Uptake of Ni(II), Eu(III) and Pu(III/IV) by Hardened Cement Paste in the Presence of Proxy Ligands for the Degradation of Polyacrylonitrile

The uptake of 63Ni(II), 152Eu(III) and 242Pu(III/IV) by hardened cement paste (HCP, CEM I) in the degradation stage II (pH ≈ 12.5 [Ca] ≈ 0.02 M) was investigated in the absence and presence of α-hydroxyisobutyric, 3-hydroxybutyric and glutaric acids. These organic ligands were previously identified as proxies for the degradation products of UP2W (a polyacrylonitrile-based material used as filter aid in nuclear power plants) under repository conditions. Sorption experiments were conducted with various ligand concentrations (10−4 M ≤ [L]tot ≤ 0.1 M) and solid-to-liquid ratios (0.5 g⋅dm–3 ≤ S:L ≤ 20 g⋅dm–3). Redox conditions in the Pu systems were buffered with either hydroquinone (HQ, pe + pH ≈ 10) or Sn(II) (pe + pH ≈ 2). Strong sorption is observed for 152Eu(III) and 242Pu(III/IV) in the absence of proxy ligands, with distribution coefficients (log Rd ≈ 2.2–4, with Rd in m3⋅kg–1) in line with data reported in the literature. No differences are observed for sorption experiments with Pu in HQ and Sn(II) systems. Lower Rd values are determined for 63Ni(II) (log R d , N i 63 ≈ 0-1), consistently with previous studies. In combination with log R d , N i determined on the basis of the concentration of stable Ni(II) in pristine HCP and in cement porewater, values of the partition coefficient (α) close to 1 are determined. This suggests that the uptake of 63Ni is possibly driven by isotopic exchange with the complete species inventory of stable Ni present in pristine HCP, including Ni in solid phases and associated with surfaces, e.g., of C-S-H phases. The presence of proxy ligands has a negligible effect on the uptake of 152Eu(III) up to [L]tot = 0.1 M. A slight decrease in the distribution ratios for 63Ni(II) and 242Pu(III/IV) is observed at [L]tot > 10−2 M, although the effect is less evident in the case of plutonium due to the dispersion of the data and the increase of the detection limits with increasing ligand concentrations. Compared to strongly complexing ligands like isosaccharinic acid or gluconate, the investigated proxy ligands show a minor capacity for radionuclide mobilization in cementitious systems, even at high concentrations.


Introduction
UP2W is a polyacrylonitrile-based polymer (PAN) used as filter aid in nuclear power plants (Dario et al., 2004;. Significant amounts of used UP2W are disposed of in SFR, the repository for short-lived low and intermediate level wastes (L/ILW) in Sweden (Dario et al., 2004;Duro et al., 2012;Keith-Roach et al., 2014;. Cement is extensively used in L/ILW repositories for the conditioning of the waste and for construction purposes. The porewater in cementitious materials is characterized by hyperalkaline pH conditions (10 ≤ pH ≤ 13.3) and Ca concentrations ranging from~10 −4 M to~0.02 M (Taylor, 1997;Hewlett and Liska, 2019). Cementitious materials, in particular calcium silicate hydrate (C-S-H) phases, are known to sorb radionuclides strongly, primarily metal ions and cationic species (Wieland, 2014;Ochs et al., 2016).
In the context of an on-going collaboration between Karlsruhe Institute for Technology-Institute for Nuclear Waste Disposal (KIT-INE) and the Swedish Nuclear Fuel and Waste Management Co. (SKB), and within Work Package 3 CORI of the EURAD EU project (CORI-SOTA, 2021), the organic compounds glutaric acid (GTA), αhydroxyisobutyric acid (HIBA) and 3-hydroxybutyric acid (HBA) were proposed as proxies for the degradation products of UP2W ( Figure 1). The structure of these proxy ligands is consistent with 13 C-NMR data obtained for the degradation leachates of UP2W under hyperalkaline conditions (Tasi et al., 2021b). Considering the conversion of nitrile into carboxylate groups in the alkaline hydrolysis of PAN (Ermakov et al., 2000;Litmanovich and Plate, 2000), GTA represents the bulk chain of the PAN fragments, whereas HIBA and HBA correspond to the possible end-groups forming in the course of the degradation process.
