Synthesis and Characterisation of Eco-Friendly Chitin-Based Superabsorbent Polymers for Fertiliser Slow Release, Water Absorption Kinetics, and Plant Growth Enhancement

Abstract With the global agricultural sector facing major challenges including; drought, water runoff, and fertiliser loss, the growing demand for sustainable practices has intensified. As a result, superabsorbent polymers (SAPs) have garnered attention due to their ability to improve soil water retention and the controlled, slow-release of nutrient molecules. This study explores the synthesis and characterisation of semi-synthetic chitin-based SAPs derived from shellfish waste, offering an eco-friendly alternative to conventional synthetic SAPs. Two SAP variants were synthesised: one without urea (SCNU) and the other incorporating urea (SCU) to enhance solubility and performance. The chemical structure, absorption kinetics, and urea slow-release behaviour were analysed using Fourier-transform infrared (FTIR) spectroscopy, zeta potential measurements, and UV/Vis spectrophotometry. Results revealed that the urea-based SAP (SCU) exhibited superior water absorption and swelling capacity, reaching a maximum absorption capacity of 355 g/g compared to 155 g/g for SCNU SAP. Both SAPs exhibited good biodegradability within just 7 days, highlighting their environmental benefit. We also investigated the SAPs’ surface charge behaviour across a range of pH values by measuring zeta potential, which provided insight into their water absorption capacity in varying environmental conditions. Absorption experiments supported the trends observed in the zeta potential analysis. As pH increases, the surface charge becomes more negative, which enhances electrostatic repulsion between polymer chains and results in greater water absorption. Desorption studies showed sustained water release over 14 days, indicating potential to reduce irrigation frequency. Post-synthetic urea loading confirmed the SAPs’ slow-release fertiliser capability. Incorporation of indole butyric acid (IBA) further enhanced plant growth, demonstrating the SAPs’ dual function in moisture retention and nutrient delivery. Graphical Abstract