Synthesis and characterization of gellan gum: chitosan biohydrogels for soil humidity control and fertilizer release

Gellan gum-chitosan superabsorbent hydrogels were obtained by crosslinking reaction throughout the ionic bond formation. The network formation between high acetyl gellan gum (HAGG) and chitosan (CH) with different proportions was confirmed by FTIR spectra. The thermal stability up to 250 degrees C was obtained from thermogravimetry and high water swelling from differential scanning calorimetry. It was observed that as the quantity of crosslinks increases, the water sorption and average molecular weight between the crosslinks decrease with a decrease of the HAGG content in the networks. The best results of water absorption of more than 218 times the dry weight of the sample were obtained for the sample HAGG-CH with 4:1 composition. This sample also evidenced the lowest amount of crosslinks when compared with the sample with 1:4 proportion, being 1.1 x 10(-4) and 11.1 x 10(-4), respectively. The water evaporation analysis showed that the samples retain water for about 500 min, i.e., more than twice as long when compared to 200 min for pure water. Moreover, the HAGG-CH samples were tested as commercial monopotassium phosphate fertilizer-release agents showing that almost complete release occurred in approximately 8 h, and its quantity is mainly influenced by the concentration of fertilizer used rather than the hydrogel composition. Finally, the samples were lyophilized and their morphology analyzed by scanning electron microscopy.