Metals Immobilization

Metals Contamination

Metal contaminants are present at many sites undergoing remediation in the United States. For example, soil and groundwater are contaminated with metals at:

  • 65 percent of 944 National Priorities List (NPL) sites1,2
  • 46 percent of 214 sites regulated under the Resource Conservation and Recovery Act (RCRA) 2,3
  • 69 percent of 3,212 U.S. Department of Defense (DOD) sites 2,4

The most common metal contaminants at all U.S. sites are lead (Pb), chromium (Cr), arsenic (As), zinc (Zn), cadmium (Cd), copper (Cu), mercury (Hg) and mixtures thereof.5 Most of these metals have documented negative human health effects at low microgram-per-liter concentrations. Contamination by radioactive elements is an additional and extremely important concern. Uranium (U) is present in soil and groundwater at 70 percent of U.S. Department of Energy sites where nuclear weapons and reactor activities were conducted.6 Uranium is also found at mining and ore processing sites throughout the United States.

The most common sources of metals contamination in groundwater are leachate from landfills, mine tailings piles and abandoned mines; sewage sludge or liquid sewage; and spills, leaks and land disposal of wastes at industrial facilities.7 Industries that typically use metals and are potential sources of contamination are plating (Cr and Cd), battery manufacturing [nickel (Ni), Cd and Pb], wood preserving (Cu, Cr, and As), chemical manufacturing (Cu, Zn, Pb and Hg) and smelting (Cu, Zn, Pb and Hg).8

Metals Remediation Processes

Metals remediation consists of ex situ (above ground) and in situ (subsurface) processes. When applicable, in situ processes are often more cost-effective and easier to implement than ex situ processes since in-ground treatment eliminates excavation, disposal and/or pump-and-treat activities. One primary method of in situ metals remediation is metals immobilization. Immobilization refers to the process of transferring aqueous phase, highly mobile metals to a solid, stable phase that becomes part of the soil. This phase transfer prevents continued migration of contaminated metals plumes and can be a permanent solution depending on the metal and site-specific geochemistry. The most common mechanisms of in situ metals immobilization are metals adsorption to soil particles or precipitation of metal solids that are chemically fixed to soil particles. Both of these mechanisms can permanently remove metals from the aqueous phase, restoring the aquifer and the desired usability of the water.