A luminescence line-narrowing spectroscopic study of the uranium( VI ) interaction with cementitious materials and titanium dioxide †

Non-selective luminescence spectroscopy and luminescence line-narrowing spectroscopy were used to study the retention of UO 22+ on titanium dioxide (TiO 2 ), synthetic calcium silicate hydrate (C-S-H) phases and hardened cement paste (HCP). Non-selective luminescence spectra showed strong inhomogeneous line broadening resulting from a strongly disordered UO 22+ bonding environment. This problem was largely overcome by using luminescence line-narrowing spectroscopy. This technique allowed un-ambiguous identi ﬁ cation of three di ﬀ erent types of UO 22+ sorbed species on C-S-H phases and HCP. Comparison with spectra of UO 22+ sorbed onto TiO 2 further allowed these species to be assigned to a surface complex, an incorporated species and an uranate-like surface precipitate. This information provides the basis for mechanistic models describing the UO 22+ sorption onto C-S-H phases and HCP and the assessment of the mobility of this radionuclide in a deep geological repository for low and intermediate level radioactive waste (L/ILW) as this kind of waste is often solidi ﬁ ed with cement prior to storage.


Introduction
Cementitious materials are commonly used worldwide for the solidification of low-and intermediate level radioactive waste (L/ILW) prior to storage in surface or deep geological repositories. 1,2For an accurate prediction of the long term fate of this radioactive waste, a comprehensive understanding of the chemical interactions of the radionuclides present in the waste with the solidification material is essential.In the past, radionuclide retention by cementitious materials has been typically attributed to adsorption on the surfaces of cement minerals (e.g.Evans 3 and references therein).However, other immobilization processes, such as incorporation into the solid matrix, may take place.Incorporation processes give rise to the isolation of the radionuclide from the pore solution and may result in an irreversible immobilization in the solid matrix.The release of incorporated radionuclides is controlled by slow processes such as recrystallization and dissolution rather than by rapid surface desorption reactions.Calcium silicate hydrates (C-S-H) are major constituents of cementitious materials.They have a tobermorite-like layered structure consisting of Ca-O sheets linked on each side to silicate chains in a "dreierkette" arrangement. 4These calcium silicate layers carry a net negative charge neutralised by either protons or Ca 2+ cations in the interlayer.C-S-H phases are further characterized by high recrystallization rates making them an ideal system for radionuclide incorporation.6][7][8][9] It is still an open question whether or not larger actinyl ions such as UO 2 2+ exhibit similar incorporation behaviour.
Uranium is an important component of intermediate-level radioactive waste and the isotopes of this radionuclide contribute significantly to its long-term dose. 10,11Under the typical redox conditions prevailing in the alkaline environment of the cementitious near-field of an ILW repository (E h = −0.23 V), 12 uranium exists predominantly in the valence state +VI as the linear dioxo uranyl ion, UO 2 2+ .
