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Speciation measurements of uranium in alkaline waters using diffusive gradients in thin films technique
Analytica Chimica Acta 575 (2006) 274–280
Speciation measurements of uranium in alkaline waters using diffusive gradients in thin films technique Weijia Li a,∗ , Jiujiang Zhao a,b , Chunsheng Li a,∗∗ , Stephen Kiser a , R. Jack Cornett a a
Radiation Protection Bureau, Health Canada, A.L. 6302D1, 775 Brookfield Rd., Ottawa, Ont., K1A 1C1 Canada b Department of Chemistry, Carlton University, 1125 Colonel by Drive, Ottawa, Ont., K1S 5B6 Canada Received 15 March 2006; received in revised form 25 May 2006; accepted 27 May 2006 Available online 3 June 2006
Abstract This work investigated the application of diffusive gradients in thin films technique (DGT) to uranium speciation measurements in natural water. Two binding phases were examined, a commercially available affinity membrane, Whatman DE 81 (DE 81), with amino binding functional groups and the conventionally used Chelex 100 beads imbedded polyacrylamide hydrogel (Chelex) with iminodiacetate functional groups. The DGT devices assembled with the binding phases of DE 81 (DE 81 DGT) and Chelex gel (Chelex DGT) were tested both in synthetic river water solutions and in local river water. DE 81 DGT and Chelex DGT measured 80% and 75% of the total uranium in synthetic river water solution, respectively, and measured 73% and 60% of the total uranium in St. Lawrence River, Canada, respectively. The binding properties of the DE 81 membrane and Chelex gel for uranium, and the diffusion of uranyl complexes in the polyacrylamide gel (PAM) were also studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Diffusive gradients in thin films (DGT); Uranium; River water
1. Introduction Uranium concentrations in natural waters are concerned over the decades due to its involvement in many nuclear activities (nuclear power generation, nuclear tests and nuclear weapon programs) and leaching from natural sources [1]. Uranium is both chemically and radiologically toxic as a heavy metal and a radionuclide. However, chemical damage to organism is more important than radiation damage due to the severe chemical damage to kidney when ingested and the long half-lives of the isotopes 234 U, 2.46 × 105 year; 235 U, 7.04 × 108 year; 238 U, 4.47 × 109 year [2–5]. It is well known that the chemical toxicity or bioavailability of metals depends on their chemical speciation [6,7]. Like other heavy metals, uranium forms various complexes in natural waters with the varying ligands present. It has been known that depending on solution pH, uranyl ions form stable hydroxide and carbonate complexes with dissolved
∗
Corresponding author. Tel.: +1 613 954 8617; fax: +1 613 952 9071. Corresponding author. Tel.: +1 613 954 0299; fax: +1 613 952 9071. E-mail addresses: weijia [email protected] (W. Li), li [email protected] (C. Li). ∗∗
0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2006.05.092
atmospheric CO2 with varying coordination numbers, most of which are in anionic forms [7,8]. Complexes with humic substances may only exist in neutral or acidic solutions, if any [8]. To obtain speciation information of uranium, the methods of grabbing samples from natural water and analysing in laboratory with preconcentration and separation procedures were used in previous publications [9,10]. However, the distribution of uranium species in water may have been changed during transport, preservation and sample process [1]. In addition, the concentrations of the species measured can only represent those at the time of sampling which may be overestimated or underestimated due to the concentration fluctuations in waters. Another way of obtaining speciation information is by computer simulation method, which is dependent on the availability of reliable equilibrium constants and the assumption that the species are in equilibrium with each other [11]. To reflect the true picture of uranium speciation in water, the in situ speciation technique with time integrated preconcentration features, diffusive gradients in thin films technique (DGT), was used in this work. There were mainly two layers in a DGT device, a diffusion layer of a polyacrylamide gel (PAM), to allow the analyte to diffuse through it, and an underlying layer of a binding phase, traditionally Chelex 100 resin imbedded in PAM gel (Chelex), to
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reduce the concentration of the analyte species on the interface to negligibly low [12,13]. Concentration of DGT labile fraction of metal (C) in waters, i.e. species that are diffusible through the diffusion layer with certain pore size and reactive to the binding phase, can be calculated by measuring the mass of a metal (M) accumulated in the binding phase for certain deployment time (t) after elution in an acidic solution using the DGT equation [12,13]: C=
Mg DAt
(1)
where A and g are the area and thickness of the diffusion layer, and D is the diffusion coefficient of the uranium ions in the diffusion layer. For the analysis of a metal ion of interest, the key of using DGT technique is to find out suitable binding phases and diffusion layers. Polymers with amino-functional groups were found to be capable to remove uranium from water [14,15]. In this work, the traditional DGT binding phase of Chelex gel and a commercially available robust membrane disc, Whatman DE 81 membrane (DE 81), were used as DGT binding phases. The PAM gel was used as a diffusion layer. This work focused on the water in St. Lawrence River in Ontario, Canada, which is from Lake Ontario where many nuclear facilities located. pH of the water is around 8+. The performance of the binding phases for DGT application of uranium analysis was investigated both in this alkaline water and in laboratory solutions with similar ion contents and pH. 2. Experimental 2.1. Reagents and material All solutions were prepared with deionised water (MilliQ) and were in equilibrium with atmospheric CO2 . All reagents used for the preparation of polyacrylamide gel (PAM) and Chelex 100 PAM gel (Chelex gel) were of analytical grade and used as received. The acrylamide monomer solution (40%), ammonium peroxosulphate, N,N,N ,N -tetramethylethylene diamine and sodium form Chelex 100 resin of 200–400 mesh were obtained from Bio-Rad. The agarose derived crosslinker (2%) and the DGT holders were purchased from DGT Research Ltd. UK. Cellulose nitrate filter membrane (0.45 m) and Whatman DE 81 membrane of 2.5 cm diameter disc with 0.2 mm thickness were purchased from Whatman International Ltd., UK, and were used as the protection cover layer and binding phase, respectively. A 1000 mg L−1 uranium standard solutions were purchased from SPEX CertiPrep Inc., US, and used as stock solutions for the membrane binding property experiments and as calibration standards after dilution for measurements. Concentrated optima nitric acid (Fisher Scientific) was used for all acidic solution preparation unless otherwise stated. The acid and sodium hydroxide (Fisher Scientific) were used to adjust solution pH. All experiments were carried out in high density polypropylene containers.
