Use of diffusive gradient in thin films for in situ measurements: A review on the progress in chemical fractionation, speciation and bioavailability of metals in waters

Long-term deployment of commonly used DGT devices for trace element measurement across laboratory and field settings

Diffusive Gradients in Thin-films (DGT) technique is a promising passive sampling technique, which was used for the determination of lots of inorganic and organic pollutants. Although many DGT devices have not been extensively tested and verified in field, DGT device such as LSNP-NP, LSNT-NP, LSNZ-NP, and LSNM-NP DGT were widely employed for the assessment of various trace elements. However, the deployment time of these DGTs were much shorter than the theoretical time in the preexisting literature. Therefore, the performance of DGT for long-term application in different water bodies is not known. This investigation utilized the four DGTs for the assessment of various trace elements across extended periods both in controlled laboratory settings and natural field environments. Synthetic soft, hard, and seawater compositions served as media for laboratory deployments. Most elements can be measured accurately in 4-7 days in soft and hard water. Deployment durations in seawater exhibited a notable reduction compared to those in freshwater matrices. LSNZ-NP DGT excelling in oxyanion determination, while LSNT-NP DGT showcased superior efficacy in phosphorus quantification. Field deployments in rivers and sea affirmed the robustness of LSNZ-NP DGT, evidencing prolonged deployment capabilities for various elements, such as Mo, Se, As, Sb, and P, spanning 7-28 days. Assessment of pollution levels across four sampling sites revealed heightened concentrations of most elements in marine waters relative to riverine environments, except for phosphorus. Notably, all assessed elements, except for phosphorus, conformed to Class I water quality standards. This study demonstrated the difference between the theoretical application time and actual application time for the first time. It raises new questions for the application of DGT in nature water bodies. The mechanization of the difference between the theoretical and actual application time should be studied in the future research. The measures to extend the application time should be studied too.

Comprehensive evaluation of the distribution, transport and ecological risk of heavy metals in intra-urban river sediments using high-resolution techniques

Determining the distribution trends, transport mechanisms, and ecological risks of heavy metals (HMs) in urban river sediments is essential for the government to conduct appropriate remediation work. In this study, we collected sediment cores from the Yayao Waterway in Foshan City, China. The vertical distribution profiles of dissolved and labile Fe, Mn, Cd, Zn, Cu, Cr, Ni, Pb, As, and Co in the sediments were obtained using the thin-film diffusive gradient (DGT) and high-resolution peeper (HR-Peeper) techniques. In addition, the transport rates, contamination levels, and ecological concerns of the HMs were evaluated using the European Community Bureau of Reference (BCR) sequential extraction technique, the DGT-induced sediment fluxes (DIFS) model, and multiple contamination evaluation metrics. The results showed that most of the DGT-labile HMs were associated with Fe/Mn (hydrogen) oxides, and in particular, Zn, Ni, and Cr showed a significant negative correlation with Fe/Mn (p < 0.001). Additionally, Cd had the highest bioavailability (89.17%), and its net diffusive flux at the sediment-water interface (SWI) was positive, which indicated a high release risk from the sediment. However, the R-value of Cd based on the DGT-induced sediment fluxes (DIFS) operation was extremely low, suggesting that although Cd had the biggest supply pool of releases, its release rate was slow. The majority of sampling sites had significantly higher total HM contents in the surface sediments than the background values. The HM contamination in the sediments originated from human activities, primarily from industrial enterprises and with a large contribution from both agricultural and domestic sources. The most polluted HM with the highest ecological danger was Cd, followed by Cu, Zn, Ni, and As when the results of the four pollution evaluation indicators were combined. Consequently, the risk of contamination by HMs in inner-city river sediments should receive more attention.

Development of a novel DGT passive sampler for measuring polycyclic aromatic hydrocarbons in aquatic systems

Polycyclic aromatic hydrocarbons (PAHs) are priority pollutants and need to be measured reliably in waters and other media, to understand their sources, fate, behaviour and to meet regulatory monitoring requirements. Conventional water sampling requires large water volumes, time-consuming pre-concentration and clean-up and is prone to analyte loss or contamination. Here, for the first time, we developed and validated a novel diffusive gradients in thin-films (DGT) passive sampler for PAHs. Based on the well-known DGT principles, the sampler pre-concentrates PAHs with typical deployment times of days/weeks, with minimal sample handling. For the first time, DGT holding devices made of metal and suitable for sampling hydrophobic organic compounds were designed and tested. They minimize sorption and sampling lag times. Following tests on different binding layer resins, a MIP-DGT was preferred - the first time applying MIP for PAHs. It samples PAHs independent of pH (3.9 -8.1), ionic strength (0.01 -0.5 M) and dissolved organic matter < 20 mg L-1, making it suitable for applications across a wide range of environments. Field trials in river water and wastewater demonstrated that DGT is a convenient and reliable tool for monitoring labile PAHs, readily achieving quantitative detection of environmental levels (sub-ng and ng/L range) when coupled with conventional GC-MS or HPLC. ENVIRONMENTAL IMPLICATIONS: PAHs are carcinogenic and genotoxic compounds. They are environmentally ubiquitous and must be monitored in waters and other media. This study successfully developed a new DGT passive sampler for reliable in situ time-integrated measurements of PAHs in waters at the ng/L level. This is the first time to use passive samplers for accurate measurements of hydrophobic organic contaminants in aquatic systems without calibration, a big step forward in monitoring PAHs. The application of this new sampler will enhance our understanding of the sources, fate, behavior and ecotoxicology of PAHs, enabling improved environmental risk assessment and management of these compounds.

Phosphorus mobilization in sulfidic sediments in the Baltic Sea

It is well-known that the geochemical behavior of phosphorus (P) in sediments is closely related to that of Fe and Mn, but their relations remain unclear in long-term sulfidic sediments such as those of the Gotland Basin in the Baltic Sea. Therefore, P mobilization in these sediments at both shallow and deep sites was investigated in relation to that of Fe, Mn, and S. To achieve that aim, classic sediment slicing and subsequent solid phase sequential extraction were combined with in situ DGT (Diffusive Gradients in Thin-films) sampling, DIFS (DGT induced flux in sediments) modelling, and Visual MINTEQ calculations. Correlations between total dissolved and labile dissolved concentrations of P, Mn, and Fe in porewater, and the associations between labile fractions of these elements in the solid phase suggested two kinds of sources for P: The dissolution of P-bearing Fe oxides and/or Mn carbonates was observed in the shallower sites, while P and Mn release from different solid sources was observed in the deepest sites. Although the formation of Fe sulfides leads to extremely low dissolved Fe level in porewater, the simultaneous release of P with Mn/Fe was confirmed by theoretical calculations with Visual MINTEQ. The DIFS model showed that the resupply ability of P from sediment solids is determined by labile pool size. The application of experimental and theoretical methods made it possible to clarify the relationships between P, Fe, Mn and S in sulfidic sediments which may contribute to a better understanding of the P cycle in other sulfidic regions.

Fractionation and speciation of metals in lakes formed by abandoned clay pits from industrial effluents (Santa Gertrudes, São Paulo, Brazil) using the diffusive gradient in thin films (DGT) technique

Environmental impacts caused by mining activities (mainly tailings and effluents) are presenting serious challenges for humanity worldwide. In Brazil, clay extraction activities in the Ceramic District of Santa Gertrudes (CDSG) have led to the formation of abandoned drainage wells causing environmental and human health concerns. In the 90's, it was discovered that in one of the production areas, known as the region of the lakes of Santa Gertrudes, several ceramic industries had contaminated lakes created by abandoned clay pits with industrial effluents containing toxic metals. In the present study, analysis of total and dissolved concentrations of Al, Cd, Co, Cu, Mn, Ni, Pb and Zn in the waters of these lakes were combined with the diffusive gradients in thin films (DGT) technique to assess the lability and bioavailability of the target elements, representing one of the first studies to investigate the real environmental impact of contamination caused by ceramic production wastes to an aquatic system. Furthermore, based on the total concentrations and main physicochemical characteristics of each lake, a speciation analysis was performed using the MINTEQ software which data was compared with other surface water systems. The results indicated the presence of metals associated with ceramic residues in total, dissolved and labile fractions. It was verified that Zn, Ni and Cu were the only target metals found in labile form and according to speciation were present in the form of "free" ions, and thus may present risk in terms of bioavailability, although the majority of the total concentrations are within the limits established by the national environmental agency.

