Rice husk ash as a green feedstock for the extraction of nano-silica and its application in the synthesis of an efficient solid biocatalyst

Every year, millions of tons of rice are produced. About 20% of the rice weight is constituted of what is called rice husks (RH), which are removed during the milling process, generating approximately 150 million tons per year of residue [1]. Generally, the disposal of this waste in open fields or landfill creates environmental and human health problems [2]. The high calorific value of RH makes it a good and low-cost source of renewable energy, but high quantities of rice husk ash (RHA) (17- 26 %) rich in silica (90 - 95 %) are generated [3]. This disadvantage, however, can be transformed into a strength, because from this waste it is possible to extract a raw material widely used for industrial applications like the production of ceramics, electronics, catalysts, pharmaceutics, and other materials, i.e., silica. The extraction process from RHA presents the fundamental advantage of having a significantly lower environmental impact as compared to its commonly employed counterpart, i.e., sand extraction. Quartz extraction from natural sand exploits a non-renewable feedstock and not only destroys the natural landscape where the quarries are placed but also introduces a great number of pollutants into the environment both during the raw material acquisition and the lengthy procedures of silica extraction. Hence, with a view to developing a circular economy, this work aimed to reuse waste (RHA) as a new feedstock for silica extraction to be used in the synthesis of a biomaterial for enzymatic immobilization. That allows to recycle of something that would otherwise be discarded and may potentially save on the costs of industrial processes. In fact, among the main advantages of enzymatic immobilization are present the stabilization and improved robustness of the immobilized enzyme, as well as the possibility of reusing it, all factors that make it possible to significantly reduce the costs of an industrial process. To achieve this purpose, the development of a solid biocatalyst based on composite polymer scaffolds containing RH nano-silica (RH/NS) for the immobilization of laccases from Trametes Versicolor was investigated. Laccases were chosen as catalysts since they are considered versatile enzymes capable of oxidizing many phenolic and non-phenolic molecules thanks to their low substrate specificity, using oxygen as the electron acceptor, and generating water as a by-product. The nano-silica extraction process obtained from rice husk was optimized and the mesoporous nano-silica was characterized by X-ray diffraction (XRD), ATR-FTIR spectroscopy, Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller analysis (BET), and Energy Dispersive X-ray spectroscopy (EDX). XRD patterns revealed the amorphous nature of the extracted nano-silica while EDX confirmed the presence of pure SiO2 containing several silanol and siloxane groups as evidenced by ATR-FTIR analysis. So, an amorphous nano-silica with a purity of 99.6 %, size of 70-100 nm, a surface area of 111 m2/g, and porosity of 77 % was obtained by thermal treatment at 550 °C and extraction process using rice husks. The nano-silica obtained was then introduced in a crosslinked chitosan-alginate scaffold and then functionalized with γ-aminopropyltriethoxysilane (APTES) to create a solid support suitable for enzymatic immobilization. The use of a three-dimensional polymer structure was aimed at guaranteeing an easier recovery of the biocatalyst from the reaction mixture as well as better stability of the enzyme under varying experimental conditions. Using thermogravimetric analysis (TGA) was demonstrated that the introduction of RH/NS allowed an improvement in the thermal and mechanical properties of the chitosan-alginate scaffold. Subsequentially, the synthesized scaffold was used for laccase covalent immobilization exploiting the amine groups inserted with APTES, which guaranteed the laccase immobilization in a conformation favorable for the interaction with the substrates (ABTS and syringic acid). The solid biocatalyst obtained evidenced high immobilized activity (3.8 U/g) and good reusability in the oxidation of ABTS (about 50 % up to 6 cycles). Furthermore, the biocatalyst was employed in syringic acid removal from water, since the use of immobilized laccases for the treatment of organic contaminants is considered a promising new and eco-friendly for bioremediation of polluted sites. The synthesized biocatalyst was able to totally oxidize syringic acid in 24 h, as shown by the HPLC-DAD analysis performed. To sum up, this work refined a method to extract highly pure nano-silica from rice husk ash and, for the first time, a crosslinked chitosan-alginate scaffold containing RH nano-silica was successfully used for laccase immobilization to obtain an efficient, cheap, and easy-to-use biocatalyst. Furthermore, it should be mentioned that the synthesized nano-silica was not only an excellent filler for composite materials and support for enzymatic immobilization but may also become a potential resource of low-cost precursors for the production of high-value-added silica-based materials for industrial uses. [1] P.P. Nayak, S. Nandi, A.K. Datta, Eng. Reports. 1 (2019) 1–13. https://doi.org/10.1002/eng2.12035. [2] R. Pode, Renew. Sustain. Energy Rev. 53 (2016) 1468–1485. https://doi.org/10.1016/j.rser.2015.09.051. [3] J.G. Buta, N. Balasubramanian, Int. J. Sci. Eng. Res. 8 (2017) 1158–1169.