Mercury depth profiles in river and marine sediments measured by the diffusive gradients in thin films technique with two different specific resins

The diffusive gradients in thin films technique (DGT) was used to measure depth profiles of mercury in river and marine sediments in situ to a spatial resolution of 0.5 cm. Agarose gel was used as the diffusive gel in the DGT probes. Two different selective resins—Chelex 100 with iminodiacetic groups and Spheron-Thiol with thiol groups incorporated in the polyacrylamide resin gel—were tested. The different capture efficiencies of the two adsorbents enabled the fractions of mercury bound in different species in sediment pore water to be estimated. Mercury concentrations obtained by DGT with Spheron-Thiol resin were very similar to those obtained after centrifugation. This indicates that DGT with Sheron-Thiol resin reports on total dissolved mercury levels. The concentration of mercury measured by DGT with Chelex-100 resin was much lower (by a factor of 5–20) for the same sediment samples. Chelex-100 does not have such a high affinity to mercury as Spheron-Thiol, and so it only reports on the content of labile mercury species, such as inorganic ions and weak complexes. The content of labile mercury species in the river sediment was approximately 20% of the total dissolved mercury in pore water, whereas in marine sediment only 7% of the mercury was present as labile species.     

Speciation measurements of uranium in alkaline waters using diffusive gradients in thin films technique

This work investigated the application of diffusive gradients in thin films technique (DGT) to uranium speciation measurements in natural water. Two binding phases were examined, a commercially available affinity membrane, Whatman DE 81 (DE 81), with amino binding functional groups and the conventionally used Chelex 100 beads imbedded polyacrylamide hydrogel (Chelex) with iminodiacetate functional groups. The DGT devices assembled with the binding phases of DE 81 (DE 81 DGT) and Chelex gel (Chelex DGT) were tested both in synthetic river water solutions and in local river water. DE 81 DGT and Chelex DGT measured 80% and 75% of the total uranium in synthetic river water solution, respectively, and measured 73% and 60% of the total uranium in St. Lawrence River, Canada, respectively. The binding properties of the DE 81 membrane and Chelex gel for uranium, and the diffusion of uranyl complexes in the polyacrylamide gel (PAM) were also studied.     

Development of the diffusive gradient in thin films technique for the measurement of labile mercury species in waters

The diffusive gradients in thin films (DGT) technique, utilizing resin gel with ion-exchange resin Duolite GT73 and new ion-exchange resin Ambersep GT74, was investigated for the accumulation of four mercury species (Hg²⁺, CH₃Hg⁺, C₂H₅Hg⁺, C₆H₅Hg⁺). The diffusion coefficients of mercury species in agarose gel calculated on the basis of Fick’s Law were mercury species-specific. The diffusion coefficients of Hg²⁺ and CH₃Hg⁺ at 25 °C (9.07 ± 0.23 × 10⁻⁶ cm² s⁻¹ and 9.06 ± 0.30 × 10⁻⁶ cm² s⁻¹, respectively) were very similar, but the diffusion coefficients of C₂H₅Hg⁺ (6.87 ± 0.23 × 10⁻⁶ cm² s⁻¹) and C₆H₅Hg⁺ (3.86 ± 0.19 × 10⁻⁶ cm² s⁻¹) were significantly lower. Influence of experimental conditions (pH, selected cations, chlorides and humic substance) on mercury species accumulation by DGT was studied. The DGT technique was applied to river water spiked with mercury species.     