The presence of organic ligands disposed with the waste in L/ILW or generated through the degradation of more complex organic materials may result in the formation of stable complexes with radionuclides, eventually affecting the retention properties of the latter in repository systems. Previous studies have investigated the impact of the UP2W degradation products on the sorption of radionuclides in cementitious systems (Dario et al., 2004;Holgersson et al., 2011;Duro et al., 2012). These studies focused on the leachates resulting from the degradation of UP2W material, and did not attempt to assess the effect of individual degradation products on the uptake of radionuclides by cement. Isosaccharinic (ISA) and gluconic (GLU) acids are polyhydroxycarboxylic acids expected in L/ILW repositories as main degradation product of cellulose and cement additive, respectively. They share some structural similarities with HIBA and HBA, with alcohol groups in αand β-positions with respect to the carboxylic group. Several studies in the literature highlight the strong complexes formed by ISA and GLU with radionuclides, including Ni(II), Ln(III) and An(III/IV/V/VI), among others (Vercammen et al., 2001;Tits et al., 2005;Gaona et al., 2008;Rojo et al., 2013;Rai and Kitamura, 2017;González-Siso et al., 2018;Tasi et al., 2018a;Tasi et al., 2018b;Rojo et al., 2021). The formation of these complexes can potentially affect the retention of radionuclides in cement-based repositories (Wieland and Van Loon, 2003;Wieland, 2014;Tasi et al., 2021a).
In this context, this study aims at investigating the uptake of 63 Ni(II), 152 Eu(III) and 242 Pu(III/IV) by HCP in the absence and presence of the proxy ligands HIBA, HBA and GTA. 59,63 Ni and 239 Pu are important contributors to the radiotoxicity of the wastes in SFR, whereas Eu(III) is often considered as inactive analogue of Am(III) and Pu(III). Experiments with plutonium are conducted under mildly and strongly reducing conditions (pe + pH ≈ 10 and 2, respectively) to capture the possible interplay between Pu(IV) and Pu(III) oxidation states in the redox boundary conditions expected in SFR (Duro et al., 2014). The Pu(IV) system in HQ-buffered solutions provides also insight on the behaviour of other tetravalent actinides expected in SFR, e.g., U(IV). The results obtained for the sorption of these radionuclides in the presence of proxy ligands are compared with analogous studies in the presence of ISA, which shares some structural similarities with HIBA and HBA. The present work complements and further expands the experimental studies by Szabo et al. dedicated to the proxy ligands of UP2W, i.e., evaluating their effect on the solubility of selected radionuclides (Szabo et al., 2022b) and assessing their uptake by HCP (Szabo et al., 2022a).

Frontiers in Nuclear Engineering
frontiersin.org in a previous study (Altmaier et al., 2008 Tasi et al., 2018a), but the choice of the +VI oxidation state for the spike of Pu aimed at avoiding high initial oversaturation conditions that may result in the formation of Pu(IV) colloids (see Tasi et al., 2021a).

Cement and cement porewater
The hardened cement paste used in this work was provided by SKB as a monolith, which was prepared from a mixture of Swedish structural Portland cement for civil engineering (CEM I 42.5N BV/ SR/LA) and deionized water at a water-to-cement weight ratio of 0.5. In a previous study (Tasi et al., 2021a), the HCP monolith was milled, sieved to a particle size of <100 μm, extensively characterized and stored under Ar atmosphere until use in the sorption experiments. Tasi and co-workers reported a (Brunauer-Emmett-Teller, BET) surface area of 79.2 m 2 ·g −1 for the HCP powder material used in the present study.
The cement porewater was prepared using the protocol described in Tasi et al. (2021a). The HCP was weighted in a 2 L Kautex flask and contacted with Milli-Q water to reach a solid-toliquid ratio (S:L) of 25 g·dm −3 . After a contact time of 1 week, the initial leachate was separated by decantation from the HCP, and the solid material was equilibrated again with fresh Milli-Q water during one additional week. The same HCP material and cement pore water were used in the solubility and sorption studies with proxy ligands for UP2W degradation products (Szabo et al., 2022a;Szabo et al., 2022b). The composition of the cement porewater is buffered by portlandite at pH ≈ 12.5 and [Ca] ≈ 0.02 M, and is characterized by a low content of Na and K (≤3·10 −4 M). A detailed description of the impact of the proxy ligands investigated in this work on the HCP is provided in our previous study dedicated to this binary system (Szabo et al., 2022a).

