Nanosized yolk-shell Fe3O4@Zr(OH)x spheres for efficient removal of Pb(II) from aqueous solution.

In this work, Fe3O4@Zr(OH)x yolk-shell nanospheres (YSNs) were synthesized via a two-step process and further examined as adsorbents for the removal of Pb(II). To understand the hollow structure on the adsorption properties of Pb(II), another adsorbent without hollow cavities, i.e., Fe3O4@SiO2@Zr(OH)x core-shell nanospheres (CSNs), was also prepared for comparison. The adsorption results showed that Fe3O4@Zr(OH)x YSNs exhibited 41.6% higher Pb(II) adsorption capacity as compared to that of Fe3O4@SiO2@Zr(OH)x CSNs. The isotherm was well fitted to Langmuir adsorption model with qmax value of 310.8 mg/g after normalized by the weight of Zr in Fe3O4@Zr(OH)x YSNs. Scanning transmission electron microscopy (STEM) mapping results revealed that the existence of cavities between Fe3O4 cores and Zr(OH)x shells is responsible for the improved adsorption performance. XPS analysis indicated the surface hydroxyl groups played a key role in the Pb(II) adsorption. The removal efficiency of Pb(II) was maintained above 90% in five consecutive adsorption-desorption cycles.