Hydrogels based on oxidized starches from different botanical sources for release of fertilizers

Herein, hydrogels based on starch from corn, sweet cassava and bitter cassava were prepared by a straightforward approach. The hydrogels were obtained by an oxidation process using the redox system KMnO₄/NaHSO₃ which leads to the formation of carbonyl and carboxyl groups with minimum polymer hydrolysis. The resulting oxidized samples as well as the native starches were extensively characterized by scanning electron microscopy, thermogravimetric analysis and wide angle X-ray scattering, among others. High gel fraction values and low degree of crystallinity were obtained in all the cases, which values vary slightly depending on the botanical source. High swelling capacity was observed for all the samples, which behave as hydrogels. The influence of the ionic strength and pH in the swelling capacity of the oxidized starches was also studied. The samples exhibit an anti-polyelectrolyte behavior and the water uptake increases at basic pH as the carboxylic groups become ionized. The hydrogels were also loaded with urea, potassium nitrate and ammonium sulfate as model fertilizers and their potential for controlled release was investigated.