Valorization of Rice Husk: Simultaneous Extraction of High-Purity Lignin and Silica via Organic Acid Treatment

Valorization of elements in agricultural rice husk waste benefits a circular society and can significantly reduce greenhouse gas (GHG) and PM 2.5 emissions from burning and landfilling. The research evinces the extraction of high-purity lignin and silica from rice husk waste using a primary acetic acid solution with a hydrochloric acid catalyst followed by calcination. The new concept reveals that treating rice husks with this acid solution under hydrothermal conditions can combine three essential processes of rice husk pretreatment, lignin extraction, and silica refinement into a single step. Consequently, it can reduce the total processing time. The treatment exhibits a synergistic mechanism wherein acetic acid facilitates dissolution while both HCl and acetic acid cooperatively catalyze the cleavage of lignin bonds. The resulting high-purity lignin nanoparticles (LNPs) obtained under condition A50-H0.3-T120 are hydrophobic, rich in aromatic compounds containing guaiacyl, syringyl, and p-hydroxyphenyl monomers in a ratio of 65.7:0.3:34, and possess a molecular weight of 2129 Da. Meanwhile, the high-purity silica obtained under condition A50-H0.6-T120 forms mesoporous silica nanoparticles (MSNs) with a specific surface area of 257 m2/g and a pore volume of 0.36 cm3/g. Finally, a techno-economic evaluation at a small- and medium-enterprise (SME) scale confirms that this process is economically favorable. The gate-to-grave life cycle assessment demonstrates that this process is a sustainable alternative for lignin and silica extraction as its enhanced biodegradable material recovery offsets total greenhouse gas emissions from the process.