Revealing controlled release mechanism of starch-based polyurethane coated urea: A kinetics insight

Bio-based coated fertilizers are promising alternatives to conventional chemical fertilizers due to their excellent performances. Identifying key influencing factors and deciphering controlled release mechanism are essential for their development. Kinetic modeling is a powerful tool for elucidating the complex mechanism governing nutrient release. However, a significant technical bottleneck lies in the gap between idealized laboratory evaluations and practical field performance. Herein, controlled release mechanism of starch-based polyurethane coated urea (BPCU), was elucidated based on the synergistic effect of coating materials and environmental factors. Nutrient release behaviors of BPCU were investigated under different temperatures and soil moisture contents. Structural changes of BPCU coating at various release stages were analyzed by using techniques such as SEM in detail, and release kinetics were evaluated using four classical models. It was found that temperature was the key factor affecting its nutrient release, and higher temperature favored the swelling rate resulting in a rapid nutrient release and the shortening release longevity. SEM and CT results indicated that the expansion of the bio-based coating in water increased the volume and number of micropores. Furthermore, the BPCU release process exhibited two kinetic patterns, namely, polymer swelling and relaxation conforming to non-Fickian diffusion in the first half and non-swelling diffusion belonging to Fickian diffusion in the second half. A release kinetics prediction model was proposed, with an average relative error of 14.5 %. This study narrows the gap between laboratory evaluation and practical field application, providing support for the development of bio-based coated fertilizers.