Application of the diffusive gradients in thin films technique for available potassium measurement in agricultural soils: Effects of competing cations on potassium uptake by the resin gel

The utilization of Amberlite (IRP-69 ion-exchange resin, 100–500 wet mesh) as the binding phase in the diffusive gradients in thin films (DGT) technique has shown potential to improve the assessment of plant-available K in soils. The binding phase has recently been optimized by using a mixed Amberlite and ferrihydrite (MAF) gel which results in linear K uptake over extended deployment periods and in solutions with higher K concentrations. As restriction of K uptake by Ca on the Amberlite based resin gel has been previously proposed, potential competing effects of Ca²⁺, Mg²⁺ and NH₄⁺ on K uptake by the MAF gel were investigated. These cations had no effect on K elution efficiency which was 85%. However, K uptake by the MAF gel was restricted in the presence of competing cations in solution. Consequently, the diffusion coefficient of K decreased in the presence of cations compared to previous studies but was stable at 1.12 × 10⁻⁵ cm² s⁻¹ at 25 °C regardless of cation concentrations. Uptake of K by the DGT device was affected by the presence of excessive Ca in more than 30% of twenty typical Australian agricultural soils. However, this problem could be circumvented by using a shorter deployment time than the normal 24 h. Moderate correlation of concentrations of K extracted by DGT with Colwell K (extracted by NaHCO₃, R² = 0.69) and NH₄OAc K (R² = 0.61) indicates that DGT measures a different pool of K in soils than that measured by the standard extractants used. In addition, the MAF gel has the ability to measure Ca and Mg simultaneously.     

Diffusion characteristics of agarose hydrogel used in diffusive gradients in thin films for measurements of cations and anions

The agarose hydrogel has been increasingly used as a diffusive layer in diffusive gradients in thin films (DGT) measurements. However its diffusive characteristics have not been examined in detail. In this study, the performance of agarose gel was tested in DGT measurements of eight cations (Fe(II), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), and Cd(II)) and eight anions (P(V), As(V), Cr(VI), Mo(VI), Sb(V), Se(VI), V(V), and W(VI)). It was found that the thickness of agarose, a key parameter in the calculation of DGT measured concentration, remained unchanged after hydration followed by storage under the following conditions: pH 2–11, ionic strength 0–1.0 M, temperature 4–40 °C, and with the storage time extending to 300 d. Enrichment of cations and repelling of anions were observed in the gel under the ionic strengths of < 2–3 mM and <1 mM (NaNO₃), respectively, which was attributed to the electrostatic interactions of these ions with the fixed negatively charged groups (mainly pyruvate) in the gel. The diffusion coefficients of cations and anions through the agarose gel (plus a PVDF filter membrane) were on average 1.10 ± 0.04 times of the reported diffusion coefficients through the agarose cross-linked polyacrylamide (APA) hydrogel, typically used in DGT technique. The working pH ranges for the agarose gel-assembled DGTs were 4–10 and 5–9 for anions and cations, respectively. The use of agarose gel, either individually or along with different filter membranes, affected the overall diffusion rates of cations and anions. The measured DGT concentrations of cations and anions in filtered natural freshwater and seawater were mostly in line with those measured directly. The results showed that the agarose gel can be used as one of the standard diffusive layers in DGT measurements for a wide range of inorganic and organic analytes.     

Use of the diffusive gradients in thin films technique to evaluate (bio)available trace metal concentrations in river water

Concentrations of Cd, Cu, Cr, Pb, Ni and Zn were monitored in the Svitava River (the Czech Republic) during April and September 2005. Total concentrations and total dissolved concentrations were obtained through regular water sampling, and the diffusive gradients in thin films technique (DGT) were used to gain information on the kinetically labile metal concentrations. Each measured concentration was compared with the corresponding average (bio)available concentration calculated from the mass of metal accumulated by the moss species Fontinalis antipyretica. The concentrations of Cd, Pb, Cr and Zn measured using DGT corresponded well with those obtained after the deployment of Fontinalis antipyretica moss bags in the Svitava River, but the concentrations of Cu and Ni did not. The calculated (bio)available Cu concentration correlated well with the total dissolved concentration of Cu, whereas no correlation was found to exist between the concentrations of Ni. Scheme of the Svitava River monitoring station, including the DGT sampling units and Fontinalis antipyretica moss bags Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Preparation of guanidinylated carboxymethyl chitosan and its application in the diffusive gradients in thin films (DGT) technique for measuring labile trace metals in water

