Gradients In Thin Films Technique Research Articles

High-Resolution, Two-Dimensional Measurement of Dissolved Reactive Phosphorus in Sediments Using the Diffusive Gradients in Thin Films Technique in Combination with a Routine Procedure

Dissolved reactive phosphorus (DRP) is the most available P form in sediments and often directly controls phytoplankton blooms in aquatic systems. In this study, a novel procedure was developed for two-dimensional (2D) measurement of DRP in sediments at a spatial resolution of 0.45 mm using the diffusive gradients in thin films (DGT) technique with a revised high-capacity binding phase (Zr oxide gel). This procedure involves DGT uptake of P in sediments, 2D slicing of the binding gel on a 0.45 × 0.45-mm grid system, elution of P from each gel square with 1 M NaOH, and microcolorimetric determination of DRP in each eluted solution using 384-microwell plates. Measurements of DRP via this procedure were tested in homogeneous solutions and sediments and produced an acceptable error (<20% relative standard deviation) for the analysis once the accumulated mass of P in each gel square reached 1.2 μg cm(-2). This method was successfully applied to produce 2D images of the DRP distribution in sediments with and without the influence of tubificid worm bioturbation, revealing a much more pronounced and localized impact from tubificid worms than that found using a one-dimensional measurement of pore water DRP concentrations at 1-cm resolution.

In vitro digestion and DGT techniques for estimating cadmium and lead bioavailability in contaminated soils: Influence of gastric juice pH

A sensitivity analysis was conducted on an in vitro gastrointestinal digestion test (i) to investigate the influence of a low variation of gastric juice pH on the bioaccessibility of Cd and Pb in smelter-contaminated soils ( F B , using the unified bioaccessibility method UBM) and fractions of metals that may be transported across the intestinal epithelium ( F A , using the diffusive gradient in thin film technique), and (ii) to provide a better understanding of the significance of pH in health risk assessment through ingestion of soil by children. The risk of metal exposure to children (hazard quotient, HQ) was determined for conditions that represent a worst-case scenario (i.e., ingestion rate of 200 mg day − 1 ) using three separate calculations of metal daily intake: estimated daily intake ( EDI), bioaccessible EDI ( EDI- F B ), and oral bioavailable EDI ( EDI- F A ). The increasing pH from 1.2 to 1.7 resulted in: (i) no significant variation in Cd- F B in the gastric phase but a decrease in the gastrointestinal phase; (ii) a decrease in soluble Pb in the gastric phase and a significant variation in Pb- F B in the gastrointestinal phase; (iii) a significant decrease in Cd- F A and no variation in Pb- F A ; (iv) no change in EDI- F B and EDI- F A HQs for Cd; (v) a significant decrease in EDI- F B HQs and no significant variation in EDI- F A HQ for Pb. In the analytical conditions, these results show that risk to children decreases when the bioavailability of Pb in soils is taken into account and that the studied pH values do not affect the EDI- F A HQs. The present results provide evidence that the inclusion of bioavailability analysis during health risk assessment could provide a more realistic estimate of Cd and Pb exposure, and opens a wide field of practical research on this topic (e.g., in contaminated site management).

Combination of Diffusive Gradient in a Thin Film Probe and IC-ICP-MS for the Simultaneous Determination of CH3Hg+ and Hg2+ in Oxic Water

A diffusive gradient in thin film technique (DGT) was combined with ion chromatography and inductively coupled plasma mass spectrometry (IC-ICP-MS) for the in situ simultaneous quantification of CH(3)Hg(+) and Hg(2+) in aquatic environments. After diffusing through an agarose diffusive layer, the Hg species accumulated in a thiol-functionalized resin layer and were extracted using acidic thiourea solution to form stable thiourea-Hg complexes that were separated and detected via ion chromatography and ICP-MS, respectively. The effective diffusion coefficients of CH(3)Hg(+) and Hg(2+) complexes in the agarose diffusion layer with chloride were 5.26 (±0.27) × 10(-6) and 4.02 (±0.10) × 10(-6) cm(2) s(-1), respectively. The effective diffusion coefficients of CH(3)Hg(+) and Hg(2+) complexes in the agarose diffusion layer with dissolved organic matter was 3.57 (±0.29) × 10(-6) and 2.16 (±0.19) × 10(-6) cm(2) s(-1), respectively. The practical method detection limits are 0.1 and 0.7 ng L(-1) for CH(3)Hg(+) and Hg(2+) respectively for three weeks deployment. Lower detection limits would be possible by employing a thinner agarose diffusive layer and/or by deploying the probes longer. The method can measure time averaged CH(3)Hg(+) and Hg(2+) concentrations simultaneously in oxic water, making it useful as an in situ monitoring tool.

