31
________________________________________________________________
Removal of contaminants using plants: A review
Current trends in Biotechnology and chemical research,1-11 (2012)
32
Global development raises new challenges, especially in the field of environmental
protection1. The demand for a country’s economic agricultural and industrial
development outweighs the demand for a safe, pure & natural environment; therefore, it
is the industrial, economic & agricultural developments that are often linked to polluting
environment2. It has been found that human activities leads to substantial accumulation
of heavy metals and other pollutants in soils. These are produced from industrial
activities like mining, smelting, refining & manufacturing process3. The industries
discharge their effluents into coastal water bodies and contribute to a variety of toxic
substances on living organisms in food chain4 by bioaccumulation and bio-
magnification5
All industries release different types of pollutants but they discharge one or two types of
substances which cause major harm. Waste water treatment is essential for health,
aesthetic, ecological and other purposes which has become a serious problem6,7 and
hence there is urgent need for water treatment.
Pulp and paper mill categorized as one of the twelve most polluting industries in India,
releases environmentally hazardous8 liquid effluents containing several toxic and non-
biodegradable organic materials and heavy metals. The heavy metals9,10 are of great
ecological significance due to their toxicity and accumulative behavior. The manufacture
of paper consumes 340-425 cubic metric water per tonne and bulk of it comes out as
waste water11. This industry releases about 80% of the used water back into the
streams8.
There are several methods employed for the treatment of waste water like reverse
osmosis, ground injection, land application, constructed wetlands etc.12-21.The traditional
physico-chemical processes for treatment involve high energy and large capital
investment, whereas aquatic plant based cost effective technologies can be adopted by
developing countries for treatment of waste water, especially contaminated by heavy
metals22-24.
33
The important aspects of phytoremediation have been summarized in several
comprehensive reviews25-30 . The word phytoremediation comes from Greek word phyto
which means plant and Latin word remediation which means to remove, which refers to
a diverse collection of plants based technologies that use either naturally occurring, or
genetically engineered plant to clean contaminants31-32. It is a clean, efficient,
inexpensive and environment friendly technology. It is a non-invasive alternative
technology for engineering-based remediation methods33. The primary motivation
behind the development of phytoremediation technologies is the potential for low-cost
remediation34-35. Phytoremediation31 is the use of green plant-based systems to
remediate contaminated soils, sediments and water. Such plants are known as pollution
mitigators.
2.1 Phytoremediation of wastewater
According to ecological classification36 on the basis of vegetative organs to air, water
and ground all non-terrestrial macrophytes are subdivided into five groups:
1. Amphibious plants
2. Plants rooted to the bottom of a water body with leaves emerging at the water
surface
3. Rooted plants with vegetative organs submerged in water
4. Plants floating at the water surface without connection to the bottom
5. Completely submerged uprooted plants.
The peculiarities of accumulation of contaminants in the plant organs are of
importance for the screening of these macrophyte groups to identify the plants
effectively accumulating contaminants.
Plant species with potential for phytoremediation should possess the following
properties:
1. These plants should accumulate, extract, transform, degrade or volatilize
contaminants at the levels that are toxic to ordinary plants.
2. The plants must have fast growth and high yield and should have ability to
remediate multiple pollutant simultaneously37-38.
34
Phytoremediation targets currently include contaminated metals, metalloids, petroleum
hydrocarbons, pesticides, explosives, chlorinated solvents and industrial by-products.
The use of aquatic plants to reduce pollutant levels from sewage and industrial effluents
has been suggested by many researchers39-40.
The aquatic plant species utilized for phytoremediation showed a large range of heavy
metal tolerance, and secondary treated wastewater did not affect these species
adversely; to the contrary, growth was prompted 41-44.One or more trace elements may
affect uptake and metabolism of macro and other elements in the plants. There is
reduction in heavy metals to below toxic levels45. There is interaction between elements
in plants and growth medium33. Excess of macronutrients may interfere with trace
elements46.
The change in pH, EC and colour unit could be due to changes in BOD, COD, TS, TDS,
TSS and lignin through total phytoremediation. However, a moderate reduction in all the
parameters in the non-phytoremediated effluents might be due to the presence of
microorganisms and their activities29. In phytoremediated effluents the probability of
microbial degradation and its further enhancement is due to the availability of more
niches in response to rhizosperic effect.
2.2 Plant-based technologies of phytoremediation
2.2.1 Rhizofiltration
Metal pollutants in industrial-process water and in groundwater are most commonly
removed by precipitation or flocculation, followed by sedimentation and disposal of the
resulting sludge 34. A promising alternative to this conventional clean-up method is
rhizofilration. Rhizofiltration removes contaminants from water and aqueous waste
streams, such as agricultural run-off, industrial discharges, and nuclear material
processing wastes29,47. Absorption and adsorption by plant roots play a key role in this
technique, and consequently large root surface areas are usually required. In research
associated with Epcot Centre, closed systems with recirculating nutrients have exhibited
35
the benefits of Rhizofiltration and biofiltration using a variety of species (such as mosses
and scented geraniums)48.
2.2.2 Phytostabilisation
It also known as phytorestoration, is a plant based remediation technique that stabilizes
wastes and prevents exposure pathway via wind and water erosion; provides hydraulic
control, which suppresses the vertical migration of contaminants into groundwater; and
physically and chemically immobilizes contaminants by root sorption and by chemical
fixation with various soil amendments25,32,49-51.
Erosion and leaching can mobilize soil contaminants resulting in aerial or waterborne
pollution of additional sites. In phytostabilization, accumulation by plant roots or
precipitation in the soil by root exudates immobilizes and reduces the availability of soil
contaminants. Plants growing on polluted sites also stabilize the soil and can serve as a
groundcover thereby reducing wind and water erosion and direct contact of the
contaminants with animals. Significant phytostabilization projects have been employed
in France and the Netherlands52-54.
The goal of phytostabilization is not to remove metal contaminants from a site, but
rather to stabilize them and reduce the risk to human health and the environment.
