Agricultural Waste-Derived Lignin and Chitosan Blend Biohydrogels for Controlled Release of Muriate of Potash for Sustainable Agricultural Applications

Hydrogels have emerged as promising materials for agricultural applications by improving soil water management and nutrient-use efficiency through a controlled nutrient release. However, most commercially available hydrogels are derived from synthetic polymers and raise environmental concerns. Guided by the 12 principles of green chemistry, we developed agro-residue waste-derived biohydrogels for slow-release fertilization. Lignin extracted from mustard straw was blended with chitosan to form a biohydrogel, into which muriate of potash (MOP) was incorporated using an in situ loading strategy. Chitosan was selected because it contains primary amine (−NH2), hydroxyl (−OH), and residual acetamide (−NHCOCH3) groups, which enable interactions with other molecules through hydrogen bonding at neutral pH. The resulting hydrogel exhibited sustained potassium release with ∼91% cumulative release in deionized water over 50 days and a loading efficiency of ∼29%. Release kinetics in both water and soil were best described by the Peppas–Sahlin model (R2 = 0.99), indicating diffusion-dominated transport. When applied to soil, lignin–chitosan biohydrogels enhanced water retention for up to ∼65 days, which decreased to ∼45 days upon MOP loading. These results demonstrate the potential of lignin–chitosan biohydrogels as sustainable platforms for simultaneous water retention and controlled potassium delivery in agricultural systems.