Construction of oxidized millimeter-sized hierarchically porous carbon spheres for U(VI) adsorption

Efficient recovery U(VI) from wastewater by a simple adsorption strategy is a key point to eliminate environmental threats and recycle nuclear energy. Herein, we presented a series of millimeter-sized hierarchically porous carbon spheres (MMCs) with different sizes fabricated via evaporation-induced self-assembly and suspension polymerization. Importantly, the abundant oxygenated groups were introduced on the surface of MMCs (OMMCs) by the HNO3 oxidation treatment. Thanks to the size effect, the OMMCs-S with size about 0.2 mm exhibits higher adsorption capacity and faster adsorption rate than that of OMMCs-L (size about 1.0 mm). The maximal U(VI) removal capacities on OMMCs-S reaches 232.45 mg.g(-1) at pH value of 4.5 and 298.15 K, which are nearly quadruple as high as that of untreated MMCs. Impressively, the OMMCs-S also exhibits sphere-shape structural stability, excellent regeneration and reusability. Furthermore, the U(VI) uptake reveals that the mainly interaction mechanism is formation of the inner-sphere surface complexation, which is also confirmed by the characterization analysis. Therefore, this research highlights a simple synthesis strategy for OMMCs-S, serving as an efficient adsorbent for the adsorption/recovery of U(VI) from radioactive wastewater.