Fundamental studies of the removal of contaminants from ground and waste waters via reduction by zero-valent metals. Annual progress report, September 1, 1996--August 31, 1997

'Contaminated groundwater is a problem throughout the US and the world. In many instances the tvpes of contamination can be directly attributed to man''s actions. For instance, the burial of wastes, casual disposal of solvents in unlined pits, and the development of irrigated agriculture have all contributed to groundwater contamination. The kinds of contaminants include chlorinated solvents and toxic trace elements that are soluble and mobile in soils and aquifers. Oxyanions of selenium. chromium. uranium. arsenic. and chlorine (as perchlorate) are frequently found as contaminants on many DOE sites. In addition. the careless disposal of cleaning solvents. such as carbon tetrachloride and trichloroethylene. has further contaminated many groundwaters at these sites. In agricultural areas of the western US, shallow groundwaters have become contaminated with high levels of selenate, chromate, and uranyl. The management of these waters requires treatment to remove the contaminants before reuse or surface water disposal. In one instance in the Central Valley of California. the discharge of selenate-contaminated shallow groundwater to a wildlife refuge caused catastrophic bird deaths and deformities of embryos. At sites where solid-propellant rocket motors were tested or disposed of, high concentrations of perchlorate and trichloroethylene are being found in the groundwater. A potential remediation method for many of these oxyanions and chlorinated-solvents is to react the contaminated water with zero-valent iron. In this reaction, the iron serves as both an electron source and as a catalyst. Elemental iron is already being used, on an experimental basis, for the reductive dechlorination of solvents and the removal of toxic trace elements. Both in situ reactive barriers and above-ground reactors are being developed for this purpose. However, the design and operation of these treatment systems requires a detailed process-level understanding of the interactions between the contaminants and the iron surfaces. Only limited success has been achieved in the field, partly because the basic surface chemical reactions are not well understood. The authors are performing fundamental investigations of the interactions of the relevant chlorinated solvents, trace elements, and trace element-containing compounds with single- and poly-crystalline Fe surfaces. The aim of this work is to develop the fundamental physical and chemical understanding that is necessary for the development of cleanup techniques and procedures. The authors are perforrming both bulk chemical measurements of the reduction reactions and surface science studies of model chemical systems. During this first year of funding, the authors have already made significant progress in both areas. Initially, they have focused primarily on the reduction of selenate by elemental iron. They have also performed some work with chromate, perchlorate, uranyl, and carbon tetrachloride, as well. In the following sections, some of the progress is described.

Fundamental studies of the removal of contaminants from ground and waste waters via reduction by zero-valent metals. 1998 annual progress report

'Contaminated groundwater and surface waters are a problem throughout the US and the world. In many instances, the types of contamination can be directly attributed to man''s actions. For instance, the burial of wastes, casual disposal of solvents in unlined pits, and the development of irrigated agriculture have all contributed to groundwater and surface water contamination. The kinds of contaminants include chlorinated solvents and toxic trace elements that are soluble and mobile in soils and aquifers. Oxyanions of selenium, chromium, uranium, arsenic, and chlorine (as perchlorate) are frequently found as contaminants on many DOE sites. In addition, the careless disposal of cleaning solvents, such as carbon tetrachloride and trichloroethylene, has further contaminated many groundwaters at these sites. Oxyanions of selenium, nitrogen, arsenic, vanadium, uranium, chromium, and molybdenum are contaminants in agricultural areas of the Western US. The management of these waters requires treatment to remove the contaminants before reuse or surface water disposal. In one instance in the Central Valley of California, the discharge of selenate-contaminated shallow groundwater to a wildlife refuge caused catastrophic bird deaths and deformities of embryos. A potential remediation method for many of these oxyanions and chlorinated-solvents is to react the contaminated water with zero-valent iron. In this reaction, the iron serves as both an electron source and as a catalyst. Elemental iron is already being used on an experimental basis. Both in-situ reactive barriers and above-ground reactors are being developed for this purpose. However, the design and operation of these treatment systems requires a detailed process-level understanding of the interactions between the contaminants and the iron surfaces. Only limited success has been achieved in the field, partly because the basic surface chemical reactions are not well understood. The authors are performing fundamental investigations of the interactions of the relevant chlorinated solvents, and trace element-containing compounds with single- and poly-crystalline Fe surfaces. The aim of this work is to develop the fundamental physical and chemical understanding that is necessary for the development of cleanup techniques and procedures. As of May 1998, they have performed both bulk chemical measurements of the reduction reactions and surface science studies of model chemical systems. During these first two years of funding, the authors have made significant progress in both areas. Initially, they focused primarily on the reduction of selenate by elemental iron. They also performed some work with chromate, perchlorate, uranyl, and carbon tetrachloride. In the following sections some of the progress is described.