Zero-valent metal reduction is a process in which a metal such as iron, platinum, or other
zero-valent metals are used to refine polluted waters. These metals are placed in the flow
of water, where they begin oxidizing, causing other chain reactions to purify the water.
Although the zero-valent metal reduction process has been known for years, it has not
been applied to contaminated soils and waters until recently. Robert Gillham, chairman
of earth sciences department at the University of Waterloo in Ontario, realized that the
degradation of metals could be exploited to clean up environmental contamination.
Gillham first tested his idea with the water contaminants of a local machine shop and
observed that the chlorinated pollutants disappeared. He soon proceeded to form
EnviroMetal Technologies Inc. (ETI) to further test and commercialize this new
technology. The basis of zero-valent metal reduction is a relatively simple idea that could
function as an ideal replacement for the costly pump-and-treat technology. Gillham states
that the use of iron degradation “doesn’t require energy, it conserves water, and although
installation costs may be comparable to other above-ground systems, the big payback is in
operation and maintenance costs.”5
The most common chlorinated contaminant, trichloroethene (TCE), has been found at
more than 790 of the 1300 sites on the National Priority List.2 To better understand zerovalent metal reduction for chlorinated hydrocarbons, such as TCE, it is important to
introduce the concepts that are being applied. Alkyl halides are heavily used in industry
as solvents and lubricants, so their interaction with industrial metals is of great interest.
The chemical equations that follow are specifically involved with reductive
dehalogenation of alkyl halides using iron as the zero-valent metal.
Zero oxidation state metallic iron, Fe0, and dissolved aqueous iron, Fe2+, form the redox
couple:
Fe2+ + 2e-  Fe0
(1)
In the presence of water, a proton donor, alkyl halides (RX) can be reduced by iron. The
typical reducing reaction for RX would be reductive dehalogenation:
RX + 2e- + H+  RH+X-
(2)
The net reaction of equations 1 and 2 is a well known reaction known as the dissolving
metal reduction:
Fe0 +RX + H+  Fe2+ + RH + X-
(3)
The above equation is the net reductive dehalogenation by iron. It is equivalent to iron
corrosion with the alkyl halide acting as the oxidizing agent.
When there are no strong oxidizing agents present, there are two reduction half-reactions
that produce a spontaneous corrosion reaction in water when coupled with equation 1.
The preferred oxidant, dissolved oxygen (eq 4), resulting in rapid corrosion (eq 5):
O2 + 2H2O + 4e-  4OH-
(4)
2Fe0 + O2 + 2H2O  2Fe2+ + 4OH- (5)
In equation 6, water alone can serve as the oxidant, causing corrosion to occur under
anaerobic conditions (eq 7):
2H2O + 2e-  H2 + 2OH-
(6)
Fe0 + 2H2O  Fe2+ + H2 + 2OH-
(7)
In weakly buffered systems, Equations 5 and 7 cause the pH to rise. This effect is more
pronounced under aerobic conditions, where corrosion occurs rapidly. The concern with
increasing pH is that it can cause precipitates of iron hydroxide, which may form a layer
on the surface of the iron, possibly inhibiting its further dissolution.3
The three major reductants in this Fe0-H20 system are iron metal, the ferrous iron, and
hydrogen produced during corrosion. These three major reductants lead to three main
pathways that contribute to alkyl halide dehalogenation. The first pathway, equation 3
involves the metal directly, implying electron transfer from the Fe0 surface to the
adsorbed alkyl halide is the method of reduction.
The second pathway involves the immediate product of corrosion in aqueous systems,
Fe2+:
2Fe2+ + RX + H+  2Fe3+ + RH + X-
(8)
Dissolved Fe3+ is capable of causing dehalogenation of some alkyl halides, but this is
usually a slow reaction.
