The effect of sample drying on the operational speciation of potentially toxic elements in sediment using the modified BCR sequential extraction procedure

The BCR sequential extraction procedure is widely used to determine the operational speciation of potentially toxic elements (PTE) in soils and sediments. When applying the approach, a dichotomy exists between maintaining a sample’s intrinsic speciation, and pre-treating the sample to make it stable and more amenable to standard laboratory operations such as sieving, cone-and-quartering etc. Sample drying in particular has been reported to cause redistribution of analytes to more readily extractable forms. A common compromise is to air-dry samples at < 25 °C. However, many studies report findings based on extraction of sediments subjected to much harsher pre-treatment regimes.
In order to assess the effects of sample drying on the operational speciation of PTE in sediment, the modified BCR sequential procedure was applied to samples obtained from the catchment of the River Derwent (an area impacted by historical mining and smelting) following various pre-treatment regimes. Samples were analysed as received, air-dried, freeze-dried, and following oven drying at 60 °C and 110 °C. Extracts were analysed for As, Cd, Cu, Fe, Mn Ni, Pb, U and Zn using an Agilent 7700x ICP-MS instrument.
Different changes in operational speciation occurred for different analytes. Freeze-drying increased the proportion of Pb associated with the exchangeable and acid soluble fraction. In general the proportion of Cd, Cu and Zn associated with the oxidisable phase increased on drying up to 60 °C but decreased at 110 °C. Manganese and Fe were less affected by drying, whereas the behaviour of As differed between samples.