Isotherms, Thermodynamics and Regeneration Studies of CO2 Adsorption on Activated Carbon Impregnated With Waste-Sourced Natural Amino Acids

This study investigated the CO₂ adsorption isotherms, thermodynamic properties and regeneration efficiency of palm shell activated carbon (AC) impregnated with waste-sourced natural amino acids from egg white (EW), namely, ACEW-30. Initially, the performance of ACEW-30 was compared with AC impregnated with fresh EW and synthetic amino acids using fixed-bed adsorption system. The results revealed that ACEW-30 prepared from waste sources demonstrated comparable performance with other adsorbents tested, suggesting its potential for waste valorisation. Afterwards, the data were fitted to various adsorption isotherm models, namely, Langmuir, Freundlich, Sips, Toth, Dubinin–Radushkevich and Temkin, to characterise the adsorbate-adsorbent interaction between CO₂ molecules and ACEW-30 at different adsorption temperatures (25–50°C) and CO₂ partial pressures (0.15–0.30 vol.%). The isotherm results were used to evaluate thermodynamic properties using Van't Hoff and Clausius–Clapeyron equations. The effect of regeneration conditions (desorption temperatures and nitrogen purging flow rate) have also been investigated prior to cyclic adsorption–desorption experiments. Overall findings indicate that CO₂ adsorption on ACEW-30 was best fitted to Freundlich isotherm, spontaneous and exothermic in nature. The isosteric heat of adsorption was within 20–24 kJ/mol, suggesting that the adsorption mechanism lies within the intermediate region between purely physical and purely chemical. Remarkably, the results obtained from regeneration studies reveal that ACEW-30 exhibited high regeneration stability at 25°C and 800 mL/min purging flow rate, with more than 87% efficiency even after 20 cyclic adsorption–desorption.