Phase-dependent phytoavailability of thallium--a synthetic soil experiment

Biochemical transformation and bioremediation of thallium in the environment

Thallium (Tl) is a toxic element associated with minerals, and its redistribution is facilitated by both geological and anthropogenic activities. In the natural environment, the transformation and migration of Tl mediated by (micro)organisms have attracted increasing attention. This review presents an overview of the biochemical transformation of Tl and the bioremediation strategies for Tl contamination. In the environment, Tl exists in various forms and originates from diverse sources. The global distribution characteristics of Tl in various media are summarized here, while its speciation and toxicity mechanism to organisms are elucidated. Interactions between (micro)organisms and Tl are commonly observed in the environment. Microbial response mechanisms to typical Tl exposure are analyzed at both species and gene levels, and the possibility of microorganisms as bio-indicators for monitoring Tl contamination is also highlighted. The processes and mechanisms involved in the microbial and benthic mediated transformation of Tl, as well as its enrichment by plants, are discussed. Additionally, in situ bioremediation strategies for Tl contamination and bio-treatment techniques for Tl-containing wastewater are summarized. Finally, the existing knowledge gaps and future research challenges are emphasized, including Tl distribution characteristics in the atmosphere and ocean, the key molecular mechanisms underlying Tl transformation by organisms, the screening of potential Tl oxidizing microorganisms and hyperaccumulators, as well as the revelation of global biogeochemical cycling pathways of Tl.

Persistent thallium enrichment and its high ecological risks developed from historical carbonaceous Hg-Tl mining waste

Thallium (Tl) is a priority pollutant with high biotoxicity and has been of great concern worldwide in recent years. The former Lanmuchang Hg-Tl mining site in southwest China is a hotspot of multiple metal(loid)s pollution that previously caused large-scale chronic Tl poisoning, mainly resulting from carbonaceous Tl-bearing mining waste. However, arable land destroyed by historical mining wastes persists at high ecological risks decades after reclamation, but little is known about the solid phase partitioning and species of Tl during soil formation of underlying mining wastes as potential Tl sources. In this study, a representative reclaimed soil profile (100 cm depth) was selected in the lowlands to explore the geochemical cycling and environmental fate of Tl in mining waste-derived subsoil. The Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis revealed an unexpected enrichment of Mn (2920-7250 mg/kg) and Tl (205-769 mg/kg) in the mining waste-derived subsoil. Results from BCR sequential extraction, X-ray Photoelectron Spectroscopy (XPS), and Electron Probe Microanalyses (EPMA) indicate that high Tl loading Mn oxide particulates (up to 15,712 ppm Tl) dominate the sequestration of Tl in the subsoil via oxidation-complexation and have a high potential for migration to both topsoil and groundwater. In addition, insights from microbial fossils and Fe-metabolizing bacteria closely related to Tl indicated that Fe (hydr)oxide particulates showing high Tl levels (up to 3865 mg/kg) point to biomineralization. Detailed mineralogical investigations revealed that hematite-siderite syngenetic particulates could serve as a promising mineralogical proxy for redox oscillations under periodic flooding and recorded the frequent groundwater level fluctuations experienced in the probed profile. Despite the potential for long-term preservation of high Tl loading Fe/Mn (hydr)oxides under HCO3-rich groundwater conditions in karst areas, the reductive release of Tl will be inevitable during flooding, implying that underlying carbonaceous mining waste will pose persistent and severe hazards to the ecosystem.

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.

Sinapis alba as a useful plant in bioremediation - studies of defense mechanisms and accumulation of As, Tl and PGEs

