Measurement of Stable and Radioactive Cesium in Natural Waters by

Jun 17, 2009 - Worsfold Water Quality Centre, Trent University, 1600 West Bank Drive, Peterborough, Ontario, K9J 7B8, Canada, Department of Environmen...
0 downloads 0 Views 184KB Size
Download PDF
Anal. Chem. 2009, 81, 5889–5895

Measurement of Stable and Radioactive Cesium in Natural Waters by the Diffusive Gradients in Thin Films Technique with New Selective Binding Phases Weijia Li,*,† Feiyue Wang,‡ Weihua Zhang,§ and Douglas Evans† Worsfold Water Quality Centre, Trent University, 1600 West Bank Drive, Peterborough, Ontario, K9J 7B8, Canada, Department of Environment and Geography, and Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada, and Radiation Protection Bureau, Health Canada, A.L.:6302D1, 775 Brookfield Road, Ottawa, Ontario, K1A 1C1, Canada A cesium-specific diffusive gradients in thin films (DGT) technique was developed using copper ferrocyanide (CFCN) as the binding agent. Two types of DGT binding phases were evaluated, one by immobilizing CFCN on Chelex 100 resin gels (Chelex-CFCN) and the other on poly(acrylic acid) gels (PAA-CFCN). Both DGT devices were successfully applied to the measurement of low levels of stable 133Cs and radioactive 137Cs in synthetic solutions and in natural river waters. In all cases, the DGT labile concentrations measured with the PAACFCN DGT agreed very well with total dissolved Cs concentrations, whereas those measured by the ChelexCFCN DGT were much lower than total Cs concentrations. The difference was attributed to the different binding kinetics of Cs+ on the two gels suggesting that this might be a promising means of measuring biologically relevant Cs concentrations in natural waters. Radiocesium is one of the most important artificial radionuclides in the environment and has been of great concern over the past few decades. It may be released from anthropogenic activities such as nuclear weapons testing,1 nuclear facility accidents,2 and nuclear waste management.3 Once entering the aquatic environment, Cs can be assimilated by aquatic organisms and thus enter the food chain because of its biochemical similarity to the essential element potassium.4 Both external and internal exposure to cesium can occur, with higher concentrations occurring usually in muscle and lower in bones.5 Among the radiocesium isotopes, 137Cs, a γ ray emitter, is of particular concern because of its moderately long (several * Corresponding author. Phone: 1-705-748-1011-7886. Fax: 1-705-748-1625. E-mail: [email protected]. † Trent University. ‡ University of Manitoba. § Health Canada. (1) Almgren, S.; Isaksson, M. J. Environ. Radioact. 2006, 91, 90–102. (2) Ra¨a¨f, C. L.; Hubbard, L.; Falk, R.; Ågren, G.; Vesanen, R. Sci. Total Environ. 2006, 367, 324–340. (3) Harjula, R.; Lehto, J.; Paajanen, A.; Brodkin, L.; Tusa, E. Nucl. Sci. Eng. 2001, 137, 206–214. (4) Anjos, R. M.; Mosquera, B.; Sanches, N.; Cambui, C. A.; Mercier, H. Environ. Exp. Bot. 2009, 65, 111–118. (5) Yamada, M.; Nagaya, Y. J. Radioanal. Nucl. Chem. 1998, 230, 111–114. 10.1021/ac9005974 CCC: $40.75  2009 American Chemical Society Published on Web 06/17/2009

