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

Thallium isotopic fractionation in soils from a historic HgTl mining area: New insights on thallium geochemistry

Thallium (Tl), a highly toxic heavy metal, which may pose significant environmental threats due to extensive discharge from anthropogenic activities. It is crucial to understand geochemical behavior of Tl in soils for initiating proper measures for Tl pollution control. For this purpose, transport behavior of Tl and its dominant factors in soils collected from a typically Tl-enriched depth profile, surrounding a historical tailing dump near an independent HgTl mine area in China, were investigated by using Tl isotope compositions. Results showed that an overall enrichment of Tl (48.68-375.21 mg/kg) was accompanied with As elevation (135.00-619.00 mg/kg) in the whole depth profile, and Tl and As exhibited co-migration behavior with Fe, S, K, and Rb. Geochemical fractionation of Tl unveiled by sequential extraction further indicated that Mn-/Fe-bearing minerals and clay minerals act as main hosts of Tl in the studied soils. Thallium isotopic composition and its fractionation pattern further revealed that the major contributors to high Tl levels in the depth profile were tailing and lorandite minerals, with mean contribution rate of 51.99% and 42.47%, respectively. These findings facilitate the understanding of Tl transport behavior in highly contaminated environment, providing valuable insights for developing new technologies in mining waste treatment and historical mine reclamation.

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.

Mitigation of thallium threat in paddy soil and rice plant by application of functional biochar

Thallium (Tl) is a highly toxic metal, and its contamination in soils entails high risks to human health via food chain. It remains largely unknown of the effects of applying biochar on Tl uptake in paddy systems despite that few studies have shown that biochar exhibits great potential for decreasing Tl bioavailability in soils. Herein, we examined the mitigating effects of the application of biochar (5 and 20 g/kg pristine biochar; 5 and 20 g/kg Fe/Mn-modified biochar) on Tl uptake in paddy soil and rice plant after an entire rice growth period. The results suggested that the application of Fe/Mn-modified biochar (FMBC) considerably mitigated the accumulation of Tl in different tissues of rice plants. Specifically, total Tl content in rice plants treated with FMBC-20 decreased by over 75% compared with control experiment. In addition, the amendment of FMBC in Tl-rich paddy soils can enhance the communities of microorganisms (Actinobacteria and Proteobacteria). Further analysis of the soil microbial symbiosis network revealed that FMBC promotes the living microorganisms to play modular synergistic interactions, which is crucial for FMBC-induced Tl stabilization in soils. All these findings indicated that FMBC is an efficient and environmentally friendly Tl-immobilization alternative material and can be potentially used in the remediation of Tl-contaminated paddy soils and/or cropland.

Increased atmospheric thallium threats to populated areas: A mini review

Air pollutants combined with Hg, Cd, Cr, Pb, etc. in many global populated areas were studied comprehensively, while our understanding towards thallium (Tl), an extremely toxic heavy metal, remains very limited. Further, the knowledge on atmospheric emissions, distribution, and the hidden risks associated with Tl is of great scarcity. Hence, this work aims to review recent data on significant sources of ambient Tl resulting from industrial activities, including Pb/Zn/Cu/Fe sulfide ore smelting, steel-making, coal burning, and cement production that involves the use of Tl-bearing wastes. Through the examination of Tl emissions and transfer pathways in the atmosphere, it is found that Tl is present at lower than ng/m3 in aerosols and air particulates but can increase to much higher levels even at 1000 μg/m3 in atmospheric fine particulate matters near the mining and smelting industrialized zones located near populated areas. This study highlights the importance of creating a comprehensive emission inventory for Tl, particularly in developing countries where this data is currently lacking. The time has come to develop a precise national emission inventory for Tl in order to prevent and mitigate the risks associated with ambient exposure to this element. This review offers novel insights for the scientific community and policy-makers in establishing effective control and management strategies to curb hidden Tl hazards derived from industrial activities.

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.

Applying thallium isotopic compositions as novel and sensitive proxy for Tl(I)/Tl(III) transformation and source apportionment

Thallium is a rare metal known for its highly toxic nature. Recent research has indicated that the precise determination of Tl isotopic compositions using Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP MS) provides new opportunities for understanding Tl geochemical behavior. While isotopic fractionation of Tl derived from anthropogenic activities (e.g., mining, smelting) have been reported, there is limited information regarding Tl influenced by both natural weathering processes and anthropogenic origins. Herein, we investigated, for the first time, the Tl isotopic compositions in soils across a representative Tl-rich depth profile from the Lanmuchang (LMC) quicksilver mine (southwest China) in the low-temperature metallogenesis zone. The results showed significant variations in Tl isotope signatures (ε205Tl) among different soil layers, ranging from -0.23 to 3.79, with heavier isotope-205Tl enrichment observed in the bottom layers of the profile (ε205Tl = 2.18-3.79). This enrichment of 205Tl was not solely correlated with the degree of soil weathering but was also partially associated with oxidation of Tl(I) by Fe (hydr)oxide minerals. Quantitative calculation using ε205Tl vs. 1/Tl data further indicated that the Tl enrichment across the soil depth profile was predominantly derived from anthropogenic origins. All these findings highlight that the robustness and reliability of Tl isotopes as a proxy for identifying both anthropogenic and geogenic sources, as well as tracing chemical alterations and redox-controlled mineralogical processes of Tl in soils. The nascent application of Tl isotopes herein not only offers valuable insights into the behavior of Tl in surface environments, but also establishes a framework for source apportionment in soils under similar circumstances.

