Valorization of rice husk agricultural waste through lignin extraction using acidic deep eutectic solvent

This work reports lignin extraction from rice husk using deep eutectic solvent (DES) made using a biodegradable hydrogen bond donor, p-toluene sulfonic acid. The stability of the deep eutectic solvent was studied by the occurrence and strength of the hydrogen bonds using density functional theory calculation. The reduced density gradient plot showed that the proposed DES formed a stable H-bonded structure with −0.038 a.u. The effects of process conditions such as reaction temperature, reaction time, solid loading ratio, and water content were examined. Reaction temperature imparted a higher influence than time on lignin's structural and chemical properties. Under all processing conditions, deep eutectic solvent was capable of extracting lignin with good homogeneity, which was evident from the low polydispersity index (<1.26). Lignin from optimized reaction conditions had a total phenolic content of 430.23 μg ferulic acid equivalent/mg and exhibited antioxidant activity with an IC50 (half – minimal inhibitory concentration) value of 0.386 mg/mL. This work demonstrates the application of a promising acidic deep eutectic solvent for lignin and cellulose valorization.