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Chemistry 14 (2008) 145–156
Journal of Industrial and EngineeringReview
Permeable reactive barrier for groundwater remediation
R. Thiruvenkatachari a, S. Vigneswaran a,*, R. Naidu b
a Faculty of Engineering, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australiab CRC CARE, University of South Australia, Australia
Received 14 December 2006; accepted 26 October 2007
Abstract
This article aims to provide an overview of the upcoming technology of permeable reactive barriers for groundwater remediation. A
comprehensive list of references and web-links are also provided for further in-depth understanding. A brief discussion on the Australian
perspective on this emerging technology is also included.
# 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Permeable reactive barriers; Groundwater; Remediation; Pollution
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
2. Sources and types of groundwater contamination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
3. Permeable reactive barrier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4. Configuration of PRBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4.1. Conventional systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4.2. Advanced methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5. Mechanism of interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6. Treatment of inorganic and organic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
7. Zero-valent iron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
8. Activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
9. Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
10. Alkaline materials-complexing agents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
11. Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
11.1. Organic carbon for denitrification and sulphate reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
12. Sequential reactive media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
13. In situ chemical oxidation (ISCO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
13.1. Use of oxidants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
14. PRB studies in Australia and New Zealand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
* Corresponding author. Tel.: +61 2 9514 2641; fax: +61 2 9514 2633.
E-mail address: [email protected] (S. Vigneswaran).
1226-086X/$ – see front matter # 2008 Published by Elsevier B.V. on behalf of
doi:10.1016/j.jiec.2007.10.001
1. Introduction
Groundwater is a limited ecological resource representing a
small percentage of the total water distribution [1,2,3]. The
contribution from groundwater is vital; perhaps as many as two
billion people depend directly upon aquifers for drinking water,
and 40% of the world’s food is produced by irrigated agriculture
The Korean Society of Industrial and Engineering Chemistry.
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156146
that relies largely on groundwater. Australia has 25,780 GL of
groundwater suitable for potable, stock and domestic use, and
irrigated agriculture that can be extracted sustainably each year.
It is extensively used for urban water supplies, agriculture,
irrigation, industry and mining. In Australia, some of the
regions like arid zones of South Australia, the Northern
Territory, and the Pilbara are entirely dependent on ground-
water. Most of the country’s premium wine districts rely on
groundwater. Due to the cap on surface water extractions in the
Murray–Darling Basin and the scarcity of surface water
resources in other areas, groundwater use across Australia has
increased significantly in the last 10 years [4]. South Australia,
New South Wales and Victoria use more than 60% of
groundwater for irrigation, while Western Australia uses
72% for urban and industrial purposes [5,6].
Studies by [4,7] identified increased demand for water in
Australia and called for proper management of groundwater.
The report also revealed that groundwater resource in Australia
has been highly committed in some places, or of poor quality in
others, and poorly investigated in others [8]. If not managed
properly, groundwater resources are highly vulnerable to
widespread contamination. There are many reports of serious
incidents of groundwater contamination due to accidental
spills, or unsatisfactory disposal of industrial chemicals,
agricultural practices, mining activities, etc.
Attempts at large-scale groundwater cleanup began in
earnest in the 1980s and the results of early remediation efforts
seldom produced the expected reduction in contamination
levels. Studies by the U.S. Environmental Protection Agency
(EPA) [9,10] found that the commonly used pump-and-treat
(P&T) technologies (pump the water and treat it at the surface)
rarely restored sites that had contaminated groundwater to
background conditions. This was confirmed in a much more
extensive 1994 National Research Council (NRC) study that
explicitly reviewed 77 sites across the United States where full-
scale pump-and-treat was being used [11,12]. One of the most
promising remediation technologies is the use of permeable
reactive barriers (PRBs) filled with reactive material(s) to
intercept and decontaminate plumes in the subsurface. In the
last decade, there has been an explosion of activities directed at
the development and implementation of PRBs. This study
presents a comprehensive review on PRBs technology.
2. Sources and types of groundwater contamination
Broadly, groundwater contaminants come from two cate-
gories of sources:
Table 1Examples of point and diffuse pollutions
(a) P oint Sources andPoint source Non-point or diffuse pollution
(b) D istributed, or Non-Point Sources.Municipal landfills, industrial waste
disposal sites, leaking gasoline storage
tanks, leaking septic tanks, and accidental
spills and leaks of petroleum products and
of dense industrial organics
Atmospheric deposition,
contaminated sediments,
and many land activities
that generate polluted runoff,
such as agriculture
(pesticides and fertilisers),
logging, and onsite sewage
disposal
Localised sources are known as point sources of contam-
ination. The contaminant interacts with the moving ground-
water and the soil and spreads out to form a plume moving in
the same direction as the groundwater. The resulting ground-
water contamination plume may extend several hundred metres
or even further away from the source of pollution.
Groundwater can also be contaminated by diffuse sources
over a wide area, for instance widespread use of fertilisers on
gardens and fields. Diffuse contamination may have greater
environmental impacts than contamination from point sources
because a much larger volume of water is affected. Pollutants
from point sources are generally related to urban development,
while diffuse sources are generally rural in nature. Some of the
examples of point and diffuse pollutions are given in Table 1.
Analysts estimate that there are between 300,000 and
400,000 sites in the USA contaminated with a wide variety of
toxic chemicals, representing clean up cost in the range of $500
billion to $1 trillion [11]. Many of these sites experience
groundwater contamination by complex mixtures of chlori-
nated solvents, fuels, metals, and/or radioactive materials.