ABSTRACT

Guanidinylated carboxymethyl chitosan (GCMCS) was prepared via the guanidinylation of carboxymethyl chitosan (CMCS). A device employing the diffusive gradients for thin films (DGT) technique was made using a GCMCS aqueous solution as the binding agent and a cellulose acetate dialysis membrane (CADM) as the diffusion phase to measure labile Cu²⁺, Pb²⁺ and Cd²⁺ in water. The percentage uptake (U%) values of labile Cu²⁺, Pb²⁺ and Cd²⁺ in a synthetic water sample were almost consistent with the theoretical values at 101.6 ± 2.8%, 104.6 ± 6.1% and 95.9 ± 4.4%, respectively. The optimum pH ranges for the measurement of labile Cu²⁺, Pb²⁺ and Cd²⁺ were 3.0–7.0, 3.0–7.0 and 4.0–8.0, respectively. The ionic strength mainly affected the diffusion of metal ions in the CADM. The diffusion rates decreased with increasing concentrations of NaNO₃ solutions. The application of GCMCS-DGT in natural water and industrial wastewater showed that dissolved organic carbon (DOC) only affects metal species, and the accurate determination of labile Cu²⁺, Pb²⁺ and Cd²⁺ can be achieved when the diffusion coefficients of these metal ions in the diffusion phase have been determined. GCMCS is suitable for DGT application as a chelating agent for metal ions.     

Application of a cellulose phosphate ion exchange membrane as a binding phase in the diffusive gradients in thin films technique for measurement of trace metals

The technique of diffusive gradients in thin films (DGT) is a newly developed analytical technique capable of measuring in situ concentrations of trace metals in the environment. The technique employs a thin film diffusive hydrogel (with well-defined diffusion properties) in contact with a binding phase capable of binding metal ions of interest. In this work, we demonstrate, for the first time, the use of a commercially available solid ion exchange membrane (Whatman P81) as the binding phase in DGT analysis. The cellulose phosphate-based Whatman P81 membrane is a strong cation exchange membrane. Its performance characteristics as a new binding phase in DGT measurement of Cu²⁺ and Cd²⁺ were systematically investigated. Several advantages over the conventional ion exchange resin-embedded hydrogel binding phases used in DGT were observed including simple preparation, ease of handling, and reusability. The binding capacities of the material to various metal ions were examined both collectively and individually. The binding phase preferentially binds to transition metal ions rather than matrix ions such as potassium, sodium, calcium and magnesium, which are competitive species in natural waters. Within the optimum pH range (pH 4.0-9.0), the maximum non-competitive binding capacities of the membrane for Cu²⁺ and Cd²⁺ were 3.22 and 3.07 μmol cm⁻², respectively. The suitability of the new membrane-based binding phase for DGT applications was validated experimentally. The experimental results demonstrated excellent agreement with theoretically predicted trends. The measurement was not degraded after four consecutive reuses of the cellulose phosphate binding phase.     

Use of the diffusive gradients in thin films technique (DGT) with various diffusive gels for characterization of sewage sludge-contaminated soils

