Surface Modification of Activated Carbon Pellets by Oxygen Plasma and Their Ionic Dye Adsorption Behavior

In this study, the surface of pitch-based activated carbon pellet was functionalized with oxygen moieties via oxygen plasma, and the adsorption characteristics of ionic dyes were evaluated as a function of the functionalization degree. Activated carbon was prepared from petroleum pitch by KOH chemical activation, pelletized (pellet form), and then subjected to oxygen plasma for 0~5 min. SEM (Scanning Electron Microscope) analysis was used to observe changes in surface roughness and cracking with reaction time, while BET (Brunauer, Emmett and Teller) analysis revealed trends in the development and collapse of micropores. Notably, the 4 min sample exhibited the smallest hysteresis width in the adsorption–desorption isotherm, indicating a micropore-dominated structure. FT-IR (Fourier transform infrared spectroscopy), EA (Elemental analyzer), and XPS (X-ray photoelectron spectroscopy) showed that the total oxygen content and the C–O/C=O ratio increased with oxygen plasma reaction time and then partially decreased at 5 min due to over-etching. In adsorption tests with ionic dyes, the kinetics for MB (Methylene Blue) adsorption rate data shows excellent fit for similar pseudo-first-order and pseudo-second-order formulas, whereas MO (Methyl Orange, anionic dye) conformed best to the pseudo-second-order model, indicating the dominance of a chemisorption mechanism. Therefore, the oxygen plasma approach can tune the functional binding sites of activated carbon pellets, enhancing selective adsorption and rates for both cationic and anionic dyes, and represents a promising technique for dye-contaminated water treatment processes.