Although soil organic matter (SOM) has an important role on oxyanions retention in soils (e.g. As, Se), the nature of interactions between those elements with SOM is not clearly known. In this study, Se (IV) was used to artificially contaminate a grassland soil (“Roth2”, Rothamsted Institute, UK). Physical and chemical extractions were then used to determine the solid partition of Se. In addition, transmission electronic microscopy (TEM) and scanning electronic microscopy (SEM) observations (Fig. 1), both coupled with EDX analyses, were realised on some isolated fractions. The Particulate Organic Matter (POM)—organic debris >50 μm– which represented 29.8% of the total soil organic carbon, was particularly responsible for more than 11% of total Se retention, although it represented only 5.6% of the total soil weight Coppin et al., 2006. Moreover, selenium sorption experiments, performed on isolated fractions (Fig. 2), revealed that the POM fraction (50–200 μm) was the most Se reactive fraction of the soil. In addition, we measured that POM50–200μm contained the highest iron concentration (4 times more Fe than POM>200μm and 2 times more than soil). As iron oxides represent an important Se carrier phase, it was expected that Se retention to POM was favoured by iron oxides surface precipitates. TEM and SEM observations showed that Se was spread onto the whole POM surface and in its core, as Fe was. Thus, Se diffused into the POM matrix and was uniformly accumulated. In addition, Se hot spots were located in the vicinity of Fe spots. Although direct Se-POM links were not excluded, microscopic observations confirmed that Se-POM associations were favored by the presence of Fe. Observations of few clay-size impurities on the POM surface did not reveal any other Se hot spots.
1-IRSN/DEI/SECRE/LRE, BP 3, 13115 St. Paul-lez-Durance, France
2-IRSN/DEI/SARG/LETS, BP 17, 92262 Fontenay aux roses, France
3-LTHE, University Grenoble I, BP 53, 38041 Grenoble, France