The diffusive gradients in thin film technique (DGT) was used for characterization of South Moravian arable soils (sampling sites Zlín, Tuřany, and Chrlice) amended by sewage sludge in the 1980s. Two types of polyacrylamide diffusive gel with different pore size (APA gels—cross-linked with agarose and RG gels—cross-linked with bis-acrylamide) were employed. The (bio)available parts of Cd, Cu, and Ni and the proportions of inorganically and organically complexed species of these metals were assessed. The degree of metal resupply from the soil solid phase to the soil solution was also determined. Metal concentrations obtained by the DGT technique were lower by almost 4 to 5 orders of magnitude in comparison with those obtained by extraction with aqua regia. DGT concentrations of metals were also lower by approximately 1 to 2 orders of magnitude in comparison with those obtained by extraction with sodium nitrate (commonly used for assessment of the (bio)available part of metals). Results obtained by DGT measurement were expected to be closer to the actual content of available metal species than results obtained by extraction with sodium nitrate. Using RG gels together with APA gels provided resolution of inorganically and organically complexed metal species and their proportional representation. Inorganic metal species (particles smaller than 1 nm) formed a predominant part of assessed metal content in all studied soil samples and horizons. However, there was the exception of the cadmium content in the middle profile of Chrlice sandy soil sample. Ratio R values indicated that resupply of Cd, Cu, and Ni from the solid phase to the soil solution varied for individual soil samples and individual depth profiles. Mobile and labile species of Cd, Cu, and Ni were much more closely related to upper rather than deeper horizons. This observation correlates very well with the mechanical treatment and amendment of the studied soils.     

Effect of competitive cation binding on the measurement of Mn in marine waters and sediments by diffusive gradients in thin films

The possible adverse effect of competitive binding on DGT (diffusive gradients in thin films) measurements of metals in marine situations was investigated. Of the divalent transition metals, manganese binds most weakly to Chelex resin and is most likely to be affected by competition. In media resembling seawater, the Chelex within DGT devices becomes saturated with Ca and Mg within 2 h, and at pH 5–6 the sensitivity of the DGT measurement for Mn is substantially reduced, due to the appreciable concentration of Mn in contact with the resin. For media resembling freshwater DGT gives a near theoretical response for Mn. Iron was shown to be capable of displacing Mn and to a more limited extent Cd from the resin when its capacity for Fe is approached. Vertical profiles of Mn in a mesocosm sediment, obtained by deploying DGT for different times, could be explained by this displacement effect. The problem only occurs when Fe concentrations are exceptionally high and can be avoided by using short deployment times, typically less than 12 h. Whilst most trace metals can be simply measured by deploying DGT in marine systems, for Mn consideration should be given to possible effects associated with the capacity of the Chelex binding layer being approached by accumulation of the other cations present.     

Performance of the diffusive gradients in thin films technique for measuring Ca and Mg in freshwater

Measurements of the major cations Ca and Mg by the technique of diffusive gradients in thin films (DGTs) were systematically evaluated. The concentration in solution was calculated using Fick’s first law of diffusion from the directly measured flux to the DGT device. A selective cation exchange resin (Bio-Rad Chelex^®100), which has been used extensively with DGT for trace metals, such as Cd²⁺, Cu²⁺ and Ni²⁺, was used for this work.

Elution of Ca and Mg from the resin with 1 M HNO₃ was very reproducible. Measurements of Ca and Mg concentrations in synthetic solutions agreed well with the theoretical predictions. The negative response on uptake caused by lowered pH was investigated. Uptake was found to decline below pH 5. The capacity of the DGT device for Ca and Mg was also investigated to establish maximum deployment times for given concentrations.

Experiments with filtered and modified lake water show that DGT can be used to measure Ca and Mg when trace metals are present in the solution. An in situ deployment of DGT combined with an ultrafiltration study suggest that the Mg concentration measured by DGT is similar to the concentration found in the fraction <1 kDa.     

Diffusive gradients in thin films technique for uranium measurements in river water

The diffusive gradients in thin films technique (DGT) was used for uranium measurements in water. DGT devices with Dowex resin binding phase (Dow DGT) were tested in synthetic river water, which gave 84% response to total uranium concentration. The devices were also deployed in natural river water and compared to devices with other types of binding phases, Chelex 100 resin beads imbedded in polyacrylamide hydrogel (Chelex DGT) and DE 81 anion exchange membrane (DE DGT), deployed in the same location at the same time. The measurement by Dow DGT was the lowest among the different types of the DGT devices, 45% of total uranium, while measurement by DE DGT was the highest, 98% of total uranium. The results achieved by the three types of DGT devices were explained by three DGT working mechanisms, equilibrium between complexes of resin/uranyl carbonates and complexes of resin/competitive ligands in water, effective reduction of uranyl carbonate concentration by the binding phase and dissociation of UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻ within the diffusive layer in a DGT device. It is hoped that by deploying the DGT devices with different binding phases in natural waters, additional information on uranium speciation could be obtained.     

