Alginate/PVA beads for the controlled release of urea with applications in agriculture

FRANCISCO GALISTEO GONZALEZ; GABRIELA AMODEO; ESTEBAN TUBERT

Congreso; Polymer World Congress; 2018

Chemical fertilizers are routinely added to crops to improve yields and enhance homogeny of plantations. Despite the benefits granted by fertilization schemes in agricultural systems, there is growing concern about their long-term sustainability and their undesired environmental effects. From the total fertilizer mass that is applied onto the field, only a small proportion is effectively used by the crop (Zhang et al., 2015), while the unused part generally contributes to soil pollution (Sharma et al., 2016) or fertilizes competing flora. In this context, there is a clear need to develop alternative fertilization practices to increase the nutrient use efficiency of agricultural and forests plantations. Among conventional fertilizers, urea is one of the most problematic ones due to its high solubility and potential toxicity to plant?s rhizospheres. In this work, we synthetized polymeric beads for the controlled release of urea, based on sodium alginate and polyvinyl alcohol (PVA) through direct and simultaneous crosslinking of alginate and PVA with calcium nitrate and sodium tetraborate respectively. To manufacture the beads, a solution containing sodium alginate (2% w/w) and polyvinyl alcohol (15% w/w), was mixed with bentonite powder (reported to reduce the release rate of molecules in alginate-based systems; He et al, 2014) and with urea alone or with a previously synthetized 6:1 ratio urea-hydroxyapatite hybrid particle suspension, reported to release urea slowly (Kottegoda et al, 2017). The mixture was added dropwise to an aqueous crosslinking solution containing 30% urea, 3% calcium nitrate tetrahydrate and 1% sodium tetraborate. The resulting beads were spherical and had a diameter of 3-5 mm. The beads were oven-dried at 70°C for 24 hs which transformed them into solid pellets of 1-2 mm in diameter. The samples had a urea content of approximately 43% w/w and were analysed by SEM/EDAX showing an overall nitrogen content of 6,5%. To evaluate the urea release rate, we added 0,1g of the dry pellets to a flask containing 25 ml of distilled water at constant agitation and measured the urea content spectrophotometrically (Hussain et al, 2002) as a function of time (60-3600 seconds) after pellet addition. Results indicate that pellets featuring urea only, released it significantly slower compared to pure urea (60 times slower) while the pellets containing the urea-hydroxyapatite hybrid particles released urea even slower, reaching 100% of release only after 3600s (120 times slower compared to pure urea). The reduced release rate of urea detected could be attributed to alginate and PVA entanglement caused by simultaneous crosslinking of both polymers. In conclusion, these results could be useful to develop advanced slow release fertilizers for urea as well as other molecules based on: i) the simplicity and scalability of the manufacturing procedure; ii) the encapsulation method which permits the integration of powders or suspensions of substances that would otherwise be difficult to dispense to agricultural soils, iii) the compatibility of the obtained pellets with conventional agricultural operations and iv) the fact that the pellets are manufactured using alginate and PVA, giving the additional benefit of complete degradability of the polymer framework.