Appraisal on the role of passive sampling for more integrative frameworks on the environmental risk assessment of contaminants

Over time multiple lines of research have been integrated as important components of evidence for assessing the ecological quality status of water bodies within the framework of Environmental Risk Assessment (ERA) approaches. One of the most used integrative approaches is the triad which combines, based on the weight-of-evidence, three lines of research, the chemical (to identify what is causing the effect), the ecological (to identify the effects at the ecosystem level) and the ecotoxicological (to ascertain the causes of ecological damage), with the agreement between the different lines of risk evidence increasing the confidence in the management decisions. Although the triad approach has proven greatly strategic in ERA processes, new assessment (and monitoring) integrative and effective tools are most welcome. In this regard, the present study is an appraisal on the boost that passive sampling, by allowing to increase information reliability, can give within each of the triad lines of evidence, for more integrative ERA frameworks. In parallel to this appraisal, examples of works that used passive samplers within the triad are presented providing support for the use of these devices in a complementary form to generate holistic information for ERA and ease the process of decision-making.

In-situ Diffusive Gradients in thin-films passive sampling coupled with ex-situ small-sized electrothermal vaporization capacitively coupled plasma microtorch optical emission spectrometry as green and white method for the simultaneous determination of labile species of toxic elements in surface water

This study presents for the first time the coupling between in-situ Diffusive Gradient in Thin-film (DGT) passive sampling technique and ex-situ small-sized instrumentation based on electrothermal vaporization capacitively coupled plasma microtorch optical emission spectrometry (SSETV-μCCP-OES) for the simultaneous determination of Cd, Pb, Cu, Zn and Hg in surface water. Unique features of the DGT-SSETV-μCCP-OES are low power and low Ar consumption for plasma generation (15 W, 150 mL min^-1) and significant improvement of the detection limits following DGT passive sampling. The new method was validated in terms of river water analysis in comparison with graphite furnace atomic absorption spectrometry and thermal decomposition atomic absorption spectrometry. Combining the abilities of preconcentration by in-situ Chelex-DGT passive sampling with plasma microtorch equipped with a low resolution microspectrometer provided multielemental simultaneous determination with detection limits of (μg L^-1) 0.01 (Cd, Zn and Hg), 0.02 (Cu) and 0.07 (Pb) in water, at least one order of magnitude better than using grab sampling without preconcentration. It was possible the quantification of labile fraction of priority hazardous metals (Cd, Pb) in river water below the instrumental limits of detection (μg L^-1) of 0.12 and 0.80 obtained in SSETV-μCCP-OES without DGT sampling. The precision of the method was in the range 15.3-22.4% (combined uncertainty), while the accuracy was 95-103% and trueness of 27-33% (expanded uncertainty, k = 2). The DGT-SSETV-μCCP-OES coupling proved to be an ideal and powerful tool for surface water analysis in compliance with green and white analytical chemistry concepts. The application of the RGB-12 algorithm provided very good red/green (AGREEprep)/blue/white scores (%) of 100/80/98/93, determined primarily by in-situ DGT passive sampling, very good detection limits and cost-effective SSETV-μCCP-OES instrumentation.

Mechanisms of S cooperating with Fe and Mn to regulate the conversion of Cd and Cu during soil redox process revealed by LDHs-DGT technology

The activity changes of Cd and Cu in paddy field were strongly influenced by the transformation of S, Fe and Mn species. However, in the process of soil redox, how S cooperates with Fe/Mn to regulate the law and mechanism of Cd and Cu speciation transformation still needs to be studied. In this study, we used DGT technology based on layer double hydroxides (LDHs) combined with pore water sampling to investigate soil redox changes, rice growth, and the effects of different forms of sulfur (S⁰, SO₄^2-) on soil Cd and Cu activities. The results showed that the concentrations of C_DGT-Cd and Cu in the soil decreased rapidly in the anaerobic stage, but increased slowly in the oxidative stage. Multiple regression analysis showed that the changes of Cu and Cd concentrations mainly depended on the changes of Fe/Mn morphology. Sulfur treatment promoted the dissolution of Fe/Mn oxides in the short term (<48 h), and the activities of C_DGT-Fe, Mn, and Cd increased simultaneously, but C_DGT-Cu was not affected. However, after long-term anaerobic conditions (>10 d), sulfur addition reduced the activities of C_DGT-Cd and Cu, and decreased the uptake of Cd and Cu by rice. During sulfate reduction, the sulfur addition treatment group resulted in a 24.5-50.2 % decrease in C_DGT-Fe, indicating that sulfur addition may delay the release of Cd and Cu after rice planting by promoting the formation of FeS/FeS₂. In addition, in the anaerobic stage, Cu formed sulfide before Cd and was fixed, and the higher thermodynamic stability of CuS would promote the dissolution of CdS in the oxidation stage. Overall, soil flooding with sulfur to enhance the generation of metal sulfides and secondary iron ores provides an opportunity to use sulfur as an environmentally friendly modifier to coordinate Fe, Mn to improve heavy metal-contaminated soils.

Combining DGT with bioaccessibility methods as tool to estimate potential bioavailability and release of PTEs in the urban soil environment

Potentially toxic elements (PTEs) in urban soil environments pose a noticeable risk to both ecosystem and human health; however, only a fraction of the elemental content is available for biota. To better know the potential risk of PTEs in the urban soil environment, geochemical fractionation, bioaccessibility, and potential bioavailability of four PTEs (Cd, Cu, Pb, and Zn) were investigated by the combined use of different methods. The results showed that a high non-residual chemical fraction is related to a high bioavailability of the selected elements. The ranges of labile concentration of Cu, Zn, Cd and Pb in all sampling sites measured by diffusive gradients in thin films (DGT) were 3.5-18.0, 14.2-26.5, 0.09-1.0, and 1.8-15.7 μg/L, respectively. The high non-residual contents pointed out a serious hazard to the urban environment. The bioaccessible concentrations in gastric and lung phases were closely positively correlated with DGT-measured content (r = 0.63-0.99, p < 0.05), suggesting the potential use of DGT for the prediction of PTEs risk to human health. Moreover, the correlation of DGT results with the soluble and reducible fractions of PTEs may allow DGT use for quick screenings of the PTEs fraction potentially mobilizable during flooding events in urban soil environments. Our study suggests that combing DGT, bioaccessibility and biogeochemical fractionation could provide a more accurate assessment of the urban environmental quality and be helpful for pollution control and urban planning.

In situ, high-resolution evidence of metals at the sediment-water interface under ice cover in a seasonal freezing lake

The ice cover in winter as a physical barrier and duration would profoundly impact on changes in endogenous metal loading, migration, and transformation. Although a gradual reduction in duration and occurrence of lake ice cover in boreal lake ecosystems has been confirmed, little attention to the ice-covered period is received compared to open water studies. In this study, novel-developed diffusive gradients in thin films (DGT, ZrO-Chelex) probes were deployed to obtain the in-situ and high-resolution information on metals (Cu, Zn, Pb, Mn, Cd, Cr, and As) at the sediment-water interface (SWI) in a seasonal ice-covered lake, Chagan Lake. In addition, “source-sink” characteristics of each metal related to their endogenous release were determined based on Fick’s first law. Concentrations of labile metals at the SWI demonstrated significant spatial heterogeneity, peaking exactly below the SWI. Compared with other similar studies, concentrations of Pb (0.55 μg/L), Cr (0.58 μg/L), and As (2.4 μg/L) were a little higher even under-ice than that in other freshwater rivers and lakes, indicating potential pollution due to the agricultural intensification and petroleum extraction. The apparent diffusive fluxes suggested that sediments acted as a sink for Pb (−0.01 mg m–2 day–1), Cr (−2.37 mg m–2 day–1), and Cd (−0.1 mg m–2 day–1), diffusing from the overlying water into the sediment, while Cu (0.12 mg m–2 day–1), Zn (0.75 mg m–2 day–1), Mn (15.89 mg m–2 day–1), and As (2.12 mg m–2 day–1) as a source from sediments into the overlying water. Dissolved oxygen was the principal factor (79.5%, P = 0.032), determining the variation of the available metals at the SWI. As the urgent need for research focused on under-ice ecosystem dynamics, this study addressed the previously unknown behavior of the labile metals at the SWI and provided a unique perspective for the lake management during the ice-cover periods when external nutrient input was cut off.