2.2.3 Phytoextraction
Phytoextraction involves the removal of toxins, especially heavy metals and metalloids,
by the roots of the plants with subsequent transport to aerial plant organs29,55. Pollutants
accumulated in stems and leaves are harvested with accumulating plants and removed
from the site. Phytoextraction can be divided into two categories: continuous and
induced29. Continuous phytoextraction requires the use of plants that accumulate
particularly high levels of the toxic contaminants throughout their lifetime. The roots of
the established plants absorb metal elements from the soil and translocate them to the
above-ground shoots where they accumulate (hyperaccumulators), while induced
phytoextraction take place if metal availability in the soil is not adequate for sufficient
plant uptake, chelates or acidifying agents may be used to liberate them into the soil
solution56-58.
36
2.2.4 Phytovolatization
Some metal contaminants such as As, Hg, and Se may exist as gaseous species in
environment. There are some naturally occurring or genetically modified plants that are
capable of absorbing elemental forms of these metals from the soil, biologically
converting them to gaseous species within the plant and volatized into the atmosphere
through the stomata59-61.
There are certain members of Brassicaceae are capable of releasing up to 40 g Se ha-1
day-1 as various gaseous compounds. Some aquatic plants such as cattail (Typha
latifolia l.) are also good for Se phytoremediation. Arabidopsis thaliana L. and tobacco
(Nicotiana tabacum L.) have been genetically modified with bacterial organomecurial
lyase and mercuric reductase genes. These plants absorb elemental Hg (II) and methyl
mercury from the soil and release volatile Hg (0) from the leaves into the
atmosphere 62-65.
This remediation method has the added benefits of minimal site disturbance, less
erosion, and no need to dispose of contaminated plant material66.
2.2.5 Phytodegredation
In phytodegredation, organic pollutants are converted by internal or secreted enzymes
into compounds with reduced toxicity29,49,59. For instance, the major water and soil
contaminant trichloroethylene (TCE) was found to be taken up by hybrid poplar trees,
Populus deltoides x nigra, which breaks down the contaminant into its metabolic
components57. TCE and other chlorinated solvents can be degraded to form carbon
dioxide, chloride ion and water 60.
37
Figure 2.1
2.3 Biodiversity prospects for phytoremediation of metals in the environment
Many hazardous waste sites contain a mixture of contaminant like salts, organics,
heavy metals, trace elements, and radioactive compounds67-69. The simultaneous clean-
up of multiple, mixed contaminants using conventional chemical and thermal methods
are both technically difficult and expensive; these methods also destroy the biotic
component of soils. Biodiversity prospects offer a several opportunities of which the
most important is to save as much as possible of the world’s immense variety of
ecosystems. It would lead to the discovery of wild plants that could clean polluted
environments of the world. The desire to capitalize on this new ideas need to provide
strong incentives for conserving nature. Aquatic plants in fresh water, marine and
estuarine systems act as receptacle for several metals70-75.
Examples of simpler phytoremediation systems that have been used for years are
constructed or engineered wetlands, often using cattails to treat acid mine drainage or
38
municipal sewage. Our work extends to more complicated remediation cases: the
phytoremediation of a site contaminated with heavy metals and/or radionuclides
involves "farming" the soil with selected plants to "biomine" the inorganic contaminants,
which are concentrated in the plant biomass25,76. For soils contaminated with toxic
organics, the approach is similar, but the plant may take up or assist in the degradation
of the organic compounds68. Several sequential crops of hyper accumulating plants
could possibly reduce soil concentrations of toxic inorganics or organics to the extent
that residual concentrations would be environmentally acceptable and no longer
considered hazardous. The potential also exists for degrading the hazardous organic
component of mixed contamination, thus reducing the waste (which may be
sequestered in plant biomass) to a more manageable radioactive one.
For treating contaminated wastewater, the phytoremediation plants are grown in a bed
of inert granular substrate, such as sand or pea gravel, using hydroponic or aeroponic
techniques. The wastewater, supplemented with nutrients if necessary, trickles through
this bed, which is ramified with plant roots that function as a biological filter and a
contaminant uptake system. An added advantage of phytoremediation of wastewater is
the considerable volume reduction attained through evapotranspiration77.
Phytoremediation is well suited for applications in low-permeability soils, where most
currently used technologies have a low degree of feasibility or success, as well as in
combination with more conventional clean up technologies (electromigration, foam
migration, etc.). In appropriate situations, phytoremediation can be an alternative to the
much harsher remediation technologies of incineration, thermal vaporization, solvent
washing, or other soil washing techniques, which essentially destroy the biological
component of the soil and can drastically alter its chemical and physical characteristics
as well, creating a relatively nonviable solid waste. Phytoremediation actually benefits
the soil, leaving an improved, functional, soil ecosystem at costs estimated at
approximately one-tenth of those currently adopted technologies.
Phytoremediation is actually a generic term for several ways in which plants can be
used to clean up contaminated soils and water. Plants may break down or degrade
39
:organic pollutants, or remove and stabilize metal contaminants. This may be done
through one of or a combination of the methods. The methods used to phytoremediate
metal contaminants are slightly different to those used to remediate sites polluted with
organic contaminants.
Tthe various type of phytoremediation process like, Phytoextraction, Rhizofiltration,
Phytostabilization, Phytovolatization, phytodegredation has been reported78. The key
factor for the success of remediation process depends on characteristics to mine waste,
geo climatic conditions, types of amendment used and selection of plants species.
Evaluation of the different fraction of bioavailable metals, their mobility in plant parts and
growth of the plant species on contaminated side could be helpful for phytoremediation
of metallic waste. Data is given in Table 2.1.
