Uranium Speciation in Drinking Water from Drilled Wells in Southern

Apr 14, 2009 - Technol. , 2009, 43 (10), pp 3941–3946. DOI: 10.1021/es803658e. Publication Date ... The results indicated good agreement between mea...
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Environ. Sci. Technol. 2009, 43, 3941–3946

Uranium Speciation in Drinking Water from Drilled Wells in Southern Finland and Its Potential Links to Health Effects O D E T T E P R A T , * ,†,‡,|,Ø T H O M A S V E R C O U T E R , †,⊥,O,2 E R I C A N S O B O R L O , †,⊥,¶,Ø P A S C A L F I C H E T , †,⊥,+,2 P A S C A L E P E R R E T , †,⊥,+,2 ¨ I V I K U R T T I O , @,9 A N D PA L A I N A S A L O N E N @,9 French Atomic Energy Commission (CEA), Institut de Biologie Environnementale et de Biotechnologie (IBEB), Laboratoire d’Etude de Prote´ines Cibles (LEPC), Directions des Etudes Nucle´aires (DEN), Laboratoire de Spe´ciation des Radionucle´ides et des Mole´cules (LSRM), Commission d’ETAblissement des Me´thodes d’Analyses (CETAMA), Laboratoire d’Analyses Nucle´aires, Isotopiques et Ele´mentaires (LANIE), and Finnish Radiation and Nuclear Safety Authority (STUK)

Received December 24, 2008. Revised manuscript received March 6, 2009. Accepted March 18, 2009.

Exceptionally high concentrations of natural uranium have been found in drinking water originating from drilled wells in Southern Finland. However, no clear clinical symptoms have been observed among the exposed population. Hence a question arose as to whether uranium speciation could be one reason for the lack of significant adverse health effects. Uranium species were determined using time-resolved laserinduced fluorescence spectroscopy. We performed multi-element chemical analyses in these water samples, and predictive calculations were carried out using up-to-date thermodynamic data. The results indicated good agreement between measurements and modeling. The low toxicity of Finnish bedrock water may be due to the predominance of two calciumdependent species, Ca2UO2(CO3)3(aq) and CaUO2(CO3)32-, whose nontoxicity for cells has been described previously. This interdisciplinary study describes chemical speciation of drinking water with elevated uranium concentrations and the potential consequence on health. From these results, it appears * Corresponding author phone: +33 4 66 79 19 14; fax: +33 4 66 79 19 05; e-mail: [email protected]. † French Atomic Energy Commission (CEA). ‡ Institut de Biologie Environnementale et de Biotechnologie (IBEB). | Laboratoire d’Etude de Prote´ines Cibles (LEPC). ⊥ Directions des Etudes Nucle´aires (DEN). O Laboratoire de Spe´ciation des Radionucle´ides et des Mole´cules (LSRM). ¶ Commission d’ETAblissement des Me´thodes d’Analyses (CETAMA). + Laboratoire d’Analyses Nucle´aires, Isotopiques et Ele´mentaires (LANIE). @ Finnish Radiation and Nuclear Safety Authority (STUK). Ø F-30207 Bagnols-sur-Ce`ze, France. 2 F-91191 Gif-sur-Yvette, France. 9 Laippati 4, PO Box 14, FI-00881 Helsinki, Finland. 10.1021/es803658e CCC: $40.75

Published on Web 04/14/2009

 2009 American Chemical Society

that modeling could be used for a better understanding of uranium toxicity of drinking water in the event of contamination.

