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

In this work, Fe 3 O 4 @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., Fe 3 O 4 @SiO 2 @Zr(OH) x core–shell nanospheres (CSNs), was also prepared for comparison. The adsorption results showed that Fe 3 O 4 @Zr(OH) x YSNs exhibited 41.6% higher Pb(II) adsorption capacity as compared to that of Fe 3 O 4 @SiO 2 @Zr(OH) x CSNs. The isotherm was well fitted to Langmuir adsorption model with q max value of 310.8   mg/g after normalized by the weight of Zr in Fe 3 O 4 @Zr(OH) x YSNs. Scanning transmission electron microscopy (STEM) mapping results revealed that the existence of cavities between Fe 3 O 4 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.