Reservatórios de arsênio em solos derivados de esteatito em Bom Sucesso, MG

Abstract

Soils derived from ultramafic rocks are poor in silica and rich in iron oxides, and may have residual concentrations of heavy metals such as nickel (Ni) and chromium (Cr), as well as metalloids such as arsenic (As), which has a high carcinogenic potential. The objective of this work was to evaluate the availability of As in soils developed from steatite in Morro Paraíso, in the ultramafic complex of Morro das Almas, near Serra de Bom Sucesso-MG. Eight representative soils of the steatite (or talc-schist, a type of soapstone) outcrop were selected, at a depth of 1 meter or until lithic or petroferric contact. The soils were classified as 3 Entisols, 2 Inceptisols and 3 Oxisols. Total content analyses were performed by X-ray fluorescence (XRF) and selective extractions with water, Mehlich-I, ammonium oxalate and nitric acid. The extracts were then analysed in an atomic absorption spectrophotometer with a graphite furnace for determination of low As concentrations (μg L-1). The total As levels analyzed by XRF ranged from 4 to 225 mg.kg-1. In increasing order of As extraction, the results were: Water (zero in all samples) < Mehlich-1 < Ammonium oxalate < Nitric acid. Despite the high total contents, the results of the selective extractions are within the acceptable range by the guidelines of Resolution 420/2013 CONAMA, with the highest content found in the B2 horizon of an Oxisol (9,86 mg.kg-1) extracted by nitric acid and 4,07 mg.kg-1 extracted by oxalate). The soil with the lowest As content was an Inceptisol with 0,07 and 0,05 mg.kg-1 extracted with Mehlich-1 and oxalate, respectively, and levels below detection for nitric acid. The highest amount of As occurs in soils and horizons with higher Fe2O3 contents. The contrast between As extracted by nitric acid or determined by XRF demonstrates that under oxidizing conditions, As can be strongly adsorbed on Fe oxides and hydroxides, especially the primary oxide minerals. In other words, even in high total concentrations, As occurs in these soils in very stable forms, with low availability to the environment, mitigating a potential risk of toxicity for flora and fauna.