Photoluminescence studies of the uranyl carbonate system

Journal

of Photochemistry

and Photobiology,

PHOTOLUMINESCENCE CARBONATE SYSTEM R. D. SAINI,

STUDIES

P. K. BHATI’ACHARYYA*

Chemistry (India}

Division,

(Received

August

Bhabha 17, 1988;

Atomic

A: Chemistry,

47 ( -989)

65 - 81

65

OF THE URANYL

and R. M. IYER

Research

Centre,

in revised form October

Trombay,

Bombay-400

085

25, 1988)

Summary An investigation was carried out on the effects of hydrolysis and complex formation on the optical absorption spectra, photoluminescence and excited state lifetimes of various uranyl species formed under different pH conditions and various carbonate to uranyl concentration ratios in aqueous solutions. The hydrolysis increases both the quantum efficiency of luminescence aL and the lifetimes 7 of the excited states up to a pH of about 6.2; the exact pH depends on the total uranyl concentration in the solution. This effect is attributed to the formation of large polywhich innuclear species with hydroxo bridges, such as (UO,),(OH),-, troduce a rigidity into the molecular structure of the emitting species and inhibit the collisional quenching due to their large size and high molecular weight. At a pH of greater than 6.2, at a total many1 concentration of 5 X 10m4 M, both (PL and 7 decline continuously. This is explained as being due to the transformation of polynuclear species with hydroxo bridges to species with 0x0 bridges, with a concomitant reduction in the molecular rigidity. Complex formation between uranyl(V1) species and carbonate ion always leads to the quenching of uranyl luminescence. The EpL value decreases with increasing carbonate to uranyl concentration ratio, so much complex is non-luminescent both in solution and so that the UOZ(C03)s4in the solid state. Experimental evidence suggests that the quenching of uranyl luminescence by carbonate takes place by an intramolecular mechanism. At a pH close to 4.8, where the formation of very large polynuclear species such as (U02)11(OH)12(C03)6 2 - has been reported in the literature, kinetic evidence for the excited state dissociation of higher polynuclear species into smaller fragments is obtained. On dilution, uranyl carbonato complexes show enhanced luminescence which is interpreted as being due to the hydrolytic reactions.

+To whom lOlO-6030/89/$3.50

correspondence

should be addressed. @ Elsevier Sequoia/Printed

in The Netherlands

66

1. Introduction A detailed study of the physical and chemical processes following the excitation of uranyl ion by the absorption of light in bicarbonatecarbonate medium is important for several reasons. In alkaline and weakly acidic solutions uranyl ion is reported to form a series of carbonato complexes ranging from well-defined species such as U02(COs)34at a pH of 8 or greater to large polynuclear species such as (UO,) ,,(OH) 12(C03)62 at a pH close to 4.8, where the solid complexes start to precipitate [l - 31. It is therefore interesting to determine how the various photophysical and photochemical processes are affected by the gradual transition in the state of aggregation of a chromophore such as the UOZ2+ ion. Such studies may also help in the understanding of the photochemical behaviour of uranium in natural waters where minute quantities of uranium are believed to exist as uranyl carbonato complexes and the source of photoexcitation is sunlight [ 41. So far, the work reported in this area has largely been exploratory in nature [4 - 61. Jorgensen and Reisfeld [ 41 have described their ‘test tube experiments’ where a strong enhancement of the uranyl luminescence was observed in bicarbonate solutions. Since aqueous bicarbonate or carbonate solutions have a basic pH, the uranyl ion in such media undergoes both hydrolytic and complex formation reactions [ 11. Wheeler and Thomas [7] have reported that the intensity of uranyl luminescence increases on hydrolysis of uranyl ion under certain conditions. The observations of Jorgensen and Reisfeld [4] necessitate further investigations to assess the contribution of hydrolysis and complex formation to the enhancement of luminescence. We investigated the photoluminescence of uranyl ion in weakly acidic and alkaline aqueous solutions both in the absence and presence of carbonate ion. The results are discussed in this paper.

