Interactions of Trace Metals with Hydrogels and Filter Membranes

Water from Thirlmere and Esthwaite Water was collected when biological production was low and filtered through 0.45 μm pore size cellulose nitrate fi...
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Environ. Sci. Technol. 2008, 42, 5682–5687

Interactions of Trace Metals with Hydrogels and Filter Membranes Used in DET and DGT Techniques ØYVIND A. GARMO, WILLIAM DAVISON, AND HAO ZHANG* Department of Environmental Science, Lancaster Environment Center (LEC), Lancaster University, Lancaster LA1-4YQ, United Kingdom

Received January 15, 2008. Revised manuscript received April 30, 2008. Accepted May 7, 2008.

Equilibrium partitioning of trace metals between bulk solution and hydrogels/filter was studied. Under some conditions, trace metal concentrations were higher in the hydrogels or filter membranes compared to bulk solution (enrichment). In synthetic soft water, enrichment of cationic trace metals in polyacrylamide hydrogels decreased with increasing trace metal concentration. Enrichment was little affected by Ca and Mg in the concentration range typically encountered in natural freshwaters, indicating high affinity but low capacity binding of trace metals to solid structure in polyacrylamide gels. The apparent binding strength decreased in the sequence: Cu > Pb > Ni ≈ Cd ≈ Co and a low concentration of cationic Cu eliminated enrichment of weakly binding trace metal cations. The polyacrylamide gels also had an affinity for fulvic acid and/or its trace metal complexes. Enrichment of cationic Cd in agarose gel and hydrophilic polyethersulfone filter was independent of concentration (10 nM to 5 µM) but decreased with increasing Ca/ Mg concentration and ionic strength, suggesting that it is mainly due to electrostatic interactions. However, Cu and Pb were enriched even after equilibration in seawater, indicating that these metals additionally bind to sites within the agarose gel and filter. Compared to the polyacrylamide gels, agarose gel had a lower affinity for metal-fulvic complexes. Potential biases in measurements made with the diffusive equilibration in thin-films (DET) technique, identified by this work, are discussed.

Introduction Open-pore hydrogels can behave almost as a stagnant film of water with respect to small substances, providing a convection-free medium where transport is well defined. They are exploited in several new techniques such as voltammetric gel-integrated microelectrodes (1), diffusive gradients in thin-films (DGT) (2), and diffusive equilibration in thin-films (DET) (3). DET is a sampling technique where the substance of interest is assumed to reach the same concentration in the water held by the hydrogel as in the external aqueous solution. For substances occurring in forms (species) of different sizes, only those small enough to diffuse through a protective filter and into the pores of the hydrogel will be collected. Their aqueous concentration is normally measured after extraction * Corresponding author phone: +44 (0) 1524 593899; fax: +44 (0) 1524 593985; e-mail: [email protected]. 5682

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of the hydrogel and subsequent analysis of the extract, but analysis of the dried gel has also been used (3). Prior to extraction, the hydrogel of DET is sliced into thin strips, or, alternatively, hydrogels can be constrained into small compartments. Short equilibration times and high spatial resolution of concentrations can be obtained by using smallvolume hydrogels (4, 5). DET has been used to measure dissolved gases, major ions, nutrients, and metals in sediment porewaters (6). The study of trace metals with DET was originally restricted to elevated concentrations of Fe and Mn in sediments, but lately, in combination with sensitive analytical techniques such as inductively coupled plasma mass spectrometry (ICP-MS), it has been used for other trace metals and metalloids occurring at much lower concentrations (5–12). Given that hydrogels contain functional groups with known affinity for metals, it is advisable to test the performance of DET at realistic concentrations of the particular metal to assess whether binding of metals could bias the measurements. Most performance tests of DET have so far been conducted at relatively high concentrations of metals and for a limited range of conditions, e.g. including only Na+ of the major cations present in natural waters (4, 5, 13, 15). Recent studies have documented interactions between metals and the solid structure of commonly used agarose and polyacrylamide hydrogel (15, 16). Two types of interactions can be distinguished: specific binding of metals to sites in the hydrogel and electrostatic interaction between metal ions and fixed charged groups in the hydrogels. The former will only affect metals that can bind to the sites, while the latter may affect the equilibrium partitioning of all ions. Maintaining overall electroneutrality in the gel requires a higher concentration of ions of opposite charge and lower concentration of ions of the same charge (positive or negative) as the hydrogel sites compared to in the bulk solution. The effect of electrostatic interaction on the partitioning (Cgel/ Cwater) of ions between hydrogel and bulk solution increases with increasing charge density (F) in the gel and decreases with increasing ionic strength in solution. It can be quantified with eqs 1 and 2 (16): Cgel/Cwater)exp(-zMFψ/RT)

