Zero-Valent Iron Shows Promise As a Strong Chemical Reducer of Perc

Written by Steve Henshaw, P.G., President & CEO, EnviroForensics, in Collaboration with Keith Gaskill, L.P.G., Project Manager and Geochemist, EnviroForensics
As seen in the May 2011 issue of Cleaner & Launderer

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Zero-valent iron (ZVI) has been used in the groundwater remediation industry for several decades.  Typically, granular scale ZVI was emplaced in trenches serving as a permeable reactive barrier wall, whereby contaminated groundwater would flow through the wall and volatile organic compounds (VOCs) like Perc would react with the material and breakdown.

This technology worked so well that scientists started applying the ZVI into contamination source areas and hot spots.  Recent advances have produced smaller scale iron particles that can be mixed into a slurry and pumped into the ground through the use of pre-existing injection technologies. 

ZVI technology can be summarized as a remedial method that produces rapid chemical reduction.  Some manufacturers produce a ZVI hybrid product where the injected slurry consists of a solid-phase controlled release carbon as a backbone for nano-scale or micro-scale ZVI or other metals (e.g., zinc).  The ZVI portion first produces powerful reducing conditions (approximately -550 mV), which completely destroy many contaminants because they become unstable in these reductive conditions.  Next, natural attenuation processes in groundwater are enhanced through the release of organic carbon as well as major, minor, and micro-nutrients.  As these compounds migrate through the treatment zone, they provide ideal conditions for growth of indigenous microorganisms.  Because the rate of carbon release is controlled, the extent of biological colonization can also be managed and maintained. 

Today there are numerous companies making ZVI products for injection into contaminated groundwater plumes.  In general the iron particles are very small in size, termed nano-scale or micro-scale.  The product is made small to provide more surface area to the particles, thereby allowing for more contact with the product and the contaminated groundwater. 

The biggest drawback to using ZVI is the cost, although some states also are resistant to the use of nano-scale ZVI technology fearing that the particles are so small that they could migrate uncontrolled offsite.  But the cost is the primary guiding principle.  To achieve maximum cost effectiveness, ZVI is an aggressive method best utilized in applications in areas with high concentrations.  The location of the perc release, or source area, has the highest concentration of contaminants and must be treated with an aggressive technology.  The larger area of the plume, with lower concentrations, can be treated with a less aggressive and less expensive technology such as enhanced reductive dechlorination.  At many sites, the use of tandem technologies produces maximum cost efficiency and desired results.  The accompanying figure shows the use of ZVI beneath a dry cleaner, in what is the contaminant source area.  The ZVI is also injected in linear patterns perpendicular to the groundwater flow.  This alignment is similar to employing a granular scale permeable reactive barrier wall but it’s injected through simpler, faster and less expensive methods.  The dissolved portion of the contaminated groundwater plume is then injected with a less expensive, longer lasting enhanced reductive dechlorination type material.

As with many injection technologies, it may be necessary to come back to the site later and perform follow-up injections to be sure the chemical processes that the first injection started can be brought to completion. 

The use of ZVI, hybrid ZVI products, and ZVI in tandem technologies have emerged as a cost effective and aggressive remedial option.