Vanadium: A Review of Different Extraction Methods to Evaluate Bioavailability and Speciation

The excessive input of heavy metals such as vanadium (V) into the environment has been one of the consequences of global industrial development. Excessive exposure to V can pose a potential threat to ecological safety and human health. Due to the heterogeneous composition and reactivity of the various elements in soils and sediments, quantitative analysis of the chemical speciation of V in different environmental samples is very complicated. The analysis of V chemical speciation can further reveal the bioavailability of V and accurately quantify its ecotoxicity. This is essential for assessing for exposure and for controlling ecological risks of V. Although the current investigation technologies for the chemical speciation of V have grown rapidly, the lack of comprehensive comparisons and systematic analyses of these types of technologies impedes a more comprehensive understanding of ecosystem safety and human health risks. In this review, we studied the chemical and physical extraction methods for V from multiple perspectives, such as technological, principle-based, and efficiency-based, and their application to the evaluation of V bioavailability. By sorting out the advantages and disadvantages of the current technologies, the future demand for the in situ detection of trace heavy metals such as V can be met and the accuracy of heavy metal bioavailability prediction can be improved, which will be conducive to development in the fields of environmental protection policy and risk management.

In situ fractionation and redox speciation of arsenic in soda lakes of Nhecolândia (Pantanal, Brazil) using the diffusive gradients in thin films (DGT) technique

In situ fractionation and redox speciation of As in three different saline-alkaline lakes (green, black and crystalline lakes) in the Pantanal of Nhecolândia (Brazil) were performed by using Diffusive Gradients in Thin films (DGT). The results indicated that As is present mainly in dissolved form. Total As concentration was similar when using different filter membranes, demonstrating that the species adsorbed by DGT devices were <10 kDa. Higher concentrations of labile total As were observed in the center of the lakes, indicating that the nature of the organic matter influences the formation of As complexes. Total As concentrations determined by using ZrO₂ DGT were consistent with As concentration in ultrafiltered water samples collected in the black lake. However, part of the data about As(III) obtained in grab samples contrasted with DGT results. The differences observed may indicate that alterations in the species occur during the storage period before analysis by ultrafiltration. As(III) concentrations measured by DGT in the black and crystalline lakes were 1-3 μg L^-1 and 4-7 μg L^-1, respectively, accounting for only 4%-8% of the total DGT inorganic As. In the green lake, As(III) concentrations were significantly higher at the center (217 μg L^-1). Both the phytoplankton community and the dissolved organic carbon influence the As speciation and bioavailability in the lakes of Nhecolândia. The DGT approach used in the present work was able to perform As speciation and demonstrates that in situ sampling analytical techniques are essential in understanding As speciation and its behavior in complex natural aquatic systems.

Effective concentration signature of Zn in a natural water derived from various speciation techniques

The uptake of nutrients or toxicants by different organisms in aquatic systems is known to correlate with different fractions of the nutrient's or toxicant's total concentration. These fractions can be provided by different analytical techniques, from which the better correlation is expected to be found for those with a characteristic length comparable to that in the considered organism uptake. An effective concentration signature can be built up with the concentration values associated to the availability (i.e. fluxes in dynamic techniques) of the nutrient or toxicant measured by various analytical techniques with different characteristic lengths. Here, this new representation was obtained for the pool of Zn complexes in the Mediterranean stream Riera d'Osor (Girona, Catalonia, Spain) with a suite of four analytical techniques. Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) and Polymer Inclusion Membrane (PIM) devices provided the free Zn concentration. Linear Anodic Stripping Voltammetry provided a labile fraction (defined here as c_LASV, higher than the free concentration), related to the diffusion layer scale. Diffusion Gradients in Thin-films provided higher labile fractions (known as DGT concentrations, c_DGT) connected to the different characteristic lengths of different configurations (e.g. one or two resin discs) longer, in any case, than that corresponding to LASV. The combination of the information retrieved by the techniques allowed to quantify lability degrees of the pool of Zn complexes and to build up the effective concentration signature for this water.

A new approach to improve the accuracy of DGT (Diffusive Gradients in Thin-films) measurements in monitoring wells

The Diffusive Gradients in Thin-films (DGT) technique represents an ideal tool for monitoring water quality of inorganic species in systems with a high flow such as rivers, streams, lakes and seas. However, in low-flow systems (non-turbulent waters), the influence of a diffusive boundary layer (DBL) formed on the surface of the DGT device has been observed, which can lead to erroneous measurements by DGT. Therefore, the use of DGT in wells for groundwater monitoring is still very limited until now. In this sense, the present study evaluates the applicability of the DGT technique in non-turbulent and low-flow water systems. We propose a new way to calculate the DBL with the objective to carry out a robust DGT analysis in environmental monitoring wells. For this purpose, DGT devices with different diffusive gel thicknesses were deployed in an experimental set-up simulating a groundwater monitoring well. A DBL thickness (for each element) was calculated from the slopes of the linear regressions between the DGT accumulated mass of metal and the deployment time (4, 8, 12, 24 and 48 h) for each of the two diffusive gel thicknesses. The mean DBL thickness (averaging the individual DBL thicknesses calculated from the slopes) was 0.06 cm. The concentrations of the analysed elements were corrected with this DBL with the result that the metal concentrations measured by DGT improved and were highly approximated to their actual total values in this non-complexing medium.

Advances in the Fate of Rare Earth Elements, REE, in Transitional Environments: Coasts and Estuaries

The production of rare earth elements, REE, has significantly increased over the past years, in parallel with the latest advances in nanotechnologies and representing a new group of emerging contaminants. They find application in construction, transport, agriculture, electronics, catalysis, and biomedicine. Their extraordinary intrinsic characteristics are fundamental for overcoming current technological challenges. The accumulation of REE is consistent in near-shore waters being affected by runoff, wastewater discharge, and proximity to built-up areas. Bioavailability in water, sediments, and accumulation in marine biota as well their endocrine disruptor effect is mostly unknown. There is a significant gap of knowledge on the ecotoxicological behaviour of REE in marine areas. The existing investigations have been performed inside well-mixed estuarine systems, due to complex hydrodynamics and multiple sediment transport situations. This hampers the definition of regulatory thresholds for REE concentrations and emissions. The review summarizes the existing information on REE geochemistry and physicochemical conditions influencing dissolution, surface complexation reactions, and distribution at the continent–ocean interface, as well as their speciation, bioavailability, and detrimental effects on living organisms. Strategies for reducing REE usage and inputs are also discussed.

Time-series sampling of trace metals in surface waters using osmotic sampler with air segmentation and preservative addition

High-resolution time-series concentrations (C_TS) are very important for the investigation of the biogeochemical processes of trace metals in the aquatic environment. However, the acquisition of C_TS of trace metals in water is still challenging because of the lack of suitable samplers. In this study, an osmotic sampler coupled with air segment injection and preservative addition was employed for time-series sampling of trace metals in surface waters. In the sampler, water sampling and preservative adding are both driven by osmotic pumps (OPs), while air segment injection is accomplished by a timer-controlled micro diaphragm pump. During deployment, the sampling OP continuously draws water through a filter and stores it in a narrow-bore coil. Simultaneously, a preservative OP slowly pushes 30% HNO₃ (v/v) into the collected sample for in situ preservation. Periodically, the micro diaphragm pump injects air into the continuous water stream to divide it into water segments, enabling accurate time-stamping. After retrieval, the time-series samples were pumped out from the coil and re-collected to analyze the C_TS of analytes. The sampler was deployed in river, reservoir, and marine waters for 26 h and one week to measure C_TS of trace metals at time resolutions of 2 h and 12 h. Results showed that the recoveries of a preloaded standard mixture (1.0 μg/L) in all samplers ranged from 93.1% to 117.8%. The measured C_TS of Cd, Co, Cr, Cu, Mn, and Ni in the waters only varied in small ranges. Accordingly, the measured C_TS data from the sampler were consistent with the obtained concentrations from grab sampling. The relative percent differences between the measurements from two samplers were less than 37.4%. These results demonstrate the reliability and accuracy of the sampler for time-series sampling of the chosen trace metals in surface waters.