2. Experimental

details

Triply-distilled water and reagent grade chemicals were used to prepare the solutions. Nuclear grade U30s was dissolved in perchloric acid (Merck, G.R. grade); the uranium(IV) present in the resultant solution was oxidized by H202 (Sarabhai M. Chemicals, G.R. grade). The excess H,O, was decomposed by heating and the solution was evaporated to obtain the crystals of uranyl perchlorate. These were washed with a small quantity of water and dried in a vacuum desiccator_ They were then dissolved in perchloric acid (lo-* M) to obtain the stock solution of uranyl perchlorate. The total uranium content of the stock solution was determined volumetrically by the method described earlier [8]. Anhydrous Na2C0s (Merck, G.R. grade) and NaHCO, (BDH, AnalaR grade) were used as received. Na4U02(C03)3 was prepared by the method described in ref. 3. An aliquot of uranyl perchlorate stock solution (0.6 M) was treated with an excess of NaHCO 3; the resultant solution was evaporated until a sufficient

67

quantity of the yellow coloured solid Na4U02(C03)s separated out. The solid was washed with a water-ethanol (Photorex grade; Baker) mixture and was dried in a current of air, followed by final drying in a vacuum desiccator. A known amount of the solid was dissolved in 0.03 M Na&03 solution and its uranium content was determined spectrophotometrically by taking the molar absorbance of U02(C03)34at 449 nm as 27 M-’ cm-’ [9, lo]; the percentage of uranium agreed with the formula Na4UOZ(C03)3 within *l%. The uranium(VI) concentration in other carbonate solutions was determined in a similar manner. A stock solution of 1 M NaC104 was prepared by neutralization of standard NaOH and HC104 solutions. It was used for adjusting the ionic strength I of the sample solutions. The starting materials for the preparation of the sample solutions were solid Na4UOZ(C03)3, 0.1 M uranyl perchlorate (pH = 0.74), 0.1 M NaHC03, 0.1 M Na&03 and 0.1 M HC104 solutions. Water-saturated nitrogen gas (purity, better than 99.9%; IOLAR grade, Indian Oxygen Ltd.) was used to purge the solutions. pH measurements were carried out using an Orion model 901 ion analyser after standardizing the instrument with standard buffers [ 111. Optical absorption spectra were recorded with a Hitachi model 330 spectrophotometer; 1 cm cuvettes were used unless otherwise specified. Luminescence spectra were recorded with an Aminco-Bowman model 4-8202 spectrofluorometer. The relative intensities and quantum yields of luminescence of the sample solutions were determined using a 0.0322 M uranyl perchlorate solution (pH 2.23) as reference. The luminescence quantum yield of this reference solution was in turn determined by comparing its luminescence spectrum with that of a 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP) solution in cyclohexane; both the solutions have practically the same optical density at the excitation wavelength. The following equation was used for these calculations [ 121. Integrated emission from sample solution

(F,)

Integrated emission from reference solution (F,) =

Area under the emission spectrum of sample solution (A2) Area under the emission spectrum of reference solution (A,) &(&J(l

- 10-oDz)(n22)

= @,l(la)(l - 10-oDl)(nl*)

(I,,, intensity of the incident light)

Thus @2( 1 10-OD2)(nzZ) F2 -42 -= -= @,1(1 - lo-onl)(n,*) Fi Al where a*, OD2 and n2 refer to the luminescence quantum yield, the optical density and the refractive index of the sample solution at the excitation wavelength X,, and +1, OD1 and n1 are the corresponding quantities for

68

the reference solution at the same wavelength. The value of the refractive index for all the aqueous solutions was assumed to be 1.33 [ll] and for the solution of POPOP in cyclohexane it was taken to be 1.42 [ll]. The fluorescence quantum yield of POPOP in cyclohexane was assumed to be 0.93 [13]. For the determination of A 1 or AZ, the emission spectrum was converted from intensity us. X to intensity vs. g (cm-‘) coordinates and the area was determined by cutting and weighing the paper under the curve. Most of the luminescence studies were carried out with X,, = 370 nm; X,, = 351 and 410 nm were also used in some cases. The kinetics of luminescence decay were investigated using an Applied Photophysics model K-347 laser kinetic spectrometer with a xenon-fluorine (351 nm) excimer laser. The transient emission signals were monitored by a lP28 photomultiplier tube whose output was fed to a Gould-Biomation model 4500 digital storage oscilloscope. The data were analysed using a PDP 11/23 computer. Further details of the K-347 laser kinetic spectrometer have been reported elsewhere [ 141. Some lifetime measurements were also carried out using the single photon counting technique. An Edinburgh instrument model 199 fluorescence spectrometer and a xenon flash lamp were employed.

