Fontes fosfáticas: diferenciação por espectroscopia IV e reciclagem por precipitação de estruvita a partir de solução aquosa

Abstract

Phosphorus is an element with multiple functions in view of the maintenance of life. Its main attributions are related to its role as a nutriente, in which it acts in organisms as a structural component of molecules and in the production of energy. Due to these assignments, the element is fundamentally required within the food chain, being essential in the development of plants, and therefore, highly demanded in the primary sector of the economy. Due to its high reactivity with oxygen, phosphorus is found in the earth's crust in the ionic form of orthophosphate 􁈺PO􀬸 􀬷􀬿􁈻, distributed as phosphate minerals that form the rocks, main sources for the production of phosphorus fertilizers. As it is a finite natural resource with increasing and accelerated consumption, their reserves tend to a pressing shortage. Because of this, there is an increasing search for innovation and optimization of secondary production processes (phosphorus recycling), as well as smart strategies for using primary sources. In this context, this work sought to expand studies on Amazonian phosphates with regard to characterizationdifferentiation, and yet, to propose an optimization in the recycling of phosphorus by struvite precipitation from aqueous solution. To achieve the first objective, phosphates from three different geological origins (igneous, weathering and biogenetics), in the Amazon region, were differentiated by infrared spectroscopy with Fourier transform (FTIR). The measurements were carried out in the near and medium infrared regions by the methods of: transmission, diffuse reflectance (DRIFT) and total attenuated (ATR). In addition, complementary analyzes of X-ray diffraction and X-ray fluorescence were also performed. The results revealed that the transmission and DRIFT methods are the most appropriate and it is recommended to use them, when possible, together. The characteristic PO4 bands were observed in all spectra in the ranges between 1200 – 984 cm-1 and 634 – 450 cm-1. The differentiation of the materials was given by the presence of bands: (CO3)2- in igneous phosphates, Al2OH in weathering phosphates and NH4 in the biogenetic origin. At the end, a spectral database for phosphates was established and spectral signatures were cataloged. To achieve the second objective, an optimization of the P recovery process from aqueous solutions through struvite synthesis was carried out, and for that, a sequential design of experiments methodology (DOE) was applied. A Plackett-Burman design followed by a Doehlert acted in the definition of the factors significantly influencing the struvite precipitation process and for optimization the response surface methodology was used in conjunction with the desirability function. The answers were: phosphorus recovery (usual chemical measure), diffractometric pattern and struvite decomposition enthalpy (physical measures not usual in this type of study, therefore an innovation). In addition, complementary analyzes of Xray fluorescence spectroscopy, infrared spectroscopy, laser granulometry and scanning electron microscopy were performed on the products. The results allowed to define the best synthesis conditions: pH (10.2), N/P ratio (≥4) and initial phosphorus concentration (183.5 mg/L), with phosphorus recovery above 70% and formation of struvite and K-struvite. Finally, it can be said that advanced proposals related to chemical and structural characterization and process optimization concerning phosphates were established based on two methods: one analytical (IR spectroscopy) and the other physical-chemical (precipitation). Both research methodologies explained in this study contribute to the search for alternative solutions for the generation and use of phosphate resources.