Determination of the Formation Constants of Ternary Complexes of

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Environ. Sci. Technol. 2006, 40, 4689-4695

Determination of the Formation Constants of Ternary Complexes of Uranyl and Carbonate with Alkaline Earth Metals (Mg2+, Ca2+, Sr2+, and Ba2+) Using Anion Exchange Method WENMING DONG AND SCOTT C. BROOKS* Oak Ridge National Laboratory, Environmental Sciences Division, P. O. Box 2008, MS 6038, Oak Ridge, Tennessee 37831-6038; 865-574-6398

The formation constants of ternary complexes (MUO2(CO3)32and M2UO2(CO3)30) of uranyl and carbonate with alkaline earth metals (M2+ denotes Mg2+, Ca2+, Sr2+, and Ba2+) were determined with an anion exchange method by varying the metal concentrations (0.1-5 mmol/L) at pH 8.1 and a constant ionic strength (0.1 mol/L NaNO3) under equilibrium with atmospheric CO2. The results indicate that the complexes of MUO2(CO3)32- and M2UO2(CO3)3 are simultaneously formed for Ca2+ and Ba2+, while Mg2+ and Sr2+ form only the MUO2(CO3)32- complex under our experimental conditions. The cumulative stability constants for the MUO2(CO3)32- complex obtained at I ) 0 are as follows: logβ113 ) 26.11 ( 0.04, 27.18 ( 0.06, 26.86 ( 0.04, and 26.68 ( 0.04 for Mg2+, Ca2+, Sr2+, and Ba2+, respectively. For M2UO2(CO3)30, the value of logβ213 at I ) 0 was measured to be 30.70 ( 0.05 and 29.75 ( 0.07 for Ca2+ and Ba2+, respectively. Based on the formation constants obtained in this study, speciation calculations indicate that at low Ca2+ concentration (e.g., 0. Assuming that the formed aqueous species MUO2(CO3)32- and M2UO2(CO3)30 are not adsorbed onto resins, the distribution coefficient in the presence of M2+ is given more specifically by the following:

DM ) [UO2(CO3)34-]R [UO2(CO3)34-]aq + [MUO2(CO3)32-]aq + [M2UO2(CO3)30]aq

(6) If D0 is a constant, combining eqs 2, 3, and 4 yields

DM )

[UO2(CO3)34-]R [UO2(CO3)34-]aq (1 + K1[M2+]aq + K2[M2+]2aq)

)

(7)

D0 1 + K1[M ]aq + K2[M2+]2aq 2+

Where [M2+]aq ) free M2+ aqueous concentration (mol/L) and is equal to the total added metals because other M2+ species such as MOH+, MCO30 (aq) and MHCO3+, MUO2(CO3)32-, and M2UO2(CO3)30 are predicted to be negligible at pH 8.1. Rearranging eq 7, one obtains

D0 - 1 ) K1[M2+]aq + K2[M2+]2aq DM

(8)

D0, DM, and [M2+]aq are measured experimentally. The values of K1 and K2 are obtained by plotting (D0/DM - 1) versus [M2+]aq and fitting eq 8 to the experimental data. If only the MUO2(CO3)32- complex is formed, the plot of D0/DM - 1 versus [M]aq is a straight line with zero intercept. The formation of M2UO2(CO3)30 leads to positively curved plots. Due to the potential adsorption of MUO2(CO3)32- and M2UO2(CO3)30 onto the resin phase, the data were corrected as detailed below. In the data analysis, both the linear and quadratic forms of eq 8 were fit to the data and the more appropriate model selected by calculating an F ratio based on the extra sum of squares principle (22-24). Correction for MUO2(CO3)32- and M2UO2(CO3)30 Adsorption. When eq 8 is applied to obtain estimates for K1 and K2, an important concern is that the MUO2(CO3)32- and M2UO2(CO3)30 formed could be adsorbed by the resins through the following ion exchange reactions:

2RNO3- + MUO2(CO3)32- )

(R+)2MUO2(CO3)32- +2NO3- (9)

M2+ + UO2(CO3)34- ) MUO2(CO3)32-, K1 )

2M2+ + UO2(CO3)34- ) M2UO2(CO3)30,

4-

[M ][UO2(CO3)3 ]