decades) radiation impact as a medium-lived fission product (τ1/2 ) 30.17 years). 137Cs has received much attention as a result of the 1986 accident at the Chernobyl Nuclear Power Plant in the former Soviet Union, which released a large plume of radioactive fallout to the adjacent and other areas including the western part of the former Soviet Union, most of Europe, and parts of eastern North America, resulting in elevated concentrations of 137Cs in water and fish.6 As of 2005, 137Cs was still the principal source of radiation in and around the zone of alienation.7 Other radioactive isotopes of cesium, 134Cs (T1/2 ) 2 years) and 135Cs (T1/2 ) 2.3 million years), are also important in the environment. Although they are less hazardous than 137Cs due to their quick decay or weak radiation,8 their unique decay half-lives can be indicators of their transport and remaining radiation activity9 in the environment. Traditional methods to measure the extremely low concentrations of cesium in waters in the presence of high concentrations of coexisting matrix cations, such as Na+, K+, Ca2+, and Mg2+, usually involve the collection10 and preconcentration11 of large amounts of water. This approach is therefore labor intensive, time-consuming, cost ineffective, and prone to cross contamination. Potentially these constraints could be improved by the application of the diffusive gradients in thin films (DGT) technique.12,13 DGT is an in situ preconcentrating device consisting mainly of a well-defined diffusive layer and a binding phase. The concentrating process in DGT also resembles biological mechanisms, and thus provides a means of estimating “bioavailable” metal concentrations in natural waters.14,15 Chang et al.16 were the first to apply the DGT technique, with a cation exchange resin (AG50W-X8) as the binding agent, for cesium measurements in natural waters. However, the practice has been problematic (6) Saxe´n, R. L. Boreal Environ. Res. 2007, 12, 17–22. (7) Gaziev, I. Y.; Kryshev, I. I.; Gaziev, Y. I.; Uvarov, A. D. Izvestiya Vysshikh Uchebnykh Zavedenii, Yadernaya Energetika 2005, 40–47. (8) Lieser, K. H. Nuclear and Radiochemistry, Fundamentals and Applications; Wiley-VCH: Berlin, Germany, 2001. (9) Taylor, V. F.; Evans, R. D.; Cornett, R. J. J. Environ. Radioact. 2008, 99, 109–118. (10) Kautsky, H. Ocean Dynamics 1976, 29, 217–221. (11) Folsom, T. R.; Mohanrao, G. J. J. Radiat. Res. 1960, 1, 150–154. (12) Davison, W.; Fones, G.; Harper, M.; Teasdale, P.; Zhang, H. In Situ Monitoring of Aquatic Systems, Chemical Analysis and Speciation; John Wiley & Sons Ltd.: Chichester, England, 2000.

Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

5889

to balance the binding rate on the interface and the diffusion rate to the interface. The binding reaction (heterogeneous reaction occurred on the binding gel surface) removes the analyte and results in the flux (F) at the binding/diffusion interface which can be expressed by Ci0 and binding reaction rate constant, k,18-20 F ) -kCi0

12

Figure 1. Schematic view of the DGT operation. Depending on the rates of the binding reactions between the binding gels and Cs+, the concentration on the interface (Ci) can be negligible (solid line) or not (dashed line). In the latter case, DGT measures concentration, CDGT ) Cw - Ci0. Ci0 is negligible only when the binding reaction is fast (solid line). δ is the diffusion boundary layer in water, which is of insignificant thickness compared with ∆g.

because of its poor selectivity and hence rapid saturation of the binding agents by matrix ions that are present in natural waters at much higher concentrations.16 A new binding agent, copper ferrocyanide (Cu2Fe(CN)6, or CFCN), was investigated in this study as a selective DGT binding agent for Cs+ in natural waters based on previous work in which CFCN immobilized resin was used for selective removal of cesium from nuclear waste.17 Two different immobilization strategies were used, one by immobilizing CFCN on a heterogeneous Chelex-100 resin-gel (Chelex-CFCN) and the other on a homogeneous poly(acrylic acid) gel (PAACFCN). The DGT devices assembled with these two types of binding gels were evaluated in both synthetic laboratory solutions and in natural river waters for their performance in measuring low levels of stable133Cs and radioactive 137Cs. DGT LABILE CONCENTRATION As shown in Figure 1, the concentration of an analyte measured by DGT, known as DGT labile concentration (CDGT), can be calculated by measuring the amount of the analyte collected in the binding phase (M) after deployment time (t) when a DGT device with a diffusive layer of thickness ∆g and exposure area A is used:12

At a steady state, the flux is supplied by the diffusion through the diffusive layer and obeys Fick’s first law,

F ) -D

M∆g DAt

(1)

where Cw and Ci0 are the analyte concentrations in the bulk solution and at the diffusion/binding interface, respectively, and D the diffusion coefficient of the analyte in the diffusion layer.12 Ci0 is usually assumed to be negligible when calculating Cw. However, this assumption holds only when binding is much more effective than diffusion supply.18 Otherwise, at a steady state, a non-negligible Ci0 is maintained (13) Davison, W.; Zhang, H. Nature (London) 1994, 367, 546–548. (14) Slaveykova, V. I.; Wilkinson, K. J. J. Environ. Chem. 2005, 2, 9–24. (15) Li, W.; Zhao, J.; Li, C.; Kiser, S.; Cornett, R. J. Anal. Chim. Acta 2006, 575, 274–280. (16) Chang, L.-Y.; Davison, W.; Zhang, H.; Kelly, M. Anal. Chim. Acta 1998, 368, 243–253. (17) Clarke, T. D.; Wai, C. M. Anal. Chem. 1998, 70, 3708–3711. (18) Van Leeuwen, H. P.; Koster, W. Physicochemical Kinetics and Transport at Biointerfaces; John Wiley & Sons, Ltd.: Chichester, England, 2004.