Effect of Organic P Forms and P Present in Inorganic Colloids on the Determination of Dissolved P in Environmental Samples by the Diffusive Gradient in Thin Films Technique, Ion Chromatography, and Colorimetry

The speciation of P in environmental samples is operationally defined, since it depends on the analytical method used. In this study, we compared four methods to measure P in solution: ion chromatography (IC), the malachite green colorimetric method (CM), the diffusive gradient in thin films technique (DGT) and, for total dissolved P, optical inductively coupled plasma (ICP). These methods were compared on three sets of solutions (filtered over <0.45 μm): solutions with model organic P compounds, suspensions of synthesized inorganic Fe and Al colloids loaded with P, and environmental samples. The environmentally relevant organic P compounds were only marginally detected by CM and IC. Substantial fractions of certain organic P compounds contributed to the DGT measurement. Colorimetric analysis of DGT eluates detected in general less P than ICP analysis, indicating that these organic P compounds sorbed on the zero sink layer. Phosphorus associated with inorganic colloids was completely recovered by CM, but not by IC and least by DGT. Measurements on a wide set of 271 environmental samples (soil pore waters, groundwaters, and surface waters) suggest that surface water P is largely present as orthophosphate and phosphate sorbed onto inorganic colloids, whereas organic P contributes more in groundwaters.

Investigation of DGT as a metal speciation tool in artificial human gastrointestinal fluids

This paper reports the results of an investigation on the performance of the diffusive gradient in thin film technique (DGT) as a speciation tool for trace elements (TEs) in artificial human gastrointestinal fluids. The validity of Cd, Pb, and Zn sampling by DGT in digestive fluids was checked. The TE bioaccessibility in highly contaminated soils was determined using the in vitro Unified Barge Method (UBM) test. DGT devices were deployed in the gastrointestinal solutions obtained after carrying out the UBM test. The computer speciation code JESS (Joint Expert Speciation System) was used to predict the metal speciation of Cd, Pb, and Zn. Combining the in vitro test with the DGT technique and JESS provided an approach to the TE species available for transport across the intestinal epithelium. The gastrointestinal absorption of ingested TE ranged from 8 to 30% for Cd, 0.6 to 11% for Pb, and 0.8 to 7% for Zn and was influenced by TE speciation. In this original approach, the DGT technique was found to be simple and reliable in the investigation of TE chemical speciation in digestive fluids. Extrapolation to the in vivo situation should be undertaken very cautiously and requires further investigation.

Labile, dissolved and particulate PAHs and trace metals in wastewater: passive sampling, occurrence, partitioning in treatment plants

The occurrence and the partitioning of polycyclic aromatic hydrocarbons (PAHs) and seven metals (Al, Cd, Cr, Cu, Ni, Pb and Zn) were investigated in activated sludge wastewater treatment plants by means of passive and active sampling. Concentrations total dissolved and particulate contaminants were determined in wastewater at several points across the treatment system by means of grab sampling. Truly dissolved PAHs were sampled by means of semipermeable membrane devices. Labile (inorganic and weakly complexed) dissolved metals were also sampled using the diffusive gradient in thin film technique. This study confirms the robustness and the validity of these two passive sampling techniques in wastewater. All contaminant concentrations decreased in wastewater along the treatment, although dissolved and labile concentrations sometimes increased for substances with less affinity with organic matter. Solid-liquid and dissolved organic matter/water partitioning constants were estimated. The high variability of both partitioning constants for a simple substance and the poor relation between K(D) and K(OW) shows that the binding capacities of particles and organic matter are not uniform within the treatment and that other process than equilibrium sorption affect contaminant repartition and fate in wastewater.

