I) to Tl(III). A ternary mixing model using Tl isotopic compositions further disclosed that industrial waste contributed predominantly to Tl contamination in the studied soil, with an average contribution rate of 73.23%. All these findings indicate that Tl isotopes can be used as an efficient tracer for fingerprinting Tl pathway in complicated scenarios even under varied redox conditions, providing significant prospect in diverse environmental applications.

Acid mine drainage and smelter-derived sources affecting water geochemistry in the upper Nakdong River, South Korea

Both the smelter and acid mine drainage (AMD) in uppermost streams impact water geochemistry and deteriorate water quality. Efficient water quality management requires identifying the contribution of each source to stream water geochemistry. In this study, we aimed to determine the natural and anthropogenic sources (AMD and smelting) affecting water geochemistry by considering seasonality. Water samples were collected, from May 2020 to April 2021, in a main channel (Nakdong River) and tributaries in a small watershed including mines and smelters. The watershed is characterized by a carbonate-rich area in the upper-middle reaches and silicate-rich area in the middle-lower reaches. On the plots of Ca/Na vs. Mg/Na and 2(Ca + Mg) vs. HCO₃ + 2SO₄, the water geochemistry was predominantly explained by the carbonate and silicate weathering associated with sulfuric and carbonic acids. According to typical δ¹⁵N values for sources, nitrate contribution from soil-N mainly impacted water geochemistry, regardless of seasonality; the contribution from agricultural activity and sewage was negligible. Water geochemistry in the main channel samples was discriminated before and after passing through the smelter. The effects of the smelter were evident in elevated SO₄, Zn, and Tl concentrations and in δ⁶⁶Zn values; this was further supported by the relationships between Cl/HCO₃ and SO₄/HCO₃ and between δ⁶⁶Zn and Zn. These results were pronounced during winter, when the flush-out effect was absent. Our results suggest that multi-isotopes and chemical composition analyses can trace multiple sources influencing the water geochemistry in watersheds containing AMD and smelters.

Dynamic retention of thallium(I) on humic acid: Novel insights into the heterogeneous complexation ability and responsiveness

Widely distributed soil humic acid (HA) would significantly affect the environmental migration behavior of Tl(I), but a quantitative and mechanistic understanding of the dynamic Tl(I) retention process on HA is limited. A unified kinetic model was established by coupling the humic ion-binding model with a stirred-flow kinetic model, which quantified the complexation constants and responsiveness coefficients during dynamic Tl(I)-HA complexation. Furthermore, the heterogeneous complexation mechanism of HA and Tl(I) was revealed by batch adsorption experiments, stirred-flow migration experiments, and 2D-FTIR-COS analysis. An increase in pH significantly improved the responsiveness of HA organic binding sites, promoting Tl(I) dynamic retention. Monodentate carboxyl groups induced rapid Tl(I) complexation (k_d = 1.9 min^-1) in strongly acidic environments. Under weakly acidic conditions, Tl(I) retention on HA was mainly attributed to the synergistic complexation effect of carboxyl and amide groups. Among the groups, multidentate carboxyl-phenolic hydroxyl sites could achieve sustained Tl(I) retention due to their stable complexing properties (logK = 4.48∼7.46) and slow response (k_d = 1.1 × 10^-3 min^-1). These findings are crucial for a comprehensive understanding of the environmental interactions of Tl(I) with humic substances in swamp environments.

Integrated physiological, biochemical, and transcriptomic analysis of thallium toxicity in zebrafish (Danio rerio) larvae

Although several studies have evaluated the effects of Thallium (Tl) in adult species of fish, the developmental toxicity of Tl has not been previously explored. In this study, zebrafish embryos (<4 h post fertilization (hpf)) were exposed to Tl at concentrations from 0.8 to 400 μg L^-1 for 7 d. The results showed that the decreased hatching rate and increased malformation rate were observed in the larvae. The swimming velocity of larvae from 200 and 400 μg L^-1 treatments was respectively reduced by ~26 % and 15 %. Histopathological analysis of liver indicated the number of cells of karyolysis (143 % and 202 %) and pyknosis (170 % and 131 %) were respectively increased in 200 and 400 μg L^-1 Tl treatments. Meanwhile, the Tl body burden and metallothionein (MT) levels in the larvae were increased with elevated Tl concentrations. The level of malondialdehyde (MDA) was increased by ~20 to 51 % in all Tl treatments and total antioxidant capacity (TAC) was decreased by ~12 % at 200 μg L^-1. The activities of Na⁺/K⁺-ATPase and protease were inhibited in 200 and 400 μg L^-1 Tl treatments. Moreover, the transcripts of genes (Nrf2, HO-1, TNF-α, IL-1β, IL-8, IL-10, TGF) were significantly altered. In addition, a total of 930 differentially expressed genes (DEGs) and 1549 DEGs were found in the 200 and 400 μg L^-1 treatments with 458 overlapped DEGs by transcriptomic analysis. The protein digestion and absorption, ECM-receptor interaction, and complement and coagulation cascades pathways were shown to be the most significantly enriched pathways. This study helps better understand the molecular mechanisms of Tl toxicity in fish.

Effect of montmorillonite biochar composite amendment on thallium bioavailability in contaminated agricultural soils and its mitigated health risk

Little information is available on the effect of clay minerals and biochar composite on the remediation and bioavailability of thallium in agricultural soils. This study thus investigated the influence of montmorillonite biochar composite (Mnt-BC) amendment on the remediation of agricultural soil contaminated artificially by Tl and its potential health risks. Herein, bok choi was cultured to estimate the efficiency of soil Mnt-BC amendments through the bioavailability of Tl of the vegetable. Results showed that Tl bioavailability was significantly reduced in Mnt-BC-amended soils, mainly ascribed to the elevated soil pH and other improved soil properties of high functional groups (-OH, -COOH), negative charges, and exchangeable cations after amendment. Specifically, the highest immobilization efficiency of Tl in soils was observed in 2.5% treated soils with 79.11%, while in plant leaves the highest reduction of Tl was estimated to be 75.1% compared to the control treatment. Hence, the amendment dosage improved the immobilization of Tl in soil and subsequently reduced Tl uptake by the vegetable. Furthermore, from target hazard quotient (THQ) estimation, Mnt-BC amendment can lower the potential health risk while consuming such cultured bok choi in Tl-contaminated soils. Considering the environmental friendliness and high efficiency of Mnt-BC, it could be used as a potential soil amendment to remediate agricultural soils contaminated by Tl.

Preliminary Screening of Soils Natural Radioactivity and Metal(loid) Content in a Decommissioned Rare Earth Elements Processing Plant, Guangdong, China

Radiological aspects such as natural radioactivity of ²³⁸U, ²³²Th, ²²⁶Ra, ⁴⁰K combined with potentially toxic metal(loid) (PTM) distribution features were seldom simultaneously investigated in rare earth element (REE) processing activities. This work was designed to investigate the distribution levels of natural radioactivity, air-absorbed dose rate of γ radiation as well as PTMs at a typical REE plant in Guangdong, China. Ambient soils around REE processing facilities were sampled, measured and assessed. The natural radioactivity of radionuclides of the samples was determined using a high-purity germanium γ-energy spectrometer while the air-absorbed dose rate of γ radiation was measured at a height of 1 m above the ground using a portable radiometric detector. The PTM content was measured by inductively coupled plasma mass spectrometry (ICP-MS). The results showed that the specific activities of the radionuclides ranged from 80.8 to 1990.2, 68.2 to 6935.0, 78.4 to 14,372.4, and 625.4 to 2698.4 Bq·kg^-1 for ²³⁸U, ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively, representing overwhelmingly higher activity concentrations than worldwide soil average natural radioactivity. The radium equivalent activity and external hazard index of most samples exceeded the limits of 370 Bq·kg^-1 and 1, respectively. The measured air-absorbed dose rate of γ radiation was in a range of 113~4004 nGy·h^-1, with most sites displaying comparatively higher values than that from some other REE-associated industrial sites referenced. The content levels of PTMs of Cu, Ni, Zn, Mn, Pb, Cd, Cr, and As were 0.7~37.2, 1.8~16.9, 20.4~2070.5, 39.4~431.3, 2.3~1411.5, 0.1~0.7, 6.7~526.1, and 59.5~263.8 mg·kg^-1, respectively. It is important to note that the PTM contents in the studied soil samples were 2.1~5.4 times higher for Zn-As and 1.4 times higher for Pb than the third level of the China soil standard while 2.5~13 times higher for Zn-As and 1.2 times higher for Pb than Canadian industry standard. The findings call for subsequent site remediation to secure the ecological environment and human health after the REE processing plant was decommissioned.

