Rate of Oxygen Isotope Exchange between Selenate and Water

Article pubs.acs.org/est

Rate of Oxygen Isotope Exchange between Selenate and Water Masanori Kaneko*,‡,† and Simon R. Poulson† †

Department of Geological Sciences and Engineering, University of Nevada−Reno, 1664 N. Virginia Street, Reno, Nevada 89557-0138, United States ABSTRACT: The rate of oxygen isotope exchange between selenate and water was investigated at conditions of 10 to 80 °C and pH −0.6 to 4.4. Oxygen isotope exchange proceeds as a first-order reaction, and the exchange rate is strongly affected by reaction temperature and pH, with increased rates of isotope exchange at higher temperature and lower pH. Selenate speciation (HSeO4− vs SeO42−) also has a significant effect on the rate of isotope exchange. The half−life for isotope exchange at example natural conditions (25 °C and pH 7) is estimated to be significantly in excess of 106 years. The very slow rate of oxygen isotope exchange between selenate and water under most environmental conditions demonstrates that selenate-δ18O signatures produced by biogeochemical processes will be preserved and hence that it will be possible to use the value of selenate-δ18O to investigate the biogeochemical behavior of selenate, in an analogous fashion to the use of sulfate−δ18O to study the biogeochemical behavior of sulfate.

1. INTRODUCTION Selenium is an essential trace element but can cause toxicity when present in higher concentrations. Anthropogenic activities such as mining, irrigation, combustion, and refining of fossil fuels can release selenium, resulting in significant environmental contamination,1−3 and a variety of processes such as filtration, chemical, and biological adsorption, and precipitation have been proposed as techniques to remove selenium from contaminated waters.4,5 In the western US, elevated aqueous selenium concentrations have been produced by irrigation in regions with naturally elevated selenium concentrations, ultimately resulting in the death and deformities of birds and fish. Hence, the study of the biogeochemical cycle of selenium is important to improve our understanding of the environmental behavior of selenium. However, selenium cycles in natural systems are sometimes poorly understood because of the complexity of selenium geochemical cycles. Selenium is in the same group as sulfur in the periodic table (Group VI), and the chemical behavior and complexity of selenium resembles that of sulfur. For example, selenium occurs in four common oxidation states (-II, 0, IV, and VI). Selenium is present in soluble forms such as selenate (VI) and selenite (IV) in natural waters and is reduced to an insoluble form, Se(0) in mildly reducing conditions. Biological activity plays an important role in the redox transformation of selenium,6 as bacteria can use selenate and selenite as electron acceptors, resulting in accumulation of selenium as Se(0).6−9 Thus, the biogeochemical behavior of selenium strongly depends on redox conditions and biological activity, as is the case for sulfur. Stable isotope analyses are a valuable tool for source identification, and possible qualification and/or quantification of biogeochemical processes, and sulfur and/or oxygen isotope analyses have been successfully used in a large number of studies investigating the biogeochemical behavior of sulfate.10,11 © 2012 American Chemical Society

Selenium stable isotope techniques have been applied to investigate selenium biogeochemistry, and a burgeoning number of studies have characterized the Se isotope fractionation factors associated with various biological and abiological processes, which is critical for understanding and interpreting selenium isotope variations.6,12−15 For example, the isotope fractionation during abiological reduction of selenate to selenite is 7 to 12‰,6,14 while the fractionation during bacterial reduction of selenate to selenite is 1.1 to 4.8‰.6,15 The different magnitude of selenium fractionation factors can be used to discriminate between these reduction pathways, illustrating how selenium isotopes can be used as a tool to investigate selenium biogeochemical cycles. The oxygen isotope systematics of selenate have not been previously studied in a quantitative fashion but have the potential to serve as an additional tool to study selenate behavior, as is the case for studies of sulfate behavior. However, in order to establish oxygen isotope analysis of selenate as a feasible geochemical tool, the nonbiological rate of oxygen isotope exchange between selenate and water must be quantified. If the isotope exchange occurs quickly, the oxygen isotopes of selenate do not represent signatures of biogeochemical cycles of selenate but merely reflect the isotopic composition of the water. However, if the rate of isotope exchange is slow, the δ18O value of selenate will preserve the δ18O signature associated with the biogeochemical process that produced the selenate. The isotope exchange rate at natural environmental conditions is expected to be sufficiently slow for three reasons. First, oxygen isotopic Received: Revised: Accepted: Published: 4539

