Green fabrication of a biodegradable magnetic-graphitic carbon nitride/chitosan–alginate composite sponge for high-performance Co(II) removal: Insights into adsorption mechanism, reusability, and response surface optimization

This study presents a new composite sponge made of magnetic graphitic-carbon nitride combined with chitosan and alginate (m-g-C₃N₄/CS-Alg). This composite is designed as a sustainable and biodegradable adsorbent to effectively remove Co(II) ions from water. Its synthesis followed an eco-friendly approach, with thorough characterization using various analytical techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscope (SEM), among others. The resulting m-g-C₃N₄/CS-Alg sponge has a highly porous structure by a surface area of 82.89 m 2 /g, enhancing its interaction with contaminants. Batch adsorption studies indicated that factors like pH, adsorbent quantity, contact time, initial Co(II) concentration, and temperature significantly influence adsorption rates. The maximum recorded monolayer capacity was 541.95 mg/g, consistent with the Langmuir model. The kinetic data aligned with the model of pseudo-second-order, indicating a chemisorption process, while thermodynamic evaluations revealed the method is endothermic and entropy-driven. The optimization of parameters was effectively achieved using Box-Behnken design with response surface methodology. Notably, this composite sponge showed excellent reusability across numerous cycles, with minimal concert degradation. Overall, the m-g-C₃N₄/CS-Alg sponge is affirmed as an efficient biopolymer-based adsorbent for sustainable water purification and heavy metal remediation.