(3)

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2RNO3- + M2UO2(CO3)30 )

(R+)2MUO2(CO3)32- +2NO3- + M2+ (10)

FIGURE 1. Distribution coefficients of U(VI) as a function of alkaline earth metal concentrations at pH 8.10 ( 0.05, total [U(VI)] ) 50 µmol/L, I ) 0.1 mol/L NaNO3, PCO2 ) 10-3.5 atm, and 25 °C. Error bars indicating ( 1 standard error that are not visible are smaller than the symbol size. where R+ represents the functional group of anion-exchange resins. To address this problem, we developed the following correction procedure. The total M2+ on the resin phase either associated with (R+)2MUO2(CO3)32- or entrained in the resin pore water was extracted into acidic solution (1% HNO3) by the aforementioned desorption procedure, and quantified by ICP-MS for Mg2+, Sr2+, and Ba2+, or by LSC for Ca (45Ca). The amount of M2+ only associated with (R+)2MUO2(CO3)32-, MR (µmoles), was obtained by eq 11

mw Fw

MR ) Cd‚Vd - Cb‚

(11)

where Cd ) total metal concentration in the desorption solution (µmol/mL), Vd ) volume of desorption solution (12 mL 1% HNO3), Cb ) metal concentration in the equilibrated bulk solution (µmol/mL) (here M2+ concentration in resin pore water is assumed to equal to that in the bulk solution at equilibrium), and Fw is the solution density (g/cm3; 1.003 for 0.1 M NaNO3) and mw and  are the mass (g), and fractional water content (0.089, measured gravimetrically) of wet resin measured after adsorption equilibrium (see details in Supporting Information). According to the stoichiometric relation, MR (µmoles) should be equal to the amount of U(VI) associated with (R+)2MUO2(CO3)32-. Thus, MR was used to correct the DM in eq 5 via subtracting MR/m from the total adsorbed [U(VI)]M and adding MR/V into the aqueous phase R [U(VI)]M . The m and V are the dried resin mass (g) and the aq volume (mL) of bulk adsorption solution, respectively. Through this correction procedure, eq 6 is satisfied, and eq 8 can be applied to obtain K1 and K2.

Results and Discussion Uranium(VI) Distribution Coefficients. Uranium adsorption onto the anion-exchange resin yielded a linear isotherm with constant D0 ) 25 760 ( 60 mL/g at pH 8.1 in the absence of alkaline earth metals (Figure S-1 in the Supporting Information) supporting the assumption indicated by eq 2. This distribution coefficient is consistent with those measured by Gu et al. (25) who reported that the distribution coefficients for U(VI) (primarily in UO2(CO3)34- form) on six strong base anion-exchange resins ranged from 13 000 to 60 000 mL/g. Uranium distribution coefficients in the presence of M2+, DM (mL/g), decreased with increasing metal concentration (Figure 1) suggesting the formation of M-UO2-CO3 complexes that are not adsorbed or weakly adsorbed by the anionexchange resin. The magnitude of the metal impact decreased

in the order Ca > Sr = Ba > Mg. As the M2+ concentration increased from 0.1 to 5.0 mmol/L, DM decreased from 19 300 to 630 mL/g for Ca, 21 700 to 2420 mL/g for Sr, 21 600 to 1840 mL/g for Ba, and 24 300 to 9020 mL/g for Mg. Correction for MUO2(CO3)32- and M2UO2(CO3)30 Adsorption. MR ranged from 0.01 to 0.8 µmoles for Ca2+ system in the concentration range of [Ca] ) 0.1-3.0 mmol/L and was used to correct the DM in eq 5. The measured values of MR were negligible for Mg2+, Sr2+ and Ba2+ systems in the concentration ranges of [Mg] ) 0.1-5.0 mmol/L, [Sr] ) 0.1-0.8 mmol/L, and [Ba] ) 0.1-2.0 mmol/L, and no further data correction was indicated (Supporting Information). In addition, our desorption control studies under the same experimental conditions but without U(VI) indicated that adsorption of the metals themselves onto the resins was negligible. Formation Constants of MUO2(CO3)32- and M2UO2(CO3)30 Complexes. Initial metal concentrations were varied in the range 0.1 to 5 mmol/L but for the purposes of determining the formation constants, the data range was restricted to those solutions that were predicted to be undersaturated with respect to the alkaline earth carbonate solids (magnesite, calcite, strontianite, witherite). Thus, the concentration range was truncated at 5 mmol/L, 3 mmol/L, 0.8 mmol/L, and 2 mmol/L for Mg, Ca, Sr, and Ba, respectively. The best fit line to the plot of (D0/DM -1) versus [M2+]aq was nonlinear for M2+ ) Ca2+ and Ba2+ suggesting that both MUO2(CO3)32- and M2UO2(CO3)30 species formed (Figure 2b,d and Table 1). In contrast, the best fit line was linear for M ) Mg2+ and Sr2+ indicating that only the MUO2(CO3)32species formed under our experimental conditions (Figure 2a,c and Table 1). The formation of UO2(CO3)34- and its cumulative stability constant at zero ionic strength (I ) 0) is taken from Guillaumont et al. (15):

