Influence of EDDS on metal speciation in soil extracts: measurement and mechanistic multicomponent modeling

The use of the [S,S]-isomer of EDDS to enhance phytoextraction has been proposed for the remediation of heavy metal contaminated soils. Speciation of metals in soil solution in the presence of EDDS and dissolved organic matter (DOM) received, however, almost no attention, whereas metal speciation plays an important role in relation to uptake of metals by plants. We investigated the influence of EDDS on speciation of dissolved metals in batch extraction experiments using fourfield-contaminated soils with pH varying between 4.7 and 7.2. Free metal concentrations were determined with the Donnan membrane technique, and compared with results obtained with the chemical speciation program ECOSAT and the NICA-Donnan model using a multicomponent approach. Addition of EDDS increased total metal concentrations in our soil extracts by a factor between 1.1 and 32 (Al), 2.1-48 (Cu), 1.1-109 (Fe), 1.1-5.5 (Ni), and 1.3-17 (Zn). In general, Al, Cu, Fe, and Zn had the largest total concentrations in the EDDS-treated extracts, but the contribution of these metals to the sum of total metal concentrations varied significantly between our soils. Free metal concentrations varied between 7.0 and 8.9 (pCd2+), 3.9-9.9 (pCu2+), 6.3-10.2 (pNi2+), and 5.2-7.0 (pZn2+). Addition of EDDS decreased free metal concentrations by a factor between 1.4 and 1.9 (Cd), 3.4-216 (Cu), 1.3-186 (Ni), and 1.3-3.3 (Zn). Model predictions of free metal concentrations were very good, and predicted values were mostly within 1 order of magnitude difference from the measured concentrations. A multicomponent approach had to be used in our model calculations, because competition between Fe and other metals for binding with EDDS was important. This was done by including the solubility of metal oxides in our model calculations. Multicomponent models can be used in chelant-assisted phytoextraction experiments to predict the speciation of dissolved metals and to increase the understanding of metal uptake by plants.