Phytoremediation
What is phytoremediation?
The word's etymology comes from
the Greek φυτο (phyto) = plant, and Latin «remedium» = restoring balance,
or remediating.
Phytoremediation consists in depolluting contaminated soils, water or air with plants able to contain, degrade or eliminate metals,
pesticides, solvents, explosives, crude oil and its derivatives, and various
other contaminants, from the mediums that contain them.
It is clean,
efficient, inexpensive and non-environmentally disruptive, as opposed to
processes that require excavation of soil.
Overview:
Phytoremediation is the use of
certain plants to clean up soil, sediment, and water contaminated with metals
and/or organic contaminants such as crude oil, solvents, and polyaromatic
hydrocarbons (PAHs).
Phytoremediation is the use of green plants to remove, contain, or
render harmless environmental contaminants. It is a promising technology that addresses clean-up
of organic solvents, PCBs, heavy metals, polyaromatic hydrocarbons, explosives
and energetics, or nutrients.
It is a name for the expansion
of an old process that occurs
naturally in ecosystems as both inorganic and organic constituents cycle
through plants.
Plant physiology, agronomy, microbiology,
hydrogeology, and engineering are combined to select the proper plant and conditions for a specific
site.
Phytoremediation is an
aesthetically pleasing mechanism that can
reduce remedial costs, restore habitat, and clean up contamination in place
rather than entombing it in place or transporting the problem to another site.
The key physiological processes in
phytoremediation include:
a.
Stimulation of
microorganism-based transformation by plant exudates and leachates, and by
fluctuating oxygen regimes
b. Slowing of contaminant transport from the vegetated zone
due to adsorption and increased evapotranspiration
c.
Plant uptake,
followed by metabolism or accumulation
Various phytoremediation processes
Phytoextraction - uptake and concentration of substances from the environment into the
plant biomass.
Phytostabilization - reducing the mobility of substances in the environment, for example by
limiting the leaching of substances from the soil.
Phytotransformation - chemical modification of environmental substances as a
direct result of plant metabolism, often resulting in their inactivation,
degradation (phytodegradation) or immobilization (phytostabilization).
Phytostimulation - enhancement of soil microbial activity for the degradation of
contaminants, typically by organisms that associate with roots. This process is
also known as rhizosphere
degradation.
Phytovolatilization - removal of substances from soil or water with release into the air,
sometimes as a result of phytotransformation to more volatile and / or less
polluting substances.
Rhizofiltration - filtering
water through a mass of roots to remove toxic substances or excess nutrients.
The pollutants remain absorbed in or adsorbed to the roots.
a.
Phytoextraction
Phytoextraction (or phytoaccumulation) uses plants to remove
contaminants from soils, sediments or water into harvestable plant biomass.
Phytoextraction has been
growing rapidly in popularity world-wide for the last twenty years or so.
Generally this process has been tried more often for extracting heavy metals than for organics. At the time of disposal
contaminants are typically concentrated in the much smaller volume of the plant
matter than in the initially contaminated soil or sediment.
'Mining with plants', or phytomining, is also being experimented
with.
The plants absorb contaminants through the root
system and store them in the root biomass and/or transport them up into the
stems and/or leaves. A living plant may continue
to absorb contaminants until it is harvested. After harvest a lower level of the
contaminant will remain in the soil, so the growth/harvest cycle must usually
be repeated through several crops to achieve a significant cleanup. After the
process, the cleaned soil can support other vegetation.
Two versions of phytoextraction:
a) natural hyper-accumulation, where
plants naturally take up the contaminants in soil unassisted, and b) induced or
assisted hyper-accumulation, in which a conditioning fluid
containing a
chelator or another agent is added to soil to increase metal solubility or
mobilization so that the plants can absorb them more easily.
Examples of phytoextraction
from soils:
Arsenic, using the
Sunflower (Helianthus annuus), or the
Chinese Brake fern (Pteris spp), a
hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.
