Biodegradation and speciation of residual SS-ethylenediaminedisuccinic acid (EDDS) in soil solution left after soil washing
Introduction
Chelating agents are potent agents for solubilizing heavy metals from polluted soils. Two different remediation methods using chelating agents are now being investigated: chelant assisted ex-situ soil washing and chelant assisted phytoextraction. In ex-situ soil washing two methods are possible, batch washing (Tandy et al., 2004, Vandevivere et al., 2001a) and heap leaching (Hauser et al., 2005). In batch washing the soil is excavated and washed in a closed system with chelating agents and then returned to the site or used otherwise. In heap leaching the soil is also excavated but treated by sprinkling solution over it which preserves the soil structure. The percolate is collected and treated off or on-site. A variation of this procedure has been proposed where a biodegradable chelating agent is used that is degraded in a permeable reactive barrier under the soil which traps the liberated metals (Finzgar et al., 2004, Kos and Lestan, 2003a, Kos and Lestan, 2004a, Kos and Lestan, 2004b).
In chelant-assisted phytoextraction chelating agents are added to the soil to increase the solubilized metals and correspondingly uptake into plants (Garbisu and Alkorta, 2001). The enormous drawback of this method is the inevitable leaching of chelants and its metal-complexes into the deeper soil layers and eventually to groundwater.
The ex-situ methods attract a lesser risk of leaching than phytoremediation as most of the chelating agent is removed from the soil before returning to the field. However some chelating agent is always left in the soil and the formation of metal complexes with this residual complexing agent is possible and in turn leaching of these metal complexes must be taken into consideration.
Previously the most used complexing agent for these methods was EDTA (Abumaizar and Khan, 1996, Peters, 1999, Thayalakumaran et al., 2003, Van Benschoten et al., 1997, Wenzel et al., 2003, Wu et al., 1999). It is however recalcitrant in the environment and leaching of metal-complexes over a long time period was possible (Bucheli-Witschel and Egli, 2001, Wenzel et al., 2003, Wu et al., 2004). SS-ethylenediaminedisuccinic acid (EDDS) is a biodegradable chelating agent that is a structural isomer of EDTA (Schowanek et al., 1997, Vandevivere et al., 2001b). It is now starting to replace EDTA in soil washing and phytoextraction (Kos and Lestan, 2003b, Tandy et al., 2004). It should not be confused with the other stereo-isomers of EDDS however (RR-, RS-, SR-), which are partly or wholly non-biodegradable (Schowanek et al., 1997, Takahashi et al., 1997). Several authors recently have carried out work on EDDS assisted phytoextraction, mainly on Pb but also Zn, Cu, Cd and Ni (Grcman et al., 2003, Kos et al., 2003, Kos and Lestan, 2003a, Kos and Lestan, 2003b, Kos and Lestan, 2004a, Kos and Lestan, 2004b, Luo et al., 2005, Meers et al., 2005, Tandy et al., in press). An immediate leaching risk is possible during this method until the EDDS has degraded (Kos and Lestan, 2004a). Three studies have also used EDDS for soil washing or heap leaching (Hauser et al., 2005, Tandy et al., 2004, Vandevivere et al., 2001a).
As leaching is only a risk as long as EDDS is present, it is important to look at how it degrades in soil. Most investigations into the biodegradation of EDDS have been carried out using standard degradation tests such as the Sturm test (Schowanek et al., 1997) or modifications, that use sewage sludge (Takahashi et al., 1997, Vandevivere et al., 2001b). Although these methods have shown that EDDS is degraded in sewage sludge it tells us nothing about the rate of degradation in soil. One study has carried out a degradation experiment using soil, this soil however was also spiked with sludge which might effect the degradation rate of EDDS due to more microorganisms being present in the soil (Schowanek et al., 1997). Soils remediated by washing or phytoremediation are also contaminated with heavy metals such as Cu, Zn, Pb, Cd and Ni. There is evidence that some metal-EDDS complexes with high stability constants (Cu, Ni and to a lesser extent Zn) are non-biodegradable when in isolation (Vandevivere et al., 2001b). As the soil in which EDDS was previously tested was not contaminated with heavy metals, the above might also lead to a difference in degradation rates or incomplete degradation. Another investigation looked indirectly at the degradation of EDDS after phytoextraction of heavy metal contaminated soils (Meers et al., 2005). Here no sewage sludge was added to the soil but high concentrations of EDDS were used in keeping with concentrations used in phytoremediaton. These levels are much higher than found in soil after soil washing and initial concentration of EDDS may influence the rate of degradation.
Our aim was to investigate the degradation of residual EDDS in soil following soil washing. We also wanted to investigate the solubility of metals in soil solution caused by residual EDDS and the speciation of EDDS complexes in soil solution to see if this would effect degradation. To achieve this we have washed three contaminated soils with EDDS and have investigated the changes in soil solution metal and EDDS concentrations over time.
Section snippets
Chemicals
All chemicals were purchased from Merck (Switzerland) and were analytical grade or HPLC grade for the solvents unless stated. SS-EDDS (Octaquest E30, 1.092 mol kg−1) was obtained from Octel (Cheshire, UK) for the experiments and from Proctor and Gamble (Belgium) as the Na3EDDS salt for the EDDS analysis (stock solution 1 mM). Fluorenylmethyl chloroformate (FMOC-chloride) (puriss) was obtained from Fluka. All solutions were made with high purity water (Millipore, Bedford, MA).
Soils
The soils were taken
Soil washing
Washing with EDDS removed significant amounts of heavy metals from the three soils. The results are presented in Table 3. Seventy four percent Cu from the most contaminated soil (soil 1) and 50% of Cu from soils 2 and 3 were extracted by EDDS. For Zn between 44–56% was extracted from soils 1 and 3, but nothing from soil 2, which was only lightly contaminated. 26% of the Pb from soil 3 (the only soil contaminated with Pb) was also extracted.
The soils used for this experiment were taken from the
Degradation of EDDS in soil solution
The soils showed a lag phase of between 7 and 11 days before degradation of EDDS started. Most biodegradation work carried out on SS-EDDS has used acclimatized sludge, where the sludge has been fed EDDS for a period prior to the experiment, so no lag phase was found (Schowanek et al., 1997, Vandevivere et al., 2001b). Where unacclimatized sludge has been used a lag phase of between 5–16 days has been found (Jaworska et al., 1999, Schowanek et al., 1997, Takahashi et al., 1997). A lag phase of 3
Conclusions
Residual EDDS from soil washing was degraded after a lag phase of 7–11 days with a half-life of 4.18–5.60 days. No influence of EDDS-speciation on the reaction rate was observed. The decrease in EDDS resulted in a corresponding decrease in solubilized metals. Changes in EDDS speciation can be related to (1) initial composition of the soil, (2) temporarily anoxic conditions in the soil slurry after soil washing, (3) exchange of EDDS complexes with Cu even in soils without elevated Cu content and
Acknowledgements
We thank Diederik Schowanek from Procter & Gamble and Raj Patel from Octel, U.K. for providing the S,S-EDDS. This work was funded in part by the Federal Office for Education and Science within COST Action 837 and the Swiss National Science Foundation in the framework of the Swiss Priority Program Environment. We thank Iso Christl for his help with ECOSAT and Werner Attinger and Anna Grünwald for their technical assistance.
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