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after more than half of the U(V1) was reduced did U(1V) begin to be retained on the filter. These results indicated that the formation of large (i.e., >0.2-pm diameter) U(1V) precipitates was not instantaneous and was not directly linked to U(V1) reduction. As U(1V) began to be retained on the filters, a portion of the remaining U(V1) was also retained, evidenced as a discrepancy between the concentration of U(V1) in the filtered and unfiltered samples (Figure 3A). This was in contrast to earlier time points in which all of the U(V1) present in the cell suspensions passed through the filters. The retention of U(V1) on the filter coincided with a decrease in the rate of U(VI) reduction. Thus, it seems likely that, after ca. 10 h of incubation, some of the remaining U(V1) had become incorporated into the U(1V) precipitate and was no longer available for microbial reduction. This phenomenon requires further study. In contrast to U(V1)-reducingsuspensions, no U(V1) was reduced and no precipitate formed in cell suspensions incubated a t 4 "C (Figure 3B). As an additional control, no U(V1) was reduced and no precipitate formed in heat-killed cell suspensions incubated a t 30 "C (data not shown). These results demonstrate that U(V1)-reducing microorganisms have the potential to readily precipitate uranium from water. Thus, this metabolism may provide a novel mechanism for the removal of uranium from a variety of contaminated waters. Previously, biosorption had been the bioremediation method of choice for removal of uranium from contaminated waters (15). Microbial U(V1) reduction which results from the direct enzymatic metabolism of U(V1) is a fundamentally different process from biosorption, which typically relies on a passive interaction between U(V1) and the biosorbant. The relative effectiveness of microbial U(V1) reduction versus biosorption strategies in treating a variety of typical uranium-contaminated waters is currently under investigation. Acknowledgments
We thank Dr. Terry Beveridge and Robert Harris for electron microscopy, Daniel Webster for X-ray diffraction,
Warren Wood for providing the groundwater, and Ed Landa and Harvey Bolton for helpful suggestions on the manuscript. Registry No. U, 7440-61-1;UOz, 1344-57-6. Literature Cited (1) Waite, D. T.; Joshi, S. R.; Sommerstad, H. Arch. Environ.
Contam. Toxic01 1989,18, 881-887. (2) Bradford, G. R.; Bakhtar, D.; Westcot, D. J. Enuiron. Qual.
1990,19, 105-108. (3) Osiensky, J. L.; Williams, R. E. Ground Water Montit. Rev. 1990, 10, 107-112. (4) Strandberg, G. W.; Shumate, S. E., 11; Parrott, J. R., Jr. Appl. Enuiron. Microbiol. 1981, 41, 237-245. (5) Lovley, D. R.; Phillips, E. J. P.; Gorby, Y. A,; Landa, E. R. Nature 1991, 350, 413-416. (6) Gorby, Y. A.; Lovley, D. R. Appl. Environ. Microbiol., in preparation. (7) Langmuir, D. Geochim. Cosmochim. Acta 1978,42,547-569. (8) Taylor, G. H. In Biogeochemical Cycling of MineralForming Elements; Trudinger, P. A., Swaine, D. J., Eds.; Elsevier: New York, 1979; Chapter 8. (9) Lovley, D. R.; Phillips, E. J. P. Appl. Environ. Microbiol. 1988,54, 1472-1480. (10) LeBlanc, D. R. Sewage plume in a sand and gravel aquifer, Cape Cod, Massachusetts. U. S. Geol. Surv. Water-Supply Pap. 1984, No. 2218. (11) Hostetler, P. B.; Garrels, R. M. Econ. Geol. 1962, 57, 137-167. (12) Durrance, E. M. Radioactivity in Geology; John Wiley & Sons: New York, 1986. (13) Lovley, D. R.; Stolz, J. F.; Nord, G. L.; Phillips, E. J. P. Nature 1987, 330, 252-254. (14) Lovley, D. R. In Iron Biominerals; Frankel, R. B., Blakemore, R. P. Eds.; Plenum Press: New York, 1990; pp 151-166. (15) Tsezos, M. In Microbial Mineral Recovery; Ehrlich, H. L., Brierley, C. L., Eds.; McGraw-Hill: New York, 1990; Chapter 14. Received for review August 7,1991. Revised manuscript received September 9,1991. Accepted September 26,1991. This study was supported by the U. S. Geological Survey Toxic Waste and Nuclear Waste Hydrology Programs.