Persistence of heavy metals in river sediments

The objective of this study was to assess heavy metals accumulation and equilibrium phenomena in river sediment, in relation to water quality, and to assess metals lability, analysing whether and how easily heavy metals are available to generic biota. The diffuse gradient in thin film technique was used to assess the effective concentration (C E). The solid–liquid distribution coefficient of the metals labile pool (K dl) was calculated to include it in risk analysis within a more realistic frame. The Esino River was chosen for the case study. The results confirmed the need to determine K d experimentally; values of K d taken from the literature resulted in errors of some orders of magnitude. Metals speciation, environmental conditions, the type and texture of the sediment, features of the water, organic fraction, pH and salinity can all influence the outcome. The results showed that the two main parameters on which K d depends are chemical oxygen demand and specific surface area. C E, which takes into account the contribution of both the water and the labile fraction in the sediment, is a key parameter and may be 5–100×the concentration in the pore water.

Solid phase extraction and diffusive gradients in thin films techniques for determination of total and labile concentrations of Cd(II), Cu(II), Ni(II) and Pb(II) in Black Sea water

Total dissolved and labile concentrations of Cd(II), Cu(II), Ni(II) and Pb(II) were determined at six locations of the Bourgas Gulf of the Bulgarian Black Sea coast. Solid phase extraction procedure based on monodisperse, submicrometer silica spheres modified with 3-aminopropyltrimethoxysilane followed by the electrothermal atomic absorption spectrometry (ETAAS) was developed and applied to quantify the total dissolved metal concentrations in sea water. Quantitative sorption of Cd, Cu, Ni and Pb was achieved in the pH range 7.5–8, for 30 min, adsorbed elements were easily eluted with 2 mL 2 mol L−1 HNO3. Since the optimal pH for quantitative sorption coincides with typical pH of Black Sea water (7.9–8.2), on-site pre-concentration of the analytes without any additional treatment was possible. Detection limits achieved for total dissolved metal quantification were: Cd 0.002 µg L−1, Cu 0.005 µg L−1, Ni 0.03 µg L−1, Pb 0.02 µg L−1 and relative standard deviations varied from 5–13% for all studied elements (for typical Cd, Cu, Ni and Pb concentrations in Black Sea water). Open pore diffusive gradients in thin films (DGT) technique was employed for in-situ sampling and pre-concentration of the sea water and in combination with ETAAS was used to determine the proportion of dynamic (mobile and kinetically labile) species of Cd(II), Cu(II), Ni(II) and Pb(II) in the sea water. Obtained results showed strong complexation for Cu and Pb with sea water dissolved organic matter. The ratios between DGT-labile and total dissolved concentrations found for Cu(II) and Pb(II) were in the range 0.2–0.4. For Cd and Ni, these ratios varied from 0.6 to 0.8, suggesting higher degree of free and kinetically labile species of these metals in sea water.

Determination of labile inorganic and organic species of Al and Cu in river waters using the diffusive gradients in thin films technique

The diffusive gradients in thin films (DGT) technique, using a diffusive gel or a restrictive gel, was evaluated for the determination of labile inorganic and organic species of Al and Cu in model synthetic solutions and river water samples. Experiments were performed both in situ and in the laboratory. In the solutions containing Al ions, the major labile fraction consisted of inorganic species. The organic complex fractions were mainly kinetically inert. For the model Cu solutions, the most labile fraction consisted of inorganic species; however, significant amounts of labile organic complexes of Cu were also present. A comparison was made between the results obtained using restrictive gel DGT and tangential flow ultrafiltration (TF-UF). The Cu fraction determined by restrictive gel DGT (corresponding to the "free" ions plus the labile fraction of small molecular size complexes) was larger than that determined by TF-UF (corresponding to all small molecular size ions), suggesting that the techniques exhibited different porosities for discrimination of inorganic species. For the river water samples analyzed in the laboratory, less than 45% of the analytes were present in labile forms, with most being organic species. For the in situ measurements, the labile inorganic and organic fractions were larger than those obtained in the laboratory analyses. These differences could have been due to errors incurred during sample collection and storage. All results were consistent with those found using two different methods, namely, solid-phase extraction and the DGT technique employing the apparent diffusion coefficient.