Cadmium and zinc, using alpine pennycress (Thlaspi
caerulescens), a hyperaccumulator of these metals at levels that would be
toxic to many plants. On the other hand, the presence of copper seems to impair
its growth.
Lead, using Indian
Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia), Hemp Dogbane (Apocynum cannabinum), or Poplar trees,
which sequester lead in its biomass.
Salt-tolerant (moderately
halophytic) barley and/or sugar beets are commonly used for the extraction of
Sodium chloride (common salt) to reclaim fields that were previously flooded by
sea water.
Uranium, using sunflowers,
as used after the Chernobyl accident.
Mercury, selenium and organic pollutants such as
polychlorinated biphenyls (PCBs) have been removed from soils by transgenic
plants containing genes for bacterial enzymes.
b.
Phytostabilization
Phytostabilization focuses on long-term
stabilization and containment of the pollutant.
For example, the plant's
presence can reduce wind erosion, or the plant's roots can prevent water
erosion, immobilize the pollutants by adsorption or accumulation, and provide a
zone around the roots where the pollutant can precipitate and stabilize.
Unlike phytoextraction, phytostabilization mainly focuses on sequestering pollutants in soil
near the roots but not in plant tissues. Pollutants become less bioavailable and livestock, wildlife, and human
exposure is reduced. An example application of this sort is using a vegetative
cap to stabilize and contain mine tailings.
c.
Phytotransformation
In the case of organic
pollutants, such as pesticides, explosives, solvents, industrial
chemicals, and other xenobiotic substances, certain plants, such as Cannas, render these substances non-toxic by their metabolism. In
other cases, microorganisms living
in association with plant roots may metabolize these substances in soil or
water.
These complex and recalcitrant compounds
cannot be broken down to basic molecules (water, carbondioxide, etc) by plant
molecules, and hence the term phytotransformation represents a change in chemical
structure without complete breakdown of the compound.
The term "Green Liver
Model" is used to describe phytotransformation, as plants behave similar
to the human liver when dealing with these xenobiotic compounds (foreign
compound/pollutant). After uptake of the xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding
functional groups such as hydroxyl groups (-OH).
This is known as Phase I metabolism, similar to the way
the human liver increases the polarity of drugs and foreign compounds (Drug
Metabolism). While in the human liver, enzymes like Cytochrome P450s are
responsible for the initial reactions, in plants
enzymes such as nitroreductases
carry out the same role.
In the second stage of
phytotransformation, known as Phase II metabolism, plant biomolecules such
as glucose and amino acids are added
to the polarized xenobiotic to further increase the polarity (known as conjugation). This is again similar to
the processes occurring in the human liver wherein glucuronidation (addition of
glucose molecules by the UGT (e.g. UGT1A1) class of enzymes) and glutathione
addition reactions occur on reactive centers of the xenobiotic.
Phase I and II reactions serve to increase the
polarity and reduce the toxicity of the compounds, although many exceptions to the rule are seen at least in the case of the
human liver. The increased polarity also allows for easy transport of the
xenobiotic along aqueous channels.
In the final stage of phytotransformation (Phase III metabolism), a
sequestration of the xenobiotic occurs within
the plant. The xenobiotics polymerize in a lignin-like manner and get a
complex structure which is sequestered in the plant. This ensures that the xenobiotic is
safely stored in the plant, and does not affect the functioning of the plant.
However, preliminary studies
have shown that these
plants can be toxic to small animals (such as snails) and hence plants involved
in phytotransformation may need to be maintained in a closed enclosure.
The human liver differs from
plants in Phase III metabolism, since the liver can transport the xenobiotics
into the bile for eventual excretion. Since plants have no excretory
mechanisms, they sequester the modified xenobiotics.
Hence, the plants reduce toxicity (with exceptions) and sequester the
xenobiotics in phytotransformation. Trinitrotoluene (TNT) phytotransformation
has been extensively researched and a transformation pathway has been proposed.
