Diffusive Gradients In Thin-films Measurements Research Articles

Comprehensive investigation of pollution levels and potential bioavailable risks of 7 potentially toxic elements in Qinghai-Tibet soils by using diffusive gradients in thin-films (DGT)

ABSTRACT Seven potentially toxic elements (PTEs) including Cr, Ni, Cd, Zn, Cu, Pb and As of soil samples from Qinghai-Tibet Plateau (QTP) were measured to assess the degree of contaminations. To have a comprehensive understanding of PTEs’ risks in QTP, the single-step nitric acid extraction and diffusive gradients in thin-films (DGT) technique were employed in this study for the determination of PTEs’ total amount as well as bioavailable content, respectively. Haibei state of Qinghai province was the most contaminated sites by PTEs. Behaviors of PTEs should be highly concerned as they can produce continuous bioavailable effects to ecological systems. High DGT concentrations of Zn and Cr showed their remarkable available risks in soils. In the environment with insufficient organic matter and slight pH variations, phosphorus level was the main contributor to affect the mobility of labile PTEs, especially for Pb and Ni. Concentration ratio between DGT measurement and soil solution and correlation analysis confirmed the high bioavailability of Pb, Zn and Cr, leading to short-term mixture effects on ecological systems, while the long-term release dynamics were found for As and Cu. Results from road-side and river-side soils of QTP evidenced the short-term bioavailable risks were possibly attributed to intensive anthropogenic activities including traveling industries and vehicle driving, whereas long-term bioavailable risks was from the natural transportation and deposition.

Influence of microplastics on the availability of antibiotics in soils

Microplastics (MPs) and antibiotics, as two major types of emerging pollutants, inevitably coexist in the soil environment due to agricultural film residue, sewage irrigation and sludge application. However, the impact of MPs on antibiotic availability in soils with varying characteristics has not been extensively studied. Therefore, in this study, an interference experiment was conducted using three types of MPs (polyethylene (PE), polyvinyl chloride (PVC) and polypropylene (PP)) in red soil, paddy soil and cinnamon soil. The available antibiotics in soils were evaluated using diffusive gradients in thin-films (DGT). Results showed that MPs had a significant impact on the amount of antibiotics adsorbed on soil solid (Cs) by providing additional binding sites or altering soil characteristics (e.g., pH and dissolved organic carbon). The most significant effects on Cs were observed in cinnamon soil, and the Cs values were dependent on concentration of MPs. The available antibiotics, as measured by DGT significantly decreased after the addition of MPs. This decrease was influenced by the soil characteristics. However, the concentration of antibiotics in soil solutions (Cd) was only slightly impacted by MPs. Therefore, the influence of MPs on the migration of antibiotics was reflected by their impact on the soil/water partition coefficient (Kd), while the resupply ability (R) from the soil solid phase was less influential. Moreover, the dosage of MPs had a significant effect on the availability of antibiotics in CS by promoting the adsorption of antibiotics on the solid phase, while in RS and PS, the soil properties played a dominate role in the changes in antibiotic availability after MP addition. These results indicate that the impact of MPs on available antibiotics mainly depends on soil properties. In addition, DGT measurement is more sensitive than soil solution to investigate the effects of coexisting pollutants on the behavior of antibiotics in soil.

Bioavailability of Cd in Agricultural Soils Evaluated by DGT Measurements and the DIFS Model in Relation to Uptake by Rice and Tea Plants

The elevated accumulation of cadmium (Cd) in rice (Oryza sativa L.) and tea (Camellia sinensis L.) grown in agricultural soils may lead to a variety of adverse health effects. This study collected and analyzed crop samples along with paired rhizosphere soil samples from 61 sites in Cd-contaminated regions in Anhui Province, China. The findings revealed that both the diffusive gradients in thin-films (DGT) and soil solution were capable of effectively predicting Cd contents in crops. Conventional chemical extraction methods were inappropriate to evaluate the bioavailability of Cd. However, the effective concentrations (CE) corrected by the DGT-induced fluxes in soils (DIFS) model exhibited the strongest correlation with crop Cd contents. Except for CE, various measurement methods yielded better results for predicting Cd bioavailability in tea compared to rice. Pearson’s correlation analysis and the random forest (RF) model identified the key influencing factors controlling Cd uptake by rice and tea, including pH, soil texture, and contents of zinc (Zn) and selenium (Se) in soils, which antagonize Cd. To reduce the potential health risk from rice and tea, the application of soil liming and/or Se-oxidizing bacteria was expected to be an effective management strategy.