Europe’s groundwater is polluted in several ways: nitrates,
pesticides, hydrocarbons, chlorinated hydrocarbons, sulphate,
phosphate and bacteria. Some of the most serious problems are
pollution by nitrates and pesticides. The key findings of
Australia: State of the Environment Report [13] highlighted that
Australia’s inland waters are under increasing pressure from
over-extraction, algal blooms, catchment modification, habitat
destruction and pollution. Also, the experiences from Europe
and North America suggest that groundwater pollution in
Australia will become a more serious issue in the future. There
are many well-documented cases of groundwater pollution in
Australia. The most significant diffuse contaminant of ground-
water throughout each state and territory in Australia is nitrates,
due to their adverse affects on people, animals and the
environment [14,15]. The main source of nitrate contamination
is through the application of fertilizers for cropping and pasture
[15]. Direct discharges of nitrogen compounds from on-site
sanitation and from sewer effluent also exacerbate the problem.
In many areas, the concentration is greater than the Australian
Drinking Water Guidelines [16] level of 50 mg/L nitrate (as
nitrate), resulting in groundwater that is unfit for drinking. In
some of the more contaminated areas, the concentration is in
excess of 100 mg/L [15].
Recent incidences of reported pesticide contamination of
groundwater in this country are listed in the [5] report. In most
affected areas, pesticides were detected in at least 20% of
samples, indicating significant contamination. However, sys-
tematic monitoring of pesticide contamination of groundwater in
Australia is limited indicating inadequate data on the quantities,
locations and types of pesticides used, as well as knowledge gaps
in the fate of pesticides in local environments [17].
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156 147
3. Permeable reactive barrier
This technology termed as Permeable Reactive Barriers is
defined [9] as:
‘An emplacement of reactive media in the subsurface
designed to intercept a contaminated plume, provide a flow
path through the reactive media, and transform the
contaminant(s) into environmentally acceptable forms to
attain remediation concentration goals down-gradient of the
barrier’
The concept of PRBs is relatively simple. A permeable
reactive barrier material consisting of permanent, semi
permanent or replaceable reactive media is placed in the
subsurface across the flow of path of a plume of contaminated
groundwater, which must move through it as it flows, typically
under its natural gradient, thereby creating a passive treatment
system. As the contaminant moves through the material,
reaction occur that transform the contaminants into less
harmful (non-toxic) or immobile species. The PRB is not a
barrier to the groundwater, but it is a barrier to the
contaminants. PRBs are designed to be more permeable than
the surrounding aquifer materials so that contaminants are
treated as groundwater readily flows through without sig-
nificantly altering groundwater hydrogeology.
PRBs potentially have several advantages over conventional
pump-and-treat methods for groundwater remediation.
� P
RBs can degrade or immobilize contaminants in situwithout any need to bring them up to the surface. Hence no
need for expensive above ground facilities for storage,
treatment, transport, or disposal other than monitoring wells.
After the installation the above ground can be re-used for
other purposes. Also, as the contaminants are not brought to
the surface; there is no potential cross media contamination.
� T
hey also do not require continuous input of energy, becausea natural gradient of groundwater flow is used to carry
contaminants through the reactive zone. Only periodic
replacement or rejuvenation of the reaction medium might
be required after its reactive capacity is exhausted or it is
clogged by precipitants and/or microorganisms. However, the
drastically reduced operating costs offsets the higher
construction cost that are typical for PRBs, which results
in an overall reduction in the life cycle cost of this technology.
� D
egradation of most of the contaminants is achieved ratherthan mere change of phase of contaminants. The barrier
provides effective contaminant remediation, much more than
simple migration control of the pollutants.
� T
echnical and regulatory problems related to ultimatedischarge requirements of effluent from pump-and-treat
systems are avoided with the PRB technology.
However, so far, limited data are available on the
performances of reactive barriers with different materials
and their comparative performances. Limited long-term field
testing data are available and field monitoring is in its infancy
[18].
4. Configuration of PRBs
4.1. Conventional systems
Two installation schemes are more frequently used in field
applications [19,20]; Continuous and Funnel-and-Gate PRB.
The continuous PRB configuration consists of a single
reactive zone installed across the contaminant plume, while the
funnel-and-grate system consist of a permeable gate (reactive
zone) placed between two impermeable walls that direct the
contaminated plume towards the reactive zone.
The choice between the two configurations depends on
both the hydrogeological characteristics of the site and the
reactive material cost [19]. When a high cost reactive material
is used, funnel-and gate configuration is preferred since the
reactive zone requires less material. However, construction
cost of continuous type barrier is much cheaper than funnel-
and-gate system. Hence a balance must be struck between the
cost of reactive material and the construction cost of the
barrier, in accordance with the target pollutant and level of
removal to be achieved. Multiple reactive medium in
succession or in series can be installed in the funnel-and
gate setup [20]. Alternatively, a relatively less expensive
method using multiple caisson gates can also be installed [20].
A caisson is a hallow, load-bearing (usually cylindrical)
enclosure generally used as an alternative method for
excavation. For the purpose of emplacing a reactive cell, a
prefabricated, steel caisson (normally 8-ft-dia or smaller) is
pushed or vibrated down into the subsurface. Once the caisson
has reached the intended depth, the soil within the caisson can
be augered out and replaced with the reactive medium. Upon
emplacement of the reactive cell/medium, the caisson can be
pulled straight out. The caisson can be installed from the
ground surface and completed without requiring personnel to
enter the excavation.
Usually, the conventional PRB installation techniques
require some degree of excavation, which limits the PRB to
fairly shallow depths of 20 m [21]. However, use of new
construction techniques, such as slurry injection and hydro-
fracturing are able to overcome this depth limitation.