Pollution characteristics and environmental availability of toxic elements in soil from an abandoned arsenic-containing mine

Understanding the pollution characteristics and assessing the ecological risk of toxic metals in mine soil are crucial to controlling and managing risks in abandoned mine areas. In this study, the profile soil pollution characteristics and modified ecological risk of As, Cd, Hg, Pb, Sb, and Tl for both the different mining functional areas and the downstream impacted areas at a large-scale abandoned arsenic-containing mine were studied. Results showed that both the profile soils at the mining functional areas and the surface layer in downstream sites are heavily polluted by As, Cd, Hg, Sb, and Tl. As, Hg, Sb, and Tl mainly accumulated on soils with a depth of 0-1.5 m. In contrast, these metals in the mining site were gradually increased with soil depth above the bedrock strata. Cd and Pb were mainly concentrated at depth of 2.5-3.5 m in the smelting with by-product processing site. The speciation of metals in the profile soils mainly occurred in residual fraction. However, high levels of potential mobile As and Sb were found in mining soils and smelting surface soils, as well as Tl in deep soils at mining functional sites and top soils at downstream sites, with their mean contents in these areas arrived to 2950 mg kg^-1, 9.64 mg kg^-1, and 0.98 mg kg^-1, respectively. In addition, the modified ecological risk assessment (NIRI_m) values revealed a substantial ecological risk of As, Cd, Hg, and Sb in both the entire profile soils at the mining, smelting sites and topsoil (0-1.5 m) at the adjacent downstream site. In summary, the pollution characteristics and potential ecological risk of toxic metals in profile soils from the different functional sites at arsenic-containing mine were significantly different and suitable control strategies for available toxic elements should be adopted in the different functional sites of mine.

Understanding stable Tl isotopes in industrial processes and the environment: A review

In this review, a compilation of the current knowledge on stable thallium (Tl) isotopes (²⁰⁵Tl and ²⁰³Tl) in specific industrial processes, soils and plants is presented. An overview of the processes that may control Tl concentration and Tl isotope fractionation is compiled, while also overviewing the ability of Tl isotopic ratios to be used as a 'fingerprint' in source apportionment. Thallium isotopic compositions not only depend on their origin, but also on soil processes that may occur over time. One of the most important phases affecting the fractionation of stable Tl isotopes in soils (or sediments) was systematically identified to be specific Mn(III,IV)-oxides (mainly birnessite), due to their potential ability of oxidative Tl sorption, i.e., indicative of redox Tl reactions to be critical controlling factor. It has been established that the Brassica family is a hyperaccumulator of Tl, with clear demonstrations of Tl isotopic fractionation occurring up the translocation pathway. A clear pattern, so far, was observed with Tl isotopic compositions in plants grown on soils that were contaminated and those grown on uncontaminated soils, indicating the importance of the growing medium on Tl uptake, translocation, and isotopic fractionation.

Effect of peat organic matter on sulfide weathering and thallium reactivity: Implications for organic environments

Weathering of Tl-containing sulfides in a model (12-week) peat pot trial was studied to better understand their geochemical stability, dissolution kinetics, alteration products and the associated release and mobility of anthropogenic Tl in organic environments. We also present the effect of industrial acid rainwater on sulfide degradation and Tl migration in naturally acidic peat. Sphalerite (ZnS) was much less stable in peat than other Tl-containing sulfides (galena and pyrite), and thus acted as a major phase responsible for Tl mobilization. Furthermore, Tl incongruently leached out over Zn from ZnS, and accumulated considerably more in the peat solutions (≤5 μg Tl/L) and the peat samples (≤0.4 mg Tl/kg) that were subjected to acid rain watering compared to a deionized H₂O regime. This finding was in good agreement with the absence of secondary Tl-containing phases, which could potentially control the Tl flux into the peat. The behavior of Tl was not as conservative as Pb throughout the trial, since a higher peat mobility and migration potential of Tl was observed compared to Pb. In conclusion, industrial acid precipitations can significantly affect the stability of ZnS even in acidic peat/organic environments, making it susceptible to enhanced weathering and Tl release in the long term.

Thallium and potentially toxic elements distribution in pine needles, tree rings and soils around a pyrite mine and indication for environmental pollution

In this study, the distributions of thallium (Tl), and other potential toxic elements, such as Cd, Co, Cu, Pb, Sr, As, Cr, Ni, Zn, and Mn in needles, tree rings and soils of pine trees in one of the largest pyrite mining areas in the world, i.e., Yunfu, China were investigated. The results showed that pseudo-total Tl concentration of the tree rings ranged from 0.41 to 2.03 mg/kg (average: 1.12 mg/kg) during the year of 1998 to 2011. This indicates an overall obvious enrichment of Tl. Further investigation of element level variations in the pine needles showed a negative correlation between Tl content and the distance from the mining area. The results of Principal Component Analysis additionally demonstrated that Tl in the tree rings was most likely derived from the pine needles. Notably, Tl contents in the tree rings exhibited generally similar distribution pattern to the annual production intensity of Yunfu pyrite mining activities. The findings suggest that metal(loid)s in particular of Tl in pine tree rings can be used as alternative proxies to approximatively reconstruct the chronological change of atmospheric environmental pollution induced by pyrite associated mining/smelting activities.

Thallium isotopic compositions as tracers in environmental studies: A review

Thallium is a highly poisonous heavy metal. Since Tl pollution control has been neglected worldwide until the present, countless Tl pollutants have been discharged into the environment, endangering the safety of drinking water, farmland soil, and food chain, and eventually posing a great threat to human health. However, the source, occurrence, pathway and fate of Tl in the environment remains understudied. As Tl in non-contaminated systems and from anthropogenic origin exhibits generally different isotopic signatures, which can provide fingerprint information and a novel way for tracing the anthropogenic Tl sources and understanding the environmental processes. This review summarizes: (i) the state-of-the-art development in highly-precise determination analytical method of Tl isotopic compositions, (ii) Tl isotopic fractionation induced by the low-temperature surface biogeochemical process, (iii) Tl isotopic signature of pollutants derived from anthropogenic activities and isotopic fractionation mechanism of Tl related to the high-temperature industrial activities, and (iv) application of Tl isotopic composition as a new tracer emerging tracer for source apportionment of Tl pollution. Finally, the limitations and possible future research about Tl isotopic application in environmental contamination is also proposed: (1) Tl fractionation mechanism in different environmental geochemistry processes and industrial activities should be further probed comprehensively; (2) Tl isotopes for source apportionment should be further applied in other different high Tl-contaminated scenarios (e.g., agricultural systems, water/sediment, and atmosphere).

