Exploring activated carbon-based microalgae residue to improve CO₂ adsorption performance

The rising atmospheric carbon dioxide (CO₂) levels have driven interest in Carbon Capture and Storage (CCS) technologies. Adsorption technology has gained significant attention because of its cost-effectiveness, high efficiency, and scalability. This study focused on producing activated carbon from Nannochloropsis gaditana microalgal residue (post-lipid extraction) using a one-step KOH activation and carbonization method. The resulting activated carbon was characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDX) to examine the surface morphology, Brunauer-Emmett-Teller (BET) analysis to determine the surface area and porosity, and Fourier Transform Infrared Spectroscopy (FTIR) to identify the surface functional groups. CO₂ adsorption performance was evaluated at different temperatures (25, 40, and 55 °C) and inlet feed flow rates (200 and 400 mL/min). The kinetics of CO₂ adsorption and regeneration of activated carbon were examined. SEM results showed successful activation of the microalgae-residue with mesopores and micropores, while EDX showed an increase in carbon content in the activated carbon compared to raw microalgae residue. BET result showed that the prepared activated carbon has a surface area of 296.96 m2/g, average pore diameter of 2.26 nm and total pore volume of 0.17 cm3/g. The presence of oxygen-containing surface functional groups such as hydroxyl (OH) and carbonyl (C=O) in activated carbon were confirmed by FTIR spectroscopy. The highest adsorption capacity of 0.55 mmol/g was obtained at 25 °C and 400 mL/min. The longest breakthrough time (7 min) was observed at 25 °C and 200 mL/min. In this study, CO₂ adsorption followed pseudo-first-order kinetics, and a regeneration study showed good stability of activated carbon over four cycles.