4.2. Advanced methods
(i) Injection system: Injection system involves creating a
treatment zone within the contaminant boundary by
drilling series of bore holes or injection wells and injecting
the reactive material (chemical/particulate mixture) into
the treatment zone. Potential advantages of this approach
are that there is no need to construct a trench and possible
aquifer access at greater depths. Ususally, two or three
rows of overlapping, interlocking columns can offer
effective barrier [22]. Nevertheless, it has to be made sure
that the contaminant plume is efficiently taken care of and
no by-passing or fingering occurs, which may impair the
remediation effect.
(ii) H
ydraulic/pneumatic fracturing: Hydraulic/pneumaticfracturing is intentional fracturing (cracking) of a subsur-
Table
Some
Grou
Orga
Me
Eth
Eth
Pro
Ar
Ot
Inorg
Tra
An
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156148
face using pumped water and/or air under high pressure.
As the confining pressures are exerted in the borehole,
fractures will open and propagate out laterally from an
initiation point. A fracture fill slurry composed of the
reactive medium, can then be injected into the fracture to
form a reactive treatment zone. More than one fracture
may be required within the treatment zones. Fractures have
a preferred direction of propagation, they are therefore
asymmetric with respect to the borehole and they climb in
the preferred direction of propagation. Fractures can be
controlled to happen either horizontally or vertically
[24,25]. Some advantage to this technique includes the
ability to emplace a barrier to a depth greater than 80 ft.
Also, fracturing causes minimal disturbance, does not
generate contaminated soils, and is inexpensive. Fractured
zones may also be applied to direct groundwater flow
towards the gates in funnel-and-gate system [26]. Some
drawbacks of emplacement by hydraulic fracturing include
difficulty in controlling the fracture direction and the
limited soil conditions in which it can be used effectively.
(iii) P
assive groundwater capture and treatment by reactorcells: This technique involves emplacement of reactor
cell(s) in the subsurface consisting of reactive medium and
capturing the contaminated plume into the reactor for
treatment [27]. This system does not involve any pumping
equipment and the plume is directed into the reactor by
siphoning or by natural gradient. Because of the passive-
mode of operation, the operating and maintenance
requirements are relatively minor.
The selection of the construction technique to be used
depends upon the site characteristics [25] such as depth of PRB,
geotechnical consideration, soil excavation: space for handling
and disposal of soil (contaminated), health and safety of
personnel. Common contaminants being treated using PRB
technologies are given in Table 2.
5. Mechanism of interaction
Proper understanding of the underlying process by which the
reactive material interact with the pollutants and the mechan-
ism of removal is very important. In general, the contaminant
2
of the common contaminants in groundwater for remediation
ps Pollutants
nic compounds
thane Tetrachloromethane, trichlo
anes Hexachloroethane, 1,1,1-tri
enes Tetrachloroethene, trichloro
panes 1,2,3-Trichloropropane, 1,2
omatics Benzene, toluene, ethylbenz
hers Hexaxhlorobutadiene, 1,2-d
anic compounds
ce metals/heavy metals Chromium, nickel, lead, ur
ion contaminants Sulphate, nitrate, phosphate
removal mechanisms can be classified broadly into three
categories [18]:
- D
ro
ch
et
-d
e
ib
an
,
egradation: Through chemical or biological reactions that
lead to decomposition or degradation of contaminants into
harmless compounds.
- P
recipitation: Immobilization of contaminants within thereaction zone by formation of insoluble compounds. Here, the
chemical state of the contaminant is not altered.
- S
orption: Immobilization of contaminants within the reactionzone by adsorption or complex formation. Here, the chemical
state of the contaminant is not altered.
More specifically, the principal processes are [29]:
� R
l
h
i
n
i
a
eductive degradation of organic pollutants.
� O
xidative degradation of organic pollutants.� R
etardation and biodegradation of organic pollutants.� S
orption of organic or inorganic pollutants.� R
eduction and/or precipitation of heavy metal compounds.In general, the types of reactive materials used for the
construction of permeable reactive barriers, are [30]:
� T
hose changing pH or redox potential,� T
hose causing precipitation,� M
aterials with high sorption capacity, and� T
hose releasing nutrients/oxygen to enhance biologicaldegradation.
A study by USEPA [31], classified the reactive barrier
materials according to the target pollutant and the mechanism
of removal (Table 3).
Zero-valent iron (Fe0) is the most common reactive material
in the current field of application [19]. A comprehensive review
on iron in the use of PRBs has been carried out recently by [32].
Apart from iron based materials, other types of materials
[25,30,33] such as organic based (activated carbon, leaf, peat,
sewage sludge, sawdust, etc.) [34,35], alkaline- complexing
agents (hydrated lime, ferrous sulphate) [30,36], phosphate
minerals like hydroxyapatite and biogenic apatite (e.g. fish
bone) [37], zeolites [38–40], clay [41], metal oxides [38,42],
methane, dichloromethane
oroethane, 1,1,2-trichloroethane 1,1-dichloroethane
ene, cis-1,2-dichloroethene
chloropropane
e
romoethane, Freon 113, N-nitrosodimethylamine
um, iron, manganese, technetium, selenium, copper, cobalt, cadmium, zinc
rsenic
Table 3
Reactive materials classified based on the target pollutant and the mechanism of removal
Target and mechanism of removal Reactive materials
Inorganics-sorption or substitution
barriers
Activated carbon, activated alumina, bauxite, exchange resin, ferric oxides and oxyhydroxides, magnetite,
peat, humate, lignite, coal, phosphates, titanium dioxide, zeolite
Inorganics-precipitation barriers Biota, dithionite, ferrous hydroxides, ferrous carbonates, ferrous sulfide, hydrogen sulfide gas, lime, flyash,
limestone, miscellaneous (Mg(OH)2, MgCO3, CaCl2, CaSO4, BaCl2, zero-valent metals
Inorganics-degradation barriers Biota, zero-valent metals
Organics-degradation barriers Ferrous minerals, oxygen release, ultramicrobacteria, zero-valent metals
Organics-sorption barriers Zeolite, activated carbon, clays
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156 149
microorganisms [43–46], polymers [47,48], innovative poly-
mer membrane (as liner) containing iron [49] are also being
evaluated.
