Water Air Soil Pollut
DOI 10.1007/s11270-010-0502-1
A Comparison Between Field Applications of Nano-, Micro-,
and Millimetric Zero-Valent Iron for the Remediation
of Contaminated Aquifers
Silvia Comba & Antonio Di Molfetta &
Rajandrea Sethi
Received: 12 October 2009 / Accepted: 28 May 2010
# Springer Science+Business Media B.V. 2010
Abstract In the last 10 years, the number of field
applications of zero-valent iron differing from permeable reactive barrier has grown rapidly and at present
are 112. This study analyzes and compares such field
applications. By using statistical analysis, especially
ANOVA and principal component analysis, this study
shows that chlorinated solvent contamination can be
treated efficiently by using zero-valent iron material
singly or associated with other technologies. In the
analyzed sample of case studies, the association with
microbial dechlorination increased significantly the
performances of nanoscale iron. This is likely due to
the synergistic effect between the two processes.
Millimetric iron was always used in association with
source zone containment; therefore, it is not possible
to distinguish the contributions of the two techniques.
The comparison also shows that catalyst addition
seems to not dramatically improve treatment efficiency and that such improvement is not statistically
significant. Finally, the injection technology is correlated to the type of iron and to the soil permeability.
Electronic supplementary material The online version of this
article (doi:10.1007/s11270-010-0502-1) contains supplementary
material, which is available to authorized users.
S. Comba (*) : A. Di Molfetta : R. Sethi
DITAG—Dipartimento del Territorio,
dell’Ambiente e delle Geotecnologie, Politecnico di Torino,
Corso Duca degli Abruzzi, 24,
10129 Turin, Italy
e-mail: silvia.comba@polito.it
Keywords Nanoscale zero-valent iron (NZVI) .
Micrometric iron . Millimetric iron . Case studies . Field
application . Injection
1 Introduction
Zero-valent iron has been used in the remediation of
contaminated aquifers for 15 years. This metal acts as
a reducing agent to transform contaminants such as
chlorinated solvents. In the process, iron is consumed
and does not itself act as a catalyst.
Millimetric particles of zero-valent iron have been
used in permeable reactive barriers (PRB), now well
established in remediation technology (Gillham and
O’Hannesin 1994; Matheson and Tratnyek 1994).
Besides millimetric particles, many other iron-based
materials have appeared. Among these are nano- and
microscale iron particles, EHC, emulsified zero-valent
iron (EZVI), catalyzed or bimetallic nano-iron particles (BNP), iron nanoparticles modified by different
polymers.
Nanoscale iron particles (NZVI) are sub-micrometer
particles of iron metal. They are highly reactive
because of their large surface area, which enables a
more rapid degradation of contaminants, if compared
to millimetric iron particles, and the ability to treat
otherwise recalcitrant contaminants (Tratnyek and
Johnson 2006). Moreover, their small size facilitates
the delivery of suspension close to the source of
Water Air Soil Pollut
contamination (Li et al. 2006; Tratnyek and Johnson
2006).
Nanoscale iron particles can be modified in several
ways: (1) BNPs are particles of NZVI that have been
coated with a catalyst, such as platinum, gold, nickel,
or palladium, to enhance reduction reactions (Keane
2009); (2) EZVI are NZVI particles that have been
coated with a membrane made from biodegradable oil
and water to facilitate the treatment of chlorinated
hydrocarbons by making the particles more hydrophobic. This allows the particles to mix directly with
dense non-aqueous phase liquids (DNAPL), such as
Trichloroethylene (TCE), to increase mass transfer
between DNAPL and NZVI through the emulsion
membrane (Keane 2009); and (3) finally, NZVI
particles can be covered in different polymers in
order to increase the suspension stability and therefore
particle mobility. (Comba and Sethi 2009) compiled
the following list of surface modifiers: xanthan gum
(Comba and Sethi 2009; Della Vecchia et al. 2009),
polyacrylate (Schrick et al. 2004; Kanel et al. 2007),
triblock copolymers (Saleh et al. 2005; Lowry et al.
2006; Saleh et al. 2007; Saleh et al. 2008), polyvinyl
alcohol-co-vinyl acetate-co-itaconic (Sun et al. 2007),
guar gum (Tiraferri et al. 2008; Tiraferri and Sethi
2009), carboxymethyl cellulose (He and Zhao 2007;
He et al. 2007), starch (He and Zhao 2005), and poly
(4-styrenesulfonate) (Hydutsky et al. 2007).
Besides millimetric and nanometric particles, miscroscale ones are also employed in aquifer remediation. Micrometric iron is either available in the form
of bare iron particles or associated to an organic
component, whose task is to stimulate the growth of
bacteria in the groundwater environment. Research is
still actively being conducted in the United States to
better understand the advantages of using nanoscale
versus microscale particles (Mace 2006).
The goal of this study is to analyze and compare
the “new” field applications of iron, meaning applications where technology differs from PRB with
millimetric iron, in order to suggest some indications
regarding the field applications of iron particles. A
debate on this topic is actually suggested and
promoted by governmental organizations, like the
European Union and the United States Environmental
Protection Agency (Müller and Nowack 2010) (United
States Environmental Protecion Agency 2010). The
study is based on documents published by government organizations like US Environmental Protection
Agency, National Aeronautics and Space Administration, Naval Facilities Engineering Command, research
papers, conference proceedings, and documents available online from Adventus and from Ars Technologies. The total number of case studies available in the
literature and analyzed in this work is 112. Specific
parameters were extracted from the literature or
calculated for each site to describe the main features
of the remediation (e.g., year, dislocation, the result of
the remediation in terms of contaminant decrease,
dimension of ZVI, surface modification, addition of
an organic component to the slurry, geology, etc.).
