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.     

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.     

In situ trace metal speciation in lake surface waters using DGT,dialysis, and filtration

In situ measurements of Fe and Mn by dialysis and diffusive gradients in thin-films (DGT) in 5 lakes (pH 4.7-7.5, ionic strength 0.3-5 mmol l(-1)) and Cu and Zn in an acidic and circumneutral lake were compared to results from on site filtration. For the most acidic lake (pH 4.7) all measurements agreed, indicating an absence of colloids and negligible complexation by organic matter. There was little difference in the Mn concentrations measured by the three techniques for any lake, consistent with it being free from complexation. Zn measured by dialysis in circumneutral water was only slightly higher than DGT measurements, appropriate to only partial complexation. Substantial differences between dialysis and DGT for Cu were consistent with complexation by fulvic and humic substances, though not to the extent predicted by the speciation code WHAM. To achieve a good fit it was necessary to adjust the pK for Cu-fulvic binding from 0.8 to 1.3 and to assume that fulvic substances dominated. The presence of low molecular weight strong binding ligands would also be consistent with the data. Differences between the three measurement methods were greatest for Fe, attributable to the presence of large oxyhydroxide colloids, organic complexation and low molecular weight, reactive hydrolysis products. Fe and Mn concentrations measured by DGT on samples returned to the laboratory were much lower than in situ concentrations, illustrating the need for in situ measurements. While use of two in situ techniques provided useful information on the speciation of these natural waters, further refinements are required for unambiguous characterization of the solution. The use of DGT with a more restricted gel that excludes complexes with humic substances should provide complementary information to in situ dialysis.     

Comparing WHAM 6 and MINEQL+ 4.5 for the chemical speciation of Cu2+ in the rhizosphere of forest soils

Metal speciation data calculated by modeling could give useful information regarding the fate of metals in the rhizospheric environment. However, no comparative study has evaluated the relative accuracy of speciation models in this microenvironment. Consequently, the present study evaluates the reliability of free Cu ion (Cu2+) activity modeled by WHAM 6 and MINEQL+ 4.5 for 18 bulk and 18 rhizospheric soil samples collected in two Canadian forested areas located near industrial facilities. The modeling of Cu speciation was performed on water extracts using pH, dissolved organic carbon (DOC), major ions, and total dissolved Al, Ca, Cu, Mg, and Zn concentrations as input data. Four scenarios representing the composition of dissolved organic substances using fulvic, humic, and acetic acids were derived from the literature and used in the modeling exercise. Different scenarios were used to contrast soil components (rhizosphere vs bulk) and soil pH levels (acidic vs neutral to alkaline). Reference Cu2+ activity values measured by an ion-selective electrode varied between 0.39 and 41 nM. The model MINEQL+ 4.5 provided good predictions of Cu2+ activities [root-mean-square residual (RMSR)= 0.37], while predictions from WHAM 6 were poor (RMSR = 1.74) because they overestimated Cu complexation with DOC. Modeling with WHAM 6 could be improved by adjusting the proportion of inert DOC and the composition of DOC (RMSR = 0.94), but it remained weaker than predictions with MINEQL+ 4.5. These results suggested that the discrepancies between speciation models were attributed to differences in the binding capacity of humic substances with Cu, where WHAM 6 appeared to be too aggressive. Therefore, we concluded that chemical interactions occurring between Cu and DOC were key factors for an accurate simulation of Cu speciation, especially in rhizospheric forest soils, where high variation of the DOC concentration and composition are observed.     

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.     

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.     

In situ measurements of metal complex exchange kinetics in freshwater

Trace metals were measured in situ in a freshwater river draining a peat catchment (DOC = 15 mg L(-1)) using diffusive gradients in thin-films (DGT) devices with a range of gel layer thicknesses (0.16-2.0 mm). The reciprocal of the accumulated mass of each metal varied linearly with the thickness of the diffusive layer. These plots allowed calculation of the thickness of an apparent diffusive boundary layer (ADBL). A constant value was obtained from the plots of Cd, Pb, and Zn. The observed increase in the ADBL for the other metals (Mn相似文献   

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.     

Dynamic aspects of DGT as demonstrated by experiments with lanthanide complexes of a multidentate ligand

Sampling of metals with the technique of diffusive gradients in thin-films (DGT) depends on the rates of diffusion and on the kinetics of interconversion of the species present. In this study the discrimination between metal complexes with different dissociation kinetics is investigated. Samplers with differentthicknesses of diffusive and resin gels were deployed in solutions containing 10 microg/L of each metal in the lanthanide (Ln) series (except Pm) and 2.0 x 10(-6) M of the ligand quin2 at an ionic strength of 0.1 M (KNO3) and pH 7.0. Diffusion coefficients of Ln3+ ions and Ln-quin2 complexes were determined in a diffusion cell experiment. The equilibrium speciation of the metals was calculated from available stability constants. The sampling rate (mass/time) was highly dependent on the dissociation-rate constant of the complexes. For complexes with dissociation kinetics that appreciably limited the uptake, the sampling rate decreased significantly with increasing deployment times (12, 24, and 76 h) and was virtually independent of the thickness of the diffusive gel. Placing a layer of diffusive gel behind the resin did not influence the accumulation of Lns in the resin gel, but doubling the thickness of the layer containing resin increased the uptake, and more so for the Lns forming less labile complexes. The Lns forming more labile complexes were enriched in the outer layer of the resin, and there was a trend toward even distribution between the outer and deeper parts of the resin layer for the Lns forming less labile complexes. The measured DGT sampling rates (mass/ time) were reasonably well predicted by a dynamic model that used independently determined kinetic constants. This new knowledge of how metal complexes behave in the sampling process paves the way for using DGT to obtain in situ kinetic information in natural waters.     

