EDTA-modified hydroxyl-functionalized covalent organic frameworks for enhanced Pb(II) removal from complex wastewater: adsorption mechanism and performance

Heavy metals frequently coexist with organic ligands in water as heavy metal complexes, which are difficult to remove routinely. In this study, a novel EDTA-based covalent organic framework (COF-EDTA) was synthesized at room temperature through the incorporation of EDTA into a hydroxyl-functionalized COF (COF-OH) for enhanced removal of both free Pb(II) and Pb(II)-citrate complexes. Structural and compositional properties of the materials were analyzed via PXRD, FTIR, and 13 C NMR, while surface morphology, elemental distribution, specific surface area, and thermal stability were examined by TEM, SEM-EDS, BET, and TGA, respectively. Pb(II) adsorption experiments were subsequently performed to assess removal efficiency. In the single Pb(II) system, EDTA modification significantly improved the adsorption performance, with the equilibrium adsorption amount increasing from 38.51 mg/g (COF-OH) to 135.6 mg/g (COF-EDTA), an increase of approximately 252%. COF-OH adsorbed Pb(II) via coordinate and hydrogen bonding, while the significant adsorption of Pb(II) by COF-EDTA was attributed to chelation and ion exchange. In the Pb(II)-citrate binary system, COF-EDTA preferentially adsorbed free Pb 2+ at a citrate-to-Pb(II) molar ratio of 0.5:1. When Pb-Citrate − is the dominant species (> 93%), COF-EDTA enhanced removal of Pb(II) through ligand displacement (primary pathway, ~75% efficiency) coupled with intact adsorption of Pb-Citrate − complexes (secondary pathway, ~25%). The DFT calculation validated the feasibility of ligand displacement mediated Pb(II) capture by COF-EDTA, while COF-OH lacked this capability. This work provides critical guidelines for exploring novel adsorbents to enhance the removal of heavy metal complexes.