This study presents the synthesis, characterization, and utilization of marine macroalgae-derived bio-carbon catalysts (BC and KOH-AC) for the efficient conversion of waste cooking oil (WCO) into biodiesel. The biochar (BC) was produced through slow pyrolysis of macroalgal biomass, which was subsequently activated with potassium hydroxide (KOH) to produce a KOH-modified activated carbon (KOH-AC) catalyst. Advanced characterization techniques, including SEM, EDX, XRD, FTIR, and TGA, were used to examine the physicochemical characteristics of the catalysts. The synthesized catalysts were utilized to produce biodiesel from WCO, and the results revealed that the highest biodiesel yields, 98.96%, and 47.54%, were obtained using KOH-AC and BC catalysts, respectively, under optimal reaction conditions of 66 °C temperature, 12.3 M/O molar ratio, 130 min time, and 3.08 wt.% catalyst loading via RSM optimization. The kinetic and thermodynamic parameters, such as k, Ea, ΔH, ΔS, and ΔG, were determined to be 0.0346 min− 1, 43.31 kJ mol− 1, 38.98 kJ mol− 1, − 158.38 J K− 1 mol− 1, and 92.58 kJ mol− 1, respectively. The KOH-AC catalyst was recycled up to five times, with a significant biodiesel yield of 80.37%. The fuel properties of the biodiesel met ASTM (D6751) specifications, ensuring that it has excellent fuel characteristics and can be used as an alternative fuel.

Process optimization, kinetic, and thermodynamic studies of biodiesel production using KOH-modified bio-carbon catalyst derived from marine macroalgae
    Abstract
        Graphical abstract
    1 Introduction
    2 Materials and methods
        2.1 Materials
        2.2 Catalyst synthesis
            2.2.1 Synthesis of biochar (BC) from marine macroalgae
            2.2.2 Synthesis of KOH-modified activated carbon (KOH-AC)
        2.3 Catalyst characterization
        2.4 Transesterification of WCO into biodiesel
        2.5 Design of experiments (DOE) and process optimization
        2.6 Kinetic and thermodynamic investigations
        2.7 Biodiesel characterization
    3 Results and discussion
        3.1 Characterization of catalysts
            3.1.1 Scanning electron microscopy (SEM)
            3.1.2 Energy dispersive X-ray spectroscopy (EDX)
            3.1.3 X-ray diffraction (XRD)
            3.1.4 Fourier-transform infrared spectroscopy (FTIR)
            3.1.5 Thermogravimetric analysis (TGA)
        3.2 Process optimization using RSM-CCD
            3.2.1 Interpretation of regression model and ANOVA analysis
            3.2.2 Effect of transesterification parameters and their interactions on biodiesel yield
            3.2.3 Optimization of process variables and validation of regression model
        3.3 Kinetic study
        3.4 Evaluation of thermodynamic parameters
        3.5 Catalyst reusability
        3.6 Biodiesel characterization and fuel properties
        3.7 Comparison with other biochar-based catalysts
    4 Conclusions
    Acknowledgements 
    References

• Muhammad Zubair Yameen(Laboratory of Alternative Fuels and Sustainability, School of Chemical and Materials Engineering, National University of Sciences and Technology, H‑12, Islamabad, Pakistan)
• Salman Raza Naqvi(Laboratory of Alternative Fuels and Sustainability, School of Chemical and Materials Engineering, National University of Sciences and Technology, H‑12, Islamabad, Pakistan)
• Hamad AlMohamadi(Department of Chemical Engineering, Faculty of Engineering, Islamic University of Madinah, Madinah, Saudi Arabia)
• Shuang Wang(School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China)