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Phytoremediation: A Green Technology to Remove Environmental Pollutants

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  • Central Luzon State University, Luzon, Philippines

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Land, surface waters, and ground water worldwide, are increasingly affected by contaminations from industrial, research experiments, military, and agricultural activities either due to ignorance, lack of vision, carelessness, or high cost of waste disposal and treatment. The rapid build-up of toxic pollutants (metals, radionuclide, and organic contaminants in soil, surface water, and ground water) not only affects natural resources, but also causes major strains on ecosystems. Interest in phytoremediation as a method to solve environmental contamination has been growing rapidly in recent years. This green technology that involved “tolerant plants” has been utilized to clean up soil and ground water from heavy metals and other toxic organic compounds. Phytoremediation involves growing plants in a contaminated matrix to remove environmental contaminants by facilitating sequestration and/or degradation (detoxification) of the pollutants. Plants are unique organisms equipped with remarkable metabolic and absorption capabilities, as well as transport systems that can take up nutrients or contaminants selectively from the growth matrix, soil or water. As extensive as these benefits are, the costs of using plants along with other concerns like climatic restrictions that may limit growing of plants and slow speed in comparison with conventional methods (i.e., physical and chemical treatment) for bioremediation must be considered carefully. While the benefits of using phytoremediation to restore balance to a stressed environment seem to far outweigh the cost, the largest barrier to the advancement of phytoremediation could be the public opposition. The long-term implication of green plant technology in removing or sequestering environmental contaminations must be addressed thoroughly. As with all new technology, it is important to proceed with caution.
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American Journal of Climate Change, 2013, 2, 71-86
http://dx.doi.org/10.4236/ajcc.2013.21008 Published Online March 2013 (http://www.scirp.org/journal/ajcc)
Phytoremediation: A Green Technology to Remove
Environmental Pollutants
Annie Melinda Paz-Alberto1, Gilbert C. Sigua2*
1Institute for Climate Change and Environmental Management, Central Luzon State University,
Science City of Muñoz, Philippines
2Coastal Plains Soil, Water & Plant Research Center, Agricultural Research Service,
United States Department of Agriculture, Florence, USA
Email: *gilbert.sigua@ars.usda.gov
Received September 8, 2012; revised December 10, 2012; accepted December 27, 2012
ABSTRACT
Land, surface waters, and ground water worldwide, are increasingly affected by contaminations from industrial, re-
search experiments, military, and agricultural activities either due to ignorance, lack of vision, carelessness, or high cost
of waste disposal and treatment. The rapid build-up of toxic pollutants (metals, radionuclide, and organic contaminants
in soil, surface water, and ground water) not only affects natural resources, but also causes major strains on ecosystems.
Interest in phytoremediation as a method to solve environmental contamination has been growing rapidly in recent
years. This green technology that involved “tolerant plants” has been utilized to clean up soil and ground water from
heavy metals and other toxic organic compounds. Phytoremediation involves growing plants in a contaminated matrix
to remove environmental contaminants by facilitating sequestration and/or degradation (detoxification) of the pollutants.
Plants are unique organisms equipped with remarkable metabolic and absorption capabilities, as well as transport sys-
tems that can take up nutrients or contaminants selectively from the growth matrix, soil or water. As extensive as these
benefits are, the costs of using plants along with other concerns like climatic restrictions that may limit growing of
plants and slow speed in comparison with conventional methods (i.e., physical and chemical treatment) for bioremedia-
tion must be considered carefully. While the benefits of using phytoremediation to restore balance to a stressed envi-
ronment seem to far outweigh the cost, the largest barrier to the advancement of phytoremediation could be the public
opposition. The long-term implication of green plant technology in removing or sequestering environmental contamina-
tions must be addressed thoroughly. As with all new technology, it is important to proceed with caution.
Keywords: Phytoremediation; Green Technology; Pollutants; Contaminants; Toxic Metals
1. Green Technology
The success of green technology in phytoremediation, in
general, is dependent upon several factors. First, plants
must produce sufficient biomass while accumulating
high concentrations of metal. In some cases, an increased
biomass will lower the total concentration of the metal in
the plant tissue, but allows for a larger amount of metal
to be accumulated overall. Second, the metal-accumula-
ting plants need to be responsive to agricultural practices
that allow repeated planting and harvesting of the metal-
rich tissues. Thus, it is preferable to have the metal ac-
cumulated in the shoots as opposed to the roots, for metal
in the shoot can be cut from the plant and removed. This
is manageable on a small scale, but impractical on a large
scale. If the metals are concentrated in the roots, the en-
tire plant needs to be removed. Yet, the necessity of full
plant removal not only increases the costs of phytoreme-
diation, due to the need for additional labor and plantings,
but also increases the time it takes for the new plants to
establish themselves in the environment and begin ac-
cumulation of metals. Table 1 lists some of the common
pollutant accumulating plants found by phytoremediation
researchers.
The availability of metals in the soil for plant uptake is
another limitation for successful phytoremediation. For
example, lead (Pb2+), an important environmental pol-
lutant, is highly immobile in soils. Lead is known to be
“molecularly sticky” since it readily forms a precipitate
within the soil matrix. It has low aqueous solubility, and,
in many cases, is not readily bioavailable. In most soils
capable of supporting plant growth, the soluble Pb2+ lev-
els are relatively low and will not promote substantial
uptake by the plant even if it has the genetic capacity to
accumulate the metal. In addition, many plants retain
Pb2+ in their roots via absorption and precipitation with
only minimal transport to the aboveground harvestable
*Corresponding author.
C
opyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA
72
Table 1. Selected pollutant accumulating plants.
SCIENTIFIC NAME COMMON NAME
Armeria maririma Seapink thrift
Ambrosia artemisiifolia Ragweed
Brassica juncea Indian mustard
Brassica napus Rape, Rutabaga, Turnip
Brassica oleracea Flowering/ornamental kale and cabbage, Broccoli
Festuca ovina Blue/sheep fescue
Helianthus annuus Sunflower
Thalspi rotundifolium Pennycress
Triticum aestivum Wheat (scout)
Zea mays Corn
plant portions. Therefore, it is important to find ways to
enhance the bioavailability of Pb2+ or to find specific
plants that can better translocate the Pb2+ into harvestable
portions [1].
Although there are some challenges associated with
the phytoremediation, it remains a very promising strat-
egy and feasible alternative. However, in many situations,
soil contamination may have unique factors that require
special evaluation. Some plants may only accumulate
these essential elements and prevent all others from en-
tering. For plants termed as “hyperaccumulators” can
extract and store extremely high concentrations (in ex-
cess of 100 times greater than non-accumulator species)
of metallic elements [2]. It is believed that these plants
initially develop the ability to hyperaccumulate non-es-
sential metallic compounds as a means of protecting
themselves from herbivorous predators that would ex-
perience serious toxic side effects from ingestion of the
hyperaccumulator’s foliage [3].
1.1. Plants as Phytoremediators
The principal application of phytoremediation is for
lightly contaminated soils and waters where the material
to be treated is at a shallow or medium depth and the area
to be treated is large. This will make agronomic tech-
niques economical and applicable for both planting and
harvesting. In addition, the site owner must be prepared
to accept a longer remediation period. Plants that are able
to decontaminate soils does one or more of the following:
1) plant uptake of contaminant from soil particles or soil
liquid into their roots; 2) bind the contaminant into their
root tissue, physically or chemically; and 3) transport the
contaminant from their roots into growing shoots and
prevent or inhibit the contaminant from leaching out of
the soil.
Moreover, the plants should not only accumulate, de-
grade or volatilize the contaminants, but should also
grow quickly in a range of different conditions and lend
themselves to easy harvesting. If the plants are left to die
in situ, the contaminants will return to the soil. So, for
complete removal of contaminants from an area, the
plants must be cut and disposed of elsewhere in a non-
polluting way. Some examples of plants used in phyore-
mediation practices are the following: water hyacinths
(Eichornia crassipes); poplar trees (Populus spp.); forage
kochia (Kochia spp); alfalfa (Medicago sativa); Ken-
tucky bluegrass (Poa pratensis); Scirpus spp, coontail
(Ceratophyllum demersum L.); American pondweed
(Potamogeton nodosus); and the emergent common ar-
rowhead (Sagittaria latifolia) amongst others [4].