][15] Furthermore, sorption was found to be linear over a wide concentration range and highly dependent on the pH and the C-S-H composition.Indeed, measured solid-liquid distribution ratios (R d ) varied between 10 6 L kg −1 at pH = 10.0 and 10 2 L kg −1 at pH = 13.3. 14,15Tits et al. 14 further demonstrated that, in the case of NpO 2 2+ , the observed dependence of R d values on the C-S-H composition can almost entirely be explained by assuming competition between the sorption process and the formation of non-sorbing highly hydrolysed anionic NpO 2 (OH) 4 2− species in solution. 14veral attempts have been made to decipher the local coordination environment of UO 2 2+ sorbed onto cementitious materials with the help of X-ray absorption spectroscopy (XAS) [16][17][18][19][20] and luminescence spectroscopy. 15These studies revealed (1) the existence of at least two types of UO 2 2+ sorbed species, (2) a sorbed UO 2 2+ structural environment dominated by O, Si and Ca atoms arranged in a similar way as in calcium uranyl silicates.The weak back-scattering contributions from neighbouring Si and Ca atoms of the C-S-H structure in the XAS spectra, however, prevented an unequivocal identification of the coordination environment and thus the determination of the dominant sorption process (surface complexation or incorporation, respectively).In addition, if more than one type of species is present, XAS data can only provide an averaged coordination environment.Hence, there is a need for complementary spectroscopic information about the coordination environment of sorbed UO 2 2+ in C-S-H phases with the aim of providing conclusive evidence for the significance of incorporation as an immobilization process for hexavalent actinides in cementitious materials.Conventional non-selective luminescence spectroscopy is commonly applied to study the speciation of UO 2 2+ in both crystalline and amorphous solids (e.g., ref. 15, 21-28).However, this technique often gives rise to severe spectral inhomogeneous broadening effects, thus hiding valuable spectroscopic information from electronic and vibronic transitions. 15,29,30Inhomogeneous broadening occurs when UO 2

2+
ions occupy sites in amorphous host matrices or at the surface of solids. 15,21,26,27In both cases the local coordination environment of the luminescing species is ill-defined resulting in a significant variation of the energies of states of the sorbed UO 2 2+ ions.Although the electronic transition line for each single sorbed UO 2 2+ ion may be sharp, the position of the electronic transition line and the spacings between the vibronic lines will vary among non-equivalent sorbed UO 2 2+ ions, which results in inhomogeneously broadened overall electronic and vibronic transition bands composed of all the sharp lines of each single sorbed UO 2 2+ ion.0][31][32][33][34] This well-established technique is called luminescence line-narrowing spectroscopy (LLNS) and has been successfully applied for the detection and characterization of organic molecules and biomolecular systems (e.g.Jankowiak 35 and references therein).A narrow-band tunable laser is used to selectively excite a small subset of a set of UO ions can be probed individually and the variation of the optical properties within the set of luminescing ions can be evaluated.
The aim of the present study is to determine the speciation of UO 2 2+ taken up by C-S-H phases in cementitious materials.
To achieve this goal, the optical properties of UO 2 2+ sorbed on C-S-H phases and hardened cement paste (HCP) on the one hand side and of UO 2 2+ adsorbed onto the surface of titanium dioxide (TiO 2 ) on the other hand side, were investigated using LLNS.7][38] Furthermore, TiO 2 is a solid phase known to be stable under alkaline conditions and, in contrast to C-S-H phases, it is characterized by a low solubility and a low recrystallization rate. 39Therefore, UO 2

2+
incorporation into the structure of this mineral is unlikely, thus making it an ideal solid to study the optical properties of UO 2 2+ surface complexes under alkaline conditions.Differences in the luminescence spectra of UO 2 2+ sorbed on TiO 2 , C-S-H phases and HCP will enable us to determine whether or not this cation is incorporated in the C-S-H structure.

Materials
All solutions and suspensions were prepared using reagentgrade chemicals and deionized, decarbonated water (Milli-Q water) generated by a Milli-Q Gradient A10 water purification system (Millipore Billerica, USA).Teflon containers and centrifuge tubes used for the preparation of the samples, were washed, left overnight in a solution of 0.1 M HCl, and thoroughly rinsed with deionized water prior to use.Sample preparation was carried out in a glovebox under an inert nitrogen atmosphere (CO 2 and O 2 <2 ppm).

Analytical techniques
Solution compositions were determined using an Applied Research Laboratory ARL 3410D inductively coupled plasma optical emission spectrometer (ICP-OES) or a Perkin Elmer ELAN 6100 inductively coupled plasma mass spectrometer (ICP-MS).A combination glass pH electrode (Metrohm, Switzerland) calibrated against dilute standard pH buffers ( pH = 7.0-11.0)was used to determine the pH.