Then, data were analyzed by using different
statistical techniques in order to derive information.
Case studies were considered as experiments in
which the “response variable Y” was the efficiency
and the “factors” were parameters like particle
dimension, presence of catalyst, and others as
reported.
Results are discussed making extensive reference
to the bottom-up approach (i.e., laboratory experiments) results reported in the literature.
2 Material and Methods
2.1 Case Studies
Field applications are reported in the “Electronic
Supplementary Material” section, together with bibliographic references. To date, the total number of
case studies known by the authors is 112.
The majority of field applications (90%) were
performed in the United States (Fig. 1), with
approximately 9% in Europe and less than 1% in
Asia.
The first reported field application of micrometric
iron was in 1999–2000. In the same 2-year period,
millimetric (or granular) iron was used in a different
application than PRB (i.e., source zone containment).
The first field application of nanosized iron occurred
in 2001. According to the graph in Fig. 2, the
application of different materials continued through
the years; therefore, to date different iron-based
technologies co-exist. The number of case studies in
the 2-year period 2009–2010 is low because the
period is recent. After a period of separate use,
nanoscale and microscale iron technologies started
to be applied together (2005–2006).
Water Air Soil Pollut
Fig. 1 Geographical distribution of case studies
2.2 Data Organization and Systematization
First, a bibliographic research was made to identify
case studies. Since the same site could have been
cited with different names in different documents and
since in the same site different treatments could have
been performed, we carefully studied and selected the
literature in order to avoid repetitions and omissions.
We univocally named each case study according to
this scheme: (1) state name (abbreviation), (2) region
(abbreviation), and (3) city or, if the city was not
available, type of activity that was performed on site
(e.g., US-AK-North Slope or EU-CZ-Industrial
Plant).
Fig. 2 Time trend of the use of different iron sizes
The quality of the documents ranges from the
completeness and accuracy of some reports of the US
Navy to the scanty information of a document
presented at a conference. While all the documents
were taken into account in studying generic aspects
like the geographic distribution of the treatments, only
the most complete and consistent ones—about 50%—
were considered in the analysis of more complex
subjects like the relationship between particle dimension and treatment performance.
Specific parameters were then extracted from the
literature or calculated for each treatment to describe
the main features of the remediation (e.g., year,
dislocation), the result of the treatment, the characteristic of the injected slurry (e.g., dimension of ZVI,
surface modification, addition of an organic component to the slurry, etc.), and the geology (e.g., soil
permeability).
To describe the success or failure of the remediation, we derived a parameter, called efficiency, as
follows:
Efficiency ¼
Ct¼0 Ct¼fin
100
Ct¼0
where Ct=0 is the contaminant concentration before
the remediation took place and Ct=fin is the contaminant concentration at the end of the remediation
project. When an immiscible contaminant was present
and its final concentration being determined in liquid
form and at a final time inferior to 9 months, the
efficiency was not considered representative (and
therefore the corresponding case study was removed
from analysis). This was because the liquid phase
concentration is not a good indicator of the amount of
immiscible contaminants. Rebound is actually a
phenomenon where the liquid contaminant concentration increases after a period of decrease, indicating
that the mass of the immiscible phase has not been
completely removed.
The contamination characteristics (e.g., pollutant
type and concentration) were also extracted from the
literature together with the hydrogeological parameters
(e.g., pH, redox conditions, ionic strength). However,
in the end, we decided not to include the item of
reactivity in the analysis because of insufficient
information and because the argument is too wide
and complex to be treated with incomplete data and
would give rise to a too long digression. Therefore,
even if the authors believe that chemical condition in
Water Air Soil Pollut
the field is a very important issue for designing a
remediation operation, its specific contribution to the
result was not quantitatively established but was
considered as a part of the unexplained variability.
2.3 Statistical Analysis
Case studies were considered as experiments in which
the “response variable Y” was the efficiency and the
“factors” were parameters like particle dimension,
presence of catalyst, and others as reported below.
Unfortunately, the number of experiments was
small compared with the number of factors, and,
worse still, data were neither “balanced” nor “orthogonal” because, obviously, the experiments were not
designed by us but were the random result of a
bibliographic research.
In order to manage to interpret results, data were
analyzed by using different statistical techniques, that
is, one-way analysis of variance (ANOVA) and
corresponding Student’s t-test (p<0.05 was taken to
be statistically significant), principal component analysis (PCA), or linear regression. The statistical
software Minitab was used.
3 Remediations with ZVI
3.1 Particle Size
Treatments where zero-valent iron metal was used
singly (i.e., no organic component added in the
suspension) were selected.
As shown in Fig. 3, the efficiency varies with iron
dimension and this variation is statistically significant
(p-value below 0.001). The best results were achieved
by millimetric iron (average efficacy 97%), which is
followed by micrometric particles (91%) and by
nanoscale iron (65%).
The lower efficiency of nanoscale iron particles is
likely due to the side effects of their distinctive
feature, i.e., high reactivity. As explained by Tratnyek
and Johnson (2006), high reactivity correlates with
low selectivity, which is particle tendency to react
with non-target substances, including dissolved oxygen and water. This ‘natural demand’, together with
the demand arising from the reaction with the target
contaminants, implies that NZVI will have a limited
lifetime in environmental porous media. On the
Fig. 3 Efficiency [%] versus particle dimension for case
studies where the only zero-valent metal was employed. The
vertical line with horizontal lines at the endpoints represents
the 95% confidence interval for the mean. There were 30 case
studies considered for the analysis
contrary, less reactive particles will sustain reducing
conditions for longer times, giving better performances. According to this rule, millimetric particles
will have the highest longevity and the lower
reactivity.
