Solubility, redox and sorption behavior of plutonium in the presence of α-D-isosaccharinic acid and cement under reducing conditions
Creators: name: Tasi, Agost Gyula
Publisher: name: Karlsruhe
Year: 2018
Subjects: Plutonium
cellulose
isosaccharinic acid
aquatic chemistry
solubility
complexation
thermodynamics

description: Nuclear waste is planned to be embedded in deep underground facilities within suitable host rock formations. In the post-operational phase, ground water may intrude from the surrounding rock and saturate the repository. The present cementitious materials, used frequently for construction and waste conditioning purposes, will buffer the pH in the alkaline range (10 ≤ pH ≤ 13.3) over a very long time-scale, whereas strongly reducing conditions are expected to simultaneously evolve in the system due to the anaerobic corrosion processes of steel and related materials.
Plutonium in the waste inventory potentially contributes to the long–term radiological risk arising from the long half–life of 239Pu (t½ 239Pu = 2.41∙104 a). In reducing, aqueous environments, the formation of Pu(III) and Pu(IV) is foreseen. However, uncertainties in the available thermodynamic data lead to a rather ill–defined redox transition border, especially under alkaline to hyperalkaline pH conditions.
Large quantities of cellulosic materials may also be disposed of along with low- and intermediate-level nuclear waste (L/ILW). Under alkaline, Ca(II)-rich aqueous solutions, these materials are unstable and observed to decompose into smaller chained organic compounds. Alpha-D-isosaccharinic acid (HISA) is generated with a great yield and was previously identified as the most relevant ligand resulting from cellulose degradation. The strong affinity of ISA for the complexation with An(III) and An(IV) together with the often large inventory of cellulose and the lack of experimental studies on the system Cement-Pu-ISA were the main leading motivations in the development of this study.
A bottom-up, step-wise approach involving the investigation on four sub-systems was considered in the study of the ternary system Cement-Pu-ISA:

1) Firstly, the redox chemistry of Pu was investigated in the absence of ISA (and Ca(II)) under alkaline reducing conditions;
2) Then, the binary system Pu-ISA was studied (in the absence of Ca(II)) via systematic solubility experiments.
3) The binary system Pu-ISA was additionally assessed also in the presence of Ca(II), as expected for cementitious environments. The main aim of all solubility studies was to derive a comprehensive chemical and thermodynamic model on the Ca(II)-Pu(III/IV)-OH-ISA system valid under the boundary conditions of interest;
4) In the next step, sorption processes prevailing in the binary systems: Cement-Pu and Cement-ISA were assessed. Finally, the ternary system Cement-Pu-ISA was analyzed, and the knowledge gained in the previous sub-systems was adapted for the interpretation of this, most complex system.
All experiments were conducted under Ar atmosphere with strict control of redox conditions using hydroquinone (HQ, with pe + pHm 9.5), Sn(II) (with pe + pHm 1.5) or Na2S2O4 (with pe + pHm 0.5) as buffering agents. The moderately reducing conditions imposed by the presence of HQ were considered as the reference case, with expected predominance of Pu(IV) in the aqueous and solid phases. The strongly reducing conditions defined by Sn(II) and Na2S2O4 were taken as representative of the post-closure period in a geological disposal facility for L-/ILW. The solubility of an aged 242Pu(IV)O2(ncr,hyd) solid phase was systematically investigated in alkaline reducing systems: (i) in the absence of ISA, (ii) in the presence of ISA and absence of Ca(II), and (iii) in the presence of ISA and Ca(II). The impact of ISA on the uptake of plutonium by cement was investigated using ordinary Portland cement (OPC, CEM I 42.5N BV/SR/LA, provided by the Swedish Nuclear Fuel and Waste Management Company, SKB). Experiments were performed with various solid-to-liquid ratios, Pu and ISA total concentrations. Special emphasis was given to investigate the impact of experimental preparation order and the reversibility of the sorption processes governing Pu concentrations in the system. The solubility and sorption experiments were complemented with characterizations via synchrotron-based techniques (in–situ XRD and Pu LIII edge XANES/EXAFS peformed at the INE–Beamline for Actinide Research at KARA synchrotron facility) and Density Functional Theory (DFT) calculations as well.