Evaluation of thallium isotopic fractionation during the metallurgical processing of sulfides: An update

In this study, we report combined Tl isotopic and Tl mineralogical and speciation data from a set of Tl-rich sulfide concentrates and technological wastes from hydrometallurgical Zn extraction. We also present the first evaluation of Tl isotopic ratios over a cycle of sulfide processing, from the ore flotation to pyro- and hydrometallurgical stages. The results demonstrate that the prevailing Tl form in all samples is Tl(I), without any preferential incorporation into sulfides or Tl-containing secondary phases, indicating an absence of Tl redox reactions. Although the Tl concentrations varied significantly in the studied samples (~9-280 mg/kg), the overall Tl isotopic variability was small, in the range of -3.1 to -4.4 ± 0.7 (2σ) ε²⁰⁵Tl units. By combining present ε²⁰⁵Tl results with the trends first found for a local roasting plant, it is possible to infer minimum Tl isotopic effects throughout the studied industrial process. As a result, the use of Tl isotopic ratios as a source proxy may be complicated or even impossible in areas with naturally high/extreme Tl background contents. On the other hand, areas with two or more isotopically contrasting Tl sources allow for relatively easy tracing, i.e., in compartments which do not suffer from post-depositional isotopic redistributions.

Quantification of smelter-derived contributions to thallium contamination in river sediments: Novel insights from thallium isotope evidence

Thallium(Tl), an extremely toxic metal, is posing great hazards to water safety through anthropogenic activities (e.g., Pb-Zn smelter) and natural weathering in riverine systems. However, the relative contribution from each source remains obscure. This study investigated enrichment pattern of Tl and its isotopic compositions in sediment profiles from a recipient river, which was continuously collecting various Tl-bearing wastes discharged from a large Pb-Zn smelter in South China. Results show that high Tl content and ultra-fine particles (~ μm) of Tl-bearing mineral assemblages, probably derived from Pb-Zn smelting wastes, were ubiquitously observed in both of the depth profiles. In addition, the sediments generally yielded intermediate ε²⁰⁵Tl values of -3.76 to 1.01, which resembled those found in smelting wastes. A ternary mixing model was for the first time proposed for quantifying relative Tl contributions from each possible source. The calculation suggests that the smelter wastes are the major contributors, contributing approximately 80% of Tl contamination. All these results indicate that Tl isotope can be used as powerful proxies for quantitatively identifying potential different contributors in the environment. This is of critical importance to further implementation of pollution control and remediation strategy for the riverine systems in the near future.

Thallium and lead variations in a contaminated peatland: A combined isotopic study from a mining/smelting area

Vertical profiles of Tl, Pb and Zn concentrations and Tl and Pb isotopic ratios in a contaminated peatland/fen (Wolbrom, Poland) were studied to address questions regarding (i) potential long-term immobility of Tl in a peat profile, and (ii) a possible link in Tl isotopic signatures between a Tl source and a peat sample. Both prerequisites are required for using peatlands as archives of atmospheric Tl deposition and Tl isotopic ratios as a source proxy. We demonstrate that Tl is an immobile element in peat with a conservative pattern synonymous to that of Pb, and in contrast to Zn. However, the peat Tl record was more affected by geogenic source(s), as inferred from the calculated element enrichments. The finding further implies that Tl was largely absent from the pre-industrial emissions (>~250 years BP). The measured variations in Tl isotopic ratios in respective peat samples suggest a consistency with anthropogenic Tl (ε²⁰⁵Tl between ~ -3 and -4), as well as with background Tl isotopic values in the study area (ε²⁰⁵Tl between ~0 and -1), in line with detected ²⁰⁶Pb/²⁰⁷Pb ratios (1.16-1.19). Therefore, we propose that peatlands can be used for monitoring trends in Tl deposition and that Tl isotopic ratios can serve to distinguish its origin(s). However, given that the studied fen has a particularly complicated geochemistry (attributed to significant environmental changes in its history), it seems that ombrotrophic peatlands could be better suited for this type of Tl research.

Geochemical distribution and speciation of thallium in groundwater impacted by acid mine drainage (Southern China)

Thallium (Tl) commonly occurs in shallow groundwater affected by acid mine drainage (AMD); however, our knowledge of the occurrence of Tl in shallow groundwater is limited. This study observes that the shallow groundwater in an AMD-impacted area in Southern China contains an elevated Tl concentration (22 μg/L) under the oxidizing conditions and a low Tl concentration (<1 μg/L) in the reducing environment. The groundwater Tl concentration is positively correlated with oxidation-reduction potential (Eh) and negatively correlated with Cl content. The modelling results of the Tl species demonstrate that Tl⁺, TlSO₄^-, TlCl, and TlNO₃ are the main forms of Tl in groundwater. Tl may precipitate as Tl(OH)₃ under weakly acidic to alkaline conditions. Drill-core analysis of wells indicates that the Tl content in the vadose zone is equal to the background soil Tl content under oxidizing conditions. However, under artificial reducing conditions, the Tl content at the 3-4 m depth below the groundwater level ranges from 1.6 to 3.5 μg/g. This finding demonstrates that Tl solute in groundwater migrates into the aquifer when redox conditions change. Mn-oxides and illite in the weak permeable aquifer are the key minerals for Tl adsorption; some major sites of illite start to uptake Tl from pH 8.0. This study highlights not only the geochemical distribution of Tl in groundwater but also the influences of changes in redox conditions caused by human activities on Tl enrichment in groundwater. Enhancing our understanding of the aqueous geochemistry of Tl is of significance for the prevention and control of Tl pollution.

Escalating health risk of thallium and arsenic from farmland contamination fueled by cement-making activities: A hidden but significant source

Soil-to-vegetable migration of toxic metal(loid)s is a pivotal pathway of human exposure to chemical intoxication. Thallium (Tl) and arsenic (As) are highly toxic metal(loid)s but their co-occurrence in soils and vegetables remain poorly understood. Herein, the present study focuses on potential health risk arising from co-occurrence of TlAs in various common vegetables cultivated in different farmlands around an industrial area featured by cement production activities. The results reveal obvious co-contamination of Tl (2.28 ± 1.39 mg/kg) and As (102.0 ± 66.7 mg/kg) in soils. Fine particles bearing sulfide and other minerals associated with Tl and As are detected in fly ash from cement plant, which can be migrated by wind over a long distance with hidden but inevitable pollution. Bioaccumulation Factor (BCF) and Enrichment Factor (EF) show that taro and corn preferentially accumulate Tl especially in underground parts. Hazard Quotient (HQ) indicates that consumption of these vegetables may result in chronic poisoning and/or even carcinogenic risk. The study highlights that the pathway and high risk of co-contamination of TlAs in the nearby farmlands posed by cement-making activities should be highly concerned.

Retention of thallium by natural minerals: A review

Though thallium (Tl) is usually present in trace amounts in natural environments, its biotoxicity is extremely high. With the development of mining, the metallurgy industry, and the growing application of Tl in high-tech fields, the threat of Tl to ecological environments and human health is increasing. Natural minerals, such as clay minerals, iron oxides, and manganese oxides, are natural Tl adsorbents due to their mineralogy and crystal structures. In this review, we discuss the mechanisms of Tl adsorption by various natural minerals, compare the adsorption capacities of common soil minerals for Tl, and describe the limitations of traditional sequential extraction methods for identifying the chemical states of Tl on minerals and source of Tl. We also provide suggestions on future directions needed in Tl research including a) additional in-depth studies on the competitive adsorption of Tl by minerals; b) more direct comparison of Tl adsorption behavior from lab-based experiments with field observations to clarify the mechanisms of Tl adsorption by minerals under environmental conditions; c) more research data are needed to support the establishment and improvement of relevant research methods based on modern leading-edge technologies such as synchrotron radiation. Further, we suggest further research is needed in adsorption technologies used for Tl treatment. This is the first review on the research progress of Tl adsorption by natural minerals with the purpose of helping understanding the mechanisms of Tl migration and transformation controlled by natural minerals, and providing theoretical supports for the development of Tl adsorbents and the treatments of Tl pollution.

