ARTICLE pubs.acs.org/est
Combination of Diffusive Gradient in a Thin Film Probe and IC-ICP-MS for the Simultaneous Determination of CH3Hg+ and Hg2+ in Oxic Water Yong Seok Hong,†,‡ Erik Rifkin,‡ and Edward J Bouwer†,* †
Department of Geography and Environmental Engineering, Johns Hopkins University, 3400 North Charles Street, Ames Hall 313, Baltimore, Maryland 21218, United States ‡ National Aquarium Conservation Center, Baltimore National Aquarium, 501 East Pratt Street, Baltimore, Maryland 21202, United States
bS Supporting Information ABSTRACT: A diffusive gradient in thin film technique (DGT) was combined with ion chromatography and inductively coupled plasma mass spectrometry (IC-ICP-MS) for the in situ simultaneous quantification of CH3Hg+ and Hg2+ in aquatic environments. After diffusing through an agarose diffusive layer, the Hg species accumulated in a thiolfunctionalized resin layer and were extracted using acidic thiourea solution to form stable thioureaHg complexes that were separated and detected via ion chromatography and ICPMS, respectively. The effective diffusion coefficients of CH3Hg+ and Hg2+ complexes in the agarose diffusion layer with chloride were 5.26 ((0.27) 106 and 4.02 ((0.10) 106 cm2 s1, respectively. The effective diffusion coefficients of CH3Hg+ and Hg2+ complexes in the agarose diffusion layer with dissolved organic matter was 3.57 ((0.29) 106 and 2.16 ((0.19) 106 cm2 s1, respectively. The practical method detection limits are 0.1 and 0.7 ng L1 for CH3Hg+ and Hg2+ respectively for three weeks deployment. Lower detection limits would be possible by employing a thinner agarose diffusive layer and/ or by deploying the probes longer. The method can measure time averaged CH3Hg+ and Hg2+ concentrations simultaneously in oxic water, making it useful as an in situ monitoring tool.
’ INTRODUCTION Mercury (Hg) is a metal contaminant that is highly toxic, globally ubiquitous, and environmentally persistent. In oxic water, mercury typically exists as inorganic forms, that is, Hg2+, which is strongly complexed with ligands, such as OH, Cl, and dissolved organic matter (DOM).1,2 In anoxic water and sediments, the inorganic mercury can be microbially transformed to methylmercury, primarily to CH3Hg+, which is more toxic and bioavailable than inorganic mercury. Because the CH3Hg+ is likely to be magnified in biota through trophic transfer,1,2 elevated Hg concentrations in fish, sometimes exceeding the concentrations safe for human consumption, are observed in aquatic systems with low dissolved Hg levels.3 Although there are great needs for monitoring CH3Hg+ and Hg2+ concentrations in water for assessing the risks associated from Hg in ecosystems,4,5 analysis of Hg is challenging due to lengthy analytical procedures, numerous potential artifacts, and the high cost of the techniques.68 Conventional procedures for Hg analysis consist of sampling (USEPA Method 1669) and the use of USEPA Method 1630 for CH3Hg+ and USEPA Method 1631 for total Hg. The low levels of Hg in natural waters mean that contamination from background sources is common during sample handling and preparation in the laboratory, and the difference between total Hg and CH3Hg+ yields Hg2+, which may not be accurate from the combined uncertainties of the two r 2011 American Chemical Society
methods.8 The complicated procedures often restrict the scope of environmental research on Hg 6 and the assessment of Hg pollution in ecosystems.8 As an alternative to the current Hg monitoring procedures, we developed a diffusive gradient in thin film technique (DGT) 9 coupled to ion chromatography inductively coupled plasma mass spectrometry (IC-ICP-MS) for the in situ measurement of CH3Hg+ and Hg2+ in oxic water, that can be summed to yield total Hg. DGT was originally developed for in situ determination of kinetically labile metal species in aquatic systems 9 and has been successfully used to measure trace metal concentrations in natural waters.10 Recently, the technique has been used to monitor total Hg 11,12 and CH3Hg+ 6,13 in various environmental matrices, such as waters and sediments. However, the previous probes only detect either total Hg or CH3Hg+ and need to be deployed in multiple numbers for the Hg speciation studies. The present paper summarizes the development of DGT coupled to an IC-ICP-MS technique for the simultaneous determination of environmentally relevant levels of CH3Hg+ and Hg2+ in oxic waters. Received: February 2, 2011 Accepted: June 16, 2011 Revised: June 13, 2011 Published: June 16, 2011 6429