pH and E h measurements
The pH was measured using a ROSS combination pH electrode (Thermo Scientific, OrionTM) calibrated against standard buffer solutions (pH = 2 to 12, Merck).
Redox potentials of plutonium systems containing either HQ or Sn(II) were measured using Pt combination electrodes (Metrohm) with Ag/AgCl as a reference system. Measurements were performed following the protocol described in the literature (Altmaier et al., 2010). The measured potentials were converted to E h values (referred to the standard hydrogen electrode, SHE) by correcting for the potential of the Ag/AgCl reference in 3.0 M KCl at T = 22°C, i.e. +207 mV. E h values were converted to pe (with pe = −log a e− ) as E h = −RT ln (10) F −1 log a e− , where R is the ideal gas constant (8.3144 J·mol −1 ·K −1 ) and F is the Faraday constant (96,485.3 C mol −1 ). Uncertainties of pH and E h measurements (calculated as 2σ of averaged measurements) ranged between ±0.05/±0.10 and ±15/± 40 mV, respectively. 152 Eu and 242 Pu in the absence and presence of proxy ligands All batch sorption samples were prepared using the HCP and cement porewater described in Section 2.2. Stock solutions of the proxy ligands were titrated with 1.0 M NaOH to pH ≈ 12.5 in order to avoid any impact on the pH of the cement porewater.

Sorption experiments with 63 Ni,
Suspensions of HCP and cement porewater (10 mL) at the targeted S:L ratio were equilibrated for 2 days before the addition of the corresponding radionuclide. In the experiments with plutonium, the HCP was mixed with cement porewater containing 2 mM of either HQ or Sn(II), and the suspension was equilibrated for 2 days, after which pH and E h were determined as described in Section 2.3. Small volumes (14-351 μL) of the diluted radionuclide stock solutions were pipetted into these systems, aiming at log [ 63 Ni(II)] 0 = −8/−9, log[ 152 Eu(III)] 0 = −8/−8.67 and log[ 242 Pu(IV)] 0 ≈ −8. After the addition of the corresponding radionuclides, the values of pH, E h (only in the case of Pu) and total radionuclide concentration [RN] aq , in the aqueous phase were monitored for ≤4 months [RN] aq was measured after phase separation, i.e., ultrafiltration with 10 kD filters at 3,400-3,900 g (Nanosep ® , Pall Life Sciences; pore size ≈ 2-3 nm) (Ni, Eu, Pu) or ultracentrifugation (694,000 g, 1 h; Beckman XL-90, rotor type 90Ti) (Eu). After each sampling, the volume of the original sample was restored with cement pore water. The aliquots obtained after phase separation were diluted with 2% HNO 3 (Suprapur). In the samples containing nickel, the acidified aliquot was mixed with 10 mL of liquid scintillation counting (LSC) cocktail (Ultima Gold XR, Perkin Elmer) and measured for 30 min in a LSC Tri-Carb 3110 TR (Perkin Elmer) equipment considering an energy window between 0 and 100 keV. Detection limits achieved with this counting time and sample characteristics were ≈6·10 −12 M, calculated as three times the standard deviation of blank solutions. The concentration of Eu was determined by gamma counting (Wizard2 10-Detector Gamma Counter, Perkin Elmer) for an energy window between 15 and 2000 keV and a counting time of 30 min. Detection limits calculated for these conditions were ≈10 −13 M. Pu was quantified by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS; type Element XR, Thermo Fisher), with a detection limit of ≈10 −13 M in the conditions of the present study. The solutions in the HCP-Ni system were additionally quantified for total nickel concentration by ICP-MS.