New resin gel for uranium determination by diffusive gradient in thin films technique

A new resin gel based on Spheron-Oxin ® chelating ion-exchanger with anchored 8-hydroxyquinoline functional groups was tested for application in diffusive gradient in thin film technique (DGT) for determination of uranium. Selectivity of uranium uptake from model carbonate loaded solutions of natural water was studied under laboratory conditions and compared with selectivity of the conventional Chelex 100 based resin gel. The affinity of Spheron-Oxin ® functional groups enables determination of the overall uranium concentration in water containing carbonates up to the concentration level of 10 2 mg L −1. The effect of uranium binding to the polyacrylamide (APA) and agarose diffusive gels (AGE) was also studied. Uranium is probably bound in both gels by a weak interaction with traces of acrylic acid groups in the structure of APA gel and with pyruvic and sulfonic acid groups in the AGE gel. These sorption effects can be eliminated to the negligible level by prolonged deployment of DGT probes or by disassembling probes after the 1–2 days post-sampling period that is sufficient for release of uranium from diffusive gel and its sorption in resin gel.

Measurement of Dissolved Reactive Phosphorus Using the Diffusive Gradients in Thin Films Technique with a High-Capacity Binding Phase

Measurement of dissolved reactive phosphorus (DRP) by the diffusive gradients in thin films (DGT) technique was investigated using a new binding phase. Half-dried amorphous zirconium oxide (with 50 ± 5% of water content) was mixed with acrylamide solution for the preparation of the new binding phase. The resulting binding gel had a high binding capacity (223 μg P cm(-2)) for phosphate. The solution of NaOH (1 M) was used for elution of phosphate from the gel, and an elution efficiency of 0.95 was obtained. A test of DGT uptake with this gel showed its dependence on temperature, and there was no influence of pH (3 to 10) and ionic strength (10 nM to 0.1 M). Its capacity for DGT response exceeded 100 μg P cm(-2), corresponding to a DRP concentration of more than 20 mg L(-1) for a 24 h deployment with a standard DGT device at 25 °C, which was at least 50 times of the Fe-oxide gel commonly used in the present DGT technique. Measurements with this high-capacity DGT technique in a laboratory microcosm of homogeneously mixed sediments gave smooth and reproducible mass-depth profiles. This technique was well demonstrated by in situ measurements in algal- and macrophyte-dominated regions of Lake Taihu. The DGT-measured concentrations of DRP were on average 20% and 40% of the DRP concentrations in pore waters, respectively, indicating a partial resupply of the sediments to the pore waters with DRP.

Characterization of sorption gels used for determination of mercury in aquatic environment by diffusive gradients in thin films technique

Abstract The influence of pH, ionic strength and selected natural ligands on the measurement of mercury by the diffusive gradients in thin films technique (DGT) using recently introduced sorption gels was determined. Sorption gels containing Chelex 100, Spheron-Thiol, Duolite GT73 and modified Iontosorb AV resins were investigated, with the sorption capacity determined for all used sorption gels. The minimum DGT measurable concentrations were calculated from 3 times the standard deviation of mercury amount in unexposed sorption gels.

The Association of Cobalt with Iron and Manganese (Oxyhydr)oxides in Marine Sediment

Formation and dissolution of authigenic Fe and Mn (oxyhydr)oxides influence cycling of trace metals in oxic/suboxic surface sediments. We used the diffusive gradients in thin films technique (DGT) to estimate the association of cobalt with iron and manganese oxides. We compared Co, Fe and Mn maxima measured by DGT in the pore waters of fresh and aged marine sediment cores and estimated the Co/Fe and Co/Mn ratios in the metal oxides. A Mn maximum was not visible in DGT concentration profiles of freshly collected sediment cores, but after ageing the sediment, we observed a distinct Mn peak, presumably due to broadening of the depth range over which the various electron acceptors occur. Estimated Co/Mn ratios from both experiments are within the range of literature values for marine sediments, but the value from the aged experiment is at the lower end of the range. This is attributed to stimulation of sulphate reduction and precipitation of cobalt sulphides. The good correlation between Co and Fe maxima in the fresh sediments is attributed to the similarity of their reactions with sulphide rather than Co being released during authigenic Fe oxide reduction.