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2.2. Instrument Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS, ThermoFinnigan, Element 2) was used for uranium measurements in solutions with detection limit of 2.8 × 10−3 nmol L−1 (3σ). Ion chromatography (DX-600, Dionex, Oakville, Ont., Canada) was used to determine the concentrations of potassium, sodium, calcium and magnesium cations and anions in water. The dissolved organic carbon (DOC) was measured with TOCVcpn Shimadzu (Mandel Scientific, Guelph, Ont., Canada). 2.3. Preparation of PAM gel, Chelex gel and pre-treatment of DE 81 membrane PAM gel and Chelex gel were prepared according to previously described procedures [16]. The thicknesses of the gels were made as 0.040 cm. The gels were cut as 2.5 cm diameter discs before use. Whatman DE 81 membrane used for all experiments were immersed in 10% nitric acid for 24 h before being rinsed with Milli-Q water and stored in Milli-Q water in order to remove any metals which may be contained. 2.4. Binding of uranium The binding of uranium to the DE 81 membrane, Chelex gel and PAM gel was examined by immersing the 2.5 cm diameter discs in 500 mL solutions containing 1.0 mg L−1 uranium with constant rate of stirring at pH 8 for 24 h. About 100 mg L−1 NaHCO3 was added to the solutions to maintain pH at 8 and to equilibrate with uranyl carbonates. Ionic strength of the solutions was adjusted as 0.01 M NaNO3 unless otherwise stated to keep constant ionic strength in the solutions. Uranium concentrations in the tested solutions were measured by SF-ICP-MS before and after immersion. Uranium bound in the membranes was also measured by SF-ICP-MS after elution in 10% HNO3 solution. All experiments were repeated three times. The effect of immersion time on the binding of uranium was investigated by immersing the DE 81 or Chelex gel discs in uranium solutions for varying periods of time intervals of 0.5–26 h. The influence of pH on the binding was tested in the pH range of 1–9. The effect of ionic strength on the binding was carried out in solutions containing various concentrations of NaNO3 ranging from 1.0 × 10−4 to 1.0 M. The PAM gel was also tested for uranium binding. Uranium solutions with NaNO3 concentrations ranged from 1 × 10−1 to 1 × 10−4 M were used. 2.5. Diffusion coefficient measurement Diffusion coefficient of uranium ions in PAM gel were measured according to the previously described method by Chang et al. [17]. A polyacrylic cell with two compartments was used. One compartment (A) initially contained a solution of 800 mL of 1.0 mg L−1 uranium and the other compartment (B) initially contained a solution of the same volume as compartment A with no uranium addition. The ionic strengths of the solutions
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in both compartments were adjusted to 0.01 M by the addition of 1 M NaHCO3 . pH of the solutions was adjusted to 3.3, 3.9, 4.9, 6.0, 7.1, 8.1 and 8.8 with 1% NaOH and 1% HNO3 . The solutions were stirred over night with exposure to atmosphere before adding to the compartments. Each compartment was mixed well with an overhead stirrer during the experiments. A 2.0 and 0.2 mL aliquots of solutions were taken from compartment A and compartment B, respectively, at 30 min intervals for 240 min. The samples were diluted accordingly before being measured by ICPMS. Diffusion coefficients of uranium ions in the PAM gel were calculated using equation, D = Mg/(CAt), derived from Eq. (1).
solution after immersion in the solution for time intervals of 21–189 h. Chelex and DE 81 DGT devices were also deployed in St. Lawrence River of Canada side located at the town of Morrisburg, ON, Canada (46◦ 20.93 N, 72◦ 26.04 W) for 24, 72, 144 and 336 h in early August 2005. Average temperature of the water was 24 ◦ C. pH of the water was 8.2. The devices were rinsed with Milli-Q water thoroughly at the end of the deployment before the binding phase discs were removed from the devices. For river water deployments, the devices were kept separately in Ziploc plastic bags and transported to laboratory in a cool box at the end of the deployment.
2.6. Deployment of DGT devices in laboratory solution and in river water
2.7. Elution of uranium from the binding phases
Water samples were taken from St. Lawrence River according standard procedures of on site filtration at the beginning, middle and end of the DGT deployments. Samples were preserved in 2% HNO3 solutions during transport and storage. pH and alkalinity of the water were measured on site. Concentrations of uranium, major cations and anions, dissolved organic carbon (DOC) and total inorganic carbon (TIC) were analyzed in laboratory using appropriate instruments (Section 2.2). All data were shown in Table 1 and used as recipe for synthetic river water preparation. he DE 81 or Chelex binding phase, the PAM hydrogel diffusion layer and a wet 100 m thickness Whatman cellulose nitrate filter membrane (0.45 m) as a protection layer were placed in order in a DGT holder according to previously described procedures [16]. To test the performance of the binding phases, the assembled DGT devices were deployed in a synthetic river water solution (St. Lawrence) of composition: [K+ ] = 1.2 mg L−1 ; [Na+ ] = 14 mg L−1 ; [Ca2+ ] = 27 mg L−1 ; [Mg2+ ] = 6.2 mg L−1 ; [Cl− ] = 42 mg L−1 ; [NO3 − ] = 54 mg L−1 ; [SO4 2− ] = 34 mg L−1 . Sufficient volume of solution (30 L) was provided to ensure that the depletion of uranium by the DGT devices was negligible. The solution was exposed to atmosphere with constant stirring over night before the devices were immersed to let the solution equilibrated with CO2 . pH of the solution was ∼8 after addition of 100 mg L−1 NaHCO3 . The devices were removed from the
Table 1 Concentration composition and other parameters of St. Lawrence River water Na+ K+ Ca2+ Mg2+ Cl− SO4 2− DOC TIC TC pH Alk
11.9 1.47 32.0 7.92 22.1 25.9 6.42 21.2 27.6 8.2 83.3
Note: Unit for ionic concentration, dissolved organic carbon (DOC), total inorganic carbon (TIC), total carbon (TC) and alkalinity (CaCO3 ) was mg L−1 .
The binding phases of both DE 81 membrane and Chelex gel discs were sunk in 2.0 mL of 2 M HNO3 solution for 24 h both for laboratory and river water deployments to elute the uranium from the binding discs after being removed from the DGT devices before being analyzed by SF-ICP-MS. The elution efficiency, or elution factor, Ef , was defined as the ratio between the amount of uranium eluted from gel discs, Me, and loss of uranium mass in the tested solutions containing uranium between beginning and end of experiments, Ms, Ef =
Me Ms
(2)
The elution factors for DE 81 and Chelex gel were determined by averaging five elution measurements of each membrane. 2.8. Speciation calculation The speciation calculation function of Stability Constants Database program [18] was used to calculate the speciation distribution of uranium as a function of pH in water solutions. The stability constants of uranium complexes and the dissociation constants of the ligands were cited from literatures [8,19–21]. For natural organic matter (NOM) in natural water, two types of binding sites were considered with stability constants of 2.39 and 1.63 and dissociation constants of -4 and -6 accounting for 1.26 and 1.16 mmol binding sites per gram of NOM, respectively [8]. 3. Results and discussion 3.1. Diffusion gel As a DGT diffusive layer, polyacrylamide gel (PAM) is required to be chemically inert to the diffusing uranium species in order for the accumulation of the uranium in the binding phase to obey DGT equation [12]. The experiments testing the inertness of PAM gel to uranium ions showed that, in the NaNO3 concentration range of 1 × 10−1 –1 × 10−4 M, the ratios of the uranium concentrations in the gel to those in the solutions were in the range of 0.91–1.1 after 24 h immersion of the gels in uranium solutions. This indicated that there was little or no interaction between the gel and uranium ions. The uranium in the gel was
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Table 2 Diffusion coefficients of U (VI) (×10−6 cm2 s−1 ) in polyacrylamide gel in varying pH solutions at 23 ◦ C room temperature
sion coefficients also showed similar trend of decrease from pH 4.7 to 5.9 [23].
pH
D
3.3 3.9 4.9 6.0 7.1 8.1 8.8
5.8 5.5 4.7 3.1 2.4 1.9 1.7
3.2. Binding of uranium to DE 81 membrane and Chelex gel
due to the equilibrium between gel and solution. Hence there were no accumulation of uranium in the gel or the elevation of uranium concentration in gel was negligible. The assumption required by DGT equation therefore holds. The diffusion coefficient of uranium ions in the diffusion gel has to be determined before calculating uranium concentration in solutions where DGT devices deployed. The diffusion coefficients of uranium ions measured by the two-compartment cell method [17] with solutions of varying pH were shown in Table 2. A clear trend of diffusion coefficient decrease as increase of pH was observed. Diffusion coefficient decreased as the pH increased from 3.3 to 8.8. This could be explained by the variation of uranium speciation distribution in solution when pH changed. As shown in Fig. 1, the distribution of uranium species in solutions with different pH was calculated using the speciation function of Stability Constants Database [18]. It is known that the diffusion coefficients are dependent on size, shape and charge of the diffusers [22]. Molecular weights are one of the factors affecting the magnitudes of diffusion coefficients, as shown in equation below [22]: D=
3.3 × 10−5 Mw1/3
(3)
The variation of the diffusion coefficients was due to the variation of uranium species present in the solution at different pH. These different types of uranium complexes with different sizes, charges and shapes were the possible cause of the variation of diffusion coefficients [22]. Garmo’s results for uranium diffu-
Fig. 1. The calculated distribution of uranium species as a function of pH using the speciation function of Stability Constant Database [18]. The constants were cited from literatures [8,19–21]. The total uranium concentration was 1.0 mg L−1 , CO3 2− concentration 20 mg L−1 .
