Facile sponge-like chitosan capsules as efficient adsorbent for anionic Congo red, Reactive brilliant blue KNR, and Methyl blue dyes: Performance and mechanism

In this study, sponge-like chitosan capsules (CTS-S) were successfully prepared as efficient adsorbent through a facile and green method. The cross-linked porous structure and abundant adsorption sites of CTS-S led to stable and remarkable adsorption efficacy for the purification of wastewater containing anionic dyes in a wide pH range. The highest maximum adsorption capacity for Congo red (CR), Reactive brilliant blue KN-R (KNR), and Methyl blue (MB) were 1372.49, 881.75, and 169.04 mg/g, respectively. Kinetics study suggested that KN-R and CR adsorption processes were best depicted by the PSO model, while PFO model was applicable for MB. Adsorption isotherms of CR, KNR, MB conformed to the Sips model, demonstrating that the adsorption of CR, KNR, MB onto CTS-S was monolayer and heterogeneous. Adsorption mechanism insights further verified that electrostatic forces, hydrogen bonds, and n-π stacking worked for efficient removal of CR, KNR, MB by CTS-S, which was supported by characterization results and DFT calculation. Overall, CTS-S provided a useful and promising option as low-cost and eco-friendly absorbents for the adsorption of synthetic dyes.