Nacre-inspired hydroxyapatite/chitosan layered composites effectively remove lead ions in continuous-flow wastewater

Design and fabrication of novel adsorbents to remove heavy metal ions in continuous-flow wastewater remained a great challenge. Inspired by the hierarchical architecture and biomineralization process of nacre, we firstly constructed hydroxyapatite/chitosan (HA/CH) layered composites. The brick-and-mortar characteristics of HA/CH layered composites improved their flexure strengths up to 3.08 MPa so that the hierarchical architectures could not be destroyed even under high-pressure drop. HA/CH layered composites had the hierarchical microstructures analogous to plate towers, facilitating the separation of adsorbents from water. The interlaminar macropores in the layered composites contributed to the transfer of continuous-flow wastewater. The Pb(II), Cd(II) and Hg(II) ions in wastewater showed similar adsorption trends, and their adsorption amounts arrived at 295.96, 192.37 and 127.38 mg g^-1 after 6 days, respectively. Among the above heavy metal ions, the HA/CH layered composites possessed the best Pb(II) adsorption ability due to forming lead hydroxyapatite rods and CH-Pb complexes. The Pb(II) adsorption performances of HA/CH layered composites matched well with Elovich equation, pseudo-first-order and pseudo-second-order kinetics models, revealing the heterogeneous chemisorption mechanism at adsorbent/wastewater interfaces. Therefore, the nacre-like HA/CH layered composites with appropriate mechanical property and excellent adsorption capacity are a novel platform for heavy metal removal in continuous-flow wastewater.