UO22+ + 3CO32- ) UO2(CO3)34-, β013 )

[UO2(CO3)34-] [UO22+][CO32-]3

) 1021.84 (12)

The formation of the ternary complexes of uranyl and carbonate with alkaline earth metals (M2+) and their cumulative stability constants are expressed as

xM2+ + UO22+ + 3CO32- ) MxUO2(CO3)3(2x-4) x ) 1 and 2 βx13 )

[MxUO2(CO3)3(2x-4)]

(13)

[M2+]x[UO22+][CO32-]3

The values of K1 and K2 determined by fitting eq 8 to the data collected at I ) 0.1 mol/L were corrected to I ) 0 using Davies equation (26) to obtain β113 and β213 at I ) 0 (Table 1). The values of logβ113 obtained in this study at I ) 0 for MUO2(CO3)32- are in the order of Ca2+ (27.18 ( 0.06) > Sr2+ (26.86 ( 0.04) > Ba2+ (26.68 ( 0.04) > Mg2+ (26.11 ( 0.04) (Table 1). The values of logβ213 for M2UO2(CO3)30 are measured to be 30.70 ( 0.05 for Ca2+ and 29.75 ( 0.07 for Ba2+. The uncertainties reported here incorporate the reported uncertainty for log β013 (21.84 ( 0.04; (15)) using accepted methods for the propagation of experimental uncertainty. These results suggest that in addition to calcium, Mg2+, Sr2+, and Ba2+ can form ternary complexes with uranyl and carbonate. The values of logβ113 and logβ213 at I ) 0 determined in this study for M2+ ) Ca2+ are compared to those previously reported (2, 3), modified to reflect the revised value of logβ013 VOL. 40, NO. 15, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Plots of (D0/DM - 1) vs [M2+]aq: (a) M2+ ) Mg2+; (b) M2+ ) Ca2+; (c) M2+ ) Sr2+; (d) M2+ ) Ba2+. The fitted parameters are presented in Table 1. Error bars indicating ( 1 standard error are smaller than the symbol size.

TABLE 1. Formation Constants of the MUO2(CO3)32- (aq) and M2UO2(CO3)30 (aq) Complexes I ) 0.1 (mol/L) log K1a

log K2a

r2b

I)0 Chi2/DoFc 10-3

P valued

Ca2+

2.56 ( 0.01 0.999 1.1 × 3.63 ( 0.04 6.29 ( 0.04 0.999 0.24 5.0 ( 0.7 2.09 ( 0.25 6.38 ( 0.24

Sr2+ Ba2+

3.30 ( 0.01 0.998 1.8 × 10-3 0.46 3.13 ( 0.02 5.34 ( 0.06 0.999 1.8 × 10-3 2.9 × 10-4