Longevity of millimetric particles is further enhanced by clay addition to porous media, which is
required by injection techniques used to deliver the
material (jet grouting and mechanical soil mixing).
Since clay addition lowers aquifer permeability, the
incoming of groundwater oxidants in the contaminated zone is also reduced. Physical containment of the
DNAPLs through clay addition is a remediation
technology in itself (Kueper et al. 2003). In this case,
it has been associated to the zero-valent iron
technology.
Regarding reactivity, the lack of this property in a
millimetric material is probably partially offset by the
injection techniques used to emplace this material.
Through such, techniques iron particles are strongly
mixed with the contaminated porous medium, thus
achieving a better contact between reactants.
3.2 Addition of Organic Component
As shown in Fig. S1, over the years, zero-valent iron
particles have been used more and more in association
with an organic component. Such substances were:
carboxymethyl cellulose, soy, polysaccharide, emulsified vegetable oil, guar gum, and organic carbon in
EHC. Upon the whole, about half of the case studies
Water Air Soil Pollut
had a significant organic component (Fig. 4), which
was added in most cases to enhance particle mobility.
To evaluate the effect of organic material and of
particle dimension on treatment efficiency, a regression analysis was performed. Since millimetric iron
was never used together with an organic component,
it was excluded. In order to perform the analysis,
factors were converted into dummy variables. Results
(Table S1) show that efficiency is directly proportional to iron dimension and to the presence of
organic component. The same results come from the
principal component analysis, as shown in Table S2
and in Fig. 5. Micrometric iron shows a better
efficiency than the nanoscale one and the presence
of organic compounds also increases efficiency. The
study of the second PC (Table S2) shows also that, in
the analyzed case studies, organic compounds are
used more with nanoscale iron rather than with a
micrometric one.
To extend the study to millimetric particles, case
studies were grouped according to particle dimension
and to the presence of organic matter. Five different
groups resulted: nanoscale iron particles, nanoscale
iron particles + organic component, microscale iron
particles, microscale iron + organic component,
millimetric iron. Then, these groups were compared
according to the treatment efficacy using one-way
ANOVA.
As shown in Fig. 6, efficacy varies with groups
and this variation is statistically significant (p-value
below 0.001). The best results are achieved again by
millimetric iron (average efficacy 98%), which is
followed by micrometric particles. The addition of an
organic component to micrometric particles does not
further increase the efficiency, which is already very
high (91% with organic components and 89% without
them), while it greatly increases the performance of
Fig. 4 Percentage of total
case studies where an organic component was added
and distribution of iron sizes
among this group
nanoscale iron, as efficiency jumped from 65% to
91%.
To explain the results, we considered a further
parameter which is the reported occurrence of
biodegradation. In eight case studies with nanoscale
iron, it was proved that biodegradation took place,
while in three it was judged possible. It is interesting
to note that in seven out these eight cases and in one
out these three cases the suspension was actually
amended with organic matter.
Therefore, there is a strong suggestion that, in our
sample, the increase in nanoscale iron performance
when an organic component was also added was due
to the stimulation of bacterial growth in groundwater.
Furthermore, data show that, even if the biotic
remediation with iron has commonly been viewed as
unrelated to abiotic processes, the two techniques
have often been used together (consciously or even
unconsciously) in field applications.
It is likely that while successful in removing large
quantities of NAPL mass in short time periods, NZVI
is not able to remove all the contaminants, especially
the immiscible part, because of its low longevity. In
this case, bioremediation is necessary for complete
remediation of a site—not only the two technologies
can show additional benefits when paired together.
First, bioremediation may be limited in areas containing large amounts of mobile DNAPL (pools). Christ et
al. (2005) modeled the relation between source longevity and ganglia-to-pool ratio. High ganglia-to-pool
ratios resulted in the greatest reduction in source
longevity by source zone bioremediation through
chlororespiring organisms (e.g., Dehalococcoides bacteria), but bioremediation was less effective for low
ganglia-to-pool ratios where enhanced dissolution is
limited by lower specific surface area over which mass
can be transferred. Therefore, a pre-treatment with
Water Air Soil Pollut
Fig. 5 Loading plot. In order to perform the analysis, factors
were converted into dummy variables. For the particle
dimension: nanoscale iron was assigned the value 0 and
micrometric the value 1; for the organic component: to the
absence was assigned the value 0, while to the presence the
value 1. There were 56 case studies considered for the analysis
NZVI that would reduce the DNAPL saturation could
be beneficial for the subsequent bioremediation. Xiu et
al. (2009) demonstrated that such treatment train is
possible, since, even if at the early stages, NZVI has an
inhibitory effect upon contact with cell surfaces; after
this stage, bacteria recover following the partial
oxidation and presumably passivation of the NZVI.
Furthermore, the oxygen-depleting and-reducing
conditions developed by NZVI are comparable to the
conditions in which anaerobic bacteria develop and
proliferate, particularly the bacteria that facilitate
biodegradation of organic compounds, including
chlorinated solvents (Mace 2006). Such bacteria are
actually strictly hydrogenotrophic (de)chlororespirers
(i.e., they require chloroorganic electron acceptors)
and cannot grow with other redox couples.
