Optimised Alkaline Extraction of Lignin from Maize Stover Agricultural Waste

Lignin, a abundant biopolymer in biomass, holds significant potential for environmental applications, particularly in heavy metal adsorption from contaminated water. In Mexico, maize stover, a major agricultural by-product in the Bajío region, is often underutilised, leading to environmental pollution. This study focuses on optimising lignin extraction from maize stover to transform this waste into a value-added material, addressing both waste management and water treatment challenges.

Lignin was extracted via alkaline hydrolysis using NaOH under varying conditions: NaOH concentration (10–40% m/V), temperature (25–60°C), and reaction time (3–72 hours). Two particle sizes (20 and 100 mesh) were tested, with mechanical agitation or sonication (40 kHz) as energy sources. The extracted lignin was characterised using FTIR spectroscopy to confirm its structural integrity.

Highest lignin yield (11%) was achieved using 40% NaOH at 25°C with sonication for 25 minutes, matching the theoretical lignin content in maize stover (11.1%). Traditional mechanical agitation at 60°C for 72 hours yielded only 8%. Ultrasonication not only improved efficiency but also reduced reaction time and energy consumption. Particle size (20 mesh) marginally enhanced yields, though handling larger particles proved more practical. FTIR analysis confirmed the characteristic lignin functional groups, including aromatic rings and hydroxyl groups. NaOH solution was successfully recovered for reuse, enhancing the sustainability of the method.

Ultrasonication significantly optimises lignin extraction from maize stover, offering a greener, faster, and more efficient alternative to conventional methods. This approach aligns with circular economy principles by minimising waste and maximising resource efficiency.