Measurement of the Absorption Constants for Nitrate in Water between 270 and 335 nm Jeffrey S. Gaffney," Nancy A. Marley, and Mary M. Cunningham
Environmental Research Division, Building 203, Argonne National Laboratory, Argonne, Illinois 60439 Introduction
Acidic species in rain have received substantial attention in recent years. Most of this attention has been placed upon the acidity (Le., H+) of the solutions, with little attention being placed upon the corresponding inorganic and organic ions ( I ) . Sulfate and nitrate are known to be the dominant inorganic species in acidic precipitation (2). However, substantial numbers and amounts of organic species can also be present, as pointed out in previous articles and books ( I , 2). A number of questions remain regarding the formation of organic acids and diacids (i.e., oxalic acid) and their subsequent aqueous deposition. The production of these acids in the gas phase is difficult, since the formation of peracids or peroxyacetyl nitrates is the dominant reactive pathway for organic oxidations ( 3 ) . Therefore, the potential for aqueous oxidation in aerosols and clouds has 0013-936X/92/0926-0207$03.00/0
been given recent attention. Zellner and co-workers have examined the potential photolysis of nitrate, nitrite, and dissolved hydrogen peroxide at 308 and 351 nm by using single-line lasers ( 4 ) . They observed that the photolysis of nitrate in acidic water yielded OH in solution and determined that when compared to aqueous nitrite and hydrogen peroxide photolysis under relevant conditions this photochemical reaction was likely to be the dominant source of this oxidant in the aqueous phase. Thus, this reaction may be an important source of organic oxidation in clouds and aerosols (5). It is now well documented that the release of chlorofluorocarbons into the troposphere affects the levels of ozone in the stratosphere (6). The reduction of stratospheric ozone will increase the levels of ultraviolet radiation in the troposphere because ozone is a key ultraviolet radiation absorber in the stratosphere (2). Thus, increases
0 1991 American Chemical Society
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1.2
0'
350
I
340
330
320
310
300
WAVEN U M B ER
290
280
270
1
260
Figure 1. Aqueous nitrate absorption band in the region of 270-335 nm.
in both gaseous and aqueous ultraviolet photochemical reactions in the troposphere are expected because of the reduction in stratospheric ozone. To model these potentially increased photochemical processes, one must have accurate absorption cross sections for both gaseous and aqueous photochemically active species. As indicated above, aqueous nitrate is one of the more important of the ultraviolet-active species present in aerosols and clouds. Band strengths have been reported for the nitrate band in water but not in detail, as these early studies were concerned with determining the type of electronic transition resulting in this adsorption band (2, 7,8). Reported here are wavelength-dependent determinations of the absorption cross sections for the aqueous nitrate absorption band between 270 and 335 nm in 1-nm increments. Experimental Section Nitrate band strengths were determined by using a Perkin-Elmer ultraviolet-visible spectrophotometer (Model Lambda 3B) in 1-nm increments from 270 to 335 nm. Extinction coefficients were determined with reagent grade sodium nitrate solutions (Fisher) in triple-distilled water a t 0.053,0.079, 0.106, 0.135, and 0.159 M by using the Beer-Lambert equation. Triple-distilled water blanks were taken as the base line for these ultraviolet absorption determinations in 1-cm Pyrex cuvettes. Absorption measurements as a function of concentration were obtained, and band strengths were determined with LOTUS-I23software and linear least-squares regression analysis. Results and Discussion Figure 1shows the aqueous nitrate absorption band of interest in the region of 270-335 nm. The Gaussian-shaped band has been assigned as a forbidden n to a* transition ( 4 , 5 , 7,8). The band strengths were determined by using the Beer-Lambert law, ( A = elc), where c is the concentration, 1 is the pathlength, and e is the extinction coefficient (molar absorptivity). A typical plot of nitrate concentration versus absorption is given in Figure 2. The extinction coefficients determined in this manner, the error, and R2 values at each wavelength are given in Table I, which shows that the band maximum occurs at 301 nm and has an extinction coefficient of 7.06 M-' cm-l, which is in excellent agreement with previous work (2, 7, 8). It is increasingly apparent that dissolved species in aerosols and cloud droplets are likely to play important roles in the chemistry of our atmosphere as well as its radiative properties. Since nitrate is a significant chemical component in both aerosols and clouds, we believe that nitrate absorption and its photochemistry may prove to 208
