Use of diffusive gradients in thin films to measure cadmium speciation in solutions with synthetic and natural ligands: comparison with model predictions

The performance of the technique of diffusive gradients in thin films (DGT) was characterized in well-defined systems containing cadmium with chloride and nitrate ions, simple organic ligands (nitrilotriacetic acid and diglycolic acid), and Suwannee river fulvic acid for the pH range 5-8. Cd was fully labile in all Cd, Cl-, and NO3- solutions tested (I= 0.1 and 0.01 M), even atvery low Cd concentrations (10 nM), consistent with there being no binding of Cd to the diffusive gel. Diffusion coefficients of Cd-nitritotriacetic acid (NTA) and Cd-diglycolic acid (DGA) species were measured and found to be ca. 25-30% lower than the equivalent coefficient for free metal ions. These values were used to calculate concentrations of labile Cd from DGT measurements in solutions of Cd with NTA or DGA. Cd-NTA and Cd-DGA species were found to be fully DGT-labile. DGT devices that used a diffusive gel with a reduced pore size, which retarded the passage of fulvic acid species through the gel, were used to estimate the proportion of Cd complexed by fulvic acid. These results were compared with predictions of the solution speciation from models with default parameter values. ECOSAT, incorporating the NICA-Donnan model, correctly predicted the magnitude of the binding and its pH dependence, while predictions from WHAM V (with humic ion binding model V) and WHAM 6 (with humic ion binding model VI) were less satisfactory at predicting the pH dependence. Reasonable fits to the data could be obtained from WHAM 6 when the effective binding constant log K(MA) was changed from 1.6 to 1.5, the value of deltaLK1 from 2.8 to 1.0 to minimize the dependence on pH, and the value of deltaLK2 from 1.48 to 1.0 to decrease the strength of the strong bidentate and tridentate binding sites.     

Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters

Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following: (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME-VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE-SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT > or = GIME-VIP > or = FTPLM > or = HFPLM approximately = DMT (>CLE-SV for Cd), detected Zn decreased as DGT > or = GIME-VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME-VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling.     

In-situ speciation of Ni and Zn in freshwaters: comparison between DGT measurements and speciation models

The technique of DGT (diffusive gradients in thin films) was used for the first time to measure in situ the distribution of Zn and Ni between inorganic species and complexes with fulvic and humic acids in natural waters. With DGT, metals are bound to a resin embedded in a layer of hydrogel after diffusive transport through an adjacent layer of hydrogel. The metal concentrations in the waters can be quantified using simple diffusion equations. By using devices with hydrogels of different pore size, large and small complex species were discriminated. Inorganic species diffuse freely through all gels, but larger organic complexes with fulvic and humic acids diffuse less freely through more restricted gels (gels with smaller pore size). Systematic differences between DGT devices containing gels of different pore size were obtained. Their calibration for the diffusion of fulvic and humic complexes allowed calculation of the concentrations of labile inorganic (Zn, 34.6 +/- 2.5 nM; Ni, 23.5 +/- 0.9 nM) and labile organic (Zn, 43.1 +/- 2.9 nM; Ni, 11.2 +/- 0.7 nM) complexes. The concentration of Zn measured by anodic stripping voltammetry in samples returned to the laboratory lay between the DGT-measured inorganic concentration and the total dissolved concentration, consistent with partial measurement of organic complexes of Zn. The speciation model WHAM successfully predicted the species distribution of Ni, Zn, and Cu, provided that competitive binding by Fe(III) was considered. Using the speciation models WHAM and ECOSAT, free ion activities of Zn and Ni were calculated from (1) the total inorganic species measured by DGT and (2) the total dissolved species and dissolved organic carbon. The calculations confirmed the good model predictions of metal-humic binding but highlighted problems with default databases used for the speciation of inorganic components.     

