ZVI-PRBs – A Passive Remedy
In-situ permeable reactive barriers (PRBs), or treatment walls, are replacing pump and treat remedies for contaminated groundwater with life cycle cost savings greater than 75%. Iron PRBs remediate chlorinated solvent contaminated groundwater by abiotic degradation of the halogenated volatile organic compounds to harmless daughter products.
Zero valent iron placed in the groundwater has been known to abiotically degrade a wide range of chlorinated compounds and energetics into non-toxic end products. Also, numerous heavy metals are immobilized and precipitated from the groundwater by the iron.
The first iron reactive treatment wall was constructed in 1991 as a field trial. In almost three decades, over two hundred (200) full scale and pilot PRBs have been installed.
The rapid increase in the number of PRBs installed reflects the maturity and continued acceptance of the zero valent iron technology and benefits of passive in-situ remediation. Iron PRB technology is now considered a well-established and proven long term solution to groundwater remediation and is acknowledged by many experts and regulatory agencies to be a preferred remedy with a life of greater than thirty years. The advantages and applicability of iron PRB technology are summarized below:
- Green and sustainable technology.
- Accepted by the Regulatory Agencies as a Preferred Remedy and has replaced Pump & Treat in a many Record of Decision (RODs).
- Environmentally benign - no pumping or energy needs - no toxic waste or emissions generated.
- Destroys a wide variety of volatile organic compounds (VOCs) to non-toxic end products, with chemistry and kinetics well understood.
- Immobilizes or precipitates numerous metals with a large effective capacity.
- Demonstrated long term performance, with expected life of >30 years, with no signs of clogging, no loss of permeability, or major changes in iron reactivity over time.
- Significant cost advantage amounting to less than the life cycle cost of pump and treat.
There are a variety of contaminants that are treatable by iron filings including chlorinated solvents, metals, dissolved metals and energetic compounds. The treatment process is dependent upon the contaminant.
VOC Contaminants Degraded
Iron filings placed in the subsurface beneath the water table degrade chlorinated solvents to non-toxic end products. The iron strips the chlorine atoms from chlorinated solvent compounds by abiotic dehalogenation. Click on the table for a list of contaminants destroyed during this process.
Metals Treated by Zero Valent Iron
Certain metals, such as hexavalent chromium, technetium-99, selenium, antimony, vanadium and uranium, are reduced and precipitated in the presence of iron. Adsorption and precipitation of cationic metals (i.e., copper, lead, cadmium, molybdenum, nickel, mercury and zinc) on iron occurs as a secondary iron corrosion process on the iron surface (e.g., iron oxides and oxyhydroxides). Once the metals are precipitated or absorbed to the iron, the metal is immobilized within the PRB and will not leach out by flushing with groundwater. Click on the link to see the metals treatable by iron.