Four heavy metal concentrations in soils (Cu, Cr, As,
and Pb) were examined to see if removal through the
process of phytoremediation was possible. Tomato and
mustard plants were able to extract different concentra-
tions of each heavy metal from the soils. The length of
time that the soils were exposed to the contaminants af-
fected the levels of heavy metals accumulation. Today,
many institutions and companies are funding scientific
efforts to test different plants' effectiveness in removing
wide ranges of contaminants. Scientists favor Brassica
juncea and Brassica olearacea, two members of the
mustard family, for phytoremediation because these
plants appeared to remove large quantities of Cr, Pb, Cu,
and Ni from the soil [5].
1.2. Grasses as Potential Phytoremediators
1.2.1. Vetiver Grass (Vetiveria zizanioides L.)
Vetiver (Vetiveria zizanioides L.) belongs to the same
grass family as maize, sorghum, sugarcane, and lemon
grass. It has several unique characteristic as reported by
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA 73
the National Research Council [6]. Vetiver grass is a
perennial grass growing two meters high, and three me-
ters deep in the ground. It has a strong dense and vertical
root system. It grows both in hydrophilic and xerophytic
conditions. The leaves sprout from the bottom of the
clumps and each blade is narrow, long and coarse. The
leaf is 45 - 100 cm long and 6 - 12 cm wide.
Vetiver grass is highly suitable for phytoremedial ap-
plication due to its extraordinary features. These include
a massive and deep root system, tolerance to extreme
climatic variations such as prolonged drought, flood,
submergence, fire, frost, and heat waves. It is also toler-
ant to a wide range of soil acidity, alkalinity, salinity,
sodicity, elevated levels of Al, Mn, and heavy metals
such as As, Cr, Ni, Pb, Zn, Hg, Se, and Cu in soils [7].
The roots of vetiver are the most useful and important
part. Its root system does not expand horizontally, but
penetrates vertically deep into the soil, whether it is the
main, secondary or fibrous roots. The horizontal expan-
sion of the vetiver grass root system is limited up to only
50 cm. The root vertical penetration expends up to 5 me-
ters. Normally, yield levels of the leaves is 15 - 30
tons·ha1 (15,000 - 30,000 kg·ha1) while vetiver grass
roots can produce a dry matter yield of about 1428.6 to
2142.9 kg·ha1 [8].
Various uses of vetiver grass are known worldwide. In
South Africa, it was used effectively to stabilize waste
and slime dams from Pt and Au mines [9]. In Australia,
vetiver grass was used to stabilize landfill and industrial
waste sites contaminated with heavy metals such as As,
Cd, Cr, Ni, Cu, Pb, and Hg [7]. In China, vetiver grass
was planted in large scale for pollution control and mine
tail stabilization [10]. In Thailand, vetiver grass is found
widely distributed naturally in all parts of the country. It
has been used for erosion control and slope stabilization.
Vetiver hedges had an important role in the process of
captivity and decontamination of pesticides, preventing
them from contaminating and accumulating in crops [11].
When compared with other plants, vetiver grass is more
efficient in absorbing certain heavy metals and chemicals
due to the capacity of its root system to reach greater
depths and widths [7]. As confirmed by Roongtanakiat
and Chairoj [12], vetiver grass was found to be highly
tolerant to an extremely adverse condition. Therefore,
vetiver grass can be used for rehabilitation of mine tail-
ings, garbage landfills, and industrial waste dumps which
are often extremely acidic or alkaline, high in heavy
metals, and low in plant nutrients.
1.2.2. Cogon Grass (Imperata cylindrica L.)
Cogon grass, generally occurs on light textured acid soils
with clay subsoil, and can tolerate a wide range of soil
pH ranging from strongly acidic to slightly alkaline [13].
It is hardy species, tolerant of shade, high salinity, and
drought. It can be found in virtually any ecosystem, es-
pecially those experiencing disturbances [8]. It is a per-
ennial grass up to 120 cm high with narrow and rigid
leaf-blades.
The roots can penetrate to a soil depth of about 58 cm
in alluvial soil. More than 80 percent of shoots can
originate from rhizomes less than 15 cm below the soil
surface. The average number of shoots of cogon grass
was about 4.5 million per hectare, producing 18,500
kg·ha1 of leaves and rhizomes (11,500 kg of leaves and
7000 kg of rhizomes) [13].
1.2.3. Carabao Grass (Paspalum conjugatum L.)
Carabao grass is a vigorous, creeping perennial grass
with long stolons and rooting at nodes. Its culms can as-
cend to about 40 to 100 cm tall, branching, solid, and
slightly compressed where new shoots can develop at
every rooted node. Under a coconut plantation, a yield of
about 19,000 kg·ha1 of green materials was obtained. It
grows from near sea-level up to 1700 m altitude in open
to moderately shaded places. It is adapted to humid cli-
mates and found growing gregariously under plantation
crops and also along stream banks, roadsides, and in dis-
turbed areas. This grass can adapt easily to a wide range
of soils [14].
2. Phytoremediation as a Cleansing Tool: An
Overview
Phytoremediation is described as a natural process car-
ried out by plants and trees in the cleaning up and stabi-
lization of contaminated soils and ground water. It is
actually a generic term for several ways in which plants
can be used for these purposes. It is characterized by the
use of vegetative species for in situ treatment of land
areas polluted by a variety of hazardous substances [15].
Garbisu [16] defined phytoremediation as an emerging
cost effective, non-intrusive, aesthetically pleasing, and
low cost technology using the remarkable ability of
plants to metabolize various elements and compounds
from the environment in their tissues. Phytoremediation
technology is applicable to a broad range of contami-
nants, including metals and radionuclides, as well as or-
ganic compounds like chlorinated solvents, polychlori-
biphenyls, polycyclic aromatic hydrocarbons, pesti-
cides/insecticides, explosives and surfactants. According
to Macek [17], phytoremediation is the direct use of green
plants to degrade, contain, or render harmless various
environmental contaminants, including recalcitrant or-
ganic compounds or heavy metals. Plants are especially
useful in the process of bioremediation because they
prevent erosion and leaching that can spread the toxic
substances to surrounding areas [18].
Several types of phytoremediation are being used to-
day. One is phytoextraction, which relies on a plant’s
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A. M. PAZ-ALBERTO, G. C. SIGUA
74
natural ability to take up certain substances (such as
heavy metals) from the environment and sequester them
in their cells until the plant can be harvested. Another is
phytodegredation in which plants convert organic pol-
lutants into a non-toxic form. Next is phytostabilization,
which makes plants release certain chemicals that bind
with the contaminant to make it less bioavailable and less
mobile in the surrounding environment. Last is phyto-
volitization, a process through which plants extract pol-
lutants from the soil and then convert them into a gas that
can be safely released into the atmosphere [19]. Rhizofil-
tration is a similar concept to phytoextraction, but mainly
use with the remediation of contaminated groundwater
rather than the remediation of polluted soils. The con-
taminants are either absorbed onto the root surface or are
absorb by the plant roots. Plants used for rhizofiltration
are not planted directly in situ, but are acclimated with
the pollutant first. Until a large root system has devel-
oped, plants are hydroponically grown in clean water
rather than in soil. Once a large root system is in place,
the water supply is substituted for polluted water supply
to acclimate the plant. After the plants become acclima-
tized, they are planted in the polluted area. As the roots
become saturated, they are harvested and disposed of
safely.
Phytoremediation is a naturally occurring process rec-
ognized and documented by humans more than 300 years
ago [2]. Since then, humans have exploited certain plant
abilities to survive in contaminated areas and to assist in
the removal of contaminants from the soil. However,
scientific study and development of these plants’ unique
qualities were not conducted until the early 1980’s [2].
At this time, it was recognized that certain species of
plants could accumulate high levels of heavy metals from
the soil while continuing to grow and proliferate nor-
mally [2]. Research has been slow and tedious due to
scientists’ incomplete understanding of the generalized
cellular mechanisms of plants. However, the advent of
new genetic technology has allowed scientists to deter-
mine the genetic basis for high rates of accumulation of
toxic substances in plants [20]. Using genetic engineer-
ing, scientists may soon be able to exploit plants’ char-
acteristics that can provide faster and more efficient
means of removing contaminants from the soil. Genetic
engineering will also be crucial for the creation of trans-
genic plants that will be able to combine the natural ag-
ronomic benefits associated with plants (ease of harvest
and rapid, expansive growth) with the remediation capa-
bilities of bacteria-a traditional organism used in biore-
mediation [21].