Finally, another possible synergistic effect between
iron and bioremediation that could have occurred is the
growth-linked aerobic oxidation of the dechlorination
products cis-CDE and VC in down-gradient zones or
in the source zone once the environment has returned
to aerobic. This kind of reactions is also feasible, but
Tetrachloroethylene (PCE) and TCE are stable under
aerobic conditions and do not support the growth of
aerobic microorganisms. Thus, the reductive transformation of PCE and TCE is critical to initiate
detoxification (Loffler and Edwards 2006), and a
sequential abiotic–biotic aerobic process might also
lead to detoxification.
As explained before, the occurrence of biotic
degradation of contaminants was, in many case
Fig. 6 Efficiency [%] versus different kinds of iron slurries.
The vertical line with horizontal lines at the endpoints
represents the 95% confidence interval for the mean. There
were 56 case studies considered for the analysis
studies, even unintentional (e.g., He et al. 2009).
Therefore, it is likely that the overall treatment
efficiency could be further improved, if the in situ
bioremediation phase is thoroughly designed. In situ
bioremediation should not be confused with natural
attenuation. It actually involves, as a first step, site
characterization, which aims at identifying eventual
bottlenecks of the detoxification process, such as
unfavorable pH and redox conditions, high salinity,
nutrient limitations, insufficient concentration of
suitable electron donors, and presence or absence of
microorganisms catalyzing the desired transformation
reaction. Then, to overcome bottlenecks, specific
actions are undertaken. Consortia capable of PCE-toethene dechlorination are used to augment sites where
microbiology limits the detoxification process. In
addition to establishing and maintaining reducing
conditions, organic substrate(s) are added. A variety
of organic substrates, including alcohols, organic
acids, emulsified vegetable oil, and complex organic
materials (e.g., molasses, corn cobs, newsprint, wood
chips, microbial biomass, chitin, etc.), have been
applied.
3.3 Catalyst
Iron nano-particles have been used together with a
catalyst (Fig. 7).
As the presence of a catalyst and of an organic
component is correlated, a PCA was performed. In
order to perform the analysis, factors were converted
into dummy variables (i.e., for the organic compo-
Water Air Soil Pollut
ordered structure can act like a catalyst, increasing
the reaction rate and iron exploitation, and consequently mask the effect of the catalyst’s presence.
Another factor that could play a role in the system
is the decreasing contribution of atomic hydrogen
mechanism on chloroethene reduction with increasing
pH values (Wang and Farrell 2003). The increase in
pH in iron-based systems results from iron corrosion
by contaminants and water. In Lien and Zhang
(2007), it is not clear whether a buffer was added in
the system, while in Liu et al. (2005) deionized water
without buffers was used.
Fig. 7 Time trend of the use of catalytic particles
4 Delivery
nent: to the absence was assigned the value 0, while
to the presence the value 1; for the catalyst: to the
absence was assigned the value 0, while to the
presence the value 1). Results indicate that efficiency
is mostly linked to the presence of an organic
component, while the presence of a catalyst is of
minor importance (Fig. 8; Table S3). The same
result was obtained by analyzing separately the
presence of catalyst and of organic component with
one-way ANOVA, where the presence of organic
component resulted as statistically significant (pvalue below 0.005), while the presence of a catalyst
is not (Fig. 9).
Lien and Zhang (2007) compared the reactivity of
iron nano-particles differing only in palladium content. According to their results, the palladium addition
increased the reactivity in a TCE-deionized water
system by a factor of about 70%. However, Liu et al.
(2005) showed that the degree of crystallinity of
nano-iron is a more important factor controlling
reactivity than the presence or absence of boron as a
catalyst. Another study (Scherer et al. 1999) showed
that the oxide layer that lies at the iron–water
interface can influence the reduction of contaminants
by acting as a passive film, which provides a physical
barrier between the metal and dissolved oxidants.
According to Scherer et al. (1999), sustained reduction of contaminants required localized defects in the
passive film; consequently, the reactivity of iron is a
function of the surface concentration of these reactive
sites (Johnson et al. 1996). Therefore, a possible
explanation to the treatment efficiency results is that
surface defects and/or the presence of a poorly
In order to treat contaminants, iron particles should be
delivered from the surface to the target subsurface
contamination. Several injection techniques and the
natural movement of groundwater can potentially be
used for this purpose.
4.1 Injection
Various conventional techniques proved to be effective in delivering iron particles to subsurface environments, while others have never been tested.
Soil mixing techniques have been used for millimetric iron:
1. Jet grouting: it consists in breaking up the soil
with a high-pressure jet in a borehole and mixing
in place the loosened soil with a stable grout to
form columns or panels. Compared to other
injection techniques, very high pressures are used.
Fig. 8 Loading plot. There were 30 case studies considered for
the analysis
Water Air Soil Pollut
Fig. 9 Efficiency [%] versus different kinds of nano-iron
slurries. The vertical line with horizontal lines at the endpoints
represents the 95% confidence interval for the mean. There
were 30 case studies considered for the analysis
2. Mechanical soil mixing: soil disaggregation and
mixing with the grout are achieved through the
use of a mechanical auger.
These technologies require sufficient overhead
space to operate the mixing equipment and remove
buried obstructions. Completed applications suggest
that soil mixing technologies can be used at depths
reaching 100 ft bgs but are most effective at depths
less than 40 ft bgs.
Moreover, the use of this technology should be
avoided when the permeability reduction of the
aquifer is undesired.
On the other hand, the following injection techniques were (or when specified have the potential to be)
applied for nano- and micro-iron:
1. Gravity injection: the slurry is poured into the
well and its distribution into the porous medium
is due to head build-up in piezometer and to
natural groundwater movement.
2. Direct push: injection is usually performed during
perforation using high-pressure pump systems.
