Halloysite@polydopamine/ZIF-8 Nanocomposites for Efficient Removal of Heavy Metal Ions

In this study, polymeric nanocomposites of zeolitic imidazolate frameworks (ZIFs) were synthesized by assembly of a biomimetic polymer-polydopamine (PDA)onto halloysite nanotubes (HNTs@PDA), followed by the in situ growth of zeolitic imidazolate framework-8 (ZIF-8) on the surface of HNTs@PDA. The obtained nanocomposites (HNTs@PDA/ZIF-8) prevented agglomeration of ZIFs and increased the number of active sites derived from PDA. The factors influencing heavy metal ions (Pb2+, Cd2+, Cu2+, and Ni2+) adsorption by HNTs@PDA/ZIF-8 were discussed. The Langmuir model was able to well describe the adsorption, and the maximum adsorption capacity of HNTs@PDA/ZIF-8 was calculated to be 285.00 mg/g for Cu2+, 515.00 mg/g for Pb2+, 185 mg/g for Cd2+ and 112.5 mg/g for Ni2+. Thermodynamic parameters confirmed that the adsorption was exothermic and spontaneous. Moreover, HNTs@PDA/ZIF-8 has good regenerability, which is very important in practical applications. The adsorption mechanism study showed that electrostatic attraction, coordination reactions and ion-exchange were the main mechanisms between the adsorbents and heavy metal ions. Hence, HNTs@PDA/ZIF-8 is a promising candidate for removing heavy metal ions from wastewater.