Phytoremediation of heavy metals from the environ-
ment serves as an excellent example of plant-facilitated
bioremediation process and its role in removing envi-
ronmental stress. Traditionally, when an area becomes
contaminated with heavy metals, the area must be exca-
vated and the soil should be removed and put to a landfill
site [2]. This process is extremely expensive and, there-
fore not entirely appealing despite recent discoveries
regarding phytoremediation [2]. Analysts have estimated
the cost of cleaning one hectacre of highly contaminated
land at a depth of one meter. The estimated cost would
range from $600,000 to $3,000,000 depending on the
extent of the pollution and the toxicity of the pollutants
[21]. The cost of phytoremediation could be as much as
20 times less expensive, making this practice far less
prohibitive than conventional methods [2]. The ideal type
of phytoremediator is a species that creates a large bio-
mass, grows quickly, extensive root system, and can be
easily cultivated and harvested [20]. The only problem is
that natural phytoremediators often lack most of the
qualities described above. Therefore, scientists have been
forced to become very creative in developing effective
transgenic phytoremediators.
Many human diseases result from the buildup of toxic
metals in soil, making remediation crucial in protecting
human health. Lead is one of the most difficult contami-
nants to be removed from the soil and one of the most
dangerous. According to Lasat [2], the presence of Pb in
the environment can have devastating effects on plant
growth and can result in serious side effects-including
seizures and mental retardation if ingested by humans or
animals. Much of the global Pb contamination has oc-
curred as a result of mining and iron smelting activities
[22]. Phytoremediation of Pb contaminated soil involves
two of the aforementioned strategies-phytostabilization
and phytoextraction. It is believed that plants’ ability to
phytoextract certain metal is a result of its dependence
upon the absorption of many metals such as Zn, Mn, Ni,
and Cu [2].
2.1. Phytoremediation of Water Pollutants
In 2005, Cortez [23] conducted a study to assess pollu-
tion and survey the potential plants that can be used as
phytoremediators of heavy metals in Nueva Ecija, Phil-
ippines. Water and plant samples were taken near the
dumpsites, which is about 500 m away from the creek.
Results of the water analysis showed that the dumpsite
and Panlasian Creek were slightly polluted with consid-
erable amount of phosphate. Results of the plant chemi-
cal analysis showed that kangkong (Ipomea aquatic) and
Hydracharitaceae (Ottelia alismoides L.) were both effi-
cient in phytoremediating Pb. Analysis of the plants fur-
ther suggests that the concentrations of Pb in morning
glory (Ipomea violacea L.) and hydracharitaceae (Ottelia
alismoides L.) was about 210% more than the concentra-
tion of Pb in the water [23].
Xia and Ma [24] in 2005 investigated the potential of
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA 75
water hyacinth (Eichhornia crassipes) in removing a
phosphorus pesticide ethion. The disappearance rate con-
stants of ethion in culture solutions were 0.01059,
0.00930, 0.00294 and 0.00201 for the non-sterile
planted, sterile planted, non-sterile unplanted and sterile
unplanted treatment, respectively. The accumulated
ethion in live water hyacinth plant decreased by 55% -
91% in shoots and 74% - 81% in roots after the plant
growing 1 week in ethion free culture solutions, suggest-
ing that the plant uptake and phytodegradation might be
the dominant process for ethion removal by the plant.
Given the promising result of the study, water hyacinth
could be utilized as an efficient, economical and eco-
logical alternative to accelerate the removal and degrada-
tion of agro-industrial wastewater polluted with ethion.
Letachowicz et al. [25] conducted a study on the phy-
toremediation capacity on heavy metals accumulation in
different organs of Typhia latifolia L. The concentrations
of Cd, Pb, Cu, Ni, Mn, Zn, and Fe were determined in
different organs of Typhia latifolia from seven water
bodies in the Nysa region in Poland. The Typhia latifolia
species that can absorb heavy metals can be used as
bio-indicator of pollutants is a macrohydrophyte and is
widely present in the entire lowland and lower mountain
sites. It is linked with nutritious water and organic or
inorganic mineral bottom sediments. Typhia latifolia is a
strongly expansive species because it can control water
space due to intensive growth of rhizomes and often cre-
ates almost mono-species group, though it can also be
found in various groups of rushes.
2.2. Phytoremediation Species in Coastal Water
The Philippines is blessed to have relatively high man-
grove diversity having 35 species [26] including five
major families, namely: Avicenniaceae; Arecaceae; Com-
bretaceae; Lythraceae; and Rhizophoraceae [27]. Though
Philippines has high mangrove diversity, it was reported
that there was a drastic decline of mangrove resources
from 450,000 hectares in 1918 to 120,000 hectares in
1995. The decrease of the mangrove forests was due to
human activities, such as fish pond conversion, human
settlement, and salt production [28]. However, with the
alarming rate of mangrove forest degradation, Philip-
pines strived to continue greater conservation of man-
groves and reforestation of the coastal areas [29].
Mangroves are higher plants, which are found mostly
in the intertidal areas of tropical and subtropical shore-
lines and show remarkable tolerance to high amounts of
salt and oxygen poor soil. The mechanisms of mangrove
to keep the salt away from the cytoplasm of the cell were
through the excretion of salt in their salt glands found in
the leaves and roots and through storage of salts in the
mature leaves, bark and wood [26]. Mangroves devel-
oped unique body features in order to cope up with harsh
environment. There are different types of roots, such as
prop roots in Rhizophora, pencil-like pneumatophores in
Avicennia, and cone-like pneumatophores in Sonneratia
that have large lenticels to permit gas exchange. The
leaves of mangroves have characteristics to survive from
dessication and conserve water like the presence of thick
epidermis, waxy cuticle, and presence of hypostomata
[26]. Mangrove ecosystem is exposed to different pol-
lutants such as heavy metal, sewage wastes, pesticides
and petroleum products. Heavy metal accumulation in
the mangrove sediment can result in biological and eco-
logical effects. Even though, mangrove trees may have
the immunity against the toxic effects of the heavy met-
als, but the animals thriving in the ecosystem are vulner-
able to the negative effects of heavy metals [30].
Few studies were conducted about phytoremediation
potential of mangroves and other wetland plant species.
However, those researchers paved the way to explore
more species of mangroves particularly the native species
present in the area, for their feasibility to accumulate
heavy metals. Zheng et al. [31] studied the different
metal concentrations of Cu, Ni, Cr, Zn, Pb, Cd, and Mn
in Rhizophora stylosa at Yingluo Bay, China. The study
showed less pollution due to relatively low concentration
of metals especially Pb, Mn, Zn and Cd.
MacFarlane and Burchett [32] examined the cellular
distribution of Cu, Pb and Zn in grey mangrove, Avicen-
nia marina (Forsk.) using scanning electron microscope
X-ray microanalysis and atomic absorption spectroscopy.
They reported that metals mostly accumulate in plants’
cell walls. Their study showed that certain parts of man-
groves have the ability to control the entrance of heavy
metals in other parts of the plants. The laboratory re-
search of MacFarlane and Burchett [33] contributed in-
formation on the accumulation, growth effect, and toxic-
ity of Cu, Pb and Zn in grey mangrove, Avicennia ma-
rina (Forsk.). Accumulation of the different metals oc-
curred at varying concentrations in the roots and leaf
tissue. In the roots, Pb accumulated lesser than the other
metals while high concentration of Zn was found in the
leaf tissue. The effects of excessive Cu and Zn on young
mangrove were reductions in seedling height, leaf num-
ber, total biomass, and root growth. The germination of
mangrove was inhibited at 800 μg·g1 Cu and 1000
μg·g1 Zn. The Pb showed only little negative effects in
the growth of the plant due to low absorption of this
metal.
Cheng [34] cited heavy metals can be absorbed by
plants using their roots, or via stems and leaves, and
stored the metals into different plant parts. Moreover, the
distribution and accumulation of heavy metals in the
plants depend on plant species and chemical factors. The
Avicennia marina, a salt-excretive mangrove, and
Rhizophora stylosa, a salt-exclusion mangrove, have dif-
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A. M. PAZ-ALBERTO, G. C. SIGUA
76
ferent accumulation potential of different heavy metals.
In terms of Pb absorption, A. marina was able to accu-
mulate more concentrations of heavy metals than in R.
stylosa. However, the purification processes of plants
were affected by different factors such as heavy metal
concentration, plant species, and exposure duration.
