Sonoluminescence of the Uranyl Ion in Aqueous
Solutions
Rachel Pflieger[a], Virginie Cousin[a], Nicole Barré[b], Philippe Moisy[c], and Sergey I.
Nikitenko*[a]
[a] Dr. R. Pflieger, V. Cousin, Dr. S. I. Nikitenko
Institute for Separation Chemistry of Marcoule (ICSM), UMR 5257 CEA-CNRS-UMII-ENSCM, BP
17171-30207 Bagnols-sur-Cèze Cedex France
Fax: 33(0)466797611
E-mail: serguei.nikitenko@cea.fr
[b] Dr. N. Barré
Institute of Nuclear Physics, Radiochemistry, CNRS-IN2P3-UPS, BP1-91406 Orsay Cedex, France
[c] Dr. P. Moisy
CEA/DEN/DRCP, Centre of Marcoule, BP 17171-30207 Bagnols-sur-Cèze Cedex, France
Supporting information for this article is available on the WWW under ……
1
For over a century and a half, the fluorescence of the uranyl ion, UO22+, has been the subject of
numerous studies. The uranyl ion can be excited by UV/Vis light[1] or by radiolysis[2] of liquid or solid
systems containing uranium. Radioluminescence of UO22+ in aqueous solutions arises from the energy
transfer from excited water molecules (excitons) to uranyl ions rather than from the direct absorption of
radiation or from Cherenkov emission.[2b] It was reported that the emission spectrum of radiolytically
excited UO22+ is identical with that of photoexcited UO22+.[2b,c] The uranyl ion displays a dense manifold
of excited states mainly occurring by excitations from the bonding σu, σg, πg, and πu orbitals to the nonbonding uranium 5fb and 5fφ orbitals, and include both singlet and triplet states.[1b] The lowest-lying
excited state is generally a triplet state and is transferred back to the ground state with light emission in
the optical range 400-650 nm.[1] The lifetime of the excited uranyl ion, (UO22+)*, in solutions depends
on a series of factors, such as temperature, solvent, presence of ligands etc., and can expand
approximately from 1 to 200 μs.[3] The quenching of (UO22+)* was interpreted as an electron-transfer
process involving a uranium(V) ion.[4]
Excited uranyl ion is a strong oxidant (E0 = 2.6 V vs. NHE) and can readily react with a variety
of organic and inorganic substrates.[1,5] Therefore, search for new approaches of uranyl ion excitation is
of interest for developing advanced actinide separation processes and radioactive waste management.
Power ultrasound has been proven to be effective for excitation of volatile and non-volatile species in
solutions.[6] The violent implosion of gas/vapor filled microbubbles in liquids submitted to ultrasonic
irradiation produces chemically active species as well as light emission, known as sonoluminescence
(SL). Multibubble SL spectra of pure water sparged with noble gases consist of OH° radical emissions
from A2+ and C2+ electronically excited states and a broad continuum spanning from the UV to the
near IR spectral range.[7] This continuum was attributed to bremsstrahlung, to H + OH° recombination,
and possibly to water molecules desexcitation or OH(B2+-A2+) emission. SL from alkali-metal salt
solutions exhibits emission from the excited alkali-metal atoms.[8] Such kind of SL was explained by
nanodroplet injections into the hot cores of collapsing bubbles. Recently, the SL of lanthanide ions was
reported in concentrated (0.5-1.0 M) lanthanide salt aqueous solutions under the effect of 20 kHz
ultrasound.[9] Sharipov et al.[9a] suggested that the SL of lanthanides would be governed by two major
processes: (i) sonophotoluminescence, when the excitation of Ln(III) ions occurs through the light
emitted from cavitation bubbles during their collapse and (ii) collisional excitation of Ln(III) ions by
radicals or excited molecules at the bubble/solution interface. However, the mechanism of metal ions
ultrasonic excitation is still under debate. Herein, we report for the first time a study of uranyl ion SL in
acidic solutions of HClO4 and H3PO4 under the effect of 20, 203 and 607 kHz ultrasound.
