Preparation of Elutriates for Aquatic Ecotoxicity Tests

For the preparation of aqueous elutriates from solid samples the standard method DIN 38414-4 [31] can be used. Ten parts of water are added to one part dry matter of the samples and the mixture is continuously shaken for 24 hours. A clear solution is obtained by centrifugation or filtration and can be used for aquatic ecotoxicity tests. If necessary a smaller amount of water can be used (for example a dilution of 1 5). 

By the standard procedure only water soluble fractions of the original sample are obtained in the elutriate. Hydrophobic and even some soluble substances, which are adsorbed or immobilised by ion exchange bonds to the organic or inorganic matrix, will be retained in the solid residue. If substances with a moderate solubility are present, it may be that in every elution the same concentration (saturated solution) may be achieved. 

It may not be of major importance in which proportion the elutriates are made from compost or soil samples. The same elution procedure should be used for all samples if the results are to be compared with or related to each other. The most suitable proportions are 1 5 and 1 10 (dry matter to water). Using the proportion 1 5 gives an increased chance of detecting small inhibition effects. 

The elution method DIN 38414-4 [31] is disputed concerning its universal use for aquatic biotests from solid samples. Hund [66] has observed significant differences in the results between terrestrial and aquatic biotests made from the same samples. From our group recently obtained research results with biodegradable materials did show in almost all cases no or very small inhibition effects, exceptions had been rare, [67, 68]. From those few results no further serious predictions could be made about the suitability of the DIN 38414-4 elution procedure for the testing of biodegradable materials. 

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