Sari et al. [35] conducted an in-situ experiment on the
bioaccumulation of Pb in two mangrove species, Avicen-
nia alba and Rhizophora apiculata using hydroponics
culture. The mangroves were grown in 0%, 15% and
30% salinity and 0.03, 0.3, and 3 mg·L1 of Pb concen-
tration. They observed that both mangroves had signifi-
cantly lower Pb accumulation in leaves than in roots.
They claimed that the mobility of Pb in the aerial part of
the plant can be related to its mechanism associated with
the accumulation of sodium in the salt glands found in
the leaves. Saenger and McConchie [36] evaluated the
accumulation trend of Pb in the tissues, barks and woods,
old and young leaves and fruits of different mangrove
species. They discovered that Pb concentrated more in
the bark than in other tissues of mangroves because of
atmospheric Pb due to vehicle exhausts from nearby ma-
jor roads.
Shete et al. [37] revealed in their study entitled, “Bio-
accumulation of Zn and Pb in Avicennia marina (Forsk.)
from urban areas of Mumbai (Bombay), India,” that the
mangrove species can bioaccumulate and survive despite
heavy metal contamination. Results showed that man-
groves have greater uptake of heavy metals. Variations
on the concentrations of Zn were found from the differ-
ent plant parts while high accumulation of Pb was fo-
cused in the roots. They found out that Pb concentrations
were present in the leaves and roots. Kamaruzzaman et al.
[38] studied the cumulative partitioning of Pb and Cu in
the Rhizophora apiculata in the Setiu mangrove forest,
Terengganu. Results showed increasing concentration of
Cu and Pb from the leaf, bark, root, and sediments. The
study by Pahalawattaarachchi et al. [39] reported the
absorption, accumulation, and partitioning of eight dif-
ferent metals specifically Cu, Cd, Cr, Fe, Mg, Ni, Pb and
Zn by mangrove species, Rhizophora mucronata (Lam.)
at Alibag, Maharashtra, India. They revealed that Cu, Mn
and Fe showed limited mobility due to their accumula-
tion in the roots while other metals (Cd, Zn, Ni and Pb)
were concentrated in the aerial part of the plant. They
concluded that Rhizophora mucronata (Lam.) was more
capable of phytostabilization rather than phytoextraction
because of low uptake capacity of different metals.
Nazli and Hashim [40] revealed that Sonneratia case-
olaris was a potential phytoremediation species for se-
lected heavy metals in Malaysian mangrove ecosystem.
The study assessed the concentrations of Cd, Cr, Cu, Pb
and Zn in Sonneratia caseolaris. Results showed that
both roots and leaves of Sonneratia caseolaris accumu-
lated and exceeded the general normal upper range of Cu
and Pb in plants. In Iran, Parvaresh et al. [41] studied the
bioavailability of different heavy metals (Ni, Cu, Cd, Pb
and Zn) in the sediments of Sirik Azini creek. The out-
come of their research revealed no heavy metal pollution
was found in the area due to low geo-accumulation index
of Pb in the sediment. They assessed that the concentra-
tion of heavy metals particularly Pb in the leaves were
higher than the concentration of Pb in the sediment.
Qui et al. [42] studied the different accumulation and
partitioning of seven trace metals, namely, As, Cd, Cr,
Cu, Hg, Pb and Zn, in mangroves and sediments from
three estuarine wetlands of Hainan Island, China. They
analyzed the sediment samples and found out that the
heavy metals present in the area were still at relatively
low levels. Furthermore, Pb analysis of mangroves
showed that this metal was found mostly in the branches
of the different mangroves. Zhang et al. [43] investigated
the physiological response of Sonneratia apetala (Buch)
to the addition of wastewater nutrients and heavy metals
(Pb, Cd, and Hg). They planted mangroves in four dif-
ferent treatments: 1) control, which has only salted water;
2) normal concentration of wastewater nutrients and
heavy metals; 3) five times the normal treatment; and 4)
ten times the normal treatment. Results revealed that
growth of mangrove increased with increasing levels of
wastewater pollution. The study showed that mangroves
were potential phytoremediator in wetland ecosystem.
The research of Nirmal et al. [44] entitled, “An as-
sessment of the accumulation potential of Pb, Zn, and Cd
by Avicennia marina (Forssk) in Vamleshwar Mangroves,
Gujarat, India,” reported that sediments in the area are
below critical soil concentration for heavy metals. A.
marina possesses the capacity to uptake selected heavy
metals, Pb, Zn and Cd, via its roots and storing them in
their leaves without any sign of complications. The con-
centrations of heavy metals in the A. marina were in
normal range except for Pb. The roots of mangrove con-
tained the highest concentration of heavy metal except
for Cd. Furthermore, A. marina had the capacity to up-
take metals via its roots and accumulates them in their
leaves without any sign of injury. The study showed
Avicennia marina as a potential phytoremediation spe-
cies for selected heavy metals in many mangrove eco-
systems. Subramanian [45] cited that mangroves gener-
ally have low concentration of heavy metal. Kathiresan
and Bingham [27] mentioned that mangroves can tolerate
metal pollution because they were poor accumulators of
heavy metals. A study on the metal uptake of Rhizophora
mangle in Sepetiba Bay, Rio de Janeiro, Brazil showed
that only one percent of the total heavy metals concentra-
tion in the sediment accumulated in the mangrove [27].
In their experiment, they used young Bruguiera gymnor-
rhiza and artificially synthesized wastewater treatments
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA 77
with different levels of Cu, Cd, Cr, Ni, and Zn. The con-
trol treatment showed higher biomass and growth than
the plants treated with wastewater.
2.3. Phytoremediation of Soil Pollutants
Phytoremediation is a cleanup technology for metal con-
taminated soils, specifically Pb. In order for this type of
remediation strategy to be successful, it is necessary to
utilize metal accumulating plants to extract environmen-
tally toxic metals from the soil, such as Pb, Ni, Cr, Cd
and Zn. Certain plants have been identified not only to
accumulate metals in the plant roots, but also to translo-
cate the accumulated metals from the root to the leaf and
to the shoot. While many plants performed this function,
some plants, known as “hyperaccumulators”, can accu-
mulate extremely high concentrations of metals in their
shoots (0.1% to 3% of their dry weight) [46]. The metal-
rich plant material can then be harvested and removed
from the site without extensive excavation, disposal costs,
and loss of topsoil that is associated with traditional
remediation practices.
Bioremediation process would be extremely slow be-
cause the rate of bioemediation is directly proportional to
growth rate while the total amount of bioremediation is
correlated with a plant’s total biomass. No plant has been
discovered yet capable of meeting all the ideal criteria of
an effective phytoremediator. These criteria are fast
growing, deep and extensive roots, high biomass, easy to
harvest and hyperaccumulators of a wide range of toxic
metals. A Pb absorption study by Huang and Cunning-
ham [47] cited corn as a perfect phytoremediator due to
its large biomass, fast rate of growth, and the existence of
extensive genomic knowledge of this crop. Introduction
of hyper accumulating genes as well as genetic informa-
tion would better prepare these species to deal with di-
verse climatic conditions [19]. The mobilization of metal
contaminants, both in the soil and the plant, is another
important factor influencing the success of phytoreme-
diation. The amount of soluble Pb2+ in the soil appears to
be a key factor to the enhancement of Pb2+ uptake by
plants [48].
Two main amendment techniques have been used to
increase the bioavailability of Pb in soils and the mobility
of Pb within plant tissue by lowering soil pH and adding
synthetic chelates. Soil pH is a significant parameter in
the uptake of metal contaminants because soil pH value
is one of the principal soil factors controlling metal
availability [49]. Maintaining a moderately acidic pH in
the soil may be attained through the use of ammonium
containing fertilizer or soil acidifiers. By this, Pb metal
bioavailability and plant uptake can increase [50-52]. In a
study performed by Cholpecka et al. [53] on metal con-
taminated soils in southwest Poland, reported that soil
samples with pH of less than 5.6 contained relatively
more metals in the exchangeable form than in soil sam-
ples with pH greater than 5.6. In addition, at lower pH,
the Pb in soil has a greater potential to translocate from a
plant’s roots into its shoots. Synthetic chelates, such as
ethylenediaminetetraacetic acid (EDTA), have been
shown to aid in the accumulation of Pb2+ in the plant
tissue. EDTA and other chelates have been used in soils
and nutrient solutions to increase the solubility of metal
cations and the translocation of Pb into shoots [54].
