The C-S-H phases were prepared in 10 mL polycarbonate centrifuge tubes (Beckman Coulter, United States). The S/L ratio achieved in the samples was determined from the dry weight of the solids formed. After a two-week synthesis period, during which the centrifuge tubes were rotated in an end-over-end rotator (Stuart Rotator SB3, United Kingdom), aliquots of the respective GLU stock solutions were added. A non-radioactive GLU stock solution was prepared by weighing the appropriate amount of sodium gluconate (Sigma-Aldrich, United States) and dissolving it in deionized water at various concentrations for the batch sorption experiments. Additionally, a ¹⁴C-GLU tracer solution was added by diluting a stock solution of 3.7 MBq/mL ¹⁴C-GLU (Hartmann Analytic, Germany) in deionized water. The contact time of GLU with the C-S-H phases was 72 h.

A first batch sorption experiment was performed as a sorption isotherm of GLU (S/L = 5 g/L) over a concentration range of 1 × 10⁻⁹ to 1 × 10⁻¹ M. In the second experiment, the S/L ratio was varried (S/L = 0.5–50 g/L) at a constant initial GLU concentration of 1 × 10⁻² M. Finally, the samples were pre-centrifuged at 3,770 × g (SIGMA 3K30, SIGMA Laborzentrifugen, Germany) for 5 min and ultracentrifuged at 108,800 × g (Avanti J-30I, Beckman-Coulter, United States) for 1 h. The final pH of each suspension was determined using an inoLab pH/Cond 720 (WTW, Germany) pH-meter, which was connected to a BlueLine 16 pH electrode (Schott Instruments, Schott AG, Germany), with 3 M KCl (Mettler-Toledo AG, Switzerland) as filling solution. A three-point calibration was carried out using reference buffer solutions with the pH values 6.87 (SI Analytics, Germany), 4.01 and 9.18 (Merck, Germany). LSC (Hidex 300 SL, Hidex, Finland) was used to analyze the supernatants of the batch sorption samples containing ¹⁴C-GLU. To 10 mL LSC cocktail Ultima Gold^TM XR (PerkinElmer LAS GmbH, Germany), 1-2 mL of the sample solutions was added. The samples were measured until a 2σ error of 2% was attained and with a resulting limit of detection (LOD) of 1 × 10⁻⁹ M for ¹⁴C.

XPS measurements were performed to determine the surface composition of the solids in presence and absence of GLU. After separation by ultracentrifugation, the C-S-H phases were dried under Ar atmosphere in a glove box at room temperature (RT) for at least 72 h. The sample powders were milled in an agate mortar and a small amount was pressed into an indium foil on a copper sample holder. The measurements were performed with a custom-built XPS system (SPECS GmbH, Germany) at RT and under a vacuum of 7 × 10⁻⁹ mbar. For photoelectron excitation, a non-monochromatic Mg Kα radiation (1,253.6 eV) from a high intensity double anode X-ray source (Al/Mg) XR-50 was used. Photoelectron detection was conducted with a constant analyzer pass energy of 13 eV using the PHOIBOS 100 hemispherical energy analyzer. The analysis of the spectra was carried out using CasaXPS (version 2.3.15). To calculate the corresponding atomic ratio n C a / n S i , the intensities I i of the Ca 2p and Si 2p signals were used (Eq. 1). The XPS measurement of a CaSiO₃ (Sigma-Aldrich, United States) standard resulted in the experimental sensitivity factor of S = 0.24. n C a n S i = I C a I S i ∙ S (1)