December 6, 2011 March 6, 2012 March 19, 2012 March 19, 2012 dx.doi.org/10.1021/es204351d | Environ. Sci. Technol. 2012, 46, 4539−4545

Environmental Science & Technology

Article

Table 1. Experimental Results To Measure the Rate of Oxygen Isotope Exchange between Selenate and Water

a

experiment

reaction temperature (°C)

pHa

1 2 3 4 5 6b 7 8 9 10 11 12b 13 14 15 16b 17b 18 19 20 21 22b 23b 24 25

80 80 80 80 80 80 50 50 50 50 50 50 25 25 25 25 25 10 10 10 10 10 10 25 50

1.00 1.50 2.00 2.77 3.32 4.35 0.16 0.87 1.75 1.93 2.69 4.00 0.15 0.84 1.58 2.74 3.76 −0.64 0.15 0.82 1.50 2.62 3.63 0.84 0.87

k (h−1) 7.4 2.2 1.7 1.2 8.9 4.5 1.2 1.5 2.1 1.0 3.2 6.9 9.6 1.3 2.5 5.6 9.1 8.4 1.6

× × × × ×

10−1 10−1 10−1 10−2 10−4

× × × × ×

10−1 10−1 10−2 10−3 10−4

× 10−2 × 10−3 × 10−4

× × × ×

10−2 10−3 10−4 10−5

× 10−3 × 10−1

(std. error) (3.2 (2.4 (3.8 (5.5 (3.4 (2.3 (5.3 (3.2 (1.6 (1.8 (1.6 (3.4 (2.6 (5.0 (4.2 (4.2 (2.9 (7.1 (6.0

× × × × ×

10−2) 10−2) 10−3) 10−4) 10−5)

× × × × ×

10−2) 10−3) 10−4) 10−4) 10−5)

× 10−3) × 10−4) × 10−5)

× × × ×

10−4) 10−5) 10−5) 10−5)

× 10−4) × 10−3)

t1/2 (hour)

log t1/2

(std. error)

0.941 3.12 4.00 55.8 775 1.53 5.96 45.4 266 6600 21.6 100 719 52.5 276 1230 7600 82.2 4.34

−0.03 0.49 0.60 1.75 2.89 0.19 0.78 1.66 2.42 3.82 1.34 2.00 2.86 1.72 2.44 3.09 3.88 1.91 0.64

(0.02) (0.05) (0.01) (0.02) (0.02) (0.02) (0.02) (0.01) (0.03) (0.08) (0.02) (0.02) (0.01)

(0.02) (0.01) (0.03) (0.14)

(0.04) (0.02)

pH values calculated using Visual MINTEQ. bExperiments had insufficient isotope exchange to calculate rate constants.

followed by addition of Na2SeO4 (>99.8%, anhydrous, Alfa Aesar) to produce 0.1 M selenate solutions. The pH of each solution was measured by a pH meter (pHTestr 30, Oakton) at room temperature, and the pH at each experimental temperature was calculated using Visual MINTEQ.22 Each solution was periodically sampled (approximately 1 mL), followed immediately by increasing the pH of the sample with NaOH solution to pH 5−7, in order to quench the isotope exchange reaction. Selenate was promptly precipitated as BaSeO4 with an excess of 1.6 M BaCl2 solution (approximately 3 mL). After stirring and standing for 5 min, BaSeO4 was filtered (0.45 μm membrane filter, Millipore) and dried at 50 °C. Under very acidic conditions (4 to 6 M HCl), selenate is reduced to selenite by HCl.23 However, the rate of selenate reduction (t1/2 = 5.7 × 103 to 4.9 × 1019 hours) is much slower than the rate of oxygen isotope exchange between selenate and water under the experimental conditions used in this study, indicating that the extent of abiotic selenate reduction is very small, and that the potential isotope fractionation associated with abiological selenate reduction can be assumed to be negligible in this study. 2.2. Oxygen Isotope Analysis. Oxygen isotope analysis was conducted using a Eurovector elemental analyzer interfaced to a Micromass IsoPrime stable isotope ratio mass spectrometer. Selenate samples (Na2SeO4 reagent or BaSeO4 experimental products) were reacted at 1300 °C in a glassy carbon reactor packed with glassy carbon and nickelized graphite, after the method of Kornexl et al. (1999).24 Selenate samples were loaded into silver capsules with a small amount of nickelized graphite to promote complete conversion of selenate to CO. Released CO was separated from a trace N2 component by gas chromatography before isotope analysis. Oxygen isotopic compositions were measured by comparison against working