The same sequence was used in the preparation of the sorption samples in the ternary systems HCP-RN-proxy ligands. Two days after the addition of the corresponding radionuclide, the suspensions were spiked with appropriate volumes of GTA, HIBA and HBA stock solutions (0.2, 5·10 −2 and 5·10 −3 M) targeting the range of initial ligand concentrations −4 ≤ log([L] 0 /M) ≤ −1. Note that the sorption of the proxy ligands on HCP at the S:L ratio used in the ternary system HCP-RN-proxy ligands is very minor (Szabo et al., 2022a), and thus [L] 0 is considered equal to the concentration of the ligand in equilibrium conditions [L] aq . After the addition of the proxy ligands, the values of pH, E h (only in the case of Pu) and [RN] aq were monitored for ≤3 months. The sampling procedure was the same as described above for the binary system HCP-RN. After each sampling, the volume of the original sample was restored with cement pore water containing the corresponding proxy ligand and ligand concentration. The sorption of Ni(II) and Eu(III) on the walls of the vessels was evaluated after terminating the sorption experiments. Hence, the where [M] 0 is the initial radionuclide concentration in the aqueous phase (M), V is the volume of sample (m 3 ) and m is the mass of HCP (kg).

Results and discussion
3.1 Uptake of Ni(II) by HCP in the absence and presence of proxy ligands Figure 2 shows the uptake of 63 Ni(II) by HCP in the absence of proxy ligands, as determined in this work for the degradation stage II of cement (pH ≈ 12.5). The figure shows also sorption data reported by Wieland and co-workers for the sorption of 63 Ni(II) by HCP in the degradation stage I (pH = 13.3), both in the absence and presence of added stable Ni(II) Wieland and Van Loon, 2003;Wieland et al., 2006).
R d values determined in this work remain mostly constant within the investigated range of S:L ratios (1-20 g·dm -3 ). Slight differences in the quantified R d values are identified between sorption experiments with [ 63 Ni] 0 = 10 −8 and 10 −9 M, i.e. (3.6 ± 1.8) m 3 ·kg -1 and (1.8 ± 0.7) m 3 ·kg -1 , although both values agree within the respective uncertainties. In contrast to this, Wieland and co-workers observed a systematic decrease of the measured R d values with increasing S:L ratio (see Figure 2). The authors proposed isotopic exchange with stable nickel present in pristine HCP as main mechanism for the uptake of 63 Ni, and used the partition coefficient (α) to describe the isotopic distribution of 63 Ni between the HCP and the cement pore water:

FIGURE 2
Uptake of 63 Ni(II) by HCP in the absence of proxy ligands as determined in this present work (p.w., red and green, pH ≈ 12.5) or reported by Wieland and co-workers (triangles and inverted triangles) Wieland and Van Loon, 2003;Wieland et al., 2006).  (Wieland et al., 2006). On the basis of these results, Wieland and co-workers argued that only a small fraction of stable Ni present in pristine HCP is available for isotopic exchange with 63 Ni.
The concentration of stable Ni in the cement porewater is by a factor of 10 or 100, respectively, higher than the added 63 Ni. At the relatively high stable Ni concentration which is most likely determined by solubility of a Ni-bearing solid phase (LDH phases or Ni(OH) 2 (am)) we can expect that Ni species are not only abundant in a solid phase and as dissolved Ni(OH) 2 (aq), but also as surface sorbed onto or incorporated into cement phases such as e.g., C-S-H. Spiked 63 Ni will undergo the same reactions as stable Ni so that isotope exchange is assumed to be the dominant retention mechanism under our experimental conditions. Note further that the partition coefficients measured in this work are consistent (within the uncertainties) with α = 1, which suggests the full availability of stable Ni in pristine HCP (in the degradation stage II) for isotopic exchange with 63 Ni.    (Wieland et al., 1998;Wieland, 2014). Greyshaded area covers the range of R d values determined in this work (S:L = 1-4 g·dm −3 ) or reported by Wieland and co-workers (S:L = 0.01-0.1 g·dm −3 ) in the absence of organic ligands. Experiments in the presence of organic ligands were conducted at S:L = 1 g·dm −3 . LOD and [p.w.] stand for limit of detection and present work, respectively.
1 This concentration is in line with previous data reported in the literature (Ochs et al., 2016) but higher than the solubility limit of β-Ni(OH) 2 (cr) determined under analogous boundary conditions (González-Siso et al., 2018;Szabo et al., 2022b). This observation suggests that this solid phase is neither present in the cement not forming within the timeframe of our experiments. Solubility equilibrium involving other solid phases (e.g., Nibearing LDH phases or Ni(OH) 2 (am)) may instead be responsible of the measured concentrations of stable Ni in the cement pore water.
