Interaction between Gaomiaozi bentonite colloid and uranium

Abstract

In a deep geological repository system, the presence of clay colloids in the geological disposal medium is considered to be a potential factor promoting nuclide migration. In this study, Gaomiaozi bentonite colloid (GMZC) was extracted using the gravity sedimentation method, and its interaction with U(VI) at neutral pH was studied. Small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), dynamic light scattering, and other characterizations methods were used to analyze and observe the microstructure and agglomeration morphology of the bentonite colloidal particles before and after the reaction with U(VI) at the macro-meso-microscopic level. The XPS results indicate that the U(VI) reacted with the Si–O and Al–O on the GMZC particle surfaces at room temperature, and the hydrolyzed product of the U(VI) covered the colloidal surfaces in the neutral water environment, resulting in a decrease in the absolute zeta potential (from ∼31 mV to ∼19 mV). The hydrodynamic diameter of the colloidal particles did not change significantly (∼336 nm–∼360 nm), but the small-angle scattering data revealed that the fractal dimension was larger after the reaction (∼2.8–∼3.1), indicating particle agglomeration. About 14% of the GMZC particles were precipitated after reacting with the U(VI). When the temperature was increased to 55 °C and 85 °C, the precipitation did not change significantly, but the fractal dimension of the mixed system increased (from ∼3.1 to ∼3.6), and the d001 peak was not observed in the SAXS results. When the temperature was decreased by 25 °C, the hydrodynamic diameter of the zeta potential and SAXS image both returned to the level before the temperature increase, indicating that the unstable change in the colloidal system caused by the temperature increase was reversible.