Binding kinetics of uranium by DE 81 and Chelex gel was shown in Fig. 2. The amount of uranium bound increased linearly as immersion time increased for DE 81 with binding rate of 13.3 nmol h−1 cm−2 until 13 h for DE 81 and 12.8 nmol h−1 cm−2 until 10 h for Chelex gel (Fig. 2). The maximum uranium uptake (binding capacity) was derived from the curve as 199 nmol cm−2 for DE 81 and 116 nmol cm−2 for Chelex gel, which corresponded to 625 nmol per disc (1.0 cm diameter opening in a DGT device) for DE 81 and 364 nmol per disc for Chelex gel. These meant that the per DGT device was able to take up uranium 625 nmol for DE 81 and 364 nmol for Chelex gel theoretically, when saturated. The lower binding capacity of Chelex gel can be explained by the different structures of binding functional groups of DE 81 and Chelex and uranium speciation in water. It was likely that more uranium was bound to amino groups of DE 81 than imino groups coexisting with carboxylic groups. The lower binding capacity of Chelex gel can also be due to its heterogeneity, i.e. the gel was prepared by imbedding the Chelex 100 resin beads in PAM gel. The density of the binding functional groups which were homogeneously distributed was less than that of DE 81. DE 81 is a diethylaminoethyl-substituted cellulose polymer membrane. The amino-functional groups of DE 81 were immobilised on the neutral cellulose polymer (Fig. 3). Chelex 100 is a styrene divinylbenzene copolymer containing paired iminodiacetate ions which act as binding functional groups [24]. At pH 7.4 and 12.3, the groups were mainly in anionic form as shown in Fig. 4 where B is the polymer backbone [24]. The predominant uranium species, on the other hand, were predicted as anionic forms of uranyl carbonates in neutral or alkaline waters according to our computer modelling (Fig. 1) and previ-
Fig. 2. Effect of immersion time on uranium uptake by DE 81 and Chelex gel discs of 2.5 cm diameter at pH 8.0, 23 ◦ C with initial uranium concentration of 1.0 mg L−1 and NaHCO3 concentration of 100 mg L−1 .
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Fig. 3. Chemical structure of Whatman DE 81 binding functional group.
Fig. 4. Anionic structures of Chelex 100 binding functional group.
ous published calculations of uranium speciation using different models [7,25], although the calculated speciation distribution were not exactly the same from author to author due to the input of the constants from different literatures and the use of different models [11]. Therefore, in neutral or alkaline solutions, the negatively charged carboxylic groups of Chelex 100 may prohibit the approaching of uranyl anions to the amino binding sites. Unlike Chelex 100, there were less electrostatic repulsions between neutral DE 81 and uranyl carbonates. The pH of the solution affected the chemical forms of both uranium ions and the functional groups of the membranes tested. The effects of pH on the binding of uranium to DE 81 and Chelex gel were shown in Fig. 5. At pH < 5, as pH decreased, both DE 81 and Chelex gel gradually lost the binding capacity. The amino groups of DE 81 (Fig. 3) and Chelex 100 (Fig. 4) were protonated [24,26]. The predominant uranyl species was UO2 2+ [7,25]. There were little interactions between the protonated ammonium forms of DE 81 and Chelex 100 with uranyl cations. This property of the membranes of being unable to bind ura-
Fig. 5. Effect of pH on the binding of uranium to DE 81 membrane and Chelex gel. Initial uranium concentration: 1.0 mg L−1 ; deployment time: 24 h; temperature: 23 ◦ C.
nium at lower pH allowed the efficient elution of bound uranium from the binding phases in acidic media, which is an important requirement for DGT. At higher pH, as more binding functional groups were converted to the form of neutral trialkylamine (DE 81) or dialkylamine (Chelex) with lone pair electrons on amino nitrogen which were capable of binding uranyl ions of anionic forms (uranyl carbonates), more uranium was uptaken both for DE 81 and Chelex gel. At pH ∼ 7, the functional groups of DE 81 and Chelex 100 were kept in neutral forms and the binding of uranium to both DE 81 and Chelex gel reached their maximum binding capacities (Fig. 2). As pH increased from 7 to 9, the binding of uranium decreased from 197 to 171 nmol cm−2 for DE 81 and from 109 to 65.1 nmol cm−2 for Chelex gel. As mentioned above, the uranium ions interacted with the binding phases through ion exchange mechanism. It was more likely that the uranyl complexes were bound directly to the binding phases without dissociation due to the high stability constants of the complexes (log βUO2 (CO3 ) 9.08, log βUO2 (CO3 )2 15.76 and log βUO2 (CO3 )3 19.77) [27]. Although the predominant uranium species was in anionic forms, at higher pH solutions [25], the numbers of CO3 2− ligands complexed to uranyl were increased. The steric hindrance prohibited the approach of the complexes to the binding phases. The affinity between uranyl complexes with more coordination numbers of ligands and the binding phases was also reduced. Therefore, the binding phases were suitable for DGT application for uranium analysis in waters with pH between 5 and 9. It is also worth to mention that, for Chelex, carboxylic group is also a binding site for uranyl cations. However, when uranium exists in the form of uranyl cations at low pH, the carboxylic group of Chelex is in its acid form and lost binding capability. Influence of ionic strength on the binding of uranium to DE 81 and Chelex gel was studied since natural waters contain a range of major cations and anions. As shown in Fig. 6, ionic strength affected the binding of uranium to the binding phases. As the NaNO3 concentration increased, uranium
Fig. 6. Effect of ionic strength on uranium binding to DE 81 membranes and Chelex gel in solution of different NaNO3 concentrations (M) at pH 8.0. Initial concentration of uranium: 1.0 mg L−1 ; deployment time: 24 h; temperature: 23 ◦ C.
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uptaken to DE 81 and Chelex gel decreased. As mentioned before, the predominant uranium species in alkaline water were in anionic forms. The most competitive ions to the binding of uranium anions would be NO3 − and OH− in the tested solutions, while the OH− concentration was fixed (pH 8). Uranium bound to DE 81 decreased from 206 nmol cm−2 at NaNO3 concentration of 1.0 × 10−3 M to 124 nmol cm−2 at NaNO3 concentration of 1.0 M, while uranium bound to Chelex gel decreased from 78.2 nmol cm−2 at NaNO3 concentration of 1.0 × 10−3 M–30.0 nmol cm−2 at NaNO3 concentration of 1.0 M. Although the interaction between NO3 − and the binding phases may not be as strong as that between uranyl anions and the binding phases, the effects of NaNO3 concentration on uranium binding were still apparent due to much higher concentrations of NO3 − than that of uranium and the smaller size of NO3 − . The higher binding capacity of DE 81 membrane to uranium in the tested NaNO3 concentration range suggest that application of DE 81 in DGT in natural waters for uranium measurements would be more feasible than Chelex, especially in higher ionic strength waters. 3.3. Elution of the membranes Uranium in the 2.0 mL elution solution from blank DE 81 and Chelex gel was below the instrument detection limit, i.e. uranium in the membranes was less than 5.5 × 10−6 nmol. DGT technique usually requires efficient elution of uranium from the binding phases. The elution efficiencies for Chelex gel and DE 81 membrane disc were determined as 67.8% and 87.7%, respectively, using Eq. (2). The elution efficiency for DE 81 is acceptable for DGT applications [16]. However, the low elution efficiency for Chelex gel made it less preferable for DGT application. Digestion of Chelex gel and DE 81 membrane in concentrated nitric acid was also tried by heating the solution to 150 ◦ C to destroy the membrane matrix. However, introducing the digested samples to SF-ICP-MS instrument failed due to the incomplete digest of the matrix and gel-like particles stick to tubing. 3.4. DGT deployment in laboratory solution To validate the DGT devices assembled with DE 81 and Chelex binding phases, the devices were deployed in synthetic river water solution (St. Lawrence River, Table 1). Fig. 7 shows the mass taken by the DGT devices at different time intervals and the comparison with the theoretical calculation (dashed line) using DGT equation assuming all uranium species were measurable. The average total uranium concentration in the solution measured at the beginning, middle and end of the DGT deployment was 8.5 g L−1 . Average uranium concentrations calculated from the experimental lines of DE 81 DGT (solid line) and Chelex DGT (thinner dashed line) as 6.8 and 6.4 g L−1 , respectively, were 80% (RSD 5.0%) and 75% (RSD 6.2%) of the total uranium concentration of 8.5 g L−1 . The computer calculated major species of uranium were UO2 (CO3 ) (0.73%), UO2 (CO3 )2 2− (80.44%) and UO2 (CO3 )3 4− (18.83%). It is known that DGT only measures
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Fig. 7. DGT mass–time curves for synthetic river water (St. Lawrence). The thicker dashed line is drawn based on theoretical calculation using DGT equation assuming all uranium species were measured. The solid line and the thinner dashed line are for DE 81 DGT Chelex DGT, respectively.