Mg2+

0.0496 6.1× 10-6

log K1e

log K2e

logβ113f

logβ213f

4.27 ( 0.01 26.11 ( 0.04 5.34 ( 0.04 8.86 ( 0.04 27.18 ( 0.06 30.70 ( 0.05 7.6 ( 0.7 29.41 ( 0.7 3.8 ( 0.25 8.95 ( 0.24 25.6 ( 0.25 30.79 ( 0.24 29.8 ( 0.7 5.02 ( 0.01 26.86 ( 0.04 4.84 ( 0.02 7.91 ( 0.06 26.68 ( 0.04 29.75 ( 0.07

reference this work this work 1 3 2 this work this work

a K and K are best fitted values using eq 8 and correspond to the best fitted line showed in Figure 2. b Correlation coefficient. c Chi2 is the sum 1 2 of the squares of the deviations of the theoretical curve from the experimental points, DoF is number of degrees of freedom. d P value indicates the probability of incorrectly accepting the quadratic form of eq 8. e Corrected values by the Davies equation (26). f At I ) 0: log β113 ) log K1 + log β013, log β213 ) log K2 + log β013, and log β013 (I ) 0) ) 21.84 ( 0.04 from ref 15.

) 21.84 ( 0.04 (15) (Table 1). The formation constant (logβ213) of Ca2UO2(CO3)30 obtained in this work is in good agreement with that obtained by Bernhard et al. (3), and larger than that obtained by Kalmykov and Choppin (2), likely due to the different methods applied to correct the activity coefficients for charged species. The specific ion interaction theory was used by Kalmykov and Choppin, whereas the Davies equation was used in this work and by Bernhard et al. (3). The formation constant of CaUO2(CO3)32- obtained in this work is 1.58 orders of magnitude larger than that obtained by Bernhard et al. (3). As discussed by the reviewer in the ref15 (Chapter 14), the log K values given by Bernhard et al. (log K1 ) 3.8 and log K2 ) 8.95) indicate that the binding constant of Ca2+ to CaUO2(CO3)32- is much larger than that of Ca2+ to UO2(CO3)34-. This is not likely in the opinion of the reviewer. In the present work, log K1 ) 5.34 and log K2 ) 8.86, indicating that the binding of Ca2+ to UO2(CO3)34- is larger than that of Ca2+ to CaUO2(CO3)32- which agrees with the expected trend in stepwise formation constants. M2UO2(CO3)30 Complexes for M ) Mg2+ or Sr2+. Based on an acceptance criterion of P < 0.05 for the F-test performed, the quadratic form of eq 8 was marginally significant for the Mg system suggesting that a Mg2UO2(CO3)30 4692

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species exists (Table 1). Adopting the nonlinear form of eq 8 yields cumulative stability constants (I ) 0) of log β113 ) 26.06 ( 0.05 and log β213 ) 28.36 ( 0.20. The present results are equivocal and, in our opinion, additional work needs to be conducted to provide more definitive evidence for the Mg2UO2(CO3)30 complex. As noted above, the data set for M2+ ) Sr2+ was truncated at [Sr2+] ) 0.8 mmol/L due to solubility considerations. Inclusion of the [Sr2+] ) 1 mmol/L data point yields an improved fit of the nonlinear form of eq 8 to the data but the linear form remains the better model (P ) 0.058) Similar experiments conducted under different conditions that allow a broader range of Sr concentrations to be considered may provide evidence for a Sr2UO2(CO3)30 complex. We feel that the more conservative approach of truncating the data set based on predicted mineral saturation state was warranted. At the higher metal concentrations, white flocs were observed in some reaction flasks after the 5 day equilibration, and smaller amounts of precipitates could have escaped visual notice. U(VI) Speciation Prediction. Using the formation constants obtained in this work, predicted U(VI) species distribution as a function of pH was calculated for comparison