Introduction Elevated concentrations of uranium have been measured in water samples from private wells in residential communities in different countries throughout the world (Greece, Australia, U.S., Germany, Finland, etc.). These sources of uranium contamination generally originate from naturally occurring geological deposits but, in some cases, can also be due to external human activities such as the presence or proximity of uranium mines. In many regions of Southern Finland, particularly high concentrations of natural uranium and 222Rn have been observed in groundwater from bedrock in uraniferous granite areas (1, 2). Currently bedrock wells are mostly used as drinking water sources in sparsely populated rural areas where municipal water networks are often difficult to build, because of wide distances. Today, nearly 4% of the Finnish population (about 200 000 inhabitants) uses permanently private drilled wells (3). Maximum concentrations of uranium in private drilled wells can reach more than 200 times those given in the current World Health Organisation (WHO) (4) guideline of 15 µg/L. Several studies focusing on health effects have been carried out among Finns who use their drilled wells as sources of drinking water. These include case-cohort studies of uranium intake and risks of leukemia, stomach, and urinary tract cancers as well as chemical toxicity studies of uranium intake and renal and bone effects (5-10). Nevertheless, none of the human studies reported so far have shown a clear association between chronic uranium exposure and cancer risk, clinical symptoms, or toxicity. Some previous studies already underlined the importance and complexity of uranium speciation in groundwaters (11) mainly in the context of uranium milling for predicting radionuclide migration and for remediation strategies on contaminated sites. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was used for the analysis of uranium speciation under such environmental conditions (12, 13) as it is a very sensitive analysis technique. While TRLFS has been widely used for the low-level detection of uranium in natural water samples including drinking water, analysis of the speciation of uranium in natural drinking water by TRLFS has not been a major concern so far despite the potential relationship of the chemical form of uranium with its toxicity. To our knowledge only one single study by Bernhard and Geipel reports the detection of a uranyl carbonate complex and a calcium uranyl carbonate complex in mineral water using a cryogenic system (T ) 100 K) (14). In Canada, Zamora et al. (15) studied the health of populations chronically ingesting water containing uranium ranging from 2 to 781 µg/L. They concluded that uranium affects kidney functions at the proximal tubule, but uranium speciation was not discussed. Recently, Katsoyiannis et al. (16) analyzed the chemical contents of groundwater in the Kalikratia area in Northern Greece where uranium concentrations were in the 0.01-10 µg/L range, but no comment was given about potential effects on human health. These concentrations are however much lower than the values measured in the present study (6-3400 µg/L). Some authors (17) have simulated the amounts of uranium potentially ingested through water and food in regions of New Mexico with high uranium concentrations of up to 1200 µg/L. They concluded that such chronic exposure will not result in significant radiological health risks, by calculating VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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annual intake, converting it into risk factors for the kidney (nephrotoxicity), and comparing it to the no-damage threshold recommended by Leggett (18), i.e. 0.3 µg/g kidney. Nevertheless, this study remains speculative from a public health point of view. Modeling of uranium speciation in drinking water with high uranium content and its consequences in the event of ingestion has already been performed in rats (19), but with synthetic mixtures of uranium without potentially speciation-modifying agents such as calcium (Ca2+), and without real environmental sampling. On the other hand, the J-Chess modeling program for speciation has been used for predicting uranium (VI) speciation in different simulated human biological fluids (saliva, gastric juice, plasma) (20). Similarly, the importance of uranium speciation on its bioavailability and toxicity has clearly appeared in numerous other studies either in vitro on target cells (kidney, bone) or in vivo by ingestion or injection in rats or mice (21-23). The aim of the present study is to analyze the chemical compositions of water samples from drilled wells and to model the chemical uranium species presents in these water samples. To our knowledge, this is the first interdisciplinary study connecting the speciation of drinking waters with very high uranium content with a discussion on species implication on health effects.

Experimental Section Study Area, Sampling and Radioactivity Counting. Representative water samples over a wide uranium concentration range (a few µg/L to several mg/L) from drilled wells were selected at 13 spots in Southern Finland (see Supporting Information Figure S1). Water samples were collected by an employee of the Finnish Radiation and Nuclear Safety Authority (STUK), a local health authority of one city, or the owners of three private wells between late September and early October 2006. Raw water samples (no water treatment) were mostly collected from kitchen taps in houses except for samples 1, 2, 3, 7, and 13 which were collected after 222Rn removal equipment, the operation of which was based on water aeration. This explains the low 222Rn concentrations of these samples compared with their high uranium concentrations. The 222Rn concentration was determined using a 1414 Guardian liquid scintillation spectrometer (24) and was calculated from the alpha spectrum using a counting efficiency of 290 ( 5%. The lower limit of detection at the 95% confidence level is 0.17 Bq/L for a 10 mL water sample and 60 min counting. Chemical Quantitative Analyses. Uranium content was determined by inductively coupled plasma mass spectrometry (ICP-MS) with PQ Excel Option S (Thermoptek, Vienna, Austria). Elements with lower molecular masses B, S, and Si were preferably studied using inductively coupled plasma atomic emission spectrophotometry (ICP-AES) with an ACTIVA M (Horiba Jobin Yvon). The values were obtained using a semi-quantitative method providing results with an uncertainty of 10% (at k ) 2). Ion concentrations were determined using the standard ion chromatography technique and ion detection was performed using conductivity measurements after suppression. Cation analyses were performed using a DX600 model (Dionex Corporation) equipped with an IonPac CS12A analytical column and a CG12A guard column (diameter 4 mm). The eluant used was methane sulfonic acid (18 mM in water) with a 1 mL/min flow. Anion analyses were performed using an ICS 2000 (Dionex Corporation) equipped with an IonPac AS17 and an AG17 guard column (diameter 4 mm). The eluant was KOH (gradient concentration in water) at a flow of 1 mL/min. TOC (total organic carbon) was determined using a chemical analyzer (1010 OI-analytical, Bioritech). A 250 µL aliquot of 3942

