Environmentally Friendly Slow-Release Urea Granules with Water-Retaining Poly(vinyl alcohol)/Sodium Carboxymethylcellulose Composite Coatings

To meet the food demands of the growing global population, enhancing the efficiency of water and nutrient utilization in agriculture is crucial. This work provides a series of poly(vinyl alcohol)/carboxymethyl cellulose sodium (PVA/CMC-Na) membrane-coated urea granules with slow-release and water retention properties. The primary objective was to fabricate a series of PVA/CMC-Na membranes by blending PVA and CMC-Na, cross-linking them using glutaraldehyde (GA) or citric acid (CA), and incorporating alkaline lignose (AL). Characterization results showed that the membranes exhibited excellent thermal stability and compatibility. Among them, the PCN/CA membrane (cross-linked with CA, and not containing AL) indicated the lowest water uptake (WU, 75.35%) and nutrient permeability (P_S, 8.73 × 10^–04 cm²·day^–1). After 63 days, the biodegradation ratios (BDEs) of the five membranes ranged from 38.76 to 71.67%, indicating excellent biodegradability. Moreover, the urea granules were coated with PVA/CMC-Na membranes using a drum granulation method. When applied to soil at 2.00 wt %, the PCN/GA/AL-CU granules (coated with the PCN/GA/AL membrane, which was cross-linked with GA and incorporated with AL) had the highest maximum water retention capacity (WRC_Max) of 53.20% and the lowest soil moisture evaporation rate (R_SME) of 72.32%. The nutrient release duration of the coated urea granules lasted approximately 30 days, with nutrient release behavior following a first-order kinetic model well. These results suggest that the urea granules coated with the PVA/CMC-Na membranes are promising alternatives as water and nutrient retaining fertilizers (WNRFs) for agriculture production.