Significance of metal exchange in EDDS-flushing column experiments

Chelating agents have been widely studied for extracting heavy metals from contaminated soils, and the effectiveness of EDDS ([S,S]-ethylene-diamine-disuccinic acid) has aroused extensive attention because of its biodegradability in the natural environment. However, in the course of EDDS-flushing, metal exchange of newly extracted metal-EDDS complexes with other sorbed metals and mineral cations may result in metal re-adsorption on the soil surfaces. Therefore, this study investigated the relative significance of metal exchange under different travel distances of chelant complexes, characteristics of soil contamination, and solution pH in the column experiments. As a result of metal exchange, the elution of Zn and Pb was retarded and the cumulative extraction was lower than those of Ni and Cu, especially over a longer travel distance. Compared with the field-contaminated soils, the effects of metal exchange were even more substantial in the artificially contaminated soil because of a greater amount of extractable metals and a larger proportion of weakly bound fractions. By contrast, metal exchange was insignificant at pH 8, probably due to less adsorption of metal-EDDS complexes. These findings highlight the conditions under which metal exchange of metal-EDDS complexes and the resulting impacts are more significant during EDDS-flushing.