Geochemical fractionation, bioavailability, and potential risk of heavy metals in sediments of the largest influent river into Chaohu Lake, China

As the largest tributary flowing into Chaohu Lake, China, the Hangbu-Fengle River (HFR) has an important impact on the aquatic environment security of the lake. However, existing information on the potential risks of heavy metals (HMs) in HFR sediments was insufficient due to the lack of bioavailability data on HMs. Hence, geochemical fractionation, bioavailability, and potential risk of five HMs (Cr, Cu, Zn, Cd, and Pb) in HFR sediments were investigated by the combined use of the diffusive gradient in thin-films (DGT), sequential extraction (BCR), as well as the physiologically based extraction test (PBET). The average contents of Cd and Zn in the HFR Basin were more than the background values in the sediments of Chaohu Lake. A large percentage of BCR-extracted exchangeable fraction was found in Cd (8.69%), Zn (8.12%), and Cu (8.05%), suggesting higher bioavailability. The PBET-extracted fractions of five HMs were all almost closely positively correlated with their BCR-extracted forms. The pH was an important factor affecting the bioavailability of HMs. The average DGT-measured contents of Zn, Cd, Cr, Cu, and Pb were 28.07, 7.7, 3.69, 2.26, 0.5 μg/L, respectively. Only DGT-measured Cd significantly negatively correlated with Eh, indicating that Cd also had a high release risk under reducing conditions, similar to the risk assessment results. Our results could provide a reference for evaluating the potential bioavailabilities and ecological hazards of HMs in similar study areas.

Inputs of Total and Labile Dissolved Metals from Six Facilities Continuously Discharging Treated Wastewaters to the Marine Environment of Gran Canaria Island (Canary Islands, Spain)

The presence of ten metals (Cd, Ni, Pb, Cr, Cu, Zn, Al, Fe, Mn, and Co) was investigated in the final discharge of six facilities, including four wastewater treatment plants, which were continuously discharging treated wastewater to the coastal environment in Gran Canaria Island. A four-day sampling campaign was carried out at each facility in July 2020, in which both the spot samplings technique and the diffusive gradient in thin-film technique (DGT) were carried out to measure total dissolved metals and the in situ labile metal fraction, respectively. After the necessary sample preparation steps, measurements were carried out by ICP-MS for both samplings. Raw data referred to the spot total dissolved and DGT-labile metal concentrations were reported. In general, the average metal concentrations were dispersed in a broad range. As expected, the highest metal contents were found in those facilities with larger industrial contributions. The values of annual average environmental quality standards (AA-EQS) were used to assess the total dissolved metal concentrations for every metal in every final discharge. In only one of the studied facilities, some metals (Ni and Zn) exceeded these EQS within the receiving waterbody, highlighting the need for more efficient treatment targeted towards a specific discharging-water quality. In addition, the total dissolved and labile metal daily fluxes of discharge were calculated to estimate the contribution of every effluent to the receiving water bodies.

Selective diffusive gradients in thin-films with molecularly imprinted polymer for measuring fluoroquinolone antibiotics in waters

Fluoroquinolones (FQs) have raised significant concerns due to their ubiquitous occurrence and promoting of antimicrobial resistance (AMR). In this study, a molecularly imprinted polymer-diffusive gradient in thin-films (MIP-DGT) sampler is developed for selective measurement of FQs in waters by using a commercial available MIP material as the binding agent. The MIP-DGT shows selective adsorption of the FQs and linearly accumulates the FQs over the deployment time. MIP-DGT measurement is independent of pH (6-8) and ionic strength (IS) (0.01-0.5 M) but is affected by DOM at higher concentrations (~10 mg•L^-1), which is due to the altered diffusion coefficients and reduced adsorption on the MIP binding gel. Significant interaction effects of DOM with pH or IS indicate that this is the predominant influence on the MIP-DGT performance, which results in a lower measurement by the MIP-DGT but this is curtailed to some extend with increasing IS or pH. The MIP-DGT measurements, however, correlate well with those by grab sampling in a wastewater treatment plant, suggesting it is reliable for measuring FQs in waters. For the first time, we demonstrate that key water chemistry parameters do have interaction effects on the DGT measurements, which should be considered for the data interpretation. The MIP-DGT is a promising tool to understand the interaction effects of the environmental parameters on the fate, behaviours and bioavailability/toxicity of organic contaminants and improve environmental risk assessments in the environment and modelling.

Development and application of diffusive gradients in thin-films for in situ sampling of the bitterest chemical - denatonium benzoate in waters

Denatonium benzoate (DB), a commonly used bitter agent in numerous products, has recently been recognized as a waterborne contaminant due to concern about its potential persistence, mobility and toxicity (PMT). However, its occurrence, levels and fate in global aquatic environments are largely unknown. In this study, a new sampling method, based on diffusive gradients in thin films (DGT) with mixed-mode cation exchange (MCX) as the binding agent, was developed for measuring DB in waters. MCX shows a rapid adsorption and high capacity for DB. DB is linearly accumulated by MCX-DGT. pH (6-8), ionic strength (0.01-0.5 M), or DOM (0-10 M) do not show any significant effect on the MCX-DGT performance, confirming its reliability. The DGT measurements in a wastewater treatment plant (WWTP) are comparable to those by paralleled grab sampling. The field results suggest DB is persistent in WWTPs and could be a potential domestic wastewater indicator. Therefore, MCX-DGT is a promising technique for understanding the environmental occurrence, levels and fate of DB. This is a first report of using DGT for DB monitoring and of DB occurrence in Chinese environments. Further exploration of DGT as a reliable passive monitoring tool for a wide range of PMT substances in different applications is warranted.

Assessing variability in the ratio of metal concentrations measured by DGT-type passive samplers and spot sampling in European seawaters

The current study evaluates the effect of seawater physico-chemical characteristics on the relationship between the concentration of metals measured by Diffusive Gradients in Thin films (DGT) passive samplers (i.e., DGT-labile concentration) and the concentrations measured in discrete water samples. Accordingly, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure the total dissolved metal concentrations in the discrete water samples and the labile metal concentrations obtained by DGT samplers; additionally, lead and cadmium conditional labile fractions were determined by Anodic Stripping Voltammetry (ASV) and total dissolved nickel was measured by Cathodic Stripping Voltammetry (CSV). It can be concluded that, in general, the median ratios of DGT/ICP and DGT/ASV(CSV) were lower than 1, except for Ni (median ratio close to 1) and Zn (higher than 1). This indicates the importance of speciation and time-integrated concentrations measured using passive sampling techniques, which is in line with the WFD suggestions for improving the chemical assessment of waterbodies. It is the variability in metal content in waters rather than environmental conditions to which the variability of the ratios can be attributed. The ratios were not significantly affected by the temperature, salinity, pH, oxygen, DOC or SPM, giving a great confidence for all the techniques used. Within a regulatory context such as the EU Water Framework Directive this is a great advantage, since the simplicity of not needing to use corrections to minimize the effects of environmental variables could help in implementing DGTs within monitoring networks.

