Schiff Base-Derived Microporous Covalent Organic Framework Aerogels as Chemo-Captors for Exceptional Rapid, High-Capacity, and Selective Cadmium Ion Adsorption

Abstract

Cadmium (Cd2+) pollution in water seriously harms ecosystems and human health, requiring efficient, cost-effective, eco-friendly, and recyclable adsorbents. Herein, three novel spongy-like chitosan-based covalent organic framework (CS-COF) aerogels (Captors 1–3) were synthesized for the selective adsorption of Cd2+ ions. Captors 1–3 were fabricated via a Schiff base reaction between CS and three aldehyde (−CHO)-containing cross-linkers (terephthalaldehyde, [1,1′-biphenyl]-4,4′-dicarbaldehyde, and 2,3-dihydroxyterephthalaldehyde), yielding ultralight monolithic aerogels (5.0–20.3 mg/cm3) with ordered 2D crystalline frameworks, uniform micropore distributions (1.37–1.60 nm), and abundant multifunctional active sites (−HC═N–, −NH2, −OH). Such features synergistically enable fast Cd2+ ion transfer and strong Cd2+-binding events. Findings showed that Captors 1–3 had high adsorption capacities (725.1–771.5 mg/g) and removed over 95% of Cd2+ within 10 s at pH 8.0. The Langmuir and pseudo-second-order models confirmed that the adsorption process is primarily driven by chemisorption. Computational modeling, XPS, and elemental mapping revealed that Cd2+ adsorption proceeds via covalent and noncovalent interactions. Selectivity over competing cations at pH 8 arises principally from speciation. Cd2+ remains fully cationic while several competitors hydrolyze to species of reduced charge. Geometric matching between the imine-rich chelating pocket and the Cd2+’s octahedral coordination preference reinforces this effect. Captors 1–3 retained >90% removal efficiency after five regeneration cycles and achieved >90% Cd2+ removal from real water matrices, confirming structural robustness and practical applicability. This work establishes a rational, scalable, and eco-friendly CS-COF aerogel platform with broad potential for environmental remediation applications.