Labile Cd complexes increase Cd availability to plants

Dissolved trace metals are present in the environment as free ions and as complexes. Commonly used models to predict metal bioavailability consider the free ion as the major bioavailable species. However, increases in metal availability in the presence of metal complexes have repeatedly been found. We measured the uptake of cadmium (Cd) by spinach (Spinacia oleracea) from solution in absence or presence of synthetic ligands. At the same free ion concentration, the uptake of Cd ranged over almost 3 orders of magnitude and was largest in treatments with fast dissociating (i.e. labile) complexes. Similar results were found for the diffusional fluxes in these solutions, as measured with the DGT technique. The observed effect of Cd complexes on the plant uptake was in agreement with model calculations in which plant uptake was assumed to be governed by the diffusional flux. These results strongly suggest that Cd uptake is rate-limited by diffusion of the free ion to the root surface, even in stirred solutions. As a result, dissolved Cd complexes can increase Cd uptake, resulting in apparent exceptions from the free ion activity model. The magnitude of this increase depends both on the concentration and on the lability of the complexes. The free ion concept should therefore be reconsidered when transport limitations of the metal ion to the uptake site prevail.     

A dynamic numerical model to characterize labile metal complexes collected with diffusion gradient in thin films devices

The speciation of metal species extracted by diffusion gradient in thin films (DGT) devices during experimental deployments in simple metal-ligand synthetic solutions was numerically modeled, analyzed at steady state, and simulated in the dynamic regime. The modeled speciation of two well-known complexes (Cu-citrate and Cu-EDTA) are in good agreement with experimental data obtained in NaNO3 solutions. For any metal complex, the rate at which the metal accumulates on the chelating resin is proportional to the concentration of free metal in solution plus a fraction of the metal complex concentration in solution equivalent to xiD(ML)/D(M). D(M) and D(ML) are the diffusion coefficients of the free and metal complex, respectively, and xi characterizes the complex lability: it is defined as the fraction of metal complex lost when diffusing from the bulk solution to the chelating resin. Numerical simulations were used to explore the variations of xi as a function of several operational and chemical parameters. Lability increases when the dissociation rate constant or the residence time of the metal complex within the hydrogel increase (i.e., by either increasing the thickness of the hydrogel or decreasing D(ML)). Overall, the fraction of metal complex extracted by the DGT decreases when D(ML) decreases, which confirms the ability of diffusion-restrictive hydrogels to separate the free metal fraction from complex solutions. Although the DGT model does not exactly comply with the voltammetric formalism, the lability criteria deltatau(1/2) used with voltammetric macroelectrodes is similar to the lability criteria xi, except that it is valid for high metal-ligand ratio. To determine free metal concentrations in aquatic systems using DGT, it is necessary to account for the presence of numerous ligands with different complexing properties. Soon, the numerical model will be improved to include additional ligands competing for the metal, and new experiments will be designed to discriminate between different labile complexes. Experimental results obtained with different DGT devices (of different hydrogel thicknesses, for example) could be interpreted in terms of conditional kinetic characteristics of the dominant metal-ligand complex in solution.     

Interpretation of in situ speciation measurements of inorganic and organically complexed trace metals in freshwater by DGT

The dynamic speciation technique, diffusive gradients in thin-films (DGT), has been used in freshwater to determine simultaneously, from a single set of in situ measurements, (1) the equilibrium distribution of metal ions between simple inorganic complexes and larger organic complexes and (2) information on the rates of dissociation of these complexes. DGT devices with different diffusion layer thicknesses (0.3, 0.54, 1.34, and 2.14 mm) were used to estimate the in situ dissociation kinetics. Information on the species distribution was obtained by using two types of gel, which allow relatively free (polyacrylamide, APA) and more retarded (restricted, RES) diffusion of the metal complexes. The full theoretical basis of the technique is developed and applied to in situ measurements of Mn, Fe, Co, Ni, Cu, Cd, and Pb in a pristine river (Wyre, U.K.), with high DOC(15mg L(-1)), assuming that organic complexes are dominated by fulvic acid. These first DGT measurements that do not rely on assumptions about complex lability or the distribution of species, are compared to total dissolved measurements, previously reported speciation calculations and measurements using alternative speciation techniques. Examination of calculation consistency suggests that the effective mean diffusion coefficients of metal complexes with organic matter under in situ conditions may be larger than those measured in the laboratory using extracted fulvic acid.     

Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil

The complexation of heavy metals with dissolved organic matter (DOM) in the environment influences the solubility and mobility of these metals. In this paper, we measured the complexation of Cu, Cd, Zn, Ni, and Pb with DOM in the soil solution at pH 3.7-6.1 using a Donnan membrane technique. The results show that the DOM-complexed species is generally more significant for Cu and Pb than for Cd, Zn, and Ni. The ability of two advanced models for ion binding to humic substances, e.g., model VI and NICA-Donnan, in the simulation of metal binding to natural DOM was assessed by comparing the model predictions with the measurements. Using the default parameters of fulvic and humic acid, the predicted concentrations of free metal ions from the solution speciation calculation using the two models are mostly within 1 order of magnitude difference from the measured concentrations, except for Ni and Pb in a few samples. Furthermore, the solid-solution partitioning of the metals was simulated using a multisurface model, in which metal binding to soil organic matter, dissolved organic matter, clay, and iron hydroxides was accounted for using adsorption and cation exchange models (NICA-Donnan, Donnan, DDL, CD-MUSIC). The model estimation of the dissolved concentration of the metals is mostly within 1 order of magnitude difference from those measured except for Ni in some samples and Pb. The solubility of the metals depends mainly on the metal loading over soil sorbents, pH, and the concentration of inorganic ligands and DOM in the soil solution.     

Evaluating the performance of diffusive gradients in thin films for predicting ni sediment toxicity

Diffusive gradients in thin films (DGTs) rapidly measure labile fractions of metal and are promoted as an assessment tool for bioavailability. Using macroinvertebrate community composition as a response, this study compared the predictive ability of DGT-measured Ni with acid volatile sulfide (AVS) and organic carbon (OC) corrected Ni [(SEM(Ni)-AVS)/f(OC)] and total Ni concentrations. In two experiments, sediments were amended with Ni and placed within either a streamside mesocosm or deployed in situ. DGT-measured Ni concentrations (C(DGT)) increased with increasing total Ni, were greater at depth, and decreased over time. Relationships between Ni C(DGT) and sediment geochemistry indicated a shift in Ni partitioning from AVS-bound to Fe- and Mn-associated Ni. In both experiments, DGT-measured Ni poorly predicted the invertebrate response to metal, whereas models that included total Ni or (SEM(Ni)-AVS)/f(OC) effectively predicted the invertebrate response for the streamside mesocosm and in situ experiments, respectively. C(DGT) overestimated the available Ni fraction, possibly due to sampling either nonbioavailable solid-phase Ni or Ni irrespective of cations competing at the biotic ligand. We suggest that C(DGT) cannot replace (SEM(Ni)-AVS)/f(OC) for predicting invertebrate response to sediment Ni, and greater understanding of metal species lability to DGTs is needed before assuming equivalence between bioavailable and DGT-labile metals in sediments.     

