Biovalorization of mandarin waste peels into silver nanoparticles and activated carbon

This work aims to upcycle mandarin (Citrus reticulum) waste peels into valuable compounds with different applications. The one-factor-at-a-time method was applied to optimize the biosynthesis of silver nanoparticles using the hot water extract of mandarin peels’ waste. The maximum production reached 2.5 g L⁻¹ in a 4-h, pH9, 100 rpm continuous stirring batch process, operating at 30 °C, under fluorescent illumination of 36 W/6400 K, using 3000 mg L⁻¹ extract solution and 2 mmol AgNO₃. Dynamic light scattering, zeta potential, X-ray diffraction, energy-dispersive X-ray, Fourier transform infrared spectroscopy, field emission scanning electron microscope and high-resolution transmission electron microscope were employed to characterize the prepared silver nanoparticles, which revealed highly stable, uniformly distributed, nonagglomerated crystalline silver nanoparticles, with spherical/oval shapes and a size range of 10–19 nm. The preliminary cost analysis proved the cost-effectiveness of the valorization of mandarine peels into silver nanoparticles, which costs approximately 7.6 US$/g green synthesized silver nanoparticles with good savings relative to the global prices of the chemically synthesized ones. Moreover, to reach the point of zero waste and maximize the profitability of the valorization, the mandarin spent waste disposed from the batch process were upcycled to activated carbon which has different applications.