| It is known that the selection of ligands is a key factor in the efficiency of the extraction process. Many ligands have been developed. However, most of them are used for ambient extraction with aqueous or organic solvents. Most ligands are classified in four classes, based on the type of reaction that occurs between the metal and the ligand. The classes are acid ligands, acid chelating ligands, ion exchangers and solvating ligands.
A suitable ligand for SFE has to fulfil several requirements. Ideally, the ligand strongly complexes with the metal under investigation, and the formed complex is highly soluble in the supercritical fluid to facilitate the extraction. In literature, only a limited number of ligands have been tested for SFE. These are mainly ß-diketones, dithiocarbamates and organo-phosphate systems.
The efficiency of SFE to extract Cd from soils was also checked by comparing seedling emergence and plant biomass in control and SFE extracted soils. In the sandy soil no significant reduction in seedling emergence nor biomass was observed with increasing concentrations Cd in the soil samples. However, the average biomass of plants grown on control soils was slightly higher (be it not statistically significant) from those grown on extracted soil. This may reflect a decrease of soil nutrients (Ca, K, Fe and Mg) in the extracted soils. The uptake of Cd in Garden cress grown on the sandy soil is shown in Figure 1 (left-hand panel). A reduced availability of Cd is only seen in the extracted soils both at the highest applied concentration. Because SFE had been shown to be very difficult to perform on the Kettering soil, and because admixture of quartz sand had been shown to provide a solution. No significant differences in seedling emergence and plant biomass could be calculated for increasing Cd concentrations in the soil samples. Moreover, both endpoints did not differ between the control and the extracted soils. This is in agreement with the analytical results on the soil composition which had shown no significant reduction of soil nutrients in the clay soil after SFE. Contaminant exposure pathways are entirely different in plant and animal species. Therefore also a soil invertebrate test was included to test the efficiency of ligand mediated SFE to remove the bioavailable fraction of Cd in soils. Both mortality and loss in biomass were considered.
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