Aminated Lignin-Reinforced Biopolymer Hydrogels for Sustained Phosphate Delivery via Struvite Encapsulation in Acidic Environments

The escalating global demand for food production, coupled with excessive fertilizer use and freshwater depletion, necessitates sustainable solutions in agriculture. The excessive utilization of fertilizers to enhance crop productivity reflects a variety of negative impacts, including environmental and economic challenges. In this study, a biodegradable, dual crosslinked hydrogel composed of polyvinyl alcohol (P), chitosan (Chi), and aminated lignin (AL) is developed to encapsulate struvite (MgNH₄PO₄·6H₂O), a slow-release phosphate fertilizer. AL is synthesized via Mannich reaction using polyethyleneimine to enhance nitrogen content and functionality. The structural and functional characterization of the hydrogels is carried out using FTIR, SEM, XRD, and TGA. All pristine formulations exhibit high water-holding capacity with non-Fickian swelling behavior, reaching swelling values up to 706 ± 20.7%. Upon struvite loading, the swelling capacities reduce significantly, reflecting enhanced matrix density and encapsulation efficiency. Phosphate release studies in acidic citric solution (pH 3.3) show sustained release over 6−7 days. Kinetic modeling confirms a super case II transport mechanism (n > 1) and dominant diffusion-controlled release (Higuchi model), while a poor fit to pseudo-first-order kinetics indicates nonconcentration-dependent behavior. This study highlights the potential of lignin-based hydrogels as eco-friendly platforms for nutrient-efficient fertilizer delivery, offering a promising pathway toward sustainable agriculture.