Sample preparation
An artificial cement pore water (ACW-I) was prepared based on an estimate of the NaOH and KOH concentration in a Portland cement porewater before any degradation of the material had occurred. 40It contains 0.18 M KOH and 0.114 M NaOH and has a pH of 13.3.A C-S-H phase with a Ca : Si mol ratio (C : S) of 1.07 was synthesized in ACW-I using a method adapted from Atkins et al.: 41 Silica fume (AEROSIL 300, Degussa-Huls AG, Baar Switzerland) was mixed with CaO in a polyethylene bottle at a weight ratio of 1 : 1.To this, ACW-I was added to achieve a solid to liquid (S : L) ratio of 2.5 × 10 −2 kg L −1 .A detailed characterization of this C-S-H phase is given in Tits et al. 42 After an ageing period of 4 weeks, the C-S-H suspension was ready for further use in sorption experiments.
A sulfate-resistant Portland cement (CEM I 52.5 N HTS) provided by Lafarge (France), was used to prepare the HCP samples.A modified ACW (ACW-II) 40  TiO 2 was obtained from Materion Advanced Chemicals Inc., Milwaukee, USA.This material is a mixture of 90% rutile and 10% anatase and has a specific surface area of 5 ± 1 m 2 g −1 . 43TiO 2 suspensions were prepared by mixing the appropriate amounts of TiO 2 powder with ACW-I to obtain a S : L ratio of 2.5 × 10 −2 kg L −1 .The suspensions were equilibrated for 1 day, centrifuged at 4000 rpm for 1 hour, and the supernatant solution replaced with fresh ACW-I.This washing procedure was repeated several times until the composition of the supernatant solution analysed by ICP-OES was constant and the TiO 2 was in equilibrium with the ACW-I solution.5][46] The subsequent radiative relaxation gives rise to an electronic transition line, E, having a frequency of radiation, ν E , in the range 21 000 cm −1 < ν E < 19 000 cm −1 .UO 2 2+ emission spectra are further characterized by a pronounced vibronic structure originating from the resolution of the vibrational energy levels in the ground state.In general, the totally symmetric stretch vibration (ν s ) of the UO 2 2+ moiety is the most pronounced vibration mode and it appears as a series of homologous lines, S 1-4 , (vibronic progression) superimposed on E (e.g.Fig. 2, spectrum a and b in the results section).The spacing (Δ) between E and the first line of this series, S 1 , results in values for ν s in the ground state as follows: ν s = ∼870 cm −1 for the UO 2 2+ aquo ion 47,48 and 700 cm −1 < ν s < 800 cm −1 for uranium minerals (silicates, carbonates) and UO 2 2+ sorbed species. .In this diagram, the electronic ground state (0,0) and the first electronic excited state (1,0) of UO 2 2+ in an ill-defined coordination environment (e.g. an amorphous matrix) are shown.Each of these electronic states supports several vibrational energy levels (vibronic states) while, to simplify the diagram, only the first of them is shown for each electronic state (0,1) and (1,1).The variation of the energies of the excited state levels within the inhomogeneously broadened absorption band is represented by the slope, Γ inh (Fig. 1).Two different cases can be identified 50 : (1) Resonant luminescence line-narrowing (red color in Fig. 1): A small subset of UO 2 2+ ions (A 1 ) is directly excited to the lowest vibrational level of the first excited state (1,0) using an energy, ν ex , in the inhomogeneously broadened absorption band.These excited ions decay radiatively directly to the ground state.The frequency, ν E(1) , of the resulting narrow electronic transition line (E(1)), is identical to the frequency of the exciting laser light (ν ex = ν E1 ).
