Science Progress (2013), 96(4), 417 – 427
Doi:10.3184/003685013X13818570960538
Current Commentary
Applications of bioremediation and phytoremediation
CHRISTOPHER J. RHODES
Fresh-lands Environmental Actions, 88 Star Road, Caversham, Berkshire RG4 5BE, UK
E-mail:cjrhodes@fresh-lands.com
1. Introduction
The decontamination of soil and water from pollutants using microorganisms (bioremediators)
is known as bioremediation1. There are essentially two approaches, described as in situ and ex
situ. In situ methods are those in which the contaminated material is treated at the site, whereas
when the material is physically removed to be treated elsewhere it is referred to as ex situ. Some
technologies that are related to bioremediation include those of phytoremediation2,3, and are
outlined below. It is possible for bioremediation to occur under natural conditions, or it can be
stimulated, e.g. by the application of fertilisers (biostimulation), and more recently it has been
shown that through the addition of matched microbe strains to the medium, the effectiveness
of the resident microbe population to decompose contaminants may be enhanced. It should not
be imagined that every type of contaminant can be disposed of by means of microorganisms.
Heavy metal contaminants, e.g. Cd2+ and Pb2+, tend to resist interception by microorganisms.
In such cases, phytoremediation is useful because the toxins are bioaccumulated into the
body of plants, above ground, which can then be harvested and removed. By measuring the
oxidation reduction potential (redox) in soil and groundwater, along with pH, temperature,
O2 tension, concentrations of electron acceptors and donors, and of decomposition products,
such as CO2, a measure of the bioremediation process can be obtained. Table 1 shows different
biological decomposition processes, the rates of which decrease in decreasing order of the
redox potential (fastest at higher potentials), although the detail of the overall bioremediation
process per se is only scantly indicated by such values. To gain insight over a larger area,
sufficient measurements should be made on and around the contaminated site such that
contours of equal redox potential can be drawn. It is further necessary to perform analyses
to ascertain that the ultimate levels of the contaminating compounds (and their products of
decomposition) are below regulatory limits.
Current Commentary
Table 1 Biodegradation processes of decreasing rate according to decreasing redox potential4
Process
Reaction
Aerobic:
Anaerobic:
Denitrification
Manganese(IV) reduction
Iron(III) reduction
Sulfate reduction
Fermentation
O2 + 4e− + 4H+ → 2H2O
2NO3− + 10e− + 12H+ → N2 + 6H2O
MnO2 + 2e− + 4H+ → Mn2+ + 2H2O
Fe(OH)3 + e− + 3H+ → Fe2+ + 3H2O
SO42− + 8e− + 10H+ → H2S + 4H2O
2CH2O → CO2 + CH4
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Redox potential
(Eh in mV)
600 ~ 400
500 ~ 200
400 ~ 200
300 ~ 100
0 ~ − 150
− 150 ~ − 220
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Bioremediation can be used in locations that cannot readily be treated other than
by excavation, e.g. spillages of petrol or chlorinated solvents which may contaminate
groundwater. This is usually a much cheaper approach than excavating material to be disposed
of elsewhere, or through other ex situ strategies, and which reduces or eliminates the need for
“pump and treat”, which is often employed where clean groundwater has been contaminated.
The process may be enhanced by the addition of appropriate oxidising or reducing amendment
agents. There is scope too for the creation of genetically modified microorganisms that are
specifically tailored for bioremediation5, e.g. the most radioresistant organism known so far,
the aptly named bacterium Deinococcus radiodurans has been modified to consume and
digest toluene and mercury cations in the presence of high level nuclear waste6.
Current Commentary
2. Some applications of microbial biodegradation
The move toward finding “green” ways to ameliorate many environmental woes, including
dealing with polluted environments, has led to a rising focus towards microbial degradation.