(1)

ψ ) (RT/zF)asinh(F/2zFc)

(2)

Where zM and z represent the valence of the trace metal ion and supporting electrolyte (assumed to be symmetrical with a concentration c in the bulk solution), respectively, F the Faraday constant, R the gas constant, T the absolute temperature, and Ψ the Donnan potential difference between the gel and the bulk solution. Both electrostatic and chemical interactions may cause overestimation of concentrations measured by DET. This is the first study of the effect of these interactions on the partitioning of trace metals in different hydrogels/filter after equilibration in solutions containing realistic trace metal concentrations and matrixes relevant to natural waters. Since the presence of a reactive diffusion layer may also affect DGT measurements, we have included in this study the hydrogels and filters commonly used in both DET and DGT techniques.

Experimental Section Preparation and Conditioning of Hydrogels/Filter. The preparation of APA gel (polyacrylamide cross-linked with the agarose derivative AcrylAide), agarose gel, and a polyacrylamide gel (cross-linked with N,N′-methylenebisacrylamide) with smaller pore size, referred to as restricted gel in 10.1021/es800143r CCC: $40.75

 2008 American Chemical Society

Published on Web 06/24/2008

TABLE 1. Electrolyte Concentration (mM) in Test Solutionsa Na very soft soft moderately soft hard Thirlmere Esthwaite Water seawater

1 1 1 1

Ca

Mg

NO3

0.015 0.06 0.24 0.96

0.01 0.04 0.16 0.64

1.05 1.2 1.8 4.2

soft moderately soft salinity 30 ppt

a The concentration of NaNO3 in the synthetic solutions was held constant at 1 mM to minimize purely electrostatic interactions between polyacrylamide gels and metals (15).

