FATE AND TRANSPORT OF ETHYLENEDIAMINETETRAACETATE CHELATED CONTAMINANTS IN SUBSURFACE ENVIRONMENTS

Decontamination efforts during weapons production has historically involved the generation of mixed waste that was composed of organically chelated radionuclides. Waste disposal has traditionally involved shallow land burial, and not until recently has the subsurface migration of the organically complexed contaminants (co-contaminants) become a significant concern. The objective of this study was to provide an improved understanding of the geochemical processes that control the fate and transport of ethylenediaminetetraacetate (EDTA) chelated contaminants in heterogeneous subsurface environments. A batch technique was utilized to investigate the kinetics and mechanisms of Co(II)EDTA2- and Sr(II)EDTA2- interaction with naturally heterogeneous saprolites and laboratory prepared pyrolusite (beta-MnO2) coated SiO2. The interaction of Co(II)EDTA2- with the subsurface media was characterized by a Mn-oxide induced oxidation of the co-contaminant to form Co(III)EDTA-. The Co(III)EDTA- complex exhibited time-dependent adsorption on the subsurface material, and this adsorption process was unaffected by differences in ionic strength. The oxidation reaction of Co(II)EDTA2- to Co(III)EDTA- by subsurface Mn-oxides was catalytic such that significantly more Co(II)EDTA2- was oxidized relative to the net Mn(IV) reduced. The interaction of SrEDTA2- with the subsurface media was characterized by a solid phase induced dissociation of the co-contaminant. Subsurface Fe and Al sources effectively dissociated the SrEDTA2- complex, and Fe(III)EDTA- and Al(III)EDTA- were formed. Competition of Al and Fe for complex formation with EDTA was time-dependent and controlled by the availability of the Al and Fe sources and the stability of the metal-EDTA complexes. The contaminant Sr remained unchelated and existed as a reactive, divalent cation. The implications of the EDTA chelate on the fate and transport of contaminants in subsurface environments are discussed.