The remobilization of metals from iron oxides and sediments by metal-EDTA complexes

Synthetic chelating agents such as EDTA form strong complexes with heavy me tals and therefore have the potential to remobilize metals from sediments a nd aquifers. In natural waters EDTA is present almost exclusively in the fo rm of metal-complexes. Therefore, remobilization of metals is always a meta l-metal-EDTA exchange reaction. We have investigated, to our knowledge for the first time, the remobilization of metals from the surface of synthetic iron oxides and from a river sediment by different metal-EDTA complexes. Th e metals are exchanged as follows: MeEDTA(dissolved) + Me-adsorbed* --> Me*EDTA(dissolved) + Me-adsorbed The order of the remobilization rate of Zn2+ from goethite is CaEDTA > Fe(I II)EDTA, reflecting the slow exchange reaction of Fe(III)EDTA. For the remo bilization of Pb2+ from goethite, the rate was found to be Fe(III)EDTA > Ca EDTA > ZnEDTA. Here, Fe(III)EDTA has surprisingly the fastest exchange rate . Only very limited remobilization of Pb2+ is possible from hydrous ferric oxide at pH 8 due to the very strong adsorption of Pb2+. The order of remob ilization of Zn2+ from a natural river sediment was found to be CaEDTA > Cu EDTA > Fe(III)EDTA. The remobilization rate of Zn2+ with Fe(III)EDTA is onl y 12% of the rate with CaEDTA, illustrating the importance of EDTA speciati on for assessing remobilization.