The use of a single tool for drilling and injection
makes the operation particularly fast.
3. Tubes a manchette: after drilling, a sleeved pipe is
installed inside the borehole. A cement suspension is first injected, its task being to prevent the
later injected fluid from rising along the borehole.
After the setting of the pipe, the injection takes
place. The fluid comes out from the sleeved tube
through small holes drilled into the pipe at fixed
intervals and covered by rubber sleeves (manchette) which open only under pressure. The
pressure is applied to the sleeves one at a time,
by inserting a double packer centered on the
sleeve in the injection pipe (Kutzner 1996). This
technique has only been applied to iron particles
once (case 46); however, this could be an
economically viable technique should the injection need to be repeated. Injection repetition is
likely to be necessary in many treatments, as it is
difficult to eliminate its causes (inadequate determination of source zone location and consequent
errors in iron dosage). However, the environmental compatibility of the tubes a manchette method
should be verified, in particular the introduction
of the cement grout in the aquifer and the
abandoning of the tubings.
4. Pressure pulse technology (PPT): PPT applies
large-amplitude pulses of pressure to porous media
at the water table or at variable depths, exciting the
media, opening pores, and thus increasing fluid
level and flow. This capability to drive liquids
through the porous medium facilitates the advancing of the iron slurry. Moreover, DNAPL is
detached from the porous medium, increasing its
specific surface area (Davidson et al. 2004).
5. Pneumatic injection and pneumatic fracturing
followed by pneumatic injection: iron powder is
introduced into the porous medium using pressurized gas, like air or N2, as carriers. Iron can be
dry or in aerosol form. In this case, the technique
is called liquid atomization injection. Pneumatic
injection can be preceded by pneumatic fracturing. Pneumatic fracturing typically involves the
injection of highly pressurized air into soil,
sediments, or bedrock to extend existing fractures
and create a secondary network of conductive
subsurface fissures and channels. This facilitates
Water Air Soil Pollut
subsequent injections and recovery (US EPA
1997).
6. Hydraulic fracturing: water or a slurry of water,
sand, and a thick gel is used to create distinct
subsurface fractures that may be filled with sand or
other granular material. The fractures are created
through the use of fluid pressure to dilate a well
borehole and open adjacent cracks. Once fluid
pressure exceeds a critical value, a fracture begins
to propagate (US EPA 1997). No case studies have
been reported.
These injection techniques can be classified
according to the level of alteration produced in the
original porous medium structure: a minimum soil
alteration occurs in permeation grouting as intergranular voids are only filled with the iron slurry; on
the contrary, in pneumatic and hydraulic fracturing
and soil mixing techniques the soil is fractured.
Pressure pulse technology and pneumatic injection
(not preceded by fracturing) can be considered
intermediate technologies, since pressure impulse
and gas elasticity produce a temporary expansion
of the pores. Generally, aggressive techniques determine a decrease in aquifer permeability and productivity, which is due, in the case of soil mixing, to
clay addition to porous medium and, in the case of
pneumatic injection, to the introduction of air
bubbles.
Relationship with particle dimension From injection
technique characteristics it is clear that particles of
bigger dimension need more aggressive techniques to
be delivered, as an example, millimetric iron needs
the soil to be fractured, while nanoscale iron can just
be poured into the well. This relationship is quite
pronounced in case studies, as shown in Fig. 10. On
the other hand, in some cases, techniques more
aggressive than the bare minimum were used; as an
example, in case study 48, hydraulic fracturing was
used for nanoscale iron.
Relationship with iron concentration The same principle described for particle dimensions applies to
the particle concentration in the slurry. Soil mixing
and pneumatic injection are able to inject pure
iron, with hydraulic fracturing up to 2.5 t m3.
However, data were not enough to be reported in
matrix form.
The capability of injecting highly concentrated
slurries is an important feature of an injection
technique, because the injection of a large volume of
water can displace the plume, as occurred in case 20.
Another advantage of using highly concentrated
suspensions is the reduction of iron passivation by
oxidants present in the dilution water, which is
reported in case 19. Pneumatic injection using N2
could also be employed to reduce passivation.
However, this method does not eliminate the risk of
contaminant displacement and proper countermeasures should be taken. For example, in case 65,
pneumatic injections were performed from the bottom
going up to minimize the potential risk of displacing
DNAPL horizontally or downward into the bedrock
water-bearing zone.
Relationship with soil permeability Impermeable and
heterogeneous porous media needs to be altered to
deliver the slurries, while for permeable porous media
less disturbing techniques can be used. This relationship is quite pronounced in case studies, as shown in
Fig. 11.
Relationship with treatment efficiency Until now,
aggressiveness has been analyzed only as a requirement to inject a certain particle dimension and
concentration in a soil characterized by a certain
permeability. However, aggressiveness in itself can
also have a direct influence on the treatment efficiency, since it can provide a major efficiency in
distributing iron in geologic media, especially in the
complex ones. Moreover, the use of aggressive
techniques would overcome the difficulties in rigorously delineating contaminant distribution within
source zones. From this point of view, the most
effective technique is soil mixing. However, it is not
possible to quantify the analysis for insufficient data.
As an example, the millimetric dimension is always
associated to soil mixing technique; therefore, single
effects cannot be separated.
4.2 Transport Through Groundwater Movement
Once iron has been placed in the subsurface with a
suitable injection technique, nano- and microscale
iron particles can be transported in the aquifer
exploiting advective and dispersive mechanisms. In
Water Air Soil Pollut
Total column
32
7
INJECTION TECHNIQUE
Soil mixing
.
