In Situ Measurement of Thallium in Natural Waters by a Technique Based on Diffusive Gradients in Thin Films Containing a δ-MnO2 Gel Layer

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.

Development and evaluation of diffusive gradients in thin-films technique with a novel titanium peroxide binding gel for in situ measurement of Tl in natural waters and sediments

Speciation of thallium (Tl) controls its fate and biogeochemical behaviors. Thus, a sensitive and accurately approach for Tl monitoring is of great demand due to its ultra-low concentration and sensitivity to redox change. In this study, diffusive gradients in thin-films technique (DGT) assembled with novel titanium peroxide (TP) binding gels (TP-DGT) was developed for in situ measurement of dissolved Tl(I) and Tl(III) in waters and sediments. Laboratory test showed a linear mass accumulation of Tl(I) and Tl(III) on the TP binding gels with the deployment time from 4 to 72 h. A fascinating performance of this novel DGT was achieved in a pH range of 4-9, ionic strength range of 0.1-200 mmol L-1, and humic acid concentration of 0-30 mg L-1 with a low detection limit of 0.3 ng L-1 for Tl(I) and 0.6 ng L-1 for Tl(III). The TP binding gels own excellent stability (1-365 d) and high capacity (73.5 μg Tl disc-1) which are suitable for long-term monitoring. Field application in a river indicated that TP-DGT could work effectively, comparable to the grab sampling in waters. For the first time, the TP-DGT coupled with oxygen optode was successfully applied to map 2D distribution patterns of Tl and oxygen simultaneously in the rhizosphere of M. verticillatum L. This study confirms TP-DGT is a promising tool for routine monitoring of Tl in waters and for investigating biogeochemical processes of Tl in sediments.

Determination of the Isotopic Composition of Aqueous Mercury in a Paddy Ecosystem Using Diffusive Gradients in Thin Films

Measuring the isotopic composition of Hg in natural waters is challenging due to the ultratrace level of aqueous Hg (ng L-1). At least 5 ng of Hg mass is required for Hg isotopic analysis. Given the low Hg concentration in natural waters, a large volume of water (>10 L) is typically needed. The conventional grab sampling method is time-consuming, laborious, and prone to contamination during transportation and preconcentration steps. In this study, a DGT (diffusive gradients in thin films) method based on aminopropyl and mercaptopropyl bi-functionalized SBA-15 nanoparticles was developed and extended to determine the concentration and isotopic composition of aqueous Hg for the first time. The results of laboratory analysis showed that Hg adsorption by DGT induces ∼ -0.2‰ mass-dependent fractionation (MDF) and little mass-independent fractionation (MIF). The magnitude of MDF exhibits a dependence on the diffusion-layer thickness of DGT. Since Hg-MDF can occur in a broad range of environmental processes, monitoring the δ202Hg of aqueous Hg using the DGT method should be performed with caution. Field results show consistent MIF signatures (Δ199Hg) between the DGT and conventional grab sampling method. The developed DGT method serves as a passive sampling method that effectively characterizes the MIF of Hg in waters to understand the biogeochemical cycle of Hg at contaminated sites.

Light- and H2O2-Mediated Redox Transformation of Thallium in Acidic Solutions Containing Iron: Kinetics and Mechanistic Insights

The redox transformation between the oxidation states of thallium (Tl(I) and Tl(III)) is the key to influencing its toxicity, reactivity, and mobility. Dissolved iron (Fe) is widely distributed in the environment and coexists at a high level with Tl in acidic mine drainages (AMDs). While ultraviolet (UV) light and H₂O₂ can directly (by inducing Tl(III) reduction) and indirectly (by inducing Fe(III) to form reactive intermediates) impact the redox cycles of Tl in Fe(III)-containing solutions, the kinetics and mechanism remain largely unclear. This study is the first to investigate the UV light- and H₂O₂-mediated Tl redox kinetics in acidic Fe(III) solutions. The results demonstrate that UV light and H₂O₂ could directly reduce Tl(III) to Tl(I), with the extent of reduction dependent on the presence of Fe(III) and the solution pH. At pH 3.0, Tl(I) was completely oxidized to Tl(III), which can be ascribed to the generation of hydroxyl radicals (^•OH) from the Fe(III) photoreduction or Fe(III) reaction with H₂O₂. The kinetics of Tl(I) oxidation were strongly affected by the Fe(III) concentration, pH, light source, and water matrix. Kinetic models incorporating Tl redox kinetics with Fe redox kinetics were developed and satisfactorily interpreted Tl(III) reduction and Tl(I) oxidation under the examined conditions. These findings emphasize the roles of the UV light- and H₂O₂-driven Fe cycles in influencing the redox state of Tl, with implications for determining its mobility and fate in the environment.

