Sustainable Lignin@ZnO Hybrid Materials: Synthesis, Characterization, and Preliminary Biostimulant Assessment in Tomato Plants

Agriculture faces pressing challenges in meeting the demands of a growing population while ensuring sustainable resource use, minimal contamination, and reduced waste. Agro-industrial waste valorization, particularly of lignin, offers promising solutions within a circular economy framework. Typically burned to sustain the bioethanol or pulp production process, technical lignins possess polyphenolic structures that make them potential biostimulants with antioxidant and growth-promoting features. This study explores the synthesis of lignin-based zinc oxide (ZnO) hybrid materials, combining the bioactive potential of lignin with the essential role of zinc as a plant micronutrient. Utilizing a Kraft lignin (HMW) as a capping agent, the synthesis of a hybrid material is optimized following an ecofriendly protocol. Comprehensive characterization by means of several analytical techniques confirmed the structural integrity of the lignin matrix and the morphology of the ZnO nanoparticles. In vivo experiments on tomato plants demonstrated enhanced dry shoot weight and zinc translocation to aerial parts of the plants, highlighting the controlled nutrient delivery capacity of the hybrid compared to ZnO alone. These findings underscore the potential of lignin@ZnO materials as innovative biostimulants, contributing to sustainable agriculture by repurposing lignin as a valuable agronomic resource.