Demethylation of Lignin from Rice-Straw Biorefinery: An Integrated Chemical and Biochemical Approach

The efficiency of lignocellulosic biorefineries is limited because of the high recalcitrance and low reactivity of lignin. The reactivity of lignin can be enhanced through various chemical and biochemical approaches. Demethylation is one of the methods that improve the availability of phenolic hydroxyl groups in lignin, thereby enhancing its reactivity and application in sustainable adhesives. The goal of this study is to integrate microbial and chemical approaches to aid in the demethylation of lignin. Towards that end, lignin was first extracted and purified from the rice straw biorefinery solid residue obtained post ethanol fermentation. This rice straw lignin was then subjected to chemical and microbial demethylation. For microbial demethylation under alkaline conditions, Pseudomonas putida and Pseudomonas fluorescens were employed, while demethylation under neutral conditions was conducted using Trametes versicolor. Integrated treatment using Pseudomonas putida followed by hydrogen iodide yielded an increase in the phenolic hydroxyl content by approximately 39–43%. Demethylation using chemical methods and biological methods alone provided approximately 18–27% increases in phenolic hydroxyl content, respectively. Furthermore, to assess the physical and chemical properties of demethylated lignin, FT-IR, TGA, and morphological analytical tools were employed.