labile analyte species in waters [12,13]. It is likely that DE 81 DGT measured UO2 (CO3 )2 2− only. This agreed with the previous work of using amino containing resin to remove uranium that the molar ratio between CO2 and uranium was close to 2 in an anion exchange resin with uranium bound [28]. However, it has to be stated that it is not confident of saying that the DE 81 DGT measured species was UO2 (CO3 )2 2− based on the data achieved so far. The preferential binding of UO2 (CO3 )2 2− to anion exchange membrane was due to the lower hydration energy of dicarbonato-uranyl complex than that of tricarbonatouranyl complex explained by Gureli and Apak [29]. The steric hindrance of the UO2 (CO3 )3 4− complexes [30] prohibited the approach of the complexes to the amino groups on the binding phases. The affinity of the complexes to the binding phases greatly decreased. Further study is required to find out whether this DGT labile concentration is UO2 (CO3 )2 2− complexes, and is the bioavailable uranyl species. However, a significant pH effects on the bioaccumulation of uranium to bivalve Corbicula fluminea were observed by Simon and Garnier-Laplace [31], indicating that only certain uranium species were uptaken by the bivalve since the speciation of uranium is pH dependent. Chelex DGT measured 75% (RSD 6.2%) of the total uranium concentration of 8.5 g L−1 . As mentioned above, Chelex 100 is cation exchange resin. Therefore, it more likely measured cation forms of uranium. As shown in Fig. 1, there was little cation forms of uranium existing in the tested solutions. The fraction of uranium measured by Chelex DGT (75%) could be explained by the dissociation mechanism proposed by Scally et al. [32]. In their study, Cu2+ -nitrilotriacetic acid (NTA) complex and Ni2+ -NTA complex were demonstrated as examples of a fully dissociateable and a partly dissociateable complexe, respectively. In our case, uranyl carbonates partially dissociated in the DGT diffusion layer due to the reduction of uranyl cations by the carboxylic groups of Chelex 100 resin on the diffusion/binding interface. This measurement by Chelex DGT would provide information on stability of the uranium carbonate complexes.
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3.5. Field deployment DGT devices with DE 81 and Chelex binding phases were deployed for 2 weeks in St. Lawrence River, located at Morrisburg, Ont., Canada, in early August, 2005, to measure DGT labile concentrations of uranium. Total uranium concentration was also measured as 0.27 g L−1 at the same location by grab sampling method. Concentrations measured by DE 81 DGT and Chelex DGT were 73% and 60% of the total uranium, respectively. The calculated uranium speciation distribution was also shown in Fig. 1, with major species of UO2 (CO3 ) (0.43%), UO2 (CO3 )2 2− (72.88%) and UO2 (CO3 )3 4− (26.70%) at pH 8.2. It indicated that the DE 81 DGT measured species was most likely UO2 (CO3 )2 2− . Concentration measured by Chelex DGT was lower than the calculated one, and can also be explained by uranium carbonate dissociation mechanism. Thickness of 0.040 cm PAM diffusion gel was used in this work. It was known that the effects of double boundary layer (DBL) in well stirred laboratory solutions and in fast flowing river water (St. Lawrence River) would be negligibly small compared to the gel thickness [12]. The use of diffusion coefficients measured in laboratory solutions with similar hydrodynamic conditions to the DGT deployment solution would minimise the DBL effects [33]. Diffusion coefficients were not corrected with temperature since the temperature of laboratory solution and the average temperature of river water were the same or close to the temperature under which the diffusion coefficients were measured [12]. Temperature effects on diffusion coefficients can be considered with Stokes-Einstein equation D1 η1 D2 η2 = T2 T1
(4)
where D1 and D2 are diffusion coefficients at absolute temperature T1 and T2 , respectively. η1 and η2 are viscosities of water at T1 and T2 , respectively [34]. 4. Conclusions and future work It has been shown that both DE 81 membrane and Chelex 100 gel can be used for DGT applications as binding phases. Comparison of uranium concentrations measured by DE 81 and Chelex DGT in laboratory solutions and in natural water with the theoretical calculations indicated that species measured by DE 81 DGT was likely UO2 (CO3 )2 2− species and Chelex DGT measured fraction of uranium dissociated from uranyl carbonates during their diffusion in the diffusion layer. Studies are still required to further identify these DGT labile uranium species and correlate them to health effects. As both speciation of uranium and chemical forms of the binding phases are highly pH dependent, it is suggested that the binding phases are to be used in alkaline fresh waters of pH greater than 7. Acknowledgement Financial supports for this study were provided by Radiation Protection Bureau, Health Canada. We also thank Ms. Heather
Broadbent in Trent University, Canada, for the kind help in measuring major cations, anions and other water quality parameters for the water samples from St. Lawrence River. References [1] H. Windom, R. Smith, F. Niencheski, C. Alexander, Mar. Chem. 68 (2000) 307–321. [2] K.H. Lieser, Nuclear and Radiochemistry, Fundamentals and Applications, Wiley-VCH, Berlin, 2001. [3] B.A. Ahier, B.L. Tracy, Environ. Health Perspect. 103 (Suppl. 9) (1995) 89–101. [4] S.C. Sheppard, M.I. Sheppard, M.-O. Gallerand, B. Sanipelli, J. Environ. Radioactiv. 79 (2005) 55–83. [5] D.C. Kocher, Health Phys. 57 (1989) 9–15. [6] A.M. Ure, C.M. Davidson (Eds.), Chemical Speciation in the Environment, Blackie Academic & Proffessional, London, 1995. [7] S.J. Markich, The Sci. World J. 2 (2002) 707–729. [8] R.J. Murphy, J.J. Lenhart, B.D. Honeyman, Colloid Surf. A: Physicochem. Eng. Aspects 157 (1999) 47–62. [9] A.P. Novikov, V.V. Tkachev, B.F. Myasoedov, Comptes Rendus Chim. 7 (2004) 1219–1225. [10] F. Caron, G. Mankarios, Mining and the Environment Conference, Sudbury, Canada, 2003. [11] F.H. Denison, J. Garnier-Laplace, Geochim. Cosmochim. Acta 69 (2005) 2183–2191. [12] W. Davison, G. Fones, M. Harper, P. Teasdale, H. Zhang, In Situ Monitoring of Aquatic Systems Chemical Analysis and Speciation, John Wiley & Sons Ltd., Chichester, 2000. [13] W. Davison, H. Zhang, Nature (London) 367 (1994) 546–548. [14] A.P. Kryvoruchko, L.Y. Yurlova, I.D. Atamanenko, B.Y. Kornilovich, Desalination 162 (2004) 229–236. [15] E. Piron, A. Domard, Int. J. Biol. Macromol. 22 (1998) 33–40. [16] H. Zhang, W. Davison, Anal. Chem. 67 (1995) 3391–3400. [17] L.-Y. Chang, W. Davison, H. Zhang, M. Kelly, Anal. Chim. Acta 368 (1998) 243–253. [18] G. Pettit, L.D. Pettit, IUPAC Stability Constants Database, Academic Software, 1999. [19] I. Grenthe, I. Puigdomenech, M.C. Sandino, M.N. Rand, Chemical Thermodynamics of Americium, Elsevier, Amsterdam, 1995. [20] A. Sanding, J. Bruno, Geochim. Cosmochim. Acta 56 (1992) 4135–4145. [21] R.J. Silva, Mater. Res. Soc. Symp. Proc. 257 (1992) 323–330. [22] J. Buffle, Complexation Reactions in Aquatic Systems: An Analytical Approach, Ellis Horwood Ltd., Chichester, UK, 1988. [23] O.A. Garmo, O. Royset, E. Steinnes, T.P. Flaten, Anal. Chem. 75 (2003) 3573–3580. [24] Bio-Rad Laboratories, Chelex 100 and Chelex 20 Chelating Ion Exchange Resin Instruction Manual, Bio-Rad, Hercules, CA. [25] C.S. Barton, D.I. Stewart, K. Morris, D.E. Bryant, J. Hazard. Mater. 116 (2004) 191–204. [26] A.A. Atia, A.M. Donia, K.Z. Elwakeel, React. Funct. Polym. 65 (2005) 267–275. [27] P.W. Jones, D.M. Taylor, L.M. Webb, D.R. Williams, Appl. Radiat. Isotop. 57 (2002) 159–165. [28] Y. Song, Y. Wang, L. Wang, C. Song, Z. Yang, A. Zhao, React. Funct. Polym. 39 (1999) 245–252. [29] L. Gureli, R. Apak, Separ. Sci. Technol. 39 (2004) 1857. [30] R.N. Sylva, M.R. Davison, J Chem. Soc., Dalton Trans. (1979) 465–471. [31] O. Simon, J. Garnier-Laplace, Aquat. Toxicol. 68 (2004) 95–108. [32] S. Scally, W. Davison, H. Zhang, Environ. Sci. Technol. 37 (2003) 1379–1384. [33] W. Li, Development of new binding phases for speciation measurements of trace metals with the diffusive gradients in thin films technique, Ph.D. Thesis, Griffith University, Australia, 2003. [34] N.E. Dorsey, Properties of Ordinary Water Substances, Reinhold Publishing Corporation, New York, 1940.
