Preparation of polyvinyl alcohol/chitosan membranes and their envelope property for relative coating urea granules

Fertilizer has been the ever-increasing demand in agricultural production in recent years, particularly with the escalating demand for food. Simultaneously, the low efficiency of nutrient utilization and environmental risks of fertilizers have posed substantial challenges in the world. Alternatively, polymer-coated fertilizers can play a pivotal role in nutrient utilization efficiency, thus reducing the fertilizer application rates and environmental pollution risks. This study aims to improve the membrane materials of typical polymer-coated fertilizers for the high nutrient utilization efficiency in conventional fertilizers. Polyvinyl alcohol (PVA) was also employed as the membrane substrate, in order to relieve the secondary pollution and uncontrollable nutrient release rates. Three types of membranes—PVA/CS membrane, PVA/CS/GA membrane, and PVA/CS/GA/nano SiO₂ membrane—were prepared using three methods: blending with chitosan (CS), crosslinking with glutaraldehyde (GA), and incorporating nano SiO₂. Additionally, pure PVA membranes were prepared for comparison. The four types of membranes were characterized using scanning electron microscopy (FE-SEM), water absorption (Q), biodegradability (D_e), and nutrient permeability coefficient (P_s). Results indicated that there were excellent dispersion and compatibility of nano SiO₂ in the PVA/CS membrane solution. Furthermore, the biodegradation rates of the four membranes also exhibited gradual increments during the initial 21 days. Subsequently, biodegradation rates escalated rapidly from the 21st to the 56th day, followed by a slower growth rate after the 56th day. The CS was incorporated to enhance the membrane biodegradation rates, while the GA crosslinking contributed to the less membrane biodegradation in soil. However, the addition of nano SiO₂ had minimally impacted the membrane biodegradation rates. Overall, all four membranes displayed favorable biodegradability, with the biodegradation rates ranging from 30% to 60% within 77 days. The water absorption of PVA/CS, PVA/CS/GA, and PVA/CS/GA/nano SiO₂ membranes decreased by 43.00%, 68.79%, and 82.73%, respectively, compared with the PVA membranes. Furthermore, the nutrient permeability coefficients of PVA/CS/GA and PVA/CS/GA/nano SiO₂ membranes decreased by 48.51% and 57.59%, respectively, compared with the PVA/CS membranes. Therefore, the addition of CS, GA crosslinking, and nano SiO₂ all contributed to the hydrophobicity of PVA membranes. Four types of coated urea (PCU) particles—PCU-PVA, PCU-PVA/CS, PCU-PVA/CS/GA, and PCU-PVA/CS/GA/nano SiO₂—were prepared, where these membrane solutions were applied to the urea particles using a drum coating machine. The nitrogen release behavior was evaluated using the soil burial. The time required for the nitrogen release of the four PCUs to reach 90% varied significantly, recorded at 5, 11, 23, and 28 days, respectively. On the 5th day, the nitrogen released from PCU-PVA was the highest at 90.64%, whereas the lowest was from PCU-PVA/CS/GA/nano SiO₂ at 49.32%. The CS, GA crosslinking and nano SiO₂ enhanced the membrane hydrophobicity, which in turn reduced the nitrogen release rate. The nitrogen release behavior of all four PCUs followed a first-order kinetic model, with the release rate constants (k) decreasing progressively—0.3 654, 0.2 333, 0.1 127, and 0.0 926, respectively—and correlating linearly with the nutrient permeability coefficient (P_s) of the membranes. The correlation coefficient (R²) was impressively high at 0.9 991. A series of polyvinyl alcohol/chitosan coating materials were introduced with excellent biodegradability and controllable nutrient release rates, suitable for use in polymer-coated fertilizers. These materials were effectively applied in the production of coated urea particles. A first-order kinetic model of PCU nitrogen release was established with the membrane nutrient permeability coefficient as a key parameter, in order to more efficiently and effectively guide the application of PCU.