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 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.

Amelioration of copper toxicity to a tropical freshwater microalga: Effect of natural DOM source and season

Australian tropical freshwaters can experience extreme seasonal variability in rainfall and run off, particularly due to pulse events such as storms and cyclones. This study investigated how seasonal variability in dissolved organic matter (DOM) quality impacted the chronic toxicity of copper to a tropical green alga (Chlorella sp.) in the presence of two concentrations of DOM (low: ∼2 mg C/L; high: ∼10 mg C/L) collected from three tropical waters. Copper speciation and lability were explored using diffusive gradients in thin-films (DGT) and modelled maximum dynamic concentrations (cdynmax) using data derived from the Windermere Humic Aqueous Model (WHAM VII). Relationships between copper lability and copper toxicity were assessed as potential tools for predicting toxicity. Copper toxicity varied significantly with DOM concentration, source and season. Copper toxicity decreased with increasing concentrations of DOM, with 50% growth inhibition effect concentrations (EC50) increasing from 1.9 μg Cu/L in synthetic test waters with no added DOM (0.34 mg C/L) up to 63 μg Cu/L at DOM concentrations of 9.9 mg C/L. Copper toxicity varied by up to 2-fold between the three DOM sources and EC50 values were generally lower in the presence of wet season DOM compared to dry season DOM. Linear relationships between DGT-labile copper and dissolved copper were significantly different between DOM source, but not concentration or season. Modelled cdynmax consistently under-predicted labile copper in high DOM treatments compared to DGT measurements but performed better in low DOM treatments, indicating that this method is DOM-concentration dependent. Neither speciation method was a good surrogate for copper toxicity in the presence of different sources of natural DOM. Our findings show that DOM source and season, not just DOM concentration, affect copper toxicity to freshwater biota. Therefore, DOM quality should be considered as a toxicity-modifying factor for future derivation of bioavailability-based site-specific water quality guideline values.

A new method based on diffusive gradients in thin films for in situ monitoring microcystin-LR in waters

The passive sampling method of diffusive gradients in thin-films (DGT) was developed to provide a quantitative and time-integrated measurement of microcystin-LR (MC-LR) in waters. The DGT method in this study used HLB (hydrophilic-lipophilic-balanced) material as a binding agent, and methanol as an eluent. The diffusion coefficient of MC-LR was 5.01 × 10−6 cm2 s−1 at 25 °C in 0.45 mm thick diffusion layer. This DGT method had a binding capacity of 4.24 μg per binding gel disk (3.14 cm2), ensuring sufficient capacity to measure MC-LR in most water matrices. The detection limit of HLB DGT was 0.48 ng L−1. DGT coupled to analysis by HPLC appears to be an accurate method for MC-LR monitoring. Comparison of DGT measurements for MC-LR in water and a conventional active sampling method showed little difference. This study demonstrates that HLB-based DGT is a useful tool for in situ monitoring of MC-LR in fresh waters.

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.

Investigation and application of diffusive gradients in thin-films technique for measuring endocrine disrupting chemicals in seawaters

Endocrine disrupting chemicals (EDCs) can be released to coastal waters and affect the endocrine system of marine organisms. To monitor their levels in seawaters, a simple, robust passive sampling method, the diffusive gradients in thin-films (DGT) technique, was developed with XDA-1 resin as a binding agent. Six EDCs (including three estrogens, two pesticides and bisphenol A) were used to assess the performance of the DGT. The XDA-1 binding gel showed adequate ability for adsorbing EDCs in seawaters. The DGT sampler exhibited linear accumulation for the EDCs during a 15-day deployment and diffusion coefficients and sampling rates were calculated. The DGT measurement was independent of pH in the range 7.0–9.0 and ionic strength in the range 0.4–0.8 M. Field applications of this DGT in a coast of Dalian (China) showed comparable results to those from grab sampling. Five EDCs were detected with concentrations ranging from 0.7 to 19.4 ng L−1. This study is a first attempt to apply DGT sampler for determining EDCs in seawaters.