Speciation measurements of uranium in alkaline waters using diffusive gradients in thin films technique Weijia Li a,∗ , Jiujiang Zhao a,b , Chunsheng Li a,∗∗ , Stephen Kiser a , R. Jack Cornett a a
Radiation Protection Bureau, Health Canada, A.L. 6302D1, 775 Brookfield Rd., Ottawa, Ont., K1A 1C1 Canada b Department of Chemistry, Carlton University, 1125 Colonel by Drive, Ottawa, Ont., K1S 5B6 Canada Received 15 March 2006; received in revised form 25 May 2006; accepted 27 May 2006 Available online 3 June 2006
Abstract This work investigated the application of diffusive gradients in thin films technique (DGT) to uranium speciation measurements in natural water. Two binding phases were examined, a commercially available affinity membrane, Whatman DE 81 (DE 81), with amino binding functional groups and the conventionally used Chelex 100 beads imbedded polyacrylamide hydrogel (Chelex) with iminodiacetate functional groups. The DGT devices assembled with the binding phases of DE 81 (DE 81 DGT) and Chelex gel (Chelex DGT) were tested both in synthetic river water solutions and in local river water. DE 81 DGT and Chelex DGT measured 80% and 75% of the total uranium in synthetic river water solution, respectively, and measured 73% and 60% of the total uranium in St. Lawrence River, Canada, respectively. The binding properties of the DE 81 membrane and Chelex gel for uranium, and the diffusion of uranyl complexes in the polyacrylamide gel (PAM) were also studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Diffusive gradients in thin films (DGT); Uranium; River water
1. Introduction Uranium concentrations in natural waters are concerned over the decades due to its involvement in many nuclear activities (nuclear power generation, nuclear tests and nuclear weapon programs) and leaching from natural sources [1]. Uranium is both chemically and radiologically toxic as a heavy metal and a radionuclide. However, chemical damage to organism is more important than radiation damage due to the severe chemical damage to kidney when ingested and the long half-lives of the isotopes 234 U, 2.46 × 105 year; 235 U, 7.04 × 108 year; 238 U, 4.47 × 109 year [2–5]. It is well known that the chemical toxicity or bioavailability of metals depends on their chemical speciation [6,7]. Like other heavy metals, uranium forms various complexes in natural waters with the varying ligands present. It has been known that depending on solution pH, uranyl ions form stable hydroxide and carbonate complexes with dissolved
∗
Corresponding author. Tel.: +1 613 954 8617; fax: +1 613 952 9071. Corresponding author. Tel.: +1 613 954 0299; fax: +1 613 952 9071. E-mail addresses: weijia [email protected] (W. Li), li [email protected] (C. Li). ∗∗
0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2006.05.092
atmospheric CO2 with varying coordination numbers, most of which are in anionic forms [7,8]. Complexes with humic substances may only exist in neutral or acidic solutions, if any [8]. To obtain speciation information of uranium, the methods of grabbing samples from natural water and analysing in laboratory with preconcentration and separation procedures were used in previous publications [9,10]. However, the distribution of uranium species in water may have been changed during transport, preservation and sample process [1]. In addition, the concentrations of the species measured can only represent those at the time of sampling which may be overestimated or underestimated due to the concentration fluctuations in waters. Another way of obtaining speciation information is by computer simulation method, which is dependent on the availability of reliable equilibrium constants and the assumption that the species are in equilibrium with each other [11]. To reflect the true picture of uranium speciation in water, the in situ speciation technique with time integrated preconcentration features, diffusive gradients in thin films technique (DGT), was used in this work. There were mainly two layers in a DGT device, a diffusion layer of a polyacrylamide gel (PAM), to allow the analyte to diffuse through it, and an underlying layer of a binding phase, traditionally Chelex 100 resin imbedded in PAM gel (Chelex), to
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reduce the concentration of the analyte species on the interface to negligibly low [12,13]. Concentration of DGT labile fraction of metal (C) in waters, i.e. species that are diffusible through the diffusion layer with certain pore size and reactive to the binding phase, can be calculated by measuring the mass of a metal (M) accumulated in the binding phase for certain deployment time (t) after elution in an acidic solution using the DGT equation [12,13]: C=
Mg DAt
(1)
where A and g are the area and thickness of the diffusion layer, and D is the diffusion coefficient of the uranium ions in the diffusion layer. For the analysis of a metal ion of interest, the key of using DGT technique is to find out suitable binding phases and diffusion layers. Polymers with amino-functional groups were found to be capable to remove uranium from water [14,15]. In this work, the traditional DGT binding phase of Chelex gel and a commercially available robust membrane disc, Whatman DE 81 membrane (DE 81), were used as DGT binding phases. The PAM gel was used as a diffusion layer. This work focused on the water in St. Lawrence River in Ontario, Canada, which is from Lake Ontario where many nuclear facilities located. pH of the water is around 8+. The performance of the binding phases for DGT application of uranium analysis was investigated both in this alkaline water and in laboratory solutions with similar ion contents and pH. 2. Experimental 2.1. Reagents and material All solutions were prepared with deionised water (MilliQ) and were in equilibrium with atmospheric CO2 . All reagents used for the preparation of polyacrylamide gel (PAM) and Chelex 100 PAM gel (Chelex gel) were of analytical grade and used as received. The acrylamide monomer solution (40%), ammonium peroxosulphate, N,N,N ,N -tetramethylethylene diamine and sodium form Chelex 100 resin of 200–400 mesh were obtained from Bio-Rad. The agarose derived crosslinker (2%) and the DGT holders were purchased from DGT Research Ltd. UK. Cellulose nitrate filter membrane (0.45 m) and Whatman DE 81 membrane of 2.5 cm diameter disc with 0.2 mm thickness were purchased from Whatman International Ltd., UK, and were used as the protection cover layer and binding phase, respectively. A 1000 mg L−1 uranium standard solutions were purchased from SPEX CertiPrep Inc., US, and used as stock solutions for the membrane binding property experiments and as calibration standards after dilution for measurements. Concentrated optima nitric acid (Fisher Scientific) was used for all acidic solution preparation unless otherwise stated. The acid and sodium hydroxide (Fisher Scientific) were used to adjust solution pH. All experiments were carried out in high density polypropylene containers.
