In Situ Selective Measurement of SeIV in Waters and Soils: Diffusive Gradients in Thin-Films with Bi-Functionalized Silica Nanoparticles

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.

Selective and simultaneous high resolution 2-D imaging of AsIII, CrIII and SbIII and dissolved oxygen by developing a new DGT technique comprising a hybrid sensor

A new diffusive gradients in thin films technique (HR-MPTS DGT) with mercapto-functionalized attapulgite in a binding gel was developed for simultaneous two-dimensional (2-D) chemical imaging of As^III, Cr^III and Sb^III selectively at the submillimeter scale, combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The HR-MPTS DGT exhibited selective accumulation of As^III, Sb^III and Cr^III (> 97%), yet negligible accumulation of As^V, Sb^V and Cr^VI (< 2%). Accumulation of As^III, Cr^III and Sb^III on the binding gel had a linear relationship (R² > 0.99) with the corresponding standardized laser ablation signals, proving the feasibility of LA-ICP-MS analysis. Analysis for As^III, Cr^III and Sb^III was provided with favorable analytical precision (relative standard deviation <10%). With the purpose of evaluating the dynamics of As^III, Cr^III, Sb^III and O₂ in the rooting zone, a hybrid sensor, which comprises the HR-MPTS gel overlying an O₂ planar optode, was deployed in rhizosphere sediments. Results showed that the consumption of both As^III and Sb^III due to the oxidation extended ~4.48 mm into the sediments, which was consistent with the extension length of the oxidized sediment layers around the roots created by O₂ leakage.

Validation and Assessment of Diffusive Gradients in Thin-Films (DGT) Technique for Measuring Nutrients in Taihu Lake Water with Algae Bloom

DGT (diffusive gradients in thin films) technique has been developed for measuring nitrogen in freshwaters and applied to assess the bioavailability of phosphorus in soils/sediments. These two elements are the main nutrients causing algae bloom, but DGT has never been used in the field water conditions with algae bloom. In our study, a pair of DGT devices were used in comparison with grab sampling to characterize the performance of this technique to measure labile NO₃-N, NH₄-N, and PO₄-P concentrations in algae-cultivated Taihu Lake water. The results showed that DGT measurement was highly affected by algae bloom and the environmental conditions using the current assemblies, especially for NH₄-N measurement. For in situ measurement of nutrients in the real environment, an improvement to the DGT technique is required. The comprehensive assessment of the level of eutrophication needs to consider a variety of environmental factors rather than just the concentration of nutrients.

A Novel In Situ Method for Simultaneously and Selectively Measuring AsIII, SbIII, and SeIV in Freshwater and Soils

Anthropogenic and climatic perturbations redistribute arsenic (As), antimony (Sb), and selenium (Se) within the environment. The speciation characteristics of these elements determine their behavior and biogeochemical cycling, but these redox-sensitive species are challenging to capture, with few methods able to harmonize measurements across the whole plant-soil-ecosystem continuum. In this study, we developed a novel diffusive gradient in thin films (DGT) method based on aminopropyl and mercaptopropyl bi-functionalized mesoporous silica spheres (AMBS) to achieve in-situ, simultaneous, and selective quantification of As^III, Sb^III, and Se^IV, three typical/toxic but difficult to measure inorganic species. When used for environmental monitoring within a river catchment, AMBS-DGT exhibited stable/accurate predictions of these species despite varying water chemistries (ionic strength 0.01-200 mmol L^-1 NO₃^-, pH 5-9 for As^III and Sb^III, and pH 5-7.5 for Se^IV). Furthermore, river deployments also showed that time-averaged species concentrations by AMBS-DGT were reproducible compared with high-frequency sampling and measurement by high performance liquid chromatography coupled with inductively coupled plasma mass spectroscopy. When AMBS-DGT was used for sub-mm scale chemical imaging of soil solute fluxes, the method resolved concomitant redox-constrained spatial patterns of As^III, Sb^III, and Se^IV associated with root O₂ penetration within anaerobic soil. Improved capabilities for measurement of compartment interfaces and microniche features are critical alongside the measurement of larger-scale hydrological processes that dictate the fine-scale effects, with the AMBS-DGT achieving this for As^III, Sb^III, and Se^IV.

Development and Applications of Novel DGT Passive Samplers for Measuring 12 Per- and Polyfluoroalkyl Substances in Natural Waters and Wastewaters

Extensive and long-term use of per- and polyfluoroalkyl substances (PFASs) has caused their widespread distribution in aquatic systems. A new diffusive gradients in thin-films (DGT) passive sampling method based on weak anion exchanger (WAX) binding layer is developed here for monitoring five perfluoroalkyl carboxylic acids (PFCAs), five perfluoroalkanesulfonic acids (PFSAs) and two PFASs (6:2 FTSA and GenX) in waters. Performance of WAX-DGTs was independent of environmental conditions, namely pH (3.03-8.96), ionic strength (1-500 mM), and DOM content (4-30 mg L^-1). Diffusion coefficients (D) of the 12 PFASs in the diffusive gels were measured, 9 for the first time. Linear correlations between D and perfluoroalkyl chain lengths (CF₂) were established to obtain D for congener chemicals with the similar functional group and structure. The binding capacity of the WAX-DGT sampler was at least 440 μg PFASs per sampler, sufficient for applications in waters across a wide range of conditions and PFASs concentrations. Successful applications of WAX based DGT samplers in a wastewater treatment plant (WWTP) and three rivers has demonstrated that DGT is a powerful tool for monitoring, surveillance and research of these 12 PFASs in aquatic systems, and can be extended to wider suites of PFs in future.