The physiological and biological mechanisms involved
in Pb uptake of plants involving root to shoot transport of
Pb may require some time to develop and become func-
tional. Since plant species can differ significantly in Pb
uptake and translocation, the success of using plants to
extract Pb from contaminated soils requires the following:
1) the identification of Pb accumulating plants that can
survive in the presence of contaminants; 2) the meas-
urement of the concentration of pollutant in the soil, and
3) knowledge of chemistry (availability or speciation) of
the metal in the soil matrix. The combination of soil
amendment and foliar fertilizer application to plants ca-
pable of absorbing and translocation of Pb may be an
effective means of remediating an area with varying lev-
els of Pb concentrations.
Other model of phytoremediators includes various va-
rieties of transgenic trees. Trees are ideal in the remedia-
tion of heavy metals because they can withstand higher
concentrations of pollutants due to their large biomass.
As such, they can accumulate large amounts of the con-
taminants in their systems because of their size capable
of reaching huge area and great depths due to their ex-
tensive root systems. Furthermore, they can stabilize an
area, prevent erosion, and minimize spread of contami-
nant because of their perennial presence. They can also
be easily harvested and removed from the area with
minimal risk, effectively taking with them a large quan-
tity of the pollutants that were once present in the soil
[19].
3. Phytoremediation, Is It Good or Bad?
Earlier discussion has illustrated many advantages and
disadvantages of transgenic phytoremediation. The pri-
mary advantages of using plants in bioremediation are as
follows: it is more cost-effective; more environmentally
friendly; and more aesthetically pleasing than conven-
tional methods. The conventional methods are usually
expensive and environmentally disruptive [55]. Plants
also offer a permanent, in situ, nonintrusive, self-sus-
taining method of soil contaminant removal. More im-
portantly, contaminants can be removed much more eas-
ily through the harvest of plants than from the soil itself.
More benefits are derived through phytoextraction. It
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA
78
enables scientists to reclaim and recycle usable materials,
including a wide variety of precious metals from the soil
[21]. Also, its potential benefits are extremely high and
extremely attractive to scientists and businessmen alike
[21]. Furthermore, phytoextraction is economical be-
cause only solar energy must be present to maintain the
system [55]. Finally, the greatest advantage of this tech-
nology is that it utilizes the inherent agronomic benefits
of plants [56]. These benefits include high biomass, ex-
tensive root systems that both stabilize the ecosystem by
preventing contaminant to spread through leaching as
well as reaching a large volume of contaminated soil and
a greater ability to withstand adverse environmental con-
ditions and interspecies competition than bacteria [56].
As extensive as these benefits are, the possible costs of
using plants for bioremediation should not be ignored.
Some concerns voiced out in response to phytoremedia-
tion include its slow speed in comparison to mechanical
methods such as soil excavation and climatic restrictions
that may limit growing many species of plants, and the
unknown long-term environmental costs [19]. Also, po-
tential danger might exist for animals that live in the ar-
eas in which phytoremediators are grown, especially if
these animals typically feed on plants being used for
phytoremediation [21].
Moreover, concerns have been raised regarding the
potential for contaminants to move up the food chain
more quickly. This problem may occur if toxic materials
are sequestered in consumable sources such as plants
[57]. Finally, issues with the disposal of these toxic ma-
terials still remain. Once contaminants have been ex-
tracted from the soil by the plants, we are still faced with
the dilemma of what to do with these contaminants. It
seems that the end result remains the same. This involves
the removal of contaminants to a landfill location where
the plants would eventually biodegrade and the contami-
nants could enter the soil system once again [57].
4. Case Study: Phytoremediation Research
in the Tropics
4.1. Phytoremediation of Lead Contaminated
Soils
The global problem concerning contamination of the
environment as a consequence of human activities is in-
creasing. Most of the environmental contaminants are
chemical by-products such as Pb. Lead released into the
environment makes its way into the air, soil and water.
Lead contributes to a variety of health effects such as
decline in mental, cognitive, and physical health of the
individual. An alternative way of reducing Pb concentra-
tion from the soil is through phytoremediation. Phytore-
mediation is an alternative method that uses plants to
clean up contaminated area. Hence, Paz-Alberto et al.
[58] conducted a study in the Philippines. The objectives
of this study were 1) to determine the survival rate and
vegetative characteristics of three grass species such as
vetiver grass, cogon grass, and carabao grass grown in
soils with different Pb levels; and 2) to determine and
compare the ability of three grass species as potential
phytoremediators in terms of Pb accumulation by plants.
The three test plants: vetiver grass (Vetiveria zizanioides
L.); cogon grass (Imperata cylindrica L.); and carabao
grass (Paspalum conjugatum L.) were grown in different
individual plastic bags containing soils with 75 mg·kg1
(37.5 kg·ha1) and 150 mg·kg1 (75 kg·ha1) of Pb, re-
spectively. The Pb contents of the test plants and the soil
were analyzed before and after experimental treatments
using an atomic absorption spectrophotometer. This
study was laid out following a 3 × 2 factorial experiment
in a completely randomized design [58].
Results of the study (Table 2) revealed that on the
vegetative characteristics of the test plants, vetiver grass
registered the highest whole plant dry matter (33.85 -
39.39 Mg·ha1). Carabao grass had the lowest herbage
mass production of 14.12 Mg·ha1 and 5.72 Mg·ha1
from soils added with 75 and 150 mg·Pb·kg1, respec-
tively. Vetiver grass also had the highest percent plant
survival which meant it best tolerated the Pb contamina-
tion in soils. Vetiver grass registered the highest rate of
Pb absorption (10.16 ± 2.81 mg·kg1). This was followed
by cogon grass (2.34 ± 0.52 mg·kg1) and carabao grass
with the mean Pb level of 0.49 ± 0.56 mg·kg1. Levels of
Pb among the three grasses (shoots + roots) did not vary
significantly with the amount of Pb added (75 and 150
mg·kg1) to the soil. Vetiver grass yielded the highest
biomass; it also has the greatest amount of Pb absorbed
(roots + shoots). This can be attributed to the highly ex-
tensive root system of vetiver grass with the presence of
an enormous amount of root hairs. Extensive root system
denotes more contact to nutrients in soils, therefore more
likelihood of nutrient absorption and Pb uptake. The effi-
ciency of plants as phytoremediators (Table 3) could be
correlated with the plants’ total biomass. This implies
that the higher the biomass, the greater the Pb uptake.
Plants characteristically exhibit remarkable capacity to
absorb what they need and exclude what they do not
need. Some plants utilize exclusion mechanisms, where
there is a reduced uptake by the roots or a restricted
transport of the metals from roots to shoots. Combination
of high metal accumulation and high biomass production
results in the most metal removal in the soil [58]. The
study indicated that vetiver grass possessed many benefi-
cial characteristics to uptake Pb from contaminated soil.
It was the most tolerant and could grow in soil contami-
nated with high Pb concentration. Cogon grass and cara-
bao grass are also potential phytoremediators since they
can absorb small amount of Pb in soils, although cogon
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA
Copyright © 2013 SciRes. AJCC
79
Table 2. Levels of Pb absorbed 1) by whole plants (roots + shoots) and estimated total uptake of Pb 2) by vetiver grass, cogon
grass and carabao grass.
Amount of Pb added (kg·ha1)
Grasses
37.5 75
Mean (LSD0.05 = 17.2)
1. Levels of Pb in whole plants (mg·kg1) (mg·kg1)
Vetiver grass 11.84 ± 2.94 8.47 ± 1.59 10.16 ± 2.81a§
Cogon grass 2.00 ± 0.19 2.68 ± 0.54 2.34 ± 0.52b
Carabao grass 0.40 ± 0.32 0.58 ± 0.25 0.49 ± 0.56c
Mean (LSD0.05 = 1.5) 4.75 ± 2.5x 3.91 ± 2.6x (LSD0.05 = 1.8)
2. Plant uptake of Pb (kg·ha1) (kg·ha1)
Vetiver grass 29.71 ± 8.71 33.78 ± 10.02 31.74 ± 9.01a§
Cogon grass 1.93 ± 0.48 2.69 ± 0.19 2.33 ± 0.53b
Carabao grass 0.19 ± 0.06 0.34 ± 0.03 0.27 ± 0.03b
Mean (LSD0.05 = 5.6) 13.95 ± 2.91x 12.27 ± 3.32x (LSD0.05 = 6.8)
§Means in respective columns (1 and 2) with the same letter(s) are not significantly different at 5% level of significance. Means in respective rows (1 and 2)
with the same letter(s) are not significantly different at 5% level of significance.
