Estratégias para obtenção de compósitos fertilizantes via solubilização biológica de minerais

Abstract

Phosphate fertilizers are widely used to improve agricultural productivity. However, the process of producing these fertilizers involves high costs and can cause environmental damage. The application of phosphate solubilising microorganisms (PSM) has been considered a potential strategy to obtain biofertilizers by biological phosphate rock (PR) solubilization, but the efficiency of this process is still low. Thus, the objective of this work was to develop strategies to increase the solubilization of RF by the action of PSM using microbial culture processes by submerged route (SmC) and solid state (SSC), besides proposing new products employing the use of PSM as agent promoter. Initially the selection of a potential PSM was performed, in which the fungus Aspergillus niger was highlighted. Then, a new mechanical-biological strategy was evaluated to increase the solubilization of the PR. In this strategy, PR Bayóvar (sedimentary origin) was mechanically activated to decrease particle size, so the solubility can be increased as a result of the larger surface area together with the local pH change. The results showed an increase of solubility of 60% when using SmC and 115% in SEC. The mechanical activation strategy was also studied with the Itafós PR (igneous origin), which showed a different behavior of Bayóvar PR, with solubility gains of 57% by SmC and 45% by ESC. Also for the PR Itafós, a strategy was proposed using a system of solids supply by fed-batch integrated to the mechanical activation step increased the solubilization of this rock in 78%. In order to elucidate the mechanism of action in the biosolubilization process, a study was carried out to evaluated the effect of organic acids under different RF sources. The results showed that citric acid and oxalic acid are the organic acids with the greatest potential to solubilize PR. A synergistic relationship was found between citric/oxalic acids as well as oxalic/gluconic acids when presented in molar ratio of 1:1 and 2:1, respectively, that potentiates the solubilization. Using the knowledge obtained, a composite-fertilizer based on starch gelatinization was developed with simultaneous dispersion of PR particles mechanically activated and encapsulation of A. niger spores. The characterization of the composites showed a high degree of dispersion of the PR particles in the starch matrix, besides a considerable content of P2O5 between 10-22%. The solubility of the phosphorus contained in the composite was greater than 70% in only 96 h. In addition, the spores remained viable after processing and storage, ensuring a minimum shelf-time of two months. Due to the potential of this technology, composites-fertilizers were also developed from mineral oxides and elemental sulfur. The results showed that the matrix was capable of carrying a high load of oxides (ZnO, MnO and CuO) and elemental sulfur, as observed when using PR. The bio-activation experiments showed a high solubilization of the oxides when sulfur was used in the starch matrix. The final product obtained was a multi-nutrient granule source of Zn, Mn, Cu and S for the plants. The multi-nutrient composite was also evaluated in the soil-plant system and the results show that they were as efficient as the commercial sources used. Concludes, that the nanocomposite-fertilizers proposed in this work constituting an innovative concept of "bioreactor in granule" with many possibilities of application in the nutrition of plants.