Pollution of the soils with toxic elements is a serious problem all over the world. One of environmentally friendly techniques of their removal is phytoremediation. This paper is a summary of literature data and the results of own studies about the potential of Sinapis alba for bioaccumulation of Tl, As and PGEs, and its usefulness in remediation of polluted environment. S. alba is characterized with low living requirements, BFs ≫ 1 and high TFs, especially for Tl (up to 3). The influence of different forms of studied elements on plants was discussed based on biomass production, morphological changes and the impact on photosynthesis activity. The plants were cultivated in hydroponics and solid media of various composition, for example, in soil supplemented with MnO₂, which resulted in BFs lower 6-7 times for leaves, and about 3-4 times for stems, as well as twice lower leaf development. Application of advanced analytical techniques was presented in studies of the detoxification mechanisms, identification of particular chemical forms of the elements and the presence of phytochelatins and their complexes with the investigated elements.Novelty StatementThe paper summarizes both literature and original data on Sinapis alba exposed to such elements as thallium, arsenic and platinum group metals. The influence of different forms of studied elements on white mustard was discussed based on biomass production and morphological changes, as well as the impact on photosynthesis activity. The study covers such aspects as bioaccumulation, phytotoxicity as well as the usefulness of white mustard in remediation of polluted environment.

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.

Higher Tl bioaccessibility in white mustard (hyper-accumulator) grown under the soil than hydroponic conditions: A key factor for the phytoextraction use

The paper deals with the thallium (Tl) access into the white mustard (Sinapis alba L.). We were comparing two approaches: A - hydroponic, B - semi-hydroponic (artificial soil). The kinetics of Tl plant uptake at different available Tl doses (0.1, 0.05 and 0.01 mg L^-1) was tested. It was revealed that the hydroponic arrangement did not accelerate the plant uptake of Tl. The concentration of plant Tl was surprisingly roughly double under the semi-hydroponic (artificial soil) conditions as compared to the hydroponic system; the highest Tl concentrations were detected in stems, proving an important role of plant grown strategy on Tl bioaccessibility. We found that almost independently of the initial dose of Tl the juvenile stadium of the mustard can preserve1-2% of the total Tl pool. Up to 95% of this Tl dose is stored in the shoots. The different strategy of the plant growing may strongly affect the path of Tl incorporation. The total Tl input into the leaf tissue in hydroponics may be from 69% (p = 0.01) explained by parallel assimilation of Ca. In contrast, the Tl entry into the leaf grown on the artificial soil could be limited by Mn path (R² = 0.91, p = 0.01).

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.

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.

Uptake and Fractionation of Thallium by Brassica juncea in a Geogenic Thallium-Amended Substrate

This study shows thallium (Tl) concentrations in Brassica juncea (Indian mustard) tissue are more than an order of magnitude higher (3830 μg/kg) than that of the substrate (100 μg/kg) and are strongly influenced by the underlying mineralogy; i.e., Tl bioaccessibility depends on the mineral structure: K-feldspar > Mn nodule > hendricksite mica. The majority of Tl for all substrates is contained in edible parts of the plant, i.e., leaves (41% of total Tl, on average) ≥ flower stems (34%) > seed pods (11%) ≈ stems (10%) > flowers (3%). We also show that Tl isotope fractionation induced by B. juncea is substantial, at nearly 10 ε²⁰⁵Tl units, and generates systematic plant-specific patterns. Progressive plant growth strongly fractionates Tl isotopes, discriminating against ²⁰⁵Tl as the plant matures. Thus, ²⁰⁵Tl values are systematically higher in the early formed stem (ε²⁰⁵Tl_avg = +2.5) than in plant elements formed later (ε²⁰⁵Tl_avg = -2.5 to +0.1), which demonstrates the large degree of translocation and the associated effects during plant growth. This study establishes the potential of Tl isotopes as a new tool for understanding heavy metal (re)distribution during anthropogenic and geologic processes and the utility of such information in environmental and health-related planning and in phytomining or bioprospecting.

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).