Table 2.1: Phytoremediation process
Mechanism Process Media Contaminants Plants References
Phytoextrac
-tion
Hyper-
accumulation
Soil,
sediment,
brown
fields
Metals: Cd,Cu, Ni,
Pb, Zn with EDTA
addition of Pb,
selenium
Indian mustard,
sunflowers,
pennycress, Crusifers,
Rape seed plants ,
barley, alyssum
62
Hyper-
accumulation
Soil
sediment
lead (Pb), cadmium
(Cd), chromium (Cr),
copper (Cu), nickel
(Ni), and zinc (Zn)
with EDTA addition
Brassica juncea (Indian
mustard) and
Helianthus anuus
(sunflower)
63,64
Hyper-
accumulation
and
Contaminant
extraction
Soil,
sediment,
Zn, Co, Cu Se , Pb
and Cd
Brassicaceae,
Fabaceae,
Euphorbiaceae,
Asteraceae,
Lamiaceae, and
50
40
Scrophulariaceae
Contaminant
extraction
and capture
Soil,
sediment,
sludges
Metals:Ag, Cd, Co,
Cr, Cu, Hg, Mn, Mo,
Ni, Pb, Zn;
Radionuclides:90Sr,
137Cs, 239Pu,
234U,238U
Indian mustard,
sunflowers, hybrid
35
Contaminant
extraction
Soil Metals: Ag, Cd, Pb Perennial ryegrass
(Lolium perenne)
65
Rhizofiltra-
tion
Rizosphere
accumulation
Ground
water ,
waste
water
logoons
or
created
weetland
s
Metals: Cd,Cu, Ni,
Pb, Zn;
Radionuclides: 90Sr,
137Cs, 238U
Aquatic plants-
emergents
(Bullrush,cattail,
pondwed, arrow root,
duckweed)
Sebmergents
(algae,,hydrilla,stonewo
rt,parrotfeather)
62
Contaminant
extraction
and capture
Ground
water
and
surface
water
Metals, radionuclides Sunflowers, Indian
mustard, water hyacinth
35
Phytostabili-
zation
Contaminant
containment
Soil,
sediment
sludges
As, Cd, Cr, Cu, Hs,
Pb, Zn
Indian mustard, hybrid
poplars, grasses
35
Complexa-
tion
Soil,
sediment
Metals: Cd,Cu, Ni,
Pb, Zn,Cr,As,Se,U;
Hydrophobic
Grasses with fibrous
root
62
41
Organism
Phytodegra-
dation
Contaminant
destruction
Soil,
sediment
sludges
Organic compounds,
chlorinated solvents,
phenols, herbicides,
munitions
Algae, stonewort,
hybrid poplar, black
willow, bald cypress
35
Degradation
in plants
Soil,
ground
water,
land fill
leachate
land
applicatio
n of
waste
water
Herbisides ( atrazine,
alachlor) Aromatics
(BTEX) Choriated
alipatics (TCE),
Nutrients ( NO3-,
NH4+,PO3
-),
Alnmunition waste
TNT, RDX
Phreatophyte trees
(Popular willow, cotton
wood, grasses rye,
Bermuda sorghum
fescue)
Legumes clover alfalfa,
cowpeas
62
Phytovolatili
zation
Contaminant
extraction
from media
and release
to air
Soil,
sediment
sludges
Chlorinated solvents,
some inorganics (Se,
Hg, and As)
Poplars, alfalfa black
locust, Indian mustard
35
Volatilization
by leaves
Soil,
sediment,
ground
water
Se, Hg, and Ti Poplars, Indian
mustard, Canola
,Tobacco plant
62
volatilization
to the
atmosphere
Soil Inorganic pollutant
Ni,Zn, Cd, As, Se,
Cu,Co,Pb,Hg, and
Radionuclides
Soil plants 66
42
A comparison of the performance of process phytoextraction has been reported by
several scientists78-80.They used the various processes to extract the contaminants from
the various plants.
Phytoextraction involves the removal of toxins, especially heavy metals and metalloids,
by the roots of the plants with subsequent transport to aerial plant organs29,50. Pollutants
accumulated in stems and leaves are harvested with accumulating plants and removed
from the site. In the case of heavy metals, chelators like EDTA assist in mobilization and
subsequent accumulation of soil contaminants such as lead (Pb), cadmium (Cd),
chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn) in Brassica juncea (Indian
mustard) and Helianthus anuus (sunflower)80-81. The ability of other metal chelators
such as CDTA, DTPA, EGTA, EDDHA, and NTA to enhance metal accumulation has
also been assessed in various plant species82-83. However, there may be risks
associated with using certain chelators considering the high water solubility of some
chelator- toxin complexes which could result in movement of the complexes to deeper
soil layers50,84 and potential ground water and estuarian contamination. There data is
also given in Table no. 1
G. M Pierzynski85explain the Applicable Contaminants/Constituents amenable to
phytoextraction include Metals, (Ag, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Zn.),
Metalloids (As, Se), Radionuclides, ( 90Sr, 137Cs, 239Pu, 238U, 234U). The accumulation of
organics and subsequent removal of biomass generally has not been examined as a
remedial strategy.
The relative degree of uptake of different metals will vary. Experimentally-determined
phytoextraction coefficients [ratio of g metal/g dry weight (DW) of shoot to g metal/g DW
of soil] for B. junce 85 indicate, for example, that lead was much more difficult to take up
than cadmium: (Table 2.2).
43
Table 2.2: Determined phytoextraction coefficients
Metal Phytoextraction Coefficient
Cr6+ 58
Cd2+ 52
Ni2+ 31
Cu2+ 7.0
Pb2+ 1.7
Cr3+ 0.1
Zn2+ 17.0
Table 2.3: Contaminated soil concentrations
Metal Soil Concentration Reference
As 1,250 mg/kg 71
Cd 9.4 mg/kg 71
11 mg/kg 72
13.6 mg/kg 73
Cd uptake in vegetables 2000 mg/kg 74
Pb
110 mg/kg 72
625 mg/kg 70
Zn 444 mg/kg 73
1,165 mg/kg 72
Se 40 mg/kg 75
44
Contaminated soil concentrations used in research studies or found in field
investigations are given below in Table 2.385-89. These are total metal concentrations;
the mobile or available concentrations would be less.
The study on assessment of heavy metal accumulation in certain aquatic macrophytes
used as biomonitors, in comparison with water and sediments (abiotic monitors) for
phytoremediation carried out by some workers90. Roots, stems and leaves of native
aquatic plants (biomonitors) represented by seven species: Ipomoea aquatica Forsk,
Eichhornia crassipes, (Mart.) Solms, Typha angustata Bory & Chaub, Echinochloa
colonum (L.) Link Hydrilla verticillata (L.f.) Royle, Nelumbo nucifera Gaerth. And
Vallisneria spiralis L. along with surface sediments and water were analyzed for Cd, Co,
Cu, Ni, Pb and Zn contamination.