Kinetics of Thallium(I) Oxidation by Free Chlorine in Bromide-Containing Waters: Insights into the Reactivity with Bromine Species

The oxidation of thallium [Tl(I)] to Tl(III) by chlorine (HOCl) is an important process changing its removal performance in water treatment. However, the role of bromide (Br^-), a common constituent in natural water, in the oxidation behavior of Tl(I) during chlorination remains unknown. Our results demonstrated that Br^- was cycled and acted as a catalyst to enhance the kinetics of Tl(I) oxidation by HOCl over the pH range of 5.0-9.5. Different Tl(I) species (i.e., Tl⁺ and TlOH(aq)) and reactive bromine species (i.e., HOBr/BrO^-, BrCl, Br₂O, and BrOCl) were kinetically relevant to the enhanced oxidation of Tl(I). The oxidation by free bromine species became the dominant pathway even at a low Br^- level of 50 μg/L for a chlorine dose of 2 mg of Cl₂/L. It was found that the reactions of Tl⁺/BrCl, Tl⁺/BrOCl, and TlOH(aq)/HOBr dominated the kinetics of Tl(I) oxidation at pH < 6.0, pH 6.0-8.0, and pH > 8.0, respectively. The species-specific rate constants for Tl⁺ reacting with individual bromine species were determined and decreased in the order: BrCl > Br₂ > BrOCl > Br₂O > HOBr. Overall, the presented results refine our knowledge regarding the species-specific reactivity of TI(I) with bromine species and will be useful for further prediction of thallium mobility in chlorinated waters containing bromide.

Geochemical distribution and speciation of Tl and other trace metals in upper Beijiang River in South China: Approach of in-situ DGT monitoring

Mining activities frequently result in severe contamination of river water. This study aimed to better understand the spatial distribution characteristics of Tl and other metals (e.g., Al, Cd, Co, Mn, Ni, Zn, Pb, V, As, Mo, and Sb), and to assess their risks to human health. Surface water samples were collected from the upper Beijiang River (South China) via grab sampling and the diffusive gradients in thin-films (DGT) technique. The concentrations of Tl measured by grab sampling and δ-MnO₂-DGT ranged from 0.045 μg L^-1 to 0.231 μg L^-1 and from 0.056 μg L^-1 to 0.131 μg L^-1, respectively. Most of the metals monitored were below the threshold levels allowed by the drinking water standard in China, except for As, Sb, and Mn at specific sampling sites. The concentrations of other metals measured by grab sampling were higher than those measured using the DGT technique because of the differences in speciation during these measurements. The hazard quotient (5.43 × 10^-4-8.0 × 10^-1 for grab sampling and 2.23 × 10^-4-2.8 × 10^-1 for DGT technique) for the monitored trace metals demonstrated minimal health risk to human beings. The pollution status of these toxic metals in the study area was generally acceptable. As was found to be potentially the most harmful metal in the studied area, with hazard quotients at some sampling sites calculated by grab sampling of >1. It has previously been suggested that As is the most important non-carcinogenic contaminant. The combination of grab sampling and the DGT technique provides a comprehensive understanding of trace metals, especially Tl, in terms of potential bioavailability and ecological assessment.

Mobilization and geochemistry of nutrients in sediment evaluated by diffusive gradients in thin films: Significance for lake management

Investigation of in-situ mobilization of both nitrogen (N) and phosphate (PO₄^3-) in sediment is important for lake management strategy. In this paper, diffusion gradients in thin films (DGT) and DGT induced flux in sediments (DIFS) model are newly designed for in-situ measurement of iron (Fe), PO₄^3-, nitrate (NO₃-N) and ammonium (NH₄-N), and nutrients' mobility in sediment in Lake Nanhu (China). According to DGT profiles together with physicochemical properties in sediment, (I) PO₄^3- is released from (i) Fe-bound P plus loosely sorbed P in anoxic sediment and (ii) the loosely sorbed P in oxic sediment; (II) anoxic sediment inhibits nitrification and NO₃-N release, but it favors denitrification and dissimilatory nitrate reduction to ammonium (DNRA), leading to NH₄-N release; (III) Eh and organic matter are two key influence factors on mobility of PO₄^3-, NO₃-N and NH₄-N. According to DIFS calculation, the dynamics of desorption and diffusion at two sites belong to (i) slow rate of resupply and (ii) fast resupply cases, respectively. Internal loadings are estimated to be 92.74 (PO₄^3-), 268.1 (NH₄-N) and -2466 kg a^-1 (NO₃-N), which reflects sediment mainly acts as a source for PO₄^3- and NH₄-N, and a sink for NO₃-N in water. Based on sediment P release risk index (SPRRI), P release risks in lake sediments are estimated, ranging from light to relative high level. DGT and SPRRI aid choice of restoration methods for sediment, including sediment dredging, phytoremediation and in-situ inactivation.