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.     

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.     

Model predictions of metal speciation in freshwaters compared to measurements by in situ techniques

Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTEQ incorporating NICA-Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available forthe fluxto GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data.     

Investigating arsenic speciation and mobilization in sediments with DGT and DET: a mesocosm evaluation of oxic-anoxic transitions

Mobilization of arsenic from freshwater and estuarine sediments during the transition from oxic to anoxic conditions was investigated using recently developed diffusive sampling techniques. Arsenic speciation and Fe(II) concentrations were measured at high resolution (1-3 mm) with in situ diffusive gradients in thin films (DGT) and diffusive equilibration in thin films (DET) techniques. Water column anoxia induced Fe(II) and As(III) fluxes from the sediment. A correlation between water column Fe(II) and As(III) concentrations was observed in both freshwater (r(s) = 0.896, p < 0.001) and estuarine (r(s) = 0.557, p < 0.001) mesocosms. Porewater sampling by DGT and DET techniques confirmed that arsenic mobilization was associated with the reductive dissolution of Fe(III) (hydr)oxides in the suboxic zone of the sediment; a relationship that was visible because of the ability to measure the coincident profiles of these species using combined DGT and DET samplers. The selective measurement of As(III) and total inorganic arsenic by separate DGT samplers indicated that As(III) was the primary species mobilized from the solid phase to the porewater. This measurement approach effectively ruled out substantial As(V) mobilization from the freshwater and estuarine sediments in this experiment. This study demonstrates the capabilities of the DGT and DET techniques for investigating arsenic speciation and mobilization over a range of sediment conditions.     

Chemical speciation and toxicity of nickel species in natural waters from the Sudbury area (Canada)

Metal complexation properties of dissolved organic carbon (DOC) in freshwaters are recognized but poorly understood. Here, we investigated the release of free nickel from Ni-DOC complexes using nickel-polluted freshwaters from Sudbury (Canada). We used the Competing Ligand Exchange Method with Chelex-100 as the competing ligand to measure the rate of free Ni2+ ion released by the dissociation of Ni-DOC complexes. The kinetic studies showed that the fastest kinetically distinguishable component representing approximately 30-95% of the total nickel had a dissociation rate coefficient similar to that reported for [Ni(H20)6]2+. High concentrations of Ca2+ and Mg2+ caused a larger amount of the DOC-bound nickel to be released as free Ni2+ ion. Growth inhibition of the freshwater alga Pseudokirchneriella subcapitata was highly correlated with the Ni/DOC ratio, the free plus labile nickel concentration, and the dissociation rate coefficient. While the levels of metals were not sufficient to kill Daphnia magna, these test organisms were immobilized in the same samples that showed algal growth inhibition. Only one sample caused 22% death of Hydra attenuata. The algal toxicity tests were consistent with the kinetic speciation results and are consistent with the hypothesis that dissolved [Ni(H20)6]2+ plus other labile nickel species are toxic forms of Ni present.     

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.     

Strong hg(II) complexation in municipal wastewater effluent and surface waters

The speciation of mercury(II) in the aquatic environment is greatly affected by the presence of ligands capable of forming extremely strong complexes with Hg(II). In this study, a novel competitive ligand exchange (CLE) technique was used to characterize Hg(II)-complexing ligands in samples collected from three municipal wastewater treatment plants, a eutrophic lake, a creek located downstream of an abandoned mercury mine, and a model water containing dissolved Suwannee River humic acid. These samples contained 3.3-15.9 mg/L dissolved organic carbon and were amended with 1.0-1.7 nM Hg(II) for CLE analysis. Results indicated that all samples contained labile Hg(II)-complexing ligands with conditional stability constants similar to those of reduced sulfur-containing ligands. Two wastewater effluent samples also contained approximately 0.5 nM of ligands that formed extremely strong Hg(II) complexes that did not dissociate in the presence of competing ligands. The conditional stability constant of these extremely strong or nonlabile complexes (i.e., (c)K(HgL)) were estimated to be greater than 10(30), for the reaction Hg(2+) + L' = HgL. The third wastewater sample and the eutrophic lake sample contained lower concentrations, 0.07-0.09 nM, of nonlabile Hg(II)-complexing ligands. The results suggested that these extremely strong Hg(ll)-complexing ligands should account for most of the dissolved Hg(II) species in municipal wastewater effluent and may dominate Hg(II) speciation in effluent-receiving waters.     