Availability of metals to DGT devices with different configurations. The case of sequential Ni complexation

The analytical technique DGT (Diffusive Gradients in Thin-films) is able to gain access to a wealth of information by carefully interpreting accumulation data from passive samplers with different configurations (i.e. different thicknesses of its constituent layers). A set of DGT devices were simultaneously deployed in solutions of Ni and nitrilotriacetic acid (NTA) of different concentrations to measure the availability of Ni in these solutions. Accumulations indicate that the availability of Ni depends on both the thickness of the resin and the thickness of the diffusive gel. In both cases, the lability degree increases as the thickness increases. As the formation of successive complexes (such as Ni(NTA)₂) proceeds, the availability of the metal decreases, which is quantitatively explained by reducing the formulation to a case with only one complex, but with an effective dissociation rate constant that decreases as the concentration of NTA increases. Simple analytical expressions are reported to quantify the lability degree in the different DGT configurations. These results indicate that a set of different DGT devices can characterize the availability of a cation in a natural sample with uptake processes at different spatial or time scales. Alternatively, and from a more fundamental point of view, information on speciation, mobilities and labilities of the species present in natural samples can be obtained with a set of DGT configurations.

Iron, thermal stratification, Eucalyptus sp., and hypoxia: drivers to water blackening in southern China reservoirs

The management of black water depends primarily on the knowledge of the dynamics of organic matter (OM), iron (Fe), sulfide (S), and manganese (Mn), at the water-sediment boundary (WSB). However, the mechanistic path of these substances leading to black water remains unsettled. In this study, a 35-day field study was conducted using the thin-film diffusion gradient technology (DGT) and the planar optrode to address the unknown combined effects of Fe, Mn, OM, S, and tannins from Eucalyptus species on Tianbao reservoir.Our results indicated that the hypolimnion was hypoxic due to thermal stratification, which caused the reduction of insoluble Fe and Mn from sediments to bottom water. Correlation analysis (Fe:S (r:0.5-0.9); Mn:S (r:0.2-0.8)) and elevated fluxes (Fe²⁺, Mn²⁺, S^2-) connoted that these parameters interacted chemically to give black matter. The content of OM, Fe²⁺, and tannic acid in the benthic region diminished remarkably (p < 0.05) from day 1 (strong stratification) to day 35 (weak stratification), connoting that these parameters also interacted chemically to give black matter. The turbidity (clarity of the water) increased from day 1 to 35 with a significant difference (p < 0.05) recorded on day 14 confirming that black water was formed on this day when the thermal structure of the reservoir was annihilated. Correlation analysis supported the assertion that the variability in oxygen and redox conditions caused changes in Fe, Mn, and OM content at the WSB.The finding from the field research provides useful information to stakeholders on how to improve the quality of freshwater management designs.

Speciation of Inorganic Compounds in Aquatic Systems Using Diffusive Gradients in Thin-Films: A Review

The speciation of trace metals in an aquatic system involves the determination of free ions, complexes (labile and non-labile), colloids, and the total dissolved concentration. In this paper, we review the integrated assessment of free ions and labile metal complexes using Diffusive Gradients in Thin-films (DGT), a dynamic speciation technique. The device consists of a diffusive hydrogel layer made of polyacrylamide, backed by a layer of resin (usually Chelex-100) for all trace metals except for Hg. The best results for Hg speciation are obtained with agarose as hydrogel and a thiol-based resin. The diffusive domain controls the diffusion flux of the metal ions and complexes to the resin, which strongly binds all free ions. By using DGT devices with different thicknesses of the diffusive or resin gels and exploiting expressions derived from kinetic models, one can determine the labile concentrations, mobilities, and labilities of different species of an element in an aquatic system. This procedure has been applied to the determination of the organic pool of trace metals in freshwaters or to the characterization of organic and inorganic complexes in sea waters. The concentrations that are obtained represent time-weighted averages (TWA) over the deployment period.

Metal speciation of the Paraopeba river after the Brumadinho dam failure

On January 25, 2019, a tailings dam at the Córrego do Feijão iron ore mine (Brumadinho, Minas Gerais, southern Brazil) ruptured and released ~12 million m³ of mine tailings into the Paraopeba River, which is an important source of drinking water to a populous region. While water potability due to a strong increase in turbidity has been well documented, possible effects of metal contamination are yet to be addressed. We investigated the speciation of metals in the river water and desorption of metals from sediments as a means of supporting risk assessment, using the diffusive gradient in thin films (DGT) technique, desorption experiments and chemical speciation calculations. The results of the in-situ DGT monitoring revealed that the labile concentrations of metals were low in relation to the respective total and dissolved concentrations. Chemical speciation calculations showed that the heavy metals were not stable in the Paraopeba River. The desorption experiments suggested that sediments may release a limited amount of As and Cu, but large amounts of Mn into the river water. Higher concentrations of Fe and Mn indicated a possible association with the impact of mine tailings. In general, the total metal concentrations during the rainy season were higher than those during the dry season, whereas the reverse was generally the case for labile forms. This pattern reveals that metal speciation is intrinsically dependent on the seasonal variation of the hydrological conditions.

Diffusive gradients in thin films (DGT): A suitable tool for metals/metalloids monitoring in continental waterbodies at the large network scale

The contribution of Diffusive Gradients in Thin films (DGT) passive sampling to continental water quality monitoring was assessed in a real measurement network (6 sampling campaigns, 17 stations). Ten metals/metalloids (Al, Zn, Ni, Cd, Cu, Pb, Cr, As, Se and Sb) were studied using the control laboratory's working conditions with grab and DGT passive sampling. The DGT field deployments were robust, with a 3% sampler loss rate and a <65% average relative deviation between duplicates. Compared to grab sampling, DGT showed a similar quantification frequency for half of the targeted elements but showed a higher frequency for the other half (e.g., Cd quantification at 20% with grab sampling vs. 97% with DGT). Similar concentration trends were established using DGT and grab sampling at most sites throughout the year. Notably, for some elements, trends were only provided by DGT sampling. A study of several DGT blanks showed that the device contamination was occasional and originated primarily from cross-contamination during the disassembly step. Considering this contamination, the operational sensitivity by DGT was at least between 1 and 5 times greater in comparison to that by grab sampling. Estimations of the economic cost revealed that measurement networks cost 2 to 3 times more when monitored by DGT compared to standard grab monitoring. However, the information obtained based on each type of sampling method is different. Grab sampling is easy to implement and can highlight high contamination peaks. The DGT concentrations are averaged over time and are relevant to chronic exposure evaluations. Considering the good performance of the DGT sampling highlighted in this study and its complementarity with grab sampling in terms of water quality assessments, a combination of these two types of sampling, which can be affordable, should improve the water quality evaluation within monitoring networks.

Development and validation of DGT passive samplers for the quantification of Ir, Pd, Pt, Rh and Ru: A challenging application in waters impacted by urban activities

Platinum group elements (PGEs) are among the least abundant in the continental crust. They have become excellent tracers of anthropogenic activities, particularly due to their use in catalytic converters or in the medical industry. However, their quantification in environmental matrices is still problematic because of their low concentrations combined with the presence of interfering elements. Preconcentration methods are therefore necessary to measure accurate concentrations. In this study, the quantification of Ir, Rh, Ru, Pd and Pt was studied in depth by focusing on two resins: AG MP-1 (anion exchange) and Purolite® S-920 (chelating) with the aim of developing passive Diffusive Gradients in Thin films (DGT) samplers as in-situ pre-concentration tools. The characteristics of both resins (e.g. adsorption, elution, selectivity, etc.) were studied and the diffusion coefficients of PGEs in different matrices were determined. For the first time, carcinostatic platinum-based drugs were also studied. Better rates and percentages of adsorption were observed for S-920 while AG MP-1 was more selective with regard to spectral interferents and easier to elute. The diffusion coefficients of PGEs were resin-dependent, particularly for carcinostatic platinum-based drugs. For the first time, the applicability of these DGT samplers dedicated to PGEs was demonstrated in the field after their deployment in two wastewater treatment plants in Northern France for which concentrations were found to range from few pg L^-1 (Ir, Ru) to few ng L^-1 (Pt).