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the water sample was acidified with H3PO4 and heated to 95 °C. The inorganic carbon is liberated as CO2(g) and driven to an infrared detector using a nitrogen gas flow. Na2S2O3 was then added to the sample for measuring the remaining organic carbon in the same way. Speciation Analysis. The speciation calculations were run using the CHESS (chemical equilibrium speciation with surface) computer simulation program (version 3) (25).The theoretical speciation of U(VI) in each sample was evaluated by considering the relevant complexation and checking the potential precipitation reactions carefully according to the different anion and cation concentrations analyzed in each sample of water (see Table 1). Thermodynamic formation data for the uranyl (UO22+) species with the main anions present in these samples (e.g., hydroxo, sulfate, nitrate phosphate, carbonate) were taken from the NEA-OECD (Nuclear Energy Agency-Organisation for Economic Cooperation and Development) thermochemical database (NEA-TDB) (26). Previous studies of theoretical speciation of uranyl in different biological media (19, 20) did not take into account calcium-uranyl-carbonato complexes, CaUO2(CO3)32- and Ca2UO2(CO3)3(aq), whereas similar studies in environmental and/or geological media (11, 12, 27) or toxicological studies (22) included these mixed complexes in their database. Therefore the equilibrium constants considered in our calculations involving uranyl, carbonate, and elemental cations such as calcium, natrium, magnesium, or strontium, which are found at significant concentrations in our samples, are reported in Table S1 of the Supporting Information. From the values of the complex formation constants and the concentrations of the alkaline earth ions in our samples, it appears that interactions with calcium are by far the most significant, whereas uranyl complex with magnesium and strontium can be neglected. Concerning the complexes CaUO2(CO3)32- and Ca2UO2(CO3)3(aq), it is important to emphasize that although they have been identified and their equilibrium constants log10K° proposed, no formation data were recommended by the NEA-TDB (26). Here, the thermodynamic association constants concerning carbonate species of uranyl were taken from the NEA-TDB and completed with recent data from Dong and Brooks for aqueous calcium-uranyl-carbonato complexes (28) (see Supporting Information Table S1). In order to test the relevance of this data set, speciation calculations were also performed using values from Bernhard et al. (29) for the formation of CaUO2(CO3)32- and Ca2UO2(CO3)3(aq) but not selected by the NEA-TDB. The ionic strength in our samples was calculated from the measured concentrations of the ions, and found to vary between 20 and 50 mM. Consequently, the formation constants of the U(VI) complexes were converted into the molar scale, and activity coefficients were calculated using the Davies formula (30). Although NEATDB selected data should be used with a more parametrized formula for ion activity corrections and better consistency, the bias in the calculation is negligible at such low ionic strengths. Time-Resolved Laser-Induced Fluorescence Spectroscopy. The laser excitation source is a quadrupled (266 nm) Nd:YAG Laser (Excel Technology, Minilite). The repetition rate is 10 Hz and the pulse duration 5 ns. The laser beam energy is about 1.5 mJ. The fluorescence is collected by a monochromator spectrograph (Acton 300i, Roper Scientific, U.S.) and detected with an intensified CCD camera (Andor, U.K.). Details of the setup are described elsewhere (31). The fluorescence spectra of U(VI) were measured at ambient temperature with a delay time and gate width adjusted to 80 and 200 ns, respectively. These parameters were chosen so as to get an optimized signal-to-noise ratio. A thousand laser pulses were used for each spectrum. The fluorescence decay curves were obtained by varying the delay time from 60 to