The hydrogen produced as a product of corrosion with water is the third model for
reductive dehalogenation by iron:
H2 + RX  RH + H+ + X-
(9)
The above equation needs an effective catalyst to work as a reductant to contribute
directly to dehalogenation. Corrosion and reduction reactions may be inhibited by
excessive H2 accumulation at the metal surface. The surface of iron, its defects, and other
solid phases in the system could supply the catalyst necessary to provide for rapid
dehalogenation by H2.3
In the paper titled, “Reductive Dehalogenation of Chlorinated Methanes by Iron Metal”,
by Matheson, laboratory model systems were created to learn more about the mechanism
and kinetics of transformations that take place when chlorinated methanes in aqueous
form come in contact with granular iron. The paper presents some helpful chemical
background, showing the main equations involved in the dehalogenation processes. The
experimental section of the paper discussed the chemicals used, described the model
reaction systems, and the methods of analyzing. The results and discussion section were
very detailed and technical, covering the subjects of corrosion, halocarbon degradation
pathways, kinetics of transformation, pathway of dechlorination by iron, the effect of pH,
the role of iron surface characteristics, the kinetics of surface reaction, and the mechanism
of dehalogenation.3
The experiment was carried out using carbon tetrachloride, chloroform, methylene
chloride, and trichloroethylene to make aqueous standard solutions. Dechlorination
experiments were carried out in closed batch systems made in 60-mL serum bottles. The
fine-grained iron was cleaned of oxides and other surface coatings with hydrochloric acid,
then was triple rinsed with deionized water while the bottle was purged with N2. A
temperature of 15 C was chosen for the experiments to reflect normal groundwater
conditions. Halocarbon stock solution was injected into the serum bottles through the
septum.
Analyses was carried out using chromatographic methods to find concentrations of
chlorinated solvents. An elemental analyzer was used to measure total carbon, nitrogen,
and sulfur contents of the metal after complete combustion and thermal conductivity
detection had taken place. Gas adsorption was the method for finding iron surface area.
The pH was determined before, during, and after the experiment. 3
The results confirmed that anaerobic corrosion took place by the reduction of the water.
About 70% of the carbon tetrachloride was reduced to chloroform by dehalogenation in
all systems containing iron metal. Methylene chloride was produced from further
reductive dehalogenation of chloroform, after the carbon tetrachloride dropped to the
detection limit. Methylene chloride accounted for about 50% of the chloroform lost. The
methylene chloride disappeared after several months, it is unknown if it was due to
dechlorination. The dominant degradative pathway for chlorinated methanes in anaerobic
iron-water systems is sequential reductive dehalogenation. With each sequential
dechlorination step, reduction rates become much less favorable. The increasing pH was
found to decrease the pseudo-first-order rate constant for carbon tetrachloride
dehalogenation by iron metals. Increasing both the mass and surface area of iron cause
the pseudo-first-order rate constants for carbon tetrachloride dehalogenation to increase.3
It was concluded that the most important predictor of the rate of dechlorination was the
iron surface area concentration. Aerobic conditions might also behave differently due to
more aggressive corrosion. Precipitation of ferric hydroxides in real groundwater
conditions sulfides and bacteria are also of a concern.. Our main critique would be that
if these issues have been important in affecting the process in real-life groundwater
conditions, then why weren’t they introduced into the experiment. It would have been
interesting to see how aerobic situations would compare with the anaerobic results.
Another question this paper left us with would be; would a large amount of iron be
cleaned with hydrochloric acid in an actual groundwater treatment project? It makes
sense that this would aid the process, but is it cost effective? It would also have been
interesting to see this experiment carried out without the acid rinse, so a comparison
could be made.
One of the first projects involving zero-valent metal reduction was a contaminated
industrial site in Sunnyvale, California taken on by ETI. In January of 1995, after being
approved by the California Regional Water Quality Control Board, a full-scale, permeable
reactive iron wall system was installed. For the price of $770,000, this system includes
slurry walls that direct the groundwater toward the reactive wall, and the reactive wall
itself with dimensions of four feet wide, forty feet long, and thirty feet deep.6 The
groundwater moves through the wall in approximately four days, which is plenty of time
for most contaminants to degrade into products that are then degraded themselves. The
specific contaminants being treated in Sunnyvale are TCE, cDCE, VC, and Freon 113.
