Uptake of anionic radionuclides onto degraded cement pastes and competing effect of organic ligands

Pointeau, Ingmar; Coreau, N.; Reiller, Pascal E.

doi:10.1524/ract.2008.1503

Cited by 54 publications

(45 citation statements)

References 23 publications

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“…In view of the recent sorption and spectroscopic studies [15,16], Np(VI) is expected to sorb weakly onto cementitious materials at pH1 3.3 (R d~1 0 3 Lؒkg -1 ) in contrast to Np(V) (R d~1 0 6 Lؒkg -1 ). A similar behavior was reported for the uptake of U(VI) by calcium silicate hydrates (C-S-H phases) and cement [52][53][54][55]. Whether interpreted as a surface complexation process [53] or as a solid solution formation with the C-S-H phases [55], the weak sorption of An(VI) is likely related to the formation of highly charged anions under hyperalkaline X. GAONA et al…”

Section: Understanding the Behavior Of Np Under Alkaline Conditions: supporting

confidence: 65%

Thermodynamic description of Np(VI) solubility, hydrolysis, and redox behavior in dilute to concentrated alkaline NaCl solutions

Gaona

Fellhauer

Altmaier

2013

Pure and Applied Chemistry

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The solubility of Np(VI) was investigated in carbonate-free NaCl solutions (0.1 M ≤ I ≤ 5.0 M) at T = 22 ± 2 °C to derive thermodynamic properties of aqueous species and solid compounds formed under alkaline conditions. The experimentally derived solubility curves can be divided into four main regions: (I) ~7 ≤ pH m ≤ ~9, showing a steep decrease in Np solubility with a slope (log [Np] vs. pH m ) of -3 or -2 (depending on NaCl concentration); (II) ~9 ≤ pH m ≤ ~10.5, with a nearly pH-independent [Np]; (III) ~10.5 ≤ pH m ≤ ~13.5, showing an increase in the solubility with a well-defined slope of +1. A region (IV) with a slope ≥ +2 was only observed at I ≥ 1.0 M NaCl and pH m ≥ ~13.5. The solubility-controlling solid Np phases were characterized by X-ray diffraction (XRD), quantitative chemical analysis, thermogravimetric analysis and scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), confirming the presence of anhydrous Na 2 Np 2 O 7 (cr) in regions II and III. The same solid phase was identified in region I except for the system in 0.1 M NaCl, where a NpO 2 (OH) 2 ؒH 2 O(cr) phase predominates. XRD patterns of this solid phase show a very good agreement with that of metaschoepite (UO 3 ؒ2H 2 O), highlighting the similarities between Np(VI) and U(VI) with respect to solid phase formation and structure.Based on the analysis of solubility data, solid phase characterization and chemical analogy with U(VI), the equilibrium reactions 0.5 Na 2 Np 2 O 7 (cr) + 1.5 H 2 O ⇔ Na + + NpO 2 (OH) 3 and 0.5 Na 2 Np 2 O 7 (cr) + 2.5 H 2 O ⇔ Na + + NpO 2 (OH) 4 2-+ H + were identified as controlling Np(VI) solubility in regions II and III, respectively. The predominance of NpO 2 + in the aqueous phase of region I (quantified by UV-vis/NIR) indicates the reductive dissolution of Np(VI) [either as Na 2 Np 2 O 7 (cr) or NpO 2 (OH) 2 ؒH 2 O(cr)] to Np(V) aq . Oxidation to Np(VII) can explain the experimental observations in region IV, although it is not included in the chemical and thermodynamic models derived.The conditional equilibrium constants determined from the solubility experiments at different ionic strengths were evaluated with both the specific ion interaction theory (SIT) and Pitzer approaches. Thermodynamic data for aqueous Np(VI) species [NpO 2 (OH) 3 and NpO 2 (OH) 42-] and solid compounds [Na 2 Np 2 O 7 (cr) and NpO 2 (OH) 2 ؒH 2 O(cr)] that are relevant under alkaline conditions were derived. These data are not currently included in the Nuclear Energy Agency-Thermochemical Database (NEA-TDB) compilation.

