Preparation and characterization on a dual-layer slow-release urea fertilizer coated using ethyl cellulose and phosphoric acid-carbamate di-esterified starch-based chitosan hydrogel

The extensive utilization of natural starch (NS) in preparation of slow-release fertilizer (SRF) remains a crucial challenge owing to its undesirable defects such as poor solubility and mechanical properties. Herein, based on the preparation of di-esterified starch derivatives (PCDS) from NS with phosphoric acid and carbamate and synthesis of PCDS cross-linked chitosan (CS) hydrogels (PCDS-CSH), a dual-layer coated urea SRF (EC/PCDS-CSH/urea-SRF) was successfully developed utilizing ethyl cellulose (EC) and PCDS-CSH as the inner/outer coating. Due to the ameliorated properties of PCDS (viscosity and gelation temperature) compared to NS, the PCDS-CSH shows significantly improved swelling performance and water retention capacity. Importantly, the corresponding EC/PCDS-CSH/urea-SRF demonstrates superior nitrogen (N) nutrient slow-release behaviors, achieving cumulative release rates of 48.5 % (12 h ) and 61.3 % (48 h) in water together with 22.9 % (10 d) and 52.9 % (29 d) in soil, respectively. Kinetics fitting analysis reveals that N release in EC/PCDS-CSH/urea-SRF follows the Higuchi model (in water) and First-order kinetics (in soil), respectively, both of which belong to non-Fickian diffusion process. Furthermore, tomato seedling pot experiments demonstrate that the EC/PCDS-CSH/urea-SRF shows a significant promotion role toward the plant growth. Consequently, this study provides a novel route for developing biomass based SRFs utilizing modified NS derivatives.