Such methods of bioremediation and biotransformation exploit the remarkable diversity of
xenobiotic metabolism by microbes. Thus, an enormous range of polluting materials may be
addressed, including hydrocarbons (e.g. from oil-spills), polychlorinated biphenyls (PCBs),
polyaromatic hydrocarbons (PAHs), heterocyclics, pharmaceutical substances, pesticides,
heavy metals (e.g. Cd2+, Pb2+, Cu2+, Zn2+) and various radionuclides (e.g. Cs+, Sr2+). Detailed
genomic, metagenomic, proteomic, bioinformatic and other high-throughput analytical
techniques, as applied to environmentally important microorganisms, have disclosed key
features of critical biodegradative pathways and the ability of such organisms to adapt to
changing environmental conditions and stress factors. To achieve a truly sustainable society,
it is mandatory to reduce the environmental impact of humans, and achieving this through the
use of the remarkable catabolic versatility of microorganisms to degrade or convert a variety
of polluting compounds is a potential “holy grail”. Through genome-based global studies,
unparalleled advances are now possible from in silico (meaning, “performed on a computer, or
by computer simulation”) views of metabolic and regulatory networks, along with providing
insight into the evolution of degradation pathways and of molecular adaptation strategies
in microorganisms, at the behest of changing environmental conditions. The degradation of
PAHs provides a good example of the use of these methods7.
While it was formerly considered that the mineralisation of aromatic hydrocarbons
and halogenated compounds is probably not possible in the absence of oxygen, the more
recent discovery of previously unknown anaerobic hydrocarbon-degrading and reductively
dehalogenating bacteria has shown that these processes do indeed occur in nature. Through
an increasing application of genomics in the field of environmental microbiology, novel
molecular insights into these new metabolic properties are now possible. The ~ 4.7 Mb
genome of the facultative denitrifying Aromatoleum aromaticum strain EbN1 was the first to
be determined for an anaerobic hydrocarbon degrader. The genome sequence revealed about
two dozen gene clusters, which included a number of paralogs, in a coding for a complex
catabolic network for anaerobic and aerobic degradation of aromatic compounds. Genomes
of anaerobic hydrocarbon degrading bacteria were recently sequenced for the iron-reducing
species Geobacter metallireducens (accession nr. NC_007517) and the perchlorate-reducing
Dechloromonas aromatica (accession nr. NC_007298). Complete genome sequences also
determined for bacteria capable of anaerobic degradation of halogenated hydrocarbons by
halorespiration are: the ~ 1.4 Mb genomes of Dehalococcoides ethenogenes strain 195 and
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Dehalococcoides sp. strain CBDB1 and the ~ 5.7 Mb genome of Desulfitobacterium hafniense
strain Y51. All these bacteria were shown8 to contain multiple paralogous genes for reductive
dehalogenases, while unprecedented insights were attained into the evolution of reductive
dehalogenation and differing strategies for niche adaptation. Previously, it was demonstrated
that Desulfitobacterium chlororespirans – originally evaluated for halorespiration on
chlorophenols – can also use some brominated compounds, such as the herbicide bromoxynil
and its major metabolite as electron acceptors for growth9. De-iodination is possible too,
although the overall reductive mechanism may differ in detail9.
A complete description of Desulfitobacterium hafniense strain PCP‑1 has been made,
which can convert pentachlorophenol (PCP) into 3‑chlorophenol, dehalogenate a number of
related chloroaromatic compounds and convert tetrachloroethene to trichloroethene. Four
gene loci, encoding putative chlorophenol-reductive dehalogenases (CprA2 to CprA5) were
detected, and the products of two of these loci have been demonstrated to dechlorinate different
chlorinated phenols. The strain PCP‑1 was used at the laboratory scale to degrade PCP, as
present in contaminated environments, and it is concluded therefore that Desulfitobacterium
hafniense PCP‑1 is an excellent candidate to exploit in the development of processes for the
bioremediation of organohalide compounds10. A report has appeared of the isolation of the
ability of the Acetobacterium sp. Strain AG, to reductively debrominate technical mixtures
of octabrominated and pentabrominated diphenyl ethers11. It should be noted that a critical
factor in effective microbial degradation is the amount of the pollutant that is accessible to
microorganisms. As an early example, O’Loughlin et al. showed12 that most soil clays and
cation-exchange resins accelerated the rate of biodegradation of 2‑picoline by Arthrobacter
sp. strain R1, as a result of adsorption of the substrate onto the clays, an apparent exception
being kaolin. The directed movement of motile organisms towards or away from chemicals
in the environment (chemotaxis) is an important physiological response that may contribute
to effective catabolism of molecules in the environment. In addition, mechanisms for the
intracellular accumulation of aromatic molecules via various transport mechanisms are also
important13.