Table 3. Estimated removal (%) of Pb by three grasses from soils amended with varying levels of Pb.
Amount of Pb added (kg·ha1)
Grasses
37.5 75
Mean (LSD0.05 = 17.2)
(%)
Vetiver grass 79.2 45.1 62.2
Cogon grass 5.1 3.6 4.4
Carabao grass 0.5 0.5 0.5
grass is more tolerant to Pb-contaminated soil compared
with carabao grass. The important implication of the
findings of this study is that vetiver grass can be used for
phytoextraction on sites contaminated with high levels of
heavy metals, particularly Pb [58].
A field survey was conducted by Bautista [59] to iden-
tify phytoremediators present in the selected cities in the
province of Nueva Ecija, Philippines. The plants found in
the heavy traffic area of Cabanatuan City were the
“balite” (Ficus bengalensis) and the “espada” (Sanasavi-
era trifasciata). In the heavy traffic area of San Jose City
the most common plants are the Bougainvillea (Bougain-
villea sp.) and the Cherry Pink plant. The Indian tree
(Polyalthia longifolia) and the bougainvillea (Bougain-
villea sp.) were the most common plants found along the
traffic islands of the Science City of Muñoz. In Caba-
natuan City, the balite absorbed 2.822 ppm of Pb, while
espada absorbed 2.352 ppm of Pb; in San Jose City, the
cherry pink plant absorbed 4.803 ppm, while the bou-
gainvillea absorbed 1.521 ppm of Pb; and in the Science
City of Muñoz, the Indian tree absorbed 0.217 ppm, and
the bougainvillea absorbed 0.528 ppm, respectively. Re-
sults of the chemical analysis proved that all of the plants
along the traffic islands of the three selected cities of
Nueva Ecija were phytoremediators of Pb. They were the
most effective phytoremediator of Pb among the plants in
the traffic area within the three selected cities.
As discussed previously, there are several different
methods through which phytoremediation can occur.
However, in order to maximize the success of a phy-
toremediation strategy, it is critical to have significant
metal bioavailability at a contaminated site as well as a
large quantity of plant biomass with high rates of growth.
Metal contaminants that are not soluble, may limit the
success of phytoremediation. In most Pb contaminated
soils usually less than 0.1% of the total Pb present is
bioavailable for plant uptake. The plants grown in a con-
taminated soil accumulated less Pb in both the roots and
shoots than the plants grown hydroponically in a solution
with a similar Pb concentration. The difference in uptake
was because the Pb in the solution was much more
bioavailable to the plants.
A. M. PAZ-ALBERTO, G. C. SIGUA
80
It should be noted that while hydroponic tests do not
reflect accurately the accumulation potential in terrestrial
applications, these tests could be valuable in the screen-
ing for Pb accumulating plant species and tolerance lev-
els. The second limitation in Pb phytoextraction is the
poor translocation of the metal from the roots to the har-
vestable shoots. In the plants that do translocate Pb,
translocation is less than 30% [22].
Research has been conducted in the field to improve
both the uptake and translocation of Pb through induced
hyperaccumulation, which involves soil pH adjustments
or the application of synthetic chelates. In general, the
more biomass that the plant has, the more metal can be
accumulated since the metal uptake is a function of the
overall biomass [60]. The use of fertilizers can help fa-
cilitate rapid plant establishment and growth. For most
Pb-contaminated soil, P availability is very low due to
the precipitation of Pb-P precipitation. Thus, a foliar P
fertilizer spray applied topically to the plant’s leaves and
stem increases phosphorous content in the plant, while
not confounding the Pb-P binding problem in the soil. In
a study reported by Huang et al. [50], soil to which
phosphate fertilizer was added directly showed dimin-
ished Pb bioavailability, presumably due to Pb-P pre-
cipitation, in contrast to hydroponic uptake. Furthermore,
although the foliar P application decreases Pb2+ concen-
tration in shoots by 55% and root-Pb2+ concentration by
20%, the total amount of Pb2+ accumulation increased by
115% in shoots and 300% in the roots. This is the result
of the large increase in biomass production made possi-
ble by overcoming phosphate limitations. These results
further emphasize the relationship between Pb2+ accu-
mulation and plant biomass in Pb2+ phytoextraction.
4.2. Phytoremediation Potential of Some Plant
Species from Mining Sites
The focus of this study were on the accumulation of
heavy metals in plants most commonly found in mine
tailings of Victoria, Manlayan, Benguet, Philippines and
identification of the different plant species within the
area of the study. These plant species were assumed to be
potential phytoremediation species [61].
The heavy metals extracted from the plants in the mine
tailing were Cu, Cd, Pb and Zn. The fourteen plant spe-
cies that were identified within the study were: Eleusine
indica L.; Amaranthus spinosus L.; Alternathera sessilis
L.; Portuluca oleracea L.; Fimbristylis meliacea L., Vahl,
Mikania cordata ((Burm. F.) B. l. Robins; Polygonun
barbatum L.; Achyranthes aspera L., Blumea sp., Cype-
rus alternifolus L.; Crassocephalum crepidioides (Benth.)
S. Moore; Cyperus compactus Retz.; Desmodium sp. and
Muntingia calabura L. These plants absorbed certain
metals at low and high levels. Among the plants species,
A. spinosus was found to have almost all the metals ex-
tracted in large amounts particularly Pb. The other plant
species with high concentration of Pb were A. sessilis,
Desmodium sp., P. oleracea, and A. aspera. E. indica has
the highest concentration of Zn together with M. cordata,
C. compactus, F. maliacea and A. spinosus. In contrast,
Cd was found in trace amount in soil, but high in the fol-
lowing species: C. crepidioides, P. oleracea, A. sessilis,
and C. alternifolius. Nickel was found high only in A.
sessilis and Blumea sp. but trace amount in Desmodium
sp. and F. meliacea. Also, high Cu concentrations were
found in A. spinosus and P. oleracea.
In this study, the phytoremediation potential was de-
pendent on population within species. The potential of
the surveyed species mentioned for phytoremediation
was remarkable and promising because of the presence
of heavy metals suspected to have accumulated in the
soil. Root system of these plants showed higher root to
shoot ratios compared to other plants found in the area
indicating high translocation of metals to the shoot.
These species also plays an important role in the phy-
tostabilization of metals to reduce leaching and run off.
Also, these may be transformed to less toxic forms.
These typical plants have dense root systems which can
be effective for phytostabilization and elimination of
contaminants such as Pb, Cd, Zn, As, Cu, and Ni in mine
tailing sites.
A similar study conducted in Poland was worth in-
cluding in this section. Wislocka et al. [62] studied the
bioaccumulation of heavy metals by selected plants from
uranium mining dumps in the Sudety Mountains, Poland.
They found out that the investigated plants from the ura-
nium dumps in the Sudety Mountains grew on acidic
soils with an unfavorable C/N ratio. However, the nutri-
ent status as well as relatively high CEC, and organic
matter of the soil allowed the growth of spontaneous
vegetation. Contamination by heavy metals (Pb, Zn, Cu,
Cd and Ni), being associated with the mineral assem-
blage of the spoil material, was found to be significant
within all dumps. All plants examined (Salix caprea,
Betula pendula and Rubus idaeus) accumulated high
amounts of heavy metals, but in general R. idaeus
showed lower concentration of heavy metals (except Mn)
in its leaves. However, Pb was accumulated to a similar
degree in both trees and R. idaeus. Among all the heavy
metals analyzed in the three species, Cd exhibited the
greatest accumulation rate and the Cd accumulation ratio
was several times higher for S. caprea, in comparison to
the other two species. B. pendula and R. idaeus exhibited
higher accumulation rates for Mn than S. caprea. How-
ever, the potential use of R. idaeus in monitoring metal
concentration in the environment requires further inves-
tigation. The significant positive correlation between Pb
in soil and leaves of the same tree suggest that S. caprea
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA
Copyright © 2013 SciRes. AJCC
81
should be employed for monitoring Pb in the environ-
ment.