The Eu(III) samples were prepared in quartz tubes (Neubert-Glas GbR, Germany). For the retention experiments, different concentrations of EuCl₃∙6H₂O (Sigma-Aldrich, United States) and sodium gluconate (meets USP testing specifications, Sigma-Aldrich, United States) solutions were mixed at a neutral pH to form Eu(III)-GLU complexes. Afterwards, 1 mL of the mixtures was added to the C-S-H samples (after an initial formation time of 14 days) to yield a final S/L ratio of 33.3 g/L. The Eu(III) concentration in each sample was adjusted to 1 × 10⁻⁶ M or 5 × 10⁻⁵ M, along with eight different GLU concentrations between 5 × 10⁻⁶ M and 5 × 10⁻³ M. After preparation, the samples were equilibrated in an end-over-end rotator (REAX 20/4, Heidolph Instruments, Germany) for a contact time of 211 days. During that time, TRLFS measurements (cf. Section 2.2.2) at fixed time intervals were carried out to monitor the retention processes. Finally, six exemplary supernatants were subjected to GLU quantification by solution NMR spectroscopy (cf. Section 2.5).

As a reference experiment, 1 × 10⁻⁶ M Eu(III) was added at the very beginning of the C-S-H preparation process to foster the incorporation of Eu(III) into the C-S-H structure. After 14 days, GLU (5 × 10⁻⁶ M to 1 × 10⁻² M) was added and the samples were shaken for additional 4 weeks. Additional reference experiments were carried out in the C-S-H pore water. For this, the C-S-H phases were separated by filtration (Whatman 540, Cytiva, United States) after a formation time of 14 days. The liquid phase was mixed with stock solutions of GLU and Eu(III) to obtain samples with a Eu(III) concentration of 5 × 10⁻⁵ M and GLU concentrations between 2.5 × 10⁻⁶ M and 1 × 10⁻² M. The pH, which had decreased due to the presence of GLU, was adjusted to the intrinsic pH of the C-S-H pore water of 9.8 with approximately 3 µL of 7 M NaOH (Sigma-Aldrich, United States). The samples were equilibrated for 8 days before the measurements.

The Eu(III) samples were analyzed using TRLFS. The Eu(III) in the C-S-H samples was excited with a pulsed Nd:YAG laser (20 Hz, Quanta Ray, Spectra Physics, United States) combined with an OPO (GWU-Lasertechnik, Germany) set to 394 nm (using the ⁵L₆←⁷F₀ transition of Eu(III) for excitation). A spectrograph (Shamrock 303i, Andor Technology, Oxford Instruments, United Kingdom) equipped with a 600 lines/mm grating (blaze: 1,000 nm) and an ICCD camera (iStar DH720-18V-73, Andor Technology, Oxford Instruments, United Kingdom) were used to detect the emitted light in the spectral range of the ⁵D₀→⁷F₀ to ⁵D₀→⁷F₄ emission bands (574 nm ≤ λ_em ≤ 711 nm).

The pore water samples were measured with a Nd:YAG/OPO system operating at 10 Hz (Quanta Ray, Spectra Physics, Germany and GWU-Lasertechnik, Germany), a spectrograph (Kymera 328i, Andor Technology, Oxford Instruments, United Kingdom) with a 300 lines/mm grating (blaze: 760 nm) and an ICCD camera (iStar sCMOS, Andor Technology, Oxford Instruments, United Kingdom). Measurements of all samples were carried out using the box-car technique with an initial delay of 10 µs, a gate width of 1,000 µs and 100 different delays scaling with a linear increasing gate step. The C-S-H samples, equilibrated in an end-over-end shaker, rested for at least 60 min before measurements to allow the solid phase to settle. Both the solid and liquid phases were examined. The pore water samples were measured under constant stirring (PTFE stirring rods: Rotilabo, Carl Roth GmbH + Co. KG, Germany; stirring unit: CuV-O-Stir 333, HELLMA GmbH & Co. KG, Germany) to whirl up the precipitated solid into the light path. The limit of detection of the TRLFS setup used was approximately 5 × 10⁻⁸ M Eu(III).

