Stability of U(VI) doped calcium silicate hydrate gel in repository-relevant brines studied by leaching experiments and spectroscopy. (March 2019)

• Record Type: Journal Article
• Authors: Wolter, Jan-Martin; Schmeide, Katja; Weiss, Stephan; Bok, Frank; Brendler, Vinzenz; Stumpf, Thorsten
• Is Part Of: Chemosphere. Volume 218(2019)
• Page Start: 241
• Page End: 251
• Publication Date: 2019-03
• Subjects: Uranium -- C-S-H -- Portlandite -- Carbonate -- Ionic strength -- TRLFS; Pollution -- Periodicals; Pollution -- Physiological effect -- Periodicals; Environmental sciences -- Periodicals; Atmospheric chemistry -- Periodicals; 551.511
• DOI: 10.1016/j.chemosphere.2018.11.074
• Languages: English
• ISSNs: 0045-6535

Abstract

Abstract: The stability of calcium silicate hydrate (C-S-H) gel doped with uranium to form calcium uranium silicate hydrate (C-U-S-H) gel was investigated in 2.5 M NaCl, 2.5 M NaCl/0.02 M Na2 SO4, 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3 solutions relevant to the geological disposal of radioactive waste. The C-U-S-H gel samples were synthesized by direct U(VI) incorporation and characterized with time-resolved laser-induced luminescence spectroscopy (TRLFS), infrared (IR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Time-dependent pH changes as well as the Ca, Si and U release from C-U-S-H gels into the brines, determined by inductively coupled plasma mass spectrometry (ICP-MS), were monitored for three calcium-to-silicon (C/S) ratios (0.99, 1.55 and 2.02) over 32 d. Subsequently, changes of the U(VI) speciation and C-S-H mineralogy caused by leaching were investigated with TRLFS, IR spectroscopy and XRD. Results indicated that composition and pH value of the leaching solution, the presence of portlandite as well as formation and solubility of calcite as secondary phase determine the U(VI) retention by C-S-H gel under high saline and alkaline conditions. At high ionic strengths, the Ca release from C-S-H and secondary phases like calcite is increased. Under hyperalkaline conditions only small amounts of U(VI) were released during leaching. A decrease of the pH due to the additional presence of carbonate was linked with an increased U(VI) release from C-S-H gel leading to the formation of aqueous calcium uranyl carbonate in the supernatant solution. Graphical abstract: Image 1 Highlights: Higher NaCl concentrations increase Ca release from CSH and secondary phases. U(VI) release depends on brine composition and pH value. Carbonate destabilizes the polyhedral CaO plane of the C-U-S-H structure. Secondary phases such as calcite contribute to U(VI) retention. Formation of Ca2 UO2 (CO3 )3 (aq) at moderate alkaline pH values.