Comportamento sortivo de selênio em solos do bioma cerrado: efeito da força iônica e do cultivo do solo

Abstract

Selenium (Se) is considered an essential element for humans and animals. It is a non-metal element usually found in trace concentrations in the environment. Selenium intake by animals and humans is directly related to its availability and accumulation in plants, where Se bioavailability is linked to its content and mobility in soil. In this context, this study aimed to evaluate the influence of ionic strength (IS) and soil cultivation on the adsorption and desorption of Se in soils as well as to determine the distribution coefficient (Kd) of Se in the Cerrado soils studied. For that, we have used sixteen soil samples collected from the Brazilian Cerrado biome, being collected eight native areas and eight cultivated areas. For the adsorption experiment, 2 g of each soil were weighted in centrifuge tubes of 50 mL. Then, it was added to the centrifuge tubes 20 mL of solution containing 100 µg L-1 and 500 µg L-1 of Se (soil : solution ratio of 1:10), as sodium selenate (Na2SeO4). The solution of sodium selenate previously had the pH adjusted to approximately 5.5 and it was prepared in solution of sodium chlorite (NaCl) in both IS tested, 15 and 150 mmol L-1 . The adsorption time was 72 h, alternating 12 h of shaking and 12 h of rest. Then, the samples were centrifuged for 20 min at 3200 rpm and the supernatant was used for Se analyses using graphite furnace atomic absorption spectrometry (GFAAS). The desorption experiment was runned as described for the adsorption, with the exception that it was only added the NaCl solution in the 2 IS studied. The results showed that in general the native soils adsorbed more Se than the cultivated ones. Also, it was observed that the Se adsorption varied depending on soil characteristics and it increases upon increasing the Se concentration added, from 100 µg L-1 to 500 µg L-1 of Se. Among the soil characteristics, clay content plays an important role for the adsorption of Se, being this adsorption higher in clay soils compared with sand soils. Regarding the influence of IS on the Se adsorption, most soils adsorbed higher Se amounts in the higher IS studied, which indicates that the main adsorption mechanism for selenate is by outer sphere complex. The desorption showed a similar tendency as reported for the adsorption, i.e., soils that adsorbed high amounts of Se also showed to desorb this element in greater amounts. In general, Kd was lower for cultivated soils compared to uncultivated soils, which shows that selenate prevails in solution than adsorbed on cultivated soils, being, as a result, more plant available in these soils.