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’ MATERIALS AND METHODS The experimental approaches are briefly described in this manuscript, and details are available in the Supporting Information. Ion Chromatographic Separation of CH3Hg+ and Hg2+. A high-pressure liquid chromatography (HPLC) system (PerkinElmer/ Sciex, Concord, ON, Canada) was used to separate CH3Hg+ and Hg2+ complexes via a cation exchange column (IonPac CG5A, Dionex, Sunnyvale, CA, USA).7 The HPLC unit was connected to a quartz cyclonic spray chamber of an Elan DRCTM II ICP-MS (PerkinElmer/Sciex, Concord, ON, Canada) through an automatic switching valve (Rheodyne, Rohnert Park, CA, USA) that allowed the system to be operated in either HPLC-ICP-MS mode or ICP-MS only mode. The ion chromatographic separation of CH3Hg+ and Hg2+ relies on the higher stability constants of the Hg species complexed to thiourea (TU) than to halides (Cl, Br) and to thiols (SH) at pH less than 1.7,8 Because TU has characteristics of both a strong Bronsted acid and a very strong Lewis base, TU is not protonated at low pH and strongly complexes with soft metals, such as Hg2+. Hence, an acidic thiourea solution (1.5% thiourea + 6.5% concentrated nitric acid + 10% glacial acetic acid) was used to form the cationic charged species of Hg, such as CH3Hg(TU)+ and Hg(TU)42+, and the complexes were then separated in a cation exchange column using the acidic thiourea solution as a mobile phase.7 Concentrated nitric acid was used to make the acidic thiourea solution instead of concentrated hydrochloric acid because halogenated acids are not recommended for the instrument. Preparation of DGT Probes. The DGT probes used in the present study employ a series of layers including a protective filter membrane, a diffusive layer, and a resin layer in a plastic unit purchased from DGT Research Ltd. (Lancaster, UK).9,14 A diffusive layer made of 1.5% agarose (cat # BP1356100, genetic analysis grade, Fisher Scientific, USA) was used in the present study because an acrylamide diffusive layer is known to adsorb Hg2+.12 Beads of 3-Mercaptopropyl Functionalized Silica Gel (3MFSG, Sigma Aldrich), which have a thiol functional group were used in the resin layer. A 0.45 μm polysulfone membrane filter (VWR) was used as the filter membrane. The probes were prepared according to the procedures developed by Zhang and Davidson 14 with slight modifications; details are available in Supporting Information. Batch Sorption and Elution Experiments. The batch sorption experiments were performed in 18 mL VWR conical-bottom Teflon centrifuge tubes to test the resin layer’s uptake efficiency of CH3Hg+ and Hg2+. Resin layer with 1.5 cm diameter was added to the centrifuge tube with 0.01 M NaNO3. Appropriate volumes of 1.0 mg L1 CH3HgOH and 1.0 mg L1 of Hg(NO3)2 were added to each tube. To neutralize the acid, an equimolar quantity of either 0.1 or 1.0 M NaOH was added. The solution was finally buffered with 0.20.5 mM NaHCO3, and the pH was kept constant at pH 8.04 ((0.03) during the experiment. The tubes were tumbled in an end-over-end tumbler at 20 °C for 24 h. After the equilibration period, the reactors were centrifuged at 1500 g for 5.0 min. The supernatant from each reactor was transferred to a 15 mL polypropylene centrifuge tube, then 1.0% of bromine monochloride solution (5% KBrO3 and 10% KBr in concentrated HCl) was added to the solution volumetrically to stabilize Hg. Hg concentrations in the supernatant were measured using ICP-MS, and resin layer uptake was determined from