Desorption kinetics of Cd, Zn, and Ni measured in soils by DGT

DGT (diffusive gradients in thin films) was used to measure the distribution and rates of exchange of Zn, Cd, and Ni between solid phase and solution in five different soils. Soil texture ranged from sandy loam to clay, pH ranged from 4.9 to 7.1, and organic carbon content ranged from 0.8% to 5.8%. DGT devices continuously remove metal to a Chelex gel layer after passage through a well-defined diffusion layer. The magnitude of the induced remobilization flux from the solid phase is related to the pool size of labile metal and the exchange kinetics between dissolved and sorbed metal. DGT devices were deployed over a series of times (4 h to 3 weeks), and the DIFS model (DGT induced fluxes in soils) was used to derive distribution coefficients for labile metal (Kdl) and the rate at which the soil system can supply metal from solid phase to solution, expressed as a response time. Response times for Zn and Cd were short generally (<8 min). They were so short in some soils (<1 min) that no distinction could be made between supply of metal being controlled by diffusion or the rate of release. Generally longer response times for Ni (5-20 min) were consistent with its slow desorption. The major factor influencing Kdl for Zn and Cd was pH, but association with humic substances in the solid phase also appeared to be important. The systematic decline, with increasing pH, in both the pool size of Ni available to the DGT device and the rate constant for its release is consistent with a part of the soil Ni pool being unavailable within a time scale of 1-20 min. This kinetic limitation is likely to limit the availability of Ni to plants.     

Trace metal speciation in brackish water using diffusive gradients in thin films and ultrafiltration: comparison of techniques

Diffusive gradients in thin films (DGT) and ultrafiltration were used to measure trace metal concentrations in the Baltic Sea. The results provide the first comparison of these two fundamentally different speciation methods for trace metals. Cd, Cu, Mn, Ni, and Zn were measured at two sites with different total trace metal concentrations. DGT units prepared with APA-gel as diffusive layer and Chelex 100 resin as binding agent were used throughout the study. The ultrafiltration was performed with Millipore Prep/Scale modules with cutoffs of 1 and 10 kDa. Concentration levels of Mn, Zn, and Cd measured by DGT agreed with the concentrations measured in 1 kDa ultrafiltered samples. For Cu and Ni the ultrafiltered concentrations exceeded the DGT-labile concentrations. The ability of DGT to preconcentrate metals was found to be an analytical advantage compared with ultrafiltration. DGT appears to be a good alternative to 1 kDa ultrafiltration for measurement of truly dissolved Mn, Cd, and Zn in the Baltic Sea.     

An evaluation of DGT performance using a dynamic numerical model

A numerical model of the transport and dynamics of metal complexes in the resin and gel layers of a DGT (diffusive gradients in thin films) device was developed and used to investigate how the chelating resin and metal-ligand complexes in solution affect metal uptake. Decreasing the stability constant or concentration of the binding resin increases the competition for free metal ions by ligands in solution, lowering the rate of mass uptake. Such effects would be rarely observed for moderately or strongly binding resins (K> 10(12)), including Chelex, which out-compete labile ligands in solution. With weakly binding resins, strongly bound solution complexes can diffuse into the resin layer before a measurable amount of dissociation occurs, such that concentrations of bound metal at the rear and front surfaces of the resin layer are equal. With more strongly binding resins, metal mainly binds to the front surface of the resin. Only complexes with the largest binding constants penetrate the gel layer containing Chelex, buttheir lack of lability means thatthe DGT sensitivity to the complex is, in any case, very low. The slow diffusion of complexes, such as those of fulvic acids, which increases the time required to establish steady state, compromises the use of the simple DGT equation. Errors are negligible for 24 h deployments, when diffusive layer thicknesses are less than 1 mm, but 3 day deployments are required to ensure accuracy with 2.4 mm thick layers. The extent to which the commonly used equation, that accounts for the concentration and diffusion of metal-complex species, overestimates DGT uptake if the rate of dissociation is slow, was estimated.     