Comparison of DGT with traditional extraction methods for assessing arsenic bioavailability to Brassica chinensis in different soils

Several predictive models and methods have been used for heavy metals bioavailability, but there is no universally accepted approach in evaluating the bioavailability of arsenic (As) in soil. The technique of diffusive gradients in thin-films (DGT) is a promising tool, but there is a considerable debate with respect to its suitability. The DGT method was compared with other traditional chemical extractions techniques (soil solution, NaHCO3, NH4Cl, HCl, and total As method) for estimating As bioavailability in soil based on a greenhouse experiment using Brassica chinensis grown in various soils from 15 provinces in China. In addition, we assessed whether these methods are independent of soil properties. The correlations between plant and soil As concentration measured with traditional extraction techniques were pH and iron oxide (Feox) dependent, indicating that these methods are influenced by soil properties. In contrast, DGT measurements were independent of soil properties and also showed a better correlation coefficient than other traditional techniques. Thus, DGT technique is superior to traditional techniques and should be preferable for evaluating As bioavailability in different type of soils.

Coupling biological assays with diffusive gradients in thin-films technique to study the biological responses of Eisenia fetida to cadmium in soil

This work sets out to investigate biological responses of Eisenia fetida to Cd, based on the bioavailable rather than total concentration of Cd, in soils. E. fetida was cultured for 14d in three selected Chinese soils amended with 0.1–40mgkg−1 Cd. Potentially bioavailable concentrations of Cd were measured in soil solution, in extractions using CaCl2 and HAc solutions, and using the technique of diffusive gradients in thin-films (DGT). Antioxidant system responses in E. fetida to Cd were measured as biological endpoints. Biological responses were more highly correlated with Cd concentrations evaluated using bioavailable methods than with total concentrations. Cd concentration measured using DGT and CaCl2 extraction provided the narrowest ranges of lowest observed effect concentration (LOEC) values based on biological responses, indicating the potential use of these measurements in management of soil quality and setting soil remediation standards. The LOEC values obtained from 15 field soils contaminated by Cd were similar to those from the three Cd-amended soils and suggested that DGT in particular can be a good tool to predict stress responses of E. fetida to Cd in soils. The study shows the potential of combining biological response and DGT measurements in risk assessment.

A Diffusive Gradient-in-Thin-Film Technique for Evaluation of the Bioavailability of Cd in Soil Contaminated with Cd and Pb.

Management of heavy metal contamination requires accurate information about the distribution of bioavailable fractions, and about exchange between the solid and solution phases. In this study, we employed diffusive gradients in thin-films (DGT) and traditional chemical extraction methods (soil solution, HOAc, EDTA, CaCl2, and NaOAc) to determine the Cd bioavailability in Cd-contaminated soil with the addition of Pb. Two typical terrestrial species (wheat, Bainong AK58; maize, Zhengdan 958) were selected as the accumulation plants. The results showed that the added Pb may enhance the efficiency of Cd phytoextraction which is indicated by the increasing concentration of Cd accumulating in the plant tissues. The DGT-measured Cd concentrations and all the selected traditional extractants measured Cd concentrations all increased with increasing concentration of the addition Pb which were similar to the change trends of the accumulated Cd concentrations in plant tissues. Moreover, the Pearson regression coefficients between the different indicators obtained Cd concentrations and plants uptake Cd concentrations were further indicated significant correlations (p < 0.01). However, the values of Pearson regression coefficients showed the merits of DGT, CaCl2, and Csol over the other three methods. Consequently, the in situ measurement of DGT and the ex situ traditional methods could all reflect the inhibition effects between Cd and Pb. Due to the feature of dynamic measurements of DGT, it could be a robust tool to predict Cd bioavaiability in complex contaminated soil.

