Dual-Responsive Hydrogel Composite Bearing Schiff-Base Linkages: Water Retention Coupled with Controlled Fertilizer Release

Conventional superabsorbent hydrogels mostly rely on synthetic polymers and are often costly and environmentally persistent, while conventional fertilizers suffer from rapid leaching and volatilization. There is a pressing need to develop materials based on natural biodegradable polymers that serve as a water reservoir as well as act as fertilizers. This study reports the development of a sustained-release fertilizer based on a chitosan polymer matrix incorporated with zinc oxide (ZnO) nanoparticles. Terephthalaldehyde was employed as a crosslinking agent, and the reaction proceeded via a Schiff-base condensation mechanism. The synthesized composite material was comprehensively characterized using Fourier transform Infrared (FTIR) spectroscopy, X-ray Diffraction (XRD), and scanning electron microscopy coupled with Energy Dispersive X-ray Analysis (SEM−EDX). BET analysis revealed a porous structure with an average pore radius of 12.04 nm, confirming the mesoporous nature of the material. Thermal stability was evaluated using Thermogravimetric Analysis (TGA), while mechanical properties were examined through rheological studies. The hydrogel exhibited pronounced pH- and temperature-responsive swelling behavior, classifying it as a smart hydrogel. A maximum swelling of 3512% was observed at neutral pH, exceeding 1000 times its initial weight, thereby demonstrating superabsorbent characteristics. The smart and superabsorbent properties of the hydrogel composite enabled efficient fertilizer loading. Urea loading was investigated under different pH conditions, with a maximum loading efficiency of 93% achieved under acidic conditions. Release studies were conducted using UV−Visible spectroscopy. Kinetic analysis using various mathematical models showed the best fit with the Korsmeyer−Peppas model. At pH 4, the release profile exhibited sustained behavior with a correlation coefficient (R2) of 0.98 and an n value of 0.27, indicating a Fickian diffusion mechanism. Further, comparative germination studies were conducted using Brassica oleracea var. viridis and Cicer arietinum seeds. The urea-loaded superabsorbent composite significantly outperformed pure urea in both cases, indicating promise for sustainable agricultural applications.