Removal of heavy metal cations (Cd2+, Hg2+, and Pb2+) from water using carboxylic acid-functionalized covalent triazine frameworks: Computational design and experimental validation

The search for efficient adsorbents to capture harmful heavy metal (HM) cations is crucial for environmental remediation. This study explores the adsorption performance of pristine and functionalized CTFs for removing Cd²⁺, Hg²⁺, and Pb²⁺ from water using both computational and experimental methods. A molecular-level understanding of host–guest interactions is essential for designing functionalized nanoporous materials. DFT calculations at the ωB97XD level predict complexation energies of the cations on CTF fragments in the order: \({\text{E}}_{{\text{Pb}}^{2+}@\text{CTF}}\) > \({\text{E}}_{{\text{Hg}}^{2+}@\text{CTF}}\)  > \({\text{E}}_{{\text{Cd}}^{2+}@\text{CTF}}\). Based on the theoretical results, carboxylic acid (COOH)-functionalized CTF (COOH-CTF) is selected as the optimal adsorbent. COOH-CTF is synthesized through ZnCl₂-catalyzed ionothermal cyclotrimerization of the 2,5-dicyanobenzoic acid monomer and characterized by XRD, FT-IR, BET, and XPS techniques. The COOH-CTF adsorbent demonstrates medium to high adsorption capacities for Cd²⁺ (88.02 mg g⁻¹), Hg²⁺ (247.28 mg g⁻¹), and Pb²⁺ (360.39 mg g⁻¹). NCI analysis shows that in the HM@CTF-1 system, HM cations interact mainly via Van der Waals and strong electrostatic forces with nitrogen atoms of the triazine ring. In HM@COOH-CTF, additional strong electrostatic interactions occur with oxygen atoms of carboxylic groups, indicating a synergistic adsorption effect at N and O sites.

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