Background, Aim and Scope:
Effect-directed analysis (EDA) is a powerful tool for the identification of key toxicants in complex environmental samples. In most cases, EDA is based on total extraction of organic contaminants leading to an erroneous prioritization with regard to hazard and risk. Bioaccessibility-directed extraction aims to discriminate between contaminants that take part in partitioning between sediment and biota in a relevant time frame and those that are enclosed in structures, that do not allow rapid desorption. Standard protocols of targeted extraction of rapidly desorbing, and thus bioaccessible fraction using TENAX® are based only on small amounts of sediment. In order to get sufficient amounts of extracts for subsequent biotesting, fractionation, and structure elucidation a large volume extraction technique needs to be developed applying one selected extraction time and excluding toxic procedural blanks.
Materials and Methods:
Desorption behaviour of sediment contaminants was determined by a consecutive solid-solid extraction of sediment using TENAX® fitting a tri-compartment model on experimental data. Time needed to remove the rapidly desorbing fraction trap was calculated to select a fixed extraction time for single extraction procedures. Up-scaling by about a factor of 100 provided a large volume extraction technique for EDA. Reproducibility and comparability to small volume approach were proved. Blanks of respective TENAX® mass were investigated using Scenedesmus vacuolatus and Artemia salina as test organisms.
Results:
Desorption kinetics showed that 12 to 30 % of sediment associated pollutants are available for rapid desorption. trap is compound dependent and covers a range of 2 to 18 h. On that basis a fixed extraction time of 24 h was selected. Validation of large volume approach was done by the means of comparison to small method and reproducibility. The large volume showed a good agreement with the small volume approach. TENAX® blanks inhibited the reproduction of Scenedesmus vacuolatus about 44 %, immobilisation assay with Artemia salina showed no effects.
Discussion:
Desorption kinetics suggest 70 to 90 % of PAHs found in the sediment belong to the slowly and very slowly desorbing pool with very limited bioavailability. Because of compound dependent desorption behaviour , individual pollution pattern, and composition of sediment, an universally valid extraction time for rapidly desorbing fraction does not excist. Selected extraction time was chosen as a time, at which even the rapidly desorbing fraction of big hydrophobic compounds should be fully desorbed. Toxicity of blanks is caused by TENAX® impurities due to production procedure requiring further powerful cleaning of TENAX® e.g. with Accelerated Solvent Extraction (ASE).
Conclusions:
For consideration of bioaccessibility in EDA a large volume TENAX® extraction method was presented. Despite several other solid phases can be used to extract bioaccessible fraction, TENAX® has unique properties for depletive extraction of rapidly desorbing fraction of large amounts of sediment. Toxicity and chemical blanks due to production residues are shortcoming of the method that can be overcome by accurate pre-cleaning e.g. with ASE.
Recommendations and
Perspectives:
Higher purity of TENAX® guaranteed by the manufactures would significantly enhance the applicability of the method. TENAX® instead of total extraction may improve key toxicant prioritization by considering exposure and effect rather than effect only.
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