Removal of methylene blue by hydrochar modified from hydrothermal carbonization technique

Abstract This study explores the adsorption potential of hydrothermally treated waste derived algal biomass for removing methylene blue (MB) dye. Synthesizing a modified hydrochar through hydrothermal carbonization (150°C, 35 bar) followed by NaOH modification, we observed enhanced thermal stability and distinctive chemical changes. Optimal conditions were determined at pH 6 and 1 h contact time. Soluble salts with cations were identified as impacting adsorption efficiency, with increased interference for higher cation charges. Thermodynamic parameters (Δ G , Δ H , and Δ S ) indicated a spontaneous and exothermic process, the calculated values (−5.417 to −6.907 kJ mol −1 , −29.0 kJ mol −1 , −73.8 J K −1 mol −1 ) aligned with this behavior. Adsorption isotherms favored the Freundlich model, revealing heterogeneous multilayer adsorption, with a maximum capacity of 97%. Kinetic studies supported the pseudo‐first‐order model. This detailed exploration provides insights into thermodynamics, kinetics, and the impact of adsorption parameters on MB removal, emphasizing the practicality of alkaline‐modified hydrochar as an effective, sustainable adsorbent.