Oxygen Plasma Functionalization of Activated Carbon Pellets for Hazardous HCl Gas Mitigation: Balancing Surface...

Hydrogen chloride (HCl) is a hazardous acidic gas released from industrial processes and waste-treatment systems, posing risks to human health, process safety, and the surrounding environment. Accordingly, there is a need for practical adsorbent materials that can reduce HCl exposure without generating secondary liquid waste. In this study, pitch-based activated carbon pellets were surface-functionalized by oxygen plasma treatment to improve fixed-bed HCl removal performance. Plasma treatment was applied for 1, 2, and 4 min, and the resulting changes in surface chemistry, pore structure, and adsorption behavior were investigated using SEM, XPS, N₂ adsorption-desorption analysis, and breakthrough experiments. Oxygen plasma treatment increased the oxygen-containing surface functionalities of the pellets while largely preserving pellet morphology. Under moderate treatment conditions (1-2 min), the BET surface area and pore volume were mostly maintained, whereas prolonged treatment (4 min) reduced the accessible pore structure. In fixed-bed adsorption tests, the sample treated for 1 min showed the longest breakthrough behavior and the highest HCl uptake among the tested samples, while the sample treated for 2 min exhibited the shortest mass transfer zone and the highest bed utilization. These results indicate that controlled oxygen plasma treatment can improve the removal of hazardous HCl gas by balancing surface functionalization and pore preservation. The findings suggest that plasma-functionalized activated carbon pellets are a promising option for toxic acidic gas mitigation in air pollution control and waste-treatment applications.