5890

Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

∂Ci0 ∂x

(3)

where x and (∂Ci0)/(∂x) are the distance from the diffusive layer/ bulk solution interface and analyte concentration gradient at the binding/diffusion interface or adjacent area, respectively. Combination of eqs 2 and 3 gives ∂Ci0 k ) Ci0 ∂x D

(4)

Under typical DGT assumptions, negligible diffusive boundary layer (DBL) and linear concentration gradient within the diffusive layer,12 (∂Ci0)/(∂x) can also be written as ∂Ci0 Cw - Ci0 ) ∂x ∆g

(5)

Combining eqs 4 and 5, Ci0 can be calculated, Ci0 )

Cw 1 + (k∆g/D)

(6)

Equation 6 was also derived in the estimation of Ci0 on the biointerface of biological uptake,18 which is similar to DGT in mechanism. Combining eqs 1 and 6, k can be estimated: k)

CDGT ) Cw - Ci0 )

(2)

(

CDGT /Cw D ∆g 1 - CDGT /Cw

)

(7)

Combining eqs 4-6, the total mass of analyte accumulated in the binding phase (M) can then be calculated,19,20 M)A

kADCw

∫ (-F dt) ) k∆g + D t t

0

(8)

When k . D/(∆g) (Ci0 is negligible),18 DGT measures Cw and eq 8 becomes the usual DGT equation12 M)

ADCw t ∆g

(9)

Therefore, the DGT labile concentration is an operationally defined term, depending on the nature of the binding phase (k) and the (19) Oldham, K. B. NATO ASI Ser., Ser. E: Appl. Sci. 1991, 197, 35–50. (20) Oldham, K. B. Personal communication, 2009.

diffusion layer (D/(∆g)) used. The time taken to reach the steady state can be estimated at the order of ((∆g)2)/D.19,20 In the case of Cs (D ) 1.92 × 10-5 cm2 s-1)16 and diffusive layer thickness of 0.05 cm, it is approximately 125 s, which is negligible comparing to the typical DGT deployment time of days or weeks. EXPERIMENTAL PROCEDURES Instrumentation. An inductively coupled plasma mass spectrometer (ICPMS, Varian Inc.; SPS-3D) was used for stable isotope 133 Cs measurements; the detection limit (3σ) was 3.6 × 10-4 µg L-1. A broad energy germanium detector (BEGe, Canberra, Inc.) with an active area of 5 000 mm2, a 1 mm thick carbon composite entrance window, and a 1 µm thick front dead layer was used for radioactive 137Cs γ ray measurements at 661.7 keV. The detector signals were processed by a multichannel analyzer (DSA 2000, Canberra, Inc.) with high stability and low drift. Counting efficiency (6.11%) was calculated with a Monte Carlo simulation package, Virtual Gamma Spectroscopy Laboratory.21 The detection limit (3σ) was 5.6 mBq for 137Cs radioactivity. Ion chromatography (DX-600, Dionex, Oakville, ON, Canada) was used to determine the concentrations of major cations and anions in water. Dissolved organic carbon (DOC) was measured with a Shimazdu TOC-Vcpn analyzer (Mandel Scientific, Guelph, ON, Canada). Preparation of the Diffusive and Binding Gels. Polyacrylamide (PAM) gels (0.051 cm thick) were prepared with 15% acrylamide (Fisher Scientific) and 0.3% agarose-derived crosslinker (DGT Research Ltd.) following published procedures.22 The PAM gel sheets were cut into 4.9 cm2 discs for use as the diffusive layer in the DGT device, as described previously.16 To prepare the CFCN binding gels, first PAM-based Chelex and PAA gels (4.9 cm2 discs of 0.051 cm thickness) were prepared following previously described procedures.22,23 CFCN was then immobilized in the gels following a procedure similar to that for making CFCN-embedded Chelex resin beads.17 In brief, Chelex (or PAA) gel discs were soaked in 30 mL of 1.0 M Cu(NO3)2 in a 250 mL polypropylene bottle and stirred gently overnight. The gel disks were washed thoroughly with Milli-Q water and transferred into a clean bottle containing 3.0 mL of Milli-Q water. A total of 3.0 mL of 0.25 M K4Fe(CN)6 was then added dropwise into the bottle, followed by the addition of an additional 3.0 mL of 0.25 M K4Fe(CN)6 until the gel color turned from blue to dark brown. The gels were then removed from the solution and washed several times with Milli-Q water and 0.1 M HCl. All the immobilized gel disks were kept in Milli-Q water with frequent replenishing of the water before use. Swelling of the Binding Gels. The swelling property of the PAA-CFCN and Chelex-CFCN gels was studied by equilibrating the gel disks at different ionic strengths (0.001-1.0 M NaNO3) and pHs (2-10). The swelling ratio (Rs) is defined as the mass after equilibration (Mwet) divided by the dry gel mass (Mdry): (21) Plenteda, R., Ph.D. Thesis, Universitaetsbibliothek der Technischen Universitaet Wien, Resselgasse 4, A-1040 Wien, Austria 118, 2002. (22) Zhang, H.; Davison, W. Anal. Chem. 1995, 67, 3391–3400. (23) Li, W.; Zhao, H.; Teasdale, P. R.; John, R.; Zhang, S. React. Funct. Polym. 2002, 52, 31–41.