(2) Non resonant line-narrowing (blue color in Fig. 1): Using the same excitation energy, ν ex , another small subset of UO  51,52 This process is possible provided that the emission spectrum of the donor UO 2 2+ ion overlaps the absorption spectrum of the acceptor UO 2 2+ ion so that several vibronic transitions in the donor have similar energies as the corresponding transitions in the acceptor; i.e. the transitions are in resonance.This energy transfer results from long-range dipole-dipole interactions between donor and acceptor and not from the transfer of a photon.Following energy transfer, the acceptor ions may decay radiatively themselves or transfer their excitation energy to other nearby unexcited ions.The energy transfer efficiency depends on the extent of overlap between the donor emission spectrum and the acceptor absorption spectrum and on the donor-acceptor distance with an inverse 6 th power law.Homoresonance energy transfer occurs in samples with high concentrations of luminescence centers and is effective at donoracceptor distances up to 10 nm for systems with large absorption coefficients (ε) and high luminescence quantum efficiencies (Φ 0 ). 52Absorption coefficients and quantum yields for Luminescence spectroscopy was performed using a pulsed OPO laser system (Spectra Physics MOPO HF).Indirect excitation of UO 2 2+ was performed at an excitation energy of 24 390 cm −1 .The range used for the direct excitation was 21 270 cm −1 < ν ex < 17 857 cm −1 .The luminescence signal was detected by an optical multichannel analyzer consisting of a Czerny Turner 300 mm polychromator (Jobin Yvon, HR 320 with gratings ranging from 300 to 1200 lines mm −1 ) and an intensified, gated photodiode array (Spectroscopy instruments, ST 180, IRY 700G) for the experiments with C-S-H phases and HCP, or a Czerny Turner spectrometer (Andor Shamrock with gratings ranging 150/mm-2400/mm) coupled to an intensified ICCD (Andor Technologies) camera optimized for high on off ratio by use of a PROXITRONIC image intensifier for the experiments with TiO 2 .The laser light and Rayleigh and Raman scattering are filtered from the luminescence emission spectra by using a pulsed laser source and applying a minimum gate delay of 10 μs between the laser pulse and camera gating.
For the determination of the luminescence lifetime, luminescence decay profiles were recorded with a delay time step of 5 μs or 25 μs and a total of 90 steps were collected for each decay profile.thermodynamic database 53 showed that the speciation of U(VI) in ACW is dominated by the strongly hydrolyzed UO 2 (OH) 4 2− complex.The uranyl-doped C-S-H phases exhibit weakly structured spectra similar to those reported in an earlier paper. 15he UO 2 2+ luminescence spectra are red shifted upon hydrolysis.This red-shift is even more pronounced for UO 2 2+ sorbed on C-S-H phases.It is caused by a significant weakening of the axial UvO bonds upon sorption due to the increased electrondonating abilities of the ligands in the equatorial plane of the sorbed UO 2 2+ ions. 15,48,54e broad luminescence bands indicate strong variation in the bonding environment of the sorbed UO 2 2+ ions.At each excitation energy, a full emission spectrum was recorded and the luminescence intensity was integrated over the emission energy range between 22 000 cm −1 and 16 000 cm −1 .The excitation spectra are poorly resolved due to the effect of inhomogeneous broadening and do not provide much spectral information.In the following, the excitation spectra of UO 2 2+ doped samples will therefore only be shown for completeness of the datasets, but they will not be considered for the spectral analysis.Two series of luminescence spectra were recorded (Fig. 3).A gate delay time of 10 μs was used in both series to filter out the emission interference originating from Rayleigh and Raman scattering.The gate width of the camera was fixed to 10 μs in the first series and 10 ms in the second series.UO 2 2+ species with a shorter luminescence lifetime will dominate the spectra recorded with the small gate width whereas UO 2 2+ species with a longer lifetime will prevail in the spectra recorded with large gate width.The spectra of the first series are again characterized by broad, weakly resolved luminescence bands without any pronounced structure (Fig. 3a).The absence of a sharp luminescence line resonant with the energy of the laser excitation in these LLN spectra, is a strong indication for the presence of homo resonance energy transfer processes between adjacent sorbed UO 2 2+ moieties.The Förster distance (the distance between donor and acceptor at which the energy transfer efficiency is 50%) for UO 2 2+ in C-S-H phases was estimated using the calculation procedure described by Hink et al. 55 and resulted in a value of 1.1 nm (see ESI †).This observation suggests that at least one luminescent UO 2 2+ species in the UO 2 2+ doped C-S-H phases is present as a (surface) precipitate with neighbouring UO 2 2+ ions at distances closer than ∼1.1 nm.Calculations using the NEA thermodynamic data for uranium 53 completed with solubility data for alkali uranates of Yamamura et al. 56  by the C-S-H phase.On the other hand side, U-U distances in uranates are in the range 3.5 Å < R U-U < 6.6 Å; 58 i.e., close enough for efficient homo resonance energy transfer.