Removal of thallium by MnOx coated limestone sand filter through regeneration of KMnO4: Combination of physiochemical and biochemical actions

Treatment of industrial thallium(Tl)-containing wastewater is crucial for mitigating environmental risks and health threats associated with this toxic metal. The incorporation of Mn oxides (MnOx) into the filtration system is a promising solution for efficient Tl(I) removal. However, further research is needed to elucidate the underlying mechanism behind MnOx-enhanced filtration and the rules of its stable operation. In this study, limestone, a cost-effective material, was selected as the filter media. Raw water with Mn(II), Tl(I), and other pollutants was prepared after a thorough investigation of actual industrial wastewater conditions. KMnO4 was added to induce the formation of MnO2 on limestone surfaces, while long-term operation led to enrichment of manganese oxidizing microorganisms (MnOM). Results revealed a dual mechanism. Firstly, most Mn(II) were oxidized by KMnO4 to form MnO2 attaching to limestone sands, and both Tl(I) and residual Mn(II) were adsorbed onto the newly formed MnO2. Subsequently, enzymes secreted by MnOM facilitated oxidation of remaining Mn(II), resulting in the generation of biogenic manganese oxides (BioMnOx) with numerous vacancies during long-term operation. The generated BioMnOx not only adsorbed Mn(II) and Tl(I) but also promoted their oxidation process. This approach offers an effective and sustainable method for removing both Mn(II) and Tl(I) from industrial wastewater, thereby addressing the challenges posed by thallium-contaminated effluents.

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.

Source-oriented risk assessment of heavy metal(loid)s in agricultural soils around a multimetal smelting area near the Yellow River, China

Heavy metal(loid) (HM) contamination in agricultural soils, particularly in areas severely impacted by smelting industries, has attracted worldwide attention. In this study, agricultural soils were collected in a flourishing multimetal smelting area near the Yellow River in central China. By an integrated approach encompassing the positive matrix factorization model, ordinary kriging interpolation and hierarchical clustering analysis (PMF-OK-HC), a total of four major sources and their mass contributions were identified, namely, soil parent material (56.6%), industrial waste and Mo smelting (24.0%), metal smelting and traffic emissions (12.8%), and coal combustion (6.7%). On this basis, the health risk of HMs was evaluated by Monte Carlo simulations and showed that a higher risk, with a higher proportion of exceeding-thresholds risk, was observed for children than for adults in terms of both noncarcinogenic and carcinogenic risks. Exposure pathways of oral ingestion in children could result in a higher attributed risk than other pathways. Furthermore, source-oriented risk assessment (SORA) revealed that the sources of coal combustion, industrial waste and Mo smelting had the highest contributions to noncarcinogenic and carcinogenic risks. Overall, for effective environmental management in agricultural soil, the framework of SORA was verified as an effective tool in the identification of the priority control of HMs and their sources.

Silver isotopes: A tool to trace smelter-derived contamination

Here, for the first time, we report the concentrations and isotopic data of Ag in a variety of ore and metallurgical samples and forest soils that have been polluted due to Ag-Pb smelter emissions. Similar to the Ag concentrations, we identified a large range of δ109Ag values (from -0.8 to +2.4‰), a ∼3‰ spread, within the primary and secondary materials (i.e., galena, fly ash, slag and matte). This phenomenon, however, is evidently unrelated to Ag isotopic fractionation during the smelting process, but it reflects the starting 109Ag/107Ag signal in ore mineral and/or the specific type of ore genesis. The two studied soil profiles differed in Ag isotopic composition, but on the other hand, they consistently showed significantly lighter Ag (≤+0.8‰) of metallurgical origin in the upper horizons compared to the bottom horizons and bedrocks, with low Ag amounts depleted of 107Ag (≤+2.9‰). This isotopic pattern can be attributed to a ternary mixing relationship involving two major anthropogenic Ag components and a minor contribution from geogenic Ag. Accordingly, we did not observe any post-depositional isotopic fractionation in our soils, since Ag was geochemically stable and it was not subjected to leaching. In summary, the Ag isotopes have a potential to trace variations in anthropogenic phases, to monitor specific geochemical processes, and are clearly applicable as anthropogenic Ag source and Ag load proxies.