previous reports followed previously established procedures (14, 17) and is described in the Supporting Information. All gels were 0.8 mm thick and were cut to circular disks of diameter 2.4 cm. The gels were thoroughly washed by immersing them in MilliQ water (water/gel volume ratio >150) and changing the water 5-6 times over 5 days to ensure the removal of excess reactants. Finally, the gels were conditioned and stored in 10 mM NaNO3. The water content of hydrated APA, agarose, and restricted PA gels is about 95%, 98%, and 87%, respectively, as determined by the weight difference of hydrated and oven-dried gels. Hydrophilic polyethersulfone membrane filters (Supor450, Pall) (pore size 0.45 µm, diameter 2.5 cm, thickness 140 µm) were used. The filters were initially rinsed in 0.1 M HNO3 (Aristar, BDH) for about 24 h and then rinsed 3-4 times in MilliQ water before being stored in MilliQ water prior to use. Equilibration of Hydrogels and Filter in Solution. Test solutions for equilibration of hydrogels and filter were prepared by diluting stock solutions of metals dissolved in MilliQ water. The ionic strength and hardness of the test solutions were adjusted with NaNO3, Mg(NO3)2, and Ca(NO3)2 (Table 1). Stock solutions containing well-characterized fulvic acid were prepared by dissolving purified and freeze-dried material extracted from Whitray Beck (18) or Suwannee River (International Humic Substances Society) in MilliQ water to a concentration of 1 g/L. Test solutions were prepared by diluting the stock solutions and adjusting the pH with NaOH. Water from Thirlmere and Esthwaite Water was collected when biological production was low and filtered through 0.45 µm pore size cellulose nitrate filters (Whatman). Seawater from Morecambe Bay was filtered through 0.2 µm pore size cellulose nitrate membrane filter. After filtration, natural waters were stored in the dark at 4 °C prior to use. Aliquots of filtered natural water and synthetic solution containing fulvic acid were spiked with metals and left 24 h to equilibrate before deployment of hydrogels/filter. Disks were placed in 100 mL of test solution in small plastic boxes (one box for each type of hydrogel/filter) and left to equilibrate by shaking for at least 48 h. The disks were then transferred to preweighed plastic tubes and weighed to obtain their wet-weight. Samples for control measurement of the total concentration of metals were taken from all test solutions and acidified by adding HNO3 to 0.1 M. Deployment of DET Assemblies in Sediment. Details are provided in the Supporting Information. Analysis. Both equilibrated and blank filter and hydrogels were eluted in 1.2 and 3 mL, respectively, of 0.1 M HNO3 (Aristar, BDH) by shaking for 48 h. Solution samples and eluates from deployments in spiked seawater were diluted 100 and 10 times, respectively, before analysis. Samples were analyzed using a Thermo Elemental X7 ICP-MS. Instrument drift during analysis was corrected for by internal standardization using 115In. The concentration of a metal in filters and hydrogels was calculated by dividing the eluted mass of metal with the volume of water associated with the particular

FIGURE 1. Mean concentration (n ) 2) of Cd in filters, APA gels, agarose gels, and restricted gels equilibrated in solutions at pH 5.5, containing 1 µg/L Cd and various concentrations of Na, Ca, and Mg (see Table 1). The concentration of 1 µg/L is represented as a horizontal line. disk before elution, calculated from the difference in mass between the wet and dry disk (assuming a constant dryweight). The concentration thus estimated comprises metal bound to solid structure as well as metal dissolved in porewater. A higher concentration of metals in diffusive gels and filters than in their deployment solution, is termed enrichment (Cgel/Cwater and Cfilter/Cwater > 1).

Results and Discussion Cd in Hydrogels/Filter after Equilibration in Solutions of Varying Composition. Earlier experiments have shown that Cd is enriched in APA and agarose gel after equilibration in solutions containing low concentrations of Cd in 0-10 mM NaNO3 (15, 16). The enrichment in agarose gel has been ascribed to a combination of specific binding and ionic strength-dependent electrostatic attraction between cationic Cd and fixed negatively charged sites within the gel (16), while enrichment in APA gel appears to be caused by specific binding alone, since ionic strength had little effect on enrichment (15). Electrostatic interactions are also likely to be negligible in polyacrylamide gels cross-linked with methylenebisacrylamide for ionic strengths higher than 1 mM, as their charge density is much lower than that of agarose (19). In natural waters divalent Ca and Mg might be expected to compete with Cd for binding within the gels because these major cations occur at much higher concentrations. Moreover, binding of Ca and Mg might diminish the density of negative charge in the agarose gel, lowering the Donnan potential difference between the gel and the bulk solution. Figure 1 shows that increasing the concentration of Ca and Mg within the concentration range typically found in freshwater (see Table 1 for exact compositions) decreased Cd enrichment in the agarose gel and the filter but only marginally affected Cd in polyacrylamide gels (APA gels produced by different workers in our group showed similar enrichment in soft water). However, when the Cd concentration was increased at fixed water hardness, Cgel/Cwater approached 1 for the APA gel but remained higher than 1 and approximately constant for the agarose gel and filter (Figure 2). According to eqs 1 and 2, the enrichment of divalent Cd caused by electrostatic interaction should be about a factor of 2 for agarose gel in a solution containing 1 mM NaNO3 (no Ca and Mg) if the gel has a fixed charge density of -73 C/kg of gel (16). The observed factors are 2 or higher in synthetic soft and very soft waters (i.e., 1 mM VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Concentrations of Cd in gels and filter divided by concentration of Cd in water after equilibration in synthetic soft water (see Table 1) at pH 5.5. The horizontal line corresponds to a Cgel/Cwater ratio of 1 while the curved line represents the best fit of an inverse first order function (Cgel/Cwater ) 1 + a/ Cwater) to the experimental data for the APA gel.

