Enhancing the slow-release performance of urea by biochar polyurethanes co-coating

To optimize the controlled release performance of polyurethane-coated fertilizers, cotton stalk biochar was incorporated on polyurethane films. Then, the effect of biochar loading positions (inner layer, middle layer, and outer layer) on the structural and functional attributes of the biochar polyurethane co-coatings fertilizer was systematically investigated. The findings reveal that the biochar loading position significantly influences the physicochemical properties of the fertilizer. Specifically, biochar incorporated on the outermost layer of the film (BPU-O) exhibits a dense coating with rough surface morphology and high hydrophobicity, thereby demonstrating optimal-controlled release performance. BPU-O demonstrates an initial release rate of merely 1.8%, with cumulative release amounts of 20.82% at 14 d, 53.03% at 28 d, and reaching 80% after 40 d. The exceptional-controlled release performance of BPU-O can be attributed to the oxygen-containing functional groups on cotton stalk biochar. These functional groups, particularly hydroxyl groups, react with residual isocyanates on the polyurethane surface, enhancing compactness and hydrophobicity through grafting and filling effects, thereby, effectively inhibiting water ingress into the core of the fertilizer. Consequently, this work demonstrates a significant role of biochar embedding in determining the efficacy of polyurethane-controlled release fertilizers, providing valuable guidance for advancement in high-efficiency-controlled release fertilizer.