3. Results and discussion 3.1. Effect of hydrolysis The absorption and emission spectra of aqueous solutions containing 5 X 10m4 M uranyl perchlorate with various pH values are shown in Figs. 1 and 2 respectively. The changes in the spectral properties with increasing pH of the solution can be explained as being due to the progressive hydrolysis of the uranyl ion which leads to the formation and coexistence of several hydrolysed species [I], each with its own absorption [15] and emission characteristics. The proportions of these species (calculated from the stability constants /3 at I = 0.1 M [3]) in the sample solutions at various pH values are shown in Table 1, together with the extinction coefficients at A,,, for some of the species [ 153. The values of the quantum efficiency of luminescence aL at different pH values are shown in Fig. 3. An examination of the data of Fig. 3 and Table 1 reveals that the increase in & with an increase in pH from 3.03 to 6.21 is associated with the formation of polynuclear species. Thus a sharp increase in the concentration of (UO,),(OH),(0.4% at pH 4.03 and 90.9% at pH 5.17) is also reflected in a sharp increase in aL_ At pH 6.21 almost all the uranium is present as (UO,),(OH)1; the maximum value of *.L is also observed at this pH. Further support for this correlation is obtained by using higher uranyl concentrations that favour the formation of the polynuclear species. For example, a 0.004 M uranyl perchlorate solution (pH 3.6, I= 0.1 M) gives & = 2.46 X 10m3 (Table 2) which is higher than that obtained from Fig. 3 at pH 3.61. A logical explanation

69 I

25

-

150

-

100

-

50

L (Cm -5 x 10-4 Fig. 1. Absorption spectra of uranyl perchlorate solutions (I = 0 .lM;T= trum 1, pH 4.03; spectrum 2, pH 5.17; spectrum 3, pH 6.21; spectrum spectrum 5, pH 7.3;spectrum 6, pH 8.13;spectrum 7, pH 9.78.

25 "C):spec4, pH 6.59;

for these observations may be given as follows. The hydroxo bridges in the polynuclear species, e.g.

OH,

/OH,

,OH,

/"\OH/M\OH/M I H20

/"" \

1 -

OH

(where M = UOz) introduce a rigidity into the molecular structure of the emitting species. The rate of collisional quenching of such a species is retarded owing to its large size and high molecular weight. Both of these factors are known to increase the quantum efficiency of the radiative process. The decrease in aL at a pH of greater than 6.21 (Fig. 3) may be explained by the loss of rigidity in the molecular frame of the emitting species.

3 4 5 6

uo22+

1 2

fuo2)3(oH)7-

(UO2)3(OH)S+

WO2)2(W2

U020H+

WOz)z(OW

Species

2t

3+

-

--2.x!

-4.39 -6.09 -15.64 -24.03

-

log P 131

_.. -

-

1.0

99.0

1.97

-

-

4.7

95.3

3.03

28.5 3.6 0.8 0.4

64.3 2.4

4.03 1.3 6.4 0.3 1.1 90.9

-

5.17

= 5 X 10m4M; I = 0.1 M; T = 25 “C)

Uranyl species (‘3%) at pH

(So) of uranyl species at different pH values (U”

Solution

Proportions

TABLE 1

0.6

99.4

-

-

6.21

= 8 (425 nm) 1151 * 250 (425 nm) [15 ] -

-

-

E (M-l cm-‘)

0

4

71 140 6

60

0 400

600

500 WAVELENGTH

6

(nm)

Fig. 2. Emission spectra of uranyl perchlorate solutions (I = 0.1 M; T = 25 C; A,, nm): spectrum 1, pH 9.78; spectrum 2, pH 5.17; spectrum 3, pH 6.21; spectrum 8.13; spectrum 5, pH 6.59; spectrum 6, pH 7.3. TABLE

2

Uranyl luminescence in aqueous strength I = 0.1 M; T = 25 “C) Solution

NazC03

carbonate

OD (351

pH

CM)