FIGURE 3. Mean concentration (n ) 2) of Cd in filters, APA gels, agarose gels, and restricted gels equilibrated in synthetic solutions and filtered natural freshwater containing 1 µg/L Cd. The concentration of 1 µg/L is represented as a horizontal line. All solutions had a pH of 6.5 except Esthwaite Water which had a pH of 6.9. Soft+FA contained 10 mg/L of fulvic acid extracted from Whitray Beck. NaNO3 in addition to some Ca and Mg) indicating that specific binding of Cd may also contribute to the enrichment. However, in 0.1 M NaNO3 solution, where the difference in Donnan potential between the agarose gel and bulk solution should be negligible, Cgel/Cwater of Cd was close to 1 for agarose gel, indicating little specific binding. Most of the observed Cd enrichment in agarose gel is therefore attributed to electrostatic interactions. The pore surface of the Supor polyethersulfone filter membrane has been reported to hold a slightly negative charge (20). It therefore appears likely that the filter enrichment of Cd in soft and very soft water is also due to electrostatic interactions and not specific binding to functional groups. The data for APA gel, however, clearly indicate presence of a low concentration of binding sites with affinity for Cd. Gels were equilibrated in synthetic soft water containing fulvic acid. Complexation of Cd by fulvics reduced the enrichment of Cd in agarose gel but increased the enrichment of Cd in the two types of polyacrylamide gels (Figure 3), indicating that they have an affinity for fulvic acid and/or its 5684

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complexes with Cd. According to speciation calculations (WHAM, Model VI (21)), about 67% of the Cd in the synthetic solution was complexed by fulvics, leaving 33% as uncomplexed Cd. Applying the enrichment factor of 3.0 that was observed for agarose gel in the control solution to the free Cd concentration and, assuming that complexed Cd was unavailable for binding in the agarose gel, gives a Cgel/Cwater for Cd in the solution containing fulvic acid of around 1.67 µg/L () 0.67 + 3 × 0.33). The observed Cgel/Cwater was 1.6 ( 0.1, indicating no binding of Cd-fulvic complexes in the agarose gel. Previously, no chemical interactions were observed between humic acid and agarose gel (16). Hydrogels and filter were equilibrated in water from Thirlmere (soft) and Esthwaite Water (moderately soft) spiked to a concentration of 1 µg/L Cd. Figure 3 shows that similar enrichments were observed for agarose gel, restricted gel, and filter in natural waters compared to the synthetic soft water control solution (note that Cd enrichment in restricted gel is slightly higher in soft water at pH 6.5 than 5.5). However, Cgel/Cwater for Cd was lower in the APA gel. The higher Cgel/ Cwater in APA gel in the presence of fulvic acid suggests that the lower ratio observed in natural waters is not caused by lowering of free Cd through complexation of Cd by humic substances but rather competition with other trace metals. Multiple Metals in Hydrogels/Filter after Equilibration in Solutions of Varying Composition. Major cations are unable to displace Cd at the concentrations they normally occur in freshwater, but other cations do. Cu and Pb bind more strongly (Figure 4) and displace Cd and Co from the polyacrylamide gels in solutions containing equimolar concentrations of a suite of metals (Co, Ni, Cu, Cd, and Pb). Lanthanoids (1 µg/L of each in the same solution) were also found to displace Cd from APA binding sites (results not shown). For agarose gel and the filter, however, Cu and Pb did not lower Cgel/Cwater for Cd compared to experiments with no competing trace metals (Figures 1–3). A high concentration of divalent Mn, which can arise in reducing sediment, did not eliminate Cu and Pb enrichment, although a slight decrease was observed for Cu in APA gel and Pb in restricted gel (Figure 4b). Addition of fulvic acid to a concentration high enough to complex about 98% of the Cu in solution tended to increase the enrichment of Cu in the two types of polyacrylamide gel, indicating that polyacrylamide gels have an affinity for fulvic acid and/or its complexes with Cu (Figure 4c), in agreement with the results for Cd (Figure 3). Enrichment of other metals in polyacrylamide gels, notably Pb, also increased in the presence of fulvic acid, but this may partly be an indirect effect caused by lower concentration of cationic Cu competing for binding sites within the gels. The enrichment of Cu and Pb in agarose gel was lower in solutions containing fulvic acid compared to inorganic solutions. However, as there is a 2-fold enrichment of Cu even when the concentration of inorganic Cu is very low, some interaction between Cu-fulvic acid complexes and agarose gel may be inferred. Figure 4d shows no enrichment of Co, Ni, Cd, and Mn for hydrogels and filter equilibrated in seawater spiked with six different metals to nominal concentrations of 100 or 500 nM, i.e. the observed enrichment of these metals in agarose gel and filter (Figure 4a) is eliminated in seawater. Major cations probably out-compete the metals for binding sites, while the negatively charged sites are completely screened at the high ionic strength (see also Figure 1). However, Cgel/ Cwater for Cu and partly Pb is higher than 1, even after equilibration in seawater, reflecting their much greater tendency to bind. Furthermore, Cu shows higher enrichment in polyacrylamide gels when present at a concentration of 500 nM compared to 100 nM. This finding contrasts with concentration dependency observed in synthetic freshwater, where Cgel/Cwater tended to decrease with increasing metal

