Anal. Chem. 1995,67,348-353
Uranium Speciation in Solution by Time-Resolved Laser-Induced Fluorescence Chrlstophe Moulin* and Piem Decambox CEA, Fuel Cycle Division, DPWSPEA, Analytical Laser Spectroscopy Group, 91 191 Gif sur Yvette, France
Val6rie Moulin and Jean Gary Decaillon CEA, Fuel Cycle Division, DESDBESD, Section of Geochemistry, 92265 Fontenay aux Roses, France
Complexation studies of radionuclides (such as uranium) are important to perform in order to predict their migration behavior in natural systems. Time-resolved laserinduced fluorescence, which is a very selective and sensitive method for actinide and lanthanide analysis, is used to study the interactions between uranyl cation and hydroxide ions at low uranium concentration (0.4 p M ) . The principle of the study is based on time resolution and spectral convolution since free uranylion (UOz2+) and 6rst hydroxide complex give specific fluorescence spectra characterized by their spectral shift and fluorescence lifetime. Spectroscopic data (main fluorescence wavelengths, half-bandwidth, lifetime) for the first hydrolyzed complex U020Ht are obtained. From experimental spectra and time-resolved spectral convolution, the proportion of each species (free uranyl, complexed uranyl) can be estimated at different pH. The understanding of radionuclide migration behavior is very important for safety assessment of nuclear waste disposal in geological formations. In particular, the aquifer system of the site will constitute the main transport of radioactivity through the geosphere. Thus, knowledge of the chemical species of the radioelements formed with the different constituents of the water is an important step for modeling radionuclide transp~rt.l-~ Indeed, it is necessary to possess methods that can not only perform ultratrace analysis of single species but can also characterize complexes present at low levels in solution. Time-resolved laser-induced fluorescence 0 is a very sensitive and selective method for actinide and lanthanide analysis which has been largely used in various fields of the nuclear fuel cycle (geology, reprocessing, waste storage, medical, environment), mainly for uranium ultratrace analysis or process c o n t r ~ l . ~These - ~ ~ deter(1)Choppin, G. R;Allard, B. Handbook on the Physics and Chemistry of the Actinides; Freeman, A J., Keller, C., Eds.; Elsevier: Amsterdam, 1985;Vol. 3, Chapter 11. (2) Buftle, J. In Complezafion Reacfions in Aquatic Systems; Chalmers, R A, Masson, M. R, Eds.; Wiley and Sons.: New York, 1988. (3)Moulin, V.; Ouzounian, G. Appl. Geochem. 1992, 1, 179-186. (4)Robbins, J. C. CIM BUN. 1978, 71,61-67. (5) Campen, W.; Bachmann, K Mikrochim. Acta 1979,2,159-170. (6)Zook,A;Collins, L. H.;Pietri, C. E. Mikrochim. Acta 1981,2, 457-461. (7) Young, J. E.; Deason, P. T.Anal. Chem. Symp. Ser. 1984, 19, 7-12. (8)Fujimori, H.; Matsui, T.; Suzuki, K. /. Nucl. Sci. Technol. 1988,25, 798804. (9)Berthoud, T.;Decambox, P.; Kirsch, B.; Mauchien, P.; Moulin, C. Anal. Chem. 1988,60,1296-1299. 348 Analytical Chemistry, Vol. 67,No. 2,January 15, 1995
minations are performed directly or with fiber optics and optodes.14-16 Besides its sensitivity (limit of detection for uranium 10-l2M), TRLIF gives both spectral data with the fluorescence spectrum (wavelength shifts, peak shape and ratio modifications) and temporal data with the fluorescence lifetime (characteristic of the molecule environment). More recently, TRLIF has been used to determine equilibrium constants between cations and humic substances using fluorescence quenchingl7-l9or fluorescence enhan~ement.~O-~~ In the former case, fluorescence of humic substances is observed as a function of added cation. In the latter case, fluorescence of the cation is observed as a function of humic substances, which leads to a fluorescence increase until saturation. Several studies have been carried out on timeresolved fluorescence of uranyl in aqueous solution as a function of pH25-27 but very few for speciation purp0ses2~~29 and none at low level. (10)Moulin, C.;Beaucaire, C.; Decambox, P.; Mauchien, P. Anal. Chim. Acta 1990,238,291-296. (11) Brina, R;Miller, A G. Anal. Chem. 1992,64,1413-1418. (12)Moulin, C.; Decambox, P.; Mauchien, P.Appl. Spectrosc. 1991,45, 116118. (13)Deniau, H.; Decambox, P.; Mauchien, P.; Moulin, C. Radiochim. Acta 1993, 61,23-28. (14)Hirschfeld, T.; Haugen, G.; Milanovitch, F. Anal. Chem. Symp. Ser. 1984, 19, 13-18. (15) Vaheau, P.T.; Duesing, R; Wangen, L.E. Appl. Spectrosc. 