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the mass balance. Hg with 201.97 atomic mass unit (AMU) was used for the ICP-MS analysis. Scan mode was peak hopping and dwell time per AMU was 200 ms resulting in an integration time of 8000 ms. The ICP-MS instrument detection limit for Hg was 1.0 ng L1, which was calculated from 3 standard deviations of the analysis of 8 blanks. The resin layers in the Teflon tubes were transferred to 1.5 mL polypropylene microcentrifuge tubes. One milliliter of acidic thiourea solution was added to each tube for a contact time of 1 day. The CH3Hg+ and Hg2+ in the acidic thiourea solution were separated and detected by the IC-ICP-MS instrument. DGT Laboratory Experiments. The linear uptake of Hg species over time and the simultaneous detection of both CH3Hg+ and Hg2+ were tested using 18 probes with 0.75 mm agarose diffusive layer. The DGT probes were exposed to one of the following solutions: (a) 1.0 μg L1 CH3Hg+, (2) 1.0 μg L1 Hg2+, and (3) 1.0 μg L1 CH3Hg+ plus 1.0 μg L1 Hg2+, containing 0.01 M NaNO3 for 2, 3, 4, 5, 6, and 7 h. Each DGT probe was placed on top of a 250 mL wide mouth Erlenmeyer flask containing 200 mL of Hg solution. The volume of Hg solution in each flask and the DGT deployment time were chosen to minimize the depletion of overlying water Hg species concentration during the experiment. Only the filter side of the DGT was exposed to the aqueous solution. Then, the probes and flasks were sealed using Teflon and Parafilm tape. The Erlenmeyer flasks were placed upside down on a shaker table and incubated for the established time periods at 270 rpm and 25 °C. The pH was adjusted to circum-neutral in a similar manner as with the batch sorption experiment and checked at the end of the experiment. A separate DGT deployment experiment using cadmium suggested that diffusive boundary layer thickness was 0.04 mm under the given mixing conditions, and the thickness was included in the total diffusion thickness. The overlying water Hg concentrations were measured at the end of the experiments using ICP-MS, and typically less than 10% Hg loss was observed during the experiments. After the experiments, resin layers were retrieved and soaked in 1.0 mL of acidic thiourea solution in 1.5 mL centrifuge tubes for 24 h. The extracted CH3Hg+ and Hg2+ were separately detected by the IC-ICP-MS technique. The effects of CH3Hg+ and Hg2+ complexes with Cl and DOM on DGT performance were investigated. Solutions with 0.001, 0.01, 0.1, and 0.6 M NaCl as the principal electrolyte with 1.0 μg L1 of CH3Hg+ plus 1.0 μg L1 of Hg2+ were prepared, and three DGT probes (0.75 mm agarose diffusive layer) were exposed to each solution for 6 h. In addition, 1.0 μg L1 of CH3Hg+ plus 1.0 μg L1 of Hg2+ were spiked with 0.1 M NaNO3 electrolyte containing 10 mg L1 dissolved organic carbon (Suwannee River DOM, International Humic Substances Society, www.ihss.gatech.edu/index.html) and aged for 2 days. Then, three DGT probes for each three different agarose diffusive layer thicknesses (0.5, 0.75, 1.0 mm) were deployed to the solution to investigate the response of DGT probes to HgDOM complexes. DGT Field Test. Seven DGT units (total 21) were successfully deployed in the field, including a fresh water (South River, Waynesboro, Virginia), an estuarine water (Chesapeake Bay, Rocky point, Maryland), and a saline water (Sarasota Bay, Sarasota, Florida), all of which are located in the U.S. The probes were attached to 10 in. long garden stakes or similar devices and suspended in the overlying water 2 to 3 cm above the sediment water interface after anchoring the garden stakes to the sediments. After retrieval from the field, the DGT units were carefully 6430