Frontiers in Nuclear Engineering frontiersin.org solubility data previously reported for β-Ni(OH) 2 (cr) in the presence of the same proxy ligands (Szabo et al., 2022b), where an increase in the solubility was only observed at [L] tot = 0.1 M. In contrast to the proxy ligands investigated in this work, Figure 3 shows that ISA decreases the retention of 63 Ni by HCP in degradation stage II at [ISA] tot > 10 −3 M . This is again in line with solubility data reported for β-Ni(OH) 2 (cr) in the presence of ISA, where the complexes Ni(OH)ISA(aq), Ni(OH) 2 (ISA)and Ni(OH) 3 (ISA) 2were reported to control the aqueous speciation of Ni(II) in alkaline conditions (González-Siso et al., 2018). These observations support that the formation of stable complexes Ni(II)-OH-L is mostly responsible for the decrease in the retention of 63 Ni(II) by HCP. Differences observed between the proxy ligands and ISA are attributed to the presence of several alcohol groups in the latter ligand, which are known to form stable chelates with hard Lewis acids like Ni(II). Figure 4 shows the uptake of 152 Eu(III) by HCP in degradation stage II, both in the absence and presence of increasing concentrations of proxy ligands. For comparative purposes, the figure shows also the sorption of Eu(III) by HCP in degradation stage I in the absence and presence of ISA and GLU, as reported by Wieland and co-workers (Wieland et al., 1998;Wieland and Van Loon, 2003;Wieland, 2014).

Uptake of Eu(III) by HCP in the absence and presence of proxy ligands
A very strong sorption is observed in the absence of proxy ligands, with R d values ranging from ≈3·10 3 m 3 ·kg −1 and ≈1·10 4 m 3 ·kg −1 at S: L = 1-4 g·dm −3 . These values are slightly higher but in line with sorption studies available in the literature. Wieland and co-workers reported R d values ≥ 10 3 m 3 ·kg −1 at S:L = 0.01 g·dm −3 and > 10 2 m 3 ·kg −1 at S:L = 0.1 g·dm −3 (Wieland et al., 1998;Wieland and Van Loon, 2003). Our results are also consistent with the uptake observed for Eu(III) in the presence of C-S-H phases Pointeau et al., 2001), thus strongly supporting that the latter phases are the main sink of Eu(III) (as well as An (III)) in cementitious systems. The uptake mechanism investigated by time-resolved laser fluorescence spectroscopy and X-ray absorption spectroscopy for Eu(III), Nd(III) and Cm(III) supports a fast adsorption with the formation of surface complexes, followed by the slower incorporation of the trivalent lanthanide or actinide in the C-S-H structure (Pointeau et al., 2001;Tits et al., 2003;Stumpf et al., 2004;Mandaliev et al., 2010a;Mandaliev et al., 2010b). Note that stable Eu(III) is also present in HCP (quantified as 2.7·10 −6 mol·kg −1 in the CEM I material used in this work), although the inventory is significantly lower than in the case of stable Ni(II) (3.7·10 −4 mol·kg −1 ) and has a negligible effect in the retention of 152 Eu(III) in the current laboratory study. 2 The uptake of Eu(III) by HCP is not affected by the presence of the proxy ligands up to ligand concentrations of [L] tot = 0.1 M, which is consistent with solubility data previously reported for Nd(OH) 3 (s) in the presence of the same proxy ligands (Szabo et al., 2022b). Although these ligands are known to form stable complexes with Ln(III)/An(III) in acidic to near-neutral pH conditions (Deelstra and Verbeek, 1964;  (Tasi et al., 2021a). Experiments in the absence of organic ligands were conducted at S:L = 0.5-2 g·dm −3 (this work). The grey-shaded area indicates the range of log R d values reported by Tasi and co-workers in the absence of organic ligands at S: L = 1 g·dm −3 (Tasi et al., 2021a). Experiments in the presence of organic ligands were conducted at S:L = 1 g·dm −3 , both in this work and in (Tasi et al., 2021a). LOD and [p.w.] stand for limit of detection and present work, respectively.