exchange for sulfate (which has similar chemical behavior to selenate) is very slow at low temperature and circum-neutral pH conditions.16−19 Second, direct qualitative measurement of the rate of oxygen isotope exchange between selenate and water indicates that the exchange does not occur in neutral or alkali conditions and appreciably occurs under very acidic conditions.20 Third, earlier work has reported that the rates of oxygen isotope exchange between selenate and water at elevated temperature (80 °C) and low pH range from 5 × 10−5 mol/L sec at pH 1.02 to 9 × 10−7 mol/L sec at pH 2.29, which is equivalent to a half-life of isotope exchange =1 to 63 h−1 for the selenate concentrations used in the study.21 As the oxygen isotope exchange rate decreases at lower temperatures and higher values of pH,21 the rate of oxygen isotope exchange should be significantly slower than these values at low temperature and circum-neutral pH conditions. In this study, the rate of oxygen isotope exchange between selenate and water has been quantified for a range of temperature and pH conditions. The isotope exchange rate under natural conditions is estimated by extrapolation of the relationship between the exchange rate vs temperature and vs pH.

2. METHODS 2.1. Oxygen Isotope Exchange Experiments between Selenate and Water. Oxygen isotope exchange experiments were conducted in polypropylene bottles. Water used in this study was prepared by mixing of 18O-enriched water (18O, 97%, Cambridge Isotope Laboratories, Inc.) and distilled, deionized water, except for two experiments (#24 and #25) which were performed with distilled, deionized water only. After pH adjustment with HCl, 100 mL of 18O-enriched water was preheated to experimental temperature (10, 25, 50, and 80 °C), 4540

dx.doi.org/10.1021/es204351d | Environ. Sci. Technol. 2012, 46, 4539−4545


Article

Table 2. Fractionation of Oxygen Isotopes during Experiments δ18O (‰), initial a

δ18O (‰), final

experiment

reaction temperature (°C)

pH

selenate

H2O

selenate

H2Od

reaction time (h)

Fc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

80 80 80 80 80 80 50 50 50 50 50 50 25 25 25 25 25 10 10 10 10 10 10 25 50

1.00 1.50 2.00 2.77 3.32 4.35 0.16 0.87 1.75 1.93 2.69 4.00 0.15 0.84 1.58 2.74 3.76 −0.64 0.15 0.82 1.50 2.62 3.63 0.84 0.87

−11.8 −11.8 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −11.8 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0 −13.0

81.0 71.1 100.2 99.6 100.3 215.0 94.6 100.3 101.6 69.5 100.0 215.2 94.0 100.1 104.3 214.4 439.6 74.8 94.6 99.4 217.2 441.9 442.8 −9.8 −9.8

77.8 69.9 98.8 99.7 90.8 −8.8 93.7 102.3 102.9 66.6 34.0 −10.2 99.2 104.6 100.6 −4.3 −9.7 83.1 96.3 68.9 41.3 −9.3 −10.2 −2.6 −4.6

80.3 70.5 99.4 98.8 99.5 215.0 93.9 99.4 100.7 68.9 99.7 215.2 93.2 99.3 103.5 214.3 439.6 74.1 93.8 98.8 216.8 441.9 442.8 −9.8 −9.8

80 79 252 1368 3461 2554 103 200 481 2046 4165 4165 337 1358 4176 4175 4171 1780 2391 2863 4176 4175 4172 2355 124

