Spectroscopic investigations on sorption of uranium onto suspended bentonite: effects of pH, ionic strength and complexing anions

Abstract

Abstract Batch sorption experiments were carried out under aerobic conditions to understand the sorption behavior of U(VI) onto bentonite clay under varying pH (2–8) and ionic strength (I = 0.01 – 1 M (NaClO4)) conditions. The influences of different complexing anions (1 · 10 –4 M) such as oxalic acid (ox), carbonate (CO3 2– ), citric acid (cit), and humic acid (HA, 10 mg/L) on the sorption behavior were also investigated. The sorption of U(VI) increased with increasing pH up to pH 6 beyond which a decrease was attributed to the formation of anionic carbonate species. Marginal influence of the change in the ionic strength of the medium on the sorption profile of uranium suggested inner-sphere complexation onto the bentonite surface. The presence of humic acid showed interesting sorption profile with varying pH. Initially, there was an enhancement in the sorption with increased pH followed by a plateau and finally a decrease thereafter due to the formation of aqueous U(VI)-humate complexes. Spectroscopic studies such as UV spectrophotometry, luminescence and extended X-ray absorption fine structure (EXAFS) measurements were also performed to understand the changes in aqueous speciation of U(VI) ion. The luminescence yields of different aqueous U(VI) species followed the order: U(VI)Hydroxy > U(VI)HumicAcid > U(VI)carbonate > U(VI)citrate. The lower luminescence yield of U(VI)carbonate complex can be attributed to the strong dynamic quenching by carbonate at room temperature. The U(VI) samples shows two distinct life-time suggesting the presence of the different luminescent U(VI) species. Similar trend was observed for U(VI)-bentonite suspension in presence/absence of the complexing ligands. There was luminescence quenching for the sorbed U(VI) due to surface complexation. These observations were further supported by spectrophotometric measurements. EXAFS spectra of U(VI) samples were recorded in luminescence mode at the U L 3 edge. There was shift in the absorbance edge which was attributed to decrease in electron density at U(VI) due to surface or ligand complexation. The R space spectra are mainly dominated by the back-scattering from the axial oxygens in the first shell. The inner-sphere multinuclear complex formation takes place during the U(VI) sorption onto bentonite.