Effect of the Thermal Activation on the Adsorption Capacity of Cationic and Anionic Dyes in Magnetic Carbon

The sustainable synthesis of multifunctional magnetic carbons was achieved using sugar cane bagasse by hydrothermal carbonization with ferric nitrate, followed by thermal activation under CO2 and N2 at 500-900 °C. Structural, magnetic, and surface characterizations were performed to evaluate their physicochemical properties and explore their potential for environmental applications, including the adsorption of cationic and anionic dyes. Activation at 700 °C significantly enhanced the material properties, particularly under N2, yielding a high specific surface area (241 m2 g-1), notable magnetization (27.4 emu g-1), and a low I D/I G ratio (0.42), indicative of graphitic domains. While CO2 activation led predominantly to magnetite formation, N2 favored the formation of iron carbide and zero-valent iron. The materials exhibited high adsorption capacities for methylene blue (MB; 81.4 mg g-1) and reactive blue 19 (RB19; 74.8 mg g-1). Adsorption kinetics followed mixed mechanisms involving both physisorption and chemisorption, while the Sips isotherm model best described the equilibrium, suggesting heterogeneous surface interactions. Activation at 700 °C under N2 was particularly effective, enhancing MB and RB19 adsorption by up to 5.5- and 15.5-fold, respectively. This performance was mainly attributed to the increased specific surface area and pore volume, which facilitate dye diffusion and retention. The N2 atmosphere limited carbon oxidation, promoting the development of mesoporous structures that efficiently adsorb both cationic and anionic dyes, underscoring the multifunctionality and sustainability of these materials for environmental applications.