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.

Thallium adsorption on three iron (hydr)oxides and Tl isotopic fractionation induced by adsorption on ferrihydrite

Thallium (Tl) is an extraordinarily toxic metal, which is usually present with Tl(I) and highly mobile in aquatic environment. Limited knowledge is available on the adsorption and isotopic variations of Tl(I) to Fe-(hydr)oxides. Herein, the adsorption behavior and mechanism of Tl(I) on representative Fe-(hydr)oxides, i.e. goethite, hematite, and ferrihydrite, were comparatively investigated kineticly and isothermally, additional to crystal structure modelling and Tl isotope composition (²⁰⁵Tl/²⁰³Tl). The results showed that ferrihydrite exhibited overall higher Tl(I) adsorption capacity (1.11-10.86 mg/kg) than goethite (0.21-1.83 mg/kg) and hematite (0.14-2.35 mg/kg), and adsorption by the three prevalent Fe-minerals presented strong pH and ionic strength dependence. The magnitude of Tl isotopic fractionation during Tl(I) adsorption to ferrihydrite (α_{solid-solution} ≈ 1.00022-1.00037) was smaller than previously observed fractionation between Mn oxides and aqueous Tl(I) (α_{solid-solution} ≈ 1.0002-1.0015). The notable difference is likely that whether oxidation of Tl(I) occurred during Tl adsorption to the mineral surfaces. This study found a small but detectable Tl isotopic fractionation during Tl(I) adsorption to ferrihydrite and heavier Tl isotope was slightly preferentially adsorbed on surface of ferrihydrite, which was attributed to the formation of inner-sphere complex between Tl and ≡Fe-OH. The findings offer a new understanding of the migration and fate of ²⁰⁵Tl/²⁰³Tl during Tl(I) adsorption to Fe (hydr)oxides.

Stable thallium (Tl) isotopic signature as a reliable source tracer in river sediments impacted by mining activities

Thallium (Tl) is an extremely toxic metal, whose geochemical behavior remains poorly understood. This study aims to clarify the migration pathway and source apportionment of Tl in sediments from a watershed downstream of an open and large-scale pyrite mine area in south China, using high-precised Tl isotopic compositions. Results showed that Tl isotopic fractionations were mainly influenced by the anthropogenic Tl sources in all the sediments as a whole from the studied watershed, while in situ mineral adsorption and biological activity were limited. Moreover, plot of ε²⁰⁵Tl vs. 1/Tl further illustrated that three possible end-members, viz. background sediments, pyrite tailings, and sewage treatment wastes were ascribed to predominant sources of Tl enrichment in the sediments. A ternary mixing model unveiled that waste from pyrite mining activities (i.e., both pyrite tailings and sewage treatment wastes) affected the downstream sediments up to 10 km. All these findings suggest that Tl isotopic signature is a reliable tool to trace Tl sources in the sediments impacted by mining activities. It is highly critical for further target-oriented and precise remediation of Tl contamination.

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 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.

Stable isotope fractionation of thallium as novel evidence for its geochemical transfer during lead‑zinc smelting activities

Thallium (Tl) is a highly toxic trace metal. Lead (Pb)‑zinc (Zn) smelting, which is a pillar industry in various countries, is regarded as one of the dominant anthropogenic sources of Tl contamination in the environment. In this study, thallium isotope data have been evaluated for raw material and a set of industrial wastes produced at different stages of Pb-Zn smelting in a representative large facility located by the North River, South China, in order to capture Tl isotope signatures of such typical anthropogenic origin for laying the foundation of tracking Tl pollution. Large variations in Tl isotopic compositions of raw Pb-Zn ores and solid smelting wastes produced along the process chain were observed. The ε²⁰⁵Tl values of raw Pb-Zn ores and return fines are -0.87 ± 0.26 and -1.0 ± 0.17, respectively, contrasted by increasingly more negative values for electrostatic precipitator dust (ε²⁰⁵Tl = -2.03 ± 0.14), lime neutralizing slag (ε²⁰⁵Tl = -2.36 ± 0.18), and acid sludge (ε²⁰⁵Tl = -4.62 ± 0.76). The heaviest ε²⁰⁵Tl (1.12 ± 0.51) was found in clinker. These results show that isotopic fractionation occurs during the smelting processes. Obviously, the lighter Tl isotope is enriched in the vapor phase (-3.75 ε²⁰⁵Tl units). Further XPS and STEM-EDS analyses show that Tl isotope fractionation conforms to the Rayleigh fractionation model, and adsorption of ²⁰⁵Tl onto hematite (Fe₂O₃) may play an important role in the enrichment of the heavier Tl isotope. The findings demonstrate that Tl isotope analysis is a robust tool to aid our understanding of Tl behavior in smelting processes and to provide a basis for source apportionment of Tl contaminations.