The luminescence spectra were corrected for the spectral sensitivity of the grating and the ICCD camera. Parallel factor analysis (PARAFAC (Bro, 1997; Drobot et al., 2015) executed in Matlab 2019b (MATLAB, 2019, The MathWorks, Inc., Natick, MA, 2019.) with a monoexponential constraint in the time dimension and non-negativity in all other dimensions) was employed to deconvolute the time-resolved luminescence spectra at different GLU concentrations and contact times in order to obtain the emission spectrum, luminescence decay kinetics, and luminescence intensity of each Eu(III) species. From the deconvoluted spectra and the luminescence decay times, the Judd-Ofelt parameters and luminescence quantum yields of each species were determined. Assuming that all Eu(III) species are detectable and have similar extinction coefficients at 394 nm, the concentration c _i of the different species can be calculated from the luminescence intensities I _i obtained by the PARAFAC analysis and the luminescence quantum yields Φ _i according to Eq. 2: c i = I i ϕ i · ∑ j = 1 n I j ϕ j · c E u I I I   t o t a l (2)

Since the Eu(III) luminescence is quenched in the presence of O-H oscillators due to a radiationless energy transfer to O-H vibrations, the number of water molecules n(H ₂ O) in the first coordination sphere of the Eu(III) ions can be estimated from the luminescence decay time τ (Eq. 3), offering additional information about the coordination environment of the Eu(III) species (Marmodée et al., 2010): n H 2 O ≈ 1.09 τ   m s − 0.75 (3)

For these experiments, the C-S-H phases were prepared as described in Supplementary Table S1 (Supplementary Material). Stock solutions of ²⁴¹Am(III), ²³²Th(IV), and ²³⁹Pu(IV) were diluted to achieve the initial concentration of 1 × 10⁻⁸ M in the batch sorption experiments. A ²³²Th ICP-MS standard (SPS Science, Canada) was diluted in 2% HNO₃. The ²⁴¹Am(III) and ²³⁹Pu(IV) stock solutions were prepared as described elsewhere (Amayri et al., 2016) and subsequently diluted in 1 M HClO₄. A Na-GLU solution was also added in the experiments with an initial concentration of 1 × 10⁻² M, where the order of reactant addition was varied: i) An(X) was equilibrated with the C-S-H phase for 72 h before the addition of GLU to the suspension and subsequent equilibration for another 72 h, referred to as (C-S-H + An(X)) + GLU in the following text, ii) GLU was equilibrated with the C-S-H phase for 72 h before the addition of An(X) to the suspension and subsequent equilibration for another 72 h, (C-S-H + GLU) + An(X), and iii) simultaneous addition of An(X) and GLU to the C-S-H suspension followed by equilibration for 72 h, (C-S-H + An(X) + GLU). The phase separation was performed as described in Section 2.1.1. For Am(III) and Th(IV), also the influence of time was investigated, and further separations took place after 59 and 71 days, respectively.

The analysis of ²⁴¹Am(III), ²³²Th(IV) and ²³⁹Pu(IV) was performed with ICP-MS (Agilent 7900ce, Agilent Technologies, United States). For this, the samples were diluted in 2% HNO₃. As an internal standard an aliquot of ¹⁹³Ir in 2% HNO₃ was added ([Ir] = 100 ppt). The LOD for ICP-MS measurements of ²⁴¹Am(III), ²³²Th(IV), and ²³⁹Pu(IV) were 1 × 10⁻¹¹ M, 3 × 10⁻¹¹ M and 2 × 10⁻¹¹ M, respectively. The distribution coefficients R _d [L/kg] were determined by Eq. 4 using the sample volume V [L], the total mass of the solids m [kg], the initial concentration [An(X)]_0, and the equilibrium concentration [An(X)]_eq of the actinides. R d = V m ∙ A n X 0 − A n X e q A n X e q (4)