Ref. [50] evaluated 124 PRB projects in the USA and
reported that in majority of projects (45%), zero-valent iron was
the material used as reactive barrier material. Another study
conducted by [51], on around 50 PRB sites across US, Canada
and Europe revealed that zero-valent iron is used in more
number of occasions. Refs. [52,53] reported that the primary
determinant of degradation rate in different in the case of iron is
the available reactive surface area. The parameter generally
used to discriminate between different irons is the specific
surface area, or the surface area per unit mass (m2/g) of iron. In
most cases, a linear relationship has been observed between
reduction rates and iron metal surface area concentration
[54,55]. Therefore commercial irons with higher surface area
are preferred [20]. Finer iron particles can also be mixed with
sand in order to increase the hydraulic conductivity. Iron with
several amendments has been tested to improve the perfor-
mance.
� D
uring the reaction, oxidation of Fe0 to Fe2+ results in anincrease in pH, which eventually leads to the formation of
precipitation of number of minerals. In order to overcome
this, pyrite or iron sulfide are added (oxidation of pyrite
produces acid) which lowers the pH and increases the organic
removal efficiency [56,57]. One negative effect of addint pH-
controlling amendments could be the increased possibility of
higher levels of dissolved iron in water downstream after the
reactive cell.
� I
nstead of using granular sized iron medium, use of colloidal-size iron (1–3 mm) or of nanosize (1–100 nm = 0.001–
0.1 mm) would significantly increase the surface area and a
lower total iron mass may be required in the treatment zone
[20,58–60]. Nanosized [61,62] or as emulsion state [63] have
also been studied. Colloidal or nono-state iron allows the
formulation of slurries that can be injected in PRB by
boreholes or fractured media.
� B
imetallic systems where several metals are plated onto zero-valent iron (ZVI) (e.g. Fe–Cu, Fe–Pd, Fe–NI) have shown to
produce greater performance compared to having ZVI alone
[64–66]. Some bimetals enhance the degradation by acting as
catalysts and as galvanic couples (increase electron activity).
However it has been cited that reactivity of bimetallic system
may be high initially but may decline gradually [20].
� A
cid pre-treatment of ZVI was also found to increase thedegradation efficiency; probably due to removal of oxide
coating layer on the iron surface or due to increase in surface
area by etching or pitting corrosion [52,67].
6. Treatment of inorganic and organic pollutants
Fundamental difference exists between organic and inor-
ganic contaminant remediation. Organic contaminants can be
broken down into innocuous elements and compounds, such as
carbon dioxide and water because they are molecules consisting
of carbon, hydrogen, halogens, oxygen, and sometimes sulphur,
phosphorous, and nitrogen. Conversely, most inorganic
contaminants are themselves elements. They cannot be
destroyed but can only change speciation [68]. Therefore
remediation strategies must focus on transforming inorganics
into forms that are non-toxic, not bioavailable, immobile, or
capable of being removed from the subsurface [68]. The
characteristics of these elements have in common is that they
can undergo redox reactions and can form solid precipitates
with carbonates, sulphide and hydroxide [68].
7. Zero-valent iron
ZVI tend to be oxidised, passing its electron to contaminants
(organic-halogenated hydrocarbons, inorganic- some metal;
U(VI), Cr(VI), etc., which undergo reductive mechanism
resulting in precipitation or degradation. The mechanism for
metal precipitation and degradation of halogenated hydro-
carbon by ZVI is given elsewhere [19,25,68]. In the case of
reductive precipitation, there is also a potential risk of
remobilization due to dissociation [33].
Interferences with reactions can occur while treating
groundwater with ZVI [20,68].
� O
xygen oxidises ZVI and causes FeO(OH) or Fe(OH)3 solidsto form. Besides making the ZVI no longer available to
interact with contaminants, the solid formation can clog the
reactive media, thus reducing the hydraulic permeability/
conductivity. In order to overcome this limitation, a pre-
treatment buffer zone of sand and pae gravel mixed with
about 10–15% ZVI by weight can be placed before the actual
treatment reactive barrier with 100% ZVI reactive cell. The
pre-treatment cell containing small amounts of ZVI will
remove the dissolved oxygen from the contaminated plume
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156150
before it reaches the actual reactive cell. Also, most of the
precipitates formed in the pre-treatment stage will be retained
and as the permeability is higher, it will not affect the
groundwater flow to the main barrier [20].
� Z
VI interaction with the contaminant plume would also resultin the increase in pH, which results in the formation of
precipitates. The carbonate and hydroxide precipitates with
species like Fe, Ca, and Mg could impede the groundwater
flow through the barrier. Solid precipitation can be favored
when it results in the removal of toxic metals from the
groundwater. However, it is detrimental if it decreases the
hydraulic conductivity through the barrier or if it interferes
with the dominant contaminant removal mechanism. To
overcome this limitation, buffering amendments to prevent
the increase in pH is applied, as also reported earlier in this
report [20]. Alternative methods like ultrasound technology
to deal with permeability problem are also being evaluated
[33,69].
