Gaomiaozi bentonite colloids: Interactions with plutonium (IV) and zirconium (IV)

Abstract

According to previous field experiments, carrier colloids represent the primary mechanism for enhancing the transport of actinides in the subsurface at former nuclear weapons facilities. However, sub-micrometer size investigations on the sorption/interactions between radionuclides and natural colloids are scarce. In the present study, interactions between Pu (IV)/Zr (IV) and the colloids formed from Chinese Gaomiaozi bentonite in water were evaluated using sorption experiments, scanning electron microscopy, transmission electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, and small-angle X-ray scattering. Stable GMZ bentonite colloids showed zeta potentials of –31.8(0.9) mV at 25–85 ℃. Small-angle X-ray scattering from these colloids showed that the d-space (2.12 nm) of the colloidal particles dispersed in water exceeded that of mineral fragments obtained from the bulk powder (1.45 nm) in air, an indication of swelling. In addition, these colloidal particles (336 nm in size) exhibited higher affinities (Kd ~ 104 mL g−1) for Pu (IV) compared with bulk GMZ fragments (Kd ~ 103 mL g−1) at pH 7. X-ray diffraction showed that montmorillonite was the primarily component of colloidal GMZ particles, while X-ray photoelectron spectroscopy indicated that this clay mineral played an important role in the interaction with Pu (IV)/Zr (IV). Small-angle X-ray scattering from Zr (IV) solutions, showed power-law scattering (Intensity ∝ q−α) exponent α that increased from 2.6 to 3.4 (0.1 nm−1 <q < 0.5 nm−1), and decreased from 1.56 to 0.87 (q ~ 0.2 nm−1) as the Zr (IV) was raised from 0 to 9.8 × 10–11 mol dm–3. These results related to the partial agglomeration and precipitation of GMZ bentonite colloids. The remaining associative colloids pose a potential threat of Pu (IV) or Zr (IV) migration in subsurface waters.