Evaluation of the diffusive gradient in a thin film technique for monitoring trace metal concentrations in estuarine waters

Monitoring trace metal concentrations in dynamic estuarine waters is not straightforward. This study demonstrated that important information could be obtained from intensive sampling of physicochemical parameters and trace metal concentrations, in the Gold Coast Broadwater, Australia. A regular pattern of variation in Cu and Ni concentrations was related to the movement of water passed point sources with tidal flows, rather than due to conventional estuarine mixing of end-member waters. However, this approach was logistically demanding and expensive. The diffusive gradients in a thin film (DGT) technique was used as an alternative method due to its continual time-integrated response to changes in trace metal concentrations. Significant correlations were found between 24 h DGT-labile measurements and 0.45-microm filterable measurements, on time-averaged composite samples (grab samples combined every 4 h for 24 h), for Cu (n = 24, r = 0.965, p < 0.001), Pb (n = 24, r = 0.799, p < 0.001), Zn (n = 17, r = 0.909, p < 0.001), and Ni (n = 23, r = 0.916, p < 0.001). DGT-labile measurements as a fraction of 0.45 microm-filterable concentrations were 21 +/- 2% for Cu, 29 +/- 11% for Pb, 28 +/- 5% for Zn, and 27 +/- 12% for Ni, demonstrating the speciation capabilities of DGT. Although DGT measurements were confirmed as being highly operationally defined, DGT was still found to be very promising as a monitoring approach, particularly for dynamic estuarine waters.     

Chemical speciation of dissolved Cu, Ni, and Co in a contaminated estuary in southwest Spain and its influence on plankton communities

Four surveys of the Huelva Estuary in southwest Spain and its sources, the Tinto and the Odiel Rivers, were carried out between 1996 and 1998. The surveys investigated the impact of metalliferous mining of sulfide-rich ores in the catchment area on metal speciation, metal concentrations in a macrophyte, and phytoplankton diversity and abundance. Chemical speciation measurements in the lower Tinto Estuary showed that metals were predominantly electrochemically labile (> 99% of total dissolved Cu, Co, and Ni at 10 microM Cu, 424 nM Co, and 500 nM Ni, S = 28). Concentrations of Cu complexing ligands and free cupric ions [Cu2+] in the Gulf of Cádiz ranged between 5.3 and 38 nM and 0.2-7.9 pM, respectively, with conditional stability constants of the ligands of log K'(CuL) = 11.7-12.6. At enhanced dissolved Cu concentrations in the lower Huelva Estuary, Cu complexing ligands were saturated with Cu, resulting in nanomolar [Cu2+], which increased upstream. Metal tissue concentrations of the macrophyte Blindingia marginata were high, and a clear relationship between dissolved labile Cu and macrophyte tissue Cu concentrations was observed. A low biodiversity was observed in the Huelva system (Shannon-Wiener indices (H) typically < 0.2). Nevertheless, the maximum biomass was observed in the lower Tinto Estuary, which showed high labile metal and nutrient concentrations and a low biodiversity (H < 0.02), thereby suggesting adaptation through evolutionary processes of the phytoplankton community to the harsh conditions.     

Comparison of copper speciation in estuarine water measured using analytical voltammetry and supported liquid membrane techniques

The supported liquid membrane (SLM) is a promising separation and preconcentration technique that is well-suited for trace metal speciation in natural waters. The technique is based on the selective complexation of metal ions by a hydrophobic ligand (carrier) dissolved in a water-immiscible organic solvent immobilized in a porous, inert membrane. This membrane separates two aqueous solutions: the test (or donor) solution and the strip (or acceptor) solution. The metal carrier complex is transported by diffusion across the membrane from the source to the strip solution where metal ions are back-extracted. The technique offers great potential to tune the selectivity by incorporating different complexing ligands in the membrane. A SLM was used to analyze the dissolved (<0.45 microm) copper speciation from two sites in the San Francisco Bay estuary; Dumbarton Bridge, [Cu]total approximately 27 nM, and San Bruno Shoals, [Cu]total approximately 23 nM. The sites were also characterized independently by differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE). The SLM employed 10 mM lasalocid, a naturally occurring carboxylic polyether ionophore, in nitrophenyl octyl ether (NPOE) asthe membrane complexing ligand, supported by a microporous, polypropylene, hydrophobic membrane. This is the first study where SLM technique has been compared with an independent speciation technique in marine waters. Results of copper speciation measurements from Dumbarton Bridge, a site in South San Francisco Bay where copper speciation has been well-characterized in previous studies using various voltammetric techniques, indicated that only about 3% (0.9 nM) of the total dissolved copper was SLM labile. The corresponding DPASV labile copper fraction was <0.4% (<0.1 nM) of total dissolved copper. The concentration of total copper binding ligands measured by the membrane technique was 471 nM as compared to 354 nM measured by DPASV, more than 1 order of magnitude higher than the total dissolved copper concentration. The SLM measurements were consistent with earlier copper speciation measurements that were made in South San Francisco Bay using other voltammetric stripping techniques.     

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.