1991,45,16521655. (16)Moulin, C.; Rougeault, S.; Hamon, D.; Mauchien, P. Appl. Spectrosc. 1993, 47,2007-2012. (17) Ryan,D. K .; Weber, J. H. Anal. Chem. 1982,54,986-990. (18)Grimm, D. M.; Azarraga, L. V.; Carreira, L. A; Susetyo, W. Enuiron. Sci. Technol. 1991,25,1427-1431. (19)Puchalski, M. M.; Morra, M. J.; von Madruska, R Enuiron. Sci. Technol. 1992.26, 1787-1792. (20) Dobbs, J. C.; Susetyo, W.; Knight, F. E.; Castles, M. A; Carreira, L. A; Azarraga, L. V. Anal. Chem. 1989,61,483-488. (21)Bidoglio, G.;Omenetto, N.; Robouch, P. Radiochim. Acta 1991,52/53,5761. (22) Moulin, C.; Decambox, P.; Mauchien, P.; Moulin, V.; Theyssier, M. Radiochim. Acta 1991,52/53, 119-125. (23)Kim, J. I., Wimmer, H.; Klenze, R Radiochim. Acta 1991,54,35-41. (24)Moulin, V.; Tits,J.; Moulin, C.; Decambox, P.; Mauchien, P.; de Ruty 0. Radiochim. Acta 1992,58/59, 121-128. (25)Moriyasu, M.; Yokohama, Y.; Ikeda, S.]. hoe.Nucl. Chem. 1977,39,21992204. (26)Deschaux, M.; Marcantonatos, M. D. Chem. Phys. Lett. 1979, 63,283288. (27)Graca, M.; Formosinho, S. J.; Cardoso, A C.; Burrows, H. D. Faraday Trans 1 1984,80,1735-1744. (28)Meinrath, G.;Kato, Y.; Yoshida, Z. j . Radioanal. Nucl. Chem. 1993, 174, 299. (29)Couston, L;Pouyat, D.; Moulin, C.; Decambox, P. Appl. Spectrosc., in press. 0003-2700/95/0367-0348$9.00/0 0 1995 American Chemical Society
As mentioned previously, an interesting feature of TRLIF is its capability of characterizing species present in solution by modification of the fluorescencespectrum (and lifetime) and thus performing speciation (determination of complexes directly in solution). These properties have recently been used for uranyl speciation in nitric acid for reprocessing applications.B From this study, it was possible to determine by spectral deconvolution the different formation constants of uranium-nitric acid complexes and to compare them favorably with literature data. For environmental purposes, TRLIF could be used for uranium speciation under conditions representative of natural systems where complexing agents can be hydroxides, carbonates, humic substances, etc. In the present study, the interest is focused on the acidic pH range (2-4) where the first hydroxide complex of uranyl cation is formed, in order to demonstrate the possibility of TRLIF to deduce u r a n i u m 0 speciation. From this study, spectroscopic data on the first hydroxide complex are obtained. With knowledge of the formation constant, uranium time-resolved fluorescence spectrum deconvolution is performed successfully. EXPERIMENTAL SECTION
Reagents. Standard solutions of u r a n i u m 0 in sodium perchlorate 0.1 M (Merck) are obtained from suitable dilution of a solution prepared by dissolution of high-purity metal with perchloric acid (Merck). The uranium concentration of the initial standard solution is vedied by mass spectrometry. Deionized ultrafiltered water is used throughout the procedure. Perchloric acid and sodium hydroxide (Merck) are used for pH adjustment. Instrumentation. Time-Resolved her-Induced Fluorescence. A nitrogen laser (Model UV 24, Molectron) operating at 337 nm and delivering about 2 mJ of energy in a 10 ns pulse with a repetition rate of 20 Hz is used as the excitation source. The laser output energy is monitored by a laser power meter (Scientech). The beam is directed into a 4 mL quartz cell. The laser beam is focused into the cell of the spectrofluorometer FLU0 2001 (Dilor, Lille, France) by a quartz lens. The radiation coming from the cell is focused on the entrance slit of the polychromator. Taking into account dispersion of the holographic grating used in the polychromator, a 200 nm measurement range in the visible spectrum is obtained. The detection is performed by an intensified photodiode (710) array cooled by the Peltier effect (-30 "C) and positioned at the polychromator exit. Recording of spectra is performed by integration of the pulsed light signal given by the intens~er.The integration time, adjustable from 0.1 to 99 s, allows for variation in detection sensitivity. Time resolution is obtained by the control unit that assures pulsed running of the intensiiier and the photodiode array. The logic circuit used to generate the signals allows measurements with a gate delay adjustable from 0.2 to 99 ps during a length of 2-999 pus. The whole system is controlled by a microcomputer. Custom software automatically calculates concentrations by the standard addition method. Spectrophotomety. Absorption spectra are obtained with a Cary 1 (Varian) spectrophotometer.
