Adsorption properties of methylene blue and Cu(II) on magnetically oxidized tannic acid cross-linked carboxymethyl chitosan gels

In this work, tannic acid was selected as a green cross-linking agent to cross-link carboxymethyl chitosan to prepare a magnetic adsorbent (CC-OTA@Fe₃O₄), which was used to remove methylene blue (MB) and Cu²⁺. CC-OTA@Fe₃O₄ was characterized by FTIR, ¹³C NMR, XRD, VSM, TGA, BET and SEM. The adsorption behavior was studied using various parameters such as pH value, contact time, initial concentration of MB and Cu²⁺, and temperature. The results showed that adsorption of MB and Cu²⁺ followed the pseudo-second-order model and the Sips model. The maximum adsorption capacities were determined to be 560.92 and 104.25 mg/g MB and Cu²⁺ at 298 K, respectively. Thermodynamic analysis showed that the adsorption is spontaneous and endothermic in nature. According to the results of FTIR and XPS analyses, the electrostatic interaction was accompanied by π-π interaction and hydrogen bonding for MB adsorption, while complexation and electrostatic interaction were the predominant mechanism for Cu²⁺ adsorption. Furthermore, CC-OTA@Fe₃O₄ displayed remarkable stability in 0.1 M HNO₃, exhibited promising recyclability, and could be easily separated from aqueous solutions in the magnetic field. This study demonstrates the potential of CC-OTA@Fe₃O₄ as an adsorbent for the removal of cationic dyes and heavy metals from wastewater.