The greater accumulation of heavy metals was observed in Nelumbo nucifera and the
poor content in Echinochloa colonum. Based on the concentration and toxicity status
observed in the lake's vegetation, the six heavy metals are arranged in the following
descending order: Zn > Cu > Pb >Ni > Co > Cd compared with the standard, normal
and critical toxicity range in plants. The detected values of Cd and Pb fall within normal
range, while that of Co and Ni were within the critical range. However, Zn and Cu
showed the highest accumulation with alarming toxicity levels, which are considered as
one of the most hazardous pollutants in Pariyej reservoir. Species like Typha angustata
and Ipomoea aquatica are also proposed as bioremediants, which are the two most
useful plant species in phytoremediation studies due to their ability to accumulate heavy
metals in high concentration in the roots. The results showed the significant differences
in accumulation of metals like Zn, Cu and Pb in different plant organs, in roots than that
of stems and leaves90. Data is given in Table no.2.4.
45
Table 2.4: Heavy metal concentration in sediments and water and ratios between
the concentration in the sediments and that in the water
Metal Sediment (ppm) Water (ppm) Sediment / Water
Cd 1.27 0.74 1.70
Co 34.88 1.76 19.81
Cu 105.78 19.67 5.38
Ni 58.08 10.13 5.73
Pb 9.47 6.11 1.55
Zn 2114.82 160.70 13.16
There are three main strategies currently exist to phytoextraction inorganic substances
from soils using plants81: (1) use of natural hyperaccumulators; (2) enhancement of
element uptake of high biomass species by chemical additions to soil and plants; and
(3) phytovolatization of elements, which often involves alteration of their chemical form
within the plant prior to volatilization to the atmosphere. Concentrating on the
techniques that potentially remove inorganic pollutants such as Ni, Zn, Cd, Cu, Co, Pb,
Hg, As, Se, and radionuclides, we review the progress in the understanding of the
processes involved and the development of the technology.(Table 2.1)
The best example of volatilization is the volatilization of mercury (Hg) by conversion to
the elemental form in transgenic Arabidopsis and yellow poplars containing bacterial
mercuric reductase91-93 (fig 2.2). In a study where the movement of volatile organics was
monitored by Fourier transform infrared spectrometry (FT-IR) in hybrid poplars (Populus
deltoides x nigra), Tamarix parviflora (saltcedar), and Medicago sativa (alfalfa),
chlorinated hydrocarbons were found to move readily through the plants, but less polar
compounds like gasoline constituents did not 53. However, amounts of the contaminant
transpired are in proportion to water flow and are relatively low, especially in the field94.
46
Found that poplar saplings can concentrate (100 ppb) and transpire methyl tertiary-butyl
ether (MTBE), a compound added to gasoline which is commonly found as a
groundwater pollutant. In a one week time period, they observed a 30% reduction in
MTBE mass in hydroponic solution by saplings at both high (1600 ppb) and low (300
ppb) MTBE concentrations, which suggested that these plants could be successful in
the phytoremediation of this toxin from groundwater94. Selenium (Se) is a special case
of a metal that is taken up by plants and volatilized. Se can also be volatilized following
conversion to dimethylselenide by microbes and algae95.
Fig 2.2: Volatilization Process
47
Applicable Contaminants to the phytovolatization include the organic contaminants such
as Chlorinated solvents include TCE, 1,1,1-trichloroethane (TCA) and carbon
tetrachloride95-96 and the inorganic contaminants Se and Hg, along with As, can form
volatile methylated species85.
Poplars have also been shown to take up the ammunition wastes 2,4,6-trinitrotoluene
(TNT), hexahydro-1,3,5-trinitro-1,3,5 triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7
tetrazocine (HMX) and partially transform them97-98. Root exudates from Datura innoxia
and Lycopersicon peruvianum containing peroxidase, laccase, and nitrilase have been
shown to degrade soil pollutants49,99 and nitroreductase and laccase together can break
down TNT, RDX, and HMX50. The plants are then able to incorpxorate the broken ring
structures into new plant material or organic soil components that are thought to be non-
hazardous.
Organic compounds are the main category of contaminants subject to
phytodegradation. In general, organic compounds with a log kow between 0.5 and 3.0
can be subject to Phytodegradation within the plant. Inorganic nutrients are also
remediated through plant uptake and metabolism. Phytodegradation outside the plant
does not depend on log kow and plant uptake.
The applicable contaminants amenable to Phytodegradation100 include Organic
contaminants Phenols, Munitions. The example of Organic contaminants is Chlorinated
solvents. TCE was metabolized to trichloroethanol, trichloroacetic acid, and
dichloroacetic acid within hybrid poplar trees. In a similar study, hybrid poplar trees were
exposed to water containing about 50 ppm TCE and metabolized the TCE within the
tree. L. A Licht et al101 explain the inorganics contaminants, nutrients: Nitrate will be
taken up by plants and transformed to proteins and nitrogen gas.
A multi-process phytoremediation system (MPPS) developed by certain workers102 that
utilizes plant/PGPR (plant growth promoting rhizobacteria) interactions to mitigate stress
ethylene effects, thereby greatly increasing plant biomass, particularly in the
48
rhizosphere. The MPPS degrades a variety of organic contaminants in soils with
accelerated remediation kinetics. Over the last two years at a petroleum impacted site in
Sarnia, ON, a decrease of ~ 50 % in CCME fractions 3 and 4 was observed. At a site in
Turner Valley, AB, 30 % remediation of total petroleum hydrocarbons was achieved in
3.5 months. Recently, we tested the MPPS in salt-impacted soils in greenhouse
experiments, with promising preliminary results.
Dushenkov S. et al.103 have been developed the subsets - phytoextration, which is
based on using high biomass crop plants in combination with a system of soil
amendments to extract heavy metals from soil, and rhizofiltration, a technology which
employs plants to remove contaminants from aqueous streams.
