Rational structural design of ZnOHF nanotube-assembled microsphere adsorbents for high-efficient Pb²⁺ removal

Adsorbents with high effective adsorption capabilities have attracted extensive interest and are promising for the removal of heavy metal ions from wastewater. Herein, we prepared well-dispersed ZnOHF microspheres with efficient adsorptive removal toward Pb²⁺ in an aqueous environment via an amino acid assisted hydrothermal method. The obtained microspheres were self-assembled by numerous nanotubes with an outer diameter of ∼10 nm and a wall thickness of about 3.5 nm. By taking advantage of the large specific surface areas of nanotube-assembled microspheres (271.06 m² g⁻¹), the maximum adsorption capacity for Pb²⁺ was up to 285.7 mg g⁻¹ in solution. Further analysis revealed the key role of the geometry structure of fine nanotube-assembled microspheres in boosting their adsorption ability. Furthermore, we proposed a potential growth mechanism of the as-prepared microspheres. The thermodynamic study of Pb²⁺ adsorbed on nanotube-assembled ZnOHF microspheres suggests that it is an endothermic and spontaneous process. This finding may open a new avenue to design efficient sorbents with high surface areas and effective heavy metal or organic dye removal ability in solution.