TABLE 1. Physicochemical Parameters, Concentrations of Inorganic Elements and Total Organic Compoundsa pH (20 °C) Eh (mV) uranium (µg/L) 222 Rn (Bq/L) boron (µM) sulfur (µM) silicon (µM) HCO3- (mM) F- (µM) Cl- (µM) NO2- (µM) NO3- (µM) SO42- (µM) PO43- (µM) Li+ (µM) Na+ (mM) NH4+ (µM) K+ (µM) Mg2+ (µM) Ca2+ (mM) Sr2+ (µM) charge balance (mmolc/L) TOC (mg/L)

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

8.24 165 37 3100 13.9 81.1 192 2.61 25.6 113 2.39 2.1 49 2.32 2.02 1.74 18.3 86.2 165 0.44 1.60 0.21 10.70

8.42 166 1754 89 12.0 437 203 3.10 25.6 305 2.17 18.1 245 2.21 1.20 1.63 17.7 57.3 317 1.00 1.85 0.39 10.77

8.51 152 2716 180 11.1 468 228 3.07 35.8 553 2.39 2.1 254 2.32 1.50 1.55 18.3 60.1 340 1.00 nd 0.13 11.34

9.04 204 60 860 10.2 343 228 1.52 34.0 347 2.17 13.4 200 2.32 1.44 1.51 16.6 74.4 167 0.37 0.74 0.34 8.80

8.40 185 613 2300 14.8 228 178 1.56 42.1 666 2.17 1.9 129 2.21 1.44 1.76 16.6 32.2 139 0.35 0.68 0.26 9.85

8.10 190 1891 230 10.2 624 174 3.36 39.5 674 2.17 40.3 356 2.21 1.40 1.67 16.6 79.5 502 1.16 0.68 0.25 12.55

8.21 193 3036 42 6.47 593 221 3.54 11.1 714 2.17 134.7 314 2.21 1.44 0.94 16.6 90.3 580 1.59 0.84 0.36 12.26

8.15 200 285 1800 20.3 343 192 1.51 146.3 217 2.17 3.1 197 2.21 1.44 1.41 16.6 33.0 181 0.41 0.68 0.37 9.50

8.22 206 60 340 5.55 122 210 2.12 65.3 69 2.17 1.9 70 4.24 1.44 0.95 16.6 50.4 315 0.50 0.68 0.24 8.09

8.03 213 5.6 6300 6.47 209 253 2.16 237.9 406 2.17 1.9 120 2.21 1.58 2.56 16.6 79.0 97 0.20 0.68 0.20 9.57

8.05 230 26 63 6.47 405 320 1.10 5.8 229 2.17 21.0 224 2.21 1.44 0.63 16.6 77.0 328 0.44 0.68 0.44 7.69

7.90 240 25 63 6.47 405 331 1.51 5.3 227 1.96 16.0 223 2.11 1.44 0.64 16.6 78.0 327 0.44 0.68 0.05 20.66

8.12 221 3410 180 11.1 499 221 3.10 33.2 502 2.17 1.9 270 2.21 1.58 1.45 16.6 66.2 359 1.02 4.27 0.12 13.39

a Uranium was measured by ICP-MS. 222Rn radioactivity was measured using a liquid scintillation spectrometer. B, Si, S were measured by ICP-AES. Ions were separated by chromatography and concentrations were determined by conductivity. Samples S2, S3, S6, S7, and S13 were collected after 222Rn removal based on aeration, which explains unusual low 222Rn concentrations compared with high uranium concentrations. Charge balance is expressed in mmol charge per liter (mmolc/L). Charge balance error is inferior to 5% for all samples except for 4, 8, and 11 where error is inferior to 7, 8, and 12% respectively.

200 ns. The fluorescence lifetimes were determined as detailed elsewhere (31) and using OriginPro 7.5 software (OriginLab Corp., U.S.).

