Preparation and characterization of activated carbon from hydrochar by hydrothermal carbonization of chickpea stem: an application in methylene blue removal by RSM optimization

Herein, mesoporous activated carbon (AC) was prepared through potassium hydroxide (KOH) activation of hydrochar derived from the hydrothermal carbonization (HTC) of chickpea stem (CS), and successfully applied to remove methylene blue (MB) dye from aqueous solutions in a batch system. The HTC-CSAC was prepared depending on different impregnation ratios (hydrochar:KOH, 50–150%), impregnation times (12–48 h), activation temperatures (400–600 (Formula presented.)) and activation times (30–60 min). To define HTC-CSAC, various analytical techniques such as iodine adsorption number (IAN), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) were used. In the removal process of MB by the best HTC-CSAC with a high IAN of 887 mg g⁻¹ obtained under conditions including impregnation ratio of 70%, activation time of 45 min, activation temperature of 600 (Formula presented.) and impregnation time of 24 h, the effects of adsorption parameters such as pH factor (2–10), adsorbent dosage (50–100 mg), initial MB concentration (40–80 mg/L) and contact time (90–180 min) were studied. Besides, a detailed evaluation of the adsorption mechanism for the removal of MB by HTC-CSAC was performed. The Langmuir model indicated the best isotherm data correlation, with a maximum monolayer adsorption capacity (Q_max) of 96.15 mg g⁻¹. The adsorption isotherm findings demonstrated that the MB removal process is feasible, and that this process takes place through the physical interaction mechanism. Additionally, the HTC-CSAC adsorbent exhibited a high regeneration and reuse performance in MB removal. After five consecutive adsorption-desorption cycles, HTC-CSAC maintained the reuse efficiency of 77.86%. As a result, the prepared HTC-CSAC with a high BET surface area of 455 m² g⁻¹ and an average pore diameter of 105 Å could be recommended as a promising and reusable adsorbent in the treatment of synthetic dyes in wastewaters.