One additional ²³⁹Pu batch sample was prepared for extended X-ray absorption fine structure (EXAFS) measurements using the same experimental procedure as described before with two exeptions: First, the total Pu concentration in the batch sample was increased to 5 × 10⁻⁶ M to make the detection of an EXAFS spectrum feasible. With a S/L ratio of 2.5 g/L, this resulted in a sample loading of 494 ppm Pu on C-S-H (C/S = 0.8). Second, to avoid the precipitation of Pu(IV) colloids at pH 10.6 at the C-S-H phase, the initial Pu oxidation state was +III. As the Pu L₃-edge X-ray absorption near edge structure (XANES) spectrum showed, the initial Pu(III) was completely oxidized to Pu(IV). This also agrees with the redox conditions during the contact time of Pu in the C-S-H suspension with an E _h value of +29 mV (SHE). The Pu L₃-edge EXAFS measurement was performed at the BM20 Rossendorf Beamline (ROBL) (Matz et al., 1999; Scheinost et al., 2021) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Details of this measurement and the EXAFS data analysis are given in the Supplementary Material.

The complexation behavior of Th(IV) with GLU at pH 10 and the influence of Ca(II) were investigated by capillary electrophoresis (CE) coupled to ICP-MS. Four samples with 4 × 10⁻⁷ M Th(IV) were investigated at pH 10 and 0.1 M ionic strength. The pH and the ionic strength were adjusted using NaOH (VWR Chemicals, United States), HClO₄ (VWR Chemicals, United States), and NaClO₄ (Fluka Analytical, Germany), respectively. Solution 1 contained only Th(IV) and no GLU and no Ca(II). Solution 2 consisted of Th(IV) and 1 × 10⁻² M GLU. Sample 3 was prepared the same as sample 2 but in C-S-H pore water, i.e., with [Ca(II)] = 1.7 × 10⁻³ M (Häußler et al., 2018). To emphasize the influence of Ca(II) on the Th(IV)-GLU complexation, the Ca(II) concentration was increased. Therefore, solution 4 had the following composition: 4 × 10⁻⁷ M Th(IV), 1 × 10⁻² M GLU, and 1 × 10⁻² M Ca(II).

The experimental setup consisted of CE (Agilent 7,100, Agilent Technologies, United States) coupled to ICP-MS (Agilent 7900 ce, Agilent Technologies, United States) with a MiraMist CE nebulizer (Burgener Research, United Kingdom) leading into a Scott-type spray chamber (AHS Analysentechnik, Germany). The fused silica capillary (id. = 50 μm, Polymicro Technologies, LLC, United States) had length l of 50 cm. The applied voltage U was 10 kV. To each sample 2-bromopropane (Merck, Germany) was added as EOF marker. Measurements and data evaluation were performed as described by Willberger et al. (2019). The electrophoretic mobility µ _e was calculated according to Eq. 5 using the retention times of ²³²Th (t _Th ) and ⁷⁹Br of the EOF marker (t _EOF ), respectively: μ e = l 2 U ∙ 1 t T h − 1 t E O F (5)

A C-S-H phase was prepared as described in Supplementary Table S1 (Supplementary Material) in a 50 mL centrifuge tube (polypropylene, Greiner bio-one, Austria) and equilibrated using an end-over-end rotator (Stuart Rotator SB3, United Kingdom). The final pH value was 10.9. For phase separation, the suspension was centrifuged at 6,800 × g for 30 min (mod. Avanti J-20 XP, Beckman Coulter, United States), and the solid phase isolated. The powder was left to dry to constant weight under N₂ atmosphere at RT and stored inside the glove box. This U(VI)-free C-S-H phase was used together with the respective supernatant solution for U(VI) retention experiments in absence and presence of GLU. Using stock solutions of U_nat (1 × 10⁻³ M UO₂Cl₂ in 5 × 10⁻³ M HCl) and GLU (1 M, sodium gluconate (Sigma-Aldrich, United States)), samples for solid-state NMR ([U(VI)] = 5 × 10⁻⁶ M, [GLU] = 1 × 10⁻² M, S/L = 10 g/L) and TRLFS ([U(VI)] = 5 × 10⁻⁷ M, [GLU] = 1 × 10⁻² M, S/L = 1 g/L) measurements were prepared. For this, the order of reactant additions was varied (cf. Table 1).

