Facile Preparation of Chitosan-Alginate Crosslinked with Calcium Chloride Hydrogel as Sustained Release Fertilizers

Abstract

The utilization of urea in the agricultural sector as a fertilizer is susceptible to losses through volatilization and leaching. A strategy to mitigate this environmental impact involves employing a sustained-release approach for urea, thereby diminishing both the quantity and frequency of its application. These agrochemicals utilize a physical barrier to curtail the dissolution of urea, and their efficacy in fertilizer release is dictated by their physical characteristics. In the current investigation, hydrogel beads comprising urea were developed by blending biodegradable polysaccharides, specifically chitosan and alginate, in the presence of calcium chloride as a crosslinker through ionotropic gelation. The Box-Behnken design was employed to analyze the impacts of independent variables (low, medium, and high molecular weight chitosan; 2.5 to 4.5 wt% alginate; and 0.10 M to 1.00 M calcium chloride crosslinker) on encapsulation efficiency and urea release. After model evaluation, the optimal conditions for encapsulation efficiency and urea release were determined as the use of the lowest molecular weight chitosan, a medium amount of alginate, and the maximum amount of crosslinker, resulting in a loading efficiency of up to 98.5% and a urea release of only 2.2% within 2 hours. The composition of the polymeric hydrogel was found to govern encapsulation efficiency and urea release, while the swelling behaviour in water remained unaffected. The hydrogel beads exhibited a spherical morphology, and Fourier transform-infrared spectroscopy confirmed the presence of urea within the beads.