The micro/nanostructured materials can be used for the high efficient adsorbents owing to their high specific surface area, high surface activity and high stability against aggregation. In this paper, standing porous nanosheet-built ZnO hollow microspheres are produced through a modified hydrothermal route. Such ZnO hollow microspheres with exposed porous nanosheets surface exhibit significantly structurally enhanced adsorption performance for heavy metal cations [Cu(II), Pb(II), Cd(II), and Ni(II), etc.], compared with the commercial ZnO nanopowders, and show much higher adsorption capacities than the surface functionalized activated carbon reported previously. The adsorption isotherms can be described by Langmuir model or Freundlich model, depending on the electronegativity of the heavy metals. This ZnO hollow microspheres with exposed porous nanosheets surface can be used as adsorbent for efficient removal of heavy metal ions from the contaminated water with weak acidity or alkalescence, and easily separated from solution. This study also deepens understanding adsorption behavior of micro/nanostructured ZnO to heavy metal cations.The micro/nanostructured ZnO hollow spheres built of porous nanosheets were fabricated through hydrothermal treatment. Such hollow spheres with exposed porous nanosheets surface exhibit significantly structurally enhanced adsorption performance for heavy metal ions compared with the commercial ZnO nanopowders. (a) ZnO hollow spheres; (b) the adsorption isotherms of Cu(II) on ZnO hollow spheres compared with that of commercial ZnO nanopowders.Highlights► The standing and cross-linked porous nanoplate-built ZnO hollow microspheres are fabricated. ► Such microspheres exhibit significantly structurally enhanced adsorption to heavy metal cations. ► This material show much higher adsorption capacity than activated carbon reported previously. ► The adsorption performance of this material depends on the electronegativity of the heavy metals.

A facile one-step hydrothermal approach to prepare carbon spheres with amino groups on their surface is reported in this work. The obtained products are composed of carbon spheres with uniform size ∼2–3 μm. It was found that the addition of ammonia in the precursor solution would lead to modification of amino groups on the surface of carbon spheres. The existence of amino groups was verified by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Importantly, the carbon spheres with amino groups showed an excellent enhanced adsorption performance for the removal of heavy metal anions. Their removal capacity of Cr(VI) is ∼9 times higher than that of carbon spheres without amino groups. The enhanced adsorption performance was assumed to be induced by positively charged and increased reducing surface by modification of amino groups. This work not only supplies a novel low-cost approach to prepare amino-functionalized carbon spheres for water purification but also gives deep insight into understanding the adsorption behavior of carbon spheres.Graphical abstractCarbon spheres with amino groups on their surface were successfully fabricated by one-step hydrothermal approach with ammonia in the precursor solution. Such carbon spheres showed excellent enhanced adsorption performance for removal of heavy metal anions. (a) Amino-functionalized carbon spheres; (b) the adsorption isotherms of Cr(VI) on amino-functionalized carbon spheres compared with that of carbon spheres without amino groups.Highlights► The amino-functionalized carbon spheres are fabricated with one-step hydrothermal treatment. ► Such carbon spheres exhibit significantly enhanced adsorption to heavy metal anions. ► The enhanced adsorption performance was demonstrated by adsorption of Cr(VI) in acidic condition. ► The enhanced adsorption depends on positive and more reducing surface modified by amino groups.