Chemical speciation of Zn,Cd, Cu,and Pb in pore waters of agricultural and contaminated soils using Donnan dialysis

Knowledge of trace metal speciation in soil pore waters is important in addressing metal bioavailability and risk assessment of contaminated soils. Numerous analytical methods have been utilized for determining trace metal speciation in aqueous environmental matrixes; however, most of these methods suffer from significant interferences. The Donnan dialysis membrane technique minimizes these interferences and has been used in this study to determine free Zn2+, Cd2+, Cu2+, and Pb2+ activities in pore waters from 15 agricultural and 12 long-term contaminated soils. The soils vary widely in their origin, pH, organic carbon content, and total metal concentrations. Pore water pM2+ activities also covered a wide range and were controlled by soil pH and total metal concentrations. For the agricultural soils, most of the free metal activities were below detection limit, apart from Zn2+ for which the fraction of free Zn2+ in soluble Zn ranged from 2.3 to 87% (mean 43%). Five of the agricultural soils had detectable free Cd2+ with fractions of free metal ranging from 59 to 102% (mean 75%). For the contaminated soils with detectable free metal concentrations, the fraction of free metal as a percentage of soluble metal varied from 9.9 to 97% (mean 50%) for Zn2+, from 22 to 86% (mean 49%) for Cd2+, from 0.4 to 32.1% (mean 5%) for Cu2+, and from 2.9 to 48.8% (mean 20.1%) for Pb2+. For the contaminated soils, the equilibrium speciation programs GEOCHEM and WHAM Model VI provided reasonable estimates of free Zn2+ fractions in comparison to the measured fractions (R2 approximately 0.7), while estimates of free Cd2+ fractions were less agreeable (R2 approximately 0.5). The models generally predicted stronger binding of Cu2+ to DOC and hence lower fractions of free Cu2+ as compared with the observed fractions. The binding of Cu2+ and Pb2+ to DOC predicted by WHAM Model VI was much strongerthan that predicted by GEOCHEM.     

Measurement and dynamic modeling of trace metal mobilization in soils using DGT and DIFS

The technique of diffusive gradients in thin-films (DGT) accumulates metals on a Chelex resin after their diffusive transport through a hydrogel. It lowers metal concentrations in soil solution adjacent to the device and induces resupply of metal associated with the solid phase. DGT devices were deployed in an alluvial gley soil for 21 different time periods between 4 h and 19.5 d. The accumulated masses of Cu, Cd, Ni, and Zn were used to calculate the distribution coefficient for labile metal, Kdl, and adsorption and desorption rate constants. Calculations were performed using a dynamic numerical model of DGT-induced fluxes in soils (DIFS). It assumes first-order exchange between solid phase and solution and diffusional transport in both the soil solution and the hydrogel. The DIFS model fitted changes in accumulated mass with time very well. Values of Kdl calculated from DIFS of 100 (Cd), 250 (Cu), 150 (Ni), and 150 (Zn) were larger than values of distribution coefficients estimated by exchange with Ca(NO3)2 but similar to those estimated by isotopic exchange (Cd and Zn only). These results suggest that the solid-phase pool of metal affected by the removal of labile metal by DGT, which operates on a time scale of minutes, is similar to the solid-phase pool of metal that can isotopically exchange with solution on a time scale of 2 d. Response times of minutes were consistent with interaction rates with surfaces, and desorption rate constants agreed with other reported values. An appraisal of the DIFS model demonstrated the importance of the labile pool size in the solid phase for controlling supply to a sink, such as DGT or a plant. As values of Kdl and kinetic parameters are obtained using DGT with minimal soil disturbance and by a similar mechanism to that involved in plant uptake, they may be pertinent to bioavailability studies.     

Measuring free metal ion concentrations in situ in natural waters using the Donnan Membrane Technique

Metal toxicity is not related to the total but rather to the free or labile metal ion concentration. One of the techniques that can be used to measure several free metal ion concentrations simultaneously is the Donnan Membrane Technique (DMT) in combination with the inductively coupled plasma-mass spectrometer (ICP-MS). However, free metal ion concentrations in natural waters are commonly below the detection limit of ICP-MS. We decreased the detection limit by making use of a ligand, and we developed a field DMT cell that can be applied in situ in natural waters. A kinetic approach can be used to calculate free metal ion concentrations when the equilibrium time becomes too large. The field DMT measured in situ in natural waters a free metal ion concentration ranging from 0.015% (Cu) to 13% (Zn) of a total metal concentration ranging from 0.06 nM (Cd) to 237 nM (Zn). The free metal ion concentrations were difficult to predict using an equilibrium speciation model, probably due to the uncertainty in the nature of the dissolved organic matter or the presence of other reactive colloids. It is shown that DMT can follow changes in the free metal ion concentration on times scales less than a day under certain conditions.     

