Development of a Bio-Based Adhesive for the Encapsulation of Urea in order to Control its Dissolution Rate in Water and Improving its Degradation Rate in Soil

Abstract

Nitrogen (N) is the most essential and very important nutrient for the plant growth. Urea contains 46% of N which is the highest content of those solid nitrogenous fertilizers, but urea has a limitation on its delivery system. Most plants require the essential nutrient N from urea throughout their lifetime. Since different plants have different lifetime, the release of N from urea must to be in a controlled manner. In this research work urea was encapsulated in a biodegradable adhesive, which was synthesized through poly-condensation reaction of lactic acid (LA) and gum acacia (GA), to control its dissolution rate in water, and to improve its degradation rate in soil. The poly condensation reaction was performed in an oven with conventional heating system at different temperatures (between 165 0C to 195 0C), reaction time (between 1.5-2.5hrs), and GA to LA mixing ratio (between 8.3 to 12.5 w/w%) based on the Box-Behnken method of design expert software. The effect of those independent variables on the grafting efficiency of lactic acid oligomer grafted gum acacia (LAO-g-GA), and on its release rate after urea encapsulation was studied. The result shows that the grafting efficiency and the urea release rate were inversely proportional. Therefore, the urea release rate of urea encapsulated LAO-g-GA (U-e-LAO-g-GA) can be controlled simply by controlling the grafting efficiency of LAO-g-GA. The grafting of lactic acid oligomer (LAO) chains at the O-H and N-H functional groups of GA backbone was confirmed with the help of FTIR and NMR analyses. The results of solubility and hydrophobicity tests of LAO-g-GA exhibits that as the grafting efficiency of LAO-g-GA increases, both its solubility in chloroform and its hydrophobicity increases. The increase in temperature of a medium, and the urea concentration encapsulated in the synthesized LAO-g-GA increases the urea release rate, but the increase in pH of the medium decreases the urea release rate. The soil degradation test result shows that all U e-LAO-g-GAs were entirely degrade in soil, and as the grafting efficiency of the LAO-g-GA increases its degradation rate decreases. The amount of urea encapsulated into the LAO-g-GA affects its soil degradation rate in a positive manner. The N release rate of U-e-LAO-g-GAs in water was studied using Kjeldhal method, and from the four studied release kinetic model equations, the Korsmeyer-Peppas equation exhibits a best fit with correlation coefficients (R2 > 0.99), and the release exponent (n) values for all U-e-LAO-g-GAs were greater than 0.89, implies the release of N from U-e-LAO-g-GAs was a combination of diffusion and erosion mechanisms.