Rs )

Mwet Mdry

(10)

Binding Properties of the Binding Gels. Binding of Cs to the PAA-CFCN and Chelex-CFCN gels was examined by immersing the gel discs for 24 h in 200 mL solutions containing 100.0 µg L-1 Cs and 0.01 M NaNO3 at pH 7.0 (continuously stirred). A 24 h immersion time was chosen according to the binding capacity/immersion time experiments, in which maximum binding capacity was achieved at 10 h. The pH of the solutions was adjusted with 2% NaOH and HCl. Cs concentrations in the solutions before and after gel immersion were measured by ICPMS as was the eluted amount of cesium bound to the gels. The influence of pH and ionic strength on the binding was studied for the pH range of 1-9 and a NaNO3 concentration range of 1.0 × 10-4 to 1.0 M. Competitive binding of ions to the gels was investigated by immersing the gel disks in a solution containing a mixture of 100 µg L-1 Cs+, Na+, K+, Ca2+, Mg2+, Ba2+, and Sr2+. All experiments were done in triplicate unless otherwise stated. Elution of Cesium from the Binding Gels. Elution of Cs from the gels was carried out by soaking the gels in a 2.0 mL of solution containing optimized concentrations of 24 mM EDTA (Fisher Scientific) and 15% NH3 · H2O (Fisher Scientific) for 24 h. The eluted solutions were diluted accordingly before being measured by ICPMS. The elution efficiency, Ef, was defined as the ratio between the eluted amount of Cs, Me, and the mass decrease of Cs in the uptake solution, ∆Ms: Ef )

Me ∆Ms

(11)

The efficiencies for the Chelex-CFCN and PAA-CFCN gels were determined as 89.0% and 84.8% (average of 5 measurements), respectively. Laboratory Validation of the DGT Devices. The DGT devices were assembled by placing, in order, a binding gel (Chelex-CFCN or PAA-CFCN), a PAM diffusion gel, and a prewetted 100 µm thick cellulose nitrate filter membrane (Whatman, 0.45 µm pore size) protective layer on a piston-type DGT holder (DGT Research Ltd.).22 The assembled PAA-CFCN (36 pieces) and Chelex-CFCN (36 pieces) DGT devices were suspended on a Styrofoam float in synthetic river water (St. Lawrence River) prepared according to the previously reported composition with the pH being maintained at 8.2 with NaHCO3.15 A sufficiently large volume of the solution (30 L) was used to ensure that depletion of Cs by the DGT devices was negligible. The solution was stirred overnight to equilibrate before the devices were immersed. Three DGT devices of each type were retrieved each time at time intervals varying between 24 and 360 h. To test the germanium detector for the measurements of radioactivity in the DGT devices, a 5.0 L solution containing 137Cs dissolved from 3.0 g of a certified reference soil (International Atomic Energy Agency, IAEA 375, 5.0 Bq g-1) was used. The soil solution was stirred overnight before being filtered; three DGT devices of each type, Chelex-CFCN and PAA-CFCN, were then immersed for a period of 72 h. Radioactivities of dissolved 137 Cs in the solution and in the DGT devices were measured using the germanium detector. Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

