FATE AND TRANSPORT OF ETHYLENEDIAMINETETRAACETATE CHELATED CONTAMINANTS IN SUBSURFACE ENVIRONMENTS

Decontamination efforts during weapons production has historically inv olved the generation of mixed waste that was composed of organically c helated radionuclides. Waste disposal has traditionally involved shall ow land burial, and not until recently has the subsurface migration of the organically complexed contaminants (co-contaminants) become a sig nificant concern. The objective of this study was to provide an improv ed understanding of the geochemical processes that control the fate an d transport of ethylenediaminetetraacetate (EDTA) chelated contaminant s in heterogeneous subsurface environments. A batch technique was util ized to investigate the kinetics and mechanisms of Co(II)EDTA2- and Sr (II)EDTA2- interaction with naturally heterogeneous saprolites and lab oratory prepared pyrolusite (beta-MnO2) coated SiO2. The interaction o f Co(II)EDTA2- with the subsurface media was characterized by a Mn-oxi de induced oxidation of the co-contaminant to form Co(III)EDTA-. The C o(III)EDTA- complex exhibited time-dependent adsorption on the subsurf ace material, and this adsorption process was unaffected by difference s in ionic strength. The oxidation reaction of Co(II)EDTA2- to Co(III) EDTA- by subsurface Mn-oxides was catalytic such that significantly mo re Co(II)EDTA2- was oxidized relative to the net Mn(IV) reduced. The i nteraction of SrEDTA2- with the subsurface media was characterized by a solid phase induced dissociation of the co-contaminant. Subsurface F e and Al sources effectively dissociated the SrEDTA2- complex, and Fe( III)EDTA- and Al(III)EDTA- were formed. Competition of Al and Fe for c omplex formation with EDTA was time-dependent and controlled by the av ailability of the Al and Fe sources and the stability of the metal-EDT A complexes. The contaminant Sr remained unchelated and existed as a r eactive, divalent cation. The implications of the EDTA chelate on the fate and transport of contaminants in subsurface environments are disc ussed.