Efficient application of rice husk ash-derived silica combined with calcined orange peel as catalysts for biodiesel production from Nigella sativa seed oil within a circular economy framework.

In this work, calcined orange peel and rice husk ash waste, an agricultural by-product were mixed to generate a solid base catalyst for biodiesel generation. This catalyst was used to create biodiesel from Nigella sativa seed oil. The objective was to create and describe a heterogeneous catalyst for transesterification that was both affordable and sustainable, utilizing orange peel and rice husk as the main waste materials. To produce ash, rice husks are first calcined for 5 h at 600 °C, followed by orange peels for 2 h at 700 °C. After that, the ash is mixed in different proportions (20 %, 30 %, and 40 %) and calcined again for 3 h at 600 °C in a muffle furnace. The 40 % ratio showed the most promising characteristics for the manufacture of biodiesel out of all the ash combinations. This specific mixture, which has the following elemental ratios according to SEM-EDS: O (41.0 %), Si (25.2 %), Ca (8.4 %), C (22.0 %), K (2.5 %) and Mg (0.5 %), was chosen due to its catalytic properties. It also has a large surface area of 3.8058 m2/g and a pore size of 585.69 Å. Wet impregnation was used to create the catalyst, which was then studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), surface area and pore size distribution characterization, and Fourier transform infrared spectroscopy (FTIR). A catalyst with an ideal methanol-to-oil molar ratio of 3:1, a 1-hour reaction period, and 7 % catalyst loading produced a 98.36 % biodiesel yield. Gas chromatography was used to ascertain the composition of fatty acid methyl ester (FAME), and the biodiesel's characteristics, including density, viscosity, acid value, and calorific value, were examined. The outcome showed that the rice husk ash catalyst supported by orange peel was very efficient at producing biodiesel and had good recyclability, holding its effectiveness for up to five cycles. The waste-derived catalyst was shown to be a better alternative for producing biodiesel when the biodiesel generated fulfilled ASTM criteria. Furthermore, the use of orange peel and rice husk in catalyst creation not only offers a sustainable option but also resolves trash disposal issues.