A further C-S-H phase was prepared by using the same procedure as described above, however, immediately after mixing the reactants with deionized water, an aliquot of the U(VI) stock solution was added to the suspension. The initial U(VI) concentration was 2 × 10⁻⁵ M. During the equilibration time of 67 days, a U(VI)-doped C-S-H phase formed. The final pH value and U(VI) loading were 10.8 and 198 ppm, respectively. To study the influence of GLU on U(VI) leaching, aliquots of the GLU stock solution were added to the suspension of U(VI)-doped C-S-H (S/L = 10 g/L) to obtain 1 × 10⁻³ M or 1 × 10⁻² M GLU. The samples were equilibrated for 160 days.

After equilibrating the retention and leaching samples by shaking end-over-end, solid and liquid phases were separated by centrifugation, and aliquots were taken from the supernatant solutions for U(VI) concentration determination by ICP-MS (NexION 350X/Elan 9,000, PerkinElmer, United States) and for GLU quantification by solution NMR spectroscopy (cf. Section 2.5). Uranyl uptake by C-S-H is given in terms of a distribution ratio (R _d) according to Eq. 4.

For TRLFS measurements of U(VI)-containing C-S-H prepared in absence or presence of GLU, the samples were transferred as wet pastes into copper sample holders with sealable quartz glass lids and measured with a tunable diode pumped solid state (DPSS) laser (Ekspla, NT230, Lithuania) at 10 K. The emitted luminescence light was directed into a spectrograph (Shamrock 303i Andor Oxford Instruments, United Kingdom) equipped with a polychromator with 300, 600 and 1,200 lines/mm gratings, and the emission was monitored with an intensified CCD camera (Andor iStar, Oxford Instruments) 10 μs after the exciting laser pulse in a time window of 10 ms. The U(VI) associated with the C-S-H phase was excited at selected wavelengths (330–335 nm, 342 nm, 357 nm). Spectra were collected at varying delay times (up to 1,200 µs) between the laser pulse and camera gating to obtain luminescence decay kinetics and to extract pure component spectra of individual U(VI) species in the samples. More specifically, the decomposition of the spectra using the measured U(VI) emission data recorded at varying delay times was done with the Origin 2019b software (OriginLab Corporation, United States). The emission spectra recorded after long delay times were taken as pure components for U(VI) species with long luminescence lifetimes. The pure component spectrum was thereafter subtracted from the recorded emission data at shorter delays, yielding pure components of the U(VI) species with shorter luminescence lifetimes. Luminescence decay kinetics were obtained by integrating the collected emission spectra using MATLAB (R2019b) (Natick, Massachusetts: The MathWorks Inc., United States). Biexponential decay functions were thereafter fitted to the decay curves to obtain luminescence lifetimes of the present species in the samples.

²⁹Si and ¹³C cross-polarization (CP) and ²⁹Si single-pulse (SP) MAS NMR spectra were recorded on a Bruker AvanceTM 400 MHz WB spectrometer operating at a field strength of 9.4 T, with corresponding ¹³C, ²⁹Si and ¹H resonance frequencies of 100.7, 79.5 and 400.3 MHz, respectively, equipped with a CP/MAS DVT probe. Samples were packed in 7 mm zirconia rotors and spun at 5 kHz rotational frequency. ²⁹Si CP spectra were acquired with 5 ms contact time, using a 80% ramp, by accumulating 2048 spectra recorded for 25 ms applying a relaxation delay of 3 s. ¹³C CP spectra were acquired with 1 ms contact time, using a 70% ramp, accumulating 16 k spectra recorded for 25 ms applying a relaxation delay of 3 s. 200 ²⁹Si SP spectra were accumulated by respectively applying 5.6 µs (30°) excitation pulse, 25 ms acquisition time, and a relaxation delay of 360 s allowing for quantitative measurements. Chemical shifts are reported in ppm relative to external tetramethylsilane (TMS).

