Structure and transport of polystyrene-b-poly(acrylic acid) micelles incorporating uranyl carbonate: a model for NOM–U(vi) colloids

A new type of uranium pseudo-colloid was prepared by mixing quasi-monodisperse polystyrene-b-poly(acrylic acid) (PS-b-PAA) micelle dispersions with uranyl carbonate (UC) in order to simulate the structure and transport of colloidal fulvic acids with uranium. A micelle model composed of a homogeneous PS core and a PAA corona was employed to derive the geometrical and contrast parameters of the PS-b-PAA micelles from small-angle X-ray scattering data. The modelled data indicated multidentate complexation of UC anions to the negatively charged PAA segments, as shown by the shrinkage and the increased density of the PAA corona with increasing UC concentrations. The hydrodynamic size of the micelles decreased from 102 nm to 73 nm with increasing UC concentration, and the zeta potential remained constant around -40 mV. The prepared pseudo-colloid of uranium has comparable size, zeta potential, and functional groups with colloidal fulvic acids. In addition, column experiment indicated the micelle-borne uranium could diffuse easily through quartz sand. This result indicates a potential risk of humic substances to promote the mobility of actinides in a deep geological nuclear waste disposal environment.