Assessment of trace metal binding kinetics in the resin phase of diffusive gradients in thin films

The dynamic technique of diffusive gradients in thin films (DGT), that measures metal speciation in situ, has found wide environmental application. Simple interpretation of the metal accumulation in terms of a solution concentration has assumed that trace metals do not penetrate beyond the surface of the binding layer, but penetration, although theoretically discussed has not yet been directly measured. Multiple binding layers were used to enable analysis of different depths of a DGT binding phase (Chelex-100 or iminodiacetate resins). In simple metal solution (no ligand) at pH 7, metal penetration to the back layer was low and similar for all metals. However, at lower pH up to 42% of an individual metal accumulated in the back resin layer. This was most noticeable for Mn at pH 4 and 5, but Cd and Co were also affected at pH 4. These results were consistent with rate limited binding, particularly for Mn. A kinetic model successfully fitted the data and allowed derivation of a binding rate constant and the mean distance that metals penetrate into a resin gel (λ_M). Only for Mn, Co and Cd were experimentally derived λ_M values greater than the diameter of a Chelex-100 resin bead. For most situations, then, the penetration into the binding layer is negligible and binding of trace metal ions can be regarded as instantaneous, validating the simple use and interpretation of DGT. For weakly binding metals at low pH the slower binding allows penetration, which may affect the DGT measurement.     

Performance characteristics of suspended particulate reagent-iminodiacetate as a binding agent for diffusive gradients in thin films

The performance characteristics of an alternative binding agent, suspended particulate reagent-iminodiacetate (SPR-IDA), for use with DGT methodology were investigated. The parameters investigated during this study included gel hydration, blank levels, elution factor (f_e), capacity, the effects of pH on the binding of trace metals by DGT. The novel application of this resin for use as a quantitative standard for laser ablation ICP-MS was also evaluated. To further constrain the results for the SPR-IDA binding agent, parallel experiments were performed using resin gel containing Chelex 100, which has been widely reported in the literature. Hydration results showed that the SPR-IDA resin gel reached a stable dimension and weight within ∼30 min and was dimensionally stable for ≤6 months. The measured DGT blanks for the SPR-IDA resin were 0.0023, 0.15, 0.21, 0.0033 and 0.011 ng disc⁻¹ for Co, Ni, Cu, Cd and Pb, respectively. The elution factor differed for the two resin types with the Chelex 100 recoveries slightly lower than previous reports and the SPR-IDA resin showing on average ∼5-9% better recoveries than DGT containing Chelex 100. The measured capacity of DGT discs containing the SPR-IDA binding agent was 0.26 mg Cd, similar to the calculated value of 0.29 mg Cd, indicating the entire resin layer was available for metal uptake.Both resin types performed equally well when deployed in 1 mM NaNO₃ solutions with DGT measurements of ∼100% of direct solution measurements for Co, Ni and Cd. However, DGT measurements of Cu and Pb systematically decreased with increasing solution pH down to ∼50% of solution values at pH 8.0, due to artifacts resulting from colloid formation during the addition of the metals. This was remedied by adding the metals as dilute salt standards and addition of Mg(NO₃)₂ to eliminate adsorption to the container walls. In the latter experiments, DGT measured concentrations of Co, Ni, Cu, Cd and Pb were in agreement with solution concentrations. Deployment of DGT in solutions with increasing concentrations of trace metals yielded linear results, suggesting that quantitative analysis using simplified laser ablation techniques should be possible using this newly characterized SPR-IDA resin gel.     

