Novel core-shell microcapsules incorporating macro/micronutrients in PVA/starch matrix

Abstract

Enhanced efficiency fertilizers (EEFs) have been widely used due to their ability to meet crop nutritional needs and reduce the main problem of leaching. In this work, we compared the effects of spray-dryer microspheres and microcapsules on nutrient encapsulation and identified the spheres and core-shell structures by fluorescence microscopy using Rhodamine B (RB) as the fluorescent material. The microsphere matrix was based on starch, and the microcapsules consisted of starch as the core and poly (vinyl alcohol) as the shell. The fluorescent images confirmed that the RB was incorporated into the starch, as the overlap of fluorescence and brightfield images covered the entire area for microspheres and only the core for microcapsules. Microspheres exhibited an irregular and non-spherical morphology, whereas microcapsules presented a spherical shape with a smooth and homogeneous surface featuring small aggregated particles. The microparticle structures remained unchanged even after incorporating KNO3, Fe, Cu, and Mn. Thermal analysis indicated lower initial mass loss temperatures for the microcapsules, suggesting polymer interactions during drying and formation. FTIR further supported this, which revealed structural changes and partial oxidation during drying, contributing to reduced thermodegradation. Notably, micronutrient-based microcapsules demonstrated a significantly lower burst release than macro/micronutrient-based microcapsules and micronutrient-based microspheres. Fe and Cu were released most rapidly from microspheres in the first hour, reaching 84.2% and 79.4%, respectively. In contrast, macro/micronutrient-based microcapsules released 66.1% and 67.7%, while micronutrient-based microcapsules released 53.1% and 61.5%. As for Mn, macro/micronutrient-based microcapsules exhibited the highest initial release at 72.4%, followed by micronutrient-based microcapsules at 62.2%, with microspheres releasing at 55.8%. Additionally, the Peppas-Sahlin model provided the best fit, indicating diffusional and polymer relaxation contributions. The morphology, release, and kinetics behaviors confirmed that the core-shell prolonged the nutrient release by reducing the initial burst release, thanks to the diffusion barrier provided by the PVA layer.