In HClO4 solutions uranyl ions exhibit extremely weak SL. Figure 1a presents the SL spectra at
203 kHz of 0.2M HClO4 and 0.1M UO22+ in 0.2M HClO4 aqueous solutions. The SL spectrum of the
HClO4 solution without uranium fits well with SL obtained in pure water under the same conditions.[7c]
In the presence of uranium the SL spectrum clearly shows the typical light absorption by UO22+ ions
below 500 nm whatever the used frequency. However, emission lines from excited (UO22+)* centred at
512 nm and 537 nm – moreover very weak - are observed only at 203 kHz ultrasonic frequency which
also exhibits the most intense SL from sonicated water in the studied range of ultrasonic frequencies. [7c]
The SL spectra obtained in HClO4 solutions at 20 kHz and 609 kHz are shown in Fig. S2 of Supporting
Information. The 2-fold dilution of UO22+ solution causes these emission lines not to be discernable
anymore from the continuum. In contrast to the ultrasonic excitation, the photoexcitation (λ = 427 nm)
of UO22+ in HClO4 solutions yields relatively strong uranium fluorescence (inset of Figure1a) in full
agreement with data published elsewhere.[1] It can thus be concluded that the SL of UO22+ is effectively
quenched in HClO4 solutions. Usually, the quenching of (UO22+)* in aqueous solutions is attributed to
the abstraction of a hydrogen atom from a water molecule coordinated to uranium: [10]
(UO22+)* + H2O → UO2+ + H+ + OH• (1)
2
The significant decrease in H2 formation rate during the sonolysis of HClO4 solutions in the presence of
UO22+ (Table 1) indicates that the excited uranyl ions can also be quenched by products of the
sonochemical water split:
H2O ―)))→ H + OH•
(2)
H + H → H2
(3)
OH• + OH• → H2O2
(4)
(UO22+)* + H2 → UO2+ + H+ + H
(5)
(UO22+)* + H → UO2+ + H+
(6)
(UO22+)* + H2O2 → UO2+ + H+ + HO2• (7)
UO2+ + OH• (HO2•, H2O2) → UO22+
(8)
where the symbol ″)))″ denotes a water molecule homolytic dissociation occurring inside a cavitation
bubble. A similar quenching phenomenon, by radiolytical products of water, is known for the
radioluminescence of uranyl ions.[2c]
Figure 1b reveals that, in contrast to HClO4 solutions, UO22+ in H3PO4 medium exhibits a strong
SL. Figure 1c demonstrates that UO22+ SL intensity is strongly influenced by the ultrasonic frequency.
As in HClO4 solutions, the strongest intensity of UO22+ SL is observed with 203 kHz ultrasound. In a
dark room the SL of UO22+ at 203 kHz can be seen by the unaided eye as a green-yellow glow (Figure
2). The light emission zone is layered indicating that there is a standing acoustic wave field inside the
sonochemical reactor: the highest intensity of UO22+ SL is emitted from the antinodal acoustic pressure
zones where the concentration of cavitation bubbles is highest.
The position of (UO22+)* emission lines in 0.5 M H3PO4 is independent from the ultrasonic
frequency and red-shifted by 4-5 nm compared to HClO4 solutions due to strong complexation of UO22+
by phosphate ions (Fig. 1c).[11] The formation of uranyl phosphate complexes causes an almost 200-fold
increase of (UO22+)* lifetime[1] as well as a 2-fold increase of UO22+ molar extinction coefficient (Figure
S2). Both parameters lead to an enhancement of UO22+ sonoluminescence, while the SL intensities of
the pure acids are of the same order of magnitude.