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2.2. Instrument Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS, ThermoFinnigan, Element 2) was used for uranium measurements in solutions with detection limit of 2.8 × 10−3 nmol L−1 (3σ). Ion chromatography (DX-600, Dionex, Oakville, Ont., Canada) was used to determine the concentrations of potassium, sodium, calcium and magnesium cations and anions in water. The dissolved organic carbon (DOC) was measured with TOCVcpn Shimadzu (Mandel Scientific, Guelph, Ont., Canada). 2.3. Preparation of PAM gel, Chelex gel and pre-treatment of DE 81 membrane PAM gel and Chelex gel were prepared according to previously described procedures [16]. The thicknesses of the gels were made as 0.040 cm. The gels were cut as 2.5 cm diameter discs before use. Whatman DE 81 membrane used for all experiments were immersed in 10% nitric acid for 24 h before being rinsed with Milli-Q water and stored in Milli-Q water in order to remove any metals which may be contained. 2.4. Binding of uranium The binding of uranium to the DE 81 membrane, Chelex gel and PAM gel was examined by immersing the 2.5 cm diameter discs in 500 mL solutions containing 1.0 mg L−1 uranium with constant rate of stirring at pH 8 for 24 h. About 100 mg L−1 NaHCO3 was added to the solutions to maintain pH at 8 and to equilibrate with uranyl carbonates. Ionic strength of the solutions was adjusted as 0.01 M NaNO3 unless otherwise stated to keep constant ionic strength in the solutions. Uranium concentrations in the tested solutions were measured by SF-ICP-MS before and after immersion. Uranium bound in the membranes was also measured by SF-ICP-MS after elution in 10% HNO3 solution. All experiments were repeated three times. The effect of immersion time on the binding of uranium was investigated by immersing the DE 81 or Chelex gel discs in uranium solutions for varying periods of time intervals of 0.5–26 h. The influence of pH on the binding was tested in the pH range of 1–9. The effect of ionic strength on the binding was carried out in solutions containing various concentrations of NaNO3 ranging from 1.0 × 10−4 to 1.0 M. The PAM gel was also tested for uranium binding. Uranium solutions with NaNO3 concentrations ranged from 1 × 10−1 to 1 × 10−4 M were used. 2.5. Diffusion coefficient measurement Diffusion coefficient of uranium ions in PAM gel were measured according to the previously described method by Chang et al. [17]. A polyacrylic cell with two compartments was used. One compartment (A) initially contained a solution of 800 mL of 1.0 mg L−1 uranium and the other compartment (B) initially contained a solution of the same volume as compartment A with no uranium addition. The ionic strengths of the solutions
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in both compartments were adjusted to 0.01 M by the addition of 1 M NaHCO3 . pH of the solutions was adjusted to 3.3, 3.9, 4.9, 6.0, 7.1, 8.1 and 8.8 with 1% NaOH and 1% HNO3 . The solutions were stirred over night with exposure to atmosphere before adding to the compartments. Each compartment was mixed well with an overhead stirrer during the experiments. A 2.0 and 0.2 mL aliquots of solutions were taken from compartment A and compartment B, respectively, at 30 min intervals for 240 min. The samples were diluted accordingly before being measured by ICPMS. Diffusion coefficients of uranium ions in the PAM gel were calculated using equation, D = Mg/(CAt), derived from Eq. (1).
solution after immersion in the solution for time intervals of 21–189 h. Chelex and DE 81 DGT devices were also deployed in St. Lawrence River of Canada side located at the town of Morrisburg, ON, Canada (46◦ 20.93 N, 72◦ 26.04 W) for 24, 72, 144 and 336 h in early August 2005. Average temperature of the water was 24 ◦ C. pH of the water was 8.2. The devices were rinsed with Milli-Q water thoroughly at the end of the deployment before the binding phase discs were removed from the devices. For river water deployments, the devices were kept separately in Ziploc plastic bags and transported to laboratory in a cool box at the end of the deployment.
2.6. Deployment of DGT devices in laboratory solution and in river water
2.7. Elution of uranium from the binding phases
Water samples were taken from St. Lawrence River according standard procedures of on site filtration at the beginning, middle and end of the DGT deployments. Samples were preserved in 2% HNO3 solutions during transport and storage. pH and alkalinity of the water were measured on site. Concentrations of uranium, major cations and anions, dissolved organic carbon (DOC) and total inorganic carbon (TIC) were analyzed in laboratory using appropriate instruments (Section 2.2). All data were shown in Table 1 and used as recipe for synthetic river water preparation. he DE 81 or Chelex binding phase, the PAM hydrogel diffusion layer and a wet 100 m thickness Whatman cellulose nitrate filter membrane (0.45 m) as a protection layer were placed in order in a DGT holder according to previously described procedures [16]. To test the performance of the binding phases, the assembled DGT devices were deployed in a synthetic river water solution (St. Lawrence) of composition: [K+ ] = 1.2 mg L−1 ; [Na+ ] = 14 mg L−1 ; [Ca2+ ] = 27 mg L−1 ; [Mg2+ ] = 6.2 mg L−1 ; [Cl− ] = 42 mg L−1 ; [NO3 − ] = 54 mg L−1 ; [SO4 2− ] = 34 mg L−1 . Sufficient volume of solution (30 L) was provided to ensure that the depletion of uranium by the DGT devices was negligible. The solution was exposed to atmosphere with constant stirring over night before the devices were immersed to let the solution equilibrated with CO2 . pH of the solution was ∼8 after addition of 100 mg L−1 NaHCO3 . The devices were removed from the
Table 1 Concentration composition and other parameters of St. Lawrence River water Na+ K+ Ca2+ Mg2+ Cl− SO4 2− DOC TIC TC pH Alk
11.9 1.47 32.0 7.92 22.1 25.9 6.42 21.2 27.6 8.2 83.3
Note: Unit for ionic concentration, dissolved organic carbon (DOC), total inorganic carbon (TIC), total carbon (TC) and alkalinity (CaCO3 ) was mg L−1 .
The binding phases of both DE 81 membrane and Chelex gel discs were sunk in 2.0 mL of 2 M HNO3 solution for 24 h both for laboratory and river water deployments to elute the uranium from the binding discs after being removed from the DGT devices before being analyzed by SF-ICP-MS. The elution efficiency, or elution factor, Ef , was defined as the ratio between the amount of uranium eluted from gel discs, Me, and loss of uranium mass in the tested solutions containing uranium between beginning and end of experiments, Ms, Ef =
Me Ms
(2)
The elution factors for DE 81 and Chelex gel were determined by averaging five elution measurements of each membrane. 2.8. Speciation calculation The speciation calculation function of Stability Constants Database program [18] was used to calculate the speciation distribution of uranium as a function of pH in water solutions. The stability constants of uranium complexes and the dissociation constants of the ligands were cited from literatures [8,19–21]. For natural organic matter (NOM) in natural water, two types of binding sites were considered with stability constants of 2.39 and 1.63 and dissociation constants of -4 and -6 accounting for 1.26 and 1.16 mmol binding sites per gram of NOM, respectively [8]. 3. Results and discussion 3.1. Diffusion gel As a DGT diffusive layer, polyacrylamide gel (PAM) is required to be chemically inert to the diffusing uranium species in order for the accumulation of the uranium in the binding phase to obey DGT equation [12]. The experiments testing the inertness of PAM gel to uranium ions showed that, in the NaNO3 concentration range of 1 × 10−1 –1 × 10−4 M, the ratios of the uranium concentrations in the gel to those in the solutions were in the range of 0.91–1.1 after 24 h immersion of the gels in uranium solutions. This indicated that there was little or no interaction between the gel and uranium ions. The uranium in the gel was
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Table 2 Diffusion coefficients of U (VI) (×10−6 cm2 s−1 ) in polyacrylamide gel in varying pH solutions at 23 ◦ C room temperature
sion coefficients also showed similar trend of decrease from pH 4.7 to 5.9 [23].