Use of diffusive gradients in thin-films for studies of chemical speciation and bioavailability

Environmental context The health of aquatic organisms depends on the distribution of the dissolved forms of chemical components (speciation) and their rates of interaction (dynamics). This review documents and explains progress made using the dynamic technique of diffusive gradients in thin-films (DGT) to meet these challenges of measuring directly chemical speciation and associated dynamics in natural waters. The relevance of these measurements to uptake by biota of chemical forms in soils, sediments and water is discussed with reference to this expanding literature. Abstract This review assesses progress in studies of chemical speciation using diffusive gradients in thin-films (DGT) by examining the contributions made by key publications in the last 20 years. The theoretical appreciation of the dynamic solution components measured by DGT has provided an understanding of how DGT measures most metal complexes, but excludes most colloids. These findings strengthen the use of DGT as a monitoring tool and provide a framework for using DGT to obtain in situ kinetic information. Generally, the capabilities of DGT as an in situ perturbation and measurement tool have yet to be fully exploited. Studies that have used DGT to investigate processes relevant to bioavailability have blossomed in the last 10 years, especially for soils, as DGT mimics the diffusion limiting uptake conditions that, under some conditions, characterise uptake by plants. As relationships between element accumulated by DGT and in plants depend on the plant species, soils studied, and the element and its chemical form, DGT is not an infallible predictive tool. Rather its strength comes from providing information on the labile species in the system, whether water, soil or sediment. Recent studies have shown good relationships between measurements of metals in periphyton and by DGT, and unified dose response curves have been obtained for biota in sediments when they are based on DGT measurements. Both these cases suggest that alternative approaches to the established ‘free ion’ approach may be fruitful in these media and illustrate the growing use of DGT to investigate environmental chemical processes.

Evaluation of trace metals bioavailability in Japanese river waters using DGT and a chemical equilibrium model

To develop efficient and effective methods of assessing and managing the risk posed by metals to aquatic life, it is important to determine the effects of water chemistry on the bioavailability of metals in surface water. In this study, we employed the diffusive gradients in thin-films (DGT) to determine the bioavailability of metals (Ni, Cu, Zn, and Pb) in Japanese water systems. The DGT results were compared with a chemical equilibrium model (WHAM 7.0) calculation to examine its robustness and utility to predict dynamic metal speciation. The DGT measurements showed that biologically available fractions of metals in the rivers impacted by mine drainage and metal industries were relatively high compared with those in urban rivers. Comparison between the DGT results and the model calculation indicated good agreement for Zn. The model calculation concentrations for Ni and Cu were higher than the DGT concentrations at most sites. As for Pb, the model calculation depended on whether the precipitated iron(III) hydroxide or precipitated aluminum(III) hydroxide was assumed to have an active surface. Our results suggest that the use of WHAM 7.0 combined with the DGT method can predict bioavailable concentrations of most metals (except for Pb) with reasonable accuracy.

Fungal Inoculation and Elevated CO2 Mediate Growth of Lolium Mutiforum and Phytolacca Americana, Metal Uptake, and Metal Bioavailability in Metal-Contaminated Soil: Evidence from DGT Measurement

Fungal inoculation and elevated CO2 may mediate plant growth and uptake of heavy metals, but little evidence from Diffusive Gradients in Thin-films (DGT) measurement has been obtained to characterize the process. Lolium mutiforum and Phytolacca americana were grown at ambient and elevated CO2 on naturally Cd and Pb contaminated soils inoculated with and without Trichoderma asperellum strain C3 or Penicillium chrysogenum strain D4, to investigate plant growth, metal uptake, and metal bioavailability responses. Fungal inoculation increased plant biomass and shoot/root Cd and Pb concentrations. Elevated CO2 significantly increased plants biomass, but decreased Cd and Pb concentrations in shoot/root to various extents, leading to a metal dilution phenomenon. Total Cd and Pb uptake by plants, and DGT-measured Cd and Pb concentrations in rhizosphere soils, were higher in all fungal inoculation and elevated CO2 treatments than control treatments, with the combined treatments having more influence than either treatment alone. Metal dilution phenomenon occurred because the increase in DGT-measured bioavailable metal pools in plant rhizosphere due to elevated CO2 was unable to match the increase in requirement for plant uptake of metals due to plant biomass increase.

