Filling in the gaps in second‑generation biorefineries: evaluating rice straw and its bioethanol residue for the production of biogenic silica nanoparticles

In the present scenario of accelerated research on production of ecofriendly bioethanol, second-generation bioethanol generation has particularly gained wide attention. These utilize agricultural feedstock and industrial residues, therefore assisting in their management without presenting any competition for food supplies. Rice straw is one such abundantly generated agricultural residue with a great potential for sustainable production of bioethanol. During this bioethanol production process, a recalcitrant residue is generated that mainly contains lignin phenolic compounds, unhydrolyzed carbohydrates and some other minor components. This lignin-rich by-product has been used to produce energy and other functional biomaterials in order to make the overall process of second-generation biorefineries more cost-effective and sustainable. In this study, we proposed a green approach for the management of rice straw and its lignin-rich bioethanol residue by the synthesis of silica nanoparticles using microbial fermentation. The presence of the as-synthesized silica was indicated by Fourier transform infrared spectroscopy (FTIR), atomic absorption spectroscopy (AAS) and UV–Visible spectroscopy. The FTIR analysis showed the Si–O–Si peak at 1070 cm− 1 indicating the presence of silica. This was also proved from the elemental composition calculated by AAS results which showed the presence of high concentrations of Si (about 397,711.7 ppm) along with other elements such as Ca (about 2749 ppm), Mg (209 ppm), Fe (about 7057.9 ppm) and Na (about 955 ppm). Therefore, based on the results of this study, it is proposed that the lignin-rich residue extracted from rice straw bioethanol fermentation process has high potential to be exploited for production of silica nanoparticles. These biomass-derived nanoparticles can be used for a wide array of applications in agriculture, nanomedicine and biosensors.