In-process synthesis of sulfur-free nano-lignin during organosolv extraction from rice husk and straw: process optimization, functionality, and valorization potential

Rice husk and straw are abundant byproducts of rice cultivation, with global annual production exceeding 8 × 10¹¹ kg and 1.5 × 10¹¹ kg respectively, predominantly in Asia and Africa. Currently, most of these lignocellulosic agro-wastes are underutilized or burned, causing significant air pollution and environmental harm, especially in northern India. This study presents a sulfur-free organosolv extraction method that enables the in-process synthesis of nano-lignin particles (< 35 nm) directly during biomass pulping, simultaneously facilitating cellulose recovery from rice husk and straw. The optimized protocol, employing a formic acid:acetic acid (FA:AA) ratio of 70:30 and a 120-minute reaction time, yielded maximum lignin recovery of 16.7% from husk and 10.11% from straw. The process produces high-purity, amorphous nano-lignin with preserved phenolic and carbonyl functionalities, confirmed by FTIR and XRD analyses. Uniquely, the extracted nano-lignin exhibits broad-spectrum antimicrobial activity against Escherichia coli and Staphylococcus aureus , with maximum inhibition observed at 0.05 g/mL, attributed to phenolic hydroxyl groups inherent in sulfur-free lignin. Additionally, biosorption tests demonstrate 95% removal of arsenic (3 mg/L) from aqueous solutions, validated by FTIR evidence of arsenic-lignin complex formation. The simplified, scalable extraction process eliminates post-extraction nanoparticle synthesis and avoids sulfur contamination, offering an economically and environmentally sustainable valorization route for rice agro-wastes. This dual-functional nano-lignin presents promising applications in agricultural bioproducts and environmental remediation, advancing circular bioeconomy goals within industrial crop sectors.