U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions – Spectroscopic investigations of retention mechanisms

• Published in: The Science of the total environment 2019-08, Vol.676, p.469-481
• Main Authors: Philipp, Thimo, Shams Aldin Azzam, Salim, Rossberg, André, Huittinen, Nina, Schmeide, Katja, Stumpf, Thorsten
• Format: Article
• Language: English
• Subjects: Ca-bentonite; EXAFS; Site-selective TRLFS; Speciation; Surface complexation; Uranium
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2019.04.274

Environmental conditions in deep geological repositories for radioactive waste may involve high pH values due to the degradation of concrete. However, the U(VI) sorption at such (hyper)alkaline conditions is still poorly understood. In this study, batch sorption experiments with Ca-bentonite in the pH range 8–13 at different carbonate concentrations were combined with spectroscopic investigations in order to gain insight into the underlying retention mechanisms. It was found that U(VI) sorption strongly correlates with the aqueous U(VI) speciation determined by time-resolved laser-induced luminescence spectroscopy (TRLFS). Increasing retention with increasing pH was accompanied by a change in aqueous speciation from uranyl carbonates to uranyl hydroxides. The occurrence of luminescence line-narrowing and a decreased frequency of the symmetric stretch vibration, deduced from site-selective TRLFS, indicate the presence of adsorbed U(VI) surface complexes. X-ray absorption fine structure (EXAFS) spectroscopy confirms that surface precipitation does not contribute significantly to the removal of U(VI) from solution but that retention occurs through the formation of two non-equivalent U(VI)-complexes on the bentonite surface. The present study demonstrates that in alkaline environments, where often only precipitation processes are considered, adsorption can provide effective retention of U(VI), despite the anionic character of prevailing aqueous species. [Display omitted] •U(VI) sorption on clay minerals at (hyper)alkaline conditions is poorly understood.•In this study, spectroscopic techniques revealed underlying retention mechanisms.•At pH 10–13, U(VI) is retained by adsorption of two independent species.•Adsorption is realized despite an anionic character of predominant aqueous species.•With increasing pH, a shift to a 4-fold coordination of U(VI) is observed.