1

2 3 4 5

= 370 4, pH

Nil

0.002 0.008 0.012 0.02

3.61

(conditions:

F nm)

0.02

4.55 7.20 9.31 10.47

solutions

0.538 3.5 1.107 0.188

(arbitrary units) 0.19 1.20 0.92 17.85 1.35 0.51 Nil

Uvl

= 0.004

brrlt~x)

@L

492 502 519 518 533 533.5 -

M; ionic

2.46

x 1O-3

2.31 1.24 5.09 Nil

x 10-j x 1O-4 x 10-S

At pH 6.21 almost all the uranium already exists as (UO,),(OH),as can be seen from Table 1. Further addition of NaOH, to increase the pH, causes the elimination ‘of water from the adjacent hydroxo groups leading to a process known as oxolation [ 31, i.e.

1 n

+

[-M-O-M-O-M-],

+ nH20

(1)

72 40

I

I

I

3c

to 0 X *i

2c

IC

C

1

2.5

0

7.5

5.0

PH Fig. 3. Dependence of I = 0.1 M; T = 25 “C).

@L on pH in uranyl

perchlorate

solutions

(Uvl

= 5 x lop4

M;

This ultimately leads to the precipitation of uranates, e.g. Na2U20,. The resulting molecular frame -M-O-M-O-Mcan provide a number of additional paths for the intramolecular degradation of the absorbed photon energy through several possible rotational and bending vibrational modes which thereby leads to a decline of luminescence quantum efficiency at a pH of greater than 6.21. Our sample solutions with a pH of greater than 7 were unstable; they became turbid within 2 - 3 h. 3.2. Combined effect of hydrolysis and complex formation The absorption spectrum of a 0.004 M uranyl perchlorate solution at pH 3.61 has several weak absorption bands in the 20 000 - 28 000 cm-’ region. On addition of different amounts of sodium carbonate, the pH of the solution increases, and changes in the absorption spectrum occur which are shown in Fig. 4. It can be seen that the molar absorbance of the solution increases, a loss of fine structure o’ccurs and the absorption maximum exhibits a red shift up to the addition of 0.008 M Na,COs, where fine structure starts to reappear_ Thereafter, the addition of carbonate decreases

73

‘C.1.

0 1.5

I

I

I

I

Y_

2.0

2.5

Ei

3.0

(Cm--‘) x 10-4

Fig. 4. Absorption spectra of 0.004 M uranyl solutions containing different amounts of sodium carbonate (I = 0.1 M; T = 25 “C): spectrum 1, carbonate = zero, pH 3.61; spectrum 2, carbonate = 0,002 M, pH 4.55; spectrum 3, carbonate = 0,008 M, pH 7.2; spectrum 4, carbonate = 0.012 M, pH 9.3; spectrum 5, carbonate = 0.02 M, pH 10.46.

the molar absorbance and an absorption spectrum similar to that of uranyl dicarbonate complex [9, lo] (which has recently been shown to be a trimer WW3(CW64 - [l]) is obtained (Fig. 4, spectrum 4). The decline in the molar absorbance and the sharpening of the fine structure continues until the characteristic spectrum of UOz(C0,),4[9, lo] is obtained after the addition of 0.02 M sodium carbonate (spectrum 5). On further addition of carbonate the pH of the solution increases monotonically and eventually leads to the precipitation of uranates. Substitution of Na&Os by NaHCO, gives similar results. However, precipitation of uranates does not occur due to the buffer action of the sodium bicarbonate. Similar results are also obtained, but in reverse order, by addition of perchloric acid to aqueous solutions of Na4U02( COJ 3. The luminescence spectrum of 0.004 M uranyl perchlorate solution (pH 3.61) exhibits three peaks, whereas only a single luminescence peak is observed in the presence of carbonate. The positions of these emission