Emergent thallium exposure from uranium mill tailings

Thallium (Tl) pollution caused by the exploitation of uranium (U) mines has long been neglected due to its low crustal abundance. However, Tl may be enriched in minerals of U ore because Tl has both sulfurophile and lithophile properties. Herein, a semi-dynamic leaching experiment combined with statistical analysis, geochemical speciation and multi-characterization provided novel insight into the distinct features and mechanisms of Tl release from uranium mill tailings (UMT). The results showed that particle size effects prevail over the pH on Tl release, and surface dissolution is the pivotal mechanism controlling Tl release based on Fick's diffusion model. The study revealed that long-term leaching and weathering can lead to the increased acid-extractable and oxidizable fractions of Tl in UMT, and that the exposure and dissolution of Tl-containing sulfides would largely enhance the flux of Tl release. The findings indicate that UMT containing (abundant) pyrite should be paid particular attention due to Tl exposure. Besides, critical concern over the potential Tl pollution in universal U mining and hydrometallurgical areas likewise may need to be seriously reconsidered.

Contamination, oral bioaccessibility and human health risk assessment of thallium and other metal(loid)s in farmland soils around a historic TlHg mining area

In this study, tweenty-nine soil samples were collected from a historic TlHg mining area, located in southwest Guizhou, China. Total concentrations of metal(loid)s in soils and in vitro extracts were analysed by ICP-MS, and the bioaccessibility of metal(loid)s was conducted by two often used in vitro extraction methods, Simplified bioaccessibility Extraction Test (SBET) and Physiologically Based Extraction Test (PBET). The health risk assessment based on total concentrations of metal(loid)s, bioaccessibility of SBET and PBET through soil ingestion were investigated. Results indicated that the collected cultivated soils contained elevated concentrations of Tl (44.8 ± 67.7 mg kg^-1), Hg (110 ± 193 mg kg^-1), As (84.4 ± 89.2 mg kg^-1) and Sb (14.8 ± 24.8 mg kg^-1), exceeding the regional background values of Guizhou province, China and the Chinese farmland risk screening values. However, the bioaccessibility of Tl, Hg, As and Sb were relatively low, usually less than 30% for most samples and varied greatly among metal(loid)s and sampling sites. The average bioaccessibility values of Tl, Hg, As and Sb by SBET were lower than those by PBET. The non-carsinogenic risk (HQ and HI) and Carcinogenic Risk (CR) values were significantly reduced when incorporating the bioaccessibiltiy of metal(loid)s into health risk assessment. It is worth noting that the health risk to children exceeded adults. Moreover, Tl and As contributed the most to the risk, indicating that more attention should be paid on Tl and As during the daily environmental regulation and management of contaminated soils in Lanmuchang.

Heavy Metal(loid)s Contamination in Ground Dust and Associated Health Risks at a Former Indigenous Zinc Smelting Area

Indigenous zinc smelting (IZS) is a backward technique that releases a great deal of heavy metal(loid)s into the environment. However, the contamination of heavy metal(loid)s in ground dust and the associated health risks in such areas are poorly known. In this study, a former IZS area in Guizhou, China, was surveyed during 2008–2018 with 15 elements (Ag, As, Bi, Cd, Co, Cr, Cu, Hg, In, Ni, Pb, Sb, Sn, Tl, Zn) being analyzed. The results indicate that most elements (e.g., Ag, As, Cd, Cu, Pb, Sb, Sn, Zn) in ground dust decreased significantly after the cessation of the IZS in 2006; nevertheless, some elements still remained at relatively high levels in 2018, e.g., Pb (average: 762 ± 647 mg/kg), Zn (average: 1287 ± 753 mg/kg), Cd (average: 7.76 ± 5.06 mg/kg), and As (average: 41.9 ± 34.8 mg/kg), indicating they might come from the local contaminated soils, slag residues and smelting potteries. In terms of the impacts on human health, children have both higher non-carcinogenic and carcinogenic risks than that of adults, with the latter subpopulation having a lower risk than the threshold values. Pb and As were the two elements with the highest non-carcinogenic risk for children, the hazard index of local children was still higher than the threshold of 1 (e.g., 1.43 for As, 2.09 for Pb) in 2018. The carcinogenic risk of As exposure to children dropped more than two times to 6.42 × 10⁻⁷ in 2018, which falls below the tolerable range (10⁻⁶–10⁻⁴). This study revealed that although the concentration of heavy metal(loid)s in ground dust and linked health risk in the IZS area has reduced dramatically after the cessation of IZS, continued removal of slag residues and smelting potteries is necessary for further decreasing the human health risk.

Atmospheric trace metal deposition to remote Northwest Ontario, Canada: Anthropogenic fluxes and inventories from 1860 to 2010

National and global inventories of anthropogenic trace element emissions to air is a comparatively recent phenomenon (post-1993 in Canada) as is the monitoring of atmospheric metal deposition, the latter being also very spatially limited. Paleo-reconstructive methods offer a contiguous record of environmental contamination providing a needed framework to establish locally relevant "pre-industrial" (~natural) conditions which can be compare with relative and quantitative deviations away from reference conditions. In this study, we reconstruct the history of the long-range, anthropogenic sourced atmospheric trace element deposition to the remote region of Northwestern Ontario Canada (Experimental Lakes Area (ELA)) using dated sediment records from five lakes. Several elements are shown to be highly enriched in lake sediments relative to pre-1860 sediments (Antimony, Lead, Tellurium, Tin, Arsenic, Bismuth, Cadmium and Mercury) and moderately (Zinc, Tungsten, Thallium, Copper, Silver, Selenium, Nickel and Vanadium). Mean decadal anthropogenic atmospheric fluxes (mg m-2 yr-1) are reconstructed for 1860-2010 and compare well with available local (ELA), regional (NW Ontario Canada, N Michigan USA) monitoring data, as well as global assessments of anthropogenic contribution to atmospheric trace metal burdens. Quantitative paleo reconstructions of atmospheric contamination history using the collective signal from multiple lakes provide a rigorous methodology to assess trends, uncertainties, evaluation with monitoring data and, provide an opportunity to explore landscape processes of contaminant transport and storage. Further study of the latter is recommended to understand the latency of legacy anthropogenic contamination of the environment.

Microbial insights into the biogeochemical features of thallium occurrence: A case study from polluted river sediments

Thallium (Tl) is a trace element with extreme toxicity. Widespread Tl pollution in riverine systems, mainly due to escalating mining and smelting activities of Tl-bearing sulfide minerals, has attracted increasing attention. Insights into the function of the microbial communities with advanced characterization tools are critical for understanding the biogeochemical cycle of Tl. Herein, microbial communities and their adaptive evolution strategies in river sediments from a representative Tl-bearing pyrite mine area in southern China were profiled via 16S rRNA gene sequence analysis and shotgun metagenomic analysis. In total, 64 phyla and 778 genera of microorganisms were observed in the studied sediments. The results showed that pH, Tl, Pb, Zn and total organic carbon (TOC) had a significant influence on microbial community structure. Some important reductive microorganisms (such as Erysipelothrix, Geobacter, desulfatiferula, desulfatihabadium and fusibacter) were involved in the biogeochemical cycle of Tl. The ruv, rec, ars and other resistance genes enhanced the tolerance of microorganisms to Tl. The study suggested that relevant C, N and S cycle genes were the main metabolic paths of microorganisms surviving in the high Tl-polluted environment. The findings were critical for establishment, operation and regulation in the microbial treatment of Tl containing or related wastewater.

