Chitosan/Starch Matrices Prepared by Ionotropic Gelation: Rheological Characterization, Swelling Behavior and Potassium Nitrate Release Kinetics

Abstract

A controlled-release fertilizer made of biodegradable beads prepared with neat chitosan or chitosan/starch blends loaded with potassium nitrate was obtained using an ionic cross-linking process. This study looks at the influence of chitosan/starch mass ratio on the dynamic rheological behavior of the polymeric material without cross-linking. Every tested sample showed a viscoelastic behavior corresponding to entanglement networks of concentrated solutions except for the gelatinized starch that presented a gel behavior. Two factors affecting the water absorption and the fertilizer release were investigated: the polymeric composition (chitosan/starch mass ratio) and the cross-linking time. It was found that the equilibrium swelling degree decreased by increasing the beads’ starch content and/or the cross-linking time. When loaded beads were immersed 16 days in water under static conditions, the fertilizer release ratio was greater than 70% of the total loading. The hydrogel composition and the cross-linking time defined the mechanism governing the potassium nitrate release which was matrix relaxation controlled. Experimental results indicated that it is possible to modify the release profile of the loaded fertilizer which it is a very promising characteristic for the development of a compelling controlled-release fertilizer.