Tunable Lignin-Based Hydrogels for Enhanced Water Retention and Sustained Urea Release in Agriculture

Agricultural hydrogels with high lignin content are promising sustainable alternatives to petroleum-based superabsorbents. Here, kraft lignin was modified by Ce(IV)-initiated graft polymerization with polyacrylamide (PAAm), poly(acrylic acid) (PAAc), and their copolymers to produce lignin-derived hydrogels with tunable network structures and performance. Hydrogel properties were controlled by varying the composition of the grafted chains and cross-linker content. Materials containing PAAc fragments exhibited higher equilibrium swelling (265–400 g g–1) and faster water uptake than PAAm-grafted hydrogels (240–300 g g–1). Incorporation of 2.5 wt % hydrogel into soil prolonged moisture retention by approximately 50% under the tested conditions. The hydrogels also exhibited high urea loading capacity and sustained release over several days, with release predominantly governed by diffusion through the hydrogel matrix. Wheat seed germination tests confirmed the absence of short-term phytotoxicity. These lignin-derived hydrogels represent cost-effective and sustainable candidates for water-retaining and fertilizer-delivery applications in agriculture.