Determinação do teor total e biodisponível de selênio em diferentes classes de solos da região do Vale do Jequitinhonha - MG

Abstract

In this work, total selenium (Se) concentration and its bioavailability were determined in A and B horizons of several classes of soils from preserved areas in the Jequitinhonha Valley, Minas Gerais state. Total Se was determined by hydride generation atomic absorption spectrometry (HG AAS) after reverse aqua regia digestion in a microwave oven. The detection and quantitation limits (LD and LQ) of the method were 0.03 mg kg-1 and 0.30 mg kg-1, respectively, with relative standard deviation of 1.10%, and recovery rate of 90 %. For different classes of soils analyzed, Se concentration ranged from 0.30 ± 0.03 to 5.97 ± 0.20 mg kg-1. The highest concentrations were determined in Spodosol and weathered soils (Red and Red-Yellow Latosols). Total Se was correlated with soil physicochemical properties (SPP) (sand, silte, clay, organic matter (OM), pH and oxides of AI, Fe and Si). The statistical analysis by Pearson's correlation, principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that Se concentration correlated significantly and positively with Al203 (r = 0.76), Fe2O3 (r = 0.57) and clay (r = 0.64), and negatively with SiO2 (r = -0.76) and sand (r = -0.54). There was no significant correlation with OM, pH and silt. To evaluate Se bioavailability were used parallel single extractions. Three extractants (ultrapure water, phosphate buffer and sodium hydroxide) were used to allow assessing water-soluble Se (W-Se), exchangeable Se (E-Se) and Se bound to soil organic matter (OM-Se), respectively. Subsequently, OM-Se extract was fractioned (using nitric acid) into Se associated with fulvic acids (FA-Se) and humic acids (HA-Se). Se concentration was determined in three fractions by total reflection X-ray fluorescence (TXRF), using 1 mg L' Ga as internal standard. Accuracy was evaluated by addition and recovery, obtaining recoveries ranged from 98% to 107%. Precision ranged from 0.97 to 2.00%. Different levels of Se bioavailable was verified in soils. OM-Se was the dominant fraction, accounted for 15.2% to 98.2% Se content.The SPP was correlated mainly with W-Se fraction. This fraction was positively correlated with pH (r = 0.49) and sand (r = 0.57), and negatively with Al203 (r = - 0.50). The Fe2O3 was strongly correlated with HA-Se (r = 0.78), Residual Se fractions (r = 0.79), FA-Se (r= 0.86) and total Se (r = 0 97) These results have significant implications in the Se bioavailability for tropical soils, that present high acidity and Al203 and Fe2O3 oxides contents.