X-ray diffractograms of a U(VI)-free and a U(VI)-containing C-S-H sample with C/S of 0.8 were collected with a MiniFlex 600 diffractometer (Rigaku, Japan) equipped with a Cu Kα X-ray source (40 keV/15 mA operation for X-ray generation) and the D/teX Ultra 1D silicon strip detector in the Bragg-Brentano θ–2θ geometry at a scanning speed of 2° per minute. The samples were mounted on a low-Si-background sample holder.

An initial Eu(III) reference experiment using TRLFS was performed in an attempt to approximate a plausible endpoint for the retention of Eu(III) in C-S-H phases. Therefore, Eu(III) was already added in the very beginning of the C-S-H formation assuming that it will enter the interlayer and the CaO layer faster. In these experiments, using the PARAFAC algorithm for the evaluation of the TRLFS data, two different Eu(III) species could be identified after a contact time of one, two, and 6 weeks, respectively. Both were located in the solid phase. The Eu(III) concentration in the liquid phase was below the limit of detection in all samples, thus indicating a high degree of retention. The two species, whose luminescence spectra are shown in comparison to the raw data in Supplementary Figure S1 (Supplementary Material), had luminescence decay times of (520 ± 40) µs and (2,100 ± 200) µs, respectively. According to Eq. 3, the shorter decay time corresponds to a sorption complex with approximately one or two water molecules in the coordination sphere of Eu(III), which may indicate sorption of Eu(III) into the interlayer of the C-S-H structure. The decay time of the second species relates to an environment without water molecules in the first coordination sphere of Eu(III), which implies substitution for Ca(II) positions in the CaO layer of the C-S-H phases. Similar results have been reported in the literature (Pointeau et al., 2001; Tits et al., 2003; Mandaliev et al., 2011). However, a third species, possibly corresponding to a surface complex was observed by Pointeau et al. (2001) and Mandaliev et al. (2011), which occurred only when Eu(III) was added after the C-S-H formation or at higher Eu(III) loadings than in the present experiments, respectively.

The PARAFAC analysis suggests that after a contact time of 6 weeks approximately (90 ± 3) % of the Eu(III) ions were sorbed into the interlayers of the C-S-H phases, while (10 ± 3) % were incorporated into the CaO layers in all samples. The addition of GLU (5 × 10⁻⁶ M–5 × 10⁻³ M) after a contact time of 14 days had no effect on the Eu(III) speciation, implying that GLU did not cause the desorption of Eu(III), even at the highest GLU concentration of 1 × 10⁻² M.

In the reference experiments with aqueous phases previously equilibrated with C-S-H phases (referred to as pore water samples), a white precipitate was formed upon the addition of Eu(III) (5 × 10⁻⁵ M) in the absence and presence of GLU. Consequently, the Eu(III) concentration in the solution dropped below the limit of detection of the used TRLFS setup of approximately 5 × 10⁻⁸ M. Similar findings have also been reported by Schlegel et al. (2004), who observed the formation of a C-S-H-like precipitate upon the addition of Eu(III) (3.5 × 10⁻⁵ M) to pore water equilibrated previously with C-S-H phases with a C/S ratio of 1.3.