74

peaks, their intensities F and the quantum yields of luminescence aL in the different solutions are shown in Table 2; the optical densities (OD) at the excitation wavelength (351 nm) are also given in this table. It can be seen that the addition of 0.002 M NazCOs to a 0.004 M uranyl perchlorate solution (pH 3.61) is accompanied by a large increase in the luminescence intensity. This may explain the preliminary observation of Jorgensen and Reisfeld [ 43. However, the data of Table 2 show that the increase in luminescence intensity is due to the enhanced optical absorption by the solution; the carbonate reduces the quantum yield of luminescence. This observation becomes clearer on examination of Fig. 3, which yields a value of aL = 1.12 X lo-’ at pH 4.55 in the absence of carbonate. This value will increase further if a higher uranyl concentration of 0.004 M is considered, which leads to more favourable conditions for the formation of polynuclear species as explained above. The observed aL value of 2.31 X 10P3 (Table 2) is significantly lower than this value. Thus the marginal decrease in the aL value of solution 2 compared with that of solution 1 (Table 2) is the net effect of two mutually opposing tendencies, namely the increase in cPL due to hydrolysis and the quenching of luminescence by carbonate. Since OH- also starts to quench the uranyl luminescence at a pH of greater than 6.21 (Fig. 3), a rapid decrease in aL occurs in solutions 3 - 5, so much so that solution 5, containing almost 100% U02(Co3)34 -3 is practically non-luminescent. Solid Na4U02(C03)s is also non-luminescent on excitation with UV light; this indicates the non-luminescent nature of the U02(C03)s4ion even in the solid state. However, an aqueous solution of this salt gives a weak luminescence due to the dissociation of U02(C03)s4 - to the dicarbonate complex according to the equilibrium uo2(cos)s4-

=

KZ

uo*(co&*

- + co32 -

(2)

Kz = 3.89 x IO@ [ 33 As is expected from eqn. (2), the luminescence excess sodium carbonate.

disappears

on addition

of

3.3. Lifetimes of excited states Aqueous uranyl solutions (0.004 M) whose absorption spectra and QL values are shown in Fig. 4 and Table 2 respectively, were excited by excimer laser (351 nm) pulses to obtain the profiles of luminescence decay. The results are shown in Figs. 5 and 6. It is observed that the luminescence decay does not follow a simple exponential curve. In all cases, with the exception of solution 2, the decay requires a double exponential of the form F = Fo{Al exp(--kit)

+ A2 exp(--k2t)]

(3)

Such double-exponential decays have also been observed in the absence of a complexing ligand at a pH of approximately 2; they have been interpreted

75 TIME 0

5

(PSI

IO

15

20

25

-0.00

-0.20

:

- 0.10

-

-0.40

_

-0.60

: 3i 5 e z ”

0.60 :: A

_I

-0.20

-

-0.30

1.00

-1.20

-0.40

-1.40

-0.50

0

2

4

6 TIME

&S,

IO

12

i

14

Fig. 5. Decay kinetics of uranyl luminescence in aqueous carbonate solutions (Uvl 0.004 M; I = 0.1 M; T = 25 “C): 1, h,, = 513 nm, carbonate = zero, pH 3.61; 2, h,, 518 nm, carbonate = 0.002 M, pH 4.55.

= =

as being due to the formation of an excimer [16, 171 or, more recently, to reversible crossing between two states of the uranyl ion [18]. In this case, however, the double-exponential decay can be attributed to the presence of more than one uranyl species in the solution. Computer fitting of TABLE

3

Kinetic parameters of the decay of uranyl luminescence (Uv’ = 0.004 M; I = 0.1 M; h,, = 351 nm; T = 25 “C)

Solution

Na2C03

pH

WI

in aqueous

carbonate

solutions

xem

kl

kz

71

72

km)

W’)

(s-l)

Us)

(PSI

12.48 a

1 2

Nil 0.002

3.61 4.55

502 518

4.90 a

x 10s

8.01 a

x 104

2.038 a

3 4 5

0.008 0.012 0.02

7.20 9.31 10.47

533 533 533

4.60 8.45 3.88

x lo6 x 106 x 107b

1.03 1.08 1.28

x 106 x 106 x 107b

0.2173 0.1183 0.026b

aThese values are given in the text. bComputed from a very weak and noisy signal.

0.967 0.9237 0.078b

76

-0.60

7

-I

-1.20

-

-1.30

-

-1.40

-

-1.50 0

I 200

I 400

I 600

I 800 TIME

I 1000

I 1200

I 1400

I!