Bioavailability and oxidative stress of cadmium to Corbicula fluminea

This work set out to study the effects of cadmium (Cd) in sediments on the antioxidant enzyme activities in the digestive gland of Asian clam Corbicula fluminea and to explore the potential for applying these responses to evaluate the Cd-contaminated sediment. Additionally, diffusive gradients in thin-films (DGT) technique was used to predict the response of its antioxidant defense system. The sediments, collected from Taihu Lake, were spiked with Cd at different concentrations (0.72, 0.91, 1.62, 2.59, 11.2, 20.4 and 40.6 mg kg(-1), dry weight). Asian clam was cultivated for 28 days. Concentrations of Cd in the body of Asian clam had a good relationship with concentrations of Cd in overlying water and sediments, as measured by DGT. Cd affected these biochemical parameters significantly, especially for superoxide (SOD), peroxidase (POD) and glutathione (GSH), which began to show higher sensitivity and have a significant difference in low dosage (0.91 mg Cd kg(-1)) compared with blank. Contents of MDA and MT, which were induced by Cd, increased with the increasing Cd concentration in sediments and reached peak values at 11.2 and 20.4 mg kg(-1), respectively, after 28 days exposure. All of these results suggested that biochemical responses cooperated in detoxifying and maintaining cellular metabolic homeostasis. The R(2) of regression analysis between the contents of MT and the concentrations of Cd measured by DGT, in sediments and soft body were 0.71, 0.94 and 0.88 after 28 days exposure. This suggested that DGT measurement could predict the response of MT. Cd accumulation, GSH and MT were indispensable biomarkers and the MDA content and DGT appeared to be promising biomarkers. The results clearly indicated that Cd could induce oxidative stress in the digestive gland of Asian clam. The combination of biomonitors with DGT can obtain different information about Cd bioavailability and confirm the significance of applying a suite of biomarkers rather than a selective index to assess the sublethal effect. It also offered theoretical methods for the prediction of sediment Cd pollution.

Evaluation of in Situ DGT Measurements for Predicting the Concentration of Cd in Chinese Field-Cultivated Rice: Impact of Soil Cd:Zn Ratios

DGT (diffusive gradients in thin-films) has been proposed as a tool for predicting Cd concentrations in rice grain, but there is a lack of authenticating data. To further explore the relationship between DGT measured Cd and concentrations in rice cultivated in challenging, metal degraded, field locations with different heavy metal pollutant sources, 77 paired soil and grain samples were collected in Southern China from industrial zones, a "cancer village" impacted by mining waste and an organic farm. In situ deployments of DGT in flooded paddy rice rhizospheres were compared with a laboratory DGT assay on dried and rewetted soil. Total soil concentrations were a very poor predictor of plant uptake. Laboratory and field deployed DGT assays and porewater measurements were linearly related to grain concentrations in all but the most contaminated samples where plant toxicity occurred. The laboratory DGT assay was the best predictor of grain Cd concentrations, accommodating differences in soil Cd, pollutant source, and Cd:Zn ratios. Field DGT measurements showed that Zn availability in the flooded rice rhizospheres was greatly diminished compared to that of Cd, resulting in very high Cd:Zn ratios (0.1) compared to commonly observed values (0.005). These results demonstrate the potential of the DGT technique to predict Cd concentrations in field cultivated rice and demonstrate its robustness in a range of environments. Although, field deployments provided important details about in situ element stoichiometry, due to the inherent heterogeneity of the rice rhizosphere soils, deployment of DGT in dried and homogenized soils offers the best possibility of a soil screening tool.