In the pore water precipitate, a total of four Eu(III) species were identified at different GLU concentrations. Their emission spectra and luminescence decay times obtained using PARAFAC are shown in Figure 2A. The shortest observed decay time of (64 ± 5) µs cannot be explained solely by quenching due to O-H oscillators in the coordination sphere, suggesting that this Eu(III) species may (additionally) have neighboring Eu(III) ions adding self-quenching through a Eu(III)-Eu(III) energy transfer (cross-relaxation) to the overall radiationless deactivation (Buijs et al., 1987). This would be in accordance with the findings by Schlegel et al. (2004), who observed a high Eu(III) content in their precipitate with a bulk composition of Si_0.99Ca_1.87Eu_4.77O₁₁∙48H₂O and confirmed the presence of neighboring Eu(III) shells using Eu L₃-edge EXAFS. The luminescence decay time of the second species ((240 ± 20) µs) may correspond to three or four water molecules in the coordination environment of Eu(III) or likewise to the presence of other Eu(III) ions. The other observed species with luminescence decay times of (800 ± 100) µs and (1,300 ± 100) µs are likely not surrounded by any neighboring Eu(III) ions or water molecules in the first coordination sphere.

The molar fractions of each Eu(III) species at varying GLU concentrations are shown in Figure 3A. It is notable that in the absence of GLU and at low GLU concentrations mainly the species with the shortest luminescence decay time ((64 ± 5) µs) was formed. However, at GLU concentrations above 1 × 10⁻⁴ M, additional species with higher decay times were present, which will be discussed in more detail in the following section. Nonetheless, the Eu(III) signal in the liquid phase remained below the limit of detection at all GLU concentrations, thus indicating that Eu(III) was effectively immobilized in the solid phase even in the presence of up to 1 × 10⁻² M GLU.

In further reference experiments in which NaCl solution (1 × 10⁻² M) was used instead of C-S-H pore water, the precipitation of Eu(III) hydroxide (at low GLU concentrations) and the formation of soluble Eu(III)-GLU complexes (at [GLU] ≥ 2 × 10⁻⁴ M) were observed. The luminescence spectra and decay times of these species are different from the Eu(III) species in the pore water samples, thus corroborating the hypothesis that a coprecipitation of Eu(III) with the pore water constituents (i.e., Ca(II) and Si(IV)) occurred in the pore water experiments, where the Ca(II) and Si(IV) concentrations are expected to be 1.0 × 10⁻³ M and 1.3 × 10⁻³ M at C/S 0.8, respectively (Lothenbach and Nonat, 2015).

In the C-S-H retention experiments, Eu(III) and GLU were added to prepared C-S-H phases (time for formation: 14 days). The TRLFS measurements were carried out on the solid as well as on the supernatant solution. Over a contact time of 1–211 days, a total of five different Eu(III) species could be identified in the presence of GLU in the solid phases, while similarly to the reference experiments (cf. Section 3.2.1; Section 3.2.2) no Eu(III) signal was detectable in the liquid phase. Hence, high log R _d values above 4.5 were obtained, even at the highest GLU concentration of 1 × 10⁻² M. Luminescence spectra of the solid phases at different GLU concentrations are shown in Supplementary Figure S2 (Supplementary Material). Interestingly, the two species found in the reference experiments, in which Eu(III) was present during the C-S-H formation, were not found when Eu(III) was added later, therefore suggesting that the Eu(III) in these C-S-H retention samples might have been immobilized through a different mechanism than the sorption into the interlayers and incorporation into the C-S-H structure. In fact, three of the observed species have similar luminescence spectra and decay times as the species observed in the pore water samples (cf. Figures 2A, B), thus implying that Eu(III) added to the C-S-H phases after their formation appeared to be immobilized mainly by coprecipitation with the C-S-H pore water constituents rather than via sorption or incorporation into the C-S-H phases.

However, in the presence of C-S-H phases, a lower Eu(III) content and thus fewer neighboring Eu(III) ions may be expected for the formation of a precipitate compared to the pore water samples, as the C-S-H phase might act as a reservoir for the possible dissolution of additional Ca(II) and Si(IV) (as well as a potential competing phase for the sorption of Eu(III)). This idea is supported by the observation that the species with the lowest luminescence decay time and therefore lowest number in neighboring Eu(III) ions only occurred in the precipitate found in the pore water samples, but not in the precipitate formed when C-S-H phases were present. Moreover, the pore water samples generally showed a higher molar fraction of Eu(III) species that might experience cross relaxation due to neighboring Eu(III) ions than the C-S-H retention samples, as can be seen in Figures 3A, B. Furthermore, two additional Eu(III) species were observed in the C-S-H retention samples with GLU that neither appeared in the reference experiments in the C-S-H pore water nor when Eu(III) was present during the C-S-H phase formation. They might correspond to a different chemical composition of the precipitate due to the presence of GLU (and greater amounts of Ca(II) and Si(IV) compared to the pore water samples), however this cannot be verified based on the present information yet.