Functionalized Mesoporous Silicon Nanomaterials in Inorganic Soil Pollution Research: Opportunities for Soil Protection and Advanced Chemical Imaging

"Innovative actions towards a pollution free-planet" is a goal of the United Nations Environment Assembly (UNEA). Aided by both the Food and Agricultural Organisation (FAO) and its Global Soil Partnership under the 3rd UNEA resolution, a consensus from > 170 countries have agreed a need for accelerated action and collaboration to combat soil pollution. This initiative has been tasked to find new and improved solutions to prevent and reduce soil pollution, and it is in this context that this review provides an updated perspective on an emerging technology platform that has already provided demonstrable utility for measurement, mapping, and monitoring of toxic trace elements (TTEs) in soils, in addition to the entrapment, removal, and remediation of pollutant sources. In this article, the development and characteristics of functionalized mesoporous silica nanomaterials (FMSN) will be discussed and compared with other common metal scavenging materials. The chemistries of the common functionalizations will be reviewed, in addition to providing an outlook on some of the future directions/applications of FMSN. The use of FMSN in soil will be considered with some specific case studies focusing on Hg and As. Finally, the advantages and developments of FMSN in the widely used diffusive gradients-in-thin films (DGT) technique will be discussed, in particular, its advantages as a DGT substrate for integration with oxygen planar optodes in multilayer systems that provide 2D mapping of metal pollutant fluxes at submillimeter resolution, which can be used to measure detailed sediment-water fluxes as well as soil-root interactions, to predict plant uptake and bioavailability.

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.

Novel in situ method based on diffusive gradients in thin-films with lanthanum oxide nanoparticles for measuring As, Sb, and V and in waters

Lanthanum oxide nanoparticles (nano-La₂O₃) was used to develop a novel binding gel within an in situ passive sampler based on diffusive gradients in thin-films technique (NL-DGT) for measuring As(V), Sb(V), and V(V). Performance characteristics of NL-DGT were independent of pH (pH: 3.1-7.9 for As, 3.1-8.5 for V, and 3.1-6.5 for Sb) and ionic strength (0.1-500 mmol L^-1 for As and V, and 0.1-200 mmol L^-1 for Sb). No obvious competition effects among As, Sb, and V with different concentration ratios were found for NL-DGT measurement. Long term storage (8-188 d) of the nano-La₂O₃ gels in 0.01 mol L^-1 NaNO₃ at 4 °C did not affect their performance. During the field deployments in Yangtze and Jiuxiang River, NL-DGT measured concentrations of As and V were similar to those measured by the grab samples, while some differences were found for Sb between DGT and grab sampling because higher pH (∼8.0) in the studied rivers caused the performance deterioration of NL-DGT. Generally, the newly developed NL-DGT is suitable for monitoring As and V in freshwater from acidic to light alkaline and Sb in acidic and neutral water.

A novel method for evaluating the potential release of trace metals associated with rainfall leaching/runoff from urban soils

The release of pollutants in soils owing to rainfall is a major challenge related to urbanization. Here, urban soils from different functional zones were collected to evaluate the release risk and estimate their annual release amounts of trace metals (Co and Ni) using multiple techniques, including diffusive gradients in thin-films (DGT), DGT-induced fluxes in sediments (DIFS) model, and Fourier-transform infrared (FTIR) spectroscopy. The results indicate that the average concentration of Co (6.55 mg kg^-1) was slightly lower than that of the local soil background, whereas for Ni, the trend was reversed. Risk assessments based on total concentrations show that the soil samples were uncontaminated with Co while uncontaminated to moderately contaminated with Ni. However, the mobility coefficients indicate that Co posed low to medium risk, while Ni posed low risk. Hence, further investigation of DGT measurements and DIFS model show that the DGT-measured Co and Ni concentrations were lower than the corresponding concentrations in solution, leading to low R values and the partial resupply of Co and Ni from the solid phase. The FTIR spectra and elemental analysis suggest that because of the electrostatic attraction and complexation of the hydrosilicate minerals and organic matters, the metal resupply was restrained, resulting in them being only a partial resupply. Moreover, the mobility of Co was mainly controlled by the resupply ability, response time, and desorption rate; while for Ni, the particle concentration and porosity played important roles in determining mobility. In addition, the release amounts of the trace metals were quantified using the binary mixing equation. The estimated annual release amounts of Co and Ni in Pingshan District were 0.44-3.54 t and 1.93-16.47 t, respectively. This study provides an effective in-situ method for estimating the release amounts of trace metals in soils during rainfall combining DGT and DIFS model.