Desenvolvimento de biohidrogéis para aplicação na liberação controlada de glifosato

Abstract

This thesis aims at the development, characterization and application of a biohydrogel consisting of the combination of two polysaccharides, sodium alginate (SA) and chitosan (CH), in the gradual release of glyphosate without the presence of any crosslinking agent. The results showed that the reaction conditions evaluated: pH (4 to 9), temperature (25 and 50 ° C) and ratio between polysaccharides (SA:CH = 2: 1, 1: 1 and 1: 2) had an important role in the properties of the obtained biohydrogel. Thus, by adjusting such conditions, it was possible to produce a biohydrogel having a swelling degree greater than 1000%, good thermal stability and compressive strength of 0.077 MPa. The biohydrogel had an adsorption of 86% glyphosate in 48 h of test. The herbicide gradual release test performed in vitro (up to 60 days) by colorimetric measurements showed a release of 73% of glyphosate in 30-35 days of testing. The cumulative release was approximately 90% at the end of 60 days, reaching equilibrium in a time of 50 days. In order to contribute to the understanding of the gelation mechanism, a mechanical-quantum computational study based on the Functional Density Theory was carried out to evaluate the interaction between SA and CH polysaccharides. The theoretical study was also applied to evaluate the interaction between the biohydrogel and glyphosate. Due to the high computational cost SA and CH were represented by disaccharide units. With the modeling, it was possible to obtain structural, electronic and energy parameters of SA and CH and the complex formed. Theoretical results indicated that the formation of the SA:CH polyelectrolyte complex were predominantly electrostatic. However, a significant covalent contribution in the energy of interaction between the dimers is indicated by the EDA calculations. According to the NBO analysis, the covalent component is present in the intermolecular H bonds formed between the ~NH3+ and the ~COO-groups, effectively contributing to stabilize the formed complex. When a glyphosate molecule is approached to the AS:CH complex, it preferably binds to the chitosan dimer. This result is in agreement with observations made by fluorescence micrographs. Another relevant point in the computational study clearly indicated the role of the sodium counter ion, still in wide discussion in the literature. According to the results analysis, Na+ coordination contributes between 37 and 43% for the stabilizing energy of the formed complex