Copper uptake kinetics in hydroponically-grown durum wheat (Triticum turgidum durum L.) as compared with soil’s ability to supply copper

This study investigated (a) net Cu uptake kinetics in durum wheat (Triticum turgidum durum L.) exposed to free Cu2+ activities in solution ranging from 0.4 to 2,420 nM and (b) the relative importance of plant uptake and soil’s ability to supply Cu2+ to the roots. Plant Cu flux showed a hyperbolic shape, enabling to estimate the Michaelis–Menten kinetic parameters (Fmax and KM) for durum wheat. Plant Cu flux was then compared with soil Cu flux as assessed by the Diffusive Gradient in Thin film technique on seven soil samples. This comparison suggested that the rate-limiting process of Cu bioavailability to durum wheat would be plant uptake kinetics in most contaminated soils with the exception of moderately contaminated, calcareous soils. However, theoretical considerations targeted soil’s ability to supply Cu as the rate-limiting process in most soils for Cu (hyper-) accumulator plants with requirement larger than that of common crop species.

A model predicting waterborne cadmium bioaccumulation in Gammarus pulex: The effects of dissolved organic ligands, calcium, and temperature

Metal bioavailability depends on the presence of organic ligands in the water and on the concentrations of competitive cations. The present study aims at testing whether the diffusive gradient in thin films technique (DGT) could be used to take into account Cd speciation and its consequences on bioavailability in a bioaccumulation model and whether the influences of the Ca concentration and temperature also should be considered. Four kinetic experiments were conducted on Gammarus pulex: a calibration of Cd turnover rates and of the DGT lability in mineral water, a study of the influence f ethylenediaminetetraacetic acid (EDTA) and humic acids (HA) on uptake rates, and two experiments testing the influence of the Ca concentrations and temperature on Cd uptake clearance rates (ku). In mineral water, where Cd was considered fully labile, the ku was 0.46 L g⁻¹ d⁻¹, and the depuration rate was 0.032 d⁻¹. The initial Cd influxes were lowered significantly by additions of 10 μg L⁻¹ of EDTA or 10 mg L⁻¹ of HA in the water but not at 5 mg L⁻¹HA, even if DGT measurements proved that Cd formed Cd-HA complexes in that treatment. Increasing Ca concentrations lowered ku values, and a competitive inhibition model between Ca and Cd fitted the data. A 30% enhancement of k, values was observed when the temperature was increased by 8°C, which appeared comparatively as a weak effect. Thus, taking into account the metal speciation and the influence of the Ca concentration should improve Cd bioaccumulation modeling in amphipods. In freshwater, where metal bioavailability is reduced by the presence of dissolved organic matter, forecasting Cd waterborne uptake using the labile concentrations should allow robust comparisons between laboratory and field studies.

Metals pollution tracing in the sewerage network using the diffusive gradients in thin films technique

Diffusive Gradients in Thin-films (DGT) is a quantitative, passive monitoring technique that can be used to measure concentrations of trace species in situ in solutions. Its potential for tracing metals pollution in the sewer system has been investigated by placing the DGT devices into sewage pumping stations and into manholes, to measure the concentration of certain metals in the catchment of a sewage treatment works with a known metals problem. In addition the methodology and procedure of using the DGT technique in sewers was investigated. Parameters such as temperature and pH were measured to ensure they were within the limits required by the DGT devices, and the optimum deployment time was examined. It was found that although the results given by the DGT technique could not be considered to be fully quantitative, they could be used to identify locations that were showing an excess concentration of metals, and hence trace pollution back to its source. The DGT technique is 'user friendly' and requires no complicated equipment for deployment or collection, and minimal training for use. It is thought that this is the first time that the DGT technique has been used in situ in sewers for metals pollution tracing.

Measurement of Stable and Radioactive Cesium in Natural Waters by the Diffusive Gradients in Thin Films Technique with New Selective Binding Phases

A cesium-specific diffusive gradients in thin films (DGT) technique was developed using copper ferrocyanide (CFCN) as the binding agent. Two types of DGT binding phases were evaluated, one by immobilizing CFCN on Chelex 100 resin gels (Chelex-CFCN) and the other on poly(acrylic acid) gels (PAA-CFCN). Both DGT devices were successfully applied to the measurement of low levels of stable 133Cs and radioactive 137Cs in synthetic solutions and in natural river waters. In all cases, the DGT labile concentrations measured with the PAA-CFCN DGT agreed very well with total dissolved Cs concentrations, whereas those measured by the Chelex-CFCN DGT were much lower than total Cs concentrations. The difference was attributed to the different binding kinetics of Cs+ on the two gels suggesting that this might be a promising means of measuring biologically relevant Cs concentrations in natural waters.

