Kinetic interactions of EDDS with soils. 1. Metal resorption and competition under EDDS deficiency

Biodegradable EDDS ([S,S]-ethylenediaminedisuccinic acid) is an emerging chelant for enhancing heavy metal extraction. During soil remediation that involves continuous flushing, metal extraction is often limited by the amount of EDDS. Under EDDS deficiency, initial extraction of Zn and Pb followed by resorption was observed in batch kinetic experiments. Speciation calculations indicated that the percentages of ZnEDDS(2-) and PbEDDS(2-) in respective dissolved metal concentrations decreased with time, whereas the contribution of CuEDDS(2-) to total EDDS increased accordingly. This pointed to the metal exchange of newly formed ZnEDDS(2-) and PbEDDS(2-) with sorbed Cu on the soil surfaces, rather than with Fe oxides. A portion of displaced Zn and Pb was resorbed on the exchangeable and carbonate fractions, whereas the rest was mainly bound to dissolved organic matter (DOM) and remained in solution. On the other hand, although dissolved Al was the major mineral cation in solution under EDDS deficiency, the resulting competitive effect on metal extraction was marginal because Al readily dissociated from EDDS complexes and predominantly existed as colloidal precipitates, DOM-complexes, or hydrolyzed species. By contrast, under EDDS excess, metal resorption was indiscernible while more significant Al and Fe dissolution influenced the EDDS speciation.