Results and Discussion 222

Rn and natural uranium concentrations are particularly high in bedrock waters in uraniferous granite areas of Southern Finland. Thirteen water samples were collected for analysis from drilled wells in this region to have a wide range (6-3400 µg/L) of uranium concentrations for our study. Supporting Information Figure S1 shows a map of Southern Finland and the locations of the wells. Natural uranium, as a heavy metal, induces chemical toxicity that is more harmful than its radiotoxicity whereas 222 Rn is a gas harmful only by its radioactivity but transfers too weakly from water to air (27). Consequently the main health concern in this study is uranium with a concentration in drinking water reaching 3400 µg/L and we focused our work on uranium chemotoxicity via its chemical speciation. The concentrations of elements found in the 13 drilled wells waters corresponded to typical values in tap water, except for uranium (see Table 1). CO32- was beneath the detection limit (16.7 µM) in all samples. Speciation Modeling. All the water samples are slightly alkaline with pH between 7.9 and 9.0. The main inorganic anions present in the water samples are HCO3-, SO42-, Cl-, NO3-, NO2-, F-, and HPO4- (Table 1). The presence of Si suggests that silicates SiO(OH)3- may be present as well. In aqueous solutions, the uranyl ion preferentially binds to oxygenated ligands such as the inorganic anions CO32-, SO42-, SiO(OH)3-, HPO4-, and NO3-, and organic molecules with oxygen functions. Fluoride is also a strong complexing agent, while chloride is a much weaker one. The chemical analyses show that higher concentrations are obtained for HCO3-, which partially dissociates at pH 8-9 into H+ and CO32-. The CO32- anion is expected to be the most powerful ligand for UO22+ compared with the other anions. Moreover, for such hydrogen carbonate concentrations (between 1 and 3.5 mM)

and at such a pH value, U(VI) hydrolysis can be neglected. The total content of organic carbon was found to be about 10 ppm (less than 1 mM), which represents a low level in natural water sources worldwide. According to Ranville et al., the transport of uranium by humic colloids is unimportant for aquifers with a pH greater than about 7 when substantial dissolved carbonate is present (32). In our case, the presence of organic matter can be neglected in the analysis of uranium speciation. Although no data has been selected by the NEA-TDB for the calcium-uranyl-carbonato complexes, the formation constants proposed by Kalmykov and Choppin for Ca2UO2(CO3)3(aq) (33) and by Bernhard et al. for CaUO2(CO3)32- and Ca2UO2(CO3)3(aq) (29), are suggested for use as guidance (Supporting Information Table S1). More recently, Dong and Brooks (28) have measured formation constants of ternary carbonate complexes of U(VI) with several divalent cations, using anion exchange chromatography. The formation constant for Ca2UO2(CO3)3(aq) (log10Ko ) 30.7) is similar to that proposed by Bernhard et al. (log10Ko ) 30.6) (29), and significantly higher than that of Kalmykov and Choppin (log10Ko ) 29.8) (33). However the formation constant for CaUO2(CO3)32- (log10Ko ) 27.2) is more than 1 order of magnitude higher than the value evaluated by Bernhard et al. (log10Ko ) 25.4). Consequently, we used both the data sets proposed by Bernhard et al., and by Dong and Brooks to model the speciation of U(VI) under our conditions. Since carbonate complexes should overcome all other complexes under the conditions of the solutions, Ca2+ content becomes one of the main speciation parameters. The relative concentrations of the main complexes UO2(CO3)34-, CaUO2(CO3)32-, and Ca2UO2(CO3)3(aq) only vary with Ca2+ concentration when equilibrium conditions are achieved according to eq 1: 2+ 2n-4 UO2(CO3)43 +nCa )CanUO2(CO3)3

(1)

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FIGURE 2. Calculated percentages of the major aqueous complexes of U(VI) in each sample.

FIGURE 1. J-Chess speciation diagrams of uranyl species, J-Chess speciation diagrams of uranyl species for a virtual average sample as a function of pH and Ca2+ content (mM). (A) according to NEA-OECD data (23) and Bernhard et al. data (26). (B) according to NEA-OECD data and to Dong et al. data (25). of our samples (20-50 mM). The percentages of U(VI) species are shown in Figure 1 as a function of [Ca2+] for a given pH 8.2 and [HCO3-] ) 2.33 mM corresponding to average values of the different elements and ions measured in the different samples. Depending on the data set used for the modeling, the CaUO2(CO3)32- complex is either a minor species, whatever the Ca2+ concentration, while UO2(CO3)34- and Ca2UO2(CO3)3(aq) can both predominate (Figure 1A), or a dominant species for [Ca2+] between 0.01 and 0.5 mM (Figure 1B). The CaUO2(CO3)32- formation constant used by Dong and Brooks is believed to be more accurate than that from Bernhard et al.’s work, where only a rough evaluation is given from the variation of their fluorescence measurements. However, they reported that they had good indication of the formation of CaUO2(CO3)32- at Ca2+ concentrations lower than 0.4 mM, which is consistent with the Dong and Brooks data. U(VI) speciation was calculated using the data set from the NEA-TDB including that of Dong and Brooks (28) for the calcium-uranyl-carbonato complexes and applied to all the samples, and the resulting percentages of species are presented in Figure 2. The CaUO2(CO3)32- and Ca2UO2(CO3)3(aq) complexes are the major U(VI) species in all the solutions. The UO2(CO3)34- complex is present at a maximum of 5% in sample S10. In agreement with the speciation modeled under average conditions, Ca2UO2(CO3)3(aq) predominates when [Ca2+] is higher than 0.5 mM (samples 2, 3 6, 7, 13), whereas 3944