Advantages and limitations
Advantages:
ü the cost of the phytoremediation is lower than
that of traditional processes both in situ and ex situ
ü the plants can be easily monitored
ü the possibility of the recovery and re-use of valuable
metals (by companies specializing in “phytomining”)
ü it is the least harmful method because it uses naturally
occurring organisms and preserves the natural state of the environment.
Limitations:
ü phytoremediation is limited to the surface area and depth
occupied by the roots.
ü slow growth and low biomass require a long-term
commitment
ü with plant-based systems of remediation, it is not
possible to completely to prevent the leaching of contaminants into the groundwater
(without the complete removal of the contaminated ground which in itself does
not resolve the problem of contamination)
ü the survival of the plants is affected by the toxicity of
the contaminated land and the general condition of the soil.
ü possible bio-accumulation of contaminants which then pass
into the food chain, from primary level consumers upwards.
Advantages and Disadvantages of
Phytoremediation
When using phytoremediation
there are many positive and negative
aspects to consider. The advantages and disadvantages are listed below.
Advantages
|
Disadvantages
|
ü Works on a variety on organic and inorganic compounds
ü Can be either In Situ/ Ex Situ
ü Easy to implement and maintain
ü Low-cost compared to other treatment methods
ü Environmentally Friendly and aesthetically pleasing to
the public
ü Reduces the amount wastes to be landfilled
|
ü May take several years to remediate
ü May depend on climatic conditions
ü Restricted to sites with shallow contamination within
rooting zone
ü Harvested biomass from phytoextraction may be
classified as a RCRA hazardous waste
ü Consumption of contaminated plant tissue is also a
concern
ü Possible effect on the food chain
|
A major advantage that is listed above is the low cost. For example, the cost of cleaning up one acre of sandy
loam soil at a depth of 50cm with plants is estimated at $60,000-$100,000
compared to $400,000 for the conventional excavation and disposal method. One
reason for this low cost is phytoremediation may not require expensive equipment or highly
specialized personnel, and can be relatively easy to implement.
One major concern with
phytoremediation is the possible affects on the food chain. For example
vegetation is used that absorbs toxic or heavy metals and moles or voles eat
the metal contaminated plants. The predators of the moles or voles then become
victims of intoxication. All though the possibilities of such scenarios are
being looked at, more fieldwork and analysis is necessary to understand the
possible effects phytoremediation can have.
Hyperaccumulators and biotic interactions
A plant is said to be a hyperaccumulator if it can concentrate
the pollutants in a minimum percentage
which varies according to the pollutant involved (for example: more than 1000
mg/kg of dry weight for nickel, copper, cobalt, chromium or lead; or more than
10,000 mg/kg for zinc or manganese.
Most of the 215
metal-hyperaccumulating species included in their review
hyperaccumulate nickel. They listed 145 hyperaccumulators of nickel (around 300 Ni
accumulators are known, 26 of cobalt, 24 of copper, 14 of zinc, four of Lead,
and two of Chromium.
This capacity for accumulation
is due to hypertolerance, or phytotolerance: the result of adaptative evolution from the plants to
hostile environments along multiple generations.
Boyd and Martens list 4 biotic
interactions that may be affected by metal hyperaccumulation, to which can be added the biofilm as a
particular aspect of micorrhizae:
a.
Protection
More and more evidence show
that the metals in hyperaccumulating plants give them some protection from various bacteria, fungi and/or insects.
For instance, with foliar Ni
concentrations as low as 93 mg/kg, the larval weight of Spodoptera exigua (Lepidoptera: Noctuidae) (beet army worm) is
reduced and time to pupation extended.