FIGURE 3. Aqueous U(VI) speciation distribution as a function of pH in the absence and presence of Ca2+ at [U(VI)] ) 1 µmol/L, I ) 0.1 mol/L NaNO3, PCO2 ) 10-3.5 atm, and 25 °C: (a) [Ca2+] ) 1 mmol/L, and (b) [Ca2+] ) 10 mmol/L. The formation constants of CaUO2(CO3)32and Ca2UO2(CO3)30 are from this work and the others from (15). (c) Aqueous U(VI) speciation distribution as a function of [Ca2+] at pH 8.0, [U(VI)] ) 1 µmol/L, I ) 0.1 mol/L NaNO3, PCO2 ) 10-3.5 atm, and 25 °C. As indicated the maximum aqueous calcium concentration is calculated to be 3.9 mmol/L at pH 8.0 with respect to calcite solubility. at a relatively low Ca concentration (1 mmol/L) and a relatively high Ca concentration (10 mmol/L). The calculations assume total U(VI) ) 1 µM, NaNO3 ) 0.1 M, equilibrium with atmospheric CO2, and allowing calcite to precipitate to maintain equilibrium with respect to calcite. At [Ca] ) 1 mmol/L the CaUO2(CO3)32- species is calculated to be dominant in the pH range of 7.6-8.5 with a maximum distribution of ∼59% at pH 7.9-8.4, and the Ca2UO2(CO3)30 species accounts for ∼27% of U at pH between 7.8 and 8.3 (Figure 3a). As Ca concentration is increased to 10 mmol/L, the Ca2UO2(CO3)30 species is dominant between pH 7.3 to 8.1 with a maximum distribution of ∼80% at pH 7.4-7.8 and CaUO2(CO3)32- becomes dominant at pH 8.2-8.5 (Figure 3b). An additional simulation conducted using similar assumptions but varying Ca concentration at a constant pH 8.0 shows that CaUO2(CO3)32- dominates in the Ca concentration range of 0.3-2.2 mmol/L, and Ca2UO2(CO3)30 becomes the major species at [Ca] > 2.2 mmol/L (Figure 3c). As [Ca] > 3.9 mmol/ L, Ca2UO2(CO3)30 and CaUO2(CO3)32- reach a stable distribution with 63 and 36%, respectively, since the available aqueous Ca2+ was limited by calcite precipitation. The speciation calculations as a function of either pH or Ca concentration (Figure 3), indicate that at low Ca2+ concentration (e.g., 2.2 mmol/L under the assumed conditions. However, the distribution of U(VI) species associated with Ca2+ as well as other alkaline earth metals in aqueous solutions depends on the specific aqueous geochemical conditions such as concentrations of alkaline earth metals, pH, PCO2, U(VI) concentration, inorganic and