LLN spectroscopy of UO
The second series of luminescence spectra recorded with a larger gate width of 10 ms are characterized by a series of sharp luminescence bands in the frequency region 20 500 cm −1 < ν < 19 000 cm −1 .These spectra indicate the presence of at least one more UO 2 2+ sorbed species with a longer lifetime.
Furthermore, the spectra of this UO 2 2+ sorbed species appear to be less disturbed by energy transfer processes suggesting a more homogenous distribution of this UO 2

2+
sorbed species in the C-S-H sample.The first sharp line in each of these spectra is the luminescence line resonant with the laser excitation energy.It represents the electronic transition line of a subset of UO 2 2+ ions directly excited by the applied laser energy.The position of this line shifts with ν ex .
The other sharp transition lines represent either non-resonant electronic transitions or vibronic transitions coupled to the electronic transitions.
A detailed spectral analysis was undertaken with the help of three typical emission spectra selected from the second series of spectra recorded with a large gate width (Fig. 4).The spectra were selected to include the spectrum measured with a high λ ex , a spectrum measured with an intermediate ν ex (arbitrarily chosen) and a spectrum measured with a low ν ex having sufficient intensity to give an exploitable spectrum.The bands in each spectrum are assigned based upon their energy relative to the resonance line.The interpretation is based upon the Jablonski diagramme described in Fig. 1 and the assignment of the lines is summarized in Table 1.
The first line ( 1) is always the luminescence line resonant with the laser energy and represents the electronic transition (E(1)) of a small subset of UO 2 2+ sorbed species, U1.A series of homologous lines (vibronic progression) having a spacing, Δ, between 700 and 800 cm −1 , is associated with this electronic transition.These vibronic lines result from the coupling between the electronic transition and the totally symmetric stretch vibration in the ground state, ν s (1), of this subset.Indeed, the first line of this homologous series appears to be line (4) at a spacing Δ 1-4 = 758 cm −1 (Table 1).The other lines of this series are not resolved.Lines ( 2) and ( 3) in the spectra must represent non-resonant electronic transition lines (E( 2) and E( 3)) of two more subsets of UO 2 2+ sorbed species.The first members of the vibronic progressions on each of these electronic transitions are the lines ( 6) and ( 7), with spacings Δ 2-6 = 744 ± 10 cm −1 and Δ 3-7 = 758 ± 10 cm −1 , respectively.There are two possible interpretations for the presence of the two non-resonant electronic transitions lines ( 2) and ( 3): both subsets belong to the same set of UO 2 2+ sorbed species or both subsets belong to two entirely different sets of UO 2 2+ sorbed species.
(2) Both subsets belong to two entirely different sets of UO 2 2+ sorbed species (U1 and U2) with clearly different coordination environments.In this case, the non-resonant lines ( 2) and ( 3) result from excitations to the second vibrational level of the first excited state of a subset of U1 and a subset of U2, respectively.The spectral characteristics of these two species are listed in Table 2.The main spectral difference between these two sets of UO 2 2+ sorbed species is the frequency of the symmetric stretch vibration in the excited state, ν′ s , being 406 ± 10 cm −1 (Δ 1-2 ) and 632 ± 10 cm −1 (Δ 1-3 ) for U1 and U2, respectively.
