A sustainable and cost-effective approach was developed for synthesizing carbon nanostructures namely carbon nanotubes (CNTs), carbon spheres (CSs), and carbon fibers (CFs). The process employed pyrolyzed hydrochar derived from treated rice straw, kaolin, zeolite or hydrochar as supports for Fe–Ni bimetallic catalysts, while hydrochar, camphor, or cotton fiber served as carbon sources. The resulting nanostructured materials were characterized using FE-SEM, XRD, FTIR, and N2 adsorption analyses. These tools demonstrated that morphology and structure of the carbon materials produced are governed by the carbon precursor, catalyst support, and catalyst-carbon interactions. The resultant carbon nanostructures have distinctive graphitic characteristics and surface functions that improve adsorption performance. The adsorption performance of the synthesized nanostructures was evaluated using methylene blue (MB) as a model pollutant. Among them, CNTs exhibited the highest adsorption capacity (~ 130 mg/g), which was attributed to its a large specific surface area and abundant π–π interaction sites. Adsorption behavior of MB dye followed the Langmuir isotherm and pseudo-second-order kinetic models, indicating monolayer chemisorption with multiple rate-controlling steps. This work highlights an efficient route for valorizing agricultural waste into functional carbon nanostructures for wastewater remediation.

Valorization of rice by-products for carbon nanostructures synthesis in dye wastewater remediation
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Materials
        2.2 Preparation of support samples
            2.2.1 Synthesis of carbon nanotubes
            2.2.2 Synthesis of carbon spheres
            2.2.3 Synthesis of carbon fibers
    2.3 Structural and surface characterization
    2.4 Equilibrium and kinetic adsorption experiments
    3 Results and discussions
        3.1 Morphology and surface features
        3.2 Rationale mechanisms for carbon nanostructures growth
        3.3 Effect of pH and isotherm adsorption studies
        3.4 Effect of contact time and kinetic studies
    4 Conclusions
    References

• Nady A. Fathy(Physical Chemistry Department, Advanced Materials Technology and Mineral Resources Research Institute National Research Centre, 33 EL Buhouth St., Dokki, CairoP.O. 12622, Egypt) Corresponding author
• Sahar M. El-Khouly(Physical Chemistry Department, Advanced Materials Technology and Mineral Resources Research Institute National Research Centre, 33 EL Buhouth St., Dokki, CairoP.O. 12622, Egypt)
• Ghada M. Mohamed(Physical Chemistry Department, Advanced Materials Technology and Mineral Resources Research Institute National Research Centre, 33 EL Buhouth St., Dokki, CairoP.O. 12622, Egypt)
• Ho-Jin Son(Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Republic of Korea, Division of Smart Energy Convergence Engineering, Korea University, Sejong 30019, Republic of Korea)