� U
nder anaerobic conditions, hydrogen gas is formed as aproduct of iron corrosion which may also temporarily
passivate the iron surface [70]. Venting of H2 bubbles may be
required to maintain water flow and iron reactivity.
Alternatively, microorganism that can utilize hydrogen as
an energy source in anaerobic environment can also be
introduced [70,71].
� N
itrate has been shown to negatively impact reaction rates byprogressively passivating iron surface [72].
� E
arly indications suggest that high concentration of dissolvedsilica also may have similar inhibitory effect on iron like
nitrates [73].
� C
ertain types of dissolved organic carbon (DOC) have beenshown to coat reactive sites on the iron, rendering it
unreactive [27].
Recent studies indicate that microorganisms with Fe0
increase the contaminant removal efficiency [74]. Application
of bio-augmentation in conjunction with the ZVI technology
and found that these two technologies can have a symbiotic
effect on each other [75–77]. Till et al. [75] identified that Fe0
can stoichiometrically reduce nitrate to ammonium and that
hydrogen produced (during anaerobic Fe0 corrosion by water)
can sustain microbial denitrification to reduce nitrate to more
innocuous products (i.e., N2O and N2).
Experiments with mixtures of contaminants have also shown
that bioaugmentation of PRBs with bacteria offers promise
when more than one contaminant is present. More complete
dechlorination occurred when the Fe0 was bioaugmented.
Batch experiments with mixtures of carbontetrachloride, Cr6+,
and nitrate showed that bioaugmentation reduced competition
by these pollutants for active sites on the Fe0 surface [74].
Permeable reactive barriers designed to enhance bacterial
sulfate reduction (sulfate reducing bacteria-SRB) and metal
sulfide precipitation have the potential to prevent acid mine
drainage and the associated release of dissolved metals. In this
situation, the conditions typically found within a reactive
barrier environment are well suited to SRB. Permeable reactive
barriers provide dissolved C, N, and P, and the plume water
entering the barrier provides high concentrations of iron and
other metals, provides the necessary condition and promotes
growth and reproduction of microorganisms [78]. A synergistic
interaction between microbial activity and ZVI obtained an
enhanced degradation efficiency of hydrocarbon pollutants,
according to the study by [79].
8. Activated carbon
Activated carbons are chemically stable materials and are
widely considered as suitable adsorbent for on-site or off-site
treatment of polluted groundwater [40]. This material presents
a high adsorption capacity for many organic and inorganic
contaminants largely due to its high surface area (about
1000 m2/g) and the presence of different types of surface
functional groups (hydroxyl, carbonyl, lactone, carboxylic
acid, etc.) [80]. In granular form, activated carbon appears to
be highly suitable for use in permeable barriers [34].
Significant removal of hexavalent chromium from contami-
nated groundwater using granular activated carbon (GAC) was
achieved by [34]. Regeneration of carbon by phosphate
extraction and acid washing also appeared to be successful
[34], allowing the possibility for repeated use of the material.
Microbial regeneration of activated carbon (used in organic
sorption) in PRB is a promising area, which needs to be
explored. A recent study by [81] with PRB using activated
carbon and microorganism for polyaromatic hydrocarbon
removal, showed that the degradation efficiency of organic
material was found to increase when the organic material was
adsorbed on the carbon. Few more studies on the organic
removal using activated carbon in PRBs have shown
promising outcomes [82]. Its effect on inorganic species is
yet to be evaluated in detail.
9. Zeolites
Zeolites are tectosilicates with three-dimensional alumino-
silicates structure containing water molecules, alkali and
alkaline earth metals in their structural framework [83]. These
minerals have very high ion-exchange, adsorbing, catalytic,
molecular sieving capacities and make them potentially useful
as treatment mineral for use in the PRBs [18,27]. As the mineral
is anionic (negatively charged), it can be used to remove cations
from aqueous solutions. Several hundred zeolitic materials
exists; Clinoptilolite, a natural zeolite, is a potential material for
remediation of aqueous solutions since it demonstrates strong
affinity for several toxic heavy metals [84] and can selectively
adsorb some radionuclides [85]. A new surfactant modified
zeolite has been tested to simultaneously remove organic and
inorganic species [19].
10. Alkaline materials-complexing agents
Hydrated lime (Ca(OH)2) is a cheap reagent which can be
used in PRBs for groundwater remediation. Previously, it has
been used for remediation of acid mine drainage [18,27].
Lime barriers cause pH to increase to 12–12.5 in order to
Table 4
Advantages and limitations of various reactive barriers used in PRB technology
Material Advantages Limitations
Zero valent iron (ZVI) Most commonly used reactive barrier material.
Vast amount of background data available
Not effective on all types of organic compounds especially
certain dense non-aqueous phase liquid compounds like
1,2-dichloroethane and dichloromethane [54]
Ability to be used in different states: as a
pile [24,90], powder/granular [62],
filing [91], colloidal [58,59], nanosized [61,62],
emulsion that can be injected [63]
Lifetime of the material could be reduced due to the
formation of surface coating due to geological condition
of the site [93]
High reactivity with organic and inorganic
contaminants [92]
Increase in pH during reaction induces corrosion and
subsequent precipitation of minerals would lead to
decreased permeability of reactive material [94]
Ability to combine with other treatment
methods, e.g. bioremediation
H2 gas produced and the microorganism (biofouling)
could reduce the porosity of reactive material
Less or no major problems associated with
occupational health and safety (OHS) in
handling this material
Limited information available on long term performance
of the system especially on the build-up of surface
precipitates and biofouling.