The values of ν s and ν′ s found for the sorbed species U1 and U2 are significantly smaller than the values found for the UO 2 2+ aquo ion and the UO 2 (OH) 4 2− hydroxo species indicating a strong weakening of the axial UvO bond 54 in agreement with the red-shift of the electronic transition line observed in the non-selective luminescence spectra in Fig. 2.This axial UvO bond weakening results from increased σ and π donating Gate width = 10 ms.The numbered transition lines are described in the text and in Table 1.Red = lines assigned to species U1, blue = lines assigned to species U2.

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abilities of the ligands coordinated to the U atom in the equatorial plane indicating strong bonding of the UO 2 2+ upon sorption. 54he LLN spectrum recorded at an excitation energy of 18 868 cm −1 exhibits a completely different shape (Fig. 4).Similarly to the first series of LLN spectra recorded with short gate width (Fig. 3a), this spectrum is characterized by a few broad, weakly resolved luminescence bands without any pronounced structure.Furthermore, it does not contain a resonant electronic transition line.These spectral features are again a strong indication for the occurrence of energy transfer processes suggesting the presence of a (surface) precipitated UO 2 2+ species, U3.This observation confirms the assumption made for the interpretation of the first series of LLN spectra recorded with a short gate width.The strong red-shift of the spectrum of this surface precipitate is in agreement with luminescence spectra of alkaline-earth uranates described in the literature. 59,60The poorly resolved transition bands of this spectrum don't allow the spectral characteristics of this species, ν s and ν′ s to be determined.The spectral characteristics of the three sorbed UO 2 2+ species, U1, U2 and U3, deduced from the LLN spectra, are summarized in Table 2.In Fig. 5, the values of ν s and ν′ s for U1 and U2 obtained from the spacings (Δ) between the different transition lines in all the luminescence spectra recorded after excitation with increasing ν ex , are plotted against ν ex .This plot reveals that the frequencies of ν s and ν′ s for U1 and U2 decrease with increasing ν ex .This means that the UvO axial distances of the subsets of U1 and U2, measured at higher excitation energies, are longer than the UvO axial distances of the subsets measured at lower excitation energies.This observation is in agreement with the decreasing values of ν E for these subsets of U1 and U2 with increasing ν ex and is an expression of the variability of the bonding environments within the two types of species, U1 and U2.
In order to further check the existence of the postulated three different types of sorbed UO 2 2+ species U1, U2 and U3, the lifetimes of the different transition lines in the luminescence spectrum obtained after excitation at an energy of 19 763 cm −1 were determined.Transition lines with the same lifetime are attributed to the same type of UO 2 2+ sorbed species, whereas transition lines with different lifetimes are attributed to another type of sorbed species.Decay profiles measured for the three sharp lines (1, 2, and 4 in Fig. 4) and the broad band (8 in Fig. 4) at higher energy are very similar; i. e., they can be fitted with a bi-exponential decay model suggesting the presence of at least two species with lifetimes of 108 ± 31 μs and 275 ± 31 μs, respectively (Fig. 6a, Table 3).The short lifetime is attributed to the uranium (surface) precipitate, U3, as this species is also dominantly present in the series of spectra recorded with a short gate width (Fig. 3a).This lifetime corresponds very well with the lifetime of 100 μs measured at 4.2 K by De Jong et al. 60 for MgUO 4 (s), an uranate having a structure very similar to the structure of the Cauranate (CaUO 4 (s)) expected to precipitate at the surface of C-S-H phases in the present system.
