Selective Adsorption of Cationic Dyes by Hydrochar Derived from Spirogyra sp. Algae via Temperature-Varying Hydrothermal Carbonization

This study synthesized hydrochar adsorbents at varying temperatures 150°C (HC150) and 250°C (HC250), using the hydrothermal carbonization (HTC) method from Spirogyra sp. (SPG) and characterized them using XRD, FTIR, SEM, and BET analyze. XRD results confirmed a dominant CaCO₃ phase (JCPDS 01-086-2334) in all samples, with new peaks and shifts in HC150 and HC250 indicating structural changes from hydrochar formation. FTIR spectra revealed functional groups such as O-H, C=O, C-O, and C-N-S in SPG, with notable reductions in O-H and phenolic signals in HC150 and HC250, suggesting chemical transformations. SEM analysis revealed that SPG exhibits a smooth, sheet-like morphology, while HC150 formed uniform particles and HC250 developed a rough, porous surface, indicating increased carbonization, surface heterogeneity, and enhanced adsorption potential. BET analysis showed a marked increase in surface area and a transition from macroporous to mesoporous structures in HC150 and HC250, enhancing their adsorption capabilities. Adsorbents exhibited selective adsorption toward methylene blue (MB). Surface charge analysis revealed similar pHpzc values slightly below neutral, promoting favorable electrostatic interactions with cationic MB. Adsorption followed pseudo second order (PSO) kinetics compared to pseudo first order (PFO), indicating chemisorption, while isotherm modeling highlighted HC250 strong fit to the Freundlich model. Thermodynamic assessments confirmed HC250 superior performance, exhibiting more negative ∆G, lower ∆H, and higher ∆S values, signifying spontaneous, energy-efficient, and affinity-driven adsorption. Regeneration tests further underscored HC250 stability, with removal efficiency maintaining above 50% after four cycles (90.14% to 53.55%). In comparison, HC150 showed good reusability (80.56% to 51.88%), while SPG declined significantly (56.23% to 50.20%) after two cycles.