Cysteine-modified silica resin in DGT samplers for mercury and trace metals assessment

Diffusive gradients in thin-films (DGT) is an in situ passive sampling technique to assess labile trace metal concentrations in different environmental matrix. The technique is consisting of a diffusive domain backed up by a resin gel that binds free metals and metal complexes that dissociate in the diffusive domain. This technique requires specific resin for special metals, for example mercury (Hg), since the classic resin (Chelex-100) gel is not applicable for Hg measurement. A simultaneous determination of Hg with other metals by the DGT was not yet reported. Two biomolecule-based resins were prepared by glutaraldehyde immobilisation of cysteine onto 3-amino-functionalised silica and 3-aminopropyl-functionalised silica, respectively. The load of functional groups on modified resins was qualitatively and quantitatively characterised. The modified resins were applied in the DGT technique and the uptake efficiency, elution efficiency, and linear accumulation of analytes of the DGT were tested. This novel DGT technique, using two cysteine-modified resins, can accumulate Hg and other metals in a broad range of pH and ionic strength in solutions. In the Belgian coastal zone (BCZ), the concentrations of Hg and other trace metals sampled by cysteine-modified resin-DGTs were similar as those by the other two DGT assemblies for Hg and other trace metals, respectively. The cysteine-modified silica resin combined the features of Chelex-100 resin and 3-mercaptopropyl silica resin and allowed simultaneous determination of labile Hg and other trace metals. The resin with a higher load of functional groups also showed higher performance in the further application in the DGT technique.

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.

Use of embedded Chelex chelating resin and sediment toxicity bioassays with medaka embryos to determine the bioavailability and toxicity of lead-contaminated sediment

The aquatic sediment acts as a reservoir for multiple sources of pollutants including toxic metals. Most analytical methods used for estimating the bioavailability of sediment heavy metals have not been biologically validated by correlation with an aquatic organism's response. A reliable whole-sediment contacting toxicity assay using vertebrate species is lacking and the exposure routes for sediment metals are unclear. This study established a novel bio-analytical approach involving the Chelex-100 resin detection system and sediment toxicity assessment with embryo-larval stages of medaka fish (Oryzias latipes) to evaluate the bioavailability and toxicity of lead (Pb) contamination in sediment to fish. Treated fish exposed to the Pb-spiked artificial sediment with whole-sediment exposure showed more dose-dependent toxic responses than those from pore- or overlying-water exposure extracted from the same sediment. The Chelex-100 resin-extractable Pb content was highly correlated with mortality, total malformation and Pb bioaccumulation in medaka embryos or hatchlings from Pb-spiked sediment at environmentally relevant concentrations. The environmental sediment with higher contents of clay or organic carbon showed lower potency of releasing Pb from sediment to overlying water, as compared to those observed with artificial sediment. Our results suggest that the bio-analytical method can be practically applied in situ to evaluate the adverse effect of heavy metal-contaminated sediment on the aquatic ecosystem.

Application of DGT/DIFS combined with BCR to assess the mobility and release risk of heavy metals in the sediments of Nansi Lake, China

The heavy metal contamination of the aquatic ecosystem is still prevalent even after reduction of the external anthropogenic inputs of the metals. The release of labile heavy metals from the sediments into the water is a potential risk, responsible for the contamination of the aquatic system. Herein, samples of sedimentary column cores were collected in Nansi Lake, and the distribution profiles of the labile and soluble metals (Cd, Cu, Ni, Pb, and Zn) were obtained by the diffusive gradient in thin films (DGT) and the high-resolution dialysis (HR-peeper) technique. Furthermore, the mobility, bioavailability and release risk of the heavy metals were assessed using the results of geochemical sequential extraction, DGT as well as the DGT-induced fluxes in sediments (DIFS) model. The results showed that the profile characteristics of the DGT-labile and soluble heavy metals showed irregular distribution in the sediment cores and Cd, Pb, Zn had an obvious positive correlation with Fe/Mn (p < 0.05). Ni, Cu, and Zn existed primarily in the residual fraction (accounting for 58-76%), while Cd and Pb existed in the reducible fraction (accounting for 50-67%). The Cd and Ni (0.027-0.185) had higher mobility coefficients compared with Pb, Cu, and Zn (0-0.011), and positive diffusive fluxes also proved that Cd and Ni were easy to be released from the sediments. In addition, the R values of five metals (0.18-0.85) ranged between R_diff to 0.95, indicating that all the metals had partially sustained case from the sediments solid phase. Based on the DIFS model, the five metals had weak mobility from the sediment to pore water, but the release risks in the Nansi Lake should also be of concern, especially for the highly mobile Cd and Ni in the Dushan Lake.

Aqueous inorganic uranium speciation in European stream waters from the FOREGS dataset using geochemical modelling and determination of a U bioavailability baseline

The concentration of the bioavailable uranium fraction (U_bio) at the European scale was deduced by geochemical modelling considering several definitions found in the literature and the FOREGS European stream waters geochemical atlas dataset to produce a U_bio baseline. A sensitivity analysis was performed using three thermodynamic databases. We also investigated the link between total dissolved uranium (U_aq) concentrations, speciation and global stream water chemistry on the one hand, and the lithology and ages of the surrounding rocks on the other. The more U-enriched the stream sediments or rock type contexts are, which tends to be the case with rocks containing silicates (4.1 mg/kg), the less U-concentrated the stream waters are (0.15 μg/L). Sedimentary rocks lead to slightly higher U_aq concentrations (0.34 μg/L) even if the concentration in sediment (U_sed) is relatively low (1.6 mg/kg). This trend is reversed for U_bio, with higher concentrations in a crystalline context. The mean estimated U_bio value ranges from 1.5.10^-3 to 65.3 ng/L and can fluctuate by 3 orders of magnitude depending on the considered definition as opposed to by 2 orders of magnitude accountable to differences between thermodynamic databases. The classification of the water in relation to the two surrounding rock lithologies makes it possible to reduce the mean variability for the U_bio concentrations. Irrespective of the definition of U_bio considered, in 59% of cases the U_bio fraction represents less than 1% of U_aq. Several threshold values relating to U_bio were proposed, assuming knowledge only of the aqueous concentrations of the major elements and U_aq.

Evaluation of diffusive gradients in thin films (DGT) technique for speciation of trace metals in estuarine waters - A multimethodological approach

Understanding the potential bioavailability of trace metals (TM) in marine systems is of prime importance to implement adapted regulations and efficiently protect our coastal and estuarine waters. In this study Diffusive Gradients in Thin films (DGT) technique with two different pore size was used to evaluate the potentially bioavailable fractions (DGT-labile) of Cd, Co, Cu, Ni, Pb and Zn at various depths of a highly stratified estuary (the Krka River estuary, Croatia) both in winter and summer. DGT-labile concentrations were compared to (1) total dissolved concentrations, (2) concentrations of labile species measured by anodic stripping voltammetry (ASV-labile) for Cu and (3) concentrations derived by chemical speciation modelling. High correlation between dissolved and DGT-labile concentrations was found for all metals, except for Zn where contamination problems prevented reliable conclusions. Percentages of DGT-labile fractions over total dissolved concentrations were (AVG ± SD): 92 ± 3%, 64 ± 2%, 23 ± 5%, 61 ± 3% and 57 ± 6% for Cd, Pb, Cu, Ni and Co, respectively. No significant difference was found between trace metal concentrations measured with an open pore and restricted pore devices, implying the predominance of kinetically labile metal complexes smaller than 1 nm. For Cu, ASV-labile and DGT labile concentrations were highly correlated (0.97) with ASV-labile concentration being around 35% lower than that of the DGT-labile. Modelling of chemical speciation reliably predicted dynamic (free, inorganic and part of organic complexes) concentration of Cd, whereas dynamic concentrations of Cu and Pb were underestimated by 32% and 65%, respectively. In view of the relative simplicity of DGT devices, they are well suited for the monitoring effort of coastal waters, informing on potentially bioavailable concentrations of TM and thereby, helping to achieve good environmental status of coastal waters, as stipulated within the EU Water Framework Directive.