Use of diffusive gradients in thin films (DGT) in undisturbed field soils

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal bioavailability in soils under laboratory conditions. In this study we used DGT to investigate the resupply kinetics of Cu and Zn under in-situ conditions in a polluted lysimeter soil and compared the results with laboratory measurements using undisturbed soil cores at defined water contents as well as homogenized soil samples. Results differed considerably between these treatments, although the same soil material was used in all experiments. A small pool of rapidly available Zn was found in the field but not in the homogenized soil. Soil solution pH and dissolved metal concentrations also varied significantly between the soil treatments. In addition, we compared the DGT-measured effective concentration with the uptake of Cu and Zn into the shoots of Lolium perenne (Ryegrass) under the same three types of conditions, i.e., field, soil cores, and homogenized soil. A close relationship was found which was not linear but could be described by a saturation-type behavior. L. perenne is a metal excluder plant, and thus, metal accumulation is limited by translocation of metals from roots to shoots. DGT predicted plant metal uptake much better than the soil solution concentration or pH. The results of this study suggest that DGT may be successfully used under field conditions to study the kinetics of metal resupply. Plant metal concentrations were not well predicted in all cases by the effective concentration CE under field conditions. Some plants took up considerably more metals than estimated by CE. Variations in metal uptake independent of their bioavailability can be caused by local variations in microsite conditions, e.g. light, temperature, water, and nutrients. To some degree, such indetermination has to be expected as an inherent feature of the system and the concept of bioavailability.     

Copper toxicity to bioluminescent Nitrosomonas europaea in soil is explained by the free metal ion activity in pore water

The terrestrial biotic ligand model (BLM) for metal toxicity in soil postulates that metal toxicity depends on the free metal ion activity in solution and on ions competing for metal sorption to the biotic ligand. Unequivocal evidence for the BLM assumptions is most difficult to obtain for native soil microorganisms because the abiotic and biotic compartments cannot be experimentally separated. Here, we report copper (Cu) toxicity to a bioluminescent Nitrosomonas europaea reporter strain that was used in a solid phase-contact assay and in corresponding soil extracts and artificial soil solutions. The Cu(2+) ion activities that halve bioluminescence (EC50) in artificial solutions ranged 10(-5) to 10(-7) M and increased with increasing activities of H(+), Ca(2+) and Mg(2+) according to the BLM concept. The solution based Cu(2+) EC50 values of N. europaea in six contaminated soils ranged 2 × 10(-6) to 2 × 10(-9) M and these thresholds for both solid phase or soil extract based assays were well predicted by the ion competition model fitted to artificial solution data. In addition, solution based Cu(2+) EC50 of the solid phase-contact assay were never smaller than corresponding values in soil extracts suggesting no additional solid phase toxic route. By restricting the analysis to the same added species, we show that the Cu(2+) in solution represents the toxic species to this bacterium.     

Measurement of heavy metal speciation over redox gradients in natural water-sediment interfaces and implications for uptake by benthic organisms