Rhizofiltration was also shown to be useful in the San Francisco Bay study directed by
Norman Terry (University of California, Berkely) and supported by Chevron104. A
wetland constructed next to the bay was shown to remove 89% of the Se from selenite
contaminated wastewater released from various oil refineries. The water flowing into the
wetland was measured to have 20–30 μg L−1selenite, while the outflow from the
wetland had less than 5 μg L−1 selenite104. In a study of Se removal from agricultural
subsoil drainage in the San Joaquin Valley105, a flow-through wetland system was
constructed with cells containing either a single species, or a combination of species
[e.g. Schoenoplectus robustus (sturdy bulrush), Juncus balticus (baltic rush), Spartina
alterniflora (smooth cordgrass), Polypogon monspeliensis (rabbit’s foot grass), Distichlis
spicata (saltgrass), Typha latifolia (cattail), Schoenoplectus acutus (Tule grass), and
Ruppia maritima (widgeon grass)]. Four years after planting, comprehensive analysis
showed that 59% of the Se remained in the wetland, mostly in the organic detrital layer
and surface sediment, 35% in the outflow, 4% in seepage and 2% to volatilization.
Wetland plant uptake of Se varies with species type, and parrot’s feather (Myriophyllum
aquaticum), iris-leaved rush (Juncus xiphioides), cattail, and sturdy bulrush were
particularly noted for high Se uptake potential105.
49
The applicable Contaminants/ Concentrations Constituents amenable to
phytoremediation106-108 include the Metals such as Lead (Pb2+), Cadmium (Cd2+)
Chromium (Cr6+), Copper (Cu2+), Nickel (Ni2+), Zinc (Zn2+) and Radionuclides such as
Uranium (U), Cesium (Ce), Strontium (Sr).
Contaminated water concentrations used in research studies or found in field
investigations are given below in Table 2.5.
Table 2.5: Contaminated water concentrations found in field investigations
Plant
Metal (water concentration) Ref-
No. Pb2+ Cd2+ Cu2+ Ni2+ Zn2+ Cr6+
Indian
mustard
roots
2mg/L 2mg/L 6mg/L 10mg/L 100mg
/L
4mg/L 87
Brassica
juncea
20 to
2000
g/L
20 to
2000
g/L
---- 20 to
2000
g/L
----- 20 to
2000
g/L
88
Myriophyllu
m spicatum
1 to 16
mg/L
1 to 16
mg/L
1 to 16
mg/L
1 to 16
mg/L
1 to 16
mg/L
----- 89
The different plants contain different metals in varying concentration. Sometimes these
metals are useful for environment and some time these are very hazardous. So by
using Rhizofiltration techniques106-108 the metals can be extracted from various plants.
Rhizofiltration is the recent using technique, which is gaining popularity in the field of
phytoremediation. Many scientists have work over this, with different concentration of
water we are able to extract these metals and then processed these for future use.
50
A basic research program91 for developing a phytostabilization revegetation strategy to
remediate mine tailings in arid and semi-arid ecosystems was carried out. The
researchers will monitor the bioavailability of metals for the native metal- and
droughttolerant plant species used, and determine the permanence of expected toxicity
reductions. Plants with high transpiration rates, such as grasses, sedges, forage plants,
and reeds are useful for phytostabilization by decreasing the amount of ground water
migrating away from the site carrying contaminants81. Combining these plants with
hardy, perennial, dense rooted or deep rooting trees (poplar, cottonwoods) can be an
effective combination92.
Phytostabilization has not generally been examined in terms of organic contaminants.
The following is a discussion of metals and metal concentrations, with implications
Arsenic: As (as arsenate), Cadmium: Cd, Chromium: Cr, Copper: Cu, Mercury: Hg.
Researchers109-112 have different views about the potential for use of phytoremediation
to clean up contaminated soils and water using plants comprised of two components,
one by the roots colonizing microbes and the other by plants themselves, which
accumulate the toxic compounds to further non toxic compounds. Plant assays are
highly sensitive to many environmental pollutants, including heavy metals and have
been used for monitoring the potential synergistic effects of mixtures of pollutants111. It
is green technology and most important because it’s by products can find a range of
other uses. Various Compounds viz. organic synthetic compounds, xenobiotics,
pesticides, hydrocarbon and heavy metals are among the contaminants that can be
effectively remediated by plants47.
Aquatic plants absorb elements through roots and shoots113-114. Much of the metal
uptake by plant tissue occurs by absorption to anionic sites in the cell wall115 and metals
do not actually enter the living plants. In aquatic systems, where pollutants inputs are
discontinuous and pollutants are quickly diluted, analysis of plant components provide
time-integrated information about the quality of the system116. Biomonitoring of
pollutants using some plants as accumulator species, accumulate relatively large
51
amounts of certain pollutants, even from much diluted solutions without obvious noxious
effects117.
Various macrophytes have been tested as phytoremediaors to purify water by removing
nitrogen and phosphorus, elements that cause eutrophication118-119. Aquatic
macrophytes can also remove sulphadimethoxine (drug)120 and metals like Sr, Cu, Cd,
Zn, Cr, Fe, Ni, Pb, Au, Pt and even radioactive elements121-125. According to the
component systems of aquatic macrophytes the order based on their accumulation
capacity of heavy metals as: Sediment> Root system> Stem system>Leaf system126.
It has been found that heavy metal accumulation in phytoremediators is responsible for
the decrease in total chlorophyll concentration and negatively affects the Chl a/Chl b
ratio127-128. However, the capacity to accumulate heavy metals in aboveground plant
tissues represents the suitability of the plants for metal phytoextraction129.
Some researchers have examined the naturally occurring hyper accumulators, plants
which can accumulate 10-500 times higher levels of elements than crops. Plants act as
hyper accumulators in following ways:
(1) The plants must be able to tolerate high levels of the element in root and shoot
cells.
(2) Hyper tolerance is the key property which makes hyper accumulation possible.
Hypertolerance is believed to result from vacuolar compartmentalization and
chelation130-131.
The plant must have the ability to translocate an element from roots to shoots at high
rates. In hyperaccumulators, shoot element concentration can exceed root levels132-134.
There must be a rapid uptake rate for the element at levels which occur in soil solution.
Different patterns have been observed in different groups of hyperaccumulators132,135.
