Calcium oxide nanoparticles from eggshell waste: A green nanotechnological strategy for microwave-assisted environmental clean up

This study presents a sustainable and innovative strategy for waste reutilization and environmental remediation through the green synthesis of calcium oxide (CaO) nanoparticles (NPs) derived from waste eggshells, predominantly composed of calcium carbonate. The CaO NPs were synthesized via a straight forward calcination process and characterized using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and UV–visible diffuse reflectance spectroscopy (UV-Vis DRS). XRD confirmed high crystallinity with an average crystallite size of 32 nm, while TEM revealed cubic nanoparticles in the range of 30–50 nm. TGA analysis demonstrated notable thermal stability up to 800 °C.The catalytic performance of the synthesized CaO NPs was evaluated via microwave-assisted degradation of Malachite Green (MG), a model organic pollutant. Under microwave irradiation, CaO NPs achieved 86% degradation within 30 minutes, significantly outperforming raw eggshells (72%). The degradation followed pseudo-first-order kinetics. Optimization studies revealed enhanced degradation efficiency (up to 93.40%) at 900 W microwave powers and 94.59% efficiency with a catalyst dose of 250 mg/L. However, increasing MG concentration from 20 to 50 mg/L resulted in a decline in degradation efficiency from 86% to 60.2%. Recyclability assessments showed 77% degradation efficiency after four consecutive cycles, indicating the catalyst’s stability and reusability. Scavenger experiments identified the involvement of reactive species, including hydroxyl radicals (•OH), superoxide anions (•O₂⁻), and photo-generated holes (h⁺), in the degradation mechanism. Furthermore, LC-MS analysis proposed a plausible degradation pathway based on intermediate m/z values. Compared to conventional thermal or chemical degradation methods, the microwave-assisted catalytic process using CaO NPs demonstrated superior efficiency, rapid reaction kinetics, and reduced energy consumption. This work highlights the potential of converting bio-waste into high-value nanomaterials for scalable, eco-friendly, and cost-effective wastewater treatment applications.