Uranium removal in aqueous solution by different sizes of commercial Fe3O4 microparticles: key role of oxygen vacancies

Fe₃O₄ microparticles have been investigated for U(VI) removal. However, the effect of the size of Fe₃O₄ microparticles on U(VI) removal has not been investigated. Herein, four different sizes (100, 200, 300 mesh, and nano) of commercial Fe₃O₄ microparticles were used to investigate the size effect on U(VI) removal. The results showed that Fe₃O₄ microparticles with a size of 200 mesh (Fe₃O₄-200) showed the best removal efficiency with a removal capacity of 19.3 mg/g. Characterization analysis showed that Fe₃O₄ microparticles with 100 and 300 mesh and nano consisted of Fe₃O₄ phase, whereas Fe₃O₄-200 was made up of Fe₂O₃ and Fe₃O₄ phase. The combination of Fe₂O₃ and Fe₃O₄ phase produced more oxygen vacancies, which endowed Fe₃O₄-200 with a superior removal capacity for U(VI). The solution pH affected U(VI) removal by Fe₃O₄-200. When the solution pH was 5.0, the U(VI) removal efficiency reached a maximum value. The effect of chloride ions was concentration-dependent, whereas sulfate ions slightly inhibited U(VI) removal. Carbonate ions strongly inhibited U(VI) removal. The U(VI) removal fitted the pseudo-second-order kinetics equation and Langmuir model well. Further thermodynamic analysis proved that Fe₃O₄-200 has high reaction ability with U(VI). Fe₃O₄-200 can be reused, and the adsorbed U(VI) can be easily recovered using sodium carbonate, sodium acetate, hydrochloric acid and sodium hydroxide. This study provides insight into U(VI) removal by different sizes of Fe₃O₄ microparticles, and paves a new avenue for designing highly efficient adsorbents for U(VI) removal from water bodies.