1 1 1 1 0.93 n.c.b 1 1 1 0.96 0.35 n.c.b 1 1 0.99 n.c.b n.c.b 1 1 0.81 0.32 n.c.b n.c.b 1 1

pH values calculated using Visual MINTEQ. bn.c. = ″not calculated″ as rate constant k could not be determined due to insufficient isotope exchange. cFinal value of F may be slightly greater than 1 due to analytical uncertainties associated with δ18O analysis of final selenate composition. d Isotopic composition of final water calculated by isotope-mass balance. a

reference gas (CO) using MassLynx software, and δ18O values are reported in the usual δ notation vs VSMOW. International isotope standards NBS-127 (BaSO4, δ18O = +8.6) and IAEAN3 (KNO3, δ18O = +25.6‰25) were used for data calibration and correction. The analytical precision (1σ) based on replicate measurement of the standards and samples are ±0.2‰ and ≤0.6‰, respectively. Preliminary experiments verified that there was no oxygen isotope fractionation during the precipitation of dissolved selenate as BaSeO4 (±0.2‰, n = 5). The 18O-enriched water used for each experiment was sampled before adding Na2SeO4 for δ18O analysis of initial water. Oxygen isotope analysis was conducted using a Micromass Aquaprep device interfaced to a Micromass dual inlet IsoPrime stable isotope ratio mass spectrometer, using the CO2−H2O equilibration method of Epstein and Mayeda (1953).26 Three water isotope standards with a range of δ18O values (UNR internal standard NV3A, δ18O = −8.6‰; MWS standard from Isotech Inc., δ18O = +108.6‰; and HWS standard from Isotech Inc., δ18O = +266.8‰) were used for data calibration and correction. The precision (1σ) of the method based on replicate measurement of the standards is ±0.2‰. The δ18Owater value at the end of the experiment was calculated using the initial and final values of δ18Oselenate, the selenate concentration value, and the initial δ18Owater by using an isotope-mass balance equation, although the calculated change of δ18Owater during the course of the isotope exchange experiments is small ( 0.986. 4541



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Figure 2. Fraction (F) of oxygen isotope exchange between selenate and water vs reaction time (hours) for a range of pH and temperature conditions.

relatively high pH (pH 4.4), δ18O of selenate increased by only 4.1‰ after 2553 h. Experiments at 50 °C and low pH conditions (0.2, 0.9 and 1.75) reached equilibrium within 300 h, while essentially no isotopic change was observed at pH 4.0 after 3262 h. Experiments at 25 °C and low pH (0.2 and 0.8) reached equilibrium within 1358 h, but no isotopic change was observed in experiments at high pH (2.7 and 3.8) after 1651 h. At 10 °C, only the experiment at very low pH (−0.6) reached equilibrium (within 871 h), while essentially no isotopic change was observed for the experiments performed at pH > 2.6. The rate of change of stable isotope compositions for a firstorder isotopic exchange reaction of element X between species AX and BX is described by27 (δAX − δeq AX )/(δi AX − δeq AX ) = e−kt

calculated rate constants, the fraction of exchanged selenate oxygen (F) can be expressed by the following equation F = 1 − e−kt

Therefore, the fraction F at time t is given by F = (δ18Ot − A + B)/B

(1)

4. DISCUSSION 4.1. The Rate of Oxygen Isotope Exchange Rate between Selenate and Water. The relationships between the half-lives (log t1/2) of the oxygen isotope exchange reaction for selenate−water vs pH at each temperature are shown in Figure 3 for data from this study as well as a previous study,21 together with data for the rate of oxygen isotope exchange between sulfate and water.17,18 For the selenate−water system, there are positive correlations between log t1/2 and pH at each temperature, although the slope of this correlation seems to change at approximately pH 2, especially at 50 and 80 °C. For example, the slope at 50 °C changes from approximately 0.9 below pH 2, to a slope of approximately 2.5 above pH 2. The value of the half-life at 80 °C and the observation of a change of slope of log t1/2 vs pH at approximately pH 2 are consistent with previous results.21 This change of slope may be explained by the change of selenate speciation at pH 2 and the difference in the activation energies required for oxygen isotope exchange between the biselenate ion (HSeO4−) and water and between the selenate ion (SeO42−) and water.

