Otimização de métodos para determinação de mercúrio lábil e total em sólidos em suspensão e metilmercúrio em ambientes de águas pretas amazônicas

Abstract

In general, this Thesis presents the study of validation and optimization of sample preparation techniques for the determination of labile mercury and its spicy fraction (methylmercury - MeHg) in Amazonian black water environments. Initially, an operational investigation was carried out in the literature in order to obtain the current State of the Art of the most applied techniques. With this, two widely known and devoted techniques for determining total mercury (HgT) were selected, namely: the Direct Mercury Analyzer (DMA) and the Cold Steam Atomic Fluorescence Spectrometry (CV-AFS). Therefore, a more detailed investigation on the behavior of mercury species in the water column was necessary according to the temperature variation, in addition to the supply of suspended solids and organic matter, which influence the permanence of mercury species in the water compartment. This Thesis aims to deepen the study on the influence of suspended solids on the labile fraction of mercury (HgL) in water samples from the Negro River, in addition to verifying the influence of pH, ionic strength and presence of interferents in the concentrations of (MeHg) in a black water lake formed by one of the sources of the Tarumã-Açu river, located at Km 923 of the BR 174 highway. In order to improve the techniques, special contour conditions were proposed so that there was no contamination or loss of samples with Hg. Thus, due to the direct relationship of the studied metal with the content of total suspended solids (SST), it was proposed to modify the standard method with the reduction of the drying temperature of the filters and analysis of the labile mercury fraction (HgL). In this first part of the study, the analyzes were based on the comparison between the results of the two techniques (DMA and CV-AFS), aiming to highlight the efficiency of the direct Hg analyzer in determining other metal fractions. Therefore, it was successfully demonstrated that the determination of HgL in the SST content is possible via DMA, as long as the temperature is adjusted and the glassware is properly cleaned. Continuing the analytical contributions, the technique of diffusion in thin films by concentration gradient (DGT) in in situ sampling was also studied. However, the application of this technique in natural waters is complicated due to the reduction of the adsorption of the traditional sampler for metals at low pH and low electrical conductivity. This part of the study evaluated the variation in MeHg concentrations retained in the 24 and 120 h period by the DGT technique. DGT measurements were also made in standard solutions in the laboratory for calculating the diffusion coefficient, pH influence, ionic strength and interfering with MeHg retention. The results showed that the DGT technique can be extended to waters with lower pH and conductivity, provided that its limitations are evaluated, being a useful tool for the integrated monitoring of labile concentrations in different aquatic systems. Based on the proposal to reduce the time and cost of analysis, as well as the generation of waste by eliminating the acid elution step of the binding gel. Thus, it was possible to develop via comparison (DMA and CV AFS). Thus, the results obtained demonstrated that the determination of MeHg is possible via DMA, which represents an important contribution to Hg speciation.