| Extraction of cadmium from contaminated soils using supercritical carbon dioxide (SC-CO2) is possible. To this purpose, two soils with different characteristics - mainly in clay and organic matter content - were freshly spiked with a Cd-salt and subjected to SC-CO2 extraction (SFE) at different conditions. As SC-CO2 was unable to extract heavy metals, a complexing agent had to be added which both binds cadmium and is soluble in SC-CO2. Three different complexing were investigated, i.e. ethylenediamine, hexafluoroacetylaceton and Bis(2,4,4-trimethylpentyl)monophosphinic acid (Cyanex 302). It is found that Cyanex 302 results in the highest extraction efficiency. The extraction efficiency is soil type dependent. The soil, containing more clay minerals and organic matter, retains more Cd. Both soils were extracted at the optimal conditions and were subjected to different ecotoxicological tests, including a bacteriological test, a plant test and a test with compost worms. The bacteriological test and the plant test with Garden cress confirm that less Cd is available in the soil after SFE. However, also some changes in soil nutrients are found affecting the growth of the Garden cress. Compost worms that obtain their food both from the soil pore water and by ingestion of soil particles, are less affected by the slight changes in soil characteristics induced by SC-CO2 in the control cultures and, as a result, benefit more from the lower Cd-content of the soil.<p>
Ecotoxicological field studies of contaminated soils are often hampered by the lack of noncontaminated matched controls. These are soils with identical characteristics as the contaminated ones, that can be used to generate the reference values for ecotoxicity testing. Being able to refer to such reference soils is crucial, because soils are very heterogeneous and only minute differences in soil composition may affect the bioavailability and hence the toxicity of pollutants. As a result, an extraction technique is needed that removes the contaminants from the soil without introducing major changes in soil characteristics. It is well-known that compounds can be extracted from a solid matrix at relatively mild conditions using supercritical carbon dioxide (SC-CO2). This extraction treatment (SFE) does not leave a residue, because the remaining CO2 becomes gaseous and evaporates from the matrix. Although SFE has been applied extensively for the extraction of organic compounds, only limited information is available on the extraction of metals. In this paper, it is investigated whether SFE can be applied to extract cadmium from contaminated soils. The research is performed on two soils and the optimal extraction conditions are determined. The extracted soils are subjected to different ecotoxicological tests, including a bacteriological test, a plant test and a test with compost worms. |