Arsenic in Soils: Natural Concentration and Adsorption by Oxisols Developed From Different Lithologies

Arsenic (As) is a toxic and a carcinogenic element naturally occurring in the environment. Therefore, studies on As natural concentration in soils and its adsorption process are important tools for the evaluation of potential risks of soil contamination in order to adoption of control actions or monitoring of potential As-contamination sources. The objective of this study was to evaluate the natural levels of As and determine the maximum adsorption capacity of As (MACAs) of six Oxisols (Latossolos) of Minas Gerais State, Brazil, developed from different lithologies. The soil sample’s total As content was determined using the USEPA 3051A method. The adsorption experiments were performed using different As concentrations, and the MACAs was measured by the Langmuir isotherm. On average, the natural As content in Oxisols was 13.13 mg kg-1, which is above the reference value of soil quality (RVQ) for As, in Brazil (8 mg kg-1). The levels of As in Oxisols originated from metamorphic/igneous rocks were significantly higher than those of Oxisols from sedimentary rocks. Globally, the evaluated soils showed a mean MACAs equal to 2,548 mg kg-1. Soil horizon Bw showed a higher MACAs than that of A horizon. In general, the levels of clay, iron oxides, iron forms (especially poorly-crystallized) and organic carbon had a positive influence on MACAs. Although the RVQ for As is well below the MACAs in all soils, the soil adsorbed As naturally present, rendering it unavailable in the soil aqueous phase. Therefore, there was no risk of contamination for human health.