Novel magnetic covalent organic framework for the selective and effective removal of hazardous metal Pb(II) from solution: Synthesis and adsorption characteristics

The removal of lead(II) from wastewater is conducive to reduce the risk of harmful metals to the environment and people. A magnetic organic framework adsorbent (Ni0.6Fe2.4O4-HT-COF) with high selectivity, repeatability and facile solid–liquid separation was devised through the condensation process of hexachlorocyclo-triphosphazene, trithiocyanuric acid and Ni0.6Fe2.4O4. At 298 K and pH = 5.0, the adsorbent showed remarkable Pb(II) adsorption capability. The removal rate reached 95.64 % at 10 min and the maximum adsorption capacity was 411.80 mg/g. The partition coefficient (KQ) of Pb(II) among the seven coexisting ions was 34079 mL/g and the adsorption efficiency was 97.15 %, indicating that the adsorbent can accurately capture Pb(II) from wastewater containing multiple ions. The PSO kinetic and the Langmuir isotherm models agreed with Pb(II) adsorption on Ni0.6Fe2.4O4-HT-COF, demonstrating that Pb(II) was removed by monolayer chemisorption. The adsorption was a spontaneous exothermic process using thermodynamic analysis. In addition, the adsorbent was able to maintain 91.77 % adsorption efficiency for Pb(II) after four adsorption–desorption cycles. Zeta potential and XPS analysis revealed that Ni0.6Fe2.4O4-HT-COF removed Pb(II) by electrostatic attraction and chelation. Density functional theory (DFT) was utilized to simulate the adsorption process, and the effects of different types of atoms on adsorption were discussed. The results demonstrate that the chelation and electrostatic interaction between Ni0.6Fe2.4O4-HT-COF and lead(II) ions were involved in the selective adsorption. The novel magnetic COF has a lot of potential for removing heavy metal Pb(II).