Current Commentary
3. Biodegradation of petroleum and its products
The ecological toxicity of petroleum is well known, for which seabirds floundering in a
thick black soup of crude oil are the “poster child”. Such marine environments are especially
vulnerable to oil spills since coastal regions and the open sea are not readily isolated, and for
example, the implementation of boom structures, intended to confine the oil, is of limited
efficacy. In addition to such human-induced environmental catastrophes as the Deepwater
Horizon disaster in the Gulf of Mexico, a remarkable 250 million litres of petroleum enter the
marine environment every year from natural seepages. Oil spills are almost never as long-lived
as they might at first appear, however, and the environment cleans them up substantially, given
enough time. Recently, some of the agents for this beneficial activity have been identified:
the hydrocarbonoclastic bacteria (HCB)14, of which Alcanivorax borkumensis was the first
to be completely genome-sequenced. It would appear that other components of petroleum,
including heterocyclic compounds, e.g. pyridine- and quinoline-derivatives, are degraded by
similar though separate mechanisms as pertain for hydrocarbons.
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4. Optimisation of bioreactors for waste treatment
The human species generates colossal quantities of waste, which needs to be processed to
protect the environment from it. Hence, in deriving a sustainable development programme, the
use of living organisms provides a “green” alternative to chemical/engineering solutions that
are costly and potentially environmentally damaging in their own right. Bioreactors provide a
highly controlled “contained” space in which biotreatment processes can be carried out, in the
avoidance of some of the limitations of more “open” systems. Such is the versatility of these
devices that a wide range of wastes can be treated under optimised conditions. However, it
is necessary to consider the genomic aspects15 of the various microorganisms involved along
with their expressed transcripts and proteins. Although this is laborious using conventional
genomic approaches, an adaptation of high-throughput methods of analysis – originally
developed for medical applications – is now available with which to evaluate biotreatment in
confined environments.
Current Commentary
5. Mycoremediation16 (bioremediation by fungi)
Fungi are an essential component of the soil food web, and provide nourishment for the
other biota that live in the soil. In the natural ecosystem, a realm of organisms from different
kingdoms make their assault on those different substrates that are present. The rate of
degradation becomes maximal when there is a good supply of nutrients present, e.g. N, P,
K and other essential inorganic elements. Aspergillus and other moulds are highly efficient
in decomposing starches, hemicelluloses, celluloses, pectins and other sugar polymers,
and some aspergilli can degrade such intractable substrates as fats, oils, chitin, and keratin.
Substrates of human origin, such as paper and textiles (cotton, jute and linen) are readily
degraded by these moulds, when the process is often referred to as biodeterioration. In
1969, when the Italian city of Florence (Firenza) in Italy flooded, it was found that 74% of
the isolates from a damaged Ghirlandaio fresco in the Ognissanti church were Aspergillus
versicolor. Fungi function through the mycelium, which exudes extracellular enzymes and
acids able to decompose lignin and cellulose, the two essential components of plant fibre. In
mycoremediation the correct fungal species must be selected to target a particular pollutant,
and it is possible thus to degrade successfully the nerve gases VX and sarin. By inoculating a
plot of soil contaminated with diesel oil, with mycelia from oyster mushrooms, it was found
that after 4 weeks, 95% of many of the PAHs had been converted to non-toxic compounds.