Unfortunately, the difference in lifetimes of the two other species is too small to be detected with a decay profile analysis.However, a close inspection of the spectra recorded at different delay times, normalized to the resonant luminescence line, clearly shows that line (1) and line (4) decay at exactly the same rate, whereas line (2) exhibits a slower decay (the line rises relative to the first line with increasing delay time) (Fig. 6b).Furthermore, the broad band (8) at lower frequency exhibits a faster decay (the band decreases relative to the first line with increasing delay time).This observation clearly confirms that transition lines (1) and ( 4) belong to the same set of UO 2 2+ sorbed species (U1) whereas transition line (2) belongs to an additional set of species (U2) with a longer luminescence lifetime relative to that of U1.The longer lifetime indicates that non-radiative de-excitation processes for this set of species are less efficient due to the presence of a lower number of quenchers (e.g.H 2 O molecules) in its proximity.The broad transition band at higher frequencies belongs to the (surface) precipitate, U3, with a shorter luminescence lifetime relative to that of U1.In summary, LLN spectroscopy of UO 2 2+ sorbed on a C-S-H phase in ACW at pH = 13.3 allowed three different sets of  species, U1, U2 and U3 to be identified.One of these species (U3) is a (surface) precipitate.This conclusion is in agreement with the results from previous time-resolved non-selective luminescence studies on a large number of C-S-H phases with varying compositions. 15mparison with UO 2 2+ surface complexation on TiO in the presence of ACW at pH = 13.3 were recorded with a delay time of 10 μs and using a gate width of 10 ms (Fig. 7a).
Reducing the gate width to 10 μs did not have a significant effect on the shape of the LLN spectra.This is in contrast to the observations made with samples containing UO ions is ∼3 nm.A rough estimate of the Förster distance in this system gives a value of ∼0.7 nm (see ESI †), significantly shorter than the distance of 3 nm calculated based upon the UO 2 2+ loading on the solid.Hence, homo resonance energy transfer cannot explain the line broadening observed in this sample.We don't have an explanation for this observation at present.At low excitation energy (18 868 cm −1 ) only the laser line is visible in the luminescence spectra.In contrast to the sample with UO 2 2+ sorbed on C-S-H phases, no luminescence signal is detected after excitation at this energy, confirming the absence of an uranate-like precipitate.LLN spectra of UO 2 2+ sorbed on HCP again exhibit better resolved spectra (Fig. 7b).A clear electronic transition line can be identified in each spectrum, followed by several additional well-resolved narrow bands.At high excitation energy (18 868 cm −1 ) a luminescence spectrum similar to that for the UO 2 2+ loaded C-S-H sample is observed indicating the presence of an uranate-like (surface) precipitate.For further interpretation of the bands in these two samples, LLN spectra of UO 2 2+ -C-S-H, UO 2 2+ -TiO 2 and UO 2 2+ -HCP recorded after excitation at 506 nm are compared in Fig. 8.The numbers used for the transition lines are the same as given in Fig. 4. Species U1 (red vertical lines) is found in the spectra of all three samples, although the spacing, Δ 1-4 , is slightly larger for the U1 species on TiO 2 compared to the C-S-H sample and the HCP sample.We thus conclude that species U1 is a surface complex.Species U2 (blue vertical lines) is only present in the spectra of the C-S-H sample and the HCP sample and completely absent in the spectra of the TiO 2 sample.The bands are sharp and well resolved indicating that energy transfer is not taking place so that this species must be homogeneously distributed in the C-S-H/HCP matrix.The absence of this sorbed species in the TiO 2 sample, its homogeneous distribution in the C-S-H/HCP matrix and its longer lifetime suggesting a lower number of quenchers in close proximity, leads to the conclusion that these bands must belong to a UO 2 2+ species incorporated in the C-S-H structure in both samples.The broad band typical for the uranate-like precipitate species, U3, in the frequency range, 18 500 cm −1 < ν < 17 000 cm −1 is only visible in the C-S-H and HCP samples and is completely absent in the TiO 2 sample.This uranate-like solid phase must therefore be a Ca-uranate-like surface precipitate only on the Ca-rich C-S-H surfaces which are present in both the C-S-H and HCP samples.

