Extração seletiva de As(III) por ponto nuvem e determinação de arsênio total em sedimentos de região de mineração empregando espectrometria de absorção atômica com geração de hidreto por injeção em fluxo.

Abstract

One of the indisputably worrying issues is the impact to Brazilian water resources. Among many impacts that compromise multiple uses and increase regional and local economic pressures on water resources, mining stands out. This activity is responsible for the significant entrance of toxic elements into surface waters, which negatively affects the quality of the sediments. The goal of this work was to develop a method for determination of inorganic arsenic in sediments samples obtained at different points in the Rio Doce region (Espírito Santo, Brazil). For this purpose, flame atomic absorption spectrometry and hydride generation by flow injection were employed after cloud point extraction (CPE-FIA-HG FAAS). The pH of the aqueous phase directly influences the extraction of the complex and studying this parameter indicated that the best response is obtained in the pH value equals to 1.8. After defining this parameter, the concentrations of the variables that influence the cloud point formation were optimized by multivariate analysis through the complete factorial planning and using response surface methodology. The evaluated variables were the concentration of Ammonium Pyrrolidine Dithiocarbamate (APDC) as a complexing agent, Triton X-114 as a non-ionic surfactant for the formation of micelles, and NaCl as electrolyte to induce the cloud point formation without the need for heating. The concentration of antifoam was evaluated in this study together with the cloud point variables, because its concentration is directly proportional to surfactant concentration. The best response for cloud point formation was obtained under the following conditions: APDC (0.70% w/v), Triton X-114 (0.21% w/v), and NaCl (0.52% w/v). The concentration of antifoam sufficient to prevent foaming in the liquid gas separator was 2.3% w/v. After defining the parameters related to the cloud point formation, a study for optimizing the variables of the hydride generation system by flow injection was carried out. The best responses were obtained using NaBH4 at the concentration of 0.50% w/v to reduce As(III) to its corresponding hydride, HCl 1.3 mol.L−1 for the acid hydrolysis reaction of NaBH4, a 30 cm reaction coil length, sample flow and reagents equal to 4 mL.min−1, sample injection volume and NaBH4 equal to 0.33 ml, and volume of HCl equals to 0.36 ml. From the optimized conditions, the method showed to be selective for the As(III) species presenting determination and quantification limits equal to 0,052 e 0,17 μg.L−1, respectively, with relative standard deviation (RSD) of 3.4% for n=10 and a preconcentration factor of 55. Recovery values were obtained between 90 - 103% for fortified samples. The evaluation of the interfering ions shows that Hg(II), Fe(III) and Cu(II) ions interfere in the determination of As(III) under the used experimental conditions, however, EDTA 0.1 mol.L−1 was used to minimize the interference effect. The procedure showed to be fast and efficient for preconcentration and determination of As(III) by CPE-FIA-HG FAAS, which can be used for inorganic arsenic speciation using 0.50% w/v thiourea in the pre-reduction of As(V) to As(III).