2 The ratio 152 Eu (total inventory): Stable Eu (inventory in HCP) ranges from 4:1 (at S:L = 1 g·L −1 ) to 1:1 (at S:L = 4 g·L −1 ). A much larger inventory of stable Eu in HCP would be required in order to make isotopic exchange a relevant retention mechanism in our experimental study, especially considering the very high log R d values determined for this system (log R d = 3-4, with R d in m 3 ·kg −1 ).
Frontiers in Nuclear Engineering frontiersin.org Wang et al., 2000;Giroux et al., 2002), they cannot compete against hydrolysis in hyperalkaline systems or against the strong sorption occurring in HCP systems. This observation is further supported by thermodynamic calculations conducted by Keith-Roach and Shahkarami (Keith-Roach and Shahkarami, 2021). In contrast to the proxy ligands investigated in this work, the uptake of Eu(III) is strongly decreased in the presence of GLU and, to a lesser extent, ISA (Wieland, 2014) (see Figure 4). Even though HIBA and HBA contain a carboxylic group with an alcohol group in αor β-position, this configuration is insufficient to form stable complexes with Eu(III) in hyperalkaline systems. As discussed in Szabo et al. (2022a), a greater number of alcohol groups (as occurs in ISA or GLU) is required to form stable chelates in hyperalkaline conditions and thus affect aqueous complexation and sorption phenomena.

Uptake of Pu(III/IV) by HCP in the absence and presence of proxy ligands
The measurements of pH and E h in the Pu systems containing HQ and Sn(II) resulted in (pe + pH) = (10.0 ± 0.5) and (2 ± 1), respectively. These values are consistent with previous studies available in the literature using the same reducing chemicals (Tasi et al., 2018a;Tasi et al., 2018c). Considering the redox distribution of plutonium in alkaline reducing conditions, Pu(IV) is expected to prevail in the aqueous phase of HQ-buffered systems, whereas both Pu(III) and Pu(IV) may coexist in the strongly reducing conditions imposed by Sn(II) (Fellhauer, 2013;Tasi et al., 2018c;Grenthe et al., 2020). Figures  5A, B (HQ-system) and (Sn(II)-system) show the uptake of Pu (in terms of log R d values) by HCP in the degradation stage II, both in the absence and presence of increasing concentrations of proxy ligands. The figure includes also sorption data reported for Pu in the presence of ISA under analogous experimental conditions (Tasi et al., 2021a).
Sorption data in the investigated Pu samples are characterized by a large dispersion. This is a common feature of An(IV) systems in pHconditions where the neutral species An(OH) 4 (aq) prevails, and it is caused by difficulties in the phase separation and the possible presence of intrinsic colloids. A very strong sorption of Pu is observed in HQsystems and absence of proxy ligands, with log R d values ranging from 2.2 to 3.5 (with R d in m 3 ·kg −1 ). These results are consistent with sorption data reported by Tasi and co-workers under analogous experimental conditions (Tasi et al., 2021a). The reference book by Ochs et al. recommends a log R d of 1.5 for the uptake of Pu(IV) by HCP in the degradation stage II, with log R d = 3 defined as upper limit (Ochs et al., 2016), also in line with our experimental observations. Strong sorption is observed also in the presence of proxy ligands, with analogous log R d values determined in HQ-and Sn(II)-systems. Considering the strong sorption described also in Section 3.2 for Eu(III), the experimental observations for Pu in Sn(II)-systems do not allow the unequivocal assignment of the oxidation state prevailing under these very reducing conditions. A slight decrease in the log R d values can be speculated at [L] tot > 10 −2 M, especially in the case of HQ-systems. However, this observation is uncertain due to the large dispersion of the data, and by the increase of the detection limit with increasing ligand concentration caused by the steady increase of the dilution factors required for ICP-MS. In spite of this, R d values remain high in all the investigated systems containing proxy ligands, i.e. log R d > 1. The weak impact of the proxy ligands on the retention of Pu by HCP in degradation stage II is in line with the results of solubility experiments in the presence of analogous concentrations of proxy ligands, in which the solubility of PuO 2 (ncr, hyd) remains unaffected up to [L] tot = 0.1 M, both in HQ-and Sn(II)-buffered systems (Szabo et al., 2022b). Although HIBA, HBA and GTA are known to form stable binary complexes with An (III/IV) under acidic conditions, these complexes can neither outcompete hydrolysis nor the strong sorption on HCP under the hyperalkaline pH conditions characteristic of the degradation stage II of cement.
