Sorption of heavy metals and radionuclides on mineral surfaces in the presence of organic co-contaminants. 1997 annual progress report

'This project fits well within the overall objectives established by the Environmental Management and Science Program to promote long-term basic research that will provide the tools for more effective and lower cost remediation efforts at DOE sites where hazardous and radioactive wastes or contamination zones are present. In order to develop the necessary remediation technology it has been recognized that a fundamental understanding of the various chemical and physical factors associated with waste treatment and contaminant transport must be established. Some of the specific topics include waste pretreatment, volume reduction, immobilization, separation methods, the interactions of actinides and heavy metals with surfaces in the presence of organic residues and co-contaminants, contaminant transport in the environment, and long-term storage site assessment. This project has direct and potential application in all these areas. The interaction and partitioning of contaminant metals and radionuclides between solution and solid- surface phases is a fundamental issue for waste treatment and predicting contaminant transport in the environment. Many factors are involved in the functional relationships describing chemical reactivity and physical distribution of chemical species. These include modification of chemical behavior by the suite of chemical co-contaminants in a system. Organic complexing agents are common components of waste mixtures and include both synthetic components specifically introduced as part of processing methods, and poorly characterized compounds that were introduced separately or evolved within the highly reactive wastes. Natural organic complexing agents such as citric acid and siderophores are common in nature and represent factors that will further influence contaminant transport in soils and aquatic systems. Knowledge of the existence of a metal-organic complex cannot automatically be used to predict changes in solid-solution partitioning of the metal or the rates of changes in partitioning as a consequence of ligand or metal exchange. Formation of metal ion-ligand-oxide surface ternary complexes can enhance sorption in some cases while highly stable metal-organic complexes in solution will decrease sorption. The preference for these two cases depends on the relative chemical properties of each component as well as system parameters such as pH and ionic strength and the relative reaction rates. In general, there is a fundamental absence of the experimental observations needed to establish the trends needed for predictive modeling and technological developments. If the authors can understand both the equilibrium partitioning in these ternary systems (i.e., metal-ligand-solid) and the types of complexes that form as functions of a set of measurable parameters that characterize the system, the authors should significantly improve the ability to estimate the mass transport rates of contaminants and natural solutes in aquifers and water columns where organic ligands are significant species. The authors will also be better prepared to devise remediation strategies where introduction of organic ligands can bring about either mobilization of contaminants or enhanced retardation. The ligands are likely to prove useful in the direct treatment and remediation of contaminated sites (Francis and Dodge, 1992; Tuin and Tels, 1990) and can be chosen for their long term stability or their ability to degrade by natural biotic processes.'