Time-Resolved Fluorescence Measurement Procedure. All fluorescence measurements are performed at 20 "C. No attempts are made to exclude air. The pH of the solution in the cell is measured with a conventional pH meter (PHN 81, Tacussel) equipped with a subminiature combined electrode (U402 M3S7, Ingold) with a modified liquid junction composition (0.1 M NaC104, NaCl M). For U0z2+fluorescence measurements,gate delay and length are set to 1 and 2 ps, respectively; for UOzOH+
450
4?0
490
510
530 550 570 Wavdapth lrml
590
610
630
650
Figure 1. Uranyl fluorescence spectra as a function of pH. Conditions: [U] 100 pgIL, 0.1 M NaC104, Gate delay 1 ,US, and gate length 240 ,US. Table 1. Uranium Spectroscopic Parameters as a Function of the Medium
medium pH 1 (0.1 M NaC104) pH 4 (0.1 M NaC104) pH 7 (0.1 M NaC104) pH 10 (0.1 M NaC104) HzS04' (4 M) &Podb (0.75 M) Fluran' (10%)
lifetime
ols)
2 80
200 400 35 200 60
main fluorescence wavelengths (nm) 487,510,533,560 497,520,544,570 516 (shoulder at 533) 507,528 (shoulder at 552) 494,516,540,565 494,516,540,565 499,520,545,570
Medium used for determination in presence of plutonium? Most usual complexing medium. 'Medium used in the presence of chlorine.10 (I
fluorescence measurements, gate delay and length are set to 40 and 200 ps, respectively, for global fluorescence measurements, gate delay and length are set to 1and 240 ps, respectively. Each experiment is performed three times for reproducibility purposes. Convolution Procedure. The global experimental fluorescence spectrum (Few) can be expressed (in our case) as being the sum of the contribution of the fluorescence of the free uranyl and the first hydroxide complex according to the following equation:
Since free uranyl and first hydroxide complex fluorescence spectra are perfectly characterized (1,t,intensity), a least-squares fit (with a and b as parameters) on each global fluorescencespectrum (for each pH) is applied. This procedure allows one to obtain the a and b coefficients and the relative percentage (100(bla) of the first hydroxide complex. This procedure can be generalized to more species. RESULTS AND DISCUSSION
Uranyl fluorescence in various media has been extensively studied for the last 50 years and is still under investigation due to the interesting and complex chemistry of uranium. Figure 1 shows the variations in the uranyl fluorescence spectrum at low concentration (100 pg/L, 4 x M) as a function of pH in a noncomplexing medium (NaC104). This concentrationhas been chosen to limit the presence of uranium polynuclear species and to avoid precipitation. Table 1 summarizes the results obtained in terms of fluorescencewavelengths and lifetimes and compares them with classical complexing reagents used for uranium ultratrace determination by T W F . Analytical Chemistry, Vol. 67, No. 2, January 15, 7995
349
2
3
4
5
6
7
8
9
PH Figure 2. Speciation diagram of uranium(V1). Conditions: [U] 100 pg/L, 0.1 M NaC104, and pCO2 10-3.5atm (with equilibrium constants from ref 30).