4.3. Phytoremediation Potential of Selected
Plants for Mutagenic Agents
Research and development has its own benefits and in-
conveniences. One of the inconveniences is the genera-
tion of enormous quantity of diverse toxic and hazardous
wastes and its eventual contamination to soil and
groundwater resources. Ethidium Bromide (EtBr) is one
of the commonly used substances in molecular biology
experiments. It is highly mutagenic and moderately toxic
substance in DNA-staining during electrophoresis. Inter-
est in phytoremediation as method to solve chemical
contamination has been growing rapidly in recent years.
The technology has been utilized to clean up soil and
groundwater from heavy metals and other toxic organic
compounds in many countries like the United States,
Russia and most of European countries. Phytoremedia-
tion requires somewhat limited resources and is very
useful in treating a wide variety of environmental con-
taminants. It is in this context that Uera et al. [63] con-
ducted a study aimed to assess the potential of selected
tropical plants as phytoremediators of EtBr.
This study used tomato (Solanum lycopersicum), mus-
tard (Brassica alba), vetiver grass (Viteveria zizanioides),
cogon grass (Imperata cylindrical), carabao grass (Pas-
palum conjugatum) and talahib (Saccharum spontaneum)
to remove EtBr from laboratory wastes. The six tropical
plants were planted in individual plastic bags containing
10% EtBr-stained agarose gel. The plants were allowed
to establish and grow in the soil for 30 days. Ethidium
Bromide content of the test plant s and the soil were
analyzed before and after soil treatment. Ethidium Bro-
mide contents of the plants and soils were analyzed using
an UV VIS spectrophotometer.
Results showed a highly significant (p 0.001) dif-
ference in the ability of the tropical plants to absorb the
EtBr from the soils. Mustard registered the highest ab-
sorption of EtBr (1.4 ± 0.12 µg·kg1) followed by tomato
and vetiver grass with average uptake of 1.0 ± 0.23 and
0.7 ± 0.17 µg·kg1 EtBr, respectively. Cogon grass, tala-
hib, and carabao grass had the least amount of EtBr ab-
sorbed (0.2 ± 0.6 µg·kg1). Ethidium bromide content of
the soil planted with mustard was reduced by 10.7%.
This was followed by tomato with an average reduction
of 8.1%. Only 5.6% reduction was obtained from soils
planted to vetiver grass. Soils planted to cogon grass,
talahib and carabao grass had the least reduction of
1.52% from its initial EtBr content (Table 4 and Figure
1). Mustard had the highest potential as phytoremediator
of EtBr in soil. However, the absorption capabilities of
the other test plant may also be considered in terms of
period of maturity and productivity. Uera et al. [63]
recommended that a more detailed and complete investi-
gation of the phytoremediation properties of the different
plants tested should be conducted in actual field experi-
ments. Plants should be exposed until they reach maturity
to establish their maximum response to the toxicity and
mutagenecity of EtBr and their absorbing capabilities.
Different plant parts should be analyzed individually to
determine the movement and translocation of EtBr from
soil to the tissues of the plants. Since this study has an
increased amount of EtBr application should be explored
in future studies. It is suggested therefore that a larger,
more comprehensive exploration of phytoremediation
application in the management of toxic and hazardous
wastes emanating from biotechnology research activities
should be considered especially on the use of vetiver
grass, a very promising tropical perennial grass.
4.4. Phytoremediation Potential of Selected
Tropical Plants for Acrylamide
Environmentally hazardous and health risk substances in
animals and humans in the environment have increased
as a result of continuing anthropogenic activities. Exam
ples of these activities are food processing, laboratory,
food production, industrial and other relative activities
that use various forms of acrylamide. All acrylamide in
Table 4. Levels of EtBr in soils and relative reduction of EtBr in soils after 30 days.
Plants (Treatments) Initial Level in Soil (μg·kg1) Final Level in Soil (μg·kg1) Percent Reduction in Soil
Tomato 19.7 18.1 ± 0.17 8.12b§
Mustard 19.7 17.6 ± 0.23 10.66a
Vetiver grass 19.7 18.6 ± 0.23 5.58b
Talahib 19.7 19.4 ± 0.15 1.52c
Carabao grass 19.7 19.4 ± 0.20 1.52c
Cogon grass 19.7 19.4 ± 0.21 1.52c
§Means in column followed by a common letter(s) are not significantly different from each other at p 0.05.
A. M. PAZ-ALBERTO, G. C. SIGUA
82
0.0
2.0
4.0
6.0
8.0
10.0
TOMATO MUSTARD VETIVER TALAHIB CARABAO COGON
TROPICAL PLANTS
PLANT UPTAKE OF EtBr (g kg
-1
)
a
b
b
c c c
Figure 1. Average uptake of EtBr by the different tropical plants. Uptake of EtBr by different tropical plants are significantly
different (p 0.05) when superscripts located at top of bars are different.
the environment are man-made. It is the building block
for the polymer, polyacrylamide, which is considered to
be a non-toxic additive. However, if the polymerization
process is not perfect and complete, the polyacrylamide
may still contain acrylamide which is toxic and may pose
risks and hazards to the environment. Another form of
acrylamide may pose danger as well in the environment
is the acrylamide monomer, also a very toxic organic
substance that could affect the central nervous system of
humans and is likely to be carcinogenic.
Phytoremediation could be a tool to somehow absorb
this neurotoxic agent and lessen the contamination in the
soil. This technology could lessen the soil and water
contamination by acrylamide thereby limiting the expo-
sure of animals and humans. This technique may also
help solve the problem of disposing of contaminated
acrylamide waste materials. Thus, Paz-Alberto et al. [63]
conducted a study 1) to evaluate phytoremediation po-
tentials of some selected tropical plants in acrylamide
contaminated soil; and 2) to compare the performance of
tropical plants in absorbing acrylamide through accumu-
lation in their roots, stems, and leaves. The 200 grams
polyacrylamide gel (PAG) was poured in each pot and
mixed thoroughly with the soil by stirring manually. The
soil was watered with 1,000 ml water and the test plants
were transplanted after three days. Plant samples were
collected at 45 days and 60 days after being planted onto
PAG contaminated soil. The mustard and pechay were
collected after 45 days of exposure while vetiver grass,
hogweeds, snake plant, and common sword fern were
collected after 60 days of exposure.
Among the plants tested, the highest concentration of
acrylamide was absorbed by the whole plant of mustard
(6512.8 mg·kg1) compared with pechay (3482.7 mg·kg1),
fern (2015.4 mg·kg1), hogweeds (1805.3 mg·kg1),
vetiver grass (1385.4 mg·kg1) and snake plants (887.5
mg·kg1). Results of the study regarding the acrylamide
absorption of the whole plants of mustard and pechay
conformed to previous findings of other studies (Figure
2). Two members of Brassica family, Brassica juncea L.
(mustard) and Brassica chinensis L. (pechay) were found
to be effective in removing wide ranges of contaminants.
Mustard, pechay, and fern plants had 60% survival rate
while hogweeds had 80% survival rate. Snake plant and
vetiver grass had 100% survival rate.
All the test plants planted in soil without acrylamide
had survival rate of 100%. The 100 percent survival rate
of vetiver grass and snake plant was due to the tolerance
of these plants to acrylamide (Table 5). These findings
could be attributed to the extraordinary features of
vetiver grass such as its massive and deep root system
and heavy biomass including its highly tolerance to ex-
treme soil conditions like heavy metal toxicities and high
metal concentration.
Results of the study proved that all the test plants are
potential phytoremediators of acrylamide. However,
mustard and pechay were the most effective as they ab-
sorbed the highest acrylamide concentrations in their
roots, shoots and the whole plants. On the other hand,
vetiver grass and snake plant had the highest uptake of
acrylamide even though these plants did not absorb the
highest acrylamide concentration. Therefore, these two
plants can be considered as the best phytoremediator of
acrylamide because they are perennial plants with heav-
ier biomass with long, dense and extended root system.
As such, these plants are capable of absorbing acryla-
mide in the soil for a long period of time.
As preventive measures and for application purposes,
vetiver grass and snake plants could be planted along and
around the wastewater treatment ponds of laboratories
Copyright © 2013 SciRes. AJCC
A. M. PAZ-ALBERTO, G. C. SIGUA 83
4.7 5.6
10.5
30.6
1.6
29.4
0
5
10
15
20
25
30
35
ACRYLAMIDE UPTAKE (kg ha
-1
)
Mustard Pechay Hogweed Vetiver Fern Snake Plant
TEST PLANTS
bb
b
c
aa
Figure 2. Comparative amount of acrylamide uptake among the different tropical plants. Acrylamide uptakes among the
different tropical plants are significantly different (p 0.05) when superscripts located at top of bars are different.
