Metal Extraction Kinetics and the Speciation of Metal and EDDS in the EDDS-enhanced Soil Flushing Process

Column and batch experiments were conducted to investigate the kinetic interactions of uncomplexed EDDS with contaminated soils during in-situ soil flushing. In column study, initial Pb extraction followed by re-adsorption was observed. Retardation of Pb compared with Cu elation was further supported by the decrease of Pb extraction in batch studies. On the other hand, the amount and rate of mineral dissolution in both column and batch studies were not significant when compared with metal extraction. Speciation calculations were then carried out and the result indicated that the percentages of ZnEDDS(2-) and PbEDDS(2-) in terms of the respective dissolved metal concentrations decreased with time, whereas the contribution of CuEDDS(2-) to total EDDS increased accordingly, which reflected the metal exchange of newly formed PbEDDS(2-) with adsorbed Cu rather than with Fe oxides on the soil surfaces. Apart from re-adsorption on soils, displaced Pb, due to heavy metal exchange, was bound to dissolved organic matter (DOM) and remained in solution. On the other hand, although dissolved Al was the major mineral cation in solution, its resulting competitive effect on metal extraction was marginal because Al had readily dissociated from EDDS complexes and predominantly existed as colloidal precipitates, DOM-complexes, or hydrolyzed species