Selenium, barium, arsenic and iodine in Brazil nut agroecosystems: soil-nut-human relationship

Abstract

Brazil nut (Bertholletia excelsa) is considered one of the most important non-timber products of the Amazon region, as its nuts are a food appreciated by local communities and internationally. The nuts of the Brazil nut tree are recognized worldwide as the richest food in selenium (Se), but there are also studies reporting high levels of barium (Ba). All over the world, there are situations of both deficiency and toxicity of Se in soils, reflected in the levels present in the food grown in these soils. The same occurs for arsenic (As), Ba and iodine (I), the first of which is important because the region is located in a basin whose soils are influenced by sediments from the Andes (volcanic). The interest with Ba is due to evidence of the presence of the mineral hollandite (Ba2Mn8O16) in reasonable quantities in Amazonian soils, which hypothetically is the cause for high concentrations of this element in the nuts. Iodine is a little researched element in the country and in the Amazon region, therefore, new information about it in soils serves as a basis for future research with biofortification. Consequently, the present study seeks to better understand and geographically characterize the contents of Se, Ba, As and I and their availability present in soils in the Amazon region, verifying the accumulation in Brazil nuts, as well as its spatial distribution in the seeds. Finally, the bioaccessible fraction of these elements are quantified in nuts from different sites. Samples of soil and nuts were previously collected in their natural production environment in native forest and farms established in different regions of the Amazon basin during the harvest season in the region between 2014 and 2017, covering sites in the states of Acre, Rondônia, Mato Grosso, Amazonas, Roraima, Amapá, and Pará. Analyzes of the total concentration of Se, Ba, As, I and other relevant trace elements in the soil and nuts were performed. Available and soluble fractions of Se and As in the soil were extracted and in vitro bioaccessibility analyzes of Se and Ba in Brazil nuts with a simulation of the gastrointestinal tract using a standardized technique. In addition, a 2D mapping study was carried out using two complementary techniques of μ-XRF (benchtop equipment and synchrotron light) to verify the spatial distribution of Se, Ba, Br, S, and P in Brazil nuts. The main results showed that soil pH and CEC are indirectly the main chemical attributes that influence As fractions in the studied soils. The total concentrations of Se in the soil vary from 0.05 to 2.48 mg kg-1 and, therefore, are within the safe range according to the Brazilian legislation. For Ba, the concentrations in the soil varied from 8 to 765 mg kg-1 and thus, reach levels above the prevention value (150 mg kg-1), with abnormal levels in the Western Amazon. The average I-TMAH in the superficial layer of the studied sites is 5.43 mg kg-1, which is above the world average of 2.8 mg kg-1. The μ-XRF technique in both approaches, with benchtop equipment and synchrotron, agree in terms of results obtained and showed that Se accumulated mainly in the tissue of the external parenchyma of the seeds of the Brazil nut forming a "ring" around the seed. The accumulation of Ba tends to behave similarly, but it is concentrated in a ring located more externally in the seed, around the epidermis. Our findings showed that the bioaccessibility of Se in the simulated gastric phase (GP) is lower than in the small intestine phase (SI-T1 and SI-T2), whereas for Ba the opposite occurs. The individual bioaccessible contents of Se and Ba in Brazil nuts vary widely between and within the studied sites, but the recommendation to ingest 1-3 nuts per day can be considered safe, considering the currently estimated uptake limits for Se and Ba.