SEQUENTIAL EXTRACTION OF SELENIUM FROM 4 SCOTTISH SOILS AND A SEWAGE-SLUDGE

A sequential extraction procedure was used to assess the availability of selenium (Se) from four Scottish soils which may receive sewage slu dge and a sewage sludge that may be applied to agricultural land. The procedure identified two exchangeable (available) and three non-exchan geable Se fractions. The exchangeable fractions were defined as solubl e (0.25M KCl) and ligand exchangeable (0.1 M KH2PO4). The non-exchange able fractions were defined as acid-extractable (4M HCl), oxidizable ( dissolution in KClO3 and concentrated HNO3), and residual (dissolution in HF and HNO3). The Faction of Se defined as acid-extractable is not readily available for plant uptake, but has the potential to become a vailable through chemical and microbial mobilization. The total Se was also determined for each soil and the sewage sludge by a one step dig estion procedure using HNO3 and HF acids. The determination of Se in t he individual fractions was performed by isotope dilution-mass spectro metry using Se-76. The total Se in the soils ranged from 0.5-0.8 mu g g(-1) and less than 5% was available. The sewage sludge had a total Se of 3.55 mu g g(-1), but <4% was available for uptake with a further 5 % potentially available. The sum of the Se concentrations in the fract ions was in good agreement (92-108%) with the total Se concentrations found by the one step digestion. With the likely increase of disposal of sewage sludge to land, the method provides a means of assessing the bioavailability of Se for plant uptake from sewage sludge and sludge amended soil.