Potential of Physical Activated Carbon Derived from Pyrolyzed Waste Coffee Grounds as an Adsorbent for Dye Removal

Abstract This study investigates the adsorption performance of activated carbon prepared from spent coffee grounds (SCG) through pyrolysis and subsequent CO₂-assisted physical activation for the removal of Methylene Blue (MB) dye from aqueous solutions. SCG was pyrolyzed at 400 °C, 500 °C, and 600 °C, and the resulting biochars were activated at 900 °C under CO₂ flow. The obtained materials were characterized using FTIR, SEM, BET, and elemental analyses to evaluate structural, morphological, and chemical properties. Among the samples, the activated carbon derived from 600 °C pyrolysis exhibited the highest surface area (44.59 m 2 /g) and was selected for detailed adsorption studies. Batch experiments were conducted to investigate the influence of pH (4–9), adsorbent dosage (2.5–15 g/L), initial dye concentration (15–50 mg/L), temperature (10–40 °C), and contact time (0–120 min) on MB removal efficiency. The maximum removal efficiency (99.15%) was obtained at 15 g/L and 45 min, while comparable performance (> 98.9%) was achieved even at lower dosages, indicating high adsorption capacity and economic feasibility. Kinetic analysis revealed that the adsorption process followed the pseudo-second-order model (R 2 > 0.94). The Freundlich isotherm (R 2 = 0.9924) provided the best fit, indicating multilayer adsorption on heterogeneous surfaces. These findings demonstrate that SCG-derived activated carbon is an effective, sustainable, and low-cost adsorbent with strong potential for dye removal and wastewater purification. The study also highlights the environmental value of coffee waste valorization within a circular economy framework, contributing to resource recovery and sustainable material development.