Compounds like silica or natural organic matter (NOM)
have passivating effect; reducing the iron reactivity [18,93]
Competitive reaction inhibits the reactivity in the presence of
certain compounds. In the presence of nitrate the
dehalogenation of chlorinated compounds is decreased
[95,96]
Activated carbon Different types (with different reactivities) of
activated carbon can be obtained from low cost
natural products, e.g. coconut shell based
Vast data on ex situ water and wastewater treatment.
But very limited data on in situ treatment under field
conditions
Effective in the treatment of organic and heavy
metal contaminants [97]
Rapid breakthrough and thus frequent carbon change-outs
or regeneration. Requires optimization studies
Excellent material to combine with biotreatment Performance highly dependent on temperature and other
extrinsic parameters
Chemically stable material Surface coatings may decrease sorption capacity
Competitive adsorption
Lime (calcium carbonate
or hydroxide)
Low cost reactive PRB material Slow reaction time
Effective in neutralization; reducing the solubility of
certain metals or conditioning hydrochemical
system to assist with other treatment processes,
e.g. bioremediation
Loss in efficiency of the system because of coating of
the limestone particles with iron precipitates
Used extensively for acid mine drainage remediation or
acidic agricultural soils
Difficulty in treating acid mine drainage with a high
ferrous–ferric ratio, and ineffectiveness in removing
manganese
Limestone treatment is generally not effective for acidities
exceeding 50 mg/L
Voluminous sludge is produced with hydrated lime
(calcium hydroxide)
Microbial bioremediation Less expensive and easy to install A perceived lack of knowledge about biodegradation
mechanism
Natural processes to treat contaminants Specific contaminants may not be amenable to
biodegradation [98]
Capability to degrade organic contaminants into relatively
less toxic end products
In the case of mixed wastes, some are amenable only under
aerobic condition and some only under aerobic condition
Reduced risk of human exposure to contaminated media Site characterization and optimization studies are required for
each contaminated site. Chemical characteristics of the
contaminants dictate the extent of biodegradability
Remediation is not restricted to the treatment zone alone
Works well for dissolved contaminants and contamination
adsorbed onto higher permeability sediments
(sands and gravels)
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156 151
facilitate the formation of metal hydroxides, which reduce the
solubility of certain metals. They have proved to be
successful in remediation of anionic and cationic pollutant
species [27].
11. Bioremediation
Remediation of pollutants using microorganisms (bioreme-
diation) is one of the promising and viable technologies, which
Table 5
Examples of sequential PRBs
� Granular zero-valent iron to treat chlorinated hydrocarbons followed by
aerobic bioremediation (using oxygen release compounds) to treat aromatic
hydrocarbons
� Granular zero-valent iron to treat chlorinated hydrocarbons followed by
nutrient addition or solid carbon source addition to promote anaerobic
biodegradation of volatile organic carbons (VOCs) that cannot be degraded by
granular iron
� Sequential anaerobic biodegradation of chlorinated solvents followed by
aerobic biodegradation for toluene degradation
� Solid carbon sources to treat nitrate followed by granular iron to treat VOCs
� A four component PRB consisting of four reactive media in series for treating
(immobilizing or destroying) multiple contaminants
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156152
utilizes naturally occurring microorganisms and biological
reactions for the degradation of toxic contaminants (mostly
organic and in some instances inorganic compounds) in the
subsurface groundwater. In situ bioremediation creates subsur-
face environmental conditions (without withdrawing the
contaminated water from under the ground), typically through
the principle of oxidation–reduction manipulation, which
induce the degradation of chemicals via microbial catalyzed
biochemical reactions [45].
Many environmental pollutants such as petroleum hydro-
carbons are highly reduced, which allows oxidation of these
pollutants to innocuous final compounds like carbon dioxide
and water. On the other hand some pollutants like chlorinated
solvents and nitrates are highly oxidized and are prone to
undergo reduction. Microorganisms mediate such redox
reactions (anaerobically or aerobically) and thrive on the
contaminant degradation process to obtain it energy and food
for its growth [33].
11.1. Organic carbon for denitrification and sulphate
reduction
Organic carbon for denitrification and sulfate reduction has
been well recognized. Robertson and Cherry [86] adapted the
use of permeable organic carbon material to stimulate
biologically mediated denitrification and sulfate reduction in
contaminated groundwater in PRB system. Denitritrifying and
sulfate reducing bacteria are ubiquitous in the environment. In
the presence of organic source, these heterotrophic bacteria
reduce nitrate to nitrogen gas and sulphate to sulphide, in the
absence of oxygen [35,87–89]. Generation of sulphide during
sulphate reduction, precipitate the soluble metals or metalloids
as low-solubility sulphide minerals. In combination, the
sulphate reduction and sulphide precipitation reaction have
the potential effect of decreasing concentrations of sulphate,
iron, and other metals and metalloids and increasing alkalinity
and pH. Table 4 shows the advantages and limitations of various
reactive barriers used in PRB technology.
12. Sequential reactive media
Two of the most common classes of organic contaminants in
groundwater are chlorinated solvents and petroleum derived
aromatics. These chemicals are known to contaminate vast
quantities of water and are difficult to remediate. Chlorinated
solvents including chlorinated ethenes and chloromethanes
have densities grater than water, allowing them to penetrate far
below the water table. In addition, they have very low
solubilities in water and are resistant to degradation. Even if
they do degrade, it is often along pathways that can lead to
products of greater concern than the original contaminants.
These characteristics have led to the phrase ‘‘dense nonaqueous
phase liquids’’ (DNAPL) to describe such chemicals [99].
