Selenite interactions with some particulate organic and mineral fractions isolated from a natural grassland soil

The nature of the main selenium (Se) carrier phases in soil is in debate, with some authors emphasizing the role of minerals, and others the role of organic material. In this study, we address the role of the different soil components in Se sorption by determining its interaction with easily separable and workable fractions of bulk soil. Thus four separated fractions (two organic and two mineral) isolated by size density fractionation from a grassland soil were taken as models for soil Se carrier phases. Two concentrations of selenite were used to estimate its reactivity during experiments. In addition, SEM‐TEM analyses (coupled with EDS) were used to determine the chemical environment of Se sorbed onto the different fractions. The results showed that for the smaller selenite concentration nearly all the fractions gave the same selenite sorption (approximately 99%). For the larger concentration, selenite sorption ranged between 44% (mineral fractions) and 93% (organic fractions), indicating that the sorption capacities of organic fractions were greater than those of mineral fractions. In all soil fractions, elementary analyses and SEM/EDS and TEM/EDS observations revealed that Se sorption and the detection of Se hot spots on the soil particles correlated with the presence of Fe and Al. Although a direct association between Se and organic material could not be excluded, we suggest that Se sorption on organic particles is indirect, mainly resulting from association with surface Fe oxides or clays. These findings question the current view of a direct correlation between organic carbon and Se in natural soil samples.

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