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rinsed with site waters and transported to the laboratory in clean polyethylene bags overnight. In the laboratory, resin layers were removed from the probes and soaked in about 1.0 mL of acidic thiourea solution for 1 day to determine CH3Hg+ and Hg2+ by IC-ICP-MS. Water samples were collected from each site at the beginning and at the end of the experiment to determine total Hg by ICP-MS, DOC by TOC analyzer (Phoenix 8000, Tekmar Dohrmann, Mason, OH, USA), and Cl concentrations by ion chromatography (DX-120, Dionex, Sunnyvale, CA, USA) with an anion exchange column (IonPacAS14, Dionex, Sunnyvale, CA, USA). Temperature, salinity, dissolved oxygen concentrations, and pH were also recorded using conductivity (Orion 013010MD), DO (Orion 083010MD), and pH (Orion 9107BNMD) electrodes connected to an Orion 5 Star portable meter (Thermo Fisher Scientific, Waltham, MA, USA). To monitor for possible mercury contamination during fabrication, transportation, deployment, and retrieval, two or three control probes were subjected to the same processes as the field deployed probes, but they were not exposed to the waters. From the control probes, no CH3Hg+ and Hg2+ were observed in the extractants indicating insignificant Hg contamination during the sample handling and processing. Estimation of Dissolved Hg Species Concentrations Using DGT Probe. Typically, the concentration of labile metal present in bulk solution can be estimated from the total amount of metal ions accumulated in the resin layer as follows:14 Cb ¼
M Δg DtA
ð1Þ
where Cb is the metal concentration in bulk solution, M is the accumulated metal in the resin, t is the deployment time, D is the metal’s diffusion coefficient in the hydrogel, A is the exposed interfacial area, and Δg is the total thickness of the diffusion layer including the diffusive boundary layer, filter membrane, and diffusive layer. The approach may not be applicable to estimate the Hg concentrations because Hg species are known to be strongly associated with Cl and DOM in aerobic environments, as the diffusion coefficients are altered in the diffusion layer.15 In such cases, the total mass of Hg accumulated in a single DGT device would be the sum of the Hg accumulated by the diffusion of Hginorganic ligand and Hgorganic ligand complexes as follows 15 MHg, DGT ¼ MHginorg + MHgorg
MHginorg
MHgorg ¼
CHgorg DHgorg t A Δg
MHg, DGT ¼
ð3Þ ð4Þ
where CHg-inorg and CHg-org are the aqueous phase concentrations of Hginorganic ligand complexes and Hgorganic ligand complexes respectively, and DHg-inorg and DHg-org are the effective diffusion coefficients of Hg complexed with inorganic and organic ligands, respectively. Introducing eq 3 and eq 4 into
tA ðCHginorg DHginorg + CHgorg DHgorg Þ Δg
ð5Þ + 16
2+ 17
and Hg in By running speciation models of CH3Hg aerobic water, which are described in the Supporting Information, the fractions of Hg species complexed with organic (fHg-org) and inorganic (fHg-inorg = 1 fHg-org) ligands can be estimated. The Hg species concentrations complexed with inorganic and organic ligands are as follows: CHginorg ¼ CTotHg fHginorg
ð6Þ
CHgorg ¼ CTotHg fHgorg
ð7Þ
where CTotHg is total CH3Hg+ or Hg2+ concentrations in the aqueous phase. Then eq 5 can be rewritten as follows by factoring out CTotHg: CTotHg t A ðfHginorg DHginorg Δg + fHgorg DHgorg Þ
MHg, DGT ¼
ð8Þ
Here, the apparent diffusion coefficient, which describes the average diffusion coefficient of Hg species depending on their fractional association with inorganic and organic ligands, is defined as follows DHg, App ¼ fHginorg DHginorg + fHgorg DHgorg +
ð9Þ
2+
and the total Hg species (CH3Hg and/or Hg ) can be estimated as follows. CTotHg ¼
MHg, DGT Δg t A DHg, App
ð10Þ
The apparent diffusion coefficient may depend on the temperature and can be corrected by the following equation 14 logDTm ¼
ð2Þ
where MHg,DGT is the total mass of Hg species (either CH3Hg+ or Hg2+) accumulated in the resin layer, MHg-inorg and MHg-org are the mass of Hg accumulated in the resin from the diffusion of Hginorganic ligand complexes and Hgorganic ligand complexes, respectively. The MHg-inorg and MHg-org can be calculated as follows: CHginorg DHginorg t A ¼ Δg
eq 2 and factoring out (t A/Δg), which is a known constant, the eq 2 can be written
f1:37 ðTm 25Þ + 8:36 104 ðTm 25Þ2 g ð109 + Tm Þ ð273 + Tm Þ + log D25 ð11Þ 298
where DTm and D25 are the diffusion coefficients of Hg at Tm °C and 25 °C, respectively.
’ RESULTS AND DISCUSSION Ion Chromatographic Separation of CH3Hg+ and Hg2+.