Figure 1c shows that the SL spectra of UO22+ are in general quite similar to its
photoluminescence spectrum (see inset). However, the relative intensity of UO22+ first emission line at
495 nm appears to be lower for SL than that for photoluminescence. This striking feature can be
explained by UO22+ SL self-absorption. The comparison of Fig. S3 and Fig. S4 reveals the overlapping
of UO22+ emission and absorption bands around 480-500 nm. In the case of UO22+ photoexcitation the
self-absorption is insignificant since the irradiation is performed with a highly intensive laser beam
using a relatively thin (1 cm) cell. By contrast, the excitation of UO22+ with power ultrasound occurs
within a reactor having an optical length of about 4 cm by means of light emitted by the cloud of
collapsing bubbles. Consequently, the self-absorption of UO22+ in the sonoreactor should be more
important than that in the case of photoexcitation. The comparison of UO22+ normalised emission
spectra defined as the difference between SL of UO22+ in H3PO4 and SL of pure H3PO4 under the same
conditions at different UO22+ concentrations (Figure S5) confirms the influence of UO22+ self-absorption
on the emission line at 495 nm: a 10-fold dilution of uranium causes the increase of its relative intensity
up to the fluorescence level.
Figure 3 exemplifies the evolution of the relative heights of UO22+ emission peaks, Ir, at 516 and
539 nm as a function of UO22+ concentration in 0.5 M H3PO4 under 203 kHz ultrasound. The Ir values
are obtained as:
I  I0
(9)
Ir  U
I 650
where Iu is the SL intensity at the considered wavelength (516 or 539 nm) of the uranyl solution in
H3PO4, I0 is the SL intensity of the acid solution without uranium at the same wavelength obtained at the
same conditions, and I650 is the intensity of the SL continuum at 650 nm. At the wavelength of 650 nm,
SL spectra of the sole acid and of UO22+ in H3PO4 perfectly superimpose. Besides, it has been checked
that the shape of the pure acid SL spectrum remained unchanged in the studied range of ultrasonic
3
intensities. Hence the SL intensity at 650 nm is proportional to the SL intensity in the optical range
where light is absorbed by uranyl. As can be seen in Figure 3a, the Ir values are practically independent
from UO22+ concentration in the range of 7·10-3-3·10-2 M. Further increase of UO22+ concentration
causes them to increase (Figure 3b). Such behaviour can be understood presuming two mechanisms of
UO22+ excitation with power ultrasound: sonophotoluminescence and direct UO22+ collisional excitation
at the interface of collapsing bubbles.
In general, the two-site model of the sonochemical reactions suggests that the ″hot″ particles
generated inside the cavitation bubbles mostly react in the thin liquid layer extending several hundred
nm from the bubble surface.[12] The high local concentration of such particles promotes their rapid
mutual deactivation in the liquid reaction zone rather than migration into the bulk solution. In this view
the direct excitation of non-volatile species with ultrasound could be achieved only at their relatively
high concentration. As an example, the Na SL is typically observed at [Na+]  0.1 M.[8] Therefore one
can assume that in more diluted solutions the observed UO22+ SL mostly arises from photoexcitation
with photons emitted by the cavitation bubbles. Such mechanism presumes that IU-I0 should be
proportional to the amount of photons absorbed by UO22+ ions. Figure 1 shows clearly total SL light
absorption by UO22+ at λ< 350 nm for [UO22+]≥1.5·10-3 M and at 400-430 nm for [UO22+]≥4.4·10-2 M.
