Uso da fluorescência de raios X e ferramentas quimiométricas para a avaliação química de solos afetados por rejeito de mineração do rompimento da Barragem de Fundão (Mariana - MG)

Abstract

X-ray fluorescence spectrometry (XRF) is a simultaneous and non-destructive multielement analysis technique that requires little or no sample preparation and can be used for the analysis of different matrices for different analytes, mainly with the use of portable instruments (pXRF), which allows different applications of this technique directly in the field. For the quantification of the total analyte contents in samples that are not easily dissolved in mineral acids, it is recommended a reference method, the decomposition by alkaline fusion, followed by acid dissolution and analysis by ICP OES. For environmental samples, the analysis of data generated by XRF using chemometric tools has been highlighted, mainly the exploratory analyzes PCA and HCA, since these tools reduce the original dimension of the data and can reveal relationships between the data in order to, for example, differentiating groups of apparently similar samples. Therefore, the present study aims to develop an analytical method for elemental determination using two portable equipment and one benchtop that use the energy- dispersive X-ray fluorescence spectrometry (EDXRF) technique in soil samples in process of revegetation of riparian zones affected by mining tailings for the quantitative determination of Fe. For this purpose, the samples were previously dried and comminuted. After the optimization process, 2 g of sample were analyzed in suitable sample holders by two portable equipment and one benchtop, with the following optimized parameters for portable equipment: voltage (50 kV); current (10.5 μA); collimator (8 mm), without using a filter and measuring time of 20 s. To determine the total content and validate the method, alkaline fusion was used, using 0.060 g of sample, 0.500 g of flux (LiBO2), dissolution with 25 mL of HNO3 10% vv-1, dilution and followed by ICP OES analysis. The results of the exploratory analyzes (PCA and HCA) showed good discrimination of the affected and reference areas for the XRF equipment, with more than 60% without variable selection. The equipment calibrations were also satisfactory, with r > 0.90 for most of the linear working ranges. LODs and LOQs ranged from 0.023 to 0.054 g kg-1 and 0.077 to 0.19 g kg-1 were obtained, respectively, using alkaline fusion and ICP OES determination as reference values. The RMSEPs ranged from 4.05 to 11.7 g kg-1, indicating good correspondence between the predicted concentrations (XRF) and those determined by the ICP OES. The mean coefficients of variation ranged from 0.46 to 1.25%, indicating good measurement precision for the analyzes performed with the XRF equipment. Therefore, given the similarities between the results of portable and benchtop equipment, attested by statistical tests, the efficiency of using pXRF can be highlighted, since these equipment allow direct and expeditious analysis of samples in the field, not being necessary sample collection and preparation.