Here, a novel nitrogen-doped carbon nano-material (N-CGNM) with hierarchically porous structure was prepared from spent coffee ground for efficient adsorption of organic dyes by a simple one-step carbonization process (the uniform mixture consists of spent coffee ground, urea, and CaCl2 with the ratio of 1:1:1, which was heated to 1000 °C with a rate of 10 °C min− 1 and held at 1000 °C for 90 min in N2 atmosphere to carry out carbonization, activation, and N-doping concurrently). The morphology and structure analysis show that the prepared N-CGNM exhibits hierarchical pore structure, high specific surface area (544 m2/ g), and large numbers of positively charged nitrogen-containing groups. This unique structure and chemical composition endow N-CGNM with an excellent adsorption capacity toward anion Congo red (623.12 ± 21.69 mg/g), which is obviously superior to that (216.47 ± 18.43 mg/g) of untreated spent coffee ground-based carbon nano-materials (CGM). Oppositely, the adsorption capacity of N-CGNM towards cation methylene blue is inferior to that of CGM due to the existence of electrostatic repulsion. These findings show a great guidance for the development of low-cost but efficient selective adsorbent.

    Abstract
    1 Introduction
    2 Experience
        2.1 Materials
        2.2 Preparation of N-CGNM
        2.3 Materials characterization
        2.4 Adsorption experiments
    3 Results and discussion
        3.1 Morphology and structure characterization
        3.2 Adsorption of dyes
        3.3 Adsorption isotherm and kinetics
    4 Conclusion
    Acknowledgements 
    References

• Zhong Dai(School of Materials Science and Engineering, Xi’an University of Technology)
• Peng‑Gang Ren(School of Materials Science and Engineering, Xi’an University of Technology, Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi’an University of Technology)
• Hua Zhang(School of Materials Science and Engineering, Xi’an University of Technology)
• Xin Gao(Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi’an University of Technology)
• Yan‑Ling Jin(School of Materials Science and Engineering, Xi’an University of Technology, Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi’an University of Technology)