Comparative study on methylene blue adsorption behavior of coffee husk-derived activated carbon materials prepared using hydrothermal and soaking methods

Coffee husk, a carbonaceous precursor and common agricultural waste in Vietnam, was utilized to produce activated carbon by hydrothermal carbonization (HTC) with high adsorption capacity for pollutants. The characteristics and adsorption capacity of the hydrochar produced at the first HTC step (180 °C, 6 h) were subjected to further hydrothermal activation (130 °C for 2 h) with a low concentration of potassium hydroxide (ACHC-KOH 1 M), and directly soaked in KOH medium (ACHC-KOH 1:1). The surface area of the activated hydrochar obtained was significantly increased from 33.7 m2/g (raw hydrochar) to 703.9 m2/g for ACHC-KOH 1 M and 743.8 m2/g for ACHC- KOH 1:1, respectively. The maximum methylene blue (MB) adsorption achieved by the activated carbon materials were 357.38, 314.05, and 103.62 mg/g by ACHC-KOH 1 M, ACHC-KOH 1:1, and raw hydrochar, respectively. Adsorption kinetics and isotherms were employed to explain the MB adsorption process. Adsorption data of MB on ACHC-KOH 1 M and ACHC-KOH 1:1 fitted well with Langmuir adsorption isotherm and pseudo-second-order kinetic model. The results show that coffee husk-based activated carbon is a promising sorbent for the removal of MB from aqueous solutions. Compared to other carbonization methods, the HTC method has advantages of using a low temperature and concentration of KOH. [Display omitted] •Porous and activated carbon was prepared from biomass of coffee husk.•Hydrothermal activation and soaking activation with KOH for activated carbon's preparation.•Activated hydrochars were significantly increased surface porosity and functional groups.•The adsorption capacity of activated hydrochar by hydrothermal activation showed good removal of MB.•Adsorption kinetics of activated carbon following the pseudo-second-order kinetic model.