In this case the sonophotoluminescence mechanism presumes that uranyl emission should not anymore
depend on UO22+ concentration, as is observed. The increase of SL intensity in more concentrated UO22+
solutions can be assigned to the contribution of UO22+ excitation mechanism by collisions with the ″hot″
particles (radicals, excited molecules) generated during the bubble collapse:
k1
P* + UO22+ → (UO22+)* → UO22+ + hν
(10)
k2
P* → Deactivation
(11)
where P* means ″hot″ particles, and their deactivation involves radical’s recombination, reactions with
scavengers, etc. This reaction pathway has some similarity with the mechanism of UO22+
radioluminescence.[2b] In the collisional excitation mechanism the reciprocal light emission by uranyl,
(IU-I0)-1, varies linearly with the reciprocal UO22+ concentration:
k
1
C
=1+ 2
(12)
IU  I 0
k1 UO22 
where C is a constant. In the studied system the contribution of the collisional mechanism is observed at
[UO22+]>0.035 M (Figure 3b). Note here that UO22+ SL cannot be studied in H3PO4 solutions at
[UO22+]>0.05 M due to the precipitation of insoluble uranyl phosphate. It is important to emphasize that
strong heating of UO22+ compounds yields non-luminescent uranium oxides.[11] Therefore, UO22+ SL
cannot be explained by local heating neither in the liquid reaction zone nor in the gas phase of the
cavitation bubbles as it was suggested for alkali metals.[12,13]
It is interesting that in spite of the essentially sonophotoluminescence origin of UO22+ excitation in
diluted solutions there is a striking difference between sono- and photoluminescence of uranyl ions. Our
study reveals that at 203 kHz ultrasound the SL intensity of UO22+ in 0.5M H3PO4 is increased by a
factor of at least 20 compared to that in 0.2M HClO4 solutions. At the same conditions the intensity of
UO22+ photoluminescence is increased only by a factor of 6 (Figure S4). This difference can be
explained by the strong contribution of the quenching processes in UO22+ sonoluminescence compared
to photoluminescence. When going from HClO4 to H3PO4 the replacement of water molecules with
phosphate ions in the inner coordination sphere of UO22+ reduces both the intramolecular quenching and
the intermolecular quenching by chemically active species generated in situ during aqueous solution
sonolysis. However, even in a strongly complexing medium some quenching of SL is clearly observed.
The decrease in the presence of UO22+ of H2 formation rate during the sonolysis of H3PO4 solutions
(Table 1) can be attributed to the quenching processes expressed by equations (5-6). Moreover, addition
of 10-2 M H2O2 to sonicated UO22+ solutions in H3PO4 causes a sharp drop (~40%) of Ir (Figure S6) in
agreement with reaction (7). These observations represent convincing evidence that the extremely low
UO22+ SL in non-complexing HClO4 medium is related to the quenching processes.


4
In summary, we have observed for the first time an ultrabright sonoluminescence of uranyl ions
in acidic aqueous solutions. The intensity of UO22+ SL is strongly influenced by the presence of
complexing agents, like phosphate ions, and by the ultrasonic frequency. The mechanism of UO22+ SL
seems to vary with uranium concentration: sonophotoluminescence dominates in diluted solutions, and
collisional excitation would add its contribution at higher UO22+ concentration. This study highlights the
importance of excited uranyl quenching processes by the products of solvent sonolysis. In light of these
findings, sonoluminescence of UO22+ ions exhibits a unique behaviour combining the properties typical
for both photoluminescence and radioluminescence.
Experimental Section
The thermostated cylindrical sonoreactor is mounted on top of a high-frequency transducer (203
or 607 kHz, 25 cm2, L-3 Communications ELAC Nautik). Ultrasonic irradiation with low frequency
ultrasound of 20 kHz is performed with 1 cm2 titanium horn (Vibra-Cell) placed reproducibly on top of
the reactor. The experiments are performed at 35 W (20 kHz) and 60-80 W (203 and 607 kHz) of loaded
power. Irradiated solutions (250 mL) are continuously sparged with argon (Air Liquide, 99.999%). The
cryostat temperature is set so as to have a steady-state temperature of 9-11°C within the sonoreactor
during the ultrasonic treatment. The emission spectra are recorded in the range 230-800 nm using a
spectrometer (SP 2356i, Roper Scientific; 300gr/mm blz. 300 and 150gr/mm blz. 500) coupled to a
liquid-nitrogen cooled CCD camera (SPEC10-100BR with UV coating, Roper Scientific). Each
spectrum is the average of at least three 300s spectra corrected for background noise and for the
quantum efficiencies of gratings and CCD. The multifrequency ultrasonic device is presented in the
Supporting Information.
Hydrogen in the outlet gas is measured simultaneously with the spectroscopic study using a
quadrupole mass spectrometer (PROLAB 300, Thermo Fisher). The Multiple Ion Monitoring mode is
used to follow the kinetics of H2 emission from irradiated solutions. Solutions of UO22+ are prepared
from UO3 samples as described in the Supporting Information.
