Injection Wells
One exciting new avenue of bioremediation research is the application of carbon sources nutrients to clean up metal toxins like Pb, Cd, Zn, and Cu and Cr. This treatment can be used to target even toxic radionuclides such as U (VI).
Why add carbon sources?
Hydrocarbons are electron donors that provide a source of electrons to reduce metals in a redox reaction. For example, the half reaction for U (VI) to U (IV) is shown below.
UO2+2 + 2e- = UO2
When metabolized by the bacteria, hydrocarbons, like acetate, provide the necessary electrons to carry out this reaction. Once reduced, the metals are less soluble and easily precipitate out of solution.
How are carbon sources added?
A common method to deliver carbon sources are injection wells.
Here is a picture of an injection well used in experiment at the University of Massachusetts.
This form of injection gets the carbon source deep underground where the necessary anaerobic conditions exist to carry out the metal reduction reaction. Any oxygen that does exist in the environment will quickly be consumed as the bacteria metabolize the injected carbon source thus allowing the metal reduction to take place.
Injection wells can also be used to bioremediate nitrogenous compounds. Adding a carbon source as a nutrient is also common in remediation of halogenated organic solvents.
Sources:
Anderson (2003); Saunders (2001); NABIR; Jaffe (2005)