(nS)

Fig. 6. Decay kinetics of uranyl luminescence in aqueous 0.004 M; I = 0.1 M; 2’ = 25 “C): 1, h,, = 533 nm, carbonate 533 nm, carbonate = 0.012 M, pH 9.31.

carbonate solutions (Uvl = 0.008 M, pH 7.2; 2, h,,

= =

eqn. (3) to the experimental data was used to evaluate the values of the rate constants k, and k, which are shown in Table 3. It can be observed that the values of k1 and k2 increase with increasing concentration of carbonate. In addition, these values are much higher than the corresponding values of kl and k2 obtained in the absence of carbonate (Table 4). These observations indicate a strong quenching of the excited uranyl species by carbonate ions. The luminescence decay of solution 2 (Table 3) shows an unusual behaviour. A computer fit to the experimental data (Fig. 5) was achieved in three steps as follows: (1) a single exponential from 1 - 6 ps yielding a lifetime r1 of 8.55 ps; (2) another single exponential from 7 - 18 ps giving from 15 - 25 ps leading to 73 = 3.6 ~.ls 7 2 = 6.16 11s; (3) a double exponential Such a complex decay indicates the presand r4 = 10.69 ps respectively. ence of several emitting species in the solution. Other important information can be obtained from the slow decay of the luminescence in the initial I - 6 ps (Fig. 5). This suggests a transient equilibrium (transient equilibrium

77 TABLE

4

Kinetic parametersa of the decay of uranyl luminescence in perchlorate medium ferent pH values (UOz(C104)2 = 2 x lop4 M; he, = 410 nm; I = 0.1 M; T = 25 “C) Solution

1 2 3 4 5

Pff

5.014 6.375 7.450 8.200 9.250

x em

k, x 10-4

k2 x 1O-4

(nm)

(S-l)

Cf)

515 515 515 515 515

10.33 8.89 5.86 7.95 9.06

2.26 3.38 2.65 5.29 1.86

9.68 11.25 17.07 12.75 11.04

at dif-

44.29 29.60 37.77 18.90 53.71

aMeasured by single photon counting. The data in this table also support the effect of hydrolysis; however, the difference in uranyl concentration should be kept in mind when correlating these data with those of Fig. 3.

is quite common in radioactive decays; see, for example, ref. 19) between two emitting species of the uranyl ion, e.g. Ur* and Ug, i.e. hv,+U1

+cJ1* K”‘t&*L+ u* + hv,

with the condition that [U,*] = [U,*] at time t = 0 and ki’< k2. Thus U,* appears to decay with a slower rate constant than k2. The condition k I’ < k, suggests that the molecular weight of U i* is higher than that of U,*. In other words the dissociation of higher polynuclear species into lower polynuclear species is indicated in the excited state. It may be recalled that the pH of solution 2 is close to 4.8 where the formation of very large polynuclear species such as (UO,), ,(OH) 12(COs)62- has been reported

t21. 3.4. Luminescence of hemicarbonate A solution of (UOz)zC03(OH)3- (uranyl hemicarbonate) can be prepared by the addition of 3.5 moles of H+ per mole of uranium to a solution of UOz(C0,),4-, followed by sparging with an inert gas to displace the dissolved CO? [20]. Displacement of CO2 is accompanied by an increase in pH and molar absorbance and leads to the formation of hemicarbonate from a solution where uranium is largely present as (UO&(OH)S* [20]. The absorption spectra of 0.01 and 0.005 M Na,UO,(CO,)s solutions treated with 3.7 moles of H+ per mole of uranium are shown in Fig. 7, both before and after purging with nitrogen gas. The pH and luminescence quantum yields aL of these solutions are shown in Table 5. It can be seen that the formation of hemicarbonate is accompanied by a decline in aL. These results suggest that the quenching of luminescence by carbonate occurs only on formation of a complex between the uranyl ion and the carbonate. The dissolved H&O3 and HCOs-, which are the products of the hydration of coz, do not have a significant effect on the quenching of uranyl lumi-