Sodium polyacrylate as a binding agent in diffusive gradients in thin-films technique for the measurement of Cu 2+ and Cd 2+ in waters

An aqueous solution containing sodium polyacrylate (PA, 0.0030 M) was used in diffusive gradients in thin-films technique (DGT) to measure DGT-labile Cu 2+ and Cd 2+ concentrations. The DGT devices (PA DGT) were validated in four types of solutions, including synthetic river waters containing metal ions with or without complexing EDTA, natural river water (Hun River, Shenyang, China) spiked with Cu 2+ and Cd 2+, and an industrial wastewater (Shenyang, China). Results showed that only free metal ions were measured by PA DGT, recovery = 98.79% for Cu 2+ and recovery = 97.80% for Cd 2+ in solutions containing only free metal ions, recovery = 51.02% for Cu 2+ and recovery = 51.92% for Cd 2+ in solution with metal/EDTA molar ratio of 2:1 and recovery = 0 in solutions with metal/EDTA molar ratio of 1:1 and 1:2. These indicated that the complexes of Cu–EDTA and Cd–EDTA were DGT-inert or not DGT-labile. The DGT performance in spiked river water (recovery = 8.47% for Cu 2+ and recovery = 27.48% for Cd 2+) and in industrial wastewater (recovery = 14.16% for Cd 2+) were also investigated. Conditional stability constants (log K) of PA–Cu and PA–Cd complexes were determined as 6.98 and 5.61, respectively, indicating strong interaction between PA and the metals.

Methylmercury speciation in the dissolved phase of a stratified lake using the diffusive gradient in thin film technique

The diffusive gradient in thin film (DGT) technique was successfully used to monitor methylmercury (MeHg) speciation in the dissolved phase of a stratified boreal lake, Lake 658 of the Experimental Lakes Area (ELA) in Ontario, Canada. Water samples were conventionally analysed for MeHg, sulfides, and dissolved organic matter (DOM). MeHg accumulated by DGT devices was compared to MeHg concentration measured conventionally in water samples to establish MeHg speciation. In the epilimnion, MeHg was almost entirely bound to DOM. In the top of the hypolimnion an additional labile fraction was identified, and at the bottom of the lake a significant fraction of MeHg was potentially associated to colloidal material. As part of the METAALICUS project, isotope enriched inorganic mercury was applied to Lake 658 and its watershed for several years to establish the relationship between atmospheric Hg deposition and Hg in fish. Little or no difference in MeHg speciation in the dissolved phase was detected between ambient and spike MeHg.

Effects of Binding of Metals to the Hydrogel and Filter Membrane on the Accuracy of the Diffusive Gradients in Thin Films Technique

The effect of metal binding to the diffusion layer of diffusive gradients in thin films (DGT) on measurements of time-integrated concentrations (CDGT) was studied experimentally and using a numerical model. Cationic divalent Cu and Cu-fulvic complexes bind to sites in the filter membrane and polyacrylamide (APA) gel, together comprising the diffusion layer of DGT. Less Cu was found in Chelex gels retrieved immediately after DGT deployment than in Chelex gels retrieved after storage of deployed DGTs. The difference was attributed to postdeployment transport of Cu from the diffusion layer to the Chelex gel. Binding of Cu in the diffusion layer delays the formation of a steady concentration gradient between solution and Chelex gel and can, depending on diffusion layer thickness, speciation, and concentration of metal, significantly bias the measurement of CDGT for short deployment times. However, C(DGT) will converge with the concentration in solution provided that the deployment time is sufficiently long (for a 1.0 mm diffusion layer, a diffusion coefficient of 5.5 x 10(-6) cm2 s(-1) and a concentration of Cu in solution of 0.1 nM, the difference is less than 11% and 5% for deployment times longer than 32 and 64 h, respectively). Minimal bias is encountered when measuring Cu contaminated waters (> or = 100 nM) using DGTs with thinner (< or = 0.5 mm) diffusion layer thicknesses. Diffusion layer binding of cationic Cd in the presence of cationic Cu was insignificant, and C(DGT) of Cd was found to converge with the concentration in solution after deployment times of 4 h and less. These findings have implications for the maximum temporal resolution that can be achieved with DGT.