Thallium isotopic fractionation in soil: the key controls

We studied the key geochemical and mineralogical factors that could affect the fractionation of stable thallium (Tl) isotopes in soil. A set of grassland soil samples enriched in geogenic Tl in combination with selected Tl-containing mineral materials from the Czech Republic (Kluky) were investigated for this purpose. The results demonstrate significant incorporation of Tl in pedogenic (specific) Mn-oxide, which led to a large accumulation of the heavy ²⁰⁵Tl isotope (∼+14 ε²⁰⁵Tl units), presumably resulting from oxidative Tl sorption. Consequently, we concluded that the Mn-oxide-controlled Tl uptake is the primary cause of the observed ²⁰⁵Tl enrichment in the middle profile zone, at the A/B soil horizon interface, with up to +4 of ε²⁰⁵Tl. Furthermore, our results displayed a clear relationship between the Tl isotopic fractionation degree and the Mn-oxide soil concentration (R² = 0.6), as derived from the oxalate-extractable data. A combination of soil and mineralogical considerations suggests that ²⁰⁵Tl enrichment in respective soil samples is also partly due to the Tl present in micaceous clay minerals, mainly illite, which is the predominant pedogenic Tl host phase. In line with our previous results, this Tl behavior can be inferred from systematic Mn-oxide degradation and the associated Tl (enriched in ²⁰⁵Tl) cycling in the studied soils and thus, presumably in the redoximorphic soils in general.

Thallium isotopic fractionation in industrial process of pyrite smelting and environmental implications

Thallium (Tl) is typical rare element with severe toxicity comparable to Hg and Pb. To track Tl pollution, isotopic fractionation of Tl was evaluated during pyrite smelting for sulfuric acid production. Large variations in Tl isotope compositions were observed among the pyrite ore (PO) and its four different smelting wastes. The starting raw PO had an ε²⁰⁵Tl value of +1.28. The fluidized-bed furnace slag generated by high-temperature smelting had the heaviest ε²⁰⁵Tl (+16.24) in the system. Meanwhile, the boiler fly ash (ε²⁰⁵Tl = +8.34), cyclone fly ash (ε²⁰⁵Tl = +2.17), and electrostatic precipitation fly ash (ε²⁰⁵Tl = -1.10), with decreasing grain sizes during the treatment processes, were characterized by elevated levels of Tl contents and substantial enrichment in the light Tl isotopes relative to the furnace slag. Further calculation and high-resolution transmission electron microscopy indicated that Tl isotope fractionation could be governed by both Rayleigh-type fractionation and adsorption of volatilized Tl by particles of various grain sizes. According to the substantial differences in the PO from its smelting wastes and the measurement precision of isotopic fractionation, it is suggested that Tl isotopes can serve as a new tool for tracing pollution of Tl.

Thallium(I) sequestration by jarosite and birnessite: Structural incorporation vs surface adsorption

Jarosite and birnessite secondary minerals play a pivotal role in the mobility, transport and fate of trace elements in the environment, although geochemical interactions of these compounds with extremely toxic thallium (Tl) remain poorly known. In this study, we investigated the sorption behavior of Tl(I) onto synthetic jarosite and birnessite, two minerals commonly found in soils and sediments as well as in mining-impacted areas where harsh conditions are involved. To achieve this, sorption and desorption experiments were carried out under two different acidic conditions and various Tl(I) concentrations to mimic natural scenarios. In addition, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and inductively coupled plasma (ICP) analyses were conducted to determine the performance of both minerals for Tl(I) sequestration. Our results indicate that both phases can effectively remove aqueous Tl by different sorption mechanisms. Jarosite preferentially incorporates Tl(I) into the structure to form Tl(I)-jarosite and eventually the mineral dorallcharite (TlFe₃(SO₄)₂(OH)₆) as increasing amounts of Tl are employed. Birnessite, however, favorably uptakes Tl(I) through an irreversible surface adsorption mechanism, underlining the affinity of Tl for this mineral in the entire concentration range studied (0.5-5 mmol L^-1). Lastly, the presence of Tl(I) in conditions where aqueous molar ratio Tl/Mn is ∼0.25 inhibits the formation of birnessite since oxidation of Tl(I) to Tl(III) followed by precipitation of avicennite (Tl₂O₃) take place. Thus, the present research may provide useful insights on the role of both jarosite and birnessite minerals in Tl environmental cycles.

Geochemical fractionation of thallium in contaminated soils near a large-scale Hg-Tl mineralised area

Enriched levels of thallium (Tl) in the environment are not only derived from anthropogenic sources but also have potential natural origins owing to Tl-rich sulphide mineralization. However, little is known regarding the geochemical fractionations of Tl in contaminated soils from geogenic sources. This study aims to reveal the Tl geochemical fractionations in different types of soils from a large-scale independent Tl mine in southwestern China, via a modified Institute for Reference Materials and Measurement (IRMM) sequential extraction (four-step) scheme. The results revealed that a large percentage of Tl was related to the labile portions (including reducible, weak-acid-exchangeable, and oxidizable fraction) of the soils (68.8-367 mg kg^-1). Further analyses by Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectrometer (STEM-EDS) found that Tl mainly existed in the Fe-containing minerals (such as jarosite and hematite) with fine particles (∼1 μm). These results highlight that, apart from the anthropogenically induced Tl pollution, the naturally occurring Tl contamination in soils may also pose significant risks to human health and ecological safety. Owing to the relatively high mobility and bioavailability of Tl in the labile fractions, it is important to understand geochemical fractionations of this element for alleviating Tl pollution and effective management of naturally occurring Tl contaminated soils.

Environmental risk related to presence and mobility of As, Cd and Tl in soils in the vicinity of a metallurgical plant - Long-term observations

Analysis of the content of As, Cd and Tl accompanying Zn-Pb ores was performed using soil material sampled around the zinc-works in Miasteczko Śląskie. It was found that the total content of As, Cd and Tl was considerably higher than the values recorded at the same sites 20 years earlier. This clearly confirms considerable pollution of the environment which continues despite pro-ecological actions taken by the zinc-works. The analysis of the spatial distribution of pollution revealed that the zinc-works itself is no longer the main source of contamination as was the case in 1998. Instead, in 2018 the main sources of As, Cd and Tl emissions were the erosion and deflation processes in old, unprotected mining and industrial waste dumps located near the zinc-works. The most polluted soil layers are the top layers in the soil profile. The content of As, Cd and Tl determined in them was 10 times greater than in the deep layers. There was also a positive change in environmental conditions, which occurred over the preceding 20 years, namely an increase in the content of sparingly soluble fractions, fractions bound with organic matter, and residual fractions. It was also found that depending on the geochemical background and readily mobile fraction values used in the calculations, there is still very high contamination and very high ecological risk with respect to As, Cd and Tl after 20 years. However, when using the quantity of phytoavailable elements as well as the quantity of cations on ion exchange positions and bound to carbonates to calculate the risk assessment code, the analysis showed that environmental conditions improved.