Successful separation of CH3Hg+ and Hg2+ was observed by ICICP-MS with detection limits of 20 ng L1 and 100 ng L1 respectively based upon a signal-to-noise ratio greater than 3 with an injection volume of 150 μL. Clear separation of the Hg species was observed as CH3Hg+ eluted at 1.2 min and Hg2+ eluted at 7.0 min with continuous mobile phase flow rate of 0.5 mL min1. The retention times of Hg species were verified by injecting blanks spiked with either CH3Hg+ or Hg2+. Example chromatograms are available in Figure S4 (Supporting Information). To decrease the analysis time, the mobile phase flow rate was increased from 0.5 to 1.0 mL min1 after 2 min. Then, the Hg2+ species were detected at 4.5 min and total analysis time was 6431
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agreement in accumulating the two Hg species without any interactions and interferences between the two Hg species in the diffusive and resin layers. The effective diffusion coefficients, which were estimated using the following rearranged form of eq 1, D¼
Figure 1. IC-ICP-MS chromatograms of blanks spiked with (a) 1.0 μg L1 CH3Hg+ and 1.0 μg L1 Hg2+ and (b) 0.1 μg L1 CH3Hg+ and 0.1 μg L1 Hg2+ species.
Figure 2. Time dependent (a) CH3Hg+ and (b) Hg2+ mass accumulation in the 3-mercaptopropyl functionalized silica gel resin layer. Hollow triangle (Δ) data are from DGT deployed in solutions containing either 1.0 μg L1 CH3Hg+ or 1.0 μg L1 Hg2+. Solid circle (b) data are from DGT deployed in solutions containing both 1.0 μg L1 CH3Hg+ and 1.0 μg L1 Hg2+.
decreased from 10 to 6 min. The typical chromatograms of blank acidic thiourea solutions spiked with 0.1 or 1.0 μg L1 of CH3Hg+ and Hg2+ are shown in parts a and b of Figure 1. Batch Sorption and Elution Experiments. The 3MPFSG resin adsorbed more than 99% of the CH3Hg+ and Hg2+ from the water over a concentration range between 20 ng L1 and 2000 ng L1 (0.617 ng Hg cm2 resin layer) (part a of Figure S7 of the Supporting Information). The resin showed a good Hg species adsorption capacity, which is consistent with previous studies,68 and the selected resin is considered to be the ideal resin for the DGT technique as it effectively accumulates both CH3Hg+ and Hg2+. The experimental results that evaluated the extraction efficiency of adsorbed metals from the resin are shown in part b of Figure S7 (Supporting Information). A linear relation close to 1:1 was found between the mass of Hg species adsorbed to the resin layer and the quantity of Hg species measured in the extractant. The average extraction efficiencies were 94.6 ((4.2) % and 101.2 ((7.0) % for CH3Hg+ and Hg2+ respectively showing good recoveries of the Hg species. These numbers are used for the later DGT experiments when the accumulated Hg species were calculated from the Hg species concentrations in the extracts. Testing the Applicability of the DGT Probe. The points on Figure 2 represent the measured CH3Hg+ and Hg2+ mass in the resin layer from acid thiourea extraction and IC-ICP-MS determination. The points overlapped well and showed good
M Δg Cb t A
ð12Þ
were 5.30 ((0.33) 106 and 4.48 ((0.30) 106 cm2 s1 for CH3Hg+ and Hg2+, respectively. The diffusive layer made of agarose or polyacrylamide has a larger pore size, so that the effective diffusion coefficient of metals in the diffusive layer is similar to the diffusivity of metals in water. The observed effective diffusion coefficient of CH3Hg+ was close to the reported effective diffusion coefficient of 5.1 ((0.30) 106 cm2 s1 of CH3Hg+ in the polyacrylamide layer at 20 °C.6 However, the effective diffusion coefficient for Hg2+ was smaller than the reported diffusivity for Hg2+ in water and in the agarose layer of 9.0 106 cm2 s1.12 The lower effective diffusion coefficient of Hg2+ in the agarose layer can be attributed to kinetically limited Hg sorption to the resin layer, creating nonzero mercury concentrations in the resin. To incorporate the smaller mass flux from the nonzero concentration in the resin, a smaller effective diffusion coefficient of Hg2+ may be needed. If this is true, increasing the thickness of the diffusive layer will decrease the Hg flux to the resin layer and a higher effective diffusion coefficient of Hg2+ may be obtained (Supporting Information for further explanation). Hence, two additional DGT probes with increased thickness of the diffusive layer to 1.0 mm, were made and deployed in the