78

2

IW

25 t

1.5

=

3.0

2.5

2.0 Wn-’

1 x lo-4

Fig. 7. Absorption spectra of Na4UOZ(C03)3 solutions treated with 3.7 moles of Hf per and after before M, pH 5.81 and 7.89, mole of uranium: spectra 1 and 2, Uvl = 0.01 spectra 3 and 4, Uv’ = 0.005 M, pH 6.16 and 8.12, purging with nitrogen respectively; before and after purging with nitrogen respectively, TABLE

5

pH and aL values of Na4l-J02(C03)3 solutions treated mole of uranium (X,, = 370 nm; I = 0.1 M; T = 25 “C) Final uranyl concentration

PH

with

3.7

moles

of HC104

per

@L

(MI

Before purging with Nz

After purging with Nz a

Be fore purging with Nz

After purging with N, a

0.01 0.005

5.81 6.16

I .89 8.12

3.93 7.01

5.50 8.28

aSolutions

containing

predominantly

uranyl

x 10-4 x 10-4

x 10-s x lo-’

hemicarbonate.

In other words, the quenching of uranyl luminescence by carbonate is intramolecular in nature. This is further supported by the nonluminescent behaviour of UO,( C03)34 - described above. nescence.

79 TABLE

6

Effects of concentration on the uranyl luminescence 351 nm; I = 0.1 M; T = 25 “C) Solution

uvl x I03 after dilution

in the presence of carbonate

PH

*L

7.20 6.52 6.49 6.40

2.38 3.34 3.65 1.99

(A,,

=

7 W)

(MI

10.00=

1 2 3 4

1.00 0.10 0.02

x x x x

lo+ 10-4 1O-4 10-j

1.95 3.70 6.24 34.45

=Stock solution prepared by treating 0.01 M Na4UOZ(C03)s with 0.037 M HC104 lowed by purging the solution with CO* for 5 min and then with nitrogen for 5 min.

i

fol-

E

I

i

i i

i

i

0

1

I.5

(

j I - 100

2

,&‘,r;/,

I’(-/,

2.0 3

!

i, i :I

i ii

:

11

2.5 (Cm-‘)

10

2.9

x 10-4

Fig. 8. Absorption spectra of aqueous uranyl solutions prepared as follows: spectrum 1, stock solution prepared by adding 0.037 M HC104 to 0.01 M Na4U02(C03)s followed by purging with CO2 for 5 min and then with nitrogen for 5 min (pH 7.2); spectrum 2, after diluting the stock solution ten times (pH 6.52); spectrum 3, after diluting the stock solution 100 times (pH 6.49); spectrum 4, after diluting the stock solution 500 times (pH 6.4). For all the solutions, I= 0.1 M and T = 25 “C.

80

3.5. Concentration

effects

The data of Table 5 show an increase in the quantum efficiency @L with a decrease in the concentration of the carbonate complex. This effect was investigated further. A solution was prepared by treating 5 ml of 0.02 M Na4U02(C03)3 solution with 3.7 ml of 0.1 M HC104 followed by the addition of 0.72 ml of 1 M NaC104. This was made up to a volume of 10 ml. Thus the resultant solution had a uranium(V1) concentration of 0.01 M and an ionic strength I of 0.1 M. This was purged with CO* for 5 min, followed by purging with nitrogen gas for 5 min to displace the dissolved CO*. The stock solution thus obtained was diluted with 0.1 M NaC104 to obtain Uvl concentrations of 1.0, 0.1 and 0.02 mM. The values of pH, aL and 7 are shown in Table 6. The data show an enhancement of luminescence on dilution and an increase in the average lifetime 7 of the excited species. A decrease in pH on dilution suggests that these effects are largely due to the hydrolysis of the complex. This is further supported by the absorption spectra shown in Fig. 8. These observations may be explained as follows. On dilution, a decrease in the concentration of Uvl is accompanied by a decrease in the equilibrium concentration of carbonate (which also includes bicarbonate). The latter faces increasing competition from OH- ions for the uranyl ion. As a result, the formation of hydrolysed species, e.g. (UO,),(OH),+ and (UO,),(OH), 2+, takes place at the expense of the formation of carbonato complexes such as U02(C03)34- and (UO&C03(0H),-. The formation of hydrolysed uranyl species liberates H+ ions which decrease the pH of the solution.

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