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.

Ecological Restoration of Wetland Polluted by Heavy Metals in Xiangtan Manganese Mine Area

Due to manganese mining and slag accumulation, the geological structure of the wetland polluted by heavy metals in Xiangtan Manganese Mine area was seriously damaged, hence biodiversity loss, severe soil, and water pollution, as well as serious heavy metal pollution to food, vegetables, and other natural sources. In order to restore the ecological environment of the mining area, in 2015, the ecological restoration test of heavy metal polluted wetlands in the mining area was carried out. The results showed that the Mn content of different parts of Koelreuteria paniculata root from high to low order: fine root > small root > medium root > large root. The Mn content of different parts of Elaeocarpus decipiens root from high to low order: large root > medium root > small root > fine root. The order of Mn content in plants of the wetland restoration from high to low is as follows: Canna warscewiezii > Thalia dealbata > Boehmeria > Pontederia cordata > Typha orientalis > Nerium oleander > Softstem bulrush > Iris germanica > Acorus calamus > Arundo donax > Phragmites australis; The order of Internal Cu content from high to low is as follows: Acorus calamus > Thalia dealbata > Softstem bulrush > Canna warscewiezii > Typha orientalis > Arundo donax > Boehmeria > Iris germanica > Pontederia cordata > Nerium oleander > Phragmites australis; Zn content from high to low order is as follows: Canna warscewiezii > Acorus calamus > Thalia dealbata > Typha orientalis > Pontederia cordata > Arundo donax > Softstem bulrush > Iris germanica > Boehmeria > Phragmites australis > Nerium oleander; Cd content from high to low order is as follows: Phragmites australis > Softstem bulrush > Thalia dealbata > Nerium oleander > Boehmeria > Canna warscewiezii > Acorus calamus > Iris germanica > Typha orientalis > Pontederia cordata > Arundo donax. The results of this study have provided a theoretical basis and decision-making reference for the evaluation of heavy metals polluted wetland restoration, protection, and reconstruction effects and the selection of ecological restoration modes.

Thallium geochemical fractionation and migration in Tl-As rich soils: The key controls

Thallium (Tl) pollution caused by mining and processing of Tl-enriched ores has become an increasing concern. This study explored the geochemical fractionation and vertical transfer of Tl in a soil profile (200 cm) from a representative Tl-As mineralized area, Southwest China. The results showed that the soils were heavily enriched by Tl and As, with concentration ranging from 3.91-17.3 and 1830-8840 mg/kg (6.79 and 2973 mg/kg in average), respectively. Approximately 50% of Tl occurred in geochemically mobile fractions in the topsoil, wherein the reducible fraction was the most enriched fraction. Further characterization using LA-ICP-MS and TEM revealed that enriched Tl and As in soils were mainly inherited from the weathering of mine tailing piles upstream. XPS characterization indicated that Fe oxides herein may play a critical role in the oxidation of Tl(I) to Tl(III) which provoked further adsorption of Tl onto Fe oxides, thereby facilitating Tl enrichment in the reducible fraction. The findings highlight that the pivotal role of Fe oxides from mineralized area in the co-mobility and migration of Tl and As in the depth profile.

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.

Possible application of stable isotope compositions for the identification of metal sources in soil

Metals in soil are potentially harmful to humans and ecosystems. Stable isotope measurement may provide "fingerprint" information on the sources of metals. In light of the rapid progress in this emerging field, we present a state-of-the-art overview of how useful stable isotopes are in soil metal source identification. Distinct isotope signals in different sources are the key prerequisites for source apportionment. In this context, Zn and Cd isotopes are particularly helpful for the identification of combustion-related industrial sources, since high-temperature evaporation-condensation would largely fractionate the isotopes of both elements. The mass-independent fractionation of Hg isotopes during photochemical reactions allows for the identification of atmospheric sources. However, compared with traditionally used Sr and Pb isotopes for source tracking whose variations are due to the radiogenic processes, the biogeochemical low-temperature fractionation of Cr, Cu, Zn, Cd, Hg and Tl isotopes renders much uncertainty, since large intra-source variations may overlap the distinct signatures of inter-source variations (i.e., blur the source signals). Stable isotope signatures of non-metallic elements can also aid in source identification in an indirect way. In fact, the soils are often contaminated with different elements. In this case, a combination of stable isotope analysis with mineralogical or statistical approaches would provide more accurate results. Furthermore, isotope-based source identification will also be helpful for comprehending the temporal changes of metal accumulation in soil systems.

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.