Table 5. Survival rate and weight of test plants at harvest.
Test Plants Survival Rate (%) Weight at Harvest (g)
Mustard (Brassica juncea L.) 60 1.3
Pechay (Brassica chinensis L.) 60 2.9
Hogweed (Portulaca oleracea L.) 80 10.5
Vetiver (Vetiveria zizaniodes L.) 100 39.7
Fern (Nephrolepsis cordifolia L.) 60 1.4
Snake plant (Sanseviera trifasciata Prain) 100 59.6
using polyacrylamide gel. These plants can prevent fur-
ther migration of pollutants to the environment aside
from making the ponds more resistant to soil erosion.
Further studies are suggested to evaluate acrylamide
contaminations from laboratory washing, primary treat-
ment pond, and seepage ponds that have earth dikes.
Vetiver grass and snake plants are recommended for fur-
ther phytoremediation studies for longer period of time to
test the reduction of acrylamide in soil. Moreover, the
outcome of acrylamide accumulation in the plants is also
recommended for further study in conjunction with la-
beled-carbon tracer to determine its effects on the plants.
5. Outlook
Phytoremediation using “green plants” has potential
benefits in restoring a balance in stressed environment. It
is an emerging low cost technology, non-intrusive, and
aesthetically pleasing using the remarkable ability of
green plants to metabolize various elements and com-
pounds from the environment in their tissues. Phytore-
mediation technology is applicable to a broad range of
contaminants, including metals and radionuclides, as
well as organic compounds like chlorinated solvents,
polycyclic aromatic hydrocarbons, pesticides, explosives,
and surfactants. However, phytoremediation technology
is still in its youthful development stages and full scale
application is still inadequate. As with all new technol-
ogy, it is important to proceed with caution.
The largest barrier to the advancement of phytoreme-
diation, however, may be public opposition to genetic
modification in general. Because all natural hyperaccu-
mulator species are small in size, genetic modification
can be used to introduce this technology to other species
or to increase the biomass of the natural hyperaccumula-
tors in order to create effective phytoremediators. This
public opposition was the same fears that surround the
issue of genetic modification of crops, and includes con-
cerns regarding decreased biodiversity, the entry of po-
tentially harmful genes into products consumed by hu-
mans, and the slippery slope created by introducing and
transferring novel, foreign DNA between non-related
species. Nonetheless, the benefits of using phytoremedia-
tion to restore balance to a stressed environment seem to
far outweigh the costs.
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... Plants can also reduce the mobility of heavy metals in the soil, a process known as phytostabilization. In this case, the pollutants may be immobilized from the soil to the surface of the roots or be in the rhizosphere of the plant [17,22]. The soils are constantly contaminated with toxic substances, including pesticide residues and heavy metals, and the surface of such land is increasing. ...
... Phytoremediation technology is based on the ability of plants to remove toxic substances from the environment or transform them into safe compounds-metabolites. It is a natural process carried out by plants to purify and stabilize pollutants in the environment [22]. Several phytoremediation methods are described below and shown in Figure 1. ...
... The depth of soil cleaning is limited by the depth of the plant's root system. Phytoextraction can remove metals such as Cr, Cd, Cu, Co, Ag, Zn, Ni, Mo, Pb, and Hg [22,25,26]. This method, unlike phytostabilization, removes contaminants from the soil and not only stabilizes it. ...
Article
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Soil and air pollution are main problems posing a serious threat to human health. Traditional physical and chemical soil remediation methods affect the soil ecosystem and are rather costly. Since the main purpose of soil remediation is not only to remove pollutants but also to restore soil health, the method of phytoremediation is becoming extremely relevant. Phytoremediation is an environmentally friendly and natural process of removing pollutants from the environment. Cleaning up contaminated sites and enabling re-use without harming future users requires the implementation of environmentally friendly and economically attractive technologies. Phytoremediation does not adversely affect the structure and biological life of the soil. Concerning on-site cleaning in situ. Hyperaccumulator plants can accumulate heavy metals from the soil, which is the so-called phytoextraction. The ability of trees and shrubs to effectively remove solid particles from the air has also been proven. However, it is not always possible to grow large plants in polluted areas. Therefore, the main goal of the research was to explore previous studies on the phytoremediation capability of herbaceous plants, in particular, their phytoextraction capacity. Another major issue was to study the main methods of improving plant phytoextraction. The results obtained show that grass can be a good solution for natural ecosystem cleanup. It is also necessary to pay attention to the impact of phytoextraction-improving substances on soil health.
... Plants can also reduce the mobility of heavy metals in the soil, a process known as phytostabilization. In this case, the pollutants may be immobilized from the soil to the surface of the roots or be in the rhizosphere of the plant [17,22]. The soils are constantly contaminated with toxic substances, including pesticide residues and heavy metals, and the surface of such land is increasing. ...
... Phytoremediation technology is based on the ability of plants to remove toxic substances from the environment or transform them into safe compounds-metabolites. It is a natural process carried out by plants to purify and stabilize pollutants in the environment [22]. Several phytoremediation methods are described below and shown in Figure 1. ...
... The depth of soil cleaning is limited by the depth of the plant's root system. Phytoextraction can remove metals such as Cr, Cd, Cu, Co, Ag, Zn, Ni, Mo, Pb, and Hg [22,25,26]. This method, unlike phytostabilization, removes contaminants from the soil and not only stabilizes it. ...
Article
Full-text available
Soil and air pollution are main problems posing a serious threat to human health. Traditional physical and chemical soil remediation methods affect the soil ecosystem and are rather costly. Since the main purpose of soil remediation is not only to remove pollutants but also to restore soil health, the method of phytoremediation is becoming extremely relevant. Phytoremediation is an environmentally friendly and natural process of removing pollutants from the environment. Cleaning up contaminated sites and enabling re-use without harming future users requires the implementation of environmentally friendly and economically attractive technologies. Phytoremediation does not adversely affect the structure and biological life of the soil. Concerning on-site cleaning in situ. Hyperaccumulator plants can accumulate heavy metals from the soil, which is the so-called phytoextraction. The ability of trees and shrubs to effectively remove solid particles from the air has also been proven. However, it is not always possible to grow large plants in polluted areas. Therefore, the main goal of the research was to explore previous studies on the phytoremediation capability of herbaceous plants, in particular, their phytoextraction capacity. Another major issue was to study the main methods of improving plant phytoextraction. The results obtained show that grass can be a good solution for natural ecosystem cleanup. It is also necessary to pay attention to the impact of phytoextraction-improving substances on soil health.
... This controls the transfer of water and solutions, including those containing heavy metals. According to Alberto et al. (2013), plants can decontaminate heavy metals through roots transported to the top. Some heavy metals will be inhibited or prevented from spreading in this process. ...
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p>The use of biochar for remediating heavy metal-polluted soils is still partial. Various methods of controlling soil pollution are currently being implemented by combining several methods. One of which is coating the biochar with humic acid to increase the effectiveness of nutrient uptake by plants. This study aimed to elucidate the effect of humic acid-coated biochar (HCB) on reducing Pb and Cu in the soil to improve plant growth. Treatments tested were combinations of two factors. The first factor was the dose of HCB, namely D0 = control (without HCB), D1 = 15 t HCB ha<sup>-1</sup>, D2 = 30 t HCB ha<sup>-1</sup>, and D3 = 45 t HCB ha<sup>-1</sup>. The second factor was the type of plants, namely spinach (P1), water spinach (P2) and mustard green (P3). Twelve treatments were arranged in a randomised block design with three replications. The parameters observed were plant height, plant leaf area, plant stem diameter, and plant fresh weight. The results showed that the best plant growth was achieved at a dose of 30 t HCB ha<sup>-1</sup>. The mustard green had the highest Pb uptake (0.025 g pot<sup>-1</sup>), and the lowest Pb uptake (0.014 g pot<sup>-1</sup>) was observed for water spinach. The highest Cu uptake (0.443 g pot<sup>-1</sup>) was observed in water spinach, followed by spinach (0.282 g pot<sup>-1</sup>) and mustard green (0.143 g pot<sup>-1</sup>). In general, the amount of Pb reduced by plants ranged from 40.04 to 87.28%, and the amount of Cu by plants ranged from 8.63 to 40.23%.</p
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