Desenvolvimento de métodos para a determinação de metais em diferentes partes do cultivo da soja: solo, fertilizante, tecido foliar, farelo e óleo

Abstract

The soybean is one of the main oilseeds grown all over the world and it has the ability to absorb metallic ions in the soil. Some of these metals may be classified as non-essential, such as Pb, Cd, Cr, Hg, and that consequently, in certain situations are considered toxic. The nonessential elements may represent an environmental contamination risk, or even the health of living beings, depending on the levels present in different parts of soybeans. One of the main supply routes of these metals to the ground is by the application of fertilizers. In this study we determined the Cu, Pb and Cd in different parts of soybean plants (leaf tissue, oil and defatted meal) cultivated in soil contaminated with the respective metals. The determination of metal content in this material was applied using optical emission spectrometry in plasma induced by microwave (MIP OES). The results indicate that the metal content absorbed by plants of different study sites were proportional to the concentration of available Cu, Pb and Cd in the soil. It is the presence of higher Cu concentration in defatted bran and soybean oil, while the larger Pb and Cd contents were found in leaf tissue. The other part of the thesis refers to the development of two methods of sample preparation of mineral fertilizers employing ultrasonic bath for metal determination. The first one focused on Hg determination obtained as optimized conditions, 150mg of sample treated with 4 mL of a 30% HCl solution (v v-1) and sonicated for 5 minutes in an ultrasonic bath. In these conditions all recovery results were higher than 92%, comparing two instrumental techniques, CV AAS and MIP OES. The second method evaluated the simultaneous extraction of Cu, Zn, Mn, Pb, Cd and Cr present in mineral fertilizer samples employing ultrasonic bath combined with heating. The experiments were conducted with the ultrasonic bath coupling in the thermostatic bath. The effect of HNO3 and HCl solutions, both at 50% (v v-1), were evaluated in a univariate mode. The best recovery efficiencies were obtained when 150 mg of sample was treated with 4 mL of 50% HCl (v v-1) and sonicated for 10 minutes in an ultrasonic bath maintained at 85 °C. The recovery efficiencies were higher than 99%, except to Cd, in which none of the trials showed high recovery efficiency than 50%. Therefore, the use of ultrasonic baths was demonstrated as a feasible tool for the sample preparation of fertilizers for metal determination by spectrometric methods.