COFFEE-GROUND–DERIVED HYDROTHERMALLY ACTIVATED CARBON: SYNTHESIS, CHARACTERIZATION, AND ADSORPTION PERFORMANCE TOWARD METHYLENE BLUE

Spent coffee grounds (SCGs), an abundant lignocellulosic agro-industrial residue, were valorized into porous carbonaceous adsorbents through hydrothermal carbonization (HTC) followed by alkaline activation, aiming at the efficient removal of methylene blue (MB) from aqueous solutions. HTC was conducted under subcritical water conditions at temperatures of 180-220 °C and residence times of 2-6 h, producing hydrochars with tunable physicochemical properties. The obtained hydrochars were subsequently activated using potassium hydroxide in a microwave reactor to enhance surface functionality and porosity. The structural, textural, and chemical characteristics of the materials were comprehensively analyzed by N₂ physisorption (BET surface area and pore size distribution), FTIR spectroscopy, SEM, XRD/Raman spectroscopy, and elemental analysis (XRF/EDX).

Batch adsorption experiments were performed to evaluate the adsorption kinetics, equilibrium isotherms, and thermodynamic behavior toward methylene blue. The activated coffee-ground-derived carbons exhibited rapid adsorption kinetics and significantly enhanced adsorption capacities compared to non-activated hydrochars. The improved performance was attributed to the synergistic effects of developed micro- and mesoporous structures and abundant oxygen-containing surface functional groups. Mechanistic analysis indicated that methylene blue uptake was governed by a combination of pore filling, electrostatic attraction, π– π electron donor–acceptor interactions between the aromatic carbon surface and dye molecules, hydrogen bonding, and intraparticle diffusion.

The results demonstrate that hydrothermal carbonization coupled with mild alkaline activation provides an effective and sustainable route for converting spent coffee grounds into high-performance adsorbents. This study elucidates the relationships between synthesis conditions, surface chemistry, textural properties, and adsorption performance, highlighting the potential of SCG-derived activated carbon for practical wastewater treatment applications.

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