(2)

Hence, the experimental results for the change of selenate-δ18O vs time were fitted with curves of the type δ18Oselenate = A − B·e−kt

(5)

Plots of F vs reaction time are presented for all experiments in Figure 2. Half-lives of the reaction are calculated using eq 4 by setting F = 0.5, and half-lives vary from 0.94 to 7600 h (Figure 2 and Table 1). Half-lives consistently increase with increasing pH and decreasing temperature.

where δAX = isotopic composition at time t, δeqAX = isotopic composition at equilibrium, δiAX = initial isotopic composition, k = the reaction rate constant for isotope exchange, and t = reaction time. Rearrangement of eq 1 yields δAX = δeq AX + (δi AX − δeq AX ) × e−kt

(4)

(3)

where A (= δ18Oeqselenate) and B (= δ18Oeqselenate − δ18Oiselenate) are constants, k is the reaction rate constant for oxygen isotope exchange between selenate and water (h−1), and t is the reaction time (hours). Curve fitting yields values of A, B, and k, with values of R2 > 0.986 for all experiments, and the calculated rate constants range from 0.000091 to 0.74 h−1. Using the 4542



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Figure 4. Arrhenius plot of calculated ln k vs 1/temperature (1/K).

this plot are similar at pH 0 (78.5 kJ/mol) and pH 1 (87.7 kJ/ mol) when HSeO4− is the dominant selenate species and are similar to values determined in a previous study,21 while the activation energy is 154.5 kJ/mol at pH 3 when SeO42− is the dominant selenate species, and demonstrates that the oxygen isotope exchange between HSeO4− and water has a lower activation energy than isotope exchange between SeO42− and water. Moreover, the slightly higher activation energy at pH 1 vs pH 0 may be due to a slightly higher SeO42−/HSeO4− ratio at pH 1 vs pH 0. Thus, oxygen isotope exchange between selenate and water proceeds through collision between HSeO4− and water at pH < pKa2, and between SeO4− and water at pH > pKa2, with the different activation energies being responsible for the different slopes of log t1/2 vs pH above/below the value of pKa2. Direct measurement of oxygen isotope exchange rates between selenate and water at natural environmental conditions (i.e., 5−30 °C, pH 5−9) is impossible due to the extremely slow exchange rates under these pH conditions and requires a significant extrapolation from measurements determined at low pH. The difficulty in extrapolating isotope exchange rates to high pH conditions is exacerbated by the fact that the dependence of the exchange rate vs pH may depend upon selenate speciation (i.e., a different dependence of the exchange rate at pH > pKa2; Figure 3). Three approaches have been employed to estimate the selenate−water oxygen isotope exchange rate at 25 °C and pH 7: (i) an experiment at 25 °C and pH 2.7 did not show any δ18O variation (±1.4‰) after 958 h, which indicates that log t1/2 at this pH should be greater than 5 (half-life = 11.4 years), based on a sensitivity analysis using eq 3 and a range of k values. This estimate is considered to be a grossly conservative underestimate, as the exchange rate at pH 7 will be much slower than at pH 2.7; (ii) rates measured at 25 °C and pH < pKa2 (three data points: Figure 3) have been linearly extrapolated to pH 7 and yield a value of log t1/2 of 8.6 (half-life = 4.5 × 104 years). This estimate is considered to be a conservative underestimate, as it is likely that the slope of log t1/2 vs pH is higher at values of pH > pKa2; and (iii) rates measured at 80 °C and pH > pKa2 (two data points: Figure 3) have been linearly extrapolated to pH 7 and yield a value of log t1/2 of 10.4 (half-life = 2.9 × 106 years). This estimate is also considered to be a conservative underestimate vs the exchange rate at 25 °C, as the exchange rate at 25 °C will be significantly

Figure 3. The kinetics of oxygen isotope exchange between selenate and water in terms of pH and log t1/2 (hours), and comparison with previously reported selenate−water system21 and sulfate−water system.17,18 Standard error for each plot was listed in Table 1.

