Melamine sponge loaded anionic covalent organic framework by sodium alginate cross-linking for selective dye removal with high adsorption capacity and reusability.

Ionic dyes are widely used and emitted in large quantities by modern industries. It is of great importance to develop efficient and practical adsorbent materials for the removal of such pollutants. Ionic covalent organic frameworks (COFs) with charged pore environments and stable backbone structures are excellent candidates for dye adsorbents. To improve the drawbacks of COF powder, which is not easy to be recycled and prone to secondary pollution, we report an effective strategy to prepare the composite material by immobilizing dispersed anionic COF on melamine foam sponge (MF@COF). Sodium alginate cross-linking method is developed as a powerful combination of COF and MF, with no powder falling off during adsorption. The composite material can quickly adsorb dyes, and the removal rate of cationic dyes can reach >99 % in 10 min; at the same time, it can selectively separate anionic dyes. The adsorption capacity of MF@COF for methylene blue (MB), crystal violet (CV), and malachite green (MG), was 947 mg g, 466 mg g and 1689 mg g in terms of the weight of COF, respectively. Compared with using the COF powder alone, the adsorption capacity of the composite material has been improved to a certain extent, with MB's adsorption capacity increasing by 6.16 %. Furthermore, MF@COF composite showed its practicality in practical water adsorption tests and could be recycled >5 times, which makes it a simple and practical adsorbent for water pollution control.