Novel coating films containing micronutrients for controlled-release urea fertilizer: release mechanisms and kinetics study

Alkyd coatings incorporated with different types of micronutrients were successfully prepared and coated on granular urea. Three sets of micronutrients, Zn(NO3)(2)center dot 6H(2)O and borax, ZnO and borax, and ZnAl-layered double hydroxide, were studied. The water solubility of micronutrients plays an important role to produce different pore sizes and control the release rate of macro- and micronutrients. The coated urea granules were characterized by using FE-SEM technique, and micronutrient distribution on coating films was performed using energy-dispersive X-ray spectroscopy. Zn, B, and nitrogen releases were evaluated in DI water according to the ISO 21263. The nutrient release kinetics was determined by comparing the data with zero-order, first-order, Higuchi (HG), Hixson-Crowell (HC), and Korsmeyer-Peppas (KP) kinetic models. The diffusion release of Zn micronutrient showed the best fit with KP model, and the diffusion release of boron (B) micronutrient was consistent with HG model. The release characteristics of the nitrogen from urea core through coating films could be best described by the KP model. These coating films are the enhancement of controlled-release fertilizers to reduce the environmental implications imposed by excessive use of fertilizers for a long-term period.