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.

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.

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.

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 (αsolid-solution ≈ 1.00022-1.00037) was smaller than previously observed fractionation between Mn oxides and aqueous Tl(I) (αsolid-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.

Microbial diversity in paddy rhizospheric soils around a large industrial thallium-containing sulfide utilization zone.

Thallium (Tl) is a rare and extremely toxic metal whose toxicity is significantly higher than cadmium (Cd), lead (Pb) and antimony (Sb). The extensive utilization of Tl-bearing minerals, such as mining activities, has led to severe Tl pollution in a variety of natural settings, while little is known to date about its effect on the microbial diversity in paddy soils. Also, the geochemical behavior of Tl in the periodical alterations between dry and wet conditions of paddy soils remains largely unknown. Herein, the sequential extraction method and 16S rRNA gene sequence analysis were adopted to analyze Tl's migration and transformation behavior and the microbial diversity in the paddy soils with different pollution levels. The results indicated that Tl was mainly concentrated in reducible fraction, which is different from other types of soils, and may be closely attributed to the abundance of Fe-Mn (hydr)oxides in the paddy rhizospheric soils. Further analysis revealed that pH, total S, Pb, Sb, Tl and Cd were the dominant environmental factors, and the enrichment level of these potentially toxic metal(loid)s (PTMs) exerted obvious impacts on the diversity and abundance of microorganism in the rhizospheric soils, and regulating microbial community. The geochemical fractionation of Tl was closely correlated to soil microorganisms such as Fe reducing bacteria (Geothrix) and sulfate reducing bacteria (Anaerolinea), playing a critical role in Tl geochemical cycle through redox reaction. Hence, further study on microorganisms of paddy rhizospheric soils is of great significance to the countermeasures for remediating Tl-polluted paddy fields and protect the health of residents.

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

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

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).

Unexpected enrichment of thallium and its geochemical behaviors in soils impacted by historically industrial activities using lead­zinc carbonate minerals.

Thallium is a trace metal with severe toxicity. Contamination of thallium (Tl) generated by steel and non-ferrous metals industry is gaining growing concern worldwide. However, little is known on Tl contamination owing to industrial activities using carbonate minerals. This study revealed abundant geochemical mobile/bioavailable Tl (> 65.7%, in average; mostly in oxidizable fraction) in soils from a carbonate-hosted PbZn ore utilizing area in China for the first time. Unexpected Tl enrichment was observed in soil accompanying with 3655, 7820, 100.1, 27.3 and 29.9 mg/kg (in average) of Pb, Zn, As, Cd and Sb, respectively. Characterization using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis further confirmed that historical industrial activities impose anthropogenic catastrophic effects on the local agricultural soil system. The ecological and health risk assessment of heavy metal(loid)s in soils proclaimed serious potential non-carcinogenic risks of Pb and V to adults, and Pb, Tl and As to children. Sequential extraction analysis showed that Tl, as well as Pb, Zn, Mn, Co, and Cd, mainly existed in the mobile fractions (exchangeable/acid-extractable, reducible and oxidizable), indicating an ecological risk of biological accumulation of multiple metal(loid)s in this area. These findings provide a theoretical basis for taking appropriate remediation measures in order to ensure safety of soils in such industrial areas likewise.

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

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

Thallium 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.