The dissociation of selenic acids in aqueous solution proceeds in two steps28 + H2SeO4 ⇔ HSeO− 4 + H (pK a1 = − 2.0 at 25°C)

(6)

2− + HSeO− 4 ⇔ SeO4 + H (pK a 2 = 1.6 at 25°C)

(7)

28

The value of pKa1 is taken from Séby et al. (2001), and values of pKa2 are calculated using the thermodynamic database in Visual MINTEQ,22 and changes from 1.49 at 10 °C, to 1.60 at 25 °C, to 2.01 at 50 °C, and to 2.32 at 80 °C. Under the experimental conditions of this study (pH between −0.6 and 4.4), selenate should exist as either the biselenate ion and/or the selenate ion, and the selenate speciation should be dominated by HSeO4− at pH < pKa2 and by SeO42− at pH > pKa2. In Figure 3, the slopes for log t1/2 vs pH at 50 and 80 °C change at pH ≈ pKa2, and there is a suggestion that this is also the case at 10 °C. This suggests that the rate of oxygen isotope exchange at a given temperature is determined by selenate speciation. A similar change of slope dependent on chemical species has been reported for oxygen isotope exchange experiment between sulfate and water and between phosphate and water.18,29 To allow for oxygen isotope exchange between selenate or biselenate and water, the Se−O bond must be broken. The activation energy required for the oxygen isotope exchange should be dependent upon selenate speciation and should affect the change of slopes in Figure 3. In general, the activation energy of a reaction (Ea) is expressed by the Arrhenius equation30 k = Ae‐(Ea / RT )

(8)

then ln k = ln A −

Ea 1 × R T

(9)

where A is the pre-exponential factor, R is the ideal gas constant, and T represents the absolute temperature (K). Values of ln k are plotted vs 1/T in an Arrhenius plot in Figure 4 and show a strong linear correlation for a given pH (R2 > 0.96). The activation energies (Ea) calculated from the slopes of 4543



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slower than the exchange rate at 80 °C (values of log t1/2 are approximately 2 units higher at 25 °C as compared to 80 °C, at values of pH 1 to 2; Figure 3). Hence, although it is not possible to definitively quantify selenate−water oxygen isotope exchange rates under natural environmental conditions, it seems likely that the half-life for this exchange rate is significantly in excess of 106 years at 25 °C and pH 7. 4.2. Comparison with Sulfate−Water Oxygen Isotope Exchange Rates, and Application of Selenate-δ18O to Environmental Studies. Previous studies of the oxygen isotope exchange rate between sulfate and water at comparable values of pH have mostly been conducted at somewhat higher pH and temperature conditions than this study17,18 (Figure 3). Hence, it is difficult to directly compare the oxygen isotope exchange rates of the selenate−water and sulfate−water systems. However, visual inspection and comparison between data sets in Figure 3 suggest that selenate−water oxygen isotope exchange rates measured in this study are somewhat slower than sulfate−water oxygen isotope exchange rates measured in one previous study18 but somewhat faster than rates measured in another study.17 Direct measurement of the sulfate−water oxygen isotope exchange rate at 25 °C and pH 7 yields a value of log t1/2 of 7.0 (half-life = 1.1 × 103 years).17 The extrapolation of measurements at 200 °C and reduced pH (4.0 to 5.5) to a pH of 7 yield a value of log t1/2 at 200 °C of 6.7 (half-life = 570 years),18 so the half-life of the sulfate−water exchange reaction at 25 °C would be predicted to be significantly longer than this when using this data set. Nevertheless, despite the difficulty in comparing the isotope exchange rates between the two systems, it appears that selenate−water and sulfate−water oxygen isotope exchange rates are qualitatively similar at natural environmental conditions, although it is possible that exchange rates could be increased by catalysis on solid surfaces present in aquifers and sediment pore waters. Since oxygen isotopes of sulfate have repeatedly been successfully used as a tool for understanding biogeochemical processes that involve sulfate, the similarly slow rate of oxygen isotope exchange between selenate and water implies that the oxygen isotope signature of selenate produced by various biogeochemical processes will be preserved and that oxygen isotopes of selenate can also be used as a technique for better understanding the biogeochemical processes that involve selenate. This study has measured the rate of oxygen isotope exchange between selenate and water for a range of temperature and pH conditions. The rate of oxygen isotope exchange between selenate and water proceeds by a first-order reaction, and the rate is strongly affected by temperature and pH. From the observation that the slope of a plot of log t1/2 vs pH changes above and below pKa2, it appears that oxygen isotope exchange between selenate and water proceeds through exchange between HSeO4− and water at pH < pKa2 and between SeO42− and water at pH > pKa2. The estimated oxygen isotope exchange rate between selenate and water under natural conditions (e.g., 25 °C and pH 7) is very slow (estimated half-life significantly in excess of 106 years), which is comparable to the oxygen isotope exchange rate between sulfate and water at the same temperature and pH. This demonstrates that the oxygen isotopic signature of selenate can preserve information concerning the biogeochemical behavior of selenate in natural environments, in an analogous fashion to