It seems that the naturally present community of microbes acts in concert with the fungi to
decompose the contaminants, finally to CO2 plus H2O (full mineralisation). In 2007, a cargo
ship spilled 58,000 gallons of fuel along the San Francisco shoreline. Hair mats, resembling
S.O.S. pads the size of a doormat, were used as sponges to soak up spilled oil. They were then
collected and layered with oyster mushroom and straw. The mushrooms broke down the oil
and in several weeks the resulting soil was good enough to be used to for roadside landscaping.
Wood-degrading fungi are extremely effective in decomposing toxic aromatic pollutants from
petroleum and also chlorinated persistent pesticides. Mycofiltration is a similar procedure, in
which mycelia are used as a filter to remove toxic materials and microorganisms from water
in the soil. A major protagonist of mycoremediation is Paul Stamets, who proposes17 there
should be “Mycological Response Teams”, who would employ the approach to recycle and
rebuild healthy soil in the area following any incident.
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6. Phytoremediation.
This may be defined as the treatment of environmental problems by using plants in situ so to avoid
the need to excavate the contaminant material for disposal elsewhere. Phytoremediation18,19
may be applied to the amelioration of contaminated soils, water, or air, using plants that
can contain, degrade, or eliminate metals, pesticides, solvents, explosives, crude oil and its
derivatives (refined fuels), and related contaminating materials. Phytoremediation has been
used successfully for the restoration of abandoned metal-mine workings, and cleaning up
sites where polychlorinated biphenyls have been dumped during manufacture, and for the
mitigation of on-going coal mine discharges. Phytoremediation uses the natural ability of
particular plants (“hyperaccumulators”, described below) to bioaccumulate, degrade, or
otherwise reduce the environmental impact of contaminants in soils, water, or air. Those
contaminants that have been successfully mitigated in phytoremediation projects worldwide
are metals, pesticides, solvents, explosives, and crude oil and its derivatives, and the technology
has become increasingly popular and has been employed at sites with soils contaminated
with lead, uranium, and arsenic. A major disadvantage of phytoremediation is that it takes a
relatively long time to achieve, because the process rests upon the ability of a plant to thrive
in an environment that is not ideal for normal plant growth.
6.1. Advantages of phytoremediation
• In terms of cost, phytoremediation is lower than that of traditional processes both in
situ and ex situ.
• The plants can be easily monitored.
• There is the possibility of the recovery and re-use of valuable metals (by companies
specialising in “phyto-mining”).
• It is potentially the least harmful method because it uses naturally occurring organisms
and preserves the environment in a more natural state.
Current Commentary
• Trees may be used in phytoremediation, since they grow on land of marginal quality,
have long life-spans and a high flood tolerance. Willows and poplars are most
commonly used, and can grow 6 – 8 feet (ca 2 metres) per year. For deep contamination,
hybrid poplars with roots extending 30 feet deep have been used, which penetrate
microscopically sized pores in the soil matrix and each tree can cycle 100 L of water
per day, functioning almost as a solar powered and self-contained pump and treatment
system.
• Phytoscreening is possible, in which plants may be used as biosensors for particular
types of contaminants, thus giving a signal of underlying contaminant plumes, e.g.
trichloroethene has been detected in the trunks of trees.
• Genetic engineering may confer improvements to phytoremediation, e.g. genes
encoding a nitroreductase from a bacterium, when inserted into tobacco, increased
the resistance of the plant to the toxic effects of TNT and improved the uptake of the
material. Plants may be genetically modified to grow in soils even when the pollution
levels in the soil are lethal for non-treated plants, and to absorb a greater concentration
of the contaminant.
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6.2. Limitations of phytoremediation
• Phytoremediation is limited to the surface area and depth occupied by the plant roots.
• Slow growth and low biomass require a long-term commitment.