pH
D
3.3 3.9 4.9 6.0 7.1 8.1 8.8
5.8 5.5 4.7 3.1 2.4 1.9 1.7
3.2. Binding of uranium to DE 81 membrane and Chelex gel
due to the equilibrium between gel and solution. Hence there were no accumulation of uranium in the gel or the elevation of uranium concentration in gel was negligible. The assumption required by DGT equation therefore holds. The diffusion coefficient of uranium ions in the diffusion gel has to be determined before calculating uranium concentration in solutions where DGT devices deployed. The diffusion coefficients of uranium ions measured by the two-compartment cell method [17] with solutions of varying pH were shown in Table 2. A clear trend of diffusion coefficient decrease as increase of pH was observed. Diffusion coefficient decreased as the pH increased from 3.3 to 8.8. This could be explained by the variation of uranium speciation distribution in solution when pH changed. As shown in Fig. 1, the distribution of uranium species in solutions with different pH was calculated using the speciation function of Stability Constants Database [18]. It is known that the diffusion coefficients are dependent on size, shape and charge of the diffusers [22]. Molecular weights are one of the factors affecting the magnitudes of diffusion coefficients, as shown in equation below [22]: D=
3.3 × 10−5 Mw1/3
(3)
The variation of the diffusion coefficients was due to the variation of uranium species present in the solution at different pH. These different types of uranium complexes with different sizes, charges and shapes were the possible cause of the variation of diffusion coefficients [22]. Garmo’s results for uranium diffu-
Fig. 1. The calculated distribution of uranium species as a function of pH using the speciation function of Stability Constant Database [18]. The constants were cited from literatures [8,19–21]. The total uranium concentration was 1.0 mg L−1 , CO3 2− concentration 20 mg L−1 .
Binding kinetics of uranium by DE 81 and Chelex gel was shown in Fig. 2. The amount of uranium bound increased linearly as immersion time increased for DE 81 with binding rate of 13.3 nmol h−1 cm−2 until 13 h for DE 81 and 12.8 nmol h−1 cm−2 until 10 h for Chelex gel (Fig. 2). The maximum uranium uptake (binding capacity) was derived from the curve as 199 nmol cm−2 for DE 81 and 116 nmol cm−2 for Chelex gel, which corresponded to 625 nmol per disc (1.0 cm diameter opening in a DGT device) for DE 81 and 364 nmol per disc for Chelex gel. These meant that the per DGT device was able to take up uranium 625 nmol for DE 81 and 364 nmol for Chelex gel theoretically, when saturated. The lower binding capacity of Chelex gel can be explained by the different structures of binding functional groups of DE 81 and Chelex and uranium speciation in water. It was likely that more uranium was bound to amino groups of DE 81 than imino groups coexisting with carboxylic groups. The lower binding capacity of Chelex gel can also be due to its heterogeneity, i.e. the gel was prepared by imbedding the Chelex 100 resin beads in PAM gel. The density of the binding functional groups which were homogeneously distributed was less than that of DE 81. DE 81 is a diethylaminoethyl-substituted cellulose polymer membrane. The amino-functional groups of DE 81 were immobilised on the neutral cellulose polymer (Fig. 3). Chelex 100 is a styrene divinylbenzene copolymer containing paired iminodiacetate ions which act as binding functional groups [24]. At pH 7.4 and 12.3, the groups were mainly in anionic form as shown in Fig. 4 where B is the polymer backbone [24]. The predominant uranium species, on the other hand, were predicted as anionic forms of uranyl carbonates in neutral or alkaline waters according to our computer modelling (Fig. 1) and previ-
Fig. 2. Effect of immersion time on uranium uptake by DE 81 and Chelex gel discs of 2.5 cm diameter at pH 8.0, 23 ◦ C with initial uranium concentration of 1.0 mg L−1 and NaHCO3 concentration of 100 mg L−1 .
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Fig. 3. Chemical structure of Whatman DE 81 binding functional group.
Fig. 4. Anionic structures of Chelex 100 binding functional group.
ous published calculations of uranium speciation using different models [7,25], although the calculated speciation distribution were not exactly the same from author to author due to the input of the constants from different literatures and the use of different models [11]. Therefore, in neutral or alkaline solutions, the negatively charged carboxylic groups of Chelex 100 may prohibit the approaching of uranyl anions to the amino binding sites. Unlike Chelex 100, there were less electrostatic repulsions between neutral DE 81 and uranyl carbonates. The pH of the solution affected the chemical forms of both uranium ions and the functional groups of the membranes tested. The effects of pH on the binding of uranium to DE 81 and Chelex gel were shown in Fig. 5. At pH < 5, as pH decreased, both DE 81 and Chelex gel gradually lost the binding capacity. The amino groups of DE 81 (Fig. 3) and Chelex 100 (Fig. 4) were protonated [24,26]. The predominant uranyl species was UO2 2+ [7,25]. There were little interactions between the protonated ammonium forms of DE 81 and Chelex 100 with uranyl cations. This property of the membranes of being unable to bind ura-
Fig. 5. Effect of pH on the binding of uranium to DE 81 membrane and Chelex gel. Initial uranium concentration: 1.0 mg L−1 ; deployment time: 24 h; temperature: 23 ◦ C.
nium at lower pH allowed the efficient elution of bound uranium from the binding phases in acidic media, which is an important requirement for DGT. At higher pH, as more binding functional groups were converted to the form of neutral trialkylamine (DE 81) or dialkylamine (Chelex) with lone pair electrons on amino nitrogen which were capable of binding uranyl ions of anionic forms (uranyl carbonates), more uranium was uptaken both for DE 81 and Chelex gel. At pH ∼ 7, the functional groups of DE 81 and Chelex 100 were kept in neutral forms and the binding of uranium to both DE 81 and Chelex gel reached their maximum binding capacities (Fig. 2). As pH increased from 7 to 9, the binding of uranium decreased from 197 to 171 nmol cm−2 for DE 81 and from 109 to 65.1 nmol cm−2 for Chelex gel. As mentioned above, the uranium ions interacted with the binding phases through ion exchange mechanism. It was more likely that the uranyl complexes were bound directly to the binding phases without dissociation due to the high stability constants of the complexes (log βUO2 (CO3 ) 9.08, log βUO2 (CO3 )2 15.76 and log βUO2 (CO3 )3 19.77) [27]. Although the predominant uranium species was in anionic forms, at higher pH solutions [25], the numbers of CO3 2− ligands complexed to uranyl were increased. The steric hindrance prohibited the approach of the complexes to the binding phases. The affinity between uranyl complexes with more coordination numbers of ligands and the binding phases was also reduced. Therefore, the binding phases were suitable for DGT application for uranium analysis in waters with pH between 5 and 9. It is also worth to mention that, for Chelex, carboxylic group is also a binding site for uranyl cations. However, when uranium exists in the form of uranyl cations at low pH, the carboxylic group of Chelex is in its acid form and lost binding capability. Influence of ionic strength on the binding of uranium to DE 81 and Chelex gel was studied since natural waters contain a range of major cations and anions. As shown in Fig. 6, ionic strength affected the binding of uranium to the binding phases. As the NaNO3 concentration increased, uranium
Fig. 6. Effect of ionic strength on uranium binding to DE 81 membranes and Chelex gel in solution of different NaNO3 concentrations (M) at pH 8.0. Initial concentration of uranium: 1.0 mg L−1 ; deployment time: 24 h; temperature: 23 ◦ C.