FIGURE 4. (a-c) Median concentration of metals in restricted gel (RPA), agarose gel (aga), APA gel, and filter equilibrated in synthetic soft water of pH 5.5 containing 10 or 100 nM of each of the metals Co, Ni, Cu, Cd, and Pb. In addition, solutions represented in part b and c contained 0.1 mM of divalent Mn and 3 mg/L of Suwannee River fulvic acid, respectively. Error bars represent the difference between the median (n ) 3) and the maximum measured concentration. (d) Median concentrations (n ) 3) of Co, Ni, Cu, Cd, Pb, and Mn in gels and filter divided by the concentrations of the same metals in water (nominal concentration of each metal was 100 or 500 nM) after equilibration in spiked seawater (salinity 30 ppt, pH 7.9) from Morecambe Bay. concentration. This discrepancy could possibly be explained by the presence of a Cu-specific ligand at a low concentration compared to added Cu, causing concentration dependent changes in Cu speciation in the seawater. Performance of APA and Agarose DET Probes in Sediment. To test if the findings of the solution experiments described above are relevant to DET deployments in sediments, four probes containing either APA gel or agarose gel were deployed in the same sediment core from Lake Windermere. The results (Supporting Information) show that after equilibration the concentrations of Cu were 2-10 times higher in DET probes containing APA gel than in those containing agarose, while the concentrations of Co were similar. The corresponding ratios from the freshwater solution experiments (Figure 4a-c) range from 1 to 6 and 0.5 to 1 for Cu and Co, respectively, depending on metal concentrations and presence of fulvic acid. Assuming that the Cu concentrations measured with DET assemblies containing agarose gel are closer to the (unknown) true porewater concentrations, it follows that Cu concentrations in APA DET probes are higher than in the porewaters. Concentrations of Cu and Co measured previously in freshwater and marine sediments with agarose DET probes have been compared to concentrations measured in extracted porewaters. In freshwater sediment both DET Cu and DET Co was generally higher than the respective porewater concentrations (9, 10). Variance in marine sediment porewater concentrations was very high, but DET Cu and DET Co tended to be higher and lower, respectively, than mean porewater concentrations (11). Since DET probes will exclude metal associated with particles, higher concentrations measured in DET compared to porewater extracts indicate enrichment of metals in hydrogel, while lower concentrations may be attributed to speciation. Collectively, these results suggest that hydrogel enrichment of metals can bias DET measurements of Cu in seawater sediments and both Cu and Co in freshwater sediments. Selectivity and Capacity of Trace Metal Binding in Hydrogels/Filter. The present study shows that divalent cations of Cu and Pb bind quite strongly to sites within the two types of polyacrylamide gels, while Cd binds more weakly. From the titration of gel with Cd (Figure 2), a binding capacity in soft water of about 50 nmol Cd per L APA gel can be