Several comments should be made. First, a pH increase is associated with a red shift and a longer lifetime. These expected effects are due to stabilization of the excited state by complexation. As seen in Figure 2, nine species appear. They are primarily hydroxide and carbonate uranyl complexes. Hence, at pH 1,only U0z2+ is present in solution; at pH 4,U020H+ appears; at pH 7, several uranyl-hydroxide complexes are present (UOzOH+,UOZ (OH)z, UOz(OH)3-, UOZ(OH)~~-, (U02)2(OH)3+)as well as uranyl-carbonate complexes (UOzCO3, UOZ(CO~)Z~-) (this explains the very broad fluorescence spectrum at this particular pH); and finally at pH 10, it is more likely that the uranyl tricarbonate complex U O Z ( C O ~ )is~ ~the - predominant species. Second, it is important to notice that lifetimes observed for pH 7 are of the same magnitude and even higher than the ones obtained for classical complexing reagents. The same comment is also valid for the wavelength shift. It is obvious from such a figure that the direct speciation at pH 7 and above (most common pH range for natural waters) is not straightforward as too many species are present. Therefore, it was decided to characterize each species individually from acidic pH and then from alkaline pH range in order to be able to perform convolutions of the experimental spectrum and then to construct the speciation diagram. This work presents the study of the speciation of uranyl between pH 2 and 4, where only U0zZ+and UOzOH+ are likely to be present in solution. Since lifetimes obtained for pH 1 (where U022+ is only present) and pH 4 (where UOzOH+ appears) are very different, namely, 2 and 80 ps, respectively, speciation by time resolution is performed. Hence, usually this feature of TRLIF is used to get rid of short lifetime fluorescence, but in our particular case, it will be used to characterize the different species. Thus, placing a gate 40 ps after the laser pulse allows us to obtain only the fluorescence due to UOzOH+ and to compare it with the fluorescence of the free uranyl obtained at pH 1, as seen in Figure 3. The same spectrum (for free uranyl) is obtained at pH 4 by placing a gate only 1ps after the laser pulse and with a short duration (to avoid the important contribution of the long-lifetime component). Through these (30) Grenthe, I.; Fuger, J.; Lemire, R J.; Muller, A B.; NguyenTrung, C.; Wanner, H. Chemical l%modynamia of Uranium, NEA-TDB, OECD Nuclear Energy Agency Data Bank,Final Draft, March 1990.
350 Analytical Chemistry, Vol. 67, No. 2,January 15, 7995
4W
470
493
510
533
563
570
590
610
830
663
we-@ (nm)
Figure 3. Uranyl fluorescence spectrum (obtained at pH 1) and first hydroxide complex fluorescence spectrum (obtained by time resolution). Integration time 60 s. Conditions: [U] 100 pglL and 0.1 M NaC104.
measurements, fluorescence wavelengths are shifted 10 nm between the free uranyl and the first hydroxide complex and lifetimes move from 2 (for U0z2+ ) to 80 ps (for UOzOH+). Moreover, a nonnegligible modification of the half-bandwidth is observed, namely, 17 nm for U020H+ as compared to 12 nm for UO22+ (as usually obtained in phosphoric acid or Fluran (complexing media)). Since the fluorescence spectrum is directly related to the fundamental state, this difference is due to a change in vibrational modes when U0z2+ evolved from a linear (or nearly linear structure) structure31to a less symmetrical one with OH equatorially coordinated to the uranium central atom.32 To our knowledge, this is the first time that the first hydroxide uranium complex has been isolated spectroscopically at this level by time-resolved fluorescence. Having defined the proper conditions to record free uranyl and first hydroxide complex in the same solution, a more precise study has been undertaken between pH 2.5 and 4 (with 0.5 pH step). Figure 4 displays (for the same experimental conditions) the progressive appearance of UOzOH+ (spectrum 11) together with the free uranyl (spectrum I) as a (31) Rabinovitch, E.;Belford, R L. In Spectroscopy and Photochemistry of Uranyl Compounds; Dunworth, J. V., Ed.; Pergamon Press: Oxford, U.K., 1964. (32) Properties of uranium ions in solutions and melts. Gmelin Handbook of Inorganic Chemistry, 8th ed.; Springer-Verlag Publishers: Berlin, 1984.
450
480
610
540
5m
BOD
m
450
480
610
WdlIm
540
5m
go
63.3
Hudlb l
pH4
FH 9 5
450
480
610
540
5m
ea,
m
4M
480
610
540
610
go
Ba
u d l Inn wdl(m Figure 4. Uranyl (I) and first hydroxide complex (11) fluorescence spectra as a function of pH together with global and convoluted fluorescence spectra. Integration time 60 s. Conditions: [U] 100 pglL and 0.1 M NaClOa.
