Preparation and application of water-soluble oxidized starch-based hydrogels for slow-release urea

In this work, an eco-friendly biodegradable oxidized starch-based hydrogel (OSH) was prepared as a slow-release carrier for urea (OSHU) to promote vegetable growth. Firstly, oxidized starch (OS) was obtained using potato starch as the raw material and NaClO as the oxidizer. Then, OSH was prepared via a crosslinking reaction with polyvinyl alcohol (PVA). The structure and properties of OSH were characterized by Fourier transform infrared spectrum (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The gel content (GC), swelling behavior, and water retention behavior of OSH were also investigated. SEM analysis revealed an interconnected porous network within the OSH. Meanwhile, GC test results indicated that the gel fraction of OSH was maintained at approximately 81 %. This hydrogel demonstrated a remarkable water absorption capacity of 2125.62 % ± 13.42 %, and sustainable swelling behavior even after 10 absorption-drying cycles. The OSHs were loaded with urea as model fertilizers and their potential for controlled release was investigated. Soil release tests of OSHU showed that cumulative nitrogen release reached 34.57 %-56.97 % after 35 days, the release kinetics of OSHU1 and OSHU2 in water followed the first-order model and conformed to Fickian diffusion. Plant growth trials involving pakchoi demonstrated the increases in biomass when compared to both the blank control group and the group treated with pure urea.