Multispecies transport of metal-EDTA complexes and chromate through undisturbed columns of weathered fractured saprolite

Laboratory-scale tracer experiments were conducted to investigate the geoch emical and hydrological processes that govern the fate and transport of org anically chelated radionuclides and toxic metals in undisturbed saturated c olumns of weathered, fractured shale saprolite. Three long-term, reactive c ontaminant injections were pulsed onto three separate soil columns, with th e following influent mixtures: (1) (109)CdEDTA(2-), (2) (109)CdEDTA(2-) and Co-57,Co-58(II)EDTA(2-), and (3) (109)CdEDTA(2-), Co-57(III)EDTA(-), and ( HCrO4-)-Cr-51. Both single and multiple species experiments were conducted to determine the importance of interaction between the contaminants and com petition for surface sites. Flow interruption was used to identify physical and chemical non-equilibrium (PNE and CNE) which were caused by multiple p ore-region flow and rate-limited chemical reactions, respectively. Reactive contaminant transport through the fractured, weathered shale was affected by sorption, redox, and dissociation reactions, which were mediated by soil organic matter and surficial oxides of Fe, Mn, and Al. The transport of Cd EDTA(2-) was significantly influenced by ligand-promoted dissolution of sub surface Fe and Al sources, resulting in the liberation of Cd2+, Al(III)EDTA (-) and Fe(III)EDTA(-). Flow interruption confirmed that the surface-mediat ed dissociation reaction was time-dependent, with the stability of the CdED TA(2-) complex dependent on its residence time within the soil. The migrati on of Co(II)EDTA(2-) was dominated by oxidization to the highly stable Co(I II)EDTA(-) species, and elevated effluent Mn2+ suggested that surficial Mn( IV) oxides likely catalyzed the redox reaction, though Fe-oxides may have a lso contributed to the reaction. Dissociation (12%) of the Co(II)EDTA(2-) c omplex was first observed during flow interruption indicating that rate-lim ited dissociation of the complex by Fe-oxides may be significant under equi librium conditions. The transport of HCrO4- was significantly altered by th e reduction of mobile Cr(VI) to irreversibly bound Cr(III). The reduction r eaction was catalyzed by surface-bound natural organic matter and flow inte rruption confirmed that the reaction was time-dependent. There was little e vidence of competitive effects between the various contaminants in the mult ispecies experiments, since each was influenced by a different geochemical process during transport through the soil. The results of this study furthe r support research findings that suggest anionic toxic metals and radionucl ide-organic complexes can be significantly influenced by soil geochemical p rocesses that can both enhance and impede the subsurface migration of these contaminants. (C) 2000 Elsevier Science B.V. All rights reserved.