Electrochemical detection of heavy metal ions in water

Heavy metal ions are one of the main sources of water pollution. Most heavy metal ions are carcinogens that pose a threat to both ecological balance and human health. With the increasing demand for heavy metal detection, electrochemical detection is favorable due to its high sensitivity and efficiency. Here, after discussing the pollution sources and toxicities of Hg(ii), Cd(ii), As(iii), Pb(ii), UO₂(ii), Tl(i), Cr(vi), Ag(i), and Cu(ii), we review a variety of recent electrochemical methods for detecting heavy metal ions. Compared with traditional methods, electrochemical methods are portable, fast, and cost-effective, and they can be adapted to various on-site inspection sites. Our review shows that the electrochemical detection of heavy metal ions is a very promising strategy that has attracted widespread attention and can be applied in agriculture, life science, clinical diagnosis, and analysis.

Distribution and availability of rare earth elements and trace elements in the estuarine waters of the Ría of Huelva (SW Spain)

Metal pollution in estuaries represents a serious environmental challenge, especially in areas affected by industrial and mining activities. This study investigates the metal partitioning and availability of rare earth elements (REE), Y and other trace metals (Ag, Tl, U and Cs) in the Ria of Huelva estuary (SW Spain), strongly affected by mining and industrial activities. A 30 h monitoring campaign was performed collecting periodic water samples and deploying diffusive gradient in thin films (DGTs) devices to determine the main factors controlling metal availability. The dissolved concentrations of U (3118-3952 ng/L) and Cs (284-392 ng/L) were in the same order of magnitude than those reported in other estuaries and coastal waters worldwide, however, REE (26-380 ng/L), Y (15-109 ng/L), Ag (14-307 ng/L) and Tl (29-631 ng/L) concentrations exceeded these values for the same salinities. Unlike most metals (i.e. Ag, Tl, U, Cs), which were mainly found in the dissolved form (87-100% of total), REE and Y were found in the particulate phase (22-36% of total). Metal lability was mainly related to the concentration in the water column following this order: U>REE>Y>Ag>Tl. A similar binding mechanism was observed for Tl and Cd, due to its chemical affinity. This relationship between chemical properties and absorption by DGT-resin was also observed for REE (and Y), Rb and Sr, which may cause bioaccumulation upon persistent exposure, considering the ability of these metals to cross the biological membranes. The lability of metals predicted by geochemical codes did not coincide with absorption of labile metals by DGTs due probably to the instability of complexes in contact with the DGT membranes, the inability of metals to form thermodynamically stable complexes or the absorption of colloids. From this work it can be concluded that DGT passive sampling should complement traditional sampling to monitor metal availability in aquatic environments.

In Situ Selective Measurement Based on Diffusive Gradients in Thin Films Technique with Mercapto-Functionalized Mesoporous Silica for High-Resolution Imaging of SbIII in Soil

In situ monitoring of Sb speciation improves the understanding of Sb biogeochemistry and toxicity in ecosystems. Precise measurement of Sb is a challenge due to its instability of oxidation and ultratrace concentration. The development of simple and reliable methods specific to Sb^III measurement is not only appealing but essential for implementing regulations. Here, we present an in situ speciation analysis method for Sb^III, using the diffusive gradients in thin films (DGT) technique, combined with mercapto-functionalized SBA-15 mesoporous silica nanoparticles (MSBA). Laboratory performance tests confirmed MSBA-DGT uptake was independent of pH (4-9) and ionic strength (0.1-200 mmol L^-1). DGT devices equipped with MSBA-based binding gels showed a theoretically linear accumulation of Sb^III and exhibited a high capacity for Sb^III at 65 μg/gel disc, with negligible accumulation of Sb^V over a 72 h deployment. Compared with commercial 3-mercaptopropyl-functionalized silica (MFS), the nanosized MSBA facilitate its even distribution in the binding gels. Furthermore, the good selectivity and high homogeneity of the MSBA gel enabled it to be applied in a rice rhizosphere in conjunction with AgI gel to investigate the effects of sulfur application on the Sb^III solubility. In summary, the newly developed MSBA-DGT provides a selective measurement of Sb^III, showing potential for environmental monitoring and further application in understanding the biogeochemical process of Sb.