function of pH. The thick-lined spectrum is the one obtained with a gate integrating the entire fluorescence (short delay and long duration) and the other is obtained by convolution of spectra I and 11. It should be noted that spectra I and I1 have been obtained by integrating the total fluorescence for each species (Le., knowing lifetimes and fluorescence measurements for both species, it was possible to recalculate the total fluorescence in each cases). At pH 2.5, the global fluorescence spectrum is dominated by free uranyl; at pH 3, the contribution of free uranyl and of the first hydrolysis complex is nearly equal and thus leads to a very broad global spectrum. In contrast, at pH 3.5 and 4, the first hydrolysis complex dominates the global fluorescence spectrum. This drastic influence of the first hydrolysis complex despite its very weak concentration (see Figure 2) is due to the very large difference in lietimes between U0z2+ (2 ps) and U020H+ (80 p s ) , which affects the fluorescence intensity. Hence from these different spectra, it was possible to link the first hydrolysis complex concentrationto the fluorescence signal as seen on Table 2, which reports (i) calculated concentration for UOzz+and UOzOH+ obtained from the speciation diagram (Figure 2) according to the equation (from ref 30)
UO?
+ 2H,O tUO,OH+ + H+
K=
and (ii) global fluorescence signals obtained from time-resolved spectra. Several comments should be made about the data in this table. First, the fluorescence signal of U0z2+ is nearly constant (9000 f 10oO) between pH 2.5 and 4 since uranyl concentration diminishes
Table 2. Fluorescence Intensity of Uranyl (UO&) and First Hydroxide Complex (UO2OH+)and Their Relative Concentration as a Function of pH* [U022+1 PH
2.5 3 3.5 4
0
4.20 x 4.18 x 4.15 x 4.04 x
fluores intens UOz2+(AU) 9200 f 500 9000% 500 9800 f 400 8000 f 600
[UOzOH+]
0
5.43 x 1.71 x 5.36 x 1.65 x
fluores intens UOzOH+ (AV) 2050 f 200 9500f400 25000 f 1000 57000 f 2000
[VI 100 hg/L, 0.1 M NaC104.
of only 4% in this pH range. On the other hand, as seen in Table 2 and Figure 5, the fluorescence signal of U020H+ follows (within experimental errors) the first hydrolysis complex concentration. Moreover, by applying the convolution procedure on the different global fluorescence spectra, knowing the uranyl fluorescence spectum (obtained at pH 1 or by time resolution) and the first hydroxide complex spectrum (obtained by time resolution; Figure 3), the percentage of UOzOH+ can be obtained as seen on Table 3, which compares theoretical and calculated percentage of the first hydroxide complex. From this table, it can be seen that the convolution procedure gives a good estimation of the UOzOH+ percentage. In the case of an unknown solution (in this pH range), the determination of the total concentration (by the standard addition method) together with this percentage (deter-. mined by the convolution procedure) would give the first hydroxide complex concentration. Analytical Chemistry, Vol. 67, No.2, January 15, 1995
351
This equation can be applied to U0z2+ and to UOzOH+, which leads to
1 FU020H+
2.5
3.5
3.0
4.0
PH
Flgure 5. First hydroxide complex concentration (0)and fluorescence intensity (measured at 520 nm) (H) as a function of pH. Conditions: [U] 100 pg/L and 0.1 M NaC104. Table 3. Comparlron between Theoretlcal and Calculated Percentage of the Flrst Hydroxide Complex as a Function of pHa % UOzOH+
PH
theoreticalb
calculatedC
2.5 3 3.5 4
0.13
0.2 0.7 1.2 9
0.41
1.3 4
[VI 100 pg/L, 0.1 M NaC104. From ref 30 (K= 10-5.2). From the convolubon procedure.
