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Synthetic chelating agents are used to increase the phytoextraction of metals from contaminated soils, however the increased bioavailability of the metals may increases the potential risk of groundwater contamination. In this study, 4 mmol EDTA kg–1 was applied using four application methods to a Cd-, Zn-, and Pb-contaminated soil to evaluate the effects of multi-dose EDTA application on the metals concentrations in the soil solution, soil (as extracted by different methods), and shoots of rainbow pink (Dianthus chinensis). Results showed that application of 4 mmol kg–1 one time or application of 2 mmol kg–1 two times, significantly decreased the concentration of Cd, Zn, and Pb in the soil solution at the end of experiment (42nd day) compared with the application of 0.67 mmol kg–1 six times during 6 weeks experiments. Application of 2 mmol kg–1 two times significantly decreased the deionized water-extractable Cd and Pb concentrations in the soil compared with the application of 0.67 mmol kg–1 six times. However, there were no any effects of the different application methods on the concentration and total removal of Cd, Zn, and Pb in the shoots of rainbow pink. Results revealed that application of 2 mmol kg–1 two times was effective to decrease the potential groundwater contamination.

Fig. 6. The effects of different EDTA applying methods on the total removal of (a) Cd, (b) Zn, and (c) Pb in the shoots of rainbow pink. The probability level of significant difference is at p = 0.05. Replicates (n) = 4. Application of 4 mmol EDTA kg–1 to the combined metals-contaminated soils at 4 mmol kg–1 at one time and 2 mmol kg–1 two times significantly decreased the Cd, Zn, and Pb concentrations in the soil solution compared with that when applied at 0.67 mmol kg–1 six times (p < 0.05). Application of 2 mmol kg–1 two times also significantly decreased the concentrations of Cd and Pb in the soil extracted by deionized water compared with the application of 0.67 mmol kg–1 six times (p < 0.05). However, no significant decrease was observed for the extraction methods using 0.05 M EDTA or 0.005 M DTPA because of their stronger extraction capacity than that of deionized water. However, good relationships were observed between the metal concentrations in the soil that were extracted by 0.05 M EDTA and 0.005 M DTPA. Although there were significant effects of the different EDTA application methods on the concentrations of metals in the soil solutions or in the soil extracted by deionized water (p < 0.05), there were no significant effects on the concentration and total removal of metals by the shoots of rainbow pink. The results also showed that when we apply a total amount of 4 mmol EDTA kg–1 to the Cd-, Zn-, and Pb-contaminated soil, or at 2 mmol kg–1 applied two times, it significantly decreased the risk of groundwater contamination compared with the other application methods.