Kinetics and Pathways of Urea Release from Granules Spray-Coated with Biodegradable PHBV/PBAT Blends

Synthetic fertilisers play a crucial role in agricultural productivity and global food security, with nitrogen-based fertilisers, particularly urea, being the most widely used. However, the nitrogen use efficiency (NUE) of urea is only ~ 50%, leading to substantial nitrogen losses to the environment, resulting in significant environmental, health, and economic impacts. To mitigate these issues, enhanced efficiency fertilisers (EEFs) have been developed, including polymer-coated controlled-release fertilisers (CRFs). However, conventional polymer coatings are nondegradable, raising concerns about microplastic pollution. In this study, solutions of biodegradable poly(butylene adipate- co -terephthalate) (PBAT), poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV), and their blends (containing 25, 50, and 75 wt% PHBV) were spray coated onto urea granules using a lab-scale fluidised bed to form coatings with thicknesses ranging from 36 to 60 μm. Contrary to expectation, PHBV-coated granules released 94 ± 4% of urea within 24 h, whereas PBAT-coated granules released only 8 ± 3% urea over the same period, with approximately 93 ± 8% released over a 168-hour period, indicating a substantially longer release duration compared to the PHBV coating. The blend formulations exhibited intermediate release behaviour between the pure PHBV and PBAT coatings, with nutrient release generally increasing as the PHBV content increased. High-resolution micro-computed tomography showed that PHBV formed a porous, uneven coating with poor adhesion to the urea surface, while PBAT produced a smooth, uniform coating with better adhesion. Increasing the PHBV content in the blends led to higher coating porosity. These findings highlight both the potential and challenges of using PHBV and PBAT biodegradable coatings for controlled-release fertilisers.