same overlying water condition for 6 h. The effective diffusion coefficients of CH3Hg+ and Hg2+ were 5.50 ((0.05) 106 and 4.51 ((0.74) 106 cm2 s1 respectively, which were not statistically different from those values evaluated with the 0.75 mm diffusive layer thickness indicating that Hg sorption to the resin layer is not kinetically limited. The most probable explanation for the observed lower effective diffusion coefficient of Hg2+ in agarose layer compared to the metal’s diffusivity at infinite dilution is the combination of Hg2+ complexation with OH and Hg2+ retardation in agarose layer. The pH of the solution was 7.8 and the dominant Hg2+ species was the Hg(OH)2(aq). The molecular weight of the species was similar with that of CH3HgOH(aq), which was the dominant CH3Hg+ species at that pH. Hence, the diffusion coefficients of Hg2+ and CH3Hg+ should be similar; however, Hg2+ had 20% smaller effective diffusion coefficient. Docekalova and Divis 12 observed 4.5 times greater Hg2+ concentrations in agarose layer compared to water Hg2+ concentrations after 8 h equilibration. The actual residence time of Hg2+ in the diffusive layer is about 10 min for a 1 mm diffusive layer and diffusion coefficient of 5 106 cm2 s1.14 Strong sorption of Hg2+ to the agarose layer is not expected, but slight retardation of Hg2+ in the layer is possible. To check whether agarose with different grade may change the effective diffusion coefficients of the mercury species, two additional DGT probes were fabricated using different agarose layers (cat # BP1423500, molecular genetics grade, Fisher Scientific, USA). From the experiment, similar effective diffusion coefficients of 5.55 ((0.01) 106 and 4.77 ((0.22) 106 cm2 s1 for CH3Hg+ and Hg2+ were obtained indicating no agarose dependence for the effective diffusion coefficients. 6432
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Table 1. Effective Diffusion Coefficients of CH3Hg+ and Hg2+ in the Agarose Diffusion Layer at 25 °C in the Presence of Chloride or Dissolved Organic Matter (DOM) Determined by Dissolved Organic Carbon (DOC); the Values Are the Average (Standard Deviation) of Triplicate Measurements water matrix
pH
agarose layer thickness (mm)
Deff,CH3Hg+ (106 cm2 s1)
Deff,Hg2+ (106 cm2 s1)
0.001 M NaCl
7.67 (0.06)
0.75
5.42 (0.16)
4.11 (0.06)
0.01 M NaCl
7.65 (0.04)
0.75
5.38 (0.20)
4.04 (0.04)
0.1 M NaCl
7.66 (0.04)
0.75
5.26 (0.39)
3.95 (0.11)
0.6 M NaCl
7.70 (0.04)
0.75
4.98 (0.07)
3.98 (0.12)
Overall 0.1 M NaNO3, 10 mg C L1
7.67 (0.04) 7.22 (0.06)
0.5
5.26 (0.27) 3.22 (0.11)
4.02 (0.10) 1.96 (0.09)
0.1 M NaNO3, 10 mg C L1
7.25 (0.05)
0.75
3.82 (0.16)
2.22 (0.04)
0.1 M NaNO3, 10 mg C L1
7.33 (0.02)
1.0
3.65 (0.10)
2.33 (0.16)
overall
7.27 (0.07)
3.57 (0.29)
2.16 (0.19)
Although the diffusion of Hg2+ behaved differently from what was expected, the experiment suggested that agarose can be used as an effective diffusion layer leading to a linear increase of mass in the resin over time which is one of the main principles behind the original work.9 This linear response of the resin should allow for good estimations of bulk water concentrations when DGT is applied to aquatic environments to determine Hg species levels. Diffusional Characteristics of HgCl and HgDOM Complexes. In natural oxic water, Hg species are strongly associated with Cl, hence Hg species are often present in their complexed forms in aquatic environments. The effective diffusion coefficients of CH3Hg+ and Hg2+ in the presence of Cl calculated from the experimental data using eq 12 are summarized in Table 1. Generally, the effective diffusion coefficients of CH3Hg+ and Hg2+ tended to decrease as the NaCl concentrations increased; however, the differences were less than 9% and were not significant indicating negligible ionic strength effect in the performance of DGT probes when Hg species were complexed with chloride. This observation was consistent with a previous study that showed insignificant ionic strength effect with DGT probes for Cd2+ detection because the maximum decrease in the diffusion coefficient was expected to be 10% in salt water compared to a freshwater system.14 Hence, the average effective diffusion coefficients of 5.26 ((0.27) 106 and 4.02 ((0.10) 106 cm2 s1 for CH3Hg+ and Hg2+ respectively can be used when Hg species are complexed with chloride regardless of ionic strength to estimate aqueous phase Hg concentrations