The effects of interaction between vermiculite and manganese dioxide on the environmental geochemical process of thallium

The interaction among various soil minerals can significantly impact on the environmental geochemical process of contaminants. Therefore, this study investigated the effects of interaction between vermiculite (VER) and manganese dioxide (MnO₂) on the migration and transformation of Tl(I). The VER exhibited typical layered structure and MnO₂ possessed a flower-like structure with serious reunion phenomenon, while the production of interaction between vermiculite and manganese dioxide, labeled VER-MnO₂, illustrated as fish scales evenly spread over a large sheet, suggesting that MnO₂ could triumphantly be anchored on the VER and the aggregation of MnO₂ was prevented. Compared with the pure MnO₂, VER acted as template substrate contributed the higher specific surface area (298.18 m²·g^-1) and the oxidation degree of Mn. VER-MnO₂ showed the highest fixation capacity (144.29 mg·g^-1) than other two materials in the order VER-MnO₂ > MnO₂ > VER, and there was no risk derived from Mn dissolution. The influence mechanism of VER-MnO₂ on Tl(I) migration and transformation lied in immobilization, ion exchange and oxidization. Fixed-bed column immobilization experiments showed that VER-MnO₂ could purify drinking water contaminated by Tl (20 μg·L^-1) and the effective breakthrough volumes were 900 bed volumes until reaching the maximum limits allowed in drinking water (0.1 μg·L^-1). VER-MnO₂ excellently catches Tl to prevent groundwater pollution. This study provides a theoretical guidance for environmental fate and restoration of soil heavy metal pollution.

Adsorption of thallium(I) on rutile nano-titanium dioxide and environmental implications

Rutile nano-titanium dioxide (RNTD) characterized by loose particles with diameter in 20–50 nm has a very large surface area for adsorption of Tl, a typical trace metal that has severe toxicity. The increasing application of RNTD and widespread discharge of Tl-bearing effluents from various industrial activities would increase the risk of their co-exposure in aquatic environments. The adsorption behavior of Tl(I) (a prevalent form of Tl in nature) on RNTD was studied as a function of solution pH, temperature, and ion strength. Adsorption isotherms, kinetics, and thermodynamics for Tl(I) were also investigated. The adsorption of Tl(I) on RNTD started at very low pH values and increased abruptly, then maintained at high level with increasing pH >9. Uptake of Tl(I) was very fast on RNTD in the first 15 min then slowed down. The adsorption of Tl(I) on RNTD was an exothermic process; and the adsorption isotherm of Tl(I) followed the Langmuir model, with the maximum adsorption amount of 51.2 mg/g at room temperature. The kinetics of Tl adsorption can be described by a pseudo-second-order equation. FT-IR spectroscopy revealed that -OH and -TiOO-H play an important role in the adsorption. All these results indicate that RNTD has a fast adsorption rate and excellent adsorption amount for Tl(I), which can thus alter the transport, bioavailability and fate of Tl(I) in aqueous environment.

Thallium pollution in China and removal technologies for waters: A review

Thallium (Tl) is a typical toxic metal, which poses a great threat to human health through drinking water and the food chain (biomagnification). China has rich Tl-bearing mineral resources, which have been extensively explored and utilized, leading to release of large amounts of Tl into the environment. However, research on Tl pollution and removal techniques is relatively limited, because Tl has not been listed within the scope of environmental monitoring in China for several decades. This paper reviewed Tl pollution in wastewater arising from various industries in China, as well as the latest available methods for treating Tl-containing industrial wastewater, in order to give an outlook on effective technologies for controlling Tl pollution. Conventional physical and chemical treatment technologies are efficient at removing trace amounts of Tl, but it proved to be difficult to achieve the stringent environmental standard (≤0.1-5 μg/L) cost-effectively. Adsorption by using newly developed nanomaterials, and metal oxide modified polymer materials and microbial fuel cells are highly promising and expected to become next-generation technologies for remediation of Tl pollution. With the potential for greater Tl contamination in the environment under accelerated growth of industrialization, researches based on lab-scale implementation of such promising treatment technologies should be further expanded to pilot and industrial scale, ensuring environmental protection and the safety of drinking water for sustainable development. Comprehensive insights into experiences of Tl pollution in China and in-depth perspectives on new frontier technologies of Tl removal from wastewaters will also benefit other nations/regions worldwide, which are susceptible to high exposure to Tl likewise.

Insights into Heavy Metals Leakage in Chelator-Induced Phytoextraction of Pb- and Tl-Contaminated Soil

Chelators including DTPA (diethylene triamine pentaacetic acid) and oxalic acid were selected for inducing phytoextraction of heavy metals (HMs) from Pb-, Tl-, and Pb-Tl- contaminated soil, in which heavy metals leakage was highly remarkable. Results showed that compared with the control group without chelating agent under planting conditions, the extraction efficiency (i.e., uptake coefficient) of Pb, Tl increased by 86%, 43% from Pb-Tl- contaminated soil in the presence of oxalic acid, and there was no significant change in heavy metal leakage under rainfall conditions. It was the best phytoremediation scheme in this work. Under rainfall conditions, the HMs concentration in the leachate showed a linear decreasing trend. Acid rain promoted the leakage of heavy metals, and the average leached amount of Tl increased by 1.47 times under acid rain conditions. However, for Pb, DTPA was the main influencing factor, followed by acid rain.

The mobility of thallium in sediments and source apportionment by lead isotopes

Thallium (Tl) is a very toxic heavy metal. As a part of ongoing investigations, the mobility, sources and fate of Tl were investigated for sediments from a watershed in the northern part of the Pearl River, South China, whose catchment has been seriously impacted by large-scale PbZn smelting activities onshore. A wide dispersion of severe Tl contamination was observed throughout the depth profiles. A modified IRMM (Institute for Reference Materials and Measurements, Europe) sequential extraction procedure of a selected depth profile uncovered an exceptionally high enrichment of Tl in geochemically-mobile fractions (i.e., weak-acid-exchangeable, reducible and oxidizable fractions), on average 5.94 ± 2.19 mg/kg (74.6% ± 5.1% of the total Tl content) not only in the surface sediments but also in deep sediments. The proximal quantitative source apportionment using Pb isotopic fingerprinting technique indicated that a majority (80%-90%) of Tl contamination along the depth profiles is anthropogenically derived from the PbZn smelting wastes. The results highlight the pivotal role of smelting activities in discharging huge amounts of geochemically-mobile Tl to the sediments down to approximately 1 m in length, which is quantitatively evidenced by Pb isotopic tracing technique. Lead isotopes combined with distribution of Tl and Pb contents identified a potential marker for a point source from the PbZn smelter in the river catchment, which also provides a theoretical framework for source apportionment of metal contamination in a larger river/marine system and in other sulfide mining/smelting areas likewise.

Bioaccumulation of Tl in otoliths of Trout-perch (Percopsis omiscomaycus) from the Athabasca River, upstream and downstream of bitumen mining and upgrading

It has been suggested that open pit mining and upgrading of bitumen in northern Alberta releases Tl and other potentially toxic elements to the Athabasca River and its watershed. We examined Tl and other trace elements in otoliths of Trout-perch (Percopsis omiscomaycus), a non-migratory fish species, collected along the Athabasca River. Otoliths were analyzed using ICP-QMS, following acid digestion, in the metal-free, ultraclean SWAMP laboratory. Compared to their average abundance in the dissolved (<0.45 μm) fraction of Athabasca River, Tl showed the greatest enrichment in otoliths of any of the trace elements, with enrichments decreasing in the order Tl, Sr, Mn, Zn, Ba, Th, Ni, Rb, Fe, Al, Cr, Ni, Cu, Pb, Co, Li, Y, V, and Mo. Normalizing Tl in the otoliths to the concentrations of lithophile elements such as Li, Rb, Al or Y in the same tissue reveals average enrichments of 177, 22, 19 and 190 times, respectively, relative to the corresponding ratios in the water. None of the element concentrations (Tl, Li, Rb, Al, Y) or ratios were significantly greater downstream of industry compared to upstream. This natural bioaccumulation of Tl most likely reflects the similarity in geochemical and biological properties of Tl⁺ and K⁺. SUMMARY OF MAIN FINDINGS: Thallium is enriched in fish otoliths, relative to the chemical composition of the river, to the same degree both upstream and downstream of industry.