• Using plants, it is not possible to prevent entirely 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).
• The survival of the plants is affected by the toxicity of the contaminated land and the
general condition of the soil.
• Bio-accumulation of contaminants, especially metals, into the plants which then
pass into the food chain, from primary level consumers upwards, or requires the safe
disposal of the affected plant material, i.e. the plants might be eaten by animals.
6.3 Hyperaccumulators and biotic interactions
If a plant is able to concentrate particular contaminant, to a given minimum concentration (>
1000 mg kg – 1 of dry weight for nickel, copper, cobalt, chromium or lead; or > 10,000 mg kg – 1
for zinc or manganese), it is categorised as a hyperaccumulator. This capacity for accumulation
is a result of genetic adaptation over many generations in hostile environments. Metal
hyperaccumulation can affect various different factors, such as protection, interferences
between different species of plants, mutualism (e.g. mycorrhizae, pollen and seed dispersal),
commensalism, and biofilm.
6.4 Different possible phytoremediation methods
Various processes that are mediated by plants or algae might be used to address environmental
problems:
• Phytoextraction – uptake and concentration of substances from the environment into
the plant biomass.
• Phytostabilisation – reducing the mobility of substances in the environment, for
example, by limiting the leaching of substances from the soil.
Current Commentary
• Phytotransformation – chemical modification of environmental substances as a
direct result of plant metabolism, often resulting in their inactivation, degradation
(phytodegradation), or immobilisation (phytostabilisation).
• 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. Phytostimulation can also involve aquatic plants
supporting active populations of microbial degraders, as in the stimulation of atrazine
degradation by hornwort.
• Phytovolatilisation – 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.
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7. Phytoextraction
In phytoextraction (or phytoaccumulation), plants or algae are used to extract contaminants
from soils, sediments or water into harvestable plant biomass (those organisms that take
larger-than-normal amounts of contaminants from the soil are called hyperaccumulators).
Phytoextraction has been used more often for extracting heavy metals than for organic
contaminants. The plants absorb contaminants through the root system which they then
contain in the root biomass and/or move them 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 clean-up. The process can be repeated to affect
further decontamination. There are two forms of phytoextraction:
• Natural hyper-accumulation, where plants take up the contaminants in soil unassisted.
• Induced (assisted) hyper-accumulation, in which a conditioning fluid containing a
chelator or another agent is added to soil to increase metal solubility or mobilisation so
that the plants can absorb them more easily. In many cases natural hyperaccumulators
are metallophyte plants that can tolerate and incorporate high levels of toxic metals.
7.1 Examples of phytoextraction:
• Arsenic, using the Sunflower (Helianthus annuus), or the Chinese Brake fern (Pteris
vittata), a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.
• Cadmium, using willow (Salix viminalis): willow has a significant potential as a
phytoextractor of of cadmium (Cd), zinc (Zn) and copper (Cu), since it has some
specific characteristics, including a high transport capacity for heavy metals from root
to shoot, and a large biomass production. Willow can also be used to produce energy
in a biomass fuelled power plant.
• Cadmium and zinc, using Alpine pennycress (Thlaspi caerulescens), a hyperaccumulator
of these metals at levels that would be toxic to many plants, although its growth appears
to be inhibited by copper.
Current Commentary
• Lead, using Indian Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia),
Hemp Dogbane (Apocynum cannabinum), or Poplar trees, which sequester lead in
their 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.
•
Cs and 90Sr contaminating a pond were removed using sunflowers, following the
1986 Chernobyl accident.
137
• Mercury, selenium and organic pollutants including polychlorinated biphenyls (PCBs)
have been removed from soils by transgenic plants containing genes for bacterial
enzymes.
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8. Phytostabilisation
In phytostabilisation, the intention is to stabilise, or contain the pollutant over the long-term.
