Large volume TENAX® extraction of the bioaccessible fraction of sediment-associated organic compounds for a subsequent effect-directed analysis

Background, Aims 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, which may lead to an erroneous prioritisation 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 the rapidly desorbing, and thus bioaccessible, fraction using TENAX® are based only on small amounts of sediment. In order to obtain sufficient extract 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.

Methods

Desorption behaviour of sediment contaminants was determined by combining consecutive extraction of sediment using TENAX® with a three-compartment desorption model. Time needed to remove the rapidly desorbing fraction, t_rap, was calculated to select a fixed extraction time for single extraction procedures. Up-scaling by about a factor of 125 provided a large volume extraction technique for EDA. Reproducibility and comparability to the small volume approach were analysed. TENAX® blanks and sediment extracts were tested for toxicity using Scenedesmus vacuolatus and Artemia salina as test organisms.

Results and Discussion

Desorption kinetics showed that 12 to 30% of sediment-associated pollutants were available for rapid desorption, while 70 to 90% of PAHs found in the sediment belong to the slowly and very slowly desorbing pool with very limited bioavailability. t_rap is compound dependent and covers a range of 2 to 18 h. A fixed extraction time of 24 h was selected as a time at which even the rapidly desorbing fraction of big hydrophobic compounds should be fully desorbed. High reproducibility of the large volume approach and good agreement with the small consecutive approach were found. Significant toxicity of procedural TENAX® blanks was found with Scenedesmus vacuolatus, which is in agreement with chemical analysis and could be reduced by pre-cleaning of TENAX® with Accelerated Solvent Extraction (ASE). Toxicity of blanks prior to ASE-clean up was about three orders of magnitude below the toxicity of sediment extracts.

Conclusions

For consideration of bioaccessibility in EDA, a large volume TENAX® extraction method was presented. Although several other solid phases can be used to extract the bioaccessible fraction, TENAX® has unique properties for depletive extraction of the rapidly desorbing fraction from large amounts of sediment. Toxicity and chemical blanks due to production residues are shortcomings 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 manufacturers would significantly enhance the applicability of the method. Using TENAX® instead of total extraction may improve key toxicant prioritisation by considering exposure and effect rather than effect only.

     

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