Distribution and Mode of Occurrence of Co, Ni, Cu, Zn, As, Ag, Cd, Sb, Pb in the Feed Coal, Fly Ash, Slag, in the Topsoil and in the Roots of Trees and Undergrowth Downwind of Three Power Stations in Poland

It is supposed that the determination of the content and the mode of occurrence of ecotoxic elements (EE) in feed coal play the most significant role in forecasting distribution of EE in the soil and plants in the vicinity of power stations. Hence, the aim of the work was to analyze the properties of the feed coal, the combustion residues, and the topsoil which are reached by EE together with dust from power stations. The mineral and organic phases, which are the main hosts of EE, were identified by microscopy, X-ray powder diffraction, inductively coupled plasma atomic emission spectrometry, and scanning electron microscope with an energy dispersive X-ray methods. The highest content of elements was observed in the Oi and Oe subhorizons of the topsoil. Their hosts are various types of microspheres and char, emitted by power stations. In the areas of long-term industrial activity, there are also sharp-edged grains of magnetite emitted in the past by zinc, lead, and ironworks. The enrichment of the topsoil with these elements resulted in the increase in the content of EE, by between 0.2 times for Co; and 41.0 times for Cd in the roots of Scots pine, common oak and undergrowth, especially in the rhizodermis and the primary cortex and, more seldom, in the axle roller and cortex cells.

Quantitative isotopic fingerprinting of thallium associated with potentially toxic elements (PTEs) in fluvial sediment cores with multiple anthropogenic sources

Thallium (Tl) is a dispersed trace metal showing remarkable toxicity. Various anthropogenic activities may generate Tl contamination in river sediments, posing tremendous risks to aquatic life and human health. This paper aimed to provide insight into the vertical distribution, risk assessment and source tracing of Tl and other potentially toxic elements (PTEs) (lead, cadmium, zinc and copper) in three representative sediment cores from a riverine catchment impacted by multiple anthropogenic activities (such as steel-making and Pb-Zn smelting). The results showed high accumulations of Tl combined with associated PTEs in the depth profiles. Calculations according to three risk assessment methods by enrichment factor (EF), geoaccumulation index (I_geo) and the potential ecological risk index (PERI) all indicated a significant contamination by Tl in all the sediments. Furthermore, lead isotopes were analyzed to fingerprint the contamination sources and to calculate their quantitative contributions to the sediments using the IsoSource software. The results indicated that a steel-making plant was the most important contamination source (∼56%), followed by a Pb-Zn smelter (∼20%). The natural parental bedrock was found to contribute ∼24%. The findings highlight the importance of including multiple anthropogenic sources for quantitative fingerprinting of Tl and related metals by the lead isotopic approach in complicated environmental systems.

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.

Persistent thallium contamination in river sediments, source apportionment and environmental implications

The adverse impacts of detrimental thallium (Tl) contamination are of increasing concerns to sustainable development. Herein, the contents, distributions and sources of Tl and potential toxic elements (PTEs) (Pb, As, Cr, Cu, Ni, Co, Sb, Cd and U) were investigated in sediments collected in Gaofeng River, which has been contaminated by long-term mining activities, located in Yunfu City, Southern China. Results indicated that excessive Tl levels were found in sediments (1.80-16.70 mg/kg). Sequential extraction procedure indicated that despite a large amount of Tl entrapped in residual fraction, a significant level of Tl (0.28-2.34 mg/kg) still exhibited in geochemically labile fractions, which was easy for Tl mobilization and availability. Pb isotope tracing method further confirmed that the pyrite exploitations may be the prime contaminated contributor (47-76%) to these sediments. These findings highlight that it is essential to establish more effective measures for Tl contamination control and call for engineered remediation countermeasures towards polluted river sediments.

Pollution and health risk assessment of toxic metal(loid)s in soils under different land use in sulphide mineralized areas

Toxic metal(loid) pollution in sulphide mineralized area has been increasingly concerned. In the present study, the pollution characteristics and the health risk of Hg, As, Tl and other metal(loid)s in soils under different land use, from a rural area impacted by Tl-Hg-rich sulphide mineralization, were assessed using statistical analysis, enrichment factor (EF), potential ecological risk index (RI) and health risk assessment model. The results showed that Tl, Hg and As were highly enriched in the mine area due to the historic sporadic mining activities, and Tl, Hg and Sb were enriched in the peripheral area. Hg and Tl pollution in soils of the mine area impacted by past mining activities posed high ecological risk. High contents and enrichment of Tl and Hg in forest/grass land had a greater impact on the ecological risk in the mine area; whereas Tl and Hg in the grain land and vegetable land dominated the soil ecological risk in the peripheral area. Human health risk assessment indicated that children are more sensitive and vulnerable to toxic metal(loid)s in soils than the adults. Hg, Tl and As have potential non-carcinogenic risk to local children and adults. The HQ levels for different exposure pathways of toxic metal(loid)s were in the order of ingestion > dermal contact > air inhalation for Tl and As, and dermal contract > ingestion > air inhalation for Hg. For carcinogenic risk, all the mean CR values of ingestion in the mine area were higher than 10^-4, indicating seriously potential risk. The descending order of ILCR via different pathways was the same as the HQ, for which ingestion was predominant, followed by dermal and air inhalation. The findings may help provide basic knowledge and guidelines for toxic metal(loid) pollution remediation in similar sulphide mineralized areas.