The chlorinated solvents contain carbon in a relatively
oxidized form, while, in contrast, the petroleum derived
organics contain carbon in a relatively reduced form. Hence the
geochemical conditions required for degradation of these
classes of chemical in an aquifer are quite different. Often
mixtures of such contaminants are encountered and in many
occasions PRBs with single media would be unable to deal with
such situations. This has led to the development of new
strategies for the cleanup of contaminated aquifer.
The PRB with sequential treatment with duel or more kinds
of reactive barriers have recently been given more attention
(Fiorenza et al., 2000). Some of the sequential PRBs adopted
are given in Table 5 [27,100,101].
13. In situ chemical oxidation (ISCO)
ISCO can be applied to a variety of soil types and sizes and
can treat volatile organic chemicals (VOCs) including
dichloroethene (DCE), trichloroethene (TCE), tetrachlor-
oethene (PCE), and benzene, toluene, ethylbenzene, and
xylene (BTEX) as well as semi-volatile organic chemicals
(SVOCs) including pesticides, polycyclic aromatic hydrocar-
bons (PAHs), and polychlorinated biphenyls (PCBs). The
oxidants such as hydrogen peroxide (H2O2), potassium
permanganete (KMnO4), ozone (O3) are applied [102,103].
The mechanism of degradation is the production of hydroxyl
radicals, which are capable of oxidizing complex organic
compounds. For most in situ reactive zone targets, hydroxyl
radical oxidation is a much faster treatment method [104].
Some of the advantages and limitations in adopting ISCO is
given in Table 6.
13.1. Use of oxidants
(i) Potassium or sodium permanganate (KMnO4/NaMnO4):
Permanaganate is an oxidizing agent and has an affinity
towards organic compounds containing carbon-carbon
bonds, aldehyde groups, or hydroxyl groups [105–107]. As
the permanganate dose increases, organic degradation also
increases and the permanganate consumption also
increases [27]. In some instances where permanganate is
used for the degradation of DNAPL, production of MnO2
(s) may significantly lower the permeability of the soil
matrix and form a coating on DNAPL [27,108]. Phase
transfer catalyst (like quaternary amines), which has both
polar and non-polar segments, are also used along with
permanganate, to assist in the removal hydrophobic
Table 6
Advantages and limitations in using ISCO method in PRB technology
Advantages Limitations
Rapid treatment Limited information is available on the operational history
Capability of contaminants to be degraded into final
innocuous compounds like carbon dioxide, water and chloride
Proper oxidant selection must be made depending on site
characteristics and nature of contaminants
Oxidants can be injected and can be applied to greater depths
compared to method which require excavation
Problems such as gas evolution (also explosive vapors), toxic by-products,
resolubilization of metals and reduction of biomass
Also suitable for complex organics (recalcitrant compounds)
and mixed waste streams
Precise control of pH, temperature, and contact time is important
Certain materials like natural organic matter (NOM), reduced inorganics such
as iron and manganese can exert a demand for oxidant
Occupational health and safety concerns
Table
Char
Techn
Physi
Key
Oxid
By-p
Reag
Subsu
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156 153
organic contaminants [104]. Permanganate reaction rate is
much slower than those of hydroxyl radical reactions,
which gives it s significant advantage in achieving greater
contact with contaminats, especially with NADPL
compounds [104].
(ii) H
ydrogen peroxide (H2O2)/fenton (H2O2–Fe) oxidation:Concentration of peroxide injection generally varies
between 3% and 35% (w/w) at a pH range of 3.5–6, with
or without a metal catalyst (usually iron). The process
involves the production of hydroxyl radicals (OH.), which
act as a powerful oxidizing agent for the degradation of
organic contaminants. The initial amount of H2O2 and Fe
catalyst also depends on the contaminant level in the soil,
and the volume of groundwater remediated [27].
Hydrogen peroxide and fenton oxidation has been
applied for the remediation of several contaminants
including petroleum hydrocarbons [109,110], hexadecane
[111], atrazine [112,113], aromatic compounds [114,115],
dioxins [116] and chlorinated solvents [117]. The
usefulness of fenton oxidation may be limited by low
soil permeability, subsurface heterogeneities, and highly
alkaline soil (where carbonate ions are free radical
scavengers) [105].
(iii) P
ersulfate: Sodium persulfate (Na2S2O8) is recently beingused in ISCO. The conversion of the persulfate results in
the production of sulfate radicals or a single electron
radical. Free radicals act as strong oxidizing agents and are
known to oxidize many VOCs. Many sites have sufficient
background conditions which allow the persulfate to
oxidize the targeted VOCs present in the soils and
groundwater without the addition of a catalyst. In some
7
acteristics of common oxidants
ology features Fenton’s reagent Permanganate
cal state as injected Liquid Liquid
oxidant OH� MnO4�
ation potential 2.8 V 1.7 V
roducts Fe(III), O2, H2O Mn(VI)
ent costs Moderate Moderate for KMn
rface fouling Possible Yes, due to MnO2
instances, chelated iron compounds have been used as
activators [27,118].
(iv) O
zone (O3): Ozone is a very powerful oxidant and iscommonly used for the remediation of hydrocarbon and
chlorinated solvent contaminants [27,119,120]. Ozone
solubility and the concentration of ozone in the injected
gas stream are the key variables affecting the rate of
treatment. Ozone degrades to oxygen and promotes
biodegradation in combination to ozone oxidation process
[27,121]. Treatment with ozone in combination of
hydrogen peroxide (termed as peroxone process) [104]
and fenton reagents [122] has also been attempted.
The characteristics of various oxidants used in the ISCO
technology are given in Table 7.
14. PRB studies in Australia and New Zealand
� Several sightings on petroleum hydrocarbon contamination
have been reported.
- BTEX (benzene, toluene, ethylbenzene and xylene)
contamination in metropolitan Perth, Western Australia.