The sediment or fauna incubation experiment (SOFIE) is an experimental research tool that was developed to analyze concentrations and chemical speciation of heavy metals in pore waters of natural, undisturbed sediments or water-sediment interfaces over time, while simultaneously conducting exposure tests with sediment-dwelling organisms. In this way, concentrations of chemical species are directly linked to accumulation by biota. It is shown that discrete gradients of redox-sensitive metals and nutrients occur over very small intervals. These gradients differ from those of free metal ion activities. Speciation affects the uptake of metals by sediment-dwelling organisms, which, in their turn, have a significant effect on metal speciation. With reaction kinetics that differ per metal, uptake of metals by organisms from the water phase may be hindered (e.g., Cu, Zn) or promoted (e.g., Ni, As). Time-varying exposure concentrations of metals were incorporated in uptake and elimination models. Body concentrations of Cd, Cu, Ni, and Zn in the aquatic oligochaete Limnodrilus could best be described by the time-varying free ion concentration in the overlying water. Body concentrations of As and Pb were best described by sediment pore water concentrations. It is concluded that SOFIE provides the necessary experimental tool to support, in a mechanistic way, environmental risk assessments of contaminants.     

Comparison of copper speciation in coastal marine waters measured using analytical voltammetry and diffusion gradient in thin-film techniques

The diffusion gradient in thin-film hydrogel (DGT) probe is a promising tool for metal speciation work. Based on a passive sampling principle, it provides the potential for large data sets in complex regimes. DGT probes were deployed in waters characterized independently using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). The CLE-ACSV used benzoyl acetone as the competitive ligand in discrete water samples collected during the deployment of the DGT probes. The DGT probes used a 15% polyacrylamide/0.4% bis-acrylamide cross-linker hydrogel and a Na-form of Chelex-100 to complex metal that fluxed into the probe through the hydrogel. Probes were deployed in locations characterized by the degree of pollution impact: the relatively pristine Vineyard Sound, MA, [Cu]total approximately 6 nM, small seasonally active harbors on Cape Cod, MA, [Cu]total = 12-64 nM, as well as a large polluted estuary, the Elizabeth River, VA, [Cu]total = 44-58 nM, and a large polluted port, San Diego Harbor, CA, [Cu]total = 23-103 nM. This is the first study where DGT probes have been compared with an independent speciation technique in marine systems and used to establish the diffusion coefficient of Cu-complexing ligands in situ. Results showed that the probes produced highly precise data sets, with substantial differences in copper accumulation between contaminated and pristine waters. Comparison of DGT results with CLE-CSV indicate that at least 10-35% of the organically complexed copper derived by CLE-ACSV measurements was DGT-labile. Diffusion coefficients (corrected to 25 degrees C) of organically complexed DGT-labile Cu through the hydrogel ranged from 0.77 x 10(-6) cm2 s(-1) in Vineyard Sound to 2.16 x 10(-6) cm2 s(-1) in the Elizabeth River estuary. Accumulation rates of copper were substantially higher in contaminated waters than in pristine waters, suggesting that the probes in their current form may be useful as tracking tools to detect episodic sources of contamination.     

DGT-Induced Copper Flux Predicts Bioaccumulation and Toxicity to Bivalves in Sediments with Varying Properties

Many regulatory frameworks for sediment quality assessment include consideration of contaminant bioavailability. However, the "snap-shots" of metal bioavailability provided by analyses of porewaters or acid-volatile sulfide-simultaneously extractable metal (AVS-SEM) relationships do not always contribute sufficient information. The use of inappropriate or inadequate information for assessing metal bioavailability in sediments may result in incorrect assessment decisions. The technique of diffusive gradients in thin films (DGT) enables the in situ measurement of metal concentrations in waters and fluxes from sediment porewaters. We utilized the DGT technique to interpret the bioavailability of copper to the benthic bivalve Tellina deltoidalis in sediments of varying properties contaminated with copper-based antifouling paint particles. For a concentration series of copper-paint contaminated sandy, silty-sand, and silty sediment types, DGT-probes were used to measure copper fluxes to the overlying water, at the sediment-water interface, and in deeper sediments. The overlying water copper concentrations and DGT-Cu fluxes were shown to provide excellent exposure concentration-response relationships in relation to lethal effects occurring to the copper-sensitive benthic bivalve, T. deltoidalis. The study demonstrates the strength of the DGT technique, which we expect will become frequently used for assessing metal bioavailability in sediments.