Sulfur-containing triazine-based covalent organic frameworks for highly efficient Hg(II) capture

Mercury pollution poses severe threats to ecosystems and human health, driving the need for efficient adsorbents. Herein, three new covalent organic frameworks (TPA-PB COF, TPA-PT COF and TDC-PT COF) were designed and synthesized via condensation polymerization, incorporating both nitrogen and sulfur heteroatoms to enhance Hg(II) affinity. Comprehensive characterization identified TDC-PT COF as the optimal material, possessing significant heteroatom loadings (N: 12.40 wt%, S: 7.12 wt%), and a unique walnut-kernel morphology. Accordingly, TDC-PT COF showed the highest Hg(II) adsorption capacity (739.5 mg/g), followed by TPA-PT COF (575.1 mg/g) and TPA-PB COF (436.3 mg/g). Furthermore, this superior capacity was accompanied by rapid adsorption kinetics, a high removal rate of 99.9% meeting the drinking water standard, equilibrium within 40 min, and excellent selectivity. The adsorption of TDC-PT COF adhered to the pseudo-second-order kinetic model and the Langmuir isotherm, suggesting that the process is dominated by chemisorption with monolayer adsorption. Thermodynamic analysis revealed spontaneous and endothermic adsorption characteristics. X-ray photoelectron spectroscopy combined with density functional theory calculations confirmed that the primary mechanism involves Hg(II) coordination with N and S sites, where electron-withdrawing effects strengthening the interaction. The adsorbent retained over 95% of its original capacity after five regeneration cycles. This work establishes a series of isostructural COFs via controlled bottom-up molecular design, enabling a systematic investigation of the individual and cooperative roles of N and S sites in Hg(II) binding, and thereby providing a clear structure-function basis for the rational design of high-performance Hg(II) adsorbents.