Chitosan–alginate-based hydrogels for agricultural and horticultural applications

Introduction: The transboundary pollution of aquatic ecosystems, promoted by the excessive utilization of nitrogen- and phosphorus-based fertilizers, is causing an overgrowth of algae. Therefore, the development of controlled-release systems is necessary to avoid the wastage of agrochemical compounds. In this context, this study develops chitosan–alginate-based hydrogels for potential use as a direct nutrient source for plants and for improving water retention.

Methods: Physicochemical properties were determined through Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA). Mechanical performance was evaluated using strain and frequency sweep tests, as well as temperature ramps, analyzing the modification through the two-step gelation process. Moreover, microstructural characterization was determined using Scanning Electron Microscopy (SEM). Finally, water uptake capacity was measured after several cycles.

Results: The results confirmed the feasibility of combining chitosan and alginate to form polyelectrolyte hydrogels with predominantly solid behavior. Particularly, hybrid systems (50-50 CH-ALG) exhibit intermediate behavior in comparison to the unitary systems, combining good mechanical properties, with an elastic modulus similar to the chitosan-based system for shorter immersion times (2500 Pa), while improving its water absorption capacity (4500 %) because of the abundance of carboxylic groups that enhance water–polymer interactions. In terms of microstructure, chitosan exhibited a spongy morphology with increasing porosity as the immersion time increased, while alginate displayed a defined laminar structure. Finally, the material demonstrates good integration and thermal stability over a temperature range of 20 – 40 ºC.

Conclusions: Chitosan–alginate-based hydrogels demonstrate potential for redefining agricultural practices, thereby mitigating ecological damage caused by conventional fertilization methods.

Acknowledgments: This research is part of the applied research and innovation project "Desarrollo de matrices proteicas para la liberación controlada de nutrientes y agua en horticultura" (SOL2024-31712), cofounded by UE – Ministerio de Hacienda y Función Pública – Fondos Europeos – Junta de Andalucía – Consejería de Universidad, Investigación e Innovación.