Nano-size core-shell magnetic SO₃H-functionalized covalent organic frameworks via a functional defect strategy for cationic dye purification: Adsorption behavior and mechanism

With the rapid expansion of textile printing and aquaculture industries, efficient dye wastewater treatment is urgently needed. Here, magnetic sulfonic acid-functionalized covalent organic frameworks (Fe 3 O 4 @TpBD-SO 3 H COFs) were synthesized via a functional defect strategy using 4-aminobenzene-1,3-disulfonic acid (ADSA) as a competing monomer, generating periodic and sulfonate-rich nano adsorbent. Adsorption experiments involving malachite green (MG) and crystal violet (CV) in single and binary systems, supported by adsorption model fitting, indicated multimolecular adsorption through non-parallel molecular orientations. In binary adsorption system, MG enhanced CV adsorption (synergistic effect), whereas CV suppressed MG adsorption (antagonistic effect). Dynamic competitive adsorption tests suggested that adsorption configuration transitions between single and binary systems influenced this co-adsorption phenomenon. Incorporating Fe 3 O 4 magnetic cores rendered Fe 3 O 4 @TpBD-SO 3 H COFs highly hydrophilic, achieving excellent adsorption capacities for MG (1010.6 mg/g) and other cationic dyes, while minimally adsorbing anionic dyes. Moreover, the material exhibited strong resistance to interference from metal salts and maintained stable adsorption performance after ten regeneration cycles. Successful purification of real wastewater samples further demonstrates its practical potential for selective cationic dye removal from complex aqueous systems.