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FIGURE 3. TRLF spectra of U(VI) in six collected samples (as indicated on the Figure). TRLFS spectra of 6 samples (as indicated on the figure) measured with a delay of 80 ns and a gate width of 200 ns. The main emission bands are 465, 484, 505, and 526 nm. These bands correspond to Ca2UO2(CO3)3(aq), as described by Bernhard et al. (11). CaUO2(CO3)32- predominates at lower [Ca2+] (samples 1, 4, 5, 8, 9, 10, 11, 12). Speciation Analysis by TRLFS. The speciation of uranium in the collected water samples was experimentally checked by monitoring the U(VI) fluorescence via TRLFS at ambient temperature. Characteristic U(VI) fluorescence spectra were detected with a small delay of 80 ns in six samples in which the uranium content was greater than 600 µg/L (Figure 3). The intensity of emission could be enhanced by reducing the delay to 30 ns, but a nonlinear baseline then strongly contributed to the signal, which complicated data processing. The main fluorescence bands were observed at 465, 484, 505, and 526 nm in all cases. The peak positions are very similar to the Ca2UO2(CO3)3(aq) ones published by Bernhard et al. (34). Compared with the aquo UO22+ ion spectrum, the bands are blue-shifted which is very unusual for U(VI) complexes and makes it a specific feature of the calciumuranyl-carbonato complex. Fluorescence decay was measured at the three main emission bands 484, 505, and 526 nm by varying the delay time (Supporting Information Figure S2). Biexponential curves were successfully fitted to the data, since monoexponential functions were less satisfactory. It should be noted that while a U(VI) spectrum was measured for the sample S5, no reliable lifetime value could be determined because the uranium content was too low (613 µg/L). Consequently, two lifetime values were obtained for