37
9
85
102
Total: 1 case
89; 90; 91; 92; 93;
94; 99; 100
Total: 8 cases
9
95
Total: 1 case
4
Hydraulic
fracturing
48
Total: 1 case
108; 108 bis
Total: 2 cases
Pneumatic
fracturing
18
Total: 1 case
61; 63; 64; 65; 76;
78; 79; 88
Total: 8 cases
9
Pneumatic
injection
24; 25; 45
Total: 3 cases
65; 69; 70; 71; 73;
74; 75; 77; 80; 81;
82; 83; 84; 85; 86;
87
Total: 16 cases
22
Pressure
Pulse
Technology
31; 46
Total: 2 cases
101
Total: 1 case
3
Pressurized
injection
2; 2 bis; 6; 13; 14;
16; 19; 20; 21; 22;
23; 26; 27; 32; 50;
51; 53 Total: 17
cases
57; 59; 60
Total: 3 cases
66; 67; 68; 72; 98;
104; 105; 106; 107
Total: 9 cases
29
Gravity
injection
1; 5; 7; 17; 28; 43;
44; 47
Total: 8 cases
58
Total: 1 case
Nano
54; 55; 56
Total: 3 cases
9
Mix of nano and
Micro
micro
PARTICLE DIMENSION
Millimetric
Total
row
Fig. 10 Relationship between injection techniques and particle dimension among case studies. Injection techniques are classified
according to their aggressivity. There were 85 case studies considered for the analysis
various field applications (e.g., cases 16, 17, 19, 26,
47, and 97), the slurry was recirculated between wells
to enhance particle movement.
Despite its importance in defining the fate of
particles in the environment and in designing a
remediation project, our current understanding of the
complex transport mechanism of iron particles is still
evolving and incomplete. This is due to the complexity and numerousness of phenomena involved, such
as: the magnetic properties of iron nanoparticles
(Hong et al. 2009), particle attachment to soil surface
as a function of surface coatings (Sirk et al. 2009), the
possible desorption of the same particles once settled
and adhered to porous media (Kim et al. 2009), and
the increase in mobility in the presence of natural
organic matter (Johnson et al. 2009). Moreover, some
phenomena which take place in the field are hardly
reproducible in laboratory tests. As an example,
while according to those tests, bare micro-iron
particles do not migrate in porous media, in one
field application (case 63) migration was supposed
to have occurred. Recognizing this, some groups
and industries have adopted the ‘precautionary’
position that in situ applications of nanoparticles
for remediation should be prohibited (Royal Society
2005), whereas others have recommended, in effect,
that research on all fronts should proceed in parallel
(US EPA 2005).
Despite these uncertainties, in various case studies
transport mechanisms were specifically exploited,
Water Air Soil Pollut
INJECTION TECHNIQUE
Total column
15
35
14
64
Soil mixing
89; 90; 91; 94; 96
Total: 5 cases
92; 99
Total: 2 cases
7
Hydraulic
fracturing
108; 108 bis
Total: 2 cases
48
Total: 1 case
3
Pneumatic
fracturing
61; 64; 65; 78
Total: 4 cases
18; 63; 79
Total: 3 cases
7
Pneumatic
injection
65; 80
Total: 2 cases
54; 55; 70; 71; 81;
82; 83
Total: 7 cases
24; 25; 56; 69; 73;
74
Total: 6 cases
15
Pressure
Pulse
Technology
31
Total: 1 case
46
Total: 1 case
101
Total: 1 case
3
Pressurized
injection
2; 2 bis; 6; 13; 20;
21; 23; 60
Total: 8 cases
14; 19; 26; 32; 50;
51; 53; 57; 59; 66;
67; 104; 106
Total: 13 cases
105
Total: 1 case
22
Gravity
injection
7; 17; 44; 58
Total: 4 cases
1; 43; 47
Total: 3 cases
.
Pervious
Semi-Pervious
7
Impervious
Total
row
RELATIVE PERMEABILITY
Fig. 11 Relationship between injection techniques and soil permeability among case studies. Injection techniques are classified
according to their aggressivity. There were 64 case studies considered for the analysis
with various benefits. The injection of particles
upstream and their subsequent migration in the target
area was profitable in contaminated zones which were
not accessible from the surface. Subsoils underlying
infrastructure (like airports or streets) which use
cannot be interrupted (case 14) or those containing
sensible elements damageable by the injection operations (case 63) are examples of zones difficult to be
treated from surface.
In addition to the aforementioned specific cases of
contamination being inaccessible, transport through
groundwater movement: (1) allows larger injection
meshes, (2) homogenizes at a local scale the distribution of injected iron particles. This is important
especially in source treatment where even DNAPL
drops constitute a source of contamination and
should therefore be destroyed; and (3) is necessary
to dilute slurries which were injected at high iron
concentration.
Despite the large benefits, caution should also be
used as groundwater transport can determine an
incomplete exploitation of the degradative capacity
of iron. Moreover, its use is limited to aquifers
characterized by homogeneous conductivity; otherwise, particles would migrate through preferential
pathways, leaving the other zones untreated (case 19),
and could seep into drinking water aquifer with
potential toxicological effects. This is the case of
industrial soils, commonly crossed by tubing and
composed of filling earth.