using DGT. Similar to chloride, Hg2+ makes strong complexes with thiol functional groups in DOM. The complexation of Hg species to DOM often complicates the DGT performance because the diffusion coefficients of DOM are typically markedly lower than those of metals complexed with small inorganic ligands.15,18 Moreover, the affinities of Hg species to thiol functional groups are much higher than those to halides (Cl, Br), so the lability of Hg species and the amount of Hg species accumulated in the resin layer may be affected by the thickness of the diffusive layer.19 The effective diffusion coefficients of CH3Hg+ and Hg2 in the presence of DOM are summarized in Table 1. As expected, the effective diffusion coefficients of CH3Hg+ and Hg2+ were lower than those observed in the absence of DOM, and the values were consistent with literature reported values of DOM diffusion coefficients. Diffusion coefficients of the Suwannee River fulvic and humic acids in water ranged between 1.9 106 and 3.5 106 cm2 s1 with values remaining constant as a
function of both pH (4.0 to 8.5) and ionic strength (0.005 to 0.5 M),20,21 and our measurements were at the lower end of the measured values. In another study,15 the diffusion coefficients of humic and fulvic acids in agarose layer varied between 1.19 106 and 1.92 106 cm2 s1 at 20 °C, which are similar to our measurements at 25 °C. DOM consists of both humic and fulvic acids, which range in size between 0.5 and 400 nm and have molar weights ranging from 200 to 105 g mol1 or greater.22 Typically, fulvic acids have smaller sizes and faster diffusion coefficients compared to humic acids. Because CH3Hg+ tends to more strongly associate with humic acid than fulvic acid,23 the faster effective diffusion coefficient for CH3Hg+ compared to Hg2+ would not be related to the association of CH3Hg+ to the smaller size fulvic acids. The latter would be expected to diffuse faster than humic acid in the agarose diffusive layer. The reason is more likely related to the higher lability of CH3Hg+, which was postulated by Cattani et al.18 CH3Hg+ is considered to have a lower affinity for the thiol functional groups in organic matter than Hg2+ because the attachment of the methyl functional group reduces the Hg’s affinity to thiols. This implies that CH3Hg+DOM complexes are more labile than Hg2+DOM complexes and have a higher dissociation rate constant. Similar behavior of metalDOM complexes was observed in a previous study.19 Hence, the faster effective diffusion coefficients of CH3HgDOM complexes probably indicates that the complexes were dissociating in the diffusion layer, and the free CH3Hg+ moves within the diffusion layer with an effective diffusion coefficient of 5.3 106 cm2 s1. This was further supported by the differences in the effective diffusion coefficients observed under different thicknesses. The effective diffusion coefficients tended to decrease (be slower) by decreasing the diffusion layer thickness. The thinner thickness means a shorter residence time of HgDOM complexes in the layer, which would not allow for the dissociation of the complexes. In contrast, the thicker diffusion layer means longer residence time for HgDOM complexes, which can allow for more dissociation. Generally, however, the differences were less than 10%, therefore, 3.57 ((0.29) 106 and 2.16 ((0.19) 106 cm2 s1 as effective diffusion coefficients for CH3Hg+ DOM and Hg2+DOM complexes can be used to determine dissolved Hg species concentrations by DGT. Field Test. The DGT combined to IC-ICP-MS technique was tested in South River, Chesapeake Bay, and Sarasota Bay. South River is a fresh water stream and is contaminated by a former fabric fiber manufacturing plant that discharged Hg to the stream during 19301950.24 Chesapeake Bay and Sarasota Bay are 6433
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Table 2. DGT Estimated CH3Hg+ and Hg2+ Concentrations in the Fresh, Estuarine, and Saline Water Environments Location parameters
South River VA
Chesapeake Bay MD
Sarasota Bay FL saline water
water
freshwater
estuarine water
deployment duration (days)
13
24
5
temp (°C)
1516
1722
2223
pH
8.058.41
8.29.2
8.28.5
conductivity (μS cm1)
169285
5402420
49 30050 400
oxygen (mg L1)
9.29.8
8.310.7
6.06.7
DOC (mg L1)
1.33.9
3.45.4
1.01.1
Cl (mM) filtered water THg c (ng L1)
0.0270.048 4.1 (1.8)
1423 3.3 (0.6)
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