Published research supporting
the hypothesis
of metal hyperaccumulation:
Researcher
|
Plant species
|
Metal
|
Organism(s) affected
|
Ernst 1987
|
Silene vulgaris (Moench) Garke
|
Cu (400 mg g-¹)
|
Hadena cucubalis Schiff. (Lepidoptera: Noctuidae)
|
Boyd et al. 1994
|
Streptanthus polygaloides Gray
|
Ni
|
Xanthomonas campestris (Gram-negative bacterium)
|
Boyd et al. 1994
|
Streptanthus polygaloides Gray
|
Ni
|
Alternaria brassicicola (Imperfect fungus)
|
Boyd et al. 1994
|
Streptanthus polygaloides Gray
|
Ni
|
Erisyphe polygoni (Powdery mildew)
|
Martens & Boyd 1994
|
Streptanthus polygaloides
|
Ni
|
(Lepidoptera: Pieridae)
|
Boyd & Martens 1994
|
Thlaspi montanum L. var. montanum
|
Ni
|
Pieris rapae
|
Pollard & Baker 1997
|
Thlaspi caerulescens J. and C. Presl.
|
Zn
|
Schistocerca gregaria (Forsk.) (Orthoptera: Acrididae)
|
Pollard & Baker 1997
|
Thlaspi caerulescens J. and C. Presl.
|
Zn
|
Deroceras carvanae (Pollonera) (Pulmonata: Limacidae)
|
Pollard & Baker 1997
|
Thlaspi caerulescens J. and C. Presl.
|
Zn
|
Pieris brassicae L. (Lepidoptera: Pieridae)
|
The defense against viruses is not always
supported. Davis et al. (2001) have compared two close species S. polygaloides Gray (Ni
hyperaccumulator) and S. insignis
Jepson (non-accumulator), inoculating them with Turnip mosaic virus. They
showed that the
presence of nickel weakens the plant's response to the virus.
Circumvention of plants'
elemental defences by their predators may occur in three ways:
(1) selective feeding on
low-metal tissues,
(2) use of a varied diet to
dilute metal-containing food (likely more efficient in large-sized herbivores),
and
(3) tolerance of high dietary
metal content.
Avoidance of an elemental defence via
selective feeding:
Mishra & Kar (1974)
reported nickel to be transported through the xylem of crop plants. Similarly,
Kramer et al. (1996) showed that Ni is transported as a complex with the
amino-acid histidine in the xylem. This implies that phloem fluid may contain
little nickel; thus phloem fluid may be used by able organisms as a rich source
of carbohydrates.
Pea aphids (Acyrthosiphon pisum [Harris]; Homoptera:
Aphididae) feeding on Streptanthus
polygaloides Gray (Brassicaceae) have equal
survival and reproduction rates for plants containing ca. 5000 mg/kg nickel
amended with NiCl2, and those containing 40 mg/kg nickel. This means that either the
phloem fluid is poor in nickel even for nickel hyperaccumulators, or that the
aphids tolerate nickel.
Moreover the aphids feeding on high
nickel-content plants only show a small increase of nickel content
in their bodies, relatively to the nickel content of aphids feeding on
low-nickel plants. On the other hand, aphids (Brachycaudus lychnidis L.) fed on the zinc-tolerant plant Silene vulgaris (Moench) Garcke
(Caryophyllaceae) - which can contain up to 1400 mg/kg zinc in its leaves –
were reported showing elevated (9000 mg/kg) zinc in their bodies.
Metal tolerance
Hopkin (1989) and Klerks (1990)
demonstrated it for animal species; Brown & Hall (1990) for fungal species;
and Schlegel & al. (1992) and Stoppel & Schlegel (1995) for bacterial
species.
Plants of Streptanthus polygaloides (Brassicaceae, Ni hyperaccumulator) can
be parasited by Cuscuta californica
var. breviflora Engelm.
(Cuscutaceae). Metal contents of Cuscuta
ranged from 540–1220 mg/kg Ni, 73-fold higher than the metal contents of Cuscuta parasitizing a co-occurring
non-hyperaccumulator plant species.
Cuscuta plants are
therefore very Ni-tolerant - 10 mg
Ni/kg is sufficient for growth to start decreasing in unadapted plants.
According to Boyd & Martens (subm.) this is "the first well-documented
instance of the transfer of elemental defences from a hyperaccumulating host to
a seed plant parasite".
b.