Biodegradation has been adopted as the remedial method.
Preferential intrinsic biodegradation of selected organic
compounds within the BTEX plume was shown to be
occurring [98].
- BTEX and other dissolved organic contamination from
gasoline spills at Kwinana field on the Swan Coastal Plain
in Western Australia. Aerobic bioremediation was adopted.
Volatalization found to be a dominant mechanism of
removal of the VOCs contaminant present. Evidence of
Persulfate Ozone
Liquid Gas
SO42� O3 and OH�
2.5 V 2.07 V/2.8 V
Sulfate Oxygen
O4; high for NaMnO4 Moderate Moderate
formation No No
R. Thiruvenkatachari et al. / Journal of Industrial and Engineering Chemistry 14 (2008) 145–156154
biodegradation of dissolved organics was also identified
from the oxygen utilization rates [123].
- Trace concentrations of petroleum hydrocarbon–nonaro-
matic hydrocarbons (BTEX) and chlorinated solvents-
halogenated aliphatic hydrocarbon (tetrachloroethene-
PCE) at the Swan Coastal Plain area, Western Australia
was treated by biodegradation method. The results from the
column studies indicated that indigenous microorganisms
were capable of degrading trace amounts of toluene and
ethylbenzene under denitrifying conditions and toluene
under sulfate-reducing conditions. Benzene was persistent
under anoxic conditions, but degraded readily under
oxygenated condition. PCE failed to degrade under either
oxic or anoxic conditions.
- A funnel and gate PRB was adopted with saturated peat
(70% biogreen humic reed sedge, 20% sphagnum peat and
10% cocoa fibre) reactive material was tested for the
remediation of toluene, ethylbenzene, xylene and n-alkane
contaminants (spirit petroleum hydrocarbon spill at the
underground storage site in South Eastern Australia). This
study [124] reported high sorptive efficiencies compared to
previous study which shows a decrease in removal
performance with time [125]. This reinforces the impor-
tance of testing local peat types prior to full-scale
application.
� L
eachate contamination: Coal washery alkaline slag lea-chate, Kemblawarra, Port Kembla, NSW, Australia [68]. The
wastes include air-cooled blast furnace slag, steel making
slag and coal washery discard. A PRB wall with coal washery
discard was chosen. The geochemical calculations indicate
that the reactive wall’s ability to control alkalinity and S2�
should outlast the slag’s ability to produce toxic leachate
[68,126].
� D
NAPL contamination: A pilot scale zero valent iron reactivebarrier was tested for the remediation of DNAPL, including
trichloroethene (TCE), tetrachloroethene (PCE), carbon
tetrachloride (CTC), 1,1,2,2-tetrachloroetane (PCA), 1,1,1-
trichloroethane, 1,2-dichloroethane, chloroform, chloroben-
zene, hexachlorobutadiene and hexachloroethane, at South of
City of Sydney, Australia [127]. Very high removal
efficiencies of VOC was observed. However, very low
removal performance was noted for compounds such as 1,2-
dichloroethane and dichloromethane [54].
� A
mmonium contaminated groundwater: Pilot scale, sequen-tial use of polymer mat reactive barrier for the remediation of
ammonium-contaminated groundwater [128]. Two polymer
mats were placed in series along the flow of the contaminant
plume. The upgradient mat delivered oxygen to induce
bacterial nitrification of the ammonium to nitrite/nitrate and
the downgradient mat delivered ethanol to induce bacterial
denitrification of the nitrite/nitrate to produce nitrogen gas.
Significant reduction (>90%) in total N was achieved.
� D
enitrification (Bardowie farm, Cambridge, North Island,New Zealand): Pilot scale denitrification wall was con-
structed by mixing a carbon source, such as sawdust (Pinus
radiata) (40 m3), in the subsurface aquifer. The added carbon
source stimulates nitrate reduction to nitrogen gas by
denitrification. However, it was found that carbon availability
was not limiting the size of the microbial population [129].
� A
cid rock drainage (AMD) treatment: Remediation of acidmine drainage was attempted at Mt Carrington silver and gold
mine, in northern New South Wales, Australia. BauxsolTM, a
product made from seawater-neutralized red mud (a by-
product of alumina refining) was used as the reactive barrier
material. This material has very low hydraulic conductivity
(as it is very fine-grained (<10 mm)), it was mixed with sand.
Over 45,000 L of ARD was treated above ground with
Bauxsol/sand PRB material. It was found that Bauxsol/sand
PRB was able to avoid clogging (which is a major problem
when using carbonates and hydroxides for acid neutraliza-
tion) and achieve reductions satisfactory with Australian
Water quality Standards. Metal removal efficiency was also
very high. Analysis of the spent Bauxsol also indicated that
the bound metals are not readily leachable, allowing the spent
material to be disposed safely in landfills.
� B
iodegradation of pesticides: Contamination of groundwaterby atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-
triazine), terbutryn (2-tert-butylamino-4-ethylamino-6-
methylthio-1,3,5-triazine) and fenamiphos (1-(methy-
lethyl)-ethyl-3-methyl-4-(methylthio) phenylphosphorami-
date) from infiltration of pesticide-laden washwater from
Dianella, Perth, Western Australia, was treated using a lab-
scale column experiments by biodegradation [130]. Polymer
mats were used to deliver oxygen into the groundwater to
induce biodegradation. Degradation rates of atrizine were
relatively high, however, no significant degradation of
terbutryn or fenamiphos was observed.
Acknowledgement
This study was funded by the CRC Care (Land and Water),
Australia. At the time of the study the first author was with the
University of Technology Sydney.
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