Studies on Agrochemical Controlled Release Behavior of Copolymer Hydrogel with PVA Blends of Natural Polymers and Their Water-Retention Capabilities in Agricultural Soil

Agricultural technical development relies exclusively on the effective delivery of agrochemicals and water to plants and on reducing the harmful effects of agrochemicals on useful organisms in the soil. In this study, super-absorbent hydrogels were prepared in the form of microspheres using gum Arabic (GA), which was copolymerized once with chitosan (CS) and once with poly (vinyl alcohol) (PVA). To impart mechanical strength to the hydrogel microspheres, a covalent cross-linker (N,N′-methylenebisacrylamide (MBA)) was used for the PVA/GA hydrogel, and an ionic cross-linker (sodium hexametaphosphate (SHMP)) was used for the CS/GA hydrogel. The prepared PVA/GA-CH and CS/GA-PH hydrogel microspheres showed different degrees of swelling (DSs) in the following solution media: deionized water (DW), river water (RW), and buffered solutions (pH 4; pH 9). The PVA/GA-CH hydrogel microspheres showed a maximum DS of 84 g/g in the RW, while the CS/GA-PH hydrogel microspheres showed a maximum DS of 63 g/g in the buffered solution at a pH 9. The water-retention capabilities of the hydrogels were studied using a mixture of 0.5% (w/w) hydrogel microspheres in agricultural soil; the composite showed an additional 20 days of water retention in comparison with a control sample consisting of soil alone. The hydrogels were loaded with urea, which is an important fertilizer in the field of agriculture. The PVA/GA-CH hydrogel microspheres showed a maximum loading percentage (L(max)%) of 89% (w/w), while the CS/GA-PH hydrogel microspheres showed an L(max)% = 79.75% (w/w) for urea. The urea-release behaviors of the hydrogel microspheres were studied under different release media and temperature conditions. In practice, the PVA/GA-CH hydrogel microspheres showed a better release profile in the RW at 10 °C, while the CS/GA-PH hydrogel microspheres showed a more controlled release in media at a pH 9 and at 30 °C. The urea-loaded microspheres, aside from those following the release, were characterized via FTIR and SEM. In contrast, virgin microspheres were characterized using XRD,(1)H NMR, (TGA and DSC), and the maximum degree of swelling, in addition to being subjected to SEM and FTIR analyses.