5891

Field Applications. Chelex-CFCN and PAA-CFCN DGT devices were deployed for 24 and 96 h in early August 2008 in the Ganaraska River at Port Hope (43° 56′ 31.41′′ N, 78° 17′ 23.22′′ W), ON, Canada, a nuclear industrial discharge site. The devices were also deployed for 70.5 h in late September 2008 in the Berezina River at Babruysk (53° 08′ 37.99′′ N, 29° 15′ 59.08′′ E), Belarus, approximately 200 km from the 1986 Chernobyl nuclear accident site in the Ukraine. At the end of the deployments, the DGT devices were rinsed thoroughly with Milli-Q water, placed into separate Ziploc plastic bags, and stored in a cooler for transport to the laboratory. The binding gels in the DGT devices from the Ganaraska River were removed from the DGT device and soaked in separate EDTA/ NH3 H2O solutions prior to ICPMS analysis. For the devices deployed in the Berezina River, radioactivities were measured directly by the germanium detector without dissembling the devices before 133Cs was eluted from the binding gel and measured by ICPMS. Water samples were also taken at the same sites at the beginning, middle, and end of the DGT deployments and filtered through a 0.45 µm cellulose membrane. Samples were analyzed for total Cs, matrix ion concentrations and other water quality parameters, such as pH and DOC for the purpose of computer modeling. Average water temperatures of 24 °C in the Ganaraska River and 11 °C in the Berezina River were measured for the diffusion coefficient correction in the Stokes-Einstein equation.24 RESULTS AND DISCUSSION Preparation of the Binding Gels. Gelation attempts of the acrylamide solution containing CFCN-immobilized Chelex resin beads were not successful even at elevated temperatures. Instead, both Chelex-CFCN and PAA-CFCN binding gels were readily prepared by immobilizing CFCN onto the PAM-based Chelex gel or PAM-PAA gel. Both types of binding gels were fully characterized for their applications in the development of a Cs-specific DGT device. Swelling of the Binding Gels. Some hydrogels swell or shrink under different solution conditions due to different water contents in the network and can cause “bursting” of the DGT devices (when swelling) or incomplete coverage of the exposure area (when shrinking). Similar to the Chelex gel,25 there was little effect of pH and ionic strength on the Chelex-CFCN gel swelling within the pH and ionic strength ranges studied, due to the polymer encapsulation of the resin functional groups and the PAM gel being sensitive neither to pH nor ionic strength.25 In contrast, ionic strength influenced the swelling of the PAA-CFCN, due likely to the “charge screening effect”.26 The electrostatic repulsion between adjacent strands on the gel network was minimized by the charges of NaNO3 in the gel allowing the strands to move closer and the gel to absorb less water. As a result, the gel swelled at lower NaNO3 concentrations (0.001 M; Rs ) 21.7) and shrank at higher NaNO3 concentrations (1.0 M; Rs ) 5.80). Swelling effects were not apparent in the NaNO3 concentration range of 0.4 to 1.0 M, indicating that a relatively constant gel size (24) Dorsey, N. E. Properties of Ordinary Water Substances; Reinhold Publishing Corporation: New York, 1940. (25) Li, W., Ph.D. Thesis, Griffith University, Australia, 2003. (26) Baker, J. P.; Blanch, H. W.; Prausnitz, J. M. Polymer 1995, 36, 1061–1069.

5892

Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

Figure 2. Effect of pH on the binding of Cs to Chelex-CFCN and PAA-CFCN gels. Initial Cs+ concentration ) 100 µg L-1, deployment time ) 24 h, T ) 23 °C.

can be maintained in high ionic strength water such as seawater or estuary water. pH also influenced swelling of the gel as a result of transformation between the acidic and charged basic forms of the groups in the gel.23 Dramatic swelling occurred at the lower pH range, from a swelling ratio Rs ) 3.77 at pH 2 to Rs ) 14.5 at pH 3; the gel swelled less at higher pH ranges, from Rs ) 22.3 at pH 6 to Rs ) 24.7 at pH 10. To ensure proper performance of the DGT devices, it is therefore suggested that the PAA-CFCN gel be equilibrated in a solution with similar pH and ionic strength to the water in which the DGT devices are to be deployed. Cs+ Binding. The binding capacity of the gels is strongly influenced by the solution pH. As shown in Figure 2, binding capacity increased significantly when pH increased from 2 to 6. This pH dependent binding may be explained by the transformation between acidic or basic forms of the ferrocyanide ligand due to variation in pH. These different forms of the ligands may bind Cs+ differently. The binding capacity declined at pH around 4, which agrees with the first acidity constant of ferrocyanide (pKa1 ) 4.227). At higher pH (4< pH < 8), more binding functional groups are present in the negative basic form, giving rise to increased binding capacity of Cs+. However, the binding capacity gradually decreased upon further increase in pH (pH > 8), due likely to the formation of the neutral CeOH complex.28 As the pH of most natural waters is in the range of 6.0-8.5, the binding gels are suitable for application of DGT for measurements of Cs. The binding capacity for Cs decreases with increasing solution ionic strength (Figure 3), particularly when the ionic strength is higher than 0.1 M. Likely this is due to competitive ionic absorption to the ferrocyanides between high concentrations of Na+ and Cs+ ions. However, in the ionic strength range typically found in natural waters, the binding capacities of the gels should be satisfactory for DGT applications. The maximum distribution coefficient, KD, defined as the ratio between the equilibrium Cs concentration in a binding gel of 0.25 mL volume (4.9 cm2 × 0.051 cm), CB, and in solution, Cw, (27) Ramounet-Le Gall, B.; Rateau, G.; Abram, M. C.; Grillon, G.; Ansoborlo, E.; Berard, P.; Delforge, J.; Fritsch, P. Radiat. Prot. Dosim. 2003, 105, 153–156. (28) Krupka, K. M.; Kaplan, D. I.; Whelan, G.; Serne, R. J.; Mattigod, S. V. Understanding Variation in Partition Coefficient, Kd, Values, EPA 402R-99-004A, U.S. Environmental Protection Agency: Washington, DC, 1999; Vol. II.