Sampling 4-chlorophenol in water by DGT technique with molecularly imprinted polymer as binding agent and nylon membrane as diffusive layer

For the first time, a diffusive gradients in thin films (DGT) device using molecularly imprinted polymer (MIP) as the binding agent and nylon membrane (NM) as the diffusive layer (NM-MIP-DGT) has been developed for sampling 4-chlorophenol (4-CP) in water. The MIP was prepared by precipitation polymerization with methacrylic acid as monomer and ethyleneglycoldimethacrylate as cross-linker. The diffusion coefficient of 4-CP through NM was obtained to be 0.788 ± 0.040 μ cm² s⁻¹ by diffusion cell method. The ratio was 1.01 ± 0.05 (mean ± standard deviation) for the concentration of 4-CP sampled by NM-MIP-DGT and analyzed by HPLC method to the total concentration of 4-CP in the synthetic solution where free 4-CP species dominated. The results showed that NM-MIP-DGT could sample 4-CP in synthetic solution accurately. The performance of NM-MIP-DGT for sampling 4-CP was independent of pH in the range of 3–7 and ionic strength in the range of 0.0001–0.1 mol L⁻¹ NaCl solution. The concentration of free form of 4-CP sampled by NM-MIP-DGT decreased with the increasing concentration of dissolved organic carbon in different water samples due to the electrostatic interaction of natural organic compounds with 4-CP. 1.8 mg L⁻¹ of the free form of 4-CP was determined by HPLC which was sampled by NM-MIP-DGT in an intermediate untreated industrial effluent. The NM-MIP-DGT can be a potential passive tool for sampling the free form of 4-CP in water.     

Characterization of mercury species in soils by HPLC-ICP-MS and measurement of fraction removed by diffusive gradient in thin films

The properties and behaviour of Hg depend on both the oxidation state and the chemical form: the bioavailability, toxicity, persistence and accumulation of mercury in the food web are strongly influenced by chemical speciation. The present work aims to determine the chemical forms of mercury present in soil and to evaluate the fraction of mercury in soil solution available to plants. In order to do this, we analyzed eight samples of contaminated soils with Hg concentrations ranging from 1.31 to 21.7 mg kg⁻¹, collected from different depths (0–10 and 40–50 cm) close to an abandoned industrial site in Val Basento (southern Italy). Two innovative analytical techniques were used: HPLC–ICP-MS and diffusive gradient in thin films (DGT). The analytical procedure was validated using ERM 580-certified sediment and spiked samples in the case of HPLC–ICP-MS, and by a performance test in the case of DGT. In all samples, the only species found in soil and soil solution was MeHg⁺ and Hg²⁺. In soil, the MeHg⁺/Hg_tot ratio ranged from 0.05% to 0.82%; total mercury in soil solution was less than 0.01% of total mercury in soil. The percentage of MeHg⁺ in soil solution varied considerably (from 0% to 50%), with a maximum concentration of 0.02 mg L⁻¹. The root available concentration evaluated by DGT is comparable to the total mercury content of the soil solution measured by HPLC–ICP-MS. The DGT results suggest that all mercury in solution is available for uptake in DGT, and that mercury is supplied from soil to solution. However, for all samples the soluble and root available (DGT-labile) fractions of mercury are generally very low with respect to the total mercury concentration. This study confirmed that both HPLC–ICP-MS and DGT techniques are suitable tools for the estimation of Hg root availability and in assessing the risk from contaminated soils.     