At this point, it is also interesting to link these different results with general fluorescence criteria in order to obtain more spectroscopic data on UOzOH+. In the case of a pulsed excitation, the fluorescence signalgcan be expressed as follows:
where k is the apparatus factor, IO is the laser intensity, to is the natural fluorescence lifetime, E and C, respectively, are the absorption coefficient and concentration of the fluorescent species, At is the time between the laser excitation and the fluorescence measurement, t is the fluorescence lifetime, 1 is the optical pathway, E’ and c’, respectively, are the absorption coefficient and concentration of species absorbing at the emission wavelength, and E and c, respectively, are the absorption coefficient and concentration of species absorbing at the excitation wavelength. In our case, prefilter (species absorbing ( E , c) at the laser wavelength, 337 nm) and postfilter (species absorbing (E’, c’) at the uranium fluorescence wavelengths, 480-560 nm) effects are negligible (verilied by spectrophotometry) and eq 1 simplifies to
This integration of eq 2 from 0 to fluorescence intensity leads to 352
m
in order to have the total
Analyiical Chemistry, Vol. 67, No. 2, January 15, 7995
= kzO-
‘OU020H+
(5) EUO,OH+CUO,OH+~UO,OH+
Using the ratio of eqs 4 and 5, it is possible to get rid of constants and to obtain
-FU020H+ FU022+
-
Z0U02z+ EU0,0H+CUOzOH+tU020H+
ZOU020H+
eU02z+CU022+tU022+
(6)
Since fluorescence lifetimes, fluorescence intensities, and concentrations for both species are known (Tables 1 and a), as well as the extinction coefficient (CUO;+ x 12 at 337 nm) and the natural fluorescence lifetime for UO++ (to = 500 p 3 9 ,it would have been interesting to calculate the natural fluorescence lifetime for UOzOH+ (and thus to be able to determine the fluorescence quantum yield ( t / r ~of) this particular species). However, its extinction coefficient is also not known, since it is not possible to have UOzOH+ at concentrationsobservable by spectrophotometry (limit of detection &OD) EC x Hence, above 10 mg/L (4.10-5 M), (UOZ)Z(OH)Z~+ dominates the speciation diagram in this pH range and UOzOH+ cannot be isolated. Thus, the numerical application of eq 6 only leads to a relation between these two data EUO~OH+
5 ~ ( ~ 0 ~ 0 ~ 0 ~ + ~ ~ 0 ~ 0 , (7) 2+)
If it is assumed that real lifetimes for both species are equivalent (which is far from being verified), it would lead to an extinction coefficient of 50, which seems rather high (for UOZ OH+). Hence, as a comparison, in the case on nitrate uranyl complexes, the extinction coefficient for UOzN03+ is around 26.33 However, photothermal techniques such as thermal lensing or photoacoustics (LOD EC x 10-5) could allow one to work at a uranium concentration (1 mg/L, 4.2 x M) where the extinction coefficient of UOzOH+ could be determined, leading to knowledge of the fluorescence quantum yield for the first hydrolysis species, using eq 7. From a general point of view, since the fluorescence lifetime is increasingwith pH (Table 1),the same procedure (time resolution) can be applied starting with alkaline pH (8-10) in order to spectroscopically identify carbonate complexes (bicarbonate and bicarbonate, see Figure 2) by varying the partial COz pressure, pH, or carbonate concentration. In order to complete uranyl speciation in the absence of organic ligands (such as humic substances), the other hydroxide complexes (UOp OH+,UOz(OH)2, U02(OH)3-) will also be characterized by (33) Deniau, H. Ddveloppement de la Spectrofluorimktrie Laser A Resolution Temporelle pour le contrdle en ligne de I’uranium dans les solutions du procede de retraitement PUREX. Thesis, Universite Pierre et Marie Curie, Paris, France, 1992.
favoring their existence under spedic conditions ([U] , pCO2, pH). Work in this direction is in progress. The usefulness of this approach, based on the spectroscopic characterization of inorganic uranium species, has considerable scope in future investigations of complexation phenomena (in particular with humic substances). Thus, this study is a f i s t step for better knowledge of uranium behavior in the environment and then, after complete characterization of all complexes present in solution, for potential in situ speciation of uranium by TRLIF. However, such in-field measurements would require knowledge of the iduence of quenching and absorbing species present in groundwaters that would respectively affect uranyl fluorescence lifetime and global intensity. Studies to characterize these different effects are in progress. CONCLUSIONS Tme-resolved laser-induced fluorescence has been used for uranium speciation in solution in the acidic pH range. Using time resolution of TRLIF, the first hydroxide complex of uranium (UOr OH+) has been spectroscopically isolated and characterized for the first time. Indeed, by time resolution, in the pH range 2.5-4, it is possible to distinguish the free uranyl from the first hydroxide complex and then to estimate its proportion. This study has been
performed in the acidic pH range, but the same procedure will be then applied in the alkaline pH range to characterize the carbonate species or other hydroxide species according to the physicochemical conditions (pH, pCO2, ...). The application of this technique W F coupled with spectral convolution) will be particularly promising to validate the existence and proportion of uranium species (inorganic and organic) under conditions relevant to natural aquifers occurring in the vicinity of nuclear waste disposals. ACKNOWLEWMENT We thank Dr. P. Reiller for realizing the uranium speciation program and Dr. P. Mauchien for helpful discussions. This research has been carried out as the continuation of a program of the Commision of the European Community (Contract FEW-
CT91-0083). Received for review September 12, 1994. October 27, 1994.@
Accepted
AC9409041 Abstract published in Advunce ACS Abstracts, December 1, 1994.
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