KD )

CB Cw

(12)

was estimated to be 4.5 × 106 for PAA-CFCN and 1.9 × 106 for Chelex-CFCN using the maximum binding capacity data in Figures 2 and 3 (3.94 µg cm-2 for PAA-CFCN and 3.50 µg cm-2 for Chelex-CFCN). This indicates a high tendency of Cs+ binding to the gels with a negligible amount of Cs remaining in the solution. The higher binding capacity of the PAA-CFCN gel could be attributed to its homogeneity, i.e., the evenly distributed functional binding groups throughout the gel, while the Chelex-CFCN gel was heterogeneously embedded within the resin beads. Competitive binding of Cs+ was also investigated in a mixed solution containing Cs+ and other metal cations including Na+, K+, Ca2+, Mg2+, Ba2+, and Sr2+. Binding selectivity of the gels, defined as the ratio between the amount of a metal ion bound on the gel (MM) to that of Cs (MCs),

selectivity )

MM × 100% MCs

Figure 4. Selectivity of the binding phases of Cs over other alkali and alkaline earth cations as a function of their ionic radii (pH ) 8, I ) 0.01 M NaNO3).

(13)

for Cs+ was 10-1000 times higher than that for other matrix cations, with the PAA-CFCN gel being more selective (Figure 4). The high selectivity for Cs+ over other ions is due likely to its larger ionic radius (Figure 4) that favors strong trapping in the cavity of the CFCN gel.29,30 DGT Performance in Synthetic River Waters. Performance of the DGT devices assembled with the Chelex-CFCN and PAACFCN binding gels was studied first in a synthetic river water solution (St. Lawrence River15) with a total Cs concentration of 11.0 µg L-1. Linear relationships between the mass accumulated in the DGT devices vs immersion time supported the use of these binding gels as the DGT binding phases (Figure 5). However, DGT devices with the two different binding phases measured markedly different concentrations of Cs in the synthetic river water. Whereas the PAA-CFCN DGT measured 98.4 ± 6.5% (mean ± standard deviation (SD)) of the total Cs concentration, the Chelex-CFCN only measured 54.2 ± 7.2% of the total Cs concentration. With measurement of radioactive 137Cs in the DGT devices, the use of a sensitive γ detector offered several advantages

Figure 3. Effect of ionic strength on Cs binding to Chelex-CFCN and PAA-CFCN gels in solutions containing different NaNO3 concentrations (M) at pH 7.0. Initial concentration of Cs ) 100 µg L-1, deployment time ) 24 h, T ) 23 °C.

Figure 5. Accumulated mass of Cs in the DGT devices (ChelexCFCN and PAA-CFCN) vs deployment time in well-stirred synthetic river water (St. Lawrence) containing 11.0 µg L-1 Cs at pH 8.2 and T ) 23 °C.

over ICPMS, including an improved detection limit and the elimination of the elution steps, avoidance of the spectral interference from 137Ba in the ICPMS measurements. To test the validity of this approach, the DGT devices were immersed in a solution containing 1.36 Bq L-1 137Cs dissolved from IAEA 375 reference soil (6.82 Bq out of 15.8 Bq). After 72 h of deployment, the radioactivities in the DGT devices were determined to be 0.148 and 0.0635 Bq in the PAA-CFCN and Chelex-CFCN DGT devices, respectively, corresponding to labile concentrations of 1.36 and 0.586 Bq L-1 in the solution. This represents 100% ± 4.3% and 43% ± 6.8% of the total 137Cs activity concentration by the PAA-CFCN and Chelex-CFCN DGT devices, respectively, and shows the similar pattern with the stable 133Cs measurements by the DGT devices as described above. The difference in the concentrations measured by the DGTs with different binding phases is commonly interpreted as the result of different reactivities of the free analyte ions and their complexes15,31 and is often used to probe the speciation and (29) Keggin, J. F.; Miles, F. D. Nature 1936, 137, 577–578. (30) Mackay, K. M.; Mackay, R. A.; Henderson, W. Introduction to Modern Inorganic Chemistry; Nelson Thornes Ltd.: Gloucestershire, U.K., 2004.

Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

5893

bioavailability of the analyte under various conditions.12 However, this is not the case for the Cs measurements, as the free Cs+ ions are the predominant species in the synthetic river water, as well as in most natural waters (see below).28,32 Speciation simulation by the Stability Constant Database program33 suggested that over the pH range of 2-9, free Cs+ dominates the Cs speciation (96.5%-98.4%). In the synthetic river water (pH 8.2), 96.5% of the total Cs is presented as free Cs+, followed by Cs complexes with humic substances (1.90%), CsCl (1.53%), and CsHPO4- (0.03%). Since Cs+ was the predominant species in the synthetic river water, the significantly different measurements obtained by the PAA-CFCN and Chelex-CFCN DGTs can be attributed only to the difference in the two binding gels when the same diffusion gel was used. On the basis of the analysis of the DGT operation in the introductory section, the nearly 100% recovery of CDGT measured by PAA-CFCN DGT suggests that the interface concentration, Ci0, is negligible with PAACFCN, whereas with Chelex-CFCN, Ci0 accounted for ∼50% of Cw. According to eq 7, the binding rate constants of Cs+ to Chelex-CFCN (kChelex-CFCN) and PAA-CFCN (kPAA-CFCN) can be estimated as kChelex-CFCN ) D/(∆g) ) 3.8 × 10-4 s-1 (CDGT/ Cw ) 50%) and kPAA-CFCN > 3.9 × 10-3 s-1 (CDGT/Cw > 91%), respectively (∆g ) 0.051 cm-2, D ) 1.92 × 10-5 cm2 s-1).16 Such a drastic difference in the binding rate constant is surprising, given that in both binding gels the actual binding agent was the same (CFCN). A plausible explanation is that the immobilization to Chelex (via the iminodiacetate groups) or PAA (via the carboxylic groups) may have determinant impact on the cavity in the gel. In other words, Chelex-CFCN gel may not be able to “hold” Cs+ ions as tightly as PAA-CFCN (see the previously mentioned binding mechanism of trapping of Cs+ ions in the cavity of the gel). The Cs+ binding on the Chelex-CFCN may therefore be reversible due to the competitive charge attractions between the Cs+ ions and varying anionic ligands codiffused to the binding gel and between the binding gel and matrix ions (ionic strength effects). The net binding rate constant is thus smaller. The apparent competitive effects of Na+ ions (Figure 3) or other ions (Figure 4) with Cs+ binding appear to support the above explanation. The hypothesis above may also be supported by previous research for 137Cs decorporation from rats using prussian blue,27 ferric ferrocyanide (Fe4[Fe(CN)6]3), with the binding group of CFCN. Incomplete removal of 137Cs in tissues (53%)27 may indicate that, similarly to Chelex-CFCN DGT, the 137Cs binding on the tissues/biological fluids interface was kinetically controlled or reversible. Field Application. In the Ganaraska River (pH 8.2) at Port Hope, ON, Canada, the PAA-CFCN and Chelex-CFCN DGT measured 97.4% ± 7.2% (mean ± SD) and 48.2% ± 6.7% of the total dissolved Cs concentration of 3.5 ng L-1, respectively. 137 Cs in both the DGT devices after 96 h deployment was below the detection limit of the germanium detector (5.6 (31) Li, W.; Li, C.; Zhao, J.; Cornett, R. J. Anal. Chim. Acta 2007, 592, 106– 113. (32) Turner, D. R.; Whitfield, M.; Dickson, A. G. Geochim. Cosmochim. Acta 1981, 45, 855–881. (33) Pettit, L. D.; Powell, K. J. IUPAC Stability Constants Database; Academic Software: Yorks, U.K., 1999.

