Controlling nitrogen pollution via encapsulation of urea fertilizer in cross-linked corn starch

Abstract

High fertilizer inputs augment the reactive nitrogen level in soil, air, and water. Unused reactive nitrogen acts as a pollutant and harms natural resources. This study focused on the thermal processing of corn starch into a coating material using disodium tetraborate and urea. The processed corn starch was coated over granular urea in a vertical bed coating reactor. The chemically modified starch, when compared with native starch, exhibited better stability and mechanical strength over time. The modified starch looked like a weak gel, and its loss modulus was dominated by the storage modulus. However, for native starch, the viscous component dominated the elastic component, especially at lower angular frequencies. The nitrogen release from the coated urea was remarkably slower than the uncoated one. A small difference in the peak and final starch viscosities in the presence of urea and borate revealed low thermal cracking of the starch molecules. The surface of the granular urea that was coated with chemically modified corn starch was uniform, dense, hard, and least porous. The uncoated urea granules became released into water in 6 min under gentle shaking, whereas the coated urea took almost 32 min to completely release.