Development of ultrasound-assisted dispersive solid-phase microextraction based on mesoporous carbon coated with silica@iron oxide nanocomposite for preconcentration of Te and Tl in natural water systems

The main objective of this study was to develop an ultrasound-assisted dispersive solid-phase microextraction (UADSPME) method for separation and preconcentration of tellurium (Te) and thallium (Tl) in environmental samples prior to inductively coupled plasma-optical emission spectrometry determination. The MPC@SiO2@Fe3O4 nanocomposite was used as a nanoadsorbent in the UADSPME method. The nanocomposite was prepared using a coprecipitation and sol–gel method, and it was characterized using scanning electron microscopy/energy-dispersive X-ray spectroscopy, transmission electron microscopy and X-ray powder diffraction techniques. The Box–Behnken design and response surface methodology were used for the optimization of experimental parameters (such as pH, extraction time and mass of adsorbent) affecting the preconcentration procedure. Under optimized conditions, the limits of detection were 0.05 and 0.02 µg L−1 and the limits of quantification were 0.17 and 0.07 µg L−1 for Te and Tl, respectively. The precision expressed as the relative standard deviation (%RSD) was 2.5% and 2.8% for Te and Tl, respectively. Finally, the developed method was applied for the analysis of Tl and Te in real samples.

Hyperaccumulation and transport mechanism of thallium and arsenic in brake ferns (Pteris vittata L.): A case study from mining area

Both thallium (Tl) and arsenic (As) bear severe toxicity. Brake fern (Pteris vittata L.) is well-known for its hyperaccumulation capacity of As, yet its role on Tl accumulation remains unknown. Herein, brake ferns growing near an As tailing site in Yunnan, Southwestern China are for the first time discovered as a co-hyperaccumulator of both Tl and As. The results showed that the brake ferns extracted both As and Tl efficiently from the soils into the fronds. The studied ferns growing on Tl and As co-polluted soils were found to accumulate extremely high levels of both As (7215-11110 mg/kg) and Tl (6.47-111 mg/kg). Conspicuously high bio-accumulation factor (BCF) was observed for As (7.8) and even higher for Tl (28.4) among these co-hyperaccumulators, wherein the contents of As and Tl in contaminated soils were 1240 ± 12 and 3.91 ± 0.01 mg/kg, respectively. The applied advanced characterization techniques (e.g. transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS)) indicated a preferential uptake of Tl(I) while simultaneous accumulation of As (III) and As(V) from the Tl(I)/Tl(III)-As (III)/As(V) co-existent rhizospheric soils. The findings benefit the phytoremediation practice and pose implications for managing and restoring Tl-As co-contaminated soils in other countries.

Properties of Concrete with Recycled Concrete Aggregate Containing Metallurgical Sludge Waste

Sand has been considered to be something of an immeasurable quantity. There are many indications that this view is no longer valid and that the limiting of natural aggregates usage is doubly justified. Firstly, the extraction of natural aggregates is expensive and has a huge impact on the environment. The main issues in sand and gravel mining are the large areas that are affected, ground water level changes, illegal mining, unsuitability of desert and marine sand, and costs of transport. Secondly, metallurgical waste can be used as a substitute for natural aggregates. This is doubly beneficial-the waste is recycled and the use of natural aggregates is reduced. Waste is stored in landfills that take up large areas and there is also the possibility of ground and groundwater pollution by hazardous compounds. The research presented in this article focuses on the technological conditions of using metallurgical waste in its original form and as a component of recycled concrete aggregate (RCA). The use of metallurgical sludge waste or crushed or round RCA to produce concrete deteriorates the consistency and does not significantly affect the air content and density of the concrete mix. RCA lowers the density of hardened concrete. Metallurgical sludge waste or RCA usage adversely affect the absorbability and permeability of concrete. Concrete containing metallurgical sludge waste is of higher compressive strength after 7 and 28 days, with up to 60% of waste as a sand replacement. RCA concrete achieved higher compressive strength also.

