Covalent organic frameworks comprise a unique class of functional materials that has recently emerged as a versatile tool for energy-related, photocatalytic, environmental, and electrochromic device applications. A plethora of structures can be designed and implemented through a careful selection of ligands and functional units. On the other hand, porous materials for heavy metal absorption are constantly on the forefront of materials science due to the significant health issues that arise from the release of the latter to aquatic environments. In this critical review, we provide insights on the correlation between the structure of functional covalent organic frameworks and their heavy metal absorption. The elements we selected were Pb, Hg, Cr, Cd, and As metal ions, as well as radioactive elements, and we focused on their removal with functional networks. Finally, we outline their advantages and disadvantages compared to other competitive systems such as zeolites and metal organic frameworks (MOFs), we analyze the potential drawbacks for industrial scale applications, and we provide our outlook on the future of this emerging field.

New heterojunction materials (HJs) were synthesized in-situ by molecularly bonding the monomers of a triazine-based covalent organic framework (bulk COF) on the template of exfoliated carbon nitride (g-C₃N₄). The photocatalysts reduced carbon dioxide to carbon monoxide in aqueous dispersions under UV irradiation. The g-C₃N₄ showed production of 6.50 $\mathsf{\mu }$mol CO g⁻¹ h⁻¹ and the bulk COF of 2.77 $\mathsf{\mu }$mol CO g⁻¹ h⁻¹. The CO yield was evaluated in sustainability photoreduction cycles and their CO₂ uptake capacity and isosteric heat of adsorption were estimated. All the heterojunction photocatalysts obtained ameliorated CO production rates compared to the bulk COF. Finally, the influence of the Pt co-catalyst on the photocatalytic activities was determined without the addition of any sacrificial agent, and the COF:g-C₃N₄ heterojunction with the ratio of 1:10 was proven to be a photocatalytic system with an optimum and selective, CO yield of 7.56 $\mathsf{\mu }$mol g⁻¹ h⁻¹.

The graphic shows the structural representation of the electrostatic assembly of oppositely charged CuFeS ₂ nanoparticles with COF layers. Their addition in the COFs is enhancing the absorption capacity, while quenching the photoluminescence.

Commercial melamine sponges were modified with a functional covalent organic framework (COF), and they were evaluated as adsorbents of divalent copper cations from aqueous solutions. A phosphazene unit successfully covered the surface of the melamine sponge, and the organic framework was subsequently formed through the nucleophilic substitution with 4,4′ bipyridine. The covalent organic framework functionalized on the melamine sponge can detect and effectively adsorb copper compounds in aqueous solutions. Its selectivity toward the adsorption of copper was demonstrated through the presence of different metal salts. Four competitive metal cations, i.e., copper, nickel, iron, and calcium, were selected to confirm the preferential binding of copper on the COF-functionalized sponge. The outcome was determined through the studies of X-Ray Fluorescence elemental analysis, X-Ray Photoelectron Spectroscopy (XPS), and Electron Paramagnetic Resonance experiments. XRF reported a copper sorption capacity of 293 μg cm^–2, which is nearly nine times higher than the performance of the pristine sponge. Q-band EPR measurements demonstrated the presence of different coordination sites with different substituents for copper on the modified sponges, when the adsorption took place in an aqueous solution containing exclusively copper cations, while only one coordination, the favorable trigonal bipyramidal geometry, was obtained in the presence of additional metals.

In this work, a multi-step approach for the fabrication of a unique sponge-based platform with multiple functionalities and applications in water-remediation technologies under dynamic conditions is presented.