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 q_max 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.