0. 1
mL of a 1.08 × 10 −2 M uranyl nitrate solution in 10 −3 M HNO 3 was added to 40 mL of the C-S-H, HCP and TiO 2 suspensions.The UO 2 2+ doped suspensions were shaken end-overend for 30 days.After this equilibration period, phase separation of the solid and liquid phase was carried out by centrifugation at 4000 rpm for 1 hour.The resulting wet pastes were transferred to a self constructed copper sample holder with a sapphire window, sealed with a Teflon disk and cooled to <15 K by a helium refrigerated cryostat (CTI-cryogenics, USA) prior to luminescence measurements to reduce homogeneous spectral broadening.Determination of the UO 2 2+ concentration in the supernatant solution by ICP-MS showed that >95% of the UO 2 2+ added to the C-S-H suspension, was retained on the solid phase resulting in a UO 2 2+ loading of ∼250 ppm (∼10 −3 mol kg −1 ).
induced via charge-transfer excitation involving the transfer of an electron from the σ u orbitals (mainly of the axial O 2− ) to 5f δ and 5f ϕ orbitals of the U 6+
UO 2 2+ luminescence strongly depend on the surrounding matrix, but they are generally quite low resulting in much smaller maximum distances for resonance energy transfer.Resonance energy transfer processes result in broadening of the transition lines.In extreme cases (e.g., the formation of UO 2 2+ precipitates), the resonant electronic transition line in LLN spectroscopy may completely disappear.The UO 2 2+ luminescence lifetime depends partially on the presence of quenching ligands near the UO 2 2+ moiety.Shorter lifetimes are an indication for increased quenching.E.g. the luminescence of UO 2 2+ clusters exhibiting concentration quenching will typically be characterized by a shorter luminescence lifetime.Excited UO 2 2+ species typically decay monoexponentially and multi-exponential luminescence decay is often an indication for the presence of more than one luminescing species in a sample.
Fig. 2c shows the normalized luminescence spectrum of uranyl ions sorbed onto C-S-H phases with a C : S ratio of 1.07 in ACW at a pH of 13.3 after indirect laser excitation at ν ex = 24 390 cm −1 .This spectrum is compared with the spectra of the free UO 2 2+ ion in 0.1 M HNO 3 and an uranyl solution in 0.5 M tetramethyl ammonium hydroxide (TMAOH) at pH = 13.7.Thermodynamic calculations performed using the NEA chemical 2 2+ doped C-S-H phases For the study of the sorption of UO 2 2+ onto C-S-H phases with LLN spectroscopy, three experimental parameters were varied: (1) The excitation energy (ν ex ): Variation of ν ex allows individual excitation of different subsets of UO 2 2+ luminescence centers within the inhomogeneously broadened electronic transition band, (2) the gate width (time period over which the luminescence signal is recorded) of the photodiode array: A change in the spectral shape with varying gate width is an indication for the presence of species with different lifetimes, (3) the delay time after the laser pulse: Luminescence signals recorded after increasing delay times after the laser pulse can be used for the construction of decay profiles allowing the determination of the lifetime of the excited state of a luminescing UO 2 2+ species.Excitation spectra and LLN spectra of UO 2 2+ sorbed on C-S-H phases in ACW-I at pH 13.3 were obtained by varying ν ex within the range of the inhomogeneously broadened electronic transition band (ν E ) observed in the non-selective emission spectrum (21 000 cm −1 -18 000 cm −1 ; Fig. 2, spectrum c).

Fig. 5
Fig. 5 Values for the UO 2 2+ totally symmetric stretch vibration in the ground state, ν s , and in the excited state, ν' s , for subsets of the sorbed UO 2 2+ species U1 and U2, as function of the excitation energy, ν ex .
35 reduce the effect of inhomogeneous broadening, LLN spectroscopy has been applied.The Jablonski diagram in Fig.1adapted from Jankowiak,35illustrates the principle of this technique applied to UO 2 2+