Selenium speciation in soils after alkaline extraction
Résumé
A speciation method for selenium in soils was developed to evaluate the distribution of Se species by using as an example a seleniferous soil from Ireland. First, a water extraction was applied in order to evaluate the Se fraction directly available to plants. Results have shown that only 2% of the total selenium was water soluble and this fraction contained mainly Se(VI). Then, an alkaline extraction was used to solubilize both humic substances and selenium. After optimization of this procedure, a single extraction with a 2 mol l-1 sodium hydroxide solution allowed the extraction of 50% of total Se from the soil, a recovery of 90% was obtained with a 10-step extraction procedure. The alkaline extract was acidified to pH 1.5 to divide the acid-soluble fraction (fulvic acids) from the humic acids fraction precipitated at this pH. Seventy percent of total soluble selenium were found in the fulvic acid fraction while 30% were present in the non-acid-soluble fraction. In each fraction, a speciation procedure was then performed in order to identify and determinate inorganic and organic selenium forms. In the acid-soluble fraction, the main Se form was Se(IV) (approx. 43%), non-negligible amounts of organic Se were found (approx. 41%) and Se(VI) represented approx. 15% of this fraction. In the non-acid-soluble fraction, only Se(IV) was determined and it represented approx. 60% of the total Se contained in this fraction.