Water Air Soil Pollut
5 Conclusions
This comparison suggests some indications regarding
the field applications of iron particles. First, it shows
that chlorinated solvent contamination can be treated
efficiently by using zero-valent iron material singly or
associated with other technologies. When used singly,
iron seemed to perform better if millimetric (average
efficacy 97%), while micrometric particles had a
slightly lower efficiency (91%) and nanoscale particles a rather poorer one (65%). In our sample of case
studies, the performances of nanoscale iron were
significantly enhanced by the association with microbial dechlorination. This is likely due to the synergistic effect between the two processes. Millimetric iron
was always used in association with source zone
containment; therefore, it is not possible to distinguish
the contributions of the two techniques.
Particle dimensions range continuously from nanoto millimetric; in some cases, the distribution of the
dimensions is large, presumably in order to associate
the reactivity of the smaller dimensions to the
longevity of the bigger ones.
The comparison also shows that catalyst addition
does not dramatically improve treatment efficiency
and that such improvement is not statistically significant. Therefore, catalytic particle use should be
evaluated in the light of costs.
The injection technology, instead, should be
chosen according to the type of iron, the slurry
concentration, and the soil permeability. However,
the injection technique in itself can also have a direct
influence on the treatment efficiency.
To conclude, these findings suggest that zerovalent iron technologies are effectively remediation
approaches that warrant further application and
exploration.
Acknowledgements This work was conducted under the
CIPE-C30 project funded by Regione Piemonte (Italy) and
partially supported by the Lagrange Grant from Fondazione
C.R.T. (Italy).
References
Christ, J. A., Ramsburg, A., Abriola, L., Pennell, K., & Löffler,
F. (2005). Coupling aggressive mass removal with
microbial reductive dechlorination for remediation
of DNAPL source zones: a review and assessment.
Environmental Health Perspectives, 113, 465–477.
Comba, S., & Sethi, R. (2009). Stabilization of highly
concentrated suspensions of iron nanoparticles using
shear-thinning gels of xanthan gum. Water Research, 43,
3717–3726.
Davidson, B., Spanos, T., & Zschuppe, R. (2004). Pressure
pulse technology: an enhanced fluid flow and delivery
mechanism. Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA.
Della Vecchia, E., Luna, M., & Sethi, R. (2009). Transport in
porous media of highly concentrated iron micro- and
nanoparticles in the presence of xanthan gum. Environmental Science & Technology, 43, 8942–8947.
Gillham, R. W., & O’Hannesin, S. F. (1994). Enhanced
degradation of halogenated aliphatics by zero-valent iron.
Ground Water, 32, 958–967.
He, F., & Zhao, D. Y. (2005). Preparation and characterization of a new class of starch-stabilized bimetallic
nanoparticles for degradation of chlorinated hydrocarbons in water. Environmental Science & Technology, 39,
3314–3320.
He, F., & Zhao, D. Y. (2007). Manipulating the size and
dispersibility of zerovalent iron nanoparticles by use of
carboxymethyl cellulose stabilizers. Environmental Science & Technology, 41, 6216–6221.
He, F., Zhao, D. Y., Liu, J. C., & Roberts, C. B. (2007).
Stabilization of Fe–Pd nanoparticles with sodium carboxymethyl cellulose for enhanced transport and dechlorination of trichloroethylene in soil and groundwater.
Industrial and Engineering Chemistry Research, 46, 29–
34.
He, F., Zhao, D., & Paul, C. (2009). Field assessment of
carboxymethyl cellulose stabilized iron nanoparticles for
in situ destruction of chlorinated solvents in source zones.
Water Research, 44, 2360–2370.
Hong, Y., Honda, R. J., Myung, N. V., & Walker, S. L. (2009).
Transport of iron-based nanoparticles: role of magnetic
properties. Environmental Science & Technology, 43,
8834–8839.
Hydutsky, B. W., Mack, E. J., Beckerman, B. B., Skluzacek, J.
M., & Mallouk, T. E. (2007). Optimization of nano- and
microiron transport through sand columns using polyelectrolyte mixtures. Environmental Science & Technology,
41, 6418–6424.
Johnson, T. L., Scherer, M. M., & Tratnyek, P. G. (1996).
Kinetics of halogenated organic compound degradation by
iron metal. Environmental Science & Technology, 30,
2634–2640.
Johnson, R. L., Johnson, G. O. B., Nurmi, J. T., & Tratnyek, P.
G. (2009). Natural organic matter enhanced mobility
of nano zerovalent iron. Environmental Science &
Technology, 43, 5455–5460.
Kanel, S. R., Nepal, D., Manning, B., & Choi, H. (2007).
Transport of surface-modified iron nanoparticle in porous
media and application to arsenic(III) remediation. Journal
of Nanoparticle Research, 9, 725–735.
Keane E (2009) Fate, transport, and toxicity of nanoscale zerovalent iron (nZVI) used during superfund remediation. US
Environmental Protection Agency.
Water Air Soil Pollut
Kim, H.-J., Phenrat, T., Tilton, R. D. & Lowry, G. V. (2009).
Fe0 nanoparticles remain mobile in porous media after
aging due to slow desorption of polymeric surface
modifiers. Environmental Science & Technology, 43,
3824–3830.
Kueper, B. H., Wealthall, G. P., Smith, J. W. N., Leharne, S. A.,
& Lerner, D. N. (2003). In E. Agency (Ed.), An illustrated
handbook of DNAPL transport and fate in the subsurface
(pp. 1–67). Bristol: Environment Agency.
Kutzner, C. (1996). Grouting of rock and soil. Rotterdam:
Balkema.
Li, X. Q., Elliott, D. W., & Zhang, W. X. (2006). Zero-valent
iron nanoparticles for abatement of environmental pollutants: materials and engineering aspects. Critical Reviews
in Solid State and Materials Sciences, 31, 111–122.