Effective removal of trace thallium from surface water by nanosized manganese dioxide enhanced quartz sand filtration

Thallium (Tl) has drawn wide concern due to its high toxicity even at extremely low concentrations, as well as its tendency for significant accumulation in the human body and other organisms. The need to develop effective strategies for trace Tl removal from drinking water is urgent. In this study, the removal of trace Tl (0.5 μg L^-1) by conventional quartz sand filtration enhanced by nanosized manganese dioxide (nMnO₂) has been investigated using typical surface water obtained from northeast China. The results indicate that nMnO₂ enhanced quartz sand filtration could remove trace Tl(I) and Tl(III) efficiently through the adsorption of Tl onto nMnO₂ added to a water matrix and onto nMnO₂ attached on quartz sand surfaces. Tl(III)-HA complexes might be responsible for higher residual Tl(III) in the effluent compared to residual Tl(I). Competitive Ca²⁺ cations inhibit Tl removal to a certain extent because the Ca²⁺ ions will occupy the Tl adsorption site on nMnO₂. Moreover, high concentrations of HA (10 mgTOC L^-1), which notably complexes with and dissolves nMnO₂ (more than 78%), resulted in higher residual Tl(I) and Tl(III). Tl(III)-HA complexes might also enhance Tl(III) penetration to a certain extent. Additionally, a higher pH level could enhance the removal of trace Tl from surface water. Finally, a slight increase of residual Tl was observed after backwash, followed by the reduction of the Tl concentration in the effluent to a "steady" state again. The knowledge obtained here may provide a potential strategy for drinking water treatment plants threatened by trace Tl.

Removal of thallium from aqueous solutions using Fe-Mn binary oxides

In this study, Fe-Mn binary oxides, which harbor the strong oxidative power of manganese dioxide and the high adsorption capacity of iron oxides, were synthesized for Tl(I) removal using a concurrent chemical oxidation and precipitation method. The adsorption of Tl onto the Fe-Mn adsorbent was fast, effective, and selective, with equilibrium sorption reaching over 95% under a broad operating pH (3-12), and high ionic strength (0.1-0.5mol/L). The adsorption can be well fitted with both Langmuir and Freundlich isotherms, and the kinetics can be well described by the pseudo-second-order model. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) spectra suggest that surface complexation, oxidation and precipitation were the main mechanisms for the removal of Tl. This study shows that the Fe-Mn binary oxides could be a promising adsorbent for Tl removal.

FeOOH-loaded MnO2 nano-composite: An efficient emergency material for thallium pollution incident

A FeOOH-loaded MnO₂ nano-composite was developed as an emergency material for Tl(I) pollution incident. Structural characterizations showed that FeOOH successfully loaded onto MnO₂, the nanosheet-flower structure and high surface area (191 m² g^-1) of material contributed to the excellent performance for Tl(I) removal. FeOOH-loaded MnO₂ with a Fe/Mn molar ratio of 1:2 exhibited a noticeable enhanced capacity for Tl(I) removal compared to that of pure MnO₂. The outstanding performance for Tl(I) removal involves in extremely high efficiency (achieved equilibrium and drinking water standard within 4 min) and the large maximum adsorption capacity (450 mg g^-1). Both the control-experiment and XPS characterization proved that the removal mechanism of Tl(I) on FeOOH-loaded MnO₂ included adsorption and oxidation: the oxidation of MnO₂ played an important role for Tl(I) removal, and the adsorption of FeOOH loaded on MnO₂ enhanced Tl(I) purification at the same time. In-depth purification of Tl(I) had reach drinking water standards (0.1 μg L^-1) at pH above 7, and there wasn't security risk produced from the dissolution of Mn²⁺ and Fe²⁺. Moreover, the as-prepared material could be utilized as a recyclable adsorbent regenerated by using NaOH-NaClO binary solution. Therefore, the synthesized FeOOH-loaded MnO₂ in this study has the potential to be applied as an emergency material for thallium pollution incident.

Significantly improving trace thallium removal from surface waters during coagulation enhanced by nanosized manganese dioxide