Topic 1: Solubility and redox behavior of plutonium under reducing, alkaline conditions
Solubility experiments in the absence of ISA resulted in very low total Pu concentrations in solution (m(Pu)tot, as expressed in molal, mol∙kgw–1: m units) (~10–11 m) both for HQ and Sn(II) systems. EXAFS, in–situ XRD and XPS results confirmed that PuO2(ncr,hyd) is the solid phase controlling the solubility in HQ systems. XANES indicated a significant contribution of Pu(III) (30 ± 5%) in the solid phases equilibrated in Sn(II) systems. Two hypothesis are proposed to explain these observations in Sn(II) systems: (i) the coexistence of PuO2(ncr,hyd) and Pu(OH)3(am) in the retrieved solid phases, or (ii) the presence of a homogenous oxygen-deficient, PuO2–x(ncr,hyd) phase. These results provide a sound baseline for the interpretation of the solubility of Pu in the presence of ISA.

Topic 2: Plutonium solubility investigations in the presence of ISA (absence of Ca)
A pronounced increase of plutonium solubility by up to 2.5 log–units was observed in the presence of ISA (and absence of Ca(II)). In HQ systems, the slope analysis of solubility data in combination with solid phase characterization and DFT calculations resulted in the developement of a chemical model including the predominance of Pu(OH)3ISA–H– and Pu(OH)3ISA–2H2– complexes below and above pHm  12, respectively. The significantly higher m(Pu)tot measured in Sn(II) systems with pHm &lt; 12 indicated the formation of 
Pu(III)-ISA complexes under the given conditions. Above this pHm, solubility data in HQ and Sn(II) systems were observed to be identical showing the prevalence of Pu(IV)-ISA complexes in both cases. A comprehensive thermodynamic model for the Pu(III/IV)-OH-ISA system was established using the Specific Ion interaction Theory (SIT) formalism and was proven to be valid for the wide-range variation of the applied pHm, total ISA concentration (m(ISA)tot) and activity of electron in solution (pe) experimental parameters. Differences identified with available literature data for Th(IV)-, U(IV)- and Np(IV)-ISA systems are discussed in terms of systematic trends along the actinide series. Although not included in the thermodynamic model derived, “Pu-ISA colloids” were found to importantly increase the solubility of Pu in the presence of ISA (and absence of Ca(II)) as detected directly in the supernatants of the experiments.

Topic 3: Plutonium solubility investigations in the presence of ISA and Ca
The presence of Ca(II) further enhanced the solubility of Pu(IV) in HQ–buffered systems (compared to Ca(II)–free, Pu-ISA system), indicating the formation of quaternary Ca(II)–Pu(IV)–OH–ISA aqueous complexes in solution. Chemical and thermodynamic models were derived for this system as well, based on the statistical analysis of solubility data and on results of solid phase characterizations, which included the formation of two quaternary complexes: Ca(II)Pu(IV)(OH)3ISA–H+ and Ca(IV)Pu(IV)(OH)3ISA–2H(aq), dominating below and above pHm  11, respectively. The proposed model slightly overestimates the experimentally measured solubility at pHm ≥ 12.4 and m(ISA)tot ≥ 0.01 m, likely due to the formation of a yet undefined Ca(II)-Pu(IV)-OH-ISA(s) solid phase. Data collected in Sn(II)–buffered systems do not support the formation of analogous Pu(III) quaternary complexes with Ca(II) ions. For the same reducing system with pHm &gt; 11, solubility data could be explained by the model derived for Ca(II)–Pu(IV)–OH–ISA in HQ systems. In contrast to the Ca(II)-free system, no evidence on the formation of “Pu-ISA colloids” was found in the present case, pointing out the key role of Ca(II) in the destabilization of colloidal fractions. The obtained results provide key inputs to understand and quantitatively evaluate the solution chemistry (solubility, complexation) of Pu in the presence of ISA, under boundary conditions representative of reducing, cementitious systems.

