Selenium contamination in soil: sorption and desortpion processes

The biological importance of selenium is mainly linked to three factors: it is an essential element in animal and, probably, vegetable metabolisms; in many geographical areas the available quantity is insufficient to satisfy animal requirements; in some areas it is present in such high concentrations in soil, water, plant, ash and aerosol that it is toxic for animals. Soil plays an important role in the cycle of Se in the geoecosystem, since it has the ability to retain Se avoiding its loss by leaching. We conducted experiments to study the sorption and the desorption processes of selenate in various soils typical of the Mediterranean area, where the biogeochemical processes of Se are not well known. The four soils used were collected in Greece, in Northern Italy, in Central Italy and in Spain, respectively, and were characterized by different physiochemical characteristics. From the data obtained in this study it appears that at lower levels of selenate in the soil-solution system, more sites were available for high affinity sorption, and selenium was firmly retained. When increasing concentrations of selenate are added, fewer affinity sites were involved in the sorption process and selenium was more easily removed by the phosphate solution. With high concentrations of Se, the soil retention is reduced and the bioavailability of selenium in the environment might increase and induce groundwater and food chain contamination. Our studies emphasized the important role of soil organic matter in the selenate sorption process. This is significant as interest in the study of selenium develops particularly when biomasses from different sources are added to soil. This practice not only introduces increasing quantities of selenium into the soil, but also relevant amounts of organic materials which can naturally influence the dynamics of the elements in the soil. The processes of sorption and desorption are of primary importance in order to understand the behaviour of selenium in soil. These studies should be considered as a starting point for the further detailed study of the biogeochemical processes of Se in the soils of the Mediterranean area. They are also important in terms of determining the links between sorption-desorption processes and bioavailability in order to quantify the toxicological hazard of Se in soil.