There may be a number of contributing factors to this, e.g. the reduction of wind (soil) erosion
by the body of the plant, but the roots of the plant can resist water (soil) erosion, immobilise
the pollutants by adsorption or accumulation, and provide a zone around the roots where
the pollutant can be deposited in an immobilised form. In contrast with phytoextraction,
phytostabilisation aims mainly to sequester pollutants in soil around the roots but not in the
plant tissues. Hence the pollutants are increasingly less bioavailable, such that exposure to
livestock, wildlife, and humans is reduced. Mine tailings may be stabilised by growing a
vegetative cap.
9. Phytotransformation
Some plants, e.g. cannas, are able to detoxify organic pollutants – pesticides, explosives,
solvents, industrial chemicals, and other xenobiotic substances – by metabolising them.
The metabolic functions of microorganisms living in association with plant roots may also
metabolise these substances, as present in soil or water. Due to the complex and recalcitrant
nature of many of these compounds, they cannot be broken down entirely (mineralised)
to basic molecules (H2O, CO2, etc.) by plants and hence the term phytotransformation
represents molecular alterations rather than the complete decomposition of the compound.
Phytotransformation may be viewed18 as a “Green Liver” because plants behave analogously
to the human liver in processing these xenobiotic compounds, introducing polar groups such
as – OH to them. This is known as Phase I metabolism, similar to the way that the human
liver increases the polarity of drugs and foreign compounds. In plants, it is enzymes such
as nitroreductases which carry out these transformations, whereas in the human liver it is
enzymes such as the Cytochrome P450s that perform the task. Phase II metabolism in the
second step in phytotransformation, in which the polarity of the xenobiotic molecule is
increased by combination with plant biomolecules such as glucose and amino-acids. This is
called “conjugation”, and is similar to processes such as glucuronidation (addition of glucose)
and glutathione addition reactions, catalysed by appropriate enzymes. The effect of the two
metabolic steps may serve to detoxify the xenobiotic and aid its mobilisation via aqueous
channels. In Phase III metabolism, the xenobiotic becomes sequestered, by incorporation in
a complex “lignin-type” structure, where it is kept apart from the normal functioning of the
plant. The phytotransformation of trinitrotoluene (TNT) has been well studied, and a detailed
mechanism proposed for it.
Current Commentary
10. Phytostimulation and rhizoremediation
This term identifies the process where compounds released from plant roots enhance microbial
activity in the rhizosphere, which is the narrow region of soil around the roots of plants, and
associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk
soil. In rhizoremediation, microorganisms degrade soil contaminants in the rhizosphere. It is
usual that those soil pollutants which are remediated by this method are highly hydrophobic
organic xenobiotics that are hence unable to enter the plant. Rather than the plant being a main
protagonist in this process, it creates a haven in which microorganisms in the rhizosphere are
able to perform the degradation. The plant acts as a solar-powered pump, which draws in both
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water and the xenobiotic agent, simultaneously producing substrates (e.g. root exudates and
root turnover) that assist the growth of the microbes which act as pollutant degrading agents.
Microbial activity is stimulated in the rhizosphere through a number of different routes: (1)
exudates, e.g. sugars, carbohydrates, amino acids, acetates, and enzymes nourish indigenous
microbe populations; (2) root systems bring oxygen into the rhizosphere, meaning that
aerobic transformations are supported; (3) the available organic carbon is enhanced through
the growth of fine-root biomass; (4) mycorrhizal fungi, which are an essential component of
the rhizosphere, provide unique enzymatic pathways lending the capacity to degrade pollutant
molecules that would not be degraded by bacteria alone; and (5) the presence of plants (and
their roots) creates a domain for microbial populations, which are activated in the rhizosphere.