Interferences with neighbour plants of
different species
Its likelihood between
hyperaccumulators and neighbouring plants was suggested but no mechanism was proposed. Gabrielli et
al. (1991), and Wilson & Agnew (1992), suggested a decrease in competition experienced by the hyperaccumulators for
the litterfall from hyperaccumulators' canopy.
This mechanism mimics allelopathy in its effects, although technically due to
redistribution of an element in the soil rather than to the plant manufacturing
an organic compound. Boyd et Martens call it ‘‘elemental allelopathy’’ - without
the autoxicity problem met in other types of allelopathy (Newman 1978).
c.
Mutualism
Two types of mutualism are
considered here, mycorrhizal associations or mycorrhizae, and animal-mediated
pollen or seed dispersal.
1 - Mycorrhizal associations
or mycorrhizae
There are two types of mycorrhizal fungi: ectomycorrhizae and endomycorrhizae.
Ectomycorrhizae form sheaths around plant roots, endomycorrhizae enter cortex
cells in the roots.
Mycorrhizae are the symbiotic relationship between a
soil-borne fungus and the roots of a plant. Some hyperaccumulators may form
mycorrhizae and, in some cases, the latter may have a role in metal treatment.
In soils with low metal levels, vesicular arbuscular
mycorrhizae enhance metal uptake of non-hyperaccumulating species. On the other
hand, some mycorrhizae increase metal tolerance by decreasing metal uptake in
some low-accumulating species.
Mycorrhizae thus assists Calluna in avoiding Cu and Zn toxicity.
Most roots need about 100 times the amount of carbon than do the hyphae of its
associated ectomycorrhizae in order to develop across the same amount of soil.
It is therefore easier for hyphae to acquire elements that have a low mobility
than it is for plant roots. Caesium-137
and strontium-90 both have low mobilities.
Mycorrhizal fungi depend on host plants for carbon, while
enabling host plants to absorb the soil's nutrients and water with more
efficiency. In mycorrhizae, nutrient uptake is enhanced for the plants while
they provide energy-rich organic compounds to the fungus. Although certain
plant species that are normally symbiotic with mycorrhizal fungi can exist
without the fungal association, the fungus greatly enhances the plant’s growth.
Hosting mycorrhizae is much more energy effective to the plant than producing
plant roots.
The Brassicaceae family
reportedly forms few mycorrhizal associations. But Hopkins (1987) notes
mycorrhizae associated with Streptanthus
glandulosus Hook. Some fungi tolerate easily the generally elevated metal
contents of serpentine soils. Some
of these fungal species are mycorrhizal. High levels of phosphate in the soil inhibit mycorrhizal
growth.
The uptake of radionuclides by fungi depends on its nutritional mechanism (mycorrhizal or
saprophytic). Pleurotus eryngii
absorbs Cs best over Sr and Co, while Hebeloma
cylindrosporum favours Co. But increasing the amount of K increases the
uptake of Sr (chemical analogue to Ca) but not that of Cs (chemical analogue to
K). Moreover, the uptake of Cs decreases with Pleurotus eryngii (mycorrhizal) and Hebeloma cylindrosporum (saprophytic) if the Cs content is
increased, but that of Sr increases if its content is increased – this would
indicate that the uptake is independent from the nutritional mechanism.
2 - Pollen and seed dispersal
Some animals obtain food from
the plant (nectar, pollen, or fruit pulp - Howe & Westley 1988). Animals
feeding from hyperaccumulors high in
metal content must either be
metal-tolerant or dilute it with a mixed diet. Alternatively
hyperaccumulators may rely on abiotic vectors or non-mutualistic animal vectors
for pollen or seed transport, but we lack information on seed and pollen
dispersal mechanisms for hyperaccumulating plants.
Jaffré & Schmid 1974;
Jaffré et al. 1976; Reeves et al. 1981; have studied metal contents of entire
flowers and/or fruits. They have recorded elevated
metal levels in these. There is an exception with Walsura monophylla Elm. (Meliaceae), originating from the
Philippines and showing 7000 mg/kg Ni in leaves but only 54 mg/kg in fruits.