Utility of 'Diffusive Gradients in Thin-Films' for the measurement of nitrate removal performance of denitrifying bioreactors

The increase in environmental nutrient availability as a result of human activities has necessitated the development of mitigation strategies for nutrient removal, such as nitrate. Current methods for determining the efficiency of different mitigation strategies required measurement of changes in nitrate concentrations, however, these methods can be expensive or do not account fully for the temporal variability of nitrate concentration. This study evaluated the utility of Diffusive Gradients in Thins-Films (DGT) for determining nitrate removal in two denitrifying bioreactors, and compared DGT performance to traditional approaches for determining performance, including high and low frequency water grab sampling. The binding layer was produced using the Purolite® A520E anion exchange resin. The uptake and elution efficiencies were 98.8% and 93.4% respectively. DGTs of three material diffusion layer thicknesses were placed in piezometers along longitudinal transects, to enable calculation of the diffusive boundary layer and provide replicates. These were removed after 16, 24 and 36 h, and the accumulated nitrate masses were extracted and quantified to calculate nitrate concentration. Concentrations were subsequently utilised to calculate nitrate removal rates in both bioreactors. Grab samples were taken at 30 and 60 min intervals over those periods, nitrate concentrations were also measured to determine nitrate removal. DGTs provided nitrate removal rates at bioreactor site one (controlled flow, wastewater treatment) of 14.83-30.75 g N m^-3 d^-1, and 1.22-3.63 g N m^-3 d^-1 at site two (variable flow, agricultural run-off). DGT determined nitrate concentrations and removal rates were in strong accordance with high frequency grab sampling, but data collection via DGTs was considerably easier. Utilising DGTs for the measurement of bioreactor performance overcame many of the challenges associated with high frequency grab sampling, and other methods, such as accounting for temporal variation in nitrate concentration and reduced analytical requirements.

DGT: A promising technology for in-situ measurement of metal speciation in the environment

Toxicity, bioavailability, and mobility of a metal depend on its speciation rather than its total concentration. In this discussion, we emphasize the need for field-based metal speciation analysis methods, given the flaws of traditional techniques. We express a science-based opinion that the diffusive gradients in thin-films (DGT) is a promising and powerful technique for performing in-situ measurement of metal species or speciation in different environmental samples. Several limitations and challenges in further application of DGT are also highlighted in this discussion.

Bioavailability and translocation of metal oxide nanoparticles in the soil-rice plant system

To determine the bioavailability and translocation of metal oxide nanoparticles (MONPs) in the soil-rice plant system, we examined the accumulation and micro-distribution of ZnO nanoparticles (NPs), CuO NPs and CeO₂ NPs (50, 100 and 500 mg/kg) in the paddy soil and rice plants under flooded condition for 30 days using single-step chemical extraction and diffusive gradients in thin films (DGT) technique combined with micro X-ray fluorescence spectroscopy (μ-XRF). The results show that various MONPs changed the soil properties, especially the redox potential was enhanced to -165.33 to -75.33 mV compared to the control. The extraction efficiency of Zn, Cu and Ce in the paddy soil from high to low was EDTA, DTPA, CaCl₂ and DGT. Moreover, exposure to 500 mg/kg CuO NPs and CeO₂ NPs induced the primary accumulation of Cu and Ce elements in rice roots as high as 235.48 mg Cu/kg and 164.84 mg Ce/kg, respectively, while the Zn concentration in shoots was up to 313.18 mg/kg under highest ZnO NPs with a 1.5 of translocation factor. The effect of MONPs on the plant growth was mainly related to the chemical species and solubility of MONPs. Micro-XRF analysis shows that Zn was mostly located in the root cortex while Cu was primarily accumulated in the root exodermis and few Ce distributed in the root. Pearson correlation coefficients indicate that only DTPA-extracted metals in soil were significantly and well correlated to the Zn, Cu and Ce accumulation in rice seedlings exposed to MONPs. This work is of great significance for evaluating the environmental risks of MONPs in soil and ensuring the safety of agricultural products.

Development of a new binding phase for the diffusive gradients in thin films technique based on an ionic liquid for mercury determination

Determining bioavailable trace concentrations of mercury (Hg) in water is still a challenging analytical task. In this study, we report a methodology for determining labile Hg in natural waters using newly developed sorbents. Silicon dioxide at a nanoparticle range (Si-np) and cellulose powder at a microparticle range (Cel-p), both modified with the ionic liquid trioctylmethylammonium thiosalicylate (TOMATS), have been tested as sorbents (sorb-TOMATS) for Hg(II) uptake from solution. These novel sorb-TOMATS materials were characterized, and parameters affecting the uptake were examined. A similar Hg(II) uptake efficiency (97%) and binding capacity (9 mg Hg/g) was obtained for both sorb-TOMATS, while only a 25% of Hg(II) was taken up using non-impregnated materials. Moreover, these sorb-TOMATS were effectively embedded in agarose gel and were tested as a novel binding phase for the Diffusive Gradients in Thin Films (DGT) technique. Research revealed Si(np)-TOMATS sorbent as a suitable binding phase in the DGT technique for Hg(II) measurements, since it also allowed the efficient elution of the bound Hg(II). This new binding phase showed strong linear correlation between the accumulated Hg(II) mass and deployment time, which is in agreement with the DGT principle. In summary, this novel sorbent has a great potential to improve Hg monitoring in natural waters when integrated it in the DGT design.

Biosafety of cadmium contaminated sediments after treated by indigenous sulfate reducing bacteria: Based on biotic experiments and DGT technique

Sulfate reducing bacteria (SRB) biostabilization has attracted particular attention due to its ability to prevent and control heavy metal pollution. In this study, biotic experiments (immobilisation test of Daphnia (D.) magna, germination experiment of cucumber seeds, and in vitro experiment using gut juices of Sipunculus (S.) nudus) and diffusive gradients in thin films (DGT) technique were performed to investigate the biosafety of cadmium (Cd) contaminated sediments after being treated by indigenous SRB. Results showed that SRB treatment reduced Cd bioaccessibility of sediment to S. nudus, Cd levels in the overlying water and Cd bioavailability to D. magna. However, the treatment increased the biotoxicity of overlying water due to significant reduction in the root length and germination index of cucumber seeds. DGT results confirmed that SRB treatment increased Cd stability in sediment, and reduced its release from the sediment into the overlying water. The biotoxicity of overlying water was not caused by Cd, but possibly by the added culture medium, SRB itself, or its metabolites. More attention is required to assess the safety of SRB treatment when it is used to remediate environmental matrix contaminated by heavy metals.

Trace metal mobility in sub-seabed sediments by CO2 seepage under high-pressure conditions

Carbon capture and storage (CCS) is the third contributor to cumulative carbon emission reductions required by the second half of this century. Although this is a promising technology for reducing atmospheric CO₂, it is only affordable if the confinement of the gas is guaranteed for hundreds of years. Hence, it is of paramount importance to figure out and predict the chemical and biological effects associated with potential CO₂ leakage, to provide decision makers with a good basis for choosing technology and potential storage sites. To this end, a titanium reactor (1.4 m³) was used to study CO₂ seepage under realistic sub-seabed conditions (30 bar pressure and 7 °C). The injection of CO₂ was calibrated to decrease the pH value from 8.1 to 7.3, which may be the pH found near a leakage point. This pH value also coincides with predictions for near-future ocean pH under current CO₂ emissions worldwide. The results from this study demonstrate that there are some elements, i.e., Fe, Co, Pb, Ce, Zn and Cu, present in deep marine sediments, that are strongly affected by the reduced pH levels related to CO₂ addition. The dissolved concentrations of Fe, Pb and, to a lesser extent, Cr increased, due probably to weakening of the Fe/Mn shuttle by increased dissolved concentrations of CO₂. Desorption processes from oxyhydroxide surfaces due to acidification may explain the release of Co, Ni and Ce observed during the experiment. The increased CO₂ concentration also led to increased metal bioavailability, suggested by higher values for labile metal species. Conversely, Cd mobility seems not to be affected by CO₂-associated acidification. It is concluded that the determination of those elements most affected by CO₂-related acidification in a sub-seabed CO₂ storage perimeter (i.e., sediment, sediment-water interface and water column) would be a simple and effective technique to verify suspected leakage.

