Environ. Sci. Technol. 2008, 42, 1123–1130
Influence of EDDS on Metal Speciation in Soil Extracts: Measurement and Mechanistic Multicomponent Modeling G E R W I N F . K O O P M A N S , * ,† WALTER D.C. SCHENKEVELD,† JING SONG,‡ YONGMING LUO,‡ JAN JAPENGA,§ AND ERWIN J.M. TEMMINGHOFF† Department of Soil Quality, Wageningen University, Wageningen University and Research Centre (WUR), P.O. Box 47, 6700 AA, Wageningen, The Netherlands, Soil and Environment Bioremediation Research Centre, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, PR China, and Alterra, WUR, P.O. Box 47, 6700 AA, Wageningen, The Netherlands
Received July 10, 2007. Revised manuscript received October 22, 2007. Accepted November 2, 2007.
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 four fieldcontaminated 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 NICADonnan 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
* Corresponding author phone: +31 317 483842; fax: +31 317 419000; e-mail:
[email protected]. † Wageningen University. ‡ Institute of Soil Science. § Alterra. 10.1021/es071694f CCC: $40.75
Published on Web 01/12/2008
2008 American Chemical Society
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.
Introduction Phytoextraction is considered a cost-effective and environmentally friendly alternative to conventional engineering technologies for the remediation of soils contaminated with heavy metals (1). Under natural soil conditions, however, availability of metals for uptake by plants is often limited, and this stimulated the use of synthetic chelants to mobilize metals from the soil solid phase to solution in combination with high biomass plants to enhance uptake of metals from soil (2). Ethylenediamine tetraacetic acid (EDTA) was most commonly used in studies of chelant-assisted phytoextraction (3). However, EDTA increases the risk of metal leaching, due to its persistent nature, and is, therefore, unsuitable for chelant-assisted phytoextraction (4). Moreover, environmental concerns have arisen over the elevated EDTA concentrations in ground- and surface waters, because of its potential to remobilize metals from aquifer material and aquatic sediments (5). Alternatively, the use of ethylenediamine disuccinic acid (EDDS), which is a structural isomer of EDTA, has been proposed for chelant-assisted phytoextraction (4) because the [S,S]-isomer of EDDS is readily biodegradable in soil (6–8). EDDS has also been investigated for phytoextraction in combination with in situ soil washing and reactive horizontal permeable barriers to prevent metal leaching (9) and for ex-situ soil washing (7, 10, 11). Extraction of metals by EDDS from soil has been intensively investigated (6–13) because metal mobilization from the soil solid phase to solution is one of the key processes in determining the effectiveness of phytoextraction and soil washing. 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 determining metal uptake by plants (12, 14). Plant uptake of metals generally correlates best with free metal concentrations in soil solution (15), although uptake of intact metal complexes with EDDS and EDTA has been demonstrated (12, 14, 16). Since in situ measurement of metal speciation is difficult to perform, models can be used instead. Metal binding with EDDS in soil solution can be predicted based on known stability constants of metal complexes with EDDS using basic chemical speciation programs. Advanced models, such as Model VI (17) and the Non Ideal Competitive Adsorption Donnan (NICA-Donnan) model (18), have been developed to predict metal binding with humic substances. These models have been used to successfully predict metal binding with DOM in soil solution (19–21). Before models can be applied in practice to predict speciation of dissolved metals in the presence of EDDS and DOM, however, they have to be validated, and this can be done by comparing model predictions with measurements of free metal concentrations. The Donnan membrane technique (DMT) has been developed to determine free metals in solution (22). For example, the DMT has been used to determine free metals in synthetic multicomponent solutions (22–24), surface waters (25), and soil solution (20, 21), and free ions of nutrients in animal slurries (26). The aim of our study was 2-fold: (i) to determine free metal concentrations in the presence of EDDS and DOM with the DMT using a batch extraction experiment approach, and (ii) to compare these results with the outcome of the chemical speciation program ECOSAT (27) which includes the NICA-Donnan model. This validation VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1123
has not been done yet for solutions containing EDDS and DOM. Dissolution of Al and Fe oxides at acidic pH is an important process in determining the speciation of dissolved metals in the presence of synthetic chelants and DOM, because Al and Fe compete with other metals for binding with EDDS and DOM (10, 11). We selected heavy metal contaminated soils with a wide range in pH in our batch extraction experiments to demonstrate competitive effects of Al and Fe. The results from our study are not only useful to increase the understanding of metal uptake by plants in the presence of EDDS, but they also provide insight on the influence of synthetic chelants on metal speciation in surface waters.
Materials and Methods Soils. Four soil samples were collected from field-contaminated sites in Bulgaria (Zlatitza), The Netherlands (Wildekamp), Denmark (Hygum), and China (Zhejiang) (see Supporting Information). Soil samples were oven-dried at 40 °C and sieved (