Zero-valent iron-manganese bimetallic nanocomposites catalyze hypochlorite for enhanced thallium(I) oxidation and removal from wastewater: Materials characterization, process optimization and removal mechanisms

Nano zero-valent metals adsorption coupled with advanced oxidation for environmental pollutants removal has been gaining attention recently. In this study, zero-valent iron-manganese (nZVIM) bimetallic nanocomposites were prepared via one-pot borohydride reduction and coupled with hypochlorite (ClO^-) oxidation for enhanced thallium (Tl) removal from wastewater. Amorphous nZVIM nanoparticles were successfully synthesized, with a specific surface area of 106.89 m²/g, and a saturation magnetization of 65.16 emu/g. In comparison with the nZVIM adsorption or ClO^- oxidation alone, the hybrid nZVIM/ClO^- process achieved much faster Tl(I) removal rate over a wide pH range from 6 to 10. Maximum Tl(I) removal capacity was as high as 990.0 mg/g. The oxidation-induced adsorption for Tl(I) removal well followed the pseudo-first kinetic order model. Stable and effective adsorbent regeneration was achieved during the cyclic adsorption-desorption tests. This process also had high resistance to the interference of external cations, can act as an effective pretreatment for Tl(I) removal from the actual saline industrial wastewater. The main mechanisms for Tl(I) removal were found to be oxidation, surface precipitation, pore retention, and surface complexation. The nZVIM coupled with ClO^- approach has great potential for Tl(I) removal from wastewater, and its application in other fields is highly anticipated.

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 contamination, health risk assessment and source apportionment in common vegetables

As an element with well-known toxicity, excessive thallium (Tl) in farmland soils, may threaten food security and induce extreme risks to human health. Identification of key contamination sources is prerequisite for remediation technologies. This study aims to examine the contamination level, health risks and source apportionment of Tl in common vegetables from typical farmlands distributed over a densely populated residential area in a pyrite mine city, which has been exploiting Tl-bearing pyrite minerals over 50 years. Results showed excessive Tl levels were exhibited in most of the vegetables (0.16-20.33 mg/kg) and alarming health risks may induce from the vegetables via the food chain. Source apportionment of Tl contamination in vegetables was then evaluated by using Pb isotope fingerprinting technique. Both vegetables and soils were characterized with overall low ²⁰⁶Pb/²⁰⁷Pb. This indicated that a significant contribution may be ascribed to the anthropogenic activities involving pyrite deposit exploitation, whose raw material and salgs were featured with lower ²⁰⁶Pb/²⁰⁷Pb. Further calculation by binary mixing model suggested that pyrite mining and smelting activities contributed 54-88% to the thallium contamination in vegetables. The results highlighted that Pb isotope tracing is a suitable technique for source apportionment of Tl contamination in vegetables and prime contamination from pyrite mining/smelting activities urges authorities to initiate proper practices of remediation.

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.

High contamination risks of thallium and associated metal(loid)s in fluvial sediments from a steel-making area and implications for environmental management

Thallium (Tl) is an uncommon toxic element, with an even greater toxicity than that of As, Hg and Cd. Steel-making industry has been identified as an emerging new significant source of Tl contamination in China. This paper presents a pilot investigation of the contamination and geochemical transfer of Tl and associated metal(loid)s in river sediments affected by long-term waste discharge from the steel-making industry. The results uncovered an overall Tl contamination (1.96 ± 0.42 mg/kg) across a sediment profile of approximately 1.5 m in length, even 10 km downstream the steel plant. Highly elevated contents of Pb, Cu, Cd, Zn and Sb were found in the fluvial sediments, displaying strong positive correlations with Tl contents. Elevated levels of geochemically mobile Tl as well as Cd, Zn, Cu and Pb occurred in the fluvial sediments, signifying anthropogenic imprints from steel production activities at high temperature. Levels of contamination and ecological risk were calculated to be moderate to considerable for Tl, Cu, Zn and high to very high for Cd, Pb, Sb. The results highlight that there is a great challenge in view of potentially considerable Tl pollution due to continuous massive steel production in many other parts of China. It is high time to initiate process-based management of Tl contamination control for the ambient aquifer system in the steel-making area.

Thallium stable isotope fractionation in white mustard: Implications for metal transfers and incorporation in plants

We studied thallium (Tl) isotope fractionation in white mustard grown hydroponically at different Tl doses. Thallium isotope signatures in plants indicated preferential incorporation of the light ²⁰³Tl isotope during Tl uptake from the nutrient solution. Negative isotope fractionation was even more pronounced in dependence on how much the available Tl pool decreased. This finding corresponds to the concept of isotope overprinting related to a high contamination level in the growing media (solution or soil). Regarding Tl translocation in plants, we observed a large Tl isotope shift with an enrichment in the heavy ²⁰⁵Tl isotope in the shoots relative to the roots in treatments with low/moderate solution Tl concentrations (0.01/0.05 mg Tl/L), with the corresponding α^205/203Tl fractionation factors of ˜1.007 and 1.003, respectively. This finding is probably a consequence of specific (plant) reactions of Tl replacing K in its cycle. The formation of the S-coordinated Tl(I) complexes, potentially affecting both Tl accumulation and Tl isotope fractionation in plants, however, was not proven in our plants, since we did not have indication for that on the basis of X-ray absorption spectroscopy, suggesting that Tl was mainly present as free/hydrated Tl⁺ ion or chemically bound to O-containing functional groups.

