Cooperative Fe–NH2 functionalized COFs for steric-hindrance-resistant capture of organophosphorus

The removal of sterically hindered diethylenetriamine penta(methylene phosphonic acid) (DTPMP) from wastewater is challenging because its bulky molecular architecture impedes both intraparticle diffusion and effective coordination with conventional adsorbents. To address this limitation, a cooperative dual-functional covalent organic framework (Fe–NH 2 /COF 4 ) was synthesized via sequential Fe incorporation followed by nitro-to-amine reduction. The resulting material exhibits a high specific surface area of 501.6 m 2 /g and a well-matched pore size of 17.16 Å, closely aligning with the DTPMP molecular dimension (∼15.7 Å). Under the optimized conditions, Fe–NH 2 /COF 4 achieves a removal efficiency of 93.85% and an adsorption capacity of 338.2 mg/g, reducing effluent total phosphorus to below 0.5 mg/L (the Grade 1 A discharge standard). Mechanistic investigations, including experiments and DFT calculations (adsorption energy: −1.07 eV), reveal synergistic contributions from Fe–phosphonate coordination, electrostatic attraction, and hydrogen bonding. Molecular dynamics simulations confirm rapid intrapore diffusion (D = 5.7 × 10 −7  cm 2 /s). The adsorbent demonstrates good reusability (82% capacity retention after five cycles) and remarkable robustness against coexisting ions and humic acid (HA), retaining an adsorption capacity of 332.5 mg/g even in simulated wastewater. Collectively, these findings establish a steric-hindrance-resistant adsorption paradigm and provide rational design guidelines for dual-site COF adsorbents targeting refractory organophosphorus pollutants.