Evaluation of diffusive gradients in thin-films (DGTs) as a monitoring tool for the assessment of the chemical status of transitional waters within the Water Framework Directive

In this contribution, the potential use of diffusive gradients in thin-films (DGTs) for the chemical evaluation of transitional water bodies within the Water Framework Directive (WFD) has been studied. The water metal concentration has been evaluated in 13 estuaries in the southeastern Bay of Biscay. The DGTs were deployed in triplicate at two stations in each estuary, delimiting the tidal influence. The DGT results were in the ranges of 2–1570ngL−1 for cadmium, 66–515ngL−1 for copper, 30–3650ngL−1 for nickel and 0.8–95.5μgL−1 for zinc. The DGTs provided reliable average labile metal concentrations in highly dynamic systems that were comparable with DGT measurements in coastal and estuarine waters worldwide. In addition, it was possible to discriminate those estuaries more susceptible to environmental impacts, being consistent with the historical contamination of each estuary. Based on the obtained results, a sampling strategy for the monitoring of transitional water bodies using DGTs is proposed.

The Role of Natural Purified Humic Acids in Modifying Mercury Accessibility in Water and Soil

Contamination of soils with mercury can be a serious problem. It can be mobilized or stabilized by humic substances (HS) containing binding sites with reduced sulfur that can have different binding capacities for CH(3)Hg(+) and for Hg(2+). In this work we investigated the influence of different humic acids (HAs, extracted from lignite, compost, and forest soil) on mercury mobility and availability, both in a model solution and in soil samples from a mercury-polluted region. The technique of diffusive gradients in thin-films (DGT), which is capable of measuring: (i) free metal in solution; (ii) dissociated metal complexes previously mobilized by HA; (iii) mobilized metal-HA complexes that liberate metals by dissociation or by exchange reaction between the metal-HA complexes and the chelating groups on the resin-gel, was used in solutions and soils. The DGT measurements in solution, together with ultrafiltration, allowed estimation of the lability of Hg-HA complexes. Ultrafiltration results were also compared with predictions made by the windermere humic-aqueous model (WHAM). According to both these different approaches, Hg(2+) resulted nearly 100% complexed by HAs, whereas results from ultrafiltration showed that 32 to 72% of the CH(3)Hg(+) was bound to the HAs, with higher values for compost and lower values for forest and Aldrich HA. The DGT-measured mercury in soils was below 0.20 microg L(-1), irrespective of the extent of the contamination. Addition of HA increased the concentration of DGT-measured mercury in soil solution up to 100-fold in the contaminated soil and up to 30-fold in the control soil. The level of the increase also depended on the HA. The smallest increase (about 10 times) was found for lignite HA in both control and contaminated soils. The addition of forest HA gave the largest increases in DGT-measured mercury, in particular for the contaminated soil. Overall, the results demonstrated that DGT can be used for estimating the lability of mercury complexes in solution and for verifying enhanced mercury mobility when HA is added to contaminated soils.

Understanding small-scale features in DGT measurements in sediments

Environmental context. Observations, using the technique of diffusive gradients in thin-films (DGT), of pronounced, small-scale (millimetre) maxima in concentrations of sulfide and metals in the pore water of sediments, have emphasised the importance of processes occurring in microniches. Modelling of the interactions between microniche sources and DGT devices within a sediment environment demonstrates how these sharp features arise and provides a basis for their quantitative interpretation. Abstract. Measurements in sediments made using DGT (diffusive gradients in thin-films) have shown small-scale (millimetre and sub-millimetre) maxima in solute concentration (e.g trace metals and sulfide). The sediment–DGT system was simulated using a dynamic model, which incorporated a spherical microniche close to the DGT surface. DGT maxima could arise when the microniche was (1) a local source with associated elevated concentration in the pore water, and (2) when, within the microniche, the Kd for the relevant solute partitioning with exchangeable solute associated with the solid phase was much higher than for the rest of the sediment. Use of realistic values of Kd and comparison with existing data suggested that the latter mechanism was unlikely to be responsible for observed DGT maxima. Locally elevated concentrations will be reasonably accurately reproduced by DGT. Peak height measured by DGT will be between 62 and 87% of the true maxima in concentration within the sediment when DGT is not present, while peak widths will be similar. This work provides, for the first time, a means for confidently interpreting the two dimensional images of DGT-measured concentrations in sediments.