Pore structure and plutonium retention in fractal-like (Ce Al)-oxide Laponite clusters

Abstract Recently, clays have been investigated for the long-term storage of spent nuclear fuel and nanominerals have been developed for the separation of radionuclides from subsurface water, but the detailed structure relating to pillaring of the sorbent is not always easily assessable by X-ray diffraction, for example, where the shift of the basal spacing to a higher value is not distinct. The purpose of this study was to gain an insight into the correlation between nanoscale structure and macroscopic sorption performance. The time dependence of 239‑plutonium sorption to Laponite, (Ce Al)-Laponite, and (Ce Al)-bentonite was studied. (Ce Al)-Laponite was characterized by TEM, XPS, XRD, N2 adsorption/desorption isotherms, and small-angle neutron scattering. Analysis of neutron scattering from dry powder and powder dispersed in water showed that (Ce Al)-Laponite has an open pore-fractal structure described by a fractal dimension Dp = 2.5, and a radius of gyration Rg = 6.5 nm. In a slightly alkaline pH condition, the strength of the sorption (Kd) of 239Pu by the different sorbents, from most to least, was (Ce Al)-bentonite (104) > (Ce Al)-Laponite (~104) > Laponite (103) > bentonite (103). The time it took for Pu sorption to reach equilibrium for (Ce Al)-Laponite was almost two times faster than that observed for the starting clays and the (Ce Al)-bentonite (8 days). A pseudo-second order rate expression was applied to the time-course data to describe the kinetic behavior. The enhanced rate of sorption was attributed to the connected pore system present in (Ce Al)-Laponite.