Expanding dye use elevates ecological risks by increasing dye-contaminated wastewater discharge. Cationic dyes resist removal owing to high stability and toxicity, but activated carbon adsorption remains an efficient and economical treatment. In this study, we examine methylene blue (MB) removal using activated carbon prepared from petroleum pitch through KOH activation, followed by nitrogen plasma treatment for 5, 10, 15, and 30 min. To characterize the surface chemistry and porosity of the nitrogen-functionalized carbon, we employed elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), and N2 adsorption–desorption (Brunauer-Emmett-Teller, BET) measurements. Batch adsorption tests were conducted at MB concentrations of 100, 200, 300, 400, and 500 mg L⁻¹. The nitrogen plasma treatment increased the surface nitrogen content (from 0.44% to 1.94% depending on the treatment conditions) and enhanced MB adsorption performance. Equilibrium data were fitted using the Langmuir and Freundlich isotherm models; the Freundlich model showed a better correlation, indicating multilayer adsorption on a heterogeneous surface and suggesting that plasmaintroduced nitrogen sites provided additional adsorption sites.

Effects of nitrogen functional groups and pore structure on high-affinity dye adsorption by plasma-treated activated carbon pellets
    Abstract
    1 Introduction
    2 Experimental
        2.1 Materials
        2.2 Preparation of petroleum pitch based activated carbon
        2.3 Pelletization of the prepared activated carbon
        2.4 Plasma treatment of the prepared activated carbon
        2.5 Adsorption removal of MB
        2.6 Characterization and evaluation of the prepared samples
    3 Results and discussion
    4 Conclusion
    References

• Min Seong Han(Department of Chemical Engineering, Daejin University, Pocheon 1007, Republic of Korea)
• Byong Chol Bai(Department of Chemical Engineering, Daejin University, Pocheon 1007, Republic of Korea) Corresponding author
• Woong Kwon(School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea)
• Youngbin Baek(Department of Biological Sciences and Bioengineering, Inha University, 100 Inha-ro, Michuhol-gu, Incheon 22212, Republic of Korea)
• Euigyung Jeong(Department of Textile System Engineering, College of Engineering, Kyungpook University, Daegu 41566, Republic of Korea)