http://www.trelleborg.com/upload/TTime/2-2012/EN01-24_T-Time%202%202012.pdf pages 19-21.
Saturday, November 30, 2013
Thursday, November 28, 2013
Applications of Bioremediation and Phytoremediation.
This article has been published in the journal Science Progress, which I am an Editor of. You can download a free pdf version, with figures etc. from this link: http://stl.publisher.ingentaconnect.com/content/stl/sciprg/2013/00000096/00000004/art00004
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 fertilizers (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 this approach, and e.g. heavy metal contaminants, such as Cd2+ and Pb2+, tend to resist interception by microorganisms, which can at best change their oxidation state so that the solubility of the cations is decreased. The better way is to physically remove heavy metals using phytoremediation, by which 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 levels of the contaminating compounds (and their products of decomposition) are below regulatory limits.
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 fertilizers (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 this approach, and e.g. heavy metal contaminants, such as Cd2+ and Pb2+, tend to resist interception by microorganisms, which can at best change their oxidation state so that the solubility of the cations is decreased. The better way is to physically remove heavy metals using phytoremediation, by which 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 levels of the contaminating compounds (and their products of decomposition) are below regulatory limits.
Process
|
Reaction
|
Redox potential
(Eh in mV)
|
O2 + 4e−
+ 4H+ → 2H2O
|
600 ~ 400
|
|
2NO3− +
10e− + 12H+ → N2 + 6H2O
|
500 ~ 200
|
|
Manganese(IV)
reduction
|
MnO2 + 2e−
+ 4H+ → Mn2+ + 2H2O
|
400 ~ 200
|
Iron(III) reduction
|
Fe(OH)3 + e−
+ 3H+ → Fe2+ + 3H2O
|
300 ~ 100
|
Sulfate
reduction
|
SO42− +
8e− +10 H+ → H2S + 4H2O
|
0 ~ −150
|
2CH2O → CO2
+ CH4
|
−150 ~ −220
|
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 radioresistent 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.
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 toward 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.
The move toward finding “green” ways to ameliorate many environmental woes, including dealing with polluted environments, has led to a rising focus toward 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 mineralization 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 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 described, 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 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.
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.
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 which 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 optimized 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.
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) 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 inocculating a
plot of soil contaminated by 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.
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.
Advantages and limitations of phytoremediation
- Advantages:
- 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 specializing 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.
- 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 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.
- Limitations:
- 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.
- The procedure is slow.
Hyperaccumulators and biotic interactions.
If a plant
is able to concentrate a particular contaminant, to a given minimum
concentration (> 1000 mg/kg of dry weight for nickel, copper, cobalt, chromium
or lead;
or > 10,000 mg/kg for zinc or manganese), it is categorized 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.
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.
- 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. Phytostimulation can also involve aquatic plants supporting active populations of microbial degraders, as in the stimulation of atrazine degradation by hornwort.
- 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.
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
cleanup. 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 mobilization 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.
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 cadmium (Cd), zinc (Zn), and copper (Cu), as willow has some specific characteristics like high transport capacity of heavy metals from root to shoot and huge amount of biomass production; can be used also for production of bioenergy in the biomass energy 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.
- 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.
- 137Cs and 90Sr contaminating a pond were removed using sunflowers, following the 1986 Chernobyl accident.
- Mercury, selenium and organic pollutants including polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants containing genes for bacterial enzymes.
- Recovery of phosphate from wastewaters by algae.
Phytostabilization.
In
phytostabilization the intention is to stabilize, 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, immobilize the pollutants by adsorption
or accumulation, and provide a zone around the roots where the pollutant can be
deposited in an immobilized form. In contrast with phytoextraction,
phytostabilization 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 stabilized by growing a vegetative cap.
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 metabolize 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 once more
similar to processes such as glucoronidation (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 mobilization
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.
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 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: (i) exudates, e.g. sugars, carbohydrates, amino acids, acetates, and enzymes, nourish indigenous microbe populations; (ii) 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) mycorrhizae 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.
Phytovolatilization.
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. Phytovolatilization20 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 phytovolatilization22. 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.
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 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 non-fodder crop
to minimize 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, fertilizer 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.
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 (editor).
(2008). Microbial Biodegradation: Genomics and Molecular Biology (1st
ed.). Caister Academic Press.
(6) Brim, H. et al. (2000) "Engineering Deinococcus radiodurans for metal
remediation in radioactive mixed waste environments." Nature
Biotechnology 18, 85.
(7) Librando, V. and Pappalardo, M.J.
(2013) “in silico bioremediation of polycyclic aromatic hydrocarbon: A frontier
in environmental chemistry.” Mol. Graph.
Model. May 14;44C:1-8. doi: 10.1016/j.jmgm.2013.04.011. [Epub
ahead