There have been five enzyme systems identified in soils: (i) dehalogenase (which acts in
dechlorination reactions of chlorinated hydrocarbons); (ii) nitroreductase (essential for the
initial step of nitroaromatic degradation); (iii) peroxidase (a critical catalyst for oxidation
reactions); (iv) laccase (able to begin the decomposition of otherwise robust aromatic ring
structures); (v) nitrilase (another key factor in oxidation processes). The method is limited in
that when there are high concentrations of pollutants present, the plants may be overwhelmed
and die. The successful use of phytostimulation has been demonstrated in the remediation of
chlorinated solvents from groundwater, petroleum hydrocarbons from soil and groundwater
and PAHs from soil.
11. Phytovolatilisation
Probably, this is the most controversial of the phytoremediation technologies, since it
involves the release of contaminants either directly, or in a metabolically modified form, into
the atmosphere. Phytovolatilisation20 has been used principally for the removal of Hg2+ ions
which are transformed into less toxic elemental mercury21. Tritium (3H), a radioactive isotope
of hydrogen with a half-life of about 12 years, decaying to helium, has also been removed by
phytovolatilisation22. A good deal more research is necessary before this strategy becomes
mainstream, since there are various negative features to be addressed. For example, mercury
that is released into the atmosphere from plants is likely to be recycled by precipitation and
thus returned the ecosystem, and the method is restricted both to sites where the concentration
of contaminants is toward the low side, and where the contamination is no deeper than the
roots of the plants being used.
Current Commentary
12. Rhizofiltration
Rhizofiltration23 involves filtering contaminated water through a mass of roots for the
extraction of contaminants, or excess nutrients, e.g. phosphorus. The contaminated water can
either be collected from a waste site and taken to where plants are being hydroponically
cultivated, or the plants may be planted in the area directly. In both cases, the roots draw up
the water and its associated contaminants. This process is very similar to phytoextraction
in that the contaminants become sequestered in the form of harvestable plant biomass.
Then new plants are grown and harvested until a satisfactory degree of decontamination is
achieved. It is the concentration and precipitation of heavy metals that is sought principally.
While noting these similarities, the fundamental difference between the two approaches is
that rhizofiltration is used in aquatic environments, while phytoextraction is applied to the
decontamination of soils. There are limitations to rhizofiltration. As usual in phytoremediation
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methods, any contaminant that is below the rooting depth will not be extracted, and if the
level of contamination is too high the plants will not grow. Depending on the type of plant
and contaminant, the process may need to be continued over a protracted period, before
regulatory levels are achieved. It is generally true that many different kinds of contaminants
will be present – in some cases a mixture of organics and heavy metals – and thus the use
of rhizofiltration alone is unlikely to succeed. Importantly, the plants chosen should be nonfodder crop to minimise poisoning animals, which might eat them in contaminated form. That
noted, the effective removal of heavy metal cations, e.g. Cu2+, Cd2+, Cr6+, Ni2+, Pb2+, and Zn2+
from aqueous solutions has been demonstrated24, and the removal of low-level radionuclides,
from liquid streams25. In that latter application, a “feeder layer” of soil is suspended above
the stream through which plants grow, from which the plant roots extend downward into the
water. In this way, fertiliser can be used to help the plants to grow, while avoiding adding
to the contamination of the stream, while the latter is cleansed of heavy metal cations26.
Rhizofiltration is cost-effective when large volumes of water must be treated containing low
concentrations of contaminants. Inclusive of the costs of the capital outlay and final waste
disposal, the cost of removing radionuclides from water using sunflowers was reckoned (at
1996 prices) at $2─6 per thousand gallons of water treated27.
Current Commentary
References
1. http://www.clu-in.org/download/citizens/a_citizens_guide_to_bioremediation.pdf
2. http://www.ars.usda.gov/is/ar/archive/jun00/soil0600.htm
3. http://en.wikipedia.org/wiki/Bioremediation
4. Meagher, R.B. (2000) Phytoremediation of toxic elemental and organic pollutants. Current Opinion in Plant
Biology, 3, 153.
5. Diaz, E. (edi.) (2008). Microbial biodegradation: genomics and molecular biology, 1st edn. Caister Academic
Press.
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