Risk assessment and vertical distribution of thallium in paddy soils and uptake in rice plants irrigated with acid mine drainage

Excess supply of sulfur mitigates thallium toxicity to rice (Oryza sativa L.) growth in hydroponic experiment

Sulfur (S) is an essential element for the growth of rice plants (Oryza sativa L.), crucial for enhancing crop yield and grain quality. However, its potential in mitigating thallium (Tl) toxicity in rice remains unclear. In this study, a hydroponic experiment was performed to investigate the effects of low, medium and high S application levels (LS, MS, HS) on Tl accumulation in rice at three Tl exposure levels (0, 0.5 and 1 mg·L-1). Our findings reveal that the exogenous S application could alleviate Tl toxicity, enhancing fresh weight and shoot length of rice plant. Additionally, HS (HS, SO42- content was 387.84 mg·L-1) group significantly increased chlorophyll and glutathione (GSH) content by 6.46 to 21.38 % and 2.15 to 7.31 % respectively, while reducing malondialdehyde (MDA) levels by 17.43 to 28.48 %, compared to MS (MS, SO42- content was 193.41 mg·L-1) group. Fe content in rice roots and iron plaque consistently increased with S provision under Tl-free and Tl-contaminated conditions. In Tl exposure environment, HS and LS (LS, SO42- content was 1.02 mg·L-1) groups exhibited significant differences in Fe contents and iron plaque in rice root. Moreover, in Tl exposure environment, S application reduced Tl concentration in iron plaque, root, and shoot, HS treatment showed Tl content reduction from 16.29 % to 25.89 %, compared to LS treatment. Our findings underscore the potential of S application in hydroponic environment to promote rice growth and mitigate Tl accumulation, offering insights for developing effective Tl remediation strategies by using S-contained fertilizers.

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.

New insights into the key role of node I in thallium accumulation in seed of coix (Coix lacryma-jobi L.)

The mechanisms underlying the distribution of many toxic metal(loid)s in shoots and metal(loid) transport to grains have been well documented in the quest for food safety but there remains a lack of knowledge on thallium (Tl) accumulation in food crops. Here, field investigations combined with a glasshouse pot experiment were conducted to investigate the characteristics of Tl distribution and accumulation in coix, a major food crop in south Guizhou province, China, and the role of node I in restricting Tl transport to the seed. Fourteen percent of coix seed samples collected from the Lanmuchang Tl-As-Hg mine contained higher Tl concentrations than the recommended limit for foods and feedstuffs in Germany (0.5 mg kg-1), with the highest exceedance rate of the metal(loid)s determined, when grown in soils surrounding the mine with a very high Tl concentration of 0.07-89.5 mg kg-1 and a general low pH of 4.19-6.48. Thallium concentrations were higher in coix nodes than in internodes, followed by roots and grains. The Tl translocation factors from node I to grains were 0.01-0.21 and were the lowest of any translocation factors between different tissues. Node I is therefore the key tissue restricting Tl transport to coix grains. Thallium was localized mainly in the diffuse vascular bundles (DVBs) in node I. The co-localization of Tl and sulfur in the DVBs and Tl contamination-induced phytochelatin (PC) accumulation indicate that Tl storage in the DVBs involving complexation with PCs in node I is an important process in Tl accumulation in coix grains. Moreover, the area of DVBs in node I increased with increasing soil Tl pollution level, providing more channels for Tl transport to the panicles and grains and thereby acting as a key factor restricting Tl transport to the grains. These results provide new insights into the key role of node I in Tl accumulation in coix grains and indicate key points to minimize Tl accumulation in grains for food safety.

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.

Assessment of the Ecotoxicity of Pollution by Potentially Toxic Elements by Biological Indicators of Haplic Chernozem of Southern Russia (Rostov region)

The content of various chemical elements such as metals, metalloids, and nonmetals in the environment is associated with natural and anthropogenic sources. It is necessary to normalize the content of metals, metalloids, and nonmetals as potentially toxic elements (PTE) in the Haplic Chernozem. The soils of the Southern Russia are of high quality and fertility. However, this type of soil, like Haplic Chernozem, is subject to contamination with a wide range of PTE. The aim of the work was to rank metals, metalloids, and nonmetals by ecotoxicity in Haplic Chernozem. To assess the ecotoxicity of chernozem, data for 15 years (2005-2020) were used. Biological indicators used to assess the ecotoxicity of Haplic Chernozem: catalase activity, cellulolytic activity, number of bacteria, Azotobacter spp. abundance, to change of length of radish's roots. Based on these biological indicators, an integral indicator of the state of Haplic Chernozem was calculated. The ecotoxicity of 23 metals (Cd, Hg, Pb, Cr, Cu, Zn, Ni, Co, Mo, Mn, Ba, Sr, Sn, V, W, Ag, Bi, Ga, Nb, Sc, Tl, Y, Yb), 5 metalloids (B, As, Ge, Sb, Te) and 2 nonmetals (F, Se) as priority pollutants. It is proposed to distinguish three hazard classes of metals, metalloids, and nonmetals to Haplic Chernozem: I class - Te, Ag, Se, Cr, Bi, Ge, Sn, Tl, Hg, Yb, W, Cd; II class - As, Co, Sc, Sb, Cu, Ni, B, Nb, Pb, Ga; III class - Sr, Y, Mo, Zn, V, Ba, Mn, F. It is advisable to use the results of the study for predictive assessment of the impact of metals, metalloids, and nonmetals on the ecological state of the soil during pollution.