Supplementary Figure S3 (Supplementary Material) shows the obtained speciation for the C-S-H retention samples with 5 × 10⁻⁵ M Eu(III) and different GLU concentrations in dependence of the contact time. It can be seen that the changes with contact time were small and the equilibrium appeared to be reached after <14 days in all samples. However, large differences in speciation were obtained with increasing GLU concentration, indicating that, although GLU did not measurably increase the Eu(III) concentration in solution, it appeared to strongly influence the Eu(III) speciation in the solid phase. While in the absence of GLU mainly the species with a short luminescence decay time of (250 ± 20) µs was present, GLU appeared to promote the formation of species with higher luminescence decay times and thus likely smaller numbers in neighboring Eu(III) ions and/or O-H-oscillators in the coordination sphere of Eu(III). This trend was observed both in the pore water samples and the C-S-H retention samples, as shown in Figures 3A, B. In analogous experiments with nitrilotriacetic acid (NTA) instead of GLU (not shown here), a similar behavior and identical species were found, ruling out that those species could be attributed to “simple” Eu(III)-GLU complexes. Instead, GLU might alter the composition of the precipitate via complexation of Ca(II) ions, which might lead to a lower Ca(II) content in the precipitate and thus to changes in the Eu(III) coordination environment. This hypothesis is supported by the findings by Bouzouaid et al. (2022), who investigated the influence of GLU on the Ca(II) and Si(IV) concentration in C-S-H pore waters at different C/S ratios. Through wet chemistry experiments and theoretical modelling, they proposed that in the presence of 2 × 10⁻¹ M GLU, free H₃SiO₄ ⁻ and CaGLU⁺ dominate the pore water speciation at a C/S ratio of 0.8, thus indicating that under these conditions the majority of dissolved Ca(II) ions is complexed by GLU (Bouzouaid et al., 2021; Bouzouaid et al., 2022). However, it must be noted that the GLU concentration in this calculation is 20 times larger than the highest concentration used here. Additionally, some of the species, whose formation seems to be promoted by GLU, also appeared in samples without GLU to a small extent. Thus, further analysis of the Ca(II) and Si(IV) concentration in the liquid phase and of the composition of the precipitate is necessary to fully understand the influence of GLU on the Eu(III) speciation.

To investigate whether these results are also representative for lower Eu(III) concentrations, similar C-S-H retention experiments were performed with a Eu(III) concentration of 1 × 10⁻⁶ M. The Eu(III) speciation for these samples at different GLU concentrations is shown in Figure 3C. Similarly to the experiments at higher Eu(III) concentration, no Eu(III) signal was measurable in the liquid phase at all GLU concentrations. Furthermore, the observed species were the same as at higher Eu(III) concentration, indicating that Eu(III) was immobilized by the same coprecipitation processes. However, a smaller molar fraction of species with short decay times was observed at lower Eu(III) concentration, which may support the cross-relaxation hypothesis, as a lower Eu(III) content in the precipitate and therefore less cross-relaxation is expected at lower Eu(III) concentration.

Analyses were complemented by GLU concentration determination in the supernatants after the full retention period (211 days) by quantitative ¹H NMR spectroscopy (spectra not shown). A summary of the calculated figures is given in Supplementary Table S3 (Supplementary Material), revealing GLU quantities sorbed to the C-S-H phase comparable to those determined in the U(VI)/GLU/C-S-H experiments (cf. Section 3.4).