calculated (by fitting the function Cgel/Cwater ) 1 + a/Cwater to the data), while the capacity in restricted gel is considerably less. However, the enrichment of Cu (Figure 4a) exceeds the estimated capacity for binding of Cd in polyacrylamide gels equilibrated in solutions spiked with Cd only, suggesting that additional binding sites are available for Cu. Binding of divalent Cu by amide groups in polyacrylamide gels has been reported, although not in gels with such a high water content as those used here (22). Another functional group capable of selectively binding transition metal ions is carboxylate, which can be introduced into the polyacrylamide network through hydrolysis of amide, forming acrylic acid (23). This reaction is to some extent triggered by the basic catalyst TEMED (24), to which the two polyacrylamide gels will be exposed for a similar time-span. Since a higher concentration of TEMED is used for catalyzing the polymerization of APA gel compared to restricted gel (16.6 versus 13.3 mM) and, more importantly, that the polymerization of APA gels occurs at higher temperature (44 °C versus 4 °C), the APA gel’s amide groups will hydrolyze to a greater extent (24), forming more acrylic acid within the gel. Mixed polyacrylamide-poly(acrylic acid) copolymer gel produced by controlled hydrolysis of APA gels are effective binding agents for trace metals (25). The capacity of polyacrylamide gels to bind inorganic cations seems relatively small since Cgel/Cwater decreases with increasing concentration, but the total capacity for binding metals is increased by additional binding of fulvic acid or its metal complexes. Adsorption of dissolved organic matter (DOM) to abiotic (26) and biotic (27) surfaces is known to occur, and our data indicate that fulvics also adsorb to the fibers of both types of polyacrylamide gel used in this study. The amide groups of polyacrylamide have both electrondonating (hydrogen-accepting) activity with the carbonyl oxygen atoms and electron-accepting (hydrogen-donating) activity with the amide protons. An adsorption mechanism involving weak hydrogen bonds with fulvic acid macromolecules is therefore a possible explanation for the increased Cgel/Cwater of Cd (Figure 3) and Cu (Figure 4c) in the presence of fulvic acid. The capacity of agarose gel and filter for enrichment of trace metal cations in soft water is not limited by a low concentration of binding sites, as Cgel/Cwater and Cfilter/Cwater VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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were unaffected by metal concentration. Fatin-Rouge et al. (16) also came to this conclusion after an investigation of diffusion and partitioning of solutes in agarose gels. They suggested that pyruvate is the most important functional group causing enrichment of metals in agarose gel through electrostatic and chemical interactions. We found that Cu and Pb bind more strongly to the agarose gel and filter than the other metals (e.g., Cd), which were displaced in the presence of high concentrations of Na, Ca, and Mg. Reference 16 reports intrinsic equilibrium constants for Pb, Cu, and Cd complexes with sites in agarose gel showing a slightly different ranking of stability: Pb > Cu ≈ Cd. Complexation by fulvic acid decreased the enrichment of metals in agarose gel, showing that the interaction between metal-fulvic complexes and agarose gel is much smaller than for inorganic metal and agarose gel, or metal-fulvic complexes and polyacrylamide gels. Enrichment of metals in the filter, however, was little affected by complexation with fulvics, suggesting that the filter has similar affinity for cationic metal and anionic metal-fulvic complexes. Sorption of cations and anions to the same filter has previously been demonstrated for many types of filters (20, 28). Implications. The higher concentration of metals under many circumstances in the hydrogel and filter than in the bulk solution indicates that these components of DET and DGT cannot always be considered as inert with respect to diffusion of trace metals. Polyacrylamide gels contain a relatively low concentration of sites that selectively bind some trace metal cations. Furthermore, they have affinity for fulvic acid and/or its metal complexes. Agarose gel and Supor filter membrane contain negatively charged sites causing ionic strength-dependent enrichment of cations in freshwater, and some trace metals (Cu, Pb) bind strongly to gel and filter sites. The affinity of agarose gel for metal-fulvic complexes is lower than for trace metal cations, while filter appears to have similar affinity for both species. The assumption of DET, that dialyzable solutes reach the same concentration in a hydrogel as in sediment porewaters, is well-tested for major ions (29, 30). It has also been verified in solutions containing high concentrations (>0.2 mg/L) of divalent Fe, Mn, and Cd by performance tests of DET assemblies containing polyacrylamide gel (4, 8, 14), and in solution containing 10 mg/L Mn in 17 mM NaCl with agaroseDET assemblies (5). The present findings imply that DET must be used cautiously for measurement of low concentrations of metals. The concentration of those metals that bind strongly to hydrogel sites (e.g., Cu and Pb) is likely to be overestimated by DET, but the technique appears to be appropriate for weakly binding metals (e.g., Re, U, Mo (see ref 8), Mn, Co, Ni, Cd) in marine sediments. In freshwater sediments, even for metals like Co, Ni, and Cd, binding in the gel may introduce systematic errors as large as a factor of 2-3 in DET measurements. Enrichment of Cd in agarose gel is mainly dependent on ionic strength, and Cd speciation, while enrichment in polyacrylamide gel is mainly dependent on concentration of Cd, metal speciation, and presence of strongly binding cations (e.g., Cu, Pb). Other weakly binding metals probably behave similarly to Cd. It may be difficult to predict the precise response of a DET probe with polyacrylamide gel for a weakly binding metal in freshwater sediment. Low concentrations of a strongly binding cation, such as Cu, may displace weakly binding cations from binding sites, but if strongly binding cations are complexed by DOM, cations of weakly binding metals may still bind. Moreover, the affinity of polyacrylamide gels for complexes with the fulvic fraction of DOM makes it difficult to predict how the presence of DOM will affect metal partitioning. For agarose gel, competition between different trace metals is insignificant and presence of DOM will not 5686

