The effectiveness of biodegradable EDDS has aroused substantial interest over the past few years, yet there has been little information on the fate of metal-EDDS complexes that, prior to biodegradation, stay in contact with the soils. This study conducted 7-day batch experiments to investigate the kinetic interactions of CuEDDS(2-), ZnEDDS(2-), PbEDDS(2-), and AIEDDS(-), which are newly formed during EDDS application, with uncontaminated and metal-contaminated soils at pH 5.5 and 8. In uncontaminated soils, metal-EDDS complexes were adsorbed and induced mineral dissolution. In contaminated soils, on the contrary, significant metal exchange with sorbed metals on the soil surfaces (i.e., Cu, Zn, and Pb) resulted in a greater extent of metal resorption of the metal-EDDS complexes. The interactions of metal-EDDS complexes, moreover, are influenced by the characteristics of the metal center. Compared with ZnEDDS(2-) and PbEDDS(2-), CuEDDS(2-) was least adsorbed or exchanged, which may be attributed to higher ionic potential and the electron configuration of Cu. In addition, AIEDDS(-) was partially exchanged on the soil surfaces at low pH while entirely dissociated in solution at high pH. Therefore, the fate of individual metal-EDDS complexes in the subsurface depends on the metal center, other sorbed metals and mineral cations on soils, and solution pH.

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.