In contrast to the proxy ligands, Figure 5 shows the remarkable impact that ISA on the uptake of Pu, which results in a decrease of up to 3 orders of magnitude in the R d values (Tasi et al., 2021a). This is attributed to the strong ternary and quaternary complexes (Ca-)Pu-OH-ISA forming in hyperalkaline conditions (Tasi et al., 2018a;Tasi et al., 2018b). Moreover, distinct trends are observed at high ISA concentrations, involving the formation of stable surface complexes than increase sorption. Tasi and co-workers explained this observation with the strong uptake of ISA by cement, which modifies the surface properties at [ISA] tot ≥ 10 −3 M. This again highlights the significantly different behaviour of proxy ligands and ISA, not only with respect to the interaction with radionuclides but also concerning the interaction with cement surfaces. Similar observations have been reported for the uptake of Pu(IV) and Th(IV) by HCP and C-S-H phases in the presence gluconate, a polyhydroxocarboxylic acid analogous to ISA (Moreton, 1993;Moreton et al., 2000;Wieland, 2014;Guidone et al., 2022).

Summary and conclusion
A comprehensive sorption study was conducted with the aim of quantitatively assessing the impact of α-hydroxyisobutyric acid, 3hydroxybutyric acid and glutaric acid on the retention of Ni(II), Eu(III) and Pu(III/IV) by HCP in degradation stage II of cement. These low molecular weight organic ligands were identified within Work Package 3 CORI of the EURAD EU project as possible degradation products of UP2W, a polyacrylonitrile-based filter aid used in nuclear power plants.
Sorption experiments conducted in the absence of proxy ligands are consistent with literature data, with log R d, 63 Ni ≈ 0-1, log R d, 152 Eu ≥ 3.4-4 and log R d, 242 Pu ≈ 2.2-3.5 (all R d in m 3 ·kg −1 ). Considering the presence of nickel in the pristine cement and based on the quantification of R d, 63 Ni ; R d, nat Ni in the current study, we conclude that isotopic exchange with the complete species inventory of stable Ni present in pristine HCP (including Ni in solid phases and associated with surfaces, e.g., of C-S-H phases) is the main retention mechanism of 63 Ni(II) in cement systems in degradation stage II. Differences compared to the retention mechanism proposed for cement in degradation stage I might be explained by differences in the surface properties of cement and the aqueous speciation of Ni(II) under these conditions.
The retention of 63 Ni(II) and 242 Pu(III/IV) is slightly decreased in the presence of proxy ligands at [L] tot > 10 −2 M. The effect is less evident for Pu due to the dispersion of the data and to the increased detection limits with increasing ligand concentration. On the contrary, the uptake of 152 Eu(III) is not affected by the presence of proxy ligands up to concentrations of [L] tot = 0.1 M. These results are consistent with solubility studies conducted with β-Ni(OH) 2 (cr), Nd(OH) 3 (s) and PuO 2 (hyd, ncr) under analogous conditions and in the presence of proxy ligands. Although HIBA and HBA share some functional Frontiers in Nuclear Engineering frontiersin.org groups with ISA and GLU, the investigated proxy ligands show a minor capacity for the mobilization of radionuclides in cementitious systems. These observations support that mono-and dicarboxylic acids can hardly outcompete the hydrolysis and sorption of Ni(II), Eu(III) and Pu(III/IV) in cement systems in the degradation stage II, except if several alcohol groups are also present in the organic ligand.
With the aim to approaching systems of even higher complexity, the last step of this project will target the impact of the complex degradation leachates of UP2W on the retention of 63 Ni(II), 152 Eu(III) and 242 Pu(III/IV) by hardened cement paste in the degradation stage II.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions
SH, AM, XG, MA, and HG defined the initial concept of the project. AT and XG contributed to conception and design of the experimental study. PS and AT performed the experiments. AT and PS conducted data evaluation. XG wrote the first draft of the manuscript. PS and AT wrote sections of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

Funding
The EURAD project leading to this application has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 847593. The work was performed in EURAD as part of the WP CORI and was partially funded by SKB. We acknowledge support by the KIT-Publication Fund of the Karlsruhe Institute of Technology.