Sequential extraction of selenium from four Scottish soils and a sewage sludge

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Abstract
A sequential extraction procedure was used to assess the availability of selenium (Se) from four Scottish soils which may receive sewage sludge and a sewage sludge that may be applied to agricultural land. The procedure identified two exchangeable (available) and three non‐exchangeable Se fractions. The exchangeable fractions were defined as soluble (0.25M KCl) and ligand exchangeable (0.1M KH2PO4). The non‐exchangeable fractions were defined as acid‐extractable (4M HCl), oxidizable (dissolution in KClO3 and concentrated HNO3), and residual (dissolution in HF and HNO3). The fraction of Se defined as acid‐extractable is not readily available for plant uptake, but has the potential to become available through chemical and microbial mobilization. The total Se was also determined for each soil and the sewage sludge by a one step digestion procedure using HNO3 and HF acids. The determination of Se in the individual fractions was performed by isotope dilution‐mass spectrometry using 76Se. The total Se in the soils ranged from 0.5–0.8 ug g‐1 and less than 5% was available. The sewage sludge had a total Se of 3.55 μg g‐1, but <4% was available for uptake with a further 5% potentially available. The sum of the Se concentrations in the fractions 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.