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uptaken to DE 81 and Chelex gel decreased. As mentioned before, the predominant uranium species in alkaline water were in anionic forms. The most competitive ions to the binding of uranium anions would be NO3 − and OH− in the tested solutions, while the OH− concentration was fixed (pH 8). Uranium bound to DE 81 decreased from 206 nmol cm−2 at NaNO3 concentration of 1.0 × 10−3 M to 124 nmol cm−2 at NaNO3 concentration of 1.0 M, while uranium bound to Chelex gel decreased from 78.2 nmol cm−2 at NaNO3 concentration of 1.0 × 10−3 M–30.0 nmol cm−2 at NaNO3 concentration of 1.0 M. Although the interaction between NO3 − and the binding phases may not be as strong as that between uranyl anions and the binding phases, the effects of NaNO3 concentration on uranium binding were still apparent due to much higher concentrations of NO3 − than that of uranium and the smaller size of NO3 − . The higher binding capacity of DE 81 membrane to uranium in the tested NaNO3 concentration range suggest that application of DE 81 in DGT in natural waters for uranium measurements would be more feasible than Chelex, especially in higher ionic strength waters. 3.3. Elution of the membranes Uranium in the 2.0 mL elution solution from blank DE 81 and Chelex gel was below the instrument detection limit, i.e. uranium in the membranes was less than 5.5 × 10−6 nmol. DGT technique usually requires efficient elution of uranium from the binding phases. The elution efficiencies for Chelex gel and DE 81 membrane disc were determined as 67.8% and 87.7%, respectively, using Eq. (2). The elution efficiency for DE 81 is acceptable for DGT applications [16]. However, the low elution efficiency for Chelex gel made it less preferable for DGT application. Digestion of Chelex gel and DE 81 membrane in concentrated nitric acid was also tried by heating the solution to 150 ◦ C to destroy the membrane matrix. However, introducing the digested samples to SF-ICP-MS instrument failed due to the incomplete digest of the matrix and gel-like particles stick to tubing. 3.4. DGT deployment in laboratory solution To validate the DGT devices assembled with DE 81 and Chelex binding phases, the devices were deployed in synthetic river water solution (St. Lawrence River, Table 1). Fig. 7 shows the mass taken by the DGT devices at different time intervals and the comparison with the theoretical calculation (dashed line) using DGT equation assuming all uranium species were measurable. The average total uranium concentration in the solution measured at the beginning, middle and end of the DGT deployment was 8.5 g L−1 . Average uranium concentrations calculated from the experimental lines of DE 81 DGT (solid line) and Chelex DGT (thinner dashed line) as 6.8 and 6.4 g L−1 , respectively, were 80% (RSD 5.0%) and 75% (RSD 6.2%) of the total uranium concentration of 8.5 g L−1 . The computer calculated major species of uranium were UO2 (CO3 ) (0.73%), UO2 (CO3 )2 2− (80.44%) and UO2 (CO3 )3 4− (18.83%). It is known that DGT only measures
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Fig. 7. DGT mass–time curves for synthetic river water (St. Lawrence). The thicker dashed line is drawn based on theoretical calculation using DGT equation assuming all uranium species were measured. The solid line and the thinner dashed line are for DE 81 DGT Chelex DGT, respectively.
labile analyte species in waters [12,13]. It is likely that DE 81 DGT measured UO2 (CO3 )2 2− only. This agreed with the previous work of using amino containing resin to remove uranium that the molar ratio between CO2 and uranium was close to 2 in an anion exchange resin with uranium bound [28]. However, it has to be stated that it is not confident of saying that the DE 81 DGT measured species was UO2 (CO3 )2 2− based on the data achieved so far. The preferential binding of UO2 (CO3 )2 2− to anion exchange membrane was due to the lower hydration energy of dicarbonato-uranyl complex than that of tricarbonatouranyl complex explained by Gureli and Apak [29]. The steric hindrance of the UO2 (CO3 )3 4− complexes [30] prohibited the approach of the complexes to the amino groups on the binding phases. The affinity of the complexes to the binding phases greatly decreased. Further study is required to find out whether this DGT labile concentration is UO2 (CO3 )2 2− complexes, and is the bioavailable uranyl species. However, a significant pH effects on the bioaccumulation of uranium to bivalve Corbicula fluminea were observed by Simon and Garnier-Laplace [31], indicating that only certain uranium species were uptaken by the bivalve since the speciation of uranium is pH dependent. Chelex DGT measured 75% (RSD 6.2%) of the total uranium concentration of 8.5 g L−1 . As mentioned above, Chelex 100 is cation exchange resin. Therefore, it more likely measured cation forms of uranium. As shown in Fig. 1, there was little cation forms of uranium existing in the tested solutions. The fraction of uranium measured by Chelex DGT (75%) could be explained by the dissociation mechanism proposed by Scally et al. [32]. In their study, Cu2+ -nitrilotriacetic acid (NTA) complex and Ni2+ -NTA complex were demonstrated as examples of a fully dissociateable and a partly dissociateable complexe, respectively. In our case, uranyl carbonates partially dissociated in the DGT diffusion layer due to the reduction of uranyl cations by the carboxylic groups of Chelex 100 resin on the diffusion/binding interface. This measurement by Chelex DGT would provide information on stability of the uranium carbonate complexes.
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3.5. Field deployment DGT devices with DE 81 and Chelex binding phases were deployed for 2 weeks in St. Lawrence River, located at Morrisburg, Ont., Canada, in early August, 2005, to measure DGT labile concentrations of uranium. Total uranium concentration was also measured as 0.27 g L−1 at the same location by grab sampling method. Concentrations measured by DE 81 DGT and Chelex DGT were 73% and 60% of the total uranium, respectively. The calculated uranium speciation distribution was also shown in Fig. 1, with major species of UO2 (CO3 ) (0.43%), UO2 (CO3 )2 2− (72.88%) and UO2 (CO3 )3 4− (26.70%) at pH 8.2. It indicated that the DE 81 DGT measured species was most likely UO2 (CO3 )2 2− . Concentration measured by Chelex DGT was lower than the calculated one, and can also be explained by uranium carbonate dissociation mechanism. Thickness of 0.040 cm PAM diffusion gel was used in this work. It was known that the effects of double boundary layer (DBL) in well stirred laboratory solutions and in fast flowing river water (St. Lawrence River) would be negligibly small compared to the gel thickness [12]. The use of diffusion coefficients measured in laboratory solutions with similar hydrodynamic conditions to the DGT deployment solution would minimise the DBL effects [33]. Diffusion coefficients were not corrected with temperature since the temperature of laboratory solution and the average temperature of river water were the same or close to the temperature under which the diffusion coefficients were measured [12]. Temperature effects on diffusion coefficients can be considered with Stokes-Einstein equation D1 η1 D2 η2 = T2 T1
(4)
where D1 and D2 are diffusion coefficients at absolute temperature T1 and T2 , respectively. η1 and η2 are viscosities of water at T1 and T2 , respectively [34]. 4. Conclusions and future work It has been shown that both DE 81 membrane and Chelex 100 gel can be used for DGT applications as binding phases. Comparison of uranium concentrations measured by DE 81 and Chelex DGT in laboratory solutions and in natural water with the theoretical calculations indicated that species measured by DE 81 DGT was likely UO2 (CO3 )2 2− species and Chelex DGT measured fraction of uranium dissociated from uranyl carbonates during their diffusion in the diffusion layer. Studies are still required to further identify these DGT labile uranium species and correlate them to health effects. As both speciation of uranium and chemical forms of the binding phases are highly pH dependent, it is suggested that the binding phases are to be used in alkaline fresh waters of pH greater than 7. Acknowledgement Financial supports for this study were provided by Radiation Protection Bureau, Health Canada. We also thank Ms. Heather
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