Impact of a severe flood on large-scale contamination of arable soils by potentially toxic elements (Serbia)

Extreme flooding in May, 2014 affected the sub-catchments of six major rivers in Serbia. The goal of the study was to evaluate the contents of potentially toxic elements (PTEs) As, Cd, Pb, Cr, Ni, Cu, and Zn in flood sediments and arable soils within the affected sub-catchments using regulatory guidelines and background levels. The sub-catchment of West Morava was selected to assess the degree of sediments and soils contamination and environmental risk [using the Pollution index (P_i), Enrichment factor, Geo-accumulation index, and Potential ecological risk index (PERI)] as well as to identify main PTEs sources by Principal component (PCA) and cluster analysis. Contents of Ni, Cr, As, Pb, and Cu above both guidelines and background levels, and of Zn and Cd above background levels were detected in the sediments and soils from all the sub-catchments. P_i indicted that about 95% of the soils and sediments were extremely polluted by Ni and about 65% slightly polluted by Cr, whereas about 90% were not polluted by As, Cd, Pb, Cu, or Zn. E_f indicated minor to moderate enrichment of the soils and sediments by Ni, and Cr. PCA differentiated a geogenic origin of Ni, Cr, As, and Pb, a mixed origin of Cd and Zn, and a predominantly anthropogenic origin of Cu. PERI of the soils and sediments suggested a low overall multi-element ecological risk. The ecological risk of the individual elements (E _r ⁱ ) for soils was Zn < Cr < Pb < Ni < Cu < As < Cd.

Solid-phase distribution and mobility of thallium in mining-metallurgical residues: Environmental hazard implications

Thallium (Tl) and its compounds are non-essential and highly toxic for living organisms, even at low concentrations. In this paper, we analyzed the presence and geochemical distribution of Tl in different mining-metallurgical and sediment samples collected from several mining zones of Mexico. A modified BCR sequential extraction procedure was also applied to the samples to investigate the geochemical behavior and potential environmental risk of Tl according to types of ore deposit and mineral processing method applied. Results revealed the presence of Tl in the majority of the mining-metallurgical samples, with labile concentrations reaching up to values of 184.4 mg kg^-1, well above the environmental standards. A comparison of Tl partitioning in different samples showed that Tl was usually found associated with labile fractions instead of entrapped in the environmentally-passive residual fraction. Specifically, high levels of Tl were extracted from the exchangeable/acid-extractable and poorly-crystalline reducible fractions, suggesting its association with both soluble and amorphous Fe-Mn oxyhydroxides, respectively. Besides, Tl was also frequently found associated with the crystalline reducible fraction, presumably bonded to manganese oxides and jarosite-like minerals. Lastly, little amounts of Tl were extracted from the oxidizable fraction. Considering the fractionation of Tl in these mining-metallurgical samples, they may pose a significant environmental hazard. This study provides useful insights into the potential sources of Tl pollution in Mexico and emphasizes the need for further research to determine the extent of its impact and to develop effective remediation protocols to protect the environment from Tl toxicity.

Impact of anthropogenic activities on mussel (Mytella guyanensis) in the Gulf of Paria, Trinidad

This body of work demonstrated the levels of selected contaminants (as well as their potential sources), in mussels (Mytella guyanensis) - a known bio-indicator of pollution, along the west coast (Gulf-of-Paria) of Trinidad. The findings demonstrated that polyaromatic hydrocarbons levels in the mussels investigated were moderate to high (201.80-1200.08 μg/kg). Notably, benzo(a)pyrene levels (1.15-9.32 μg/kg) exceeded the European Union guidelines at various sites. The distribution of PAHs and trace elements in the Gulf of Paria are impacted by tidal currents and local source discharge. Significant risks were identified from toxic carcinogenic equivalent levels (3.01 to 22.37 μg/kg) and mutagenic equivalent levels (3.98 to 38.61 μg/kg). The concentration of trace elements, Zn (46.05-56.36 mg/kg) and Cd (0.46-1.17 mg/kg), exceeded both local and international guidelines at certain sites investigated. The analysis revealed a high bioavailability of the identified pollutants in the Gulf of Paria from industrial, urban and marine activities.

Thallium in flowering cabbage and lettuce: Potential health risks for local residents of the Pearl River Delta, South China

Thallium (Tl), a rare metal, is universally present in the environment with high toxicity and accumulation. Thallium's behavior and fate require further study, especially in the Pearl River Delta (PRD), where severe Tl pollution incidents have occurred. One hundred two pairs of soil and flowering cabbage samples and 91 pairs of soil and lettuce samples were collected from typical farmland protection areas and vegetable bases across the PRD, South China. The contamination levels and spatial distributions of soil and vegetable (flowering cabbages and lettuces) Tl across the PRD were investigated. The relative contributions of soil properties to the bioavailability of Tl in vegetables were evaluated using random forest. Random forest is an accurate learning algorithm and is superior to conventional and correlation-based regression analyses. In addition, the health risks posed by Tl exposure via vegetable intake for residents of the PRD were assessed. The results indicated that rapidly available potassium (K) and total K in soil were the most important factors affecting Tl bioavailability, and the competitive effect of rapidly available K on vegetable Tl uptake was confirmed in this field study. Soil weathering also contributed substantially to Tl accumulation in the vegetables. In contrast, organic matter might not be a major factor affecting the mobility of Tl in most of the lettuce soils. Fe and manganese (Mn) oxides also contributed little to the bioavailability of Tl. A risk assessment suggested that the health risks for Tl exposure through flowering cabbage or lettuce intake were minimal.

Thallium contamination of desert soil in Namibia: Chemical, mineralogical and isotopic insights

We studied arid desert soils from Namibia (Rosh Pinah) that were contaminated with up to 7 mg kg^-1 of thallium (Tl) via dust emitted from a local flotation tailing dam. Chemical extractions of waste and soil materials indicated that most of the Tl is strongly bound, in accordance with X-ray diffraction and X-ray absorption spectroscopy data that point to the predominant association of Tl with metal sulfides and phyllosilicates. The isotope fractionation factor ε²⁰⁵Tl of the soil samples (from -0.4 to +3.8) shows a positive linear relationship (R² = 0.62) with 1/Tl, indicative for the mixing of two major Tl pools, presumably anthropogenic Tl and geogenic Tl. The ε²⁰⁵Tl value for the topmost soil samples (∼+3) closely matches the ε²⁰⁵Tl value for post-flotation waste particles with a diameter of <0.05 mm, whereas the bulk flotation waste exhibits a significantly larger ε²⁰⁵Tl value (∼+6). These variations are in accordance with predominant atmospheric transfer of Tl from the tailings to the adjacent soils via fine (dust) particles. The identified minimal Tl alteration in soils indicates that only a small part of the Tl could be potentially released and passively enter the vegetation, local population and/or food chain in the long term. From this viewpoint, Tl does not represent such an important environmental concern as other (abundant) contaminants at the locality. Furthermore, there could be a relevance for other alkaline desert soils, including those where Tl pollution plays a major role.