Some plants may thus have a mechanism by which metal or other contaminants is
excluded from their reproductive structures.
d.
Commensalism
This is an interaction
benefiting one organism while being of neutral value to another. The most
likely one with hyperaccumulators would be epiphytism. But this is most noticeable in humid habitats,
whereas only a few detailed field studies of hyperaccumulators have been
conducted in such habitats, and those studies (mostly to do with humid
tropical forests on serpentine soils) pay little or no attention to
that point (e.g., Proctor et al. 1989; Baker et al. 1992).
Proctor et al. (1988) studied
the tree Shorea tenuiramulosa, which
can accumulate up to 1000 mg Ni/kg dry weight in leaf material.
They estimated covers of
epiphytes on the boles of trees in Malaysia, but did not report values for
individual species. Boyd et al. (1999) studied the occurrence of epiphytes on
leaves of the Ni hyperaccumulating tropical shrub Psychotria douarrei (Beauvis.).
Epiphyte load increased
significantly with increasing leaf age, up to 62% for the oldest leaves. An
epiphyte sample of leafy liverworts removed from P. douarrei, was found to contain 400 mg Ni /kg dry weight (far
less than the host plant, whose oldest and most heavily epiphytized leaves
contained a mean value of 32,000 mg Ni/kg dry weight). High doses of Ni therefore do not prevent
colonization of Psychotria douarrei
by epiphytes.
Chemicals that mediate
host-epiphyte interactions are most likely to be located in the outermost
tissues of the host (Gustafsson & Eriksson 1995). Also, most of
the metal accumulates in epidermal or subepidermal cell walls or vacuoles
(Ernst & Weinert 1972; Vazquez et al. 1994; Mesjasz- Rzybylowicz et al.
1996; Gabrielli et al. 1997).
These findings suggest that
epiphytes would experience higher metal levels when growing on hyperaccumulator
leaves. But Severne (1974) measured the release of metal via leaching of leaves from the Ni
hyperaccumulator Hybanthus
floribundus (Lindl.) F. Muell. (Violaceae) from western Australia; he
concluded that its leaves do not easily leach Ni.
In theory another commensal interaction could exist, if
the high metal content of the soil under hyperaccumulator plants was needed for
another plant species to establish itself. No evidence is known showing such
effect.
The biofilm
A biofilm is a layer of organic matter
and microorganism formed by the attachment and proliferation of bacteria on the
surface of the object. Biofilms are characterised by the presence of
bacterial
extracellular polymers glyocalyx that create
a thin visible slimy layer on solid
surface.
The role of genetics
Breeding programs and genetic
engineering are powerful methods for enhancing natural phytoremediation capabilities, or for introducing new
capabilities into plants.
Genes for phytoremediation may
originate from a micro-organism or may be transferred from one plant to another
variety better adapted to the environmental conditions at the cleanup site.
For example, genes encoding
a nitroreductase from a bacterium were inserted into tobacco and showed faster
removal of TNT and enhanced resistance to the toxic effects of TNT.
Regulatory issues
As of now phytoremediation is
too new to be approved by regulatory agencies such as the EPA (USA).
Eventually the main question
that regulators will focus on is: will phytoremediation remediate the site to the standards and
reduce the risk to human health and the environment?
In developing regulations for
phytoremediation the following questions will need answering.
Can it cleanup the site below
action levels? On what scale?
Does it create any toxic
intermediate or products?
Is it cost effective as
alternative methods?
Does the public accept the
technology?
References
Phytoremediation:
Transformation and Control of Contaminants, edited by Steven C. McCutcheon and
Jerald L. Schnoor
The significance of metal
hyperaccumulation for biotic interactions, by R.S. Boyd and S.N. Martens
EPA citizens guide to
phytoremediation - http://clu-in.org/PRODUCTS/CITGUIDE/Phyto.htm
HSRC's phytoremediation page
-http://www.engg.ksu.edu/HSRC/phytorem/
Edenspace -
http://www.edenspace.com
Phytokinetics - http://www.phytokinetics.com
