Redox-driven ultrafast synthesis of lignin-based hydrogel: Optimization, characterization and prospects for slow NPK-fertilizer release under varied environmental conditions

Lignin, a highly abundant natural polymer, offers a promising path for creating eco-friendly hydrogels with versatile industrial applications, particularly in agriculture. In this study, a super-fast method was developed to synthesize alkali-lignin (AL) hydrogel using acrylamide (AM) as a monomer, N,N′-methylenebisacrylamide (MBA) as a cross-linker, and a new pair of redox initiator system composed of ammonium persulfate (APS) and ferrous sulfate (FS). This approach yields an instant, homogeneous hydrogel at room temperature, achieving gelation in approximately two minutes. The resulting AM-g-AL-MBA hydrogel exhibited exceptional water absorption capabilities, with structural analysis confirming the hydrogel’s characteristics. Varying concentrations of AL and MBA influenced its swelling capacity, porosity, and crosslinking density. Porosity also increased with lignin content, peaking at around 72 % with 60 % lignin. Furthermore, incorporating nitrogen, potassium, and phosphorus (NPK) fertilizer into the AM-g-AL-MBA hydrogel enabled a slow-release mechanism that operated efficiently across different conditions such as salinity, pH, and dosage. Over a 14-day period, the hydrogels released 43 % of the fertilizer. Additionally, the absence of antibacterial activity renders these hydrogels a safer option for agricultural applications, as their potential biodegradability minimizes environmental impact while still offering advantages in moisture retention and efficient nutrient delivery following non-Fickian diffusion mechanism. Ultrafast Synthesis of a Lignin-Based Slow-Release NPK Fertilizer for Sustainable Agriculture • The ammonium persulfate-ferrous sulfate redox pair enabled ultrafast formation of hydrogel. • NPK fertilizer incorporated into the lignin hydrogel structure by in-situ approach. • Up to 40 % lignin incorporation enhanced hydrogel production with superior water absorption. • Release study showed ˂50 % nutrients release following non-Fickian diffusion mechanism. • Lack of antibacterial activity implied biodegradability and safer agricultural use.