the application of sulfate-δ18O to study the biogeochemical behavior of sulfate.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81−46−867−9812. Fax: +81−46−867−9775. Email: [email protected]. Present Address ‡

Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2−15 Natsushima, Yokosuka 237−0061, Japan. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research has been partially supported by NSF grant EAR0738912. We thank three anonymous reviewers for their careful and constructive comments and suggestions and Chris Sladek for assistance in the laboratory.

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REFERENCES

(1) Stillings, L. L.; Amacher, M. C. Kinetics of selenium release in mine waste from the Meade Peak Phosphatic shale, Phosphoria Formation, Wooley Valley, Idaho, USA. Chem. Geol. 2010, 269, 113− 123. (2) Engberg, R. A.; Sylvester, M. A. Concentrations, distributions, and sources of selenium from irrigated lands in western United States. J. Irrig. Drain. Eng. 1993, 119, 522−536. (3) Gates, T. K.; Cody, B. M.; Donnelly, J. P.; Herting, A. W.; Bailey, R. T.; Price, J. M. Assessing selenium contamination in the irrigated stream-aquifer system of the Arkansas River, Colorado. J. Environ. Qual. 2009, 38, 2344−2356. (4) Wu, Y.; Chi, Y.; Bai, H.; Qian, G.; Cao, Y.; Zhou, J.; Xu, Y.; Liu, Q.; Xu, Z. P.; Qiao, S. Effective removal of selenate from aqueous solutions by the Friedel phase. J. Hazard. Mater. 2010, 176, 193−198. (5) Jegadeesan, G.; Mondal, K.; Lalvani, S. B. Selenate removal from sulfate containing aqueous solutions. Environ. Technol. 2005, 26, 1181−1187. (6) Johnson, T. M.; Bullen, T. D. Mass−dependent fractionation of selenium and chromium isotopes in low temperature environments. In Reviews in Mineralogy and Geochemistry; Johnson, C. M., Beard, B. L., Albarade, F., Eds.; Mineralogical Society of America: Washington, DC, 2004; Vol. 55, pp 289−318. (7) Maiers, D. T.; Wichlacz, P. L.; Thompson, D. L.; Bruhn, D. F. (1988) Selenate reduction by bacteria from a selenium-rich environment. Appl. Environ. Microbiol. 1988, 54, 2591−2593. (8) Oremland, R. S.; Blum, J. S.; Culbertson, C. W.; Visscher, P. T.; Miller, L. G.; Dowdle, P.; Strohmaier, F. E. Isolation, growth, and metabolism of an obligately anaerobic, selenate-respiring bacterium, strain SES-3. Appl. Environ. Microbiol. 1994, 60, 3011−3019. (9) Watts, C. A.; Ridley, H.; Dridge, E. J.; Leaver, J. T.; Reilly, A. J.; Richardson, D. J.; Butler, C. S. Microbial reduction of selenate and nitrate: common themes and variations. Biochem. Soc. Trans. 2005, 33, 173−175. (10) Aharon, P.; Fu, B. Microbial sulfate reduction rates and sulfur and oxygen isotope fractionations at oil and gas seeps in deepwater Gulf of Mexico. Geochim. Cosmochim. Acta 2000, 64 (2), 233−246. (11) Gammons, C. H.; Duaime, T. E.; Parker, S. R.; Poulson, S. R.; Kennelly, P. Geochemistry and stable isotope investigation of acid mine drainage associated with abandoned coal mines in central Montana, USA. Chem. Geol. 2010, 269, 100−112. (12) Ellis, A. S.; Johnson, T. M.; Herbel, M. J.; Bullen, T. D. Stable isotope fractionation of selenium by natural microbial consortia. Chem. Geol. 2003, 195, 119−129. (13) Rouxel, O.; Ludden, J.; Carignan, J.; Marin, L.; Fouquet, Y. Natural variations of Se isotopic composition determined by hydride

