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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