52
The water hyacinth was chosen because it is well-known for its adaptability,
alkalescence resistance, fertilizer resistance and tolerance to diseases. Water hyacinth
sustains a natural growth in water with a pH of 9. Apart from improving water quality
and pollution control, chemical analysis has shown that water hyacinth is rich in
nutrition; with organic matter particularly protein content, vitamins, minerals, fertilizer,
chemicals and energy (in the form of biogas) which reduced its nuisance value136.
Investigations on bioaccumulation and toxicity of Cu, and Cd and uptake of Pb by
Vallisneria spirallis has been performed under the laboratory conditions137-138, which
indicates V.spirallis as potential as a phytoremediator. Duckweed is an aquatic, floating
plant. It is widely distributed in the world from tropical to the temperate zones, from fresh
water to brackish estuaries. Duckweed possess physiological properties like small size,
high multiplication rates and vegetative propagation and can be used in a wide range of
pH (3.5-10) which make them an ideal test system. It can accumulate certain chemicals
and may serve as biological monitors139.
2.4 Phytoremediators
2.4.1 Eichhornia crassipes (Water hyacinth)
Eichhornia crassipes is a floating plant and has an astonishing reproductive rate and
its roots can directly absorb the suspended particulate. Research over the past
decades has proved that some floating plants, such as water hyacinth (E.crassipes),
water lettuce (Pistia stratiotes), pennywort (Hydrocotyle umbellate), duckweed
(Lemna minor), water peanut (Alternanthera philoxeroides) and lidded cleistocalyx
(Cleistocalyx operculatus), have the greatest effects on purifying eutrophic water140-
142. Therefore, water hyacinth and duckweed were applied in the treatment of
wastewater143-144.
The potential of Water hyacinth on the nutrient regime of a lake ecosystem for
sewage treatment has been found out by many researchers145-146. Water hyacinth is
a prolific aquatic weed of cosmopolitan distribution with a huge potential for the
removal of vast range of pollutants from waste water147-151. All these models show
logistic growth in the plant, and are dependent on environmental factors.
53
The ability of E.crassipes to take up and translocate As (V), Cd (VI), Cr (VI), Cu (II),
Ni (II) and Sc (VI) under controlled conditions has been studied by some workers152.
According to Jain et al.136 the water hyacinth has ideal characteristics for water
purification and pollution control. The production of high quality vegetable protein,
vitamins, minerals, fertilizers, chemicals and energy in the form of biogas from water
hyacinth had reduced its nuisance value and made it potential provider.
Eichhornia crassipes has a high capacity for the uptake of heavy metals, including
Cd, Cr, Ni, Co, Pb, and Hg which could make it suitable for the biocleaning of
industrial water153-154.
Although water hyacinth is an invasive plant in most countries all over the world, it is
also used as a resource in agricultural production and waste water treatment155.
Water hyacinth is also observed to accumulate Cr (III) in root and shoot tissues in
nutrient culture supplied with Cr (VI). Reduction in nontoxic form appeared to occur
in the fine lateral roots156.In addition to heavy metals, Eichhornia crassipes can also
remove other toxins, such as cyanide which is environmentally beneficial in areas
that have endured gold mining operations157.
Eichhornia crassipes reduce COD and BOD from paper mill effluent. The
percentage of removal was doubled with a detention time of two days and argued
that such reduction is related to both physical setting and plant absorption and the
removal of nitrogen and phosphorus by biomass production was correlated with
factor favoring this production. Among floating aquatic plants, water hyacinth has
been extensively studied at the laboratory and pilot levels and evaluated on a large
scale for removing organic matter from wastewater158-159.
E.crassipes, P.stratiotes, L.minor, A. pinnata and S. polyrhiza have wide spread
availability and have capability to remove pollutants139,160-163. E.crassipes was the
most efficient accumulator among these aquatic macrophytes154.
E.crassipes effectively removes appreciable quantities of heavy metals from
wastewater, especially at low concentrations164. A plant with relatively high biomass
54
may have a greater metal uptake capacity, due to lower metal concentration in its
tissues because of a growth rate that exceeds its uptake rate165. There is higher
accumulation of heavy metals in broad –leaved plant E.crassipes 154.
2.4.2 Vallisneria spirallis (Channel grass)
The elemental composition of certain aquatic plants by X-rays166 and found high
level of heavy metals such as Al, Si, Mn and Fe being found accumulated in
Vallsneria spirallis, Hydrilla vertcillata and Azolla pinnata. Phytoaccumulation of
heavy metals by selected fresh water macrophytes viz. Eichhornia crassipes,
Ipomoea aquatic, Typha angustata, Hydrilla verticillata and Vallisneria spiralis was
studied to assess the phytoremediation of six heavy metals in Nal Sarovar Bird
Sanctuary and Pariyej Community Reserve, Gujarat, India90,167 and found that it is
necessary to carry out phytoremediation of heavy metal contamination and
sediments.
Aquatic macrophytes Hydrilla verticillata and Vallisneria spiralis were studied168 to
see the change in protein profile by the effect of lead and mercury. The
accumulation of metals increased with the increasing treatment concentrations,
although the amount of Hg accumulated was less than that of Pb in both plants.
Phytoremediated potential of Vallisneria spirallis was observed 150 on the industrial
effluents. They observed that reduction in BOD, COD, TS, TDS, TSS and lignin due
to phytodegradation, phytoextraction and phytovolatilization and reduction in Na and
K content in the effluents due to phytoextraction, rhizofiltration and
phytostabilization. The change in pH, EC and colour unit could be due to changes in
above mentioned parameters through total phytoremediation29.
Three water weeds, water hyacinth, pseudo water hyacinth and Lemna species were
tested for the removal of chromium from tannery effluent169. Water hyacinth
accumulated the most chromium (38ppm) followed by Lemna and pseudo water
55
hyacinth. In all three species the root accumulated higher amounts of chromium than
the foliage.
A freshwater submerged, rooted wetland species plants of Vallisneria spiralis were
tested for Cr accumulation and found that these plants can effectively remove Cr by
adsorption and absorption into plant tissues.170 and its harvested wetland plant
biomass used in biogas production171,172.
