Lignin Valorisation: Synthesis of Compounds and Applications in Environmental Engineering

Maize straw, the principal agricultural residue in the Bajío region of Mexico, is an abundant source of lignocellulosic biomass suitable for lignin recovery and subsequent valorisation in environmental engineering applications. This work reports the extraction and characterisation of lignin obtained from maize straw, with emphasis on process optimisation and on the study of the lignin surface as a preliminary step towards its use as an adsorbent material for heavy metals in water. Lignin was isolated by alkaline delignification using NaOH solutions, followed by controlled acidification of the black liquor, centrifugation, washing and drying, achieving yields close to 11%, in agreement with the theoretical lignin content reported for this agricultural residue. Characterisation by stereoscopic microscopy revealed a heterogeneous granular surface morphology with aggregates of different sizes, consistent with technical lignins. Complementary FTIR spectra confirmed the presence of functional groups typical of lignin, including bands associated with aromatic rings, C–O linkages and phenolic and aliphatic hydroxyl groups, indicating that the aromatic structure is preserved under the extraction conditions employed. The combination of suitable yield, accessible surface and a functional structure rich in potential coordination sites positions maize-straw lignin as a promising platform for the development of heavy-metal adsorbent materials within the framework of circular-economy strategies and the integral utilisation of agro-industrial residues.