Thallium (Tl) is an element of high toxicity and significant accumulation in human body. There is an urgent need for the development of appropriate strategies for trace Tl removal in drinking water treatment plants. In this study, the efficiency and mechanism of trace Tl (0.5 μg/L) removal by conventional coagulation enhanced by nanosized manganese dioxide (nMnO₂) were explored in simulated water and two representative surface waters (a river water and a reservoir water obtained from Northeast China). Experimental results showed that nMnO₂ significantly improve Tl(I) removal from selected waters. The removal efficiency was dramatically higher in the simulated water, demonstrating by less than 0.1 μg/L Tl residual. The enhancement of trace Tl removal in the surface waters decreased to a certain extent. Both adjusting water pH to alkaline condition and preoxidation of Tl(I) to Tl(III) benefit trace Tl removal from surface waters. Data also indicated that competitive cation of Ca²⁺ decreased the efficiency of trace Tl removal, resulting from the reduction of Tl adsorption on nMnO₂. Humic acid could largely low Tl removal efficiency during nMnO₂ enhanced coagulation processes. Trace elemental Tl firstly adsorbed on nMnO₂ and then removed accompanying with nMnO₂ settling. The information obtained in the present study may provide a potential strategy for drinking water treatment plants threatened by trace Tl.

Isotopic Tracing of Thallium Contamination in Soils Affected by Emissions from Coal-Fired Power Plants

Here, for the first time, we report the thallium (Tl) isotope record in moderately contaminated soils with contrasting land management (forest and meadow soils), which have been affected by emissions from coal-fired power plants. Our findings clearly demonstrate that Tl of anthropogenic (high-temperature) origin with light isotope composition was deposited onto the studied soils, where heavier Tl (ε(205)Tl ∼ -1) naturally occurs. The results show a positive linear relationship (R(2) = 0.71) between 1/Tl and the isotope record, as determined for all the soils and bedrocks, also indicative of binary Tl mixing between two dominant reservoirs. We also identified significant Tl isotope variations within the products from coal combustion and thermo-desorption experiments with local Tl-rich coal pyrite. Bottom ash exhibited the heaviest Tl isotope composition (ε(205)Tl ∼ 0), followed by fly ash (ε(205)Tl between -2.5 and -2.8) and volatile Tl fractions (ε(205)Tl between -6.2 and -10.3), suggesting partial Tl isotope fractionations. Despite the evident role of soil processes in the isotope redistributions, we demonstrate that Tl contamination can be traced in soils and propose that the isotope data represent a possible tool to aid our understanding of postdepositional Tl dynamics in surface environments for the future.

Evaluation of the ability of black nightshade Solanum nigrum L. for phytoremediation of thallium-contaminated soil

Thallium (Tl) pollution in agricultural areas can pose hidden danger to humans, as food consumption is the key exposure pathway of Tl. Owing to the extreme toxicity of Tl, removal of Tl from soil becomes necessary to minimize the Tl-related health effects. Phytoremediation is a cost-effective method to remove heavy metals from soil, but not all plants are appropriate for this purpose. Here, the ability of Solanum nigrum L., commonly known as black nightshade, to remediate Tl-contaminated soil was evaluated. The accumulation of Tl in different organs of S. nigrum was measured under both field and greenhouse conditions. Additionally, the growth and maximal quantum efficiency of photosystem II (Fv/Fm) under different Tl concentrations (1, 5, 10, 15, and 20 mg kg(-1)) were examined after 4-month pot culture. Under both field and greenhouse conditions, Tl accumulated in S. nigrum was positively correlated with Tl concentration in the soil. Thallium mostly accumulated in the root, and bioconcentration factor was greater than 1, indicating the good capability of S. nigrum to extract Tl. Nonetheless, the growth and Fv/Fm of S. nigrum were reduced at high Tl concentration (>10 mg kg(-1)). Given the good tolerance, fast growth, high accumulation, and global distribution, we propose that S. nigrum is a competent candidate to remediate moderately Tl-contaminated soil (<10 mg kg(-1)) without causing far-reaching ecological consequences.

