Effect of Fe3O4@PDA morphology on the U(VI) entrapment from aqueous solution

Highlights: • Different Fe₃O₄@PDA spheres were prepared by in situ polymerization method. • The Fe₃O₄@PDA spheres show great adsorption capacity for the removal of U(VI). • Fe₃O₄@PDA nano-spheres own its highest adsorption capacity for U(VI) removal. • The U(VI) adsorption onto Fe₃O₄@PDA spheres is exothermic and spontaneous. • The adsorption is achieved via interaction between U(VI) and functional groups. In this work, two kinds of core–shell Fe₃O₄@PDA materials with different morphologies were successfully prepared by a simple in situ polymerization method based on two kinds of pristine Fe₃O₄ spheres, which were obtained by hydrothermal method. Among the four obtained samples, Fe₃O₄@PDA porous nano-spheres (MNS-4) have the best U(VI) entrapment performance, which can not only be owing to the structure characteristics (higher porosity and lower density) but also to the PDA shell with large amount of functional groups. Additionally, the maximum removal capacity of MNS-4 for U(VI) entrapment can be of 193.27 mg·g⁻¹ at 298 K, which exceeds the adsorption capacity of most other reported magnetic materials. Moreover, the solution pH values have a significant effect on the adsorption performance of the adsorbents for U(VI) pre-concentration while that originated from ionic strength can be negligible, suggesting that the adsorption process can be achieved through inner-sphere surface complexation. Thermodynamic studies reveal that the U(VI) adsorption process is exothermic and spontaneous. The analysis of adsorption mechanism indicates that U(VI) pre-concentration process can be achieved by the interactions between OH groups, pyridinic N and CNH₂ and U(VI). This work not only offers a new point of synthesizing undertaking adsorbents with targeting structures, but also provides a facile and versatile approach towards designing Fe₃O₄-based hybrid materials for their potential environmental applications.