Hydrothermal сarbonization of <i>Spirogyra</i> sp. algae for adsorption and regeneration of malachite green dye

In this study, we prepared hydrochar (HCSP) from Spirogyra sp. algae (SP) as adsorbent to eliminate malachite green (MG) dye and conducted the adsorbent regeneration study. SP and HCSP were characterized using FTIR, XRD, SEM, and BET. The adsorption performance was evaluated under various conditions of pH, contact time, concentration, and temperature. SP and HCSP were optimum at pH 8 for MG adsorption. The suitable adsorption kinetics model was PSO, and the adsorption isotherm model tended to be the Freundlich model. The maximum adsorption capacity (Qmax) obtained from Langmuir calculation at 30 °C, SP and HCSP were 34.96 mg/g and 99.01 mg/g, respectively. The thermodynamic parameters indicated endothermic and spontaneous processes, with HCSP exhibiting greater adsorption capacity and spontaneity than SP. Regeneration studies demonstrated that HCSP and SP materials could be reused up to the 4th cycle with the remaining efficiency of 69.37% and 49.30%, respectively. Compared to SP, HCSP has better adsorption capacity.