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Section: Understanding the Behavior Of Np Under Alkaline Conditions: supporting

confidence: 65%

Thermodynamic description of Np(VI) solubility, hydrolysis, and redox behavior in dilute to concentrated alkaline NaCl solutions

Gaona

Fellhauer

Altmaier

2013

Pure and Applied Chemistry

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“…The same EXAFS study [22] also provided indications for incorporation of Np(VI) in the C-S-H interlayers based upon the presence of a neighbouring Si shell at short distance. Further information on the uptake of hexavalent actinides in C-S-H phases resulted from sorption studies with U(VI) on cementitious materials (e.g., [22][23][24][25][26][27][28] and 10 6 L kg −1 and to depend on pH, aqueous speciation and the composition of C-S-H phases. The latter phases are considered to be responsible for U(VI) immobilization in cementitious materials.…”

Section: Introductionmentioning

confidence: 99%

Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates

Tits¹,

Laube²,

Wieland

et al. 2014

Radiochimica Acta

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Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates Abstract: Wet chemistry experiments were carried out to investigate the effect of the redox state and aqueous speciation on the uptake of neptunium by titanium dioxide (TiO 2 ) and by calcium silicate hydrates (C-S-H) under alkaline conditions. TiO 2 was chosen as a reference sorbent to determine the surface complexation behaviour of neptunium under alkaline conditions. C-S-H phases are important constituents of cement and concrete. They may contribute significantly to radionuclide retention due to their high recrystallization rates making incorporation the dominating sorption mechanism for many radionuclides (e.g. the actinides) on these materials. The sorption of neptunium on both solids was found to depend strongly on the degree of hydrolysis. On TiO In addition to the effect of hydrolysis, the presence of Ca is found to promote Np(V) and Np(VI) sorption on TiO 2 whereas on C-S-H phases, the present wet chemistry data do not give unambiguous evidence. Thus, the aqueous speciation appears to have a similar influence on the sorption of the actinides on both types of solids despite the different sorption mechanism.The similar d values for Np(IV,V,VI) sorption at pH = 10 can be explained qualitatively by invoking inter-ligand electrostatic repulsion between OH groups in the coordination sphere of Np(V) and Np(VI). This mechanism was proposed earlier in the literature for the prediction of actinide complexation constants with inorganic ligands. A limiting coordination number for each Np redox state, resulting from the inter-ligand electrostatic repulsion, allows the weaker sorption of the highest hydrolysed Np(V,VI) species to be explained.

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“…Various approaches provided information on uranium in the cement system since many years [3][4][5][6][7][8][9][10]. Independent on the composition of the contacting leachant, high retention coefficients were found.…”

Section: Introductionmentioning

confidence: 99%

Chemical status of U(VI) in cemented waste forms under saline conditions

Kienzler¹,

Metz

Brendebach

et al. 2010

Ract

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Uranium / Retention / Cement / BrineSummary. Retention of U(VI) in cemented waste forms reacting with NaCl and MgCl 2 brines is investigated in longterm leaching experiments on full scale monoliths. Solution compositions were monitored over a period of 17 to 18 years. After termination of the leaching experiments, chemical and mineralogical compositions of solid reaction products were studied intensively. XRD, TRLFS and XANES/EXAFS analyses indicate uranophane (Ca(UO 2 ) 2 (SiO 3 OH) 2 ·5H 2 O) to be the dominant uranium bearing phase in the corroded cement. Other possible uranium phases such as soddyite, metaschoepite, and di-uranate phases could not be identified by combining the results of the various experimental techniques.

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Uptake of anionic radionuclides onto degraded cement pastes and competing effect of organic ligands

Cited by 54 publications

References 23 publications

Thermodynamic description of Np(VI) solubility, hydrolysis, and redox behavior in dilute to concentrated alkaline NaCl solutions

Thermodynamic description of Np(VI) solubility, hydrolysis, and redox behavior in dilute to concentrated alkaline NaCl solutions

Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates

Chemical status of U(VI) in cemented waste forms under saline conditions

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