Topic 4: Plutonium retention in the system Cement-Pu-ISA
Sorption studies performed under reducing conditions in the absence of ISA confirmed the strong uptake of Pu(IV) by the OPC solid phase (under conditions simulating cement degradation stage II). Distribution ratios (Rd) determined in present work were in good agreement with related data available in the literature for An(IV). For the uptake of ISA by cement under analogous conditions, a two-site Langmuir-isotherm was developed, which provided an empirical tool to evaluate the equilibrium concentration of ISA in solution at various S:L ratios.
Independent solubility experiments with PuO2(ncr,hyd) using the porewater composition in equilibrium with cement were conducted at various ISA concentrations (in the absence of cement) to set upper concentration limits for Pu, to be considered in the interpretation of sorption experiments. These results further confirmed the validity of the thermodynamic model on the system Ca2+–Pu4+–OH––Cl––ISA––H2O(l).derived in the course of the solubility studies of the present work.
Sorption experiments conducted in the presence of cement and ISA with the higher initially introduced total Pu concentration of log [Pu]in ≈ –5.5 (as expressed in molar units) were observed to be solubility controlled, and thus, main conclusions on the uptake of Pu by cement were derived from sorption experiments with log [Pu]in ≈ –8.5. Experimental results determined a relevant impact for the order of addition of components (Pu / ISA / cement) on sorption results (especially at log [ISA]tot = –2), with the sequence “(Pu + cement) + ISA” showing significantly higher log Rd values (≈ 1.5 log-units greater) than the other sequence: “(Pu + ISA) + cement”.
Two different cases could be defined based upon experimental results obtained in the presence of cement and ISA with log [Pu]in = –8.5:

- Case I shows the strongest sorption and has been observed only in desorption experiments and sorption experiments following the sequence “(Cement + Pu) + ISA”. Data in Case I represent lowest sorption reduction factors (Fred) and can be explained approximately by a simplified sorption model, which considers log Rd,in determined experimentally in the absence of ISA, and assumes a decreased sorption caused only by formation of dissolved Ca(II)–Pu(IV)–OH–ISA complexes. Thermodynamic data derived in this PhD thesis are used to calculate the concentrations of the complexes Ca(II)–Pu(IV)–OH–ISA forming in solution.
- Case II results in systematically higher Pu aqueous concentrations in solution, and accordingly lower Rd and higher Fred. It corresponds to sorption experiments performed following the order “(Pu + ISA) + Cement”. These observations are explained by a kinetic stabilization of aquatic Ca-Pu-ISA species or modification of the cement surface by the adsorbed ISA.

Although a solubility control may appear inconvenient in sorption experiments, note that Pu concentrations used in this study (log [Pu]in ≈ –8.5) were targeted to be in the range of those potentially expected in a repository for L/ILW [4]. Experiments with lower Pu concentrations (possibly using 238Pu or 239Pu) could help in providing a more insightful view on the sorption phenomena controlling Pu retention / mobility in cementitious systems.

This work demonstrates the significant impact of ISA on the retention of Pu by cement under reducing, alkaline conditions, but also reflects the high complexity of the ternary system cement-Pu-ISA. Although the results obtained in this PhD thesis represent a sound empirical basis to quantitatively assess the impact of ISA on the uptake of Pu by ISA, an unequivocal mechanistic understanding of the uptake process(es) is not yet possible.

The present PhD study was conducted in the framework of an international project by KIT–INE with Amphos 21 Consulting Agency (Spain). The project was funded by the Swedish Nuclear Fuel and Waste Management Company (SKB).