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lead to increased metal enrichment. Agarose gel may therefore be a better choice for DET probes in freshwater sediment. A bias of a factor 2-3 that can be accommodated in the calibration may be acceptable if it is relatively constant, especially if the principal aim is to study relative changes in concentration profiles in sediments. We recommend that realistic (with respect to concentration and matrix) testing of DET is undertaken in the laboratory before it is used for previously untested metals. Binding of metals in polyacrylamide gel and filter, together comprising the diffusion layer most commonly applied in DGT, should not affect the steady flux of metal from solution to sorbent in the DGT device after the binding capacity is reached. It will, however, prolong the time necessary to establish the steady flux. Furthermore, because the underlying sorbent layer is a very efficient metal accumulator, metal bound in the diffusive layer may well be relocated to the sorbent if the sorbent layer is not retrieved immediately after deployment of the DGT sampler. Further research is being undertaken to investigate the significance of the present findings for DGT.

Acknowledgments We thank Debbie Hurst for help with the collection of samples in the field. NERC provided funding (NE/D001145/1).

Supporting Information Available Description of the experimental procedures for preparation of hydrogels, deployment of DET probes in sediment, results from the sediment experiment, and a tabulated summary of findings for each hydrogel/filter. This information is available free of charge via the Internet at http://pubs.acs.org.

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