The photoexcitation of UO22+ ions is performed at λ = 427 nm (E = 4 mJ) using an OPO laser
pumped with Nd-YAG laser (10 Hz). The photoluminescence spectra are collected by means of a Pro
300i spectrometer (600 gr/mm) coupled with CCD camera (Princeton Instruments). The UO22+
absorption spectra were measured with GBC spectrophotometer in 1 cm quartz Suprasil cells.
Acknowledgements
This work was supported by French ANR program (ANR-10-BLAN- 0810). V. Cousin thanks
CEA/DAM France for financial support. The authors thank L. Couston (CEA/DEN/DRCP/Marcoule)
and H.-P. Brau (ICSM) for valuable discussions, A. Arbona (Verre Equipements) for help building up
the setup, and J.-L. Jeannin (CEA/DEN/Marcoule) for making photographs.
______________________________________
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Figure legends
Figure 1. (a): Sonoluminescence spectra of 0.2M HClO4 (65 W) and 0.1M UO22+ in 0.2M HClO4 (81
W) at 203 kHz, 10°C, Ar. The inset shows the emission spectrum of 0.2M UO22+ in 0.2 M HClO4 after
photoexcitation at λ = 427 nm; (b): Sonoluminescence spectra of 0.5M H3PO4 and 5·10-2 M UO22+ in
0.5M H3PO4 at 203 kHz, 61W, 10°C, Ar; (c): Effect of the ultrasonic frequency on 5·10-2 M UO22+
sonoluminescence in 0.5M H3PO4. 20 kHz: 35 W, 10°C; 203 kHz: 82 W, 10°C; 609 kHz: 79 W, 14°C.
At 203 and 609 kHz the spectra were collected from the middle of the window. At 20 kHz the emission
spectra were collected in the top part of the cavitation cloud where (UO22+)* exhibits highest emission
intensity. Variation of loaded power in the range of 30-50 W has no significant influence on the
emission intensity at 20 kHz ultrasound. The inset shows the emission spectrum of 5·10-2 M UO22+ in
0.5 M H3PO4 after photoexcitation at λ = 427 nm.
6
Figure 2. Photograph of UO22+ sonoluminescence (3.1·10-2 M) in 0.5M H3PO4 at 203kHz, 63W, 10°C,
Ar, exposure time 30 sec. The picture is taken through the quartz window of the sonoreactor. The halo
around the major light emission zone originates from light reflection in the reactor’s cooling jacket.
Scale bar 2.5 cm.
Figure 3. Variation of (UO22+)* relative emission peak heights at 516nm and 539 nm as a function of
UO22+ concentration in 0.5 M H3PO4 at 203 kHz, 82W, 10°C, Ar. (a): 7.5·10-3 – 1.7·10-2 M UO22+, (b):
3.0·10-2 – 5.0·10-2 M UO22+.
Tables
Table 1. Hydrogen formation rate during the sonolysis of UO22+ solutions in HClO4 and H3PO4 at 203
kHz, 80 W. T = 10°C, Ar 80 mL·min-1, σ = ±15%.
Acid
[UO22+], M
W(H2), nM·sec-1
HClO4
0.05
18.0
(0.2M)
0.1
10.5
H3PO4
0.0072
17.0
(0.5M)
0.01
16.0
0.017
11.0
0.037
10.0
Table of Contents
Sonoluminescence
Ultrasound excites uranium: The uranyl ion exhibits
an ultrabright multibubble sonoluminescence in cooled
H3PO4 solutions submitted to high-frequency
ultrasound. The mechanism of UO22+ excitation seems
to
vary
with
uranium
concentration:
sonophotoluminescence
dominates
in
diluted
solutions, and collisional excitation would contribute
in concentrated UO22+ solutions.
Keywords: Uranium, fluorescence, sonoluminescence, sonochemistry, uranyl
7