Lien, H. L., & Zhang, W. X. (2007). Nanoscale Pd/Fe
bimetallic particles: catalytic effects of palladium on
hydrodechlorination. Applied Catalysis B, Environmental,
77, 110–116.
Liu, Y. Q., Choi, H., Dionysiou, D., & Lowry, G. V. (2005).
Trichloroethene hydrodechlorination in water by highly
disordered monometallic nanoiron. Chemistry of Materials, 17, 5315–5322.
Loffler, F. E., & Edwards, E. A. (2006). Harnessing microbial
activities for environmental cleanup. Current Opinion in
Biotechnology, 17, 274–284.
Lowry, G. V., Saleh, N., Sirk, K., Phenrat, T., Dufour, B.,
Matyjaszewski, K., & Tilton, R. D. (2006). Triblock
copolymer coatings enhances nanoiron transport and
localizes nanoiron at the DNAPL/water interface. Division
of Geochemistry, 231st ACS National Meeting, Atlanta,
GA, March 26–30, 2006.
Mace, C. (2006). Controlling groundwater VOCs: do nanoscale
ZVI particles have any advantages over microscale ZVI or
BNP? Pollution Engineering, 38, 24–27.
Matheson, L. J., & Tratnyek, P. G. (1994). Reductive
dehalogenation of chlorinated methanes by iron metal.
Environmental Science & Technology, 28, 2045–2053.
Müller, N., & Nowack, B. (2010). Nano zero valent iron –
THE solution for water and soil remediation?. Report
of workshop held in Zurich (Switzerland), November
24th 2009. At: http://www.observatorynano.eu/project/
filesystem/files/nZVI_final_vsObservatory.pdf.
Royal Society (2005). Report of workshop on potential health,
environmental, and societal impacts of nanotechnologies.
London, 25 November 2005.
Saleh,N.,Phenrat,T.,Sirk,K.,Dufour,B.,Ok,J.,Sarbu,T.,etal.(2005).
Adsorbedtriblockcopolymersdeliverreactiveironnanoparticles
totheoil/waterinterface.Nano Letters, 5, 2489–2494.
Saleh,N.,Sirk,K.,Liu,Y.Q.,Phenrat,T.,Dufour,B.,Matyjaszewski,K.,
et al.(2007).Surfacemodificationsenhancenanoirontransport
and NAPL targeting in saturated porous media. Environmental
Engineering Science, 24, 45–57.
Saleh, N., Kim, H. J., Phenrat, T., Matyjaszewski, K., Tilton, R.
D., & Lowry, G. V. (2008). Ionic strength and composition
affect the mobility of surface-modified FeO nanoparticles
in water-saturated sand columns. Environmental Science &
Technology, 42, 3349–3355.
Scherer, M., Balko, B. A., & Tratnyek, P. G. (1999). The role of
oxides in reduction reactions at the metal–water interface.
Mineral–water interfacial reactions (pp. 301–322). Washington, DC: American Chemical Society.
Schrick, B., Hydutsky, B. W., Blough, J. L., & Mallouk, T. E.
(2004). Delivery vehicles for zerovalent metal nanoparticles in soil and groundwater. Chemistry of Materials,
16, 2187–2193.
Sirk, K. M., Saleh, N. B., Phenrat, T., Kim, H.-J., Dufour, B.,
Ok, J., Golas, P. L., Matyjaszewski, K., Lowry, G. V., &
Tilton, R. D. (2009). Effect of adsorbed polyelectrolytes
on nanoscale zero valent iron particle attachment to soil
surface models. Environmental Science & Technology, 43,
3803–3808.
Sun, Y. P., Li, X. Q., Zhang, W. X., & Wang, H. P. (2007). A
method for the preparation of stable dispersion of zerovalent iron nanoparticles. Colloids and Surfaces A,
Physicochemical and Engineering Aspects, 308, 60–66.
Tiraferri, A., & Sethi, R. (2009). Enhanced transport of
zerovalent iron nanoparticles in saturated porous media by
guar gum. Journal of Nanoparticle Research, 11, 635–645.
Tiraferri, A., Chen, K. L., Sethi, R., & Elimelech, M. (2008).
Reduced aggregation and sedimentation of zero-valent
iron nanoparticles in the presence of guar gum. Journal of
Colloid and Interface Science, 324, 71–79.
Tratnyek, P. G., & Johnson, R. L. (2006). Nanotechnologies for
environmental cleanup. Nano Today, 1, 44–48.
United States Environmental Protection Agency (2010) Contaminated site clean-up information. Available at http://
www.cluin.org.
US EPA (1997). Analysis of selected enhancements for soil
vapor extraction. Contract Report: EPA-542-R-97-007. At:
http://207.86.51.66/download/remed/sveenhmt.pdf.
US EPA (2005). Nanotechnology Workgroup / EPA's Science
Policy Council. Nanotechnology White Paper, 68-70, US
Environmental Protection Agency. December 2, 2005. At:
http://www.epa.gov/OSA/pdfs/EPA_nanotechnology_
white_paper_external_review_draft_12-02-2005.pdf.
Wang, J., & Farrell, J. (2003). Investigating the role of
atomic hydrogen on chloroethene reactions with iron
using Tafel analysis and electrochemical impedance
spectroscopy. Environmental Science & Technology, 37,
3891–3896.
Xiu, Z. M., Jin, Z. H., Li, T. L., Mahendra, S., Lowry, G. V.,
& Alvarez, P. J. J. (2009). Effects of nano-scale zerovalent iron particles on a mixed culture dechlorinating
trichloroethylene. Bioresource Technology, 101, 1141–
1146.