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Table I. Extinction Coefficients for Aqueous Nitrate Solutions in the Wavelength Region 270-335 rima wavelength, nm
ext coeff, M-' cm-I
335 334 333 332 331 330 329 328 327 326 325 324 323 322 321 320 319 318 317 316 315 314 313 312 311 310 309 308 307 306 305 304 303 302 301 300 299 298 297 296 295 294 293 292 291 290 289 288 287 286 285 284 283 282 281 280 279 278 277 276 275 274 273 272 271 270
0.398 0.458 0.542 0.636 0.750 0.875 1.01 1.13 1.30 1.52 1.73 1.96 2.21 2.47 2.74 3.03 3.31 3.61 3.88 4.17 4.43 5.04 5.29 5.56 5.83 6.06 6.27 6.46 6.63 6.77 6.93 6.98 7.03 7.05 7.06 7.03 6.97 6.89 6.80 6.67 6.54 6.38 6.21 6.03 5.84 5.63 5.44 5.24 5.02 4.81 4.60 4.39 4.19 3.98 3.80 3.60 3.41 3.24 3.07 2.89 2.73 2.58 2.44 2.30 2.17 2.03
error, cm-'
M-I
0.0220 0.0247 0.0206 0.0234 0.0226 0.0289 0.0222 0.0534 0.0526 0.0522 0.0517 0.0534 0.0553 0.0538 0.0457 0.0463 0.0505 0.0439 0.0458 0.0463 0.0444 0.0252 0.0207 0.0228 0.0226 0.0257 0.0260 0.0257 0.0254 0.0339 0.0623 0.0341 0.0312 0.0341 0.0340 0.0371 0.0379 0.0373 0.0458 0.0437 0.0438 0.0472 0.0476 0.0511 0.0565 0.0570 0.0594 0.0596 0.0620 0.0686 0.0677 0.0701 0.0736 0.0821 0.0826 0.0834 0.0869 0.0905 0.0928 0.0979 0.0984 0.0941 0.0977 0.0996 0.1018 0.1053
R2
0.991 0.991 0.996 0.996 0.997 0.997 0.999 0.993 0.995 0.996 0.997 0.998 0.998 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.998 0.998 0.998 0.997 0.997 0.996 0.996 0.995 0.994 0.993 0.992
Coefficienb and errors were determined from the least-squares analysis of the Beer-Lambert plots of absorption versus concentration.
be important. As Zellner and co-workers have pointed out ( 4 ) ,the photolysis of nitrate to form hydroxyl radical in solution is 2-3 times more effective than photolysis of
idations in tropospheric aerosol and cloud chemistries, particularly with the expected increases in ultraviolet (UV-B)radiation due to the depletion of stratospheric ozone by chlorofluorocarbons.
O
I
I
I
0.05
01
0.15
C0N C E NTRAT 1 0N Figure 2. Typical Beer-Lambert plot of nitrate concentration versus absorption used to determine the molar extinction coefficient at each wavelength in aqueous solution. Plot shown is for 301 nm.
nitrite or hydrogen peroxide in aqueous solution. The fact that OH is a very effective oxidizing agent may explain the formation of numerous oxidized organic species in aqueous solution, including oxalate via the aqueous oxidation of soluble diketones and aldehydic species formed in gasphase oxidations (1,2).These data should prove valuable to those researchers attempting to model and unravel the relative importance of nitrate aqueous photochemical ox-
Literature Cited (1) Gaffney, J. S.; Streit, G. E.; Spall, W. D.; Hall, J. H. Environ. Sci. Technol. 1987, 21, 519-524. (2) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; John Wiley & Sons: New York, 1986. (3) Gaffney, J. S.; Marley, N. A.; Prestbo, E. W. Handbook of Environmental Chemistry;Hutzinger, O., Ed.; SpringerVerlag: Berlin, 1989; pp 1-38. (4) Zellner, R.; Exner, M.; Herrmann, H. J. Atmos. Chem. 1990, 10,411-425. (5) Zepp, R. G.; Hoigne, J.; Bader, H. Enuiron. Sci. Technol. 1987,21, 443-450. (6) Rowland, F. S. Environ. Sci. Technol. 1991,25, 622-628. (7) Meyerstein, D.; Treinin, A. Trans. Faraday SOC.1961,57, 2 104-2 112. (8) Rotlevi, E.; Treinin, A. J. Phys. Chem. 1965,69,2645-2648. Received for review September 17, 1991. Accepted October 8, 1991. This work was supported by the Atmospheric Chemistry Program (ACP) of the U.S. Department of Energy’s Office of Health and Environmental Research under Contract W-31109-ENG-38. This work was performed at Argonne National Laboratory.
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