Operational DGT threshold values for metals in seawater from protected coastal areas in Sardinia (Western Mediterranean)

Diffusive gradients in thin films (DGTs) were used for monitoring metal (Cd, Cu, Ni, and Pb) concentrations in protected and non-protected coastal areas in Sardinia (Western Mediterranean). The deployment of DGTs in relatively undisturbed areas enabled calculation of operational DGT threshold values, which can be used for assessments of the environmental quality of coastal areas. The DGT thresholds were defined as the median metal concentrations that were found in protected areas, which ensured consideration of the natural variability of the different study sites. The calculated DGT thresholds were 11.6 ng L^-1 for Pb, 5.1 ng L^-1 for Cd, 63 ng L^-1 for Cu and 152 ng L^-1 for Ni. A comparison of the calculated DGT thresholds with previous DGT studies in the area demonstrated their suitability for identifying sites of environmental concern in the Western Mediterranean.

The performance of diffusive gradient in thin film probes for the long-term monitoring of trace level total mercury in water

The potential of diffusive gradient in thin film (DGT) as a long-term monitoring tool to assess trace level mercury (Hg) in surface waters was evaluated. A piston type DGT sampler and a plate-type device that could hold 15 DGTs were designed. The device contained piston type DGT samplers with varying diffusive gel thicknesses, that is, 0.5, 0.75, and 1.0 mm, respectively. Three DGT devices were deployed in a lake for 5 weeks, and two were deployed in a stream for 3 weeks. In the lake, the total Hg (THg) mass accumulated in the DGT varied between 0.05 and 0.15 ng, which increased with an increase in deployment time and decreased with an increase in agarose diffusion gel thickness. The DGT concentration in the lake water for a 2 week period was estimated to be about 0.8-1.0 ng/L, which was close to the measured value of 1.1 (± 0.13) ng/L, using the grab sampling technique. However, the DGT estimated at 4 and 6 weeks showed a concentration of about 0.5-0.7 ng/L, which is about twice as small as that measured by grab sampling. This underestimation of the THg levels in water appear to be caused by additional thicknesses of the physical diffusive boundary layer (0.15, 0.5, 1.29 mm) and biofilm, outside the DGT filter. The predicted DGT concentration in the upper stream of the Nakdong River was estimated to be about 0.8-1.4 ng/L, which is similar to the value of 1.22 (± 0.29) ng/L measured in the field by grab sampling. The concentration of THg was estimated to be about 1.0-1.2 ng/L, which is similar to the values measured by grab sampling. The additional diffusion thickness formed outside the DGT filter was 0.018 mm and 0.093 mm at 1 and 3 weeks, respectively, which is not larger than the diffusion gel thickness (0.5-1.0 mm). This was because DGT was installed in a region where the flow velocity is high, and the thickness of the diffusion boundary layer outside the filter is negligible.

Environmental geochemistry and bioaccumulation/bioavailability of uranium in a post-mining context - The Bois-Noirs Limouzat mine (France)

Knowledge on the bioavailability of trace elements is essential in developing environmental quality standards. The purpose of this study was to explore the relationships between trace elements (in particular Uranium (U)) in sediments, porewater and their bioaccumulation by Chironomus riparius on a uranium mining site and river sediments upstream of the mine. The mobility and speciation of U in sediments was investigated using DGT. Geochemical modelling using CHESS provided insight on sorption behavior of U on ironoxyhydrite (HFO) and aqueous speciation of U. In the upstream site U concentrations found were 0.05 μmol g^-1 in surface sediment, 0.84 nmol L^-1 in porewater and 2.4 nmol g^-1 in Chironomus riparius whereas in the ferrihydrite deposits on the mining sites the concentrations found were up to 9.4 μmol g^-1 in surface sediment, 0.37 μmol L^-1 in porewater and 0.684 μmol g^-1 in in Chironomus riparius. Despite the large differences in concentrations of U between the two sites, sediment to dissolved partitioning coefficients, bioconcentration factor (BCF) and biota sediment accumulation factors (BSAF) were very comparable. In the upstream sediment binding of U to organic matter controls sorption and aqueous speciation of U, whereas in the HFO rich sediments, sorption on HFO and the formation of HFO colloids are the determining factors. The low BSAF factors and high BCF factors indicate that the bioaccumulation is due to uptake from the dissolved phase. The DGT probes with different binding resins provide information on the colloidal nature and lability of the dissolved U species.

Flow period influence on uranium and trace elements release in water from the waste rock pile of the former La Commanderie uranium mine (France)

Uranium mining activities expose uranium ore and mine tailings to the surface environment, where the release of radionuclides is facilitated by weathering at rates exceeding those typically found in nature. Therefore, close to former uranium mining sites, radionuclides and especially uranium concentrations in water may surpass local background levels. The methodology proposed herein, entails coupling, gamma-ray mapping, water sampling and chemical analyses including DGT (Diffusive Gradient in Thin Film) measurements, provides new insights into describing the environment of the La Commanderie site (France). Gamma-ray mapping allows identifying water seepage, output from a waste rock pile, as a potential pathway for radionuclides into the environment. Water seepage monitoring has shown: a low pH value (4.2), high sulfate content (179 mg.L^-1) and high uranium concentrations of up to 436 μg.L^-1. These recordings indicate that an acid mining drainage (AMD) process is occurring inside or under the oxidized parts of the waste rock pile. Monitoring data over three flow periods revealed the release of the highest uranium concentrations during a high-flow period downstream of the site, which is compliant with local regulations. The AMD process is also responsible for the release of significant amounts of Fe, Mn and As within the immediate environment in both dissolved and particulate forms. Changes in dissolved oxygen concentration and redox potential during low flow periods, modify the speciation of Fe (in AMD waters) which acts as a scavenger for other elements such as As, Mn and U. The use of DGT under environmental conditions, and specifically AMD waters, seems to be relevant in comparison to filtered spot water sampling strategies. Moreover, based on DGT measurements, the dissolved part of the released uranium is considered as labile with concentrations above the environmental standards for freshwater organisms.

Development and evaluation of a sensitive, Diffusive Gradients in Thin-Films (DGT) method for determining microcystin-LR concentrations in freshwater and seawater

A Diffusive Gradients in Thin-Films (DGT) passive sampling technique was developed for microcystin-LR (MC-LR), one of the most common and toxic microcystins. Three types of resins (HP20, SP700, and XAD18) were evaluated for MC-LR uptake kinetics, capacities, and extraction efficiencies and simple procedures were developed for determining MC-LR concentration in binding disc extracts by Adda-ELISA (U.S. EPA Method 546). The XAD18-DGT/Adda-ELISA method had a 7-d deployment time detection limit of ≈0.05 μg/L and capacity of >250 μg/L of MC-LR in water samples which encompass U.S. EPA and WHO advisory concentrations for drinking and recreational waters. The XAD18-DGT/Adda-ELISA method determined time-averaged MC-LR concentrations in waters with wide ranging pH (4.9-8.3) and ionic strength (0.04-0.8 M) under well-stirred and quiescent conditions with 90-101% accuracy. In addition to high sensitivity and accuracy, the method is simple, inexpensive, and applicable for determining MC-LR and related MCs concentrations in waterbodies with wide ranging chemical characteristics and hydrodynamic conditions.

Aqueous copper bioavailability linked to shipwreck-contaminated reef sediments

Pollution from the grounding or sinking of ships can have long lasting effects on the recovery and dynamics of coastal ecosystems. Research on the impact of copper (Cu) pollution from the 2011 MV Rena shipwreck at the Astrolabe Reef (Otaiti), New Zealand, 5 years after the grounding, followed a multi-method and multi-disciplinary approach. Three independent measures of aqueous Cu using trace-element-clean-techniques substantiate the presence of high total, total dissolved (<2 µm) and elevated bioavailable Cu in the water column immediately above the aft section of the wreck where the highest sedimentary load of Cu was located. Intermittently elevated concentrations of strong Cu-binding ligands occurred in this location, and their binding strength was consistent with ligands actively produced by organisms in response to Cu induced stress. The recruitment of benthic invertebrates was modified at the high-Cu location. Taxonomic groups usually considered robust to pollution were restricted to this site (e.g. barnacles) or were the most abundant taxa present (e.g. foraminifera). Our results demonstrate that Cu-contaminated sediments can impose a persistent point source of Cu pollution in high-energy reef environments, with the potential to modify the composition and recovery of biological communities.