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

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

Contamination of rice crop with potentially toxic elements and associated human health risks-a review

Production of rice, a major staple food crop, should be maintained both quantitatively and qualitatively to assure global food security. In recent decades, various natural (biogeochemical weathering of rocks) and anthropogenic (increased application of agrochemicals, solid and liquid waste discharges from domestic and industrial areas, vehicular pollution, etc.) activities have deteriorated soil and water resources by contributing potentially toxic elements (PTEs) to the environment. Shortage of land resources and requirements of the ever-increasing human population has led to increasing global trend of rice cultivation in contaminated soils, causing accumulation of various PTEs such as arsenic (As), mercury (Hg), cobalt (Co), cadmium (Cd), copper (Cu), lead (Pb), and nickel (Ni) in rice crop, especially in the grains. Rice plants uptake and accumulate PTEs leading to their entry into the food chain. Consumption of rice contaminated with PTEs disturbs the human metabolism as PTEs interfere with different physiological/molecular mechanisms causing various health problems such as weak bones; skin problems; respiratory, cardiovascular, endocrine, nervous, reproductive, and hepatic disorders; and cancer. Possible non-carcinogenic and carcinogenic health risks have been determined in some studies by following the guidelines provided by various governmental or non-governmental agencies. Considering these facts, the present study was conducted to give a broader perspective on rice contamination with various potentially toxic elements, their bioconcentration in rice, associated health risks in human beings, and strategies for bioremediation of soil and water resources to eliminate PTEs.

Different responses in metallothionein gene expression and antioxidative enzyme activity lead to more ROS accumulation in rice exposed to Tl(III) than to Tl(I)

This is the first study to investigate the reduction mechanism of Tl (III) to Tl(I) in the presence of plants, especially rice. Smaller plant density could effectively reduce the content of organic acids in the hydroponic system to keep the stability of Tl(III). As the plant density was reduced from 40 seedlings to 10 seedlings in 100 mL Tl(III) solution, the content of oxalate was declined to one-third of the original, and the ratio of Tl(III)/total Tl was increased from 39.6% to 81.0% in the first 2 h treatment. Then the differences in antioxidant capacity of rice exposed to the two Tl species were studied. The contents of malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and superoxide anion (O₂˙^-) of rice roots exposed to Tl(III) were all higher than those to Tl(I). Meanwhile, the catalase (CAT) activity was significantly depressed and peroxidase (POD) was increased by Tl(III), whereas superoxide dismutase (SOD) showed a rise in both Tl(I) and Tl(III) with no significant difference between them. The expression of metallothionein gene OsMT1a to Tl(I) was upregulated to 255.5 times of Tl(III) though OsMT2c was downregulated to 0.39 times of Tl(III). Overall, the different responses in metallothionein gene expression and antioxidative enzyme activation might result in more ROS accumulation to rice roots by Tl(III) treatment than those by Tl(I).

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.

Trace elements and stable sulfur isotopes in plants of acid mine drainage area: Implications for revegetation of degraded land

The abundances of trace elements, a low pH of water and soil in areas impacted by the acid mine drainage (AMD) may cause an excessive uptake of potentially toxic elements and nutritional imbalances in plants. Metal-tolerant, native plants are used for revegetation of degraded mining areas. We established levels of selected trace elements and stable sulfur isotopes in the above-ground plant biomass collected in a mining area in south-central Poland. In 2016, 20 samples of the most common species were collected from sites with a different influence of acid mine drainage and analyzed for trace elements by the inductively coupled plasma mass spectrometry technique. On the basis of the results obtained in 2016, the most contaminated site was selected for a more detailed study, in which sulfur contents and stable sulfur isotope ratios were determined together with trace elements in 17 samples. The results confirmed that the plants native to the AMD area efficiently accumulated trace elements, especially As and rare earth elements. Mosses showed the highest content of trace elements, but exhibited the lowest concentrations of sulfur accompanied by the highest δ³⁴S values. It has been shown for the first time that stable sulfur isotope composition of AMD plants in south-central Poland is significantly depleted in the ³⁴S isotope showing an average δ³⁴S value of -10.5‰ in comparison with positive δ³⁴S values in local vegetation growing outside the AMD area and in local precipitation.

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 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.

Distribution, fractions, and potential release of thallium in acidic soils nearby a waste copper mining site from southern China

Although thallium (Tl) is a highly toxic element, little information is available on the environmental risks of Tl in agricultural soils with intensive practices, particularly nearby mining sites. Therefore, we investigated the potential release of Tl in acidic soils with intensive cultivation nearby a waste copper mining site from southern China based on its level and chemical fractions as well as simulated release under artificial acid rain. Results showed that the average Tl content was 1.31 mg/kg in the studied area, which significantly exceeds the permissible thallium value of 1 mg/kg for agricultural soil in China. Some vertical increases of soil Tl from different land uses indicate the potential transport of Tl downward to groundwater. High positive correlations between surficial soil Tl and rubidium (Rb) and copper (Cu) indicated that Tl has the lithophile and chalcophile behavior. Tl in soils is mainly entrapped in residual fraction. The exchangeable fraction of Tl in agricultural soils was less than undisturbed natural soils and copper mined soils. Additionally, the percentage of Tl release from undisturbed natural soils and soils of copper ore area was more than that from agricultural soils in simulated acid rain. Furthermore, the releases of Tl from the soils increased with the acidity of artificial acid rain. Thus, more attention must be paid to land management of this similar area to avoid the risk of Tl impact on human health.