Since installation, the water leaving the treatment wall meets EPA standards.6
After proving success at Sunnyvale, EnviroMetal Technologies Inc. has used this
treatment process on several contaminated sites throughout North America and one site in
Ireland. One other example of their improvements can be seen in a similar industrial
facility in New York. Here, a pilot-scale funnel and gate system was installed to treat
300ppb of TCE. The cost for this installation was $250,000, which included $30,000 for
45 tons of iron.6 The results can be observed on the following graph.
As is evident from the results, the advancements in zero-valent metal reduction have
immense potential for treating contaminated groundwater and soils.
The zero-valent iron reduction procedure has since been experimented with to treat soils
and has successfully removed contaminants, specifically VOCs. In the soil remediation
process, a twin auger mechanism breaks up and mixes soil to a depth of 30 feet. The
addition of iron is deployed by injecting a water slurry containing metallic iron particles
(less than 0.3 millimeters in diameter) into the soil with the soil mixing equipment.4
Results reveal that within two weeks of the addition of iron, the concentrations of carbon
tetrachloride, chloroform, 1,2-dichloroethene, and trichloroethene had decreased to below
analytical detection limits.
The potential use for this new technology could be integrated into hundreds of different
industries. For example, in addition to treating chlorinated solvents, iron can degrade
certain banned pesticides, which still contaminate the soil. Iron can also degrade dye
waste, and could most likely be used to clear up water tainted with dye effluents from
textile mills. Another possible use of iron reduction is to rid soils of technetium, a
radionucloetide polluting several DOE sites.5
Further uses of iron remediation are possible for the treatment of soils contaminated with
heavy metals. One study tested with a 50-50 sand and iron mixture reduced chromate in
groundwater to below detection levels.5 One last example of reduced iron remediation in
soil is the removal of nitrates. This is a prevalent problem due to the fact that nitrate
contamination is one of the most extensive soil contaminants worldwide, especially in
agricultural areas where they use fertilizers.5
Some of the disadvantages of using the metal reduction process are the gradual corrosion
of the treatment walls, an increase in pH, and the toxicity and high costs of metals such as
tin, zinc, and palladium. Using iron, which is a much cheaper metal and also less toxic,
easily solves the high cost and toxicity problems. This corrosion of the wall’s zero-valent
metal is inherent due to the dechlorination process, which leads to precipitation on the
metal surface. With precipitation, the wall’s dechlorination is reduced and the
permeability is decreased. Although most barriers are designed to operate for years with
minimal maintenance, the aging limitations have not yet been determined.2 Some of the
industry estimates that the reactive material may require expensive replacement within
five years while others claim that even though the porous medium between the metal
granules get filled with precipitate, and the porosity of the material lessens, the activity of
the metal does not decrease. In one experiment the porosity decreased by 10% and still
made no difference in the efficiency of the metal to treat the water inflow. Eventually the
porosity will decrease to such an extent that it will make a difference, and hence
technological advancements are being made to prevent the deterioration of the permeable
walls.4
One such advancement includes enhancing the metal treatment by the use of ultrasound.
The process, developed at the Kennedy Space Center (KSC), provides a relatively simple
and economical method to regenerate and maintain the activity of the walls. The term
“ultrasound“ refers to stress waves that occur at frequencies exceeding 20kHz. Results of
this developing technology show that after a short exposure to low frequency, low-level
ultrasound, the metal particles are cleansed of the by-products that tend to inhibit
remediation. Hence, the treatment walls’ effectiveness is returned to its original level.
This new advancement is still being tested and has not yet been applied to real world
problems, although NASA hopes to stimulate development of commercial applications in
the near future.2
Zero-valent metal reduction seems to be a large step in the fight for helping out the water
pollution problem. Much work is going into the experimentation of different reductions
and ways to lengthen the metal wall lifespan. The process is yet not fully developed, but
seems to be an upcoming innovation that will hopefully be introduced and put into use
soon.