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.

Response of microbial communities and interactions to thallium in contaminated sediments near a pyrite mining area

Thallium (Tl) is a well-recognized hazardous heavy metal with very high toxicity. It is usually concentrated in sulfide minerals, such as pyrite (FeS₂), sphalerite (ZnS), chalcopyrite (CuS) and galena (PbS). Here, this study was carried out to investigate the indigenous microbial communities via 16S rRNA gene sequence analysis in typical surface sediments with various levels of Tl pollution (1.8-16.1 mg/kg) due to acid mine drainage from an active Tl-containing pyrite mining site in South China. It was found with more than 50 phyla from the domain Bacteria and 1 phyla from the domain Archaea. Sequences assigned to the genera Ferroplasma, Leptospirillum, Ferrovum, Metallibacterium, Acidithiobacillus, and Sulfuriferula manifested high relative abundances in all sequencing libraries from the relatively high Tl contamination. Canonical correspondence analysis further uncovered that the overall microbial community in this area was dominantly structured by the geochemical fractionation of Tl and geochemical parameters such as pH and Eh. Spearman's rank correlation analysis indicated a strong positive correlation between acidophilic Fe-metabolizing species and Tl_total, Tl_oxi, and Tl_res. The findings clarify potential roles of such phylotypes in the biogeochemical cycling of Tl, which may facilitate the development of in-situ bioremediation technology for Tl-contaminated sediments.

Thallium contamination in farmlands and common vegetables in a pyrite mining city and potential health risks

Thallium (Tl) is a trace metal of severe toxicity. Its health concerns via consumption of contaminated vegetables have often been overlooked or underestimated. This study was designed to gain insight into the actual level and distribution characteristics of Tl and metal (loid)s (Pb, Cd, Cr, Sb, Mn, Cu, Zn, Ni, and Co) in agricultural soils and common vegetables cultivated in different zones (upstream, midstream, and downstream) of a densely populated residential area in a typical mine city, which has been open-pit exploiting Tl-bearing pyrite minerals since 1960s. The results show that most of the agricultural soils exhibit contaminated levels of Tl, with Tl contents (upstream: 1.35-4.31 mg/kg, midstream: 2.43-5.19 mg/kg, and downstream: 0.65-2.33 mg/kg) mostly exceeding the maximum permissible level (MPL) for agricultural land use (1 mg/kg). Sequential extraction procedure indicates that even Tl is predominantly retained in the residual fraction, significant levels of Tl are still present in the geochemically mobile fractions. Besides, metals like Cu, Cd, Mn, and Co are mostly distributed in the labile fractions. Almost all metal (loid)s in edible parts of the vegetables exceed their corresponding MPL for consumption. The chronic daily intake (CDI) and hazard quotient (HQ) values calculated for inhabitants at different ages indicate non-negligible Tl risks via consumption of local vegetables, especially for children. Therefore, it is critical to establish effective measures for hazardous waste management and enforceable regulations in Tl-polluted area to mitigate potential severe impacts of Tl on human health through food chain.

Multivariate receptor models and robust geostatistics to estimate source apportionment of heavy metals in soils

Absolute principal component score/multiple linear regression (APCS/MLR) and positive matrix factorization (PMF) were applied to a dataset consisting of 10 heavy metals in 300 surface soils samples. Robust geostatistics were used to delineate and compare the factors derived from these two receptor models. Both APCS/MLR and PMF afforded three similar source factors with comparable contributions, but APCS/MLR had some negative and unidentified contributions; thus, PMF, with its optimal non-negativity results, was adopted for source apportionment. Experimental variograms for each factor from two receptor models were built using classical Matheron's and three robust estimators. The best association of experimental variograms fitted to theoretical models differed between the corresponding APCS and PMF-factors. However, kriged interpolation indicated that the corresponding APCS and PMF-factor showed similar spatial variability. Based on PMF and robust geostatistics, three sources of 10 heavy metals in Guangrao were determined. As, Co, Cr, Cu, Mn, Ni, Zn, and partially Hg, Pb, Cd originated from natural source. The factor grouping these heavy metals showed consistent distribution with parent material map. 43.1% of Hg and 13.2% of Pb were related to atmosphere deposition of human inputs, with high values of their association patterns being located around urban areas. 29.6% concentration of Cd was associated with agricultural practice, and the hotspot coincided with the spatial distribution of vegetable-producing soils. Overall, natural source, atmosphere deposition of human emissions, and agricultural practices, explained 81.1%, 7.3%, and 11.6% of the total of 10 heavy metals concentrations, respectively. Receptor models coupled with robust geostatistics could successfully estimate the source apportionment of heavy metals in soils.

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.