Diyun Chen

Influence of aqueous sulfide on speciation of U(VI) adsorbed to nanomagnetite

Heterogeneous reduction of U(VI) by structural Fe2+ and S2− is a key process influencing its fate and transport in subsurface environment. In this study, the adsorption of U(VI) on nanomagnetite was primarily conducted and then, it was desorbed by adding S2−. The desorption of absorbed U(VI) significantly increased with the increase in S2−, whereas S2− consumption was higher than the desorbed amount of U(VI) due to the volatilization (H2S) and sulfidization (FeS) at low and high pH, respectively. The XRD analysis demonstrated that the main components of corrosion products for nanomagnetite after adding S2− at pH 3.0 and 7.0 are lepidocrocite and mackinawite, respectively, after a reaction time of 128 days. According to analyses of ITFA and XANES spectra, the main speciation of uranium at pH 3.0 and 7.0 were U(V) and U(IV) species, respectively. The required energy of U(V) desorbed from lepidocrocite (−12.78 kcal mol−1) at pH 3.0 after 128 days was higher than that of U(IV) (−26.16 kcal mol−1) by theoretical calculations, indicating that U(V) can be stably incorporated into the secondary phase. These findings are crucial for understanding the speciation of uranium at low pH and reducing conditions.

Influence of carbonate on sequestration of U(VI) on perovskite

Cubic perovskite (CaTiO3) was successfully synthesized by a facile solvothermal method and was utilized to sequestrate U(VI) from aqueous solutions. The batch experiments revealed that carbonate inhibited U(VI) sequestration at pHu2009>u20096.0 due to the formation of uranyl-carbonate complexes. The maximum sequestration capacity of U(VI) on perovskite was 119.3u2009mg/g (pH 5.5). The sequestration mechanism of U(VI) on perovskite were investigated by XPS and EXAFS techniques. According to XPS analysis, the presence of U(IV) and U(VI) oxidation states revealed the photocatalytic reduction of U(VI) by perovskite under UV-vis irradiation. In addition, photocatalytic reduction performance significantly decreased in the presence of carbonate. Based on EXAFS analysis, the occurrence of U-Ti and U-U shells revealed the inner-sphere surface complexation and reductive precipitation of U(VI) on perovskite. These findings herein are crucial for the application of perovskite-based composites in the decontamination of U(VI) in aquatic environmental cleanup.

Impact of water chemistry on surface charge and aggregation of polystyrene microspheres suspensions

The discharge of microplastics into aquatic environment poses the potential threat to the hydrocoles and human health. The fate and transport of microplastics in aqueous solutions are significantly influenced by water chemistry. In this study, the effect of water chemistry (i.e., pH, foreign salts and humic acid) on the surface charge and aggregation of polystyrene microsphere in aqueous solutions was conducted by batch, zeta potentials, hydrodynamic diameters, FT-IR and XPS analysis. Compared to Na+ and K+, the lower negative zeta potentials and larger hydrodynamic diameters of polystyrene microspheres after introduction of Mg2+ were observed within a wide range of pH (2.0-11.0) and ionic strength (IS, 0.01-500mmol/L). No effect of Cl-, HCO3- and SO42- on the zeta potentials and hydrodynamic diameters of polystyrene microspheres was observed at low IS concentrations (<5mmol/L), whereas the zeta potentials and hydrodynamic diameters of polystyrene microspheres after addition of SO42- were higher than that of Cl- and HCO3- at high IS concentrations (>10mmol/L). The zeta potentials of polystyrene microspheres after HA addition were decreased at pH2.0-11.0, whereas the lower hydrodynamic diameters were observed at pH<4.0. According to FT-IR and XPS analysis, the change in surface properties of polystyrene microspheres after addition of hydrated Mg2+ and HA was attributed to surface electrostatic and/or steric repulsions. These investigations are crucial for understanding the effect of water chemistry on colloidal stability of microplastics in aquatic environment.