Use of Saccharomyces cerevisiae immobilized in agarose gel as a binding agent for diffusive gradients in thin films

A new binding agent, consisting of the yeast Saccharomyces cerevisiae immobilized in agarose, is proposed for use in diffusive gradients in thin films (DGT). Different gel compositions, containing from 4.5% to 20% (m/v) of S. cerevisiae and 1.5-5.0% (m/v) of agarose, were prepared and tested for uptake of Cd(II). For gels containing 20% (m/v) of S. cerevisiae, a mass of 14,900 ng has been attributed as the uptake limit of Cd for each disk. Determination of the Cd retained in the binding agent was readily carried out using a slurry of the agarose-yeast disk introduced directly into the inductively coupled plasma optical emission spectrometer. The performance characteristics of the DGT samplers, which were assembled with the proposed binding agent (25 mm disk containing 20% of S. cerevisiae and 1.5% of agarose) and a diffusive layer of cellulose (chromatographic paper 3MM Chr of 25 mm diameter), were evaluated by measuring the Cd(II) uptake at various pH values and ionic strengths. Very consistent results were found within the pH range 4.5-7.5 and at ionic strengths ≥0.005 mol L⁻¹. The precision of DGT measurements was characterized by relative standard deviations of <8%. No changes in the uptake of Cd(II) were observed in the samplers that were assembled with recently prepared disks or 35-day-old stored disks. The proposed material has been applied to the analyses of river and sea water samples. For determination of Cd(II), excellent agreement between the results obtained from devices assembled with the proposed material and those assembled with conventional material (Chelex-100 resin) were obtained, strongly validating the use of the agarose-yeast gel disk as a new binding agent for DGT.     

Laboratory and field evaluation of diffusive gradient in thin films (DGT) for monitoring levels of dissolved mercury in natural river water

The diffusive gradient in the thin films (DGT) technique was tested to measure dissolved mercury (Hg) both in laboratory aqueous solutions and in situ in river water. For this purpose, a commercial ready-to-use and specific-for-Hg DGT device was used. Each sampler consisted of a filter membrane-agarose gel as the diffusive layer and a Spheron-Thiol resin in polyacrylamide gel as the binding agent. Basic performance assays at the laboratory with this type of DGT unit confirmed the applicability of Fick's first law for DGT measurements. The diffusion coefficient of MeHg in the agarose diffusive gel was 8.50?×?10^?6?cm² s^?1 at 25°C. Several field studies were also carried out in two different rivers of the Ebro River basin (NE Spain) affected by Hg wastes released by the chlor-alkali industry. Hg concentrations determined by DGT were generally much lower than the results obtained through direct measurements of the river water. In addition, the results of a time series experiment also performed in the field show that the amount of Hg accumulated in the resin does not increase at all with the exposure time. This may be explained by the underestimation of the truly dissolved Hg fraction due to the formation of a biofilm layer on the surface of the samplers, thus clogging the filter and preventing Hg species from diffusing through it. Consequently, it was demonstrated that the DGT technique presents important limitations for measuring Hg in polluted rivers characterised by a high biomass load (eutrophic), whereas its performance was demonstrated to be correct in oligotrophic waters.     

A method for measurement of free cadmium species in waters using diffusive gradients in thin films technique with an ion-imprinted sorbent

A diffusive gradients in thin films (DGT) device for the analysis of free Cd(II) species, based on Cd(II) ion-imprinted sorbent (IIS) as the binding agents and commercial polyethersulfone membrane (PES) as diffusion layer, was developed (PES/IIS-DGT). DGT time-series experiments showed that the mass of free Cd(II) species accumulated by PES/IIS-DGT was linear vs. time (R² = 0.9953) and the concentration of free Cd(II) species by PES/IIS-DGT was in good agreement with the total dissolved concentrations of free Cd(II) species in simple synthetic solutions where free ionic species dominated. PES/IIS-DGT performance was independent in the range of pH 4.5–7.5 and ionic strength range from 1.0 × 10⁻³ to 0.7 mol L⁻¹. The measurement of free Cd(II) species in synthetic solution containing different concentrations of ligands by PES/IIS-DGT showed an excellent agreement with the value measured by Cd(II) ion selective electrodes (Cd-ISE), indicating that PES/IIS-DGT method is more suitable than Cd-ISE for the measurement of low concentration of free Cd(II) species due to the enrichment of IIS for the analytes.