Sulfonated Cellulose: A Strategy for Effective Methylene Blue Sequestration

This study investigates the sulfonation modification of cellulose for the removal of methylene blue (MB) from aqueous solutions. The prepared biosorbent was characterized, and its sorption capacity, kinetics, and thermodynamics were systematically evaluated. Fourier-transform infrared (FTIR) spectroscopy analyzed structural modifications, while scanning electron microscopy (SEM) examined the surface properties. The optimal sorbent dosage was determined as 0.05 g. MB removal efficiency increased from 11% at pH 1 to 70% at pH 2, reaching 99% within the pH range of 3 to 7. Kinetic studies revealed rapid sorption, achieving 99% removal within 3 min. Among various isotherm models, the Langmuir model provided the best fit (<i>R</i> ² = 0.9989), indicating monolayer sorption with a maximum capacity of 37.65 mg/g. Thermodynamic analysis showed negative Δ<i>G</i>° values, confirming a spontaneous sorption process, while an enthalpy change (Δ<i>H</i>°) of -33.5 kJ/mol indicated exothermic behavior. The entropy change (Δ<i>S</i>°) of -82.6 J mol^-1·K^-1 suggested decreased disorder during sorption. Regeneration studies demonstrated that 0.2 M HCl combined with ethanol achieved the highest desorption efficiency, and after three cycles, the MB removal efficiency remained above 99%. The presence of -SO₃ ^- groups played a crucial role in MB sorption via ion exchange and may also contribute through hydrogen bonding, thereby enhancing MB sorption. These findings highlight sulfonated cellulose as an efficient and regenerable biosorbent for MB removal, offering valuable insights into its sorption mechanisms.