Thallium speciation and extractability in a thallium- and arsenic-rich soil developed from mineralized carbonate rock

We investigated the speciation and extractability of Tl in soil developed from mineralized carbonate rock. Total Tl concentrations in topsoil (0-20 cm) of 100-1000 mg/kg are observed in the most affected area, subsoil concentrations of up to 6000 mg/kg Tl in soil horizons containing weathered ore fragments. Using synchrotron-based microfocused X-ray fluorescence spectrometry (μ-XRF) and X-ray absorption spectroscopy (μ-XAS) at the Tl L3-edge, partly Tl(I)-substituted jarosite and avicennite (Tl2O3) were identified as Tl-bearing secondary minerals formed by the weathering of a Tl-As-Fe-sulfide mineralization hosted in the carbonate rock from which the soil developed. Further evidence was found for the sequestration of Tl(III) into Mn-oxides and the uptake of Tl(I) by illite. Quantification of the fractions of Tl(III), Tl(I)-jarosite and Tl(I)-illite in bulk samples based on XAS indicated that Tl(I) uptake by illite was the dominant retention mechanism in topsoil materials. Oxidative Tl(III)uptake into Mn-oxides was less relevant, probably because the Tl loadings of the soil exceeded the capacity of this uptake mechanism. The concentrations of Tl in 10 mM CaCl2-extracts increased with increasing soil Tl contents and decreasing soil pH, but did not exhibit drastic variations as a function of Tl speciation. With respect to Tl in contaminated soils, this study provides first direct spectroscopic evidence for Tl(I) uptake by illite and indicates the need for further studies on the sorption of Tl to clay minerals and Mn-oxides and its impact on Tl solubility in soils.

Thallium contamination of soils/vegetation as affected by sphalerite weathering: a model rhizospheric experiment

The environmental stability of Tl-rich sphalerite in two contrasting soils was studied. Rhizospheric conditions were simulated to assess the risk associated with sulfide microparticles entering agricultural (top)soils. The data presented here clearly demonstrate a significant effect of 500 μM citric acid, a model rhizospheric solution, on ZnS alteration followed by enhanced Tl and Zn release. The relative ZnS mass loss after 28 days of citrate incubation reached 0.05 and 0.03 wt.% in Cambisol and Leptosol samples respectively, and was up to 4 times higher, compared to H2O treatments. Incongruent (i.e., substantially increased) mobilization of Tl from ZnS was observed during the incubation time. Generally higher (long-term) stability of ZnS with lower Tl release is predicted for soils enriched in carbonates. Furthermore, the important role of silicates (mainly illite) in the stabilization of mobilized Tl, linked with structural (inter)layer Tl-K exchange, is suggested. Thallium was highly bioavailable, as indicated by its uptake by white mustard; maximum Tl amounts were detected in biomass grown on the acidic Cambisol. Despite the fact that sulfides are thought as relatively stable phases in soil environments, enhanced sulfide dissolution and Tl/trace element release (and bioaccumulation) can be assumed in rhizosphere systems.

Tracing anthropogenic thallium in soil using stable isotope compositions

Thallium stable isotope data are used in this study, for the first time, to apportion Tl contamination in soils. In the late 1970s, a cement plant near Lengerich, Germany, emitted cement kiln dust (CKD) with high Tl contents, due to cocombustion of Tl-enriched pyrite roasting waste. Locally contaminated soil profiles were obtained down to 1 m depth and the samples are in accord with a binary mixing relationship in a diagram of Tl isotope compositions (expressed as ε(205)Tl, the deviation of the (205)Tl/(203)Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 10(4)) versus 1/[Tl]. The inferred mixing endmembers are the geogenic background, as defined by isotopically light soils at depth (ε(205)Tl ≈ -4), and the Tl emissions, which produce Tl-enriched topsoils with ε(205)Tl as high as ±0. The latter interpretation is supported by analyses of the CKD, which is also characterized by ε(205)Tl ≈ ± 0, and the same ε(205)Tl value was found for a pyrite from the deposit that produced the cocombusted pyrite roasting waste. Additional measurements for samples from a locality in China, with outcrops of Tl sulfide mineralization and associated high natural Tl backgrounds, reveal significant isotope fractionation between soils (ε(205)Tl ≈ +0.4) and locally grown green cabbage (ε(205)Tl between -2.5 and -5.4). This demonstrates that biological isotope fractionation cannot explain the isotopically heavy Tl in the Lengerich topsoils and the latter are therefore clearly due to anthropogenic Tl emissions from cement processing. Our results thus establish that isotopic data can reinforce receptor modeling for the toxic trace metal Tl.