5894

Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

mBq, corresponding to a 137Cs concentration of 13.7 mBq L-1 in water). In the Chernobyl accident-impacted Berezina River (pH 7.6), radioactivities of 137Cs accumulated in both types of the DGT devices after 70.5 h deployment were also below the detection limit of the germanium detector, indicating that the 137Cs concentration in the river was below 18.7 mBq L-1. This agrees with a recent report that 137Cs concentrations are reduced significantly in the waters adjacent to the Chernobyl accident site.34 For stable 133Cs concentrations, PAA-CFCN and Chelex CFCN DGTs measured 95.0% ± 5.6% and 47.6% ± 7.7% of the total concentration of 2.1 ng L-1, respectively. These field applications agree very well with the previous laboratory studies on the synthetic river water, i.e., PAA-CFCN and Chelex-CFCN DGT measured total and partial free Cs+ concentrations, respectively. These consistent measurements (SD ) 4.6%) of ∼50% of free Cs+ by Chelex-CFCN DGT also confirmed the proposed mechanism that the Cs concentration on the diffusive/binding interface was reduced to Ci0, instead of being negligibly low. Detection Limit. In addition to providing speciation information, the DGT technique is also a useful tool for quantitative preconcentration,22 because the mass accumulated in the binding phase, M, is proportional to the deployment time, t, (eq 9). Extended time of deployment of the DGT devices thus allows the measurement of very low concentrations of Cs in water. With the germanium detector detection limit of 5.6 mBq (M), a DGT detection limit of 1.8 mBq L-1 (5.9 × 10-10 µg L-1) can be achieved for a 4-week deployment when the traditional DGT device with a diffusive layer of 3.14 cm2 exposure area and 0.05 cm thickness is used. On the basis of a binding capacity of 3.50 µg cm-2, the DGT devices with both types of binding gels having a 0.05 cm thickness and 3.14 cm2 exposure area could theoretically be deployed in natural waters of 0.1 µg L-1 Cs for up to 1 year before being saturated (eq 9) assuming only Cs is bound and the actual deployment time is not limited by the biofouling effects on the gels.25 Alternatively, the DGT detection limit can be further lowered by increasing the exposure area of the diffusive layer (M ) (DCwt)/(∆g)A). For example, the same DGT device with a 104fold increase in surface area would result in a detection limit of 1.8 × 10-4 mBq L-1 or 5.9 × 10-13 µg L-1 for the same deployment period. CONCLUSIONS AND FUTURE WORK It has been demonstrated that both PAA-CFCN and ChelexCFCN gels can be used as binding phases for Cs measurements in the DGT technique. Both gels are capable of selectively binding Cs+ in the presence of much higher concentrations of alkali and alkaline earth metal ions. This work has shown that the DGT labile concentration, CDGT ) Cw - Ci0, is dependent on the binding rate (k) of the binding phase used, which may help explain other DGT results when the binding between analytes and binding phases are relatively weak. (34) Kinley, D. Chernobyl’s Legacy; Health, Environmental and Socio-Economic Impacts and Recommendations to the Governments of Belarus, the Russian Federation and Ukraine: Vienna, Austria, 2006. (35) Campell, P. G. C. Metal Speciation and Bioavailability in Aquatic Systems; Wiley: New York, 1995.

The DGT technique reported here can be readily coupled with a γ detector for the direct determination of radioactive 137Cs or, following elution, to an ICPMS for stable 133Cs measurements. On site measurement of radioactive 137Cs is also possible by coupling with a portable γ detector, which is advantageous by eliminating the elution steps. The detection limit could be improved readily either by extending the deployment time (up to 1 year in the absence of biofouling of the gels) or by increasing the surface area of the gels. Furthermore, these two DGT setups, Chelex-CFCN and PAA-CFCN DGT, could be of particular significance for modeling biological uptake with the similar suggestion by the free ion activity model (FIAM)35 and the biotic ligand model (BLM)36 that metal uptake by the plasma membrane is the limiting step when compared to metal transport in the bulk solution or in the cell protective layer.18,37 Further studies on the comparison between DGT and biological uptake are ongoing and may have significant implications for the practice of using Chelex-CFCN DGT to mimic selective biological uptake (36) Van Leeuwen, H. P. Environ. Sci. Technol. 1999, 33, 3743–3748.

of cesium or using Chelex-CFCN like reagents for decorporation of 137Cs.4 ACKNOWLEDGMENT Financial support for this study was provided by an Ontario Postdoctoral Research Fellowship and a NSERC Operating Grant. We thank Prof. K. B. Oldham for his great help in derivation of the equations with kinetic binding considerations, Ms. H. Broadbent for her assistance in measuring major water quality parameters of the river water samples, Dr. A. Izmer for the help in getting DGT samples from the Berezina River at Babruysk, Ukraine, and Mr. J. Balch for his help in the experiments.

Received for review March 23, 2009. Accepted June 5, 2009. AC9005974 (37) Campbell, P. G. C.; Erre´calde, O.; Fortin, C.; Hiriart-Baer, V. P.; Vigneault, B. Comp. Biochem. Physiol., Part C: Toxicol. Pharmacol. 2002, 133, 189– 206.

Analytical Chemistry, Vol. 81, No. 14, July 15, 2009

5895