every measured sample (Supporting Information Table S2), which suggests the presence of two species of U(VI). This interpretation is consistent with the presence of CaUO2(CO3)32- and Ca2UO2(CO3)3(aq) as predicted by our speciation calculation (Figure 1B and Figure 2). Indeed, the longer lifetime was found between 37 and 42 ns, which is consistent with the presence of the complex Ca2UO2(CO3)3(aq) to which a lifetime of 43 ( 12 ns was attributed (34). The shorter lifetime, ranging between 9 and 13 ns, may be attributed to CaUO2(CO3)32-. No lifetime value has been reported so far for this complex. The presence of UO2(CO3)34- might also explain this additional contribution to the fluorescence at short delay times, but this complex was assumed to exhibit no fluorescence at room temperature according to Bernhard et al (11). The CaUO2(CO3)32- and Ca2UO2(CO3)3(aq) complexes would then have different lifetimes, measurable at room temperature, i.e., without altering the sample by freezing, but identical fluorescence spectra. Such similarity means that they cannot be discriminated by TRLFS unless the fluorescence lifetimes of both U(VI) species are measured. Speciation and Uranium Toxicity. Katsoyiannis et al. (16) report that the higher uranium concentrations apparently occur in oxidizing groundwaters with Eh values (272-352 mV), pH 6.7-7.5 and 6-12 mM HCO3 alkalinity. But the authors noticed that the presence of calcium (0.6-3.2 mM) also influences uranium speciation and can increase uranium mobilization through the supposed formation of calciumuranyl-carbonato complexes. According to Zheng et al (35). an increase in Ca concentration induces less U(VI) adsorption on quartz, meaning that Ca2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers. According to our calculations, these Greek waters might contain a mixture of 1:1 complex CaUO2(CO3)32- and 2:1 complex Ca2UO2(CO3)3(aq) in view of their pH (7-8) and elevated calcium content (>0.5 mM). The Finnish waters are similar oxidizing waters (Eh values ranging 152-240 mV), but much less carbonated than the Greek waters with concentrations of 1-3.5 mM HCO3. They contain only half the calcium content, 0.2-1.6 mM, but with a more alkaline pH (8-9) and the mixed formation of 1:1 CaUO2(CO3)32- and the 2:1 Ca2UO2(CO3)3(aq) complexes is still dominant at such concentrations of uranium. According to the recent Human Alimentary Tract model produced by the International Commission on Radiological Protection (ICRP 100) (36, 37), at least 98% of the uranium ingested in soluble form is discharged in faeces. Consequently only a very small part of ingested soluble uranium (0.1-2%) is transferred to the blood because of the very low level of absorption of uranium by the gastro-intestinal tract (38). From this maximum of 2% ingested and absorbed into the blood, 66% is eliminated in the urine within 24 h and the rest is then distributed and stored in the kidneys (12-25%), bones (10-15%) and a lesser proportion in soft tissues. On the basis of an average consumption of two liters per day and a range of uranium content in these Finnish water samples between 6 and 3400 µg/L, an exposed person (70 kg) will have an annual intake of between 0.06 and 36 mg/kg. This average absorption is certainly dependent on seasonal variations in the uranium content of the water in the drilled wells. According to the ICRP 69 model (39), annual deposition in the kidneys (on the basis of an average transfer of 18% from blood to kidney) should be between 0.05 to 30 µg/g kidney. The WHO recommendation for maximum uranium content in water consumption (15 µg/L) corresponds to an annual value of 0.1 µg/g kidney. Consequently these values (0.05-30 µg/g kidney) calculated for persons who are ingesting uranium from drinking water are much higher than the derived WHO recommendation (0.1 µg/g kidney) or even

the proposed guidance by Leggett (0.3 µg/g kidney) by a factor which can go up to 100 (16). Most toxicological studies of uranium have been made with animals, essentially after acute exposure and using synthetic solutions rather than real environmental samples. Consequently in the event of chronic ingestion by humans of low uranium concentrations, its toxicity is still questionable. According to our findings, the uranium in drilled wells in Southern Finland is mainly in forms of Ca2UO2(CO3)3(aq) and CaUO2(CO3)32-. Previous studies carried out on rat or porcine proximal kidney cells have shown the effect of uranium speciation on cytotoxicity: under the experimental conditions used, uranium carbonate UO2(CO3)34-, and uranium citrate UO2Cit22- were assumed to be the more cytotoxic compounds, whereas Ca2UO2(CO3)3(aq) and CaUO2(CO3)32- behave as nontoxic or nonbioavailable chemical forms (22, 40). One hypothesis, to be tested experimentally in vivo using environmental water, is that uranium speciation in water in the form of Ca2UO2(CO3)3(aq) and CaUO2(CO3)32-, would decrease chemical uranium toxicity. Although stomach acidity can destroy calcium-uranyl-carbonato complexes, the point is that the pH in the small intestine increases from 5 to 8 by secretion of hydrogen carbonate ions. These chemical species might be reconstituted in this more basic environment before to be eliminated through faeces. This assumption is in agreement with the human studies previously carried out among well users in Finland indicating only slight biological effects in spite of elevated uranium ingestion through drinking water (7-9). These studies were conducted to identify biological parameters linked to an uranium-induced chemotoxicity, but any significant clinical effects on health could not be found. In conclusion, specific uranium speciation has been conducted on Finnish drinking water samples. The results showed good recovery between measurements using TRLFS and modeling. The apparent lack of a clear human toxicity of these drinking waters, in spite of their elevated uranium concentrations, may be due to the predominant presence of both species Ca2UO2(CO3)3(aq) and CaUO2(CO3)32-. This type of modeling could be used for a better understanding of toxicity of drinking water in the event of contamination by uranium.

Acknowledgments We thank the CEA Nuclear Toxicological Program for funding this research. We also thank Marcoule occupational medicine for technical help with ICP-MS measurements of uranium and Florence Casanova from CEA Saclay for TOC measurements.

Supporting Information Available Tables S1-S3 and Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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