FIGURE 4. (a) U(VI) adsorption onto anion-exchange resin as a function of pH in the absence and presence of 3.0 mmol/L Ca2+ at [U(VI)] ) 1.0 µmol/L, I ) 0.1 mol/L (NaNO3), PCO2 ) 10-3.5 atm, and 25 °C. Error bars indicate ( 1 standard deviation of replicate measurements; (b) Aqueous speciation distribution of U(VI) as a function of pH at [Ca2+] ) 0 under [U(VI)] ) 1.0 µmol/L, I ) 0.1 mol/L NaNO3, PCO2 ) 10-3.5 atm, and 25 °C. The dashed line indicates the cumulative mole fraction of uranium present as negatively charged aqueous species, and (c) [Ca2+] ) 3 mmol/L, other condition as in (b). organic ligands, and ionic strength. These need to be carefully specified to better understand the importance of alkaline earth metal-uranyl-carbonate complexes in natural and contaminated sites. The pH-Dependent U(VI) Adsorption. To investigate the influence of Ca-U(VI)-CO3 complexes on U(VI) adsorption, pH-dependent U(VI) adsorption studies onto the anionexchange resin were completed in the absence and presence of 3 mmol/L Ca (Figure 4a). Uranium adsorption increased from ∼20 to 90% in the pH range 5.5-6.5. In the absence of Ca, U(VI) adsorption increased with pH reaching a maximum value (∼98%) at pH ∼7 and remained at this value up to pH 10. This behavior is in contrast to that reported for U(VI) adsorption onto single mineral phases and aquifer solids under similar conditions where U(VI) adsorption begins to decrease as the pH exceeds ∼7.5 (8, 27-29). This behavior is explained by the amphoteric nature of the mineral surfaces where the net surface charge varies from positive to negative with increasing pH, whereas the anion-exchange resins retain a positively charged surface functional group across a broad pH range (30). In the presence of 3 mmol/L Ca, uranium adsorption was substantially lower in the pH range 7-8.1 with deviations between the treatments evident at pH 6.5 (Figure 4a) suggesting that the uncharged Ca2UO2(CO3)30 species is responsible for this decline in adsorption. Precipitates formed starting at pH 8.2 in the presence of Ca. The increased loss of U(VI) from solution at this and higher pH likely resulted from a combination of adsorption and (co)precipitation. Given that the aqueous negatively charged species of U(VI) are responsible for U(VI) adsorption by the anion-exchange resin, the pH-dependent adsorption (Figure 4a) can be explained in terms of the predicted distribution of negatively charged species (Figure 4b,c). In the absence of Ca, the VOL. 40, NO. 15, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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percentage of U(VI) present as negatively charged species (primarily (UO2)2CO3(OH)3-, UO2(CO3)34-, and UO2(CO3)22-) steadily increases with pH (Figure 4b). Approximately 88% of the U(VI) is present as negatively charged species at pH 7 and 100% by pH 8.1, approximating the same general shape of the U adsorption edge. Uranium species distribution in the presence of 3 mmol/L Ca is unchanged at pH < ∼7 and the negatively charged species are dominated by (UO2)2CO3(OH)3- in this pH range. As the pH increases above 7, the proportion of negatively charged U species drops steeply to pH 7.6 and continues a gradual decrease to pH 8.1, mirroring the formation of the Ca2UO2(CO3)30 complex (using the stability constants measured in this study). The dominant negatively charged U(VI) species in this pH range is CaUO2(CO3)32- (Figure 4c). The onset of calcite precipitation at pH ∼8.1 limits Ca concentration and the Ca2UO2(CO3)30 concentration drops sharply, replaced by CaUO2(CO3)32- and UO2(CO3)34- as the dominant aqueous species (Figure 4c). These results indicate that the formation of the uncharged Ca2UO2(CO3)30 species can inhibit the U(VI) adsorption onto anion-exchange resin and similar results were also reported previously (9, 10), where Ca2UO2(CO3)30 species were indicated to suppress the U(VI) sorption onto soils and sediment. Expected Role of MUO2(CO3)32- and M2UO2(CO3)30 Complexes. Alkaline earth metals and carbonate are ubiquitous components of natural and U contaminated groundwater at many sites nationally and internationally. At the Field Research Center for the U.S. Department of Energy’s Environmental Remediation Sciences Division located on the Oak Ridge site in eastern Tennessee, groundwater has been contaminated with uranium-bearing nitric acid wastes that percolated through carbonate host rock. The resulting calcium, magnesium, and strontium concentrations at the site can be as high as 300 mmol/L, 79 mmol/L, and 0.27 mmol/L, respectively, and carbonate alkalinity ranges from 1 to 10 mmol/L (http://public.ornl.gov/nabirfrc/ dataqueryloc2.cfm; accessed 2 Feb 06). Similar concentrations of Ca, Mg, and alkalinities are found at Uranium Mill Tailing Remedial Action (UMTRA) sites (31). At the Hanford site in Washington state (U.S.A.), calcite is a common mineral component, with which porewater is often at equilibrium (10). The long-lived radionuclide 90Sr is a significant cocontaminant with U at the Hanford nuclear waste disposal site, at nuclear weapons testing sites, and at the other largescale nuclear incident sites (32, 33). In addition, barium (Ba) is added to some uranium mine effluents during the wastewater treatment process (34). In these sites and similar other sites, the ternary complexes of alkaline earth metals with uranyl and carbonate (i.e., MUO2(CO3)32- and M2UO2(CO3)30) are expected to play an important role in U(VI) sorption, transport, and bioreduction as reported previously (5, 9, 10, 35). The formation constants for MUO2(CO3)32- and M2UO2(CO3)30) reported here will enable the reevaluation of the potential role of alkaline earth metals in the environmental chemistry of uranium.

Acknowledgments This research was funded by the U.S. Department of Energy’s Office of Science Biological and Environmental Research, Natural and Accelerated Bioremediation Research (NABIR) Program. Oak Ridge National Laboratory is managed by UTBattelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. The authors thank Xiangping Yin at ORNL for the assistance with ICP-MS analysis.

Supporting Information Available Additional details on experimental methods and a figure. This material is free of charge via the Internet at http:// pubs.acs.org. 4694

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Received for review March 17, 2006. Revised manuscript received May 10, 2006. Accepted May 12, 2006. ES0606327

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