Microcolumn-based speciation analysis of thallium in soil and green cabbage

Thallium (Tl) is a toxic trace metal, whose geochemical behavior and biological effects are closely controlled by its chemical speciation in the environment. However, little tends to be known about this speciation of Tl in soil and plant systems that directly affect the safety of food supplies. In this context, the objective of the present study was to elaborate an efficient method to separate and detect Tl(I) and Tl(III) species for soil and plant samples. This method involves the selective adsorption of Tl(I) on microcolumns filled with immobilized oxine, in the presence of DTPA (diethylenetriaminepentaacetic acid), followed by DTPA-enhanced ultrasonic and heating-induced extraction, coupled with ICP-MS detection. The method was characterized by a LOD of 0.037 μg/L for Tl(I) and 0.18 μg/L for Tl(III) in 10  mL samples. With this method, a second objective of the research was to assess the speciation of Tl in pot and field soils and in green cabbage crops. Experimental results suggest that DTPA extracted Tl was mainly present as Tl(I) in soils (>95%). Tl in hyperaccumulator plant green cabbage was also mainly present as Tl(I) (>90%). With respect to Tl uptake in plants, this study provides direct evidence that green cabbage mainly takes up Tl(I) from soil, and transports it into the aboveground organs. In soils, Tl(III) is reduced to Tl(I) even at the surface where the chemical environment promotes oxidation. This observation is conducive to understanding the mechanisms of Tl isotope fractionation in the soil-plant system. Based on geochemical fraction studies, the reducible fraction was the main source of Tl getting accumulated by plants. These results indicate that the improved analytical method presented in this study offers an economical, simple, fast, and sensitive approach for the separation of Tl species present in soils at trace levels.

Thallium isotopes in metallurgical wastes/contaminated soils: A novel tool to trace metal source and behavior

Thallium (Tl) concentration and isotope data have been recorded for contaminated soils and a set of industrial wastes that were produced within different stages of Zn ore mining and metallurgical processing of Zn-rich materials. Despite large differences in Tl levels of the waste materials (1-500mgkg^-1), generally small changes in ε²⁰⁵Tl values have been observed. However, isotopically lighter Tl was recorded in fly ash (ε²⁰⁵Tl∼-4.1) than in slag (ε²⁰⁵Tl∼-3.3), implying partial isotope fractionation during material processing. Thallium isotope compositions in the studied soils reflected the Tl contamination (ε²⁰⁵Tl∼-3.8), despite the fact that the major pollution period ended more than 30 years ago. Therefore, we assume that former industrial Tl inputs into soils, if significant, can potentially be traced using the isotope tracing method. We also suggest that the isotope redistributions occurred in some soil (subsurface) horizons, with Tl being isotopically heavier than the pollution source, due to specific sorption and/or precipitation processes, which complicates the discrimination of primary Tl. Thallium isotope analysis proved to be a promising tool to aid our understanding of Tl behavior within the smelting process, as well as its post-depositional dynamics in the environmental systems (soils).

Dispersion and solubility of In, Tl, Ta and Nb in the aquatic environment and intertidal sediments of the Scheldt estuary (Flanders, Belgium)

Certain specialty elements are indispensable in modern technologies for their particular properties. Yet, potential risks associated to the release of these elements at any stage, remains unknown. Therefore, the dispersion of indium (In), thallium (Tl), tantalum (Ta) and niobium (Nb) in the aquatic environment of the Scheldt estuary (Flanders, Belgium) was studied. Maximum concentrations in intertidal sediments of 101 ± 15 μg kg^-1 for In, 481 ± 37 μg kg^-1 for Tl, 88 ± 19 μg kg^-1 for Ta and 1162 ± 4 μg kg^-1 for Nb appeared on the sampling location closest to the river mouth, i.e. 57.5 km upstream. Their distribution in the intertidal sediments depends on the physicochemical sediment characteristics along the flow of the river Scheldt. The same was the case for most other metals and aluminum as their occurrence also correlated (p < 0.05) with the occurrence of In, Tl and Nb. While in general, studied elements correlate to the OM content and sulfur and phosphorus herein included, a relative enrichment of In, Tl and Nb was seen at Rupelmonde (92.0 km from the river mouth). Mainly the intertidal sediment silt fraction is capable of retaining the elements by exchanging with other ions in the mineral interlayer. Increasing salinity towards the river mouth can furthermore induce the formation of insoluble chloride species. Overall, the solubility of In, Tl, Ta and Nb appeared extremely low upon extraction of pore water from intertidal sediments saturated to 100% field capacity.

Effect of competitive adsorption on zinc removal from aqueous solution and zinc smelting effluent by eucalyptus leaf-based magnetic biosorbent

The objectives of this study were to investigate the competitive sorption behaviors and mechanisms of heavy metals onto ELMB, a novel eucalyptus-leaf-based magnetic biosorbent, and to study the potential application of ELMB in the treatment of actual zinc smelting effluent after a necessary pretreatment process. ELMB and ELMB-metals systems were characterized using several techniques. Competitive sorption of Zn²⁺ with Pb²⁺, Cu²⁺ and Cd²⁺ onto ELMB was studied by batch experiments and the used sorbent was separated under a magnetic field. The results show that the ELMB can be considered as paramagnetic material with various functional groups on its surface. The presence of Pb²⁺, Cu²⁺ and Cd²⁺ significantly decreases the sorption of Zn²⁺ in either the binary system or multimetal systems. The order of adsorption preference is Pb²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ in multimetal systems and the sequence of competitive ability to zinc is: Pb²⁺ > Cu²⁺ > Cd²⁺. Non-competitive Langmuir multicomponent isotherm model fits to the adsorption data of Pb²⁺ and Cu²⁺ well in aqueous solution. The co-existent Ca²⁺ and SO₄^2- decrease the removal efficiencies of heavy metals while the presence of Na⁺ and Cl^- shows little effect in the multimetal solution. In the case of actual zinc smelting effluent, "pretreatment + ELMB sorption" is successfully applied to remove heavy metals and the contents of Zn²⁺ and its associated metals are well below discharge limits.

Redox-controlled release dynamics of thallium in periodically flooded arable soil

To our knowledge, this is the first work to mechanistically study the impact of the redox potential (E_H) and principal factors, such as pH, iron (Fe), manganese (Mn), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), chlorides (Cl^-) and sulfates (SO₄^2-), on the release dynamics of thallium (Tl) in periodically flooded soil. We simulated flooding using an automated biogeochemical microcosm system that allows for systematical control of pre-defined redox windows. The E_H value was increased mechanistically at intervals of approximately 100 mV from reducing (-211 mV) to oxidizing (475 mV) conditions. Soluble Tl levels (0.02-0.28 μg L^-1) increased significantly with increases in E_H (r = 0.80, p < 0.01, n = 30). Thallium mobilization was found to be related to several simultaneous processes involving the gradual oxidation of Tl-bearing sulfides, reductive dissolution of Fe-Mn oxides and desorption from mineral sorbents. Manganese oxides did not appear to have a considerable effect on Tl retention under oxidizing conditions. Before conducting the microcosm experiment, Tl geochemical fractionation was assessed using the modified BCR sequential extraction procedure. The BCR revealed a majority of Tl in the residual fraction (77.7%), followed by reducible (13.3%) and oxidizable fractions (5.9%). By generating high levels of Tl toxicity at low doses, Tl released under oxidizing conditions may pose an environmental threat. In the future, similar studies should be conducted on various soils along with a determination of the Tl species and monitoring of the Tl content in plants to achieve more detailed insight into soluble Tl behavior.