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generation multiple collector inductively coupled plasma mass spectrometry. Geochim. Cosmochim. Acta 2002, 66, 3191−3199. (14) Rees, C. E.; Thode, H. G. Selenium isotope effects in the reduction of sodium selenite and of sodium selenate. Can. J. Chem. 1966, 44, 419−427. (15) Herbel, M. J.; Johnson, T. M.; Oremland, R. S.; Bullen, T. D. Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions. Geochim. Cosmochim. Acta 2000, 64, 3701−3709. (16) Hoering, T. C.; Kennedy, J. W. The exchange of oxygen between sulfuric acid and water. J. Am. Chem. Soc. 1957, 79, 56−60. (17) Lloyd, R. M. Oxygen isotope behavior in the sulfate-water system. J. Geophys. Res. 1968, 73, 6099−6110. (18) Chiba, H.; Sakai, H. Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal temperatures. Geochim. Cosmochim. Acta 1985, 49, 993−1000. (19) Seal, R. R. II; Alpers, C. N.; Rye, R. O. Stable isotope systematics of sulfate minerals. In: Sulfate Minerals: Crystallography, Geochemistry, and Environmental Significance. In Reviews in Mineralogy and Geochemistry; Alpers, C. N., Jambor, J. L., Nordstrom, D. K., Eds.; Mineralogical Society of America: Washington, DC, 2000; Vol. 40, pp 541−602. (20) Hall, N. F.; Alexander, O. R. Oxygen exchange between anions and water. J. Am. Chem. Soc. 1940, 64, 3455−3462. (21) Okumura, A.; Okazaki, N. Kinetics of oxygen exchange between selenate ions and water. Bull. Chem. Soc. Jpn. 1973, 46, 1080−1084. (22) Allison, J. D.; Brown, E. S.; Novo-Gradac, K. J. MINTEQA2/ PRODEFA2, A Geochemical Assessment Model for Environmental Systems: Version 3.0 User Guide; EPA/600/3-91/021; US EPA: Athens, Georgia, 1991. (23) Pettersson, J.; Olin, Ǻ . The rate of reduction of selenium (VI) to selenium (IV) in hydrochloric acid. Talanta 1991, 38, 413−417. (24) Kornexl, B. E.; Gehre, M.; Höfling, R.; Werner, R. A. On-line δ18O measurement of organic and inorganic substances. Rapid Commun. Mass Spectrom. 1999, 13, 1685−1693. (25) Böhlke, J. K.; Mroczkowski, S. J.; Coplen, T. B. Oxygen isotopes in nitrate: new reference materials for O-18: O-17: O-16 measurements and observations on nitrate-water equilibration. Rapid Commun. Mass Spectrom. 2003, 17, 1835−1846. (26) Epstein, S.; Mayeda, T. Variation of O18 content of water from natural sources. Geochim. Cosmochim. Acta 1953, 4, 213−224. (27) Cole, D. R.; Chakraborty, S. Rates and Mechanisms of Isotopic Exchange. In Reviews in Mineralogy and Geochemistry; Valley, J. W., Cole, D. R., Eds.; Mineralogical Society of America: Washington, DC, 2001; Vol. 43, pp 83−223. (28) Séby, F.; Potin-Gautier, M.; Giffaut, E.; Borge, G.; Donard, O. F. X. A critical review of thermodynamic data for selenium species at 25°C. Chem. Geol. 2001, 171, 173−194. (29) O’Neil, J. R.; Vennemann, T. W.; McKenzie, W. F. Effects of speciation on equilibrium fractionations and rates of oxygen isotope exchange between (PO4)aq and H2O. Geochim. Cosmochim. Acta 2003, 67, 3135−3144. (30) Lasaga, A. C. Kinetic Theory in the Earth Sciences; Princeton University Press: Princeton, NJ, 1998.

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Rate of Oxygen Isotope Exchange between Selenate and Water

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