2.4.3 Lemna minor (Duckweed)
Duckweed is a small, free floating aquatic plant belonging to Lemnaceae family173 It
has been found that lemna species have many unique properties ideal for
phytoremediation plant species: they have fast growth and primary production; high
bioaccumulation capacity; ability to transform or degrade contaminant; ability to
regulate chemical speciation and bioavailability of some contaminant in their milieu;
resilient to extreme contaminant concentration; and can be applied on multiple
pollutants simultaneously. In addition, they have properties significant for public
health livestock production and aquaculture and ecological function. Duckweed
wastewater treatment systems have been studied for a wide range of wastewater
types. Most of the studies have focused on nutrient removal efficiencies and removal
rates between 50-95% have been reported for duckweed covered systems174-176.
The duckweed mat, which fully covers the water surface, results in three zones i.e.
aerobic, anoxic and anaerobic zone177. In the aerobic zone, organic materials are
oxidized by aerobic bacteria using atmospheric oxygen transferred by duckweed
roots 178.Nitrification and denitrification takes place in anoxic bacteria into ammonium
and ortho-phosphate, which are intermediate products used as nutrients by the
duckweed179. Duckweed is used in water quality studies to monitor heavy metals
and other aquatic pollutants, because it may selectively accumulate certain
chemicals and may serve as biological monitors140.
56
Aquatic weed-based wastewater treatment plants revealed maximum reduction in
suspended solids, BOD and COD, nitrogen, phosphorus, oil and grease180-181.
Similar report has been made by Rose et al.,182 who inferred that Lemna minor is
efficient in removing BOD, solids and nutrient from the wastewater.
Higher concentration of nutrients in wastewater resulted in growth and further
accumulation of heavy metals E.crassipes and L.minor showed the high
accumulation of Cd among broad-leaved and small-leaved plants. Higher removal of
heavy metals by E.crassipes and Lemna minor may be due to their luxuriant growth
in nutrient and heavy metal rich media. Physicochemical analysis shows secondary
treated wastewater has high concentrations of nutrients as well as heavy metals45.
The elimination of organic material in terms of BOD and COD is lower in Lemna sp.
In comparison to other vascular plants and rich in cellulose but the nitrogen removal
is same or higher183. Under experimental conditions, L.minor is a good accumulator
of Cd, Se and Cu but a moderate accumulator of Cr and poor accumulator of Ni and
Pb184. Lemna spp. have potential of accumulation of U as well as As in surface
waters of decommissioned uranium mining185-186.
Lemna minor found suitable for surface water quality assessment as selected
endpoints showed consistency among each other with respect to different water
samples. The consistency among observed endpoints might lie in the highly
homogeneous plant material; due to predominantly vegetation reproduction of
duckweed, new fronds are formed by clonal propagation thus producing a population
of genetically homogeneous plants. Moreover, water and substances to be tested
are taken up directly through the leafy fronds187. Most Lemna plants are capable of
withstanding an extreme concentration of contaminants by sequestrating and
compartmentalizing them into cell organelles188-189. The immature cells of the
enclosed daughter fronds contained large deposits with Cd and S as C-
Phytochelatin. Excess Ca2+ in L.minor and L.gibba is deposited in the cells As Ca
oxalate190-191.
57
2.5 Phytoremediated plant biomass as a source of energy
Water hyacinth due to its rapid growth192 has been widely employed for the treatment of
a variety of wastewaters193-195 and has also been a good biogas producer196-197.
The high growth rate of water hyacinth makes it an attractive feed for energy recovery
through biogas198. However, water hyacinth has a low solids content (in the range of 7%
TS) and needs to be concentrated for efficient use in biogas production199. As
Eichhornia crassipes has abundant nitrogen content, it can be used as a substrate for
biogas production and the sludge obtained for the biogas. The application of sewage
sludge to fermenting organic residues enhanced biogas generation200. The animal dung
served as a good seeding material for the biogas fermentation process201.
In phytoremediation the plants absorb metals and other toxic materials from
wastewaters for metabolic use and sequestering in their body which could make them
suitable for the biocleaning of industrial effluent131,153. The rate of biogas production
depends on the status, type and constituents of organic materials undergoing
fermentation202-205 as well as on the digester operating conditions206. In addition to these
factors anaerobic digestion and biogas production was influenced by pH, nutrient
addition207-208 C/N ratio209-210, C/P ratio205, trace elements like iron, vitamins211, and
natural inhabitants205 which are present in the biomass. Generally the C/N ratio is used
as an index for the suitability of organic feed212. The proposed ideal range of C/N ratio
varies from 12 to 72 205,213.
Anaerobic digestion is proven as a relatively efficient conversion process for producing
a collectable biogas mixture with average 60% methane content196,205,214. The biogas
used as a substitute of fuel in boilers and the resultant slurry left over as end product
used for agricultural application as it has high N, P and K content, for agricultural
application215.
Bioenergy potential of eight aquatic weeds Salvinia molesta, Hydrilla verticillata,
Nymphaea stellata, Ceratopteris sp., Azolla pinnata, Scirpus sp., Cyperus sp. and
58
Utricularia reticulate was assessed by Abbasi et al. 216. Natural stands of Salvinia such
as the one employed in study, would yield methane of the order of 10.8 Kcal ha-1 year-1,
while Azolla, scirpus, Hydrilla and Nymphea had energy potentials of the order of 10
Kcal ha-1 year-1.In order to digest a mixture of harvested aquatic plant biomass and
primary sludge for biogas generation by anaerobic digestion was used by Chynoweth et
al. 217
Potential productivity of water hyacinth and water chestnut in nutrient enriched waste
water has lead to its selection for phytoremediation of various industrial effluents151,218.
Since these plants have high nitrogen content therefore, produced biomass used as a
feed stock for biogas production to achieve economic success in energy produced from
them. In 1971 it was pointed out by Widyanto et al.219 that utilization of Eichhornia for
biogas production was advantageous because of its high water content, soft organic
matter and a favourable C/N ratio i.e.20:1-30:1.
Biogas generation in certain aquatic weeds is relatively more and quicker due to their
high water content220. The greater biogas production from Vallisneria spirallis (Channel
grass) could be accounted for their submerged natures which have resulted in fine
slurry and finer particles resulted in greater biogas production197.
2.6 Air pollution
Suspended particulate matter (SPM), a complex mixture of organic and inorganic
substances, arising from both natural and anthropogenic sources, is a ubiquitous air
pollutant. These are the particles present in air having particle size ranging from 10-3 µm
to100µm.These particles have serious impacts on climate221222, visibility223,
biogeochemical cycling in ecosystems224 and health225-226. Epidemiologic studies have
shown that these particles are responsible for the various problems of the respiratory
tract227. Recent studies have shown that elevated fine PM is associated with increased
mortality and morbidity228-230.
59
Particulate matter having size 10µm or less in diameter are called as respirable
suspended particulate matter (RSPM), since it penetrates the respiratory system. RSPM
is grouped into three types depending upon their size i.e. ultra fine (less than 0.1µm),
fine (0.7-1 µm) and course (1-10µm) 231-232. Major constituents of fine particles often
consist of sulphates, carbonaceous materials, nitrates and trace elements233-234.
Organic substances are the second major constituents of fine PM, constituting approx.
26-47%.235. The coarse fraction mostly consists of aluminium, silicon, sulphur, calcium,
potassium and iron (40-50%) while sulphates, nitrates, ammonium ions, elemental and
organic carbon consist of approx. 10-20%.
The level of total suspended particulate is one of the basic and useful indicators for
judging the degree of air pollution236. In order to monitor aerosol quantity and quality the
national air quality standards are being reviewed in many countries so as to maintain
healthy environment. In India air pollution studies around Delhi revealed that TSPM
exceeds the air standards prescribed by Central Pollution Control Board, New Delhi237
The physical and chemical characteristics of suspended particulate matter depend upon
the source from which they are emitted. In a study on particle size distribution of SPM,
carried out in ambient air of Nagpur238 and trace metals such as Fe, Mn, Cr, Pb, and Zn
were determined. These particle size ranges such as less than 2.1 µm and 2.1µm. The
analysis showed that Fe and Mn are associated with coarse fraction of particulates
whereas Cr, Pb and Zn having anthropogenic origin are associated with fine fraction.
Level of air pollution in the ambient air of Amritsar was studied239.The study reveals that
the average values of combustible matter at 600±10ºc to SPM ranged from 32.6-51.3%
which may be due to the partially burnt fuel particles in the SPM. The average values of
ratios of benzene soluble particulate matter (BSPM) to SPM at various sites ranged
from 3.4-8.3% and overall average for the entire city is 6.4±5.2%. The mean value of
BSPM content appears to be affiliated only to industrial activity and the attached
vehicular traffic. It was observed that industrial emissions and automobile exhaust may
be causal agents of lead240. The average value of Nickel is highest in the industrial
zone. Cadmium has been found in traces in the air. There have been similar studies in
60
other cities of India as well viz. vishakhapatnam241-243, Ahmedabad244, Sindri245,
Firozabad246, Kanpur247, and Patiala248.
The increment in the level of ambient air aerosol and gaseous pollution in and around
Patiala was studied249 due to wheat and rice crop stubble burning practices. The crop
stubble burning performed after the wheat and rice crop harvesting had changed the
chemistry of ambient air in Patiala in 2007 by increasing the concentration of aerosols,
SO2 and NO2 in the province. Although levels of SO2 and NO2 were fluctuating at
different monitoring sites, high concentrations were obtained in the months of April and
May (wheat crop stubble burning months) as well as October and November (rice crop
stubble burning months)
.
2.6.1 Sources for Particulate Matter
1. The ultrafine particles less than 0.1 µm are formed by condensation of low
vapour pressure substances formed by high temperature vaporization or by
chemical reactions to form new particles, such as from wood smoke, automobile
exhaust and emission from generators or diesel engines250-252.
2. Fine particles of size range from 0.7-1 µm are formed by coagulation of ultra fine
particles. The fine particles concentration was 58-68% in close vicinity of road,
due to exhaust released from traffic253.
3. Biomass burning is an important source of fine organic aerosol and gaseous
pollution in the atmosphere254-257.
4. Coarse particles from 1- 200µm are emitted into atmosphere by mechanical
sparing of rock or soil material. These particles are also found in the wind blown
dust from agricultural processes, mining operation and uncovered soil. In urban
environment dust arise due to agitation of soil through vehicular movement258
and earth moving234.
61
5. During harvesting periods, open burning of agricultural residues releases a large
amount of pollutants to the atmosphere, including aerosols and hydrocarbons259-
260. Due to traffic in urban region about 80% of coarse dust particles from traffic
settle with in 150m, about 40% at 200-270 m and about 20% at about 1500m
from the road253.
In 1987, the primary standard for Total suspended Particles was replaced with PM10,
which includes the particles with diameter of 10µm or less. Then in 1997, the primary
standard for PM10 was replaced with PM2.5 standard.
PM 10 fraction originated from three sources261 have significant effect on health
1. Primary fine particles from industrial and combustion sources mainly traffic.
2. Secondary aerosol formed from photochemical reactions, mainly ammonium
sulphate and ammonium nitrate.
3. Windblown soil and resuspended street dust present in course fraction 2.5-10
µm.
4. Pollutants emanated from biomass burning can also affect properties, materials
and moreover health problems when they inhaled, causing respiratory
problems262-265.
Particles deposit within the respiratory tract by several mechanisms225-226 inertial
impaction, sedimentation, diffusion, electrostatic precipitation and interception.
The interception or depth to which these particles can penetrate in the respiratory
system depends upon the particle size as shown in figure. Electrostatic
precipitation is deposition related to particle charge.
62
Figure 2.3: Respiratory system, showing the depth to which particles can
penetrate
Source: U.S. EPA, Air Pollution Training Institute APTI, Course No. 435
63
It has been observed that smaller particles are typically man-made. Total Suspended
particles have a bimodal distribution, with naturally occurring particles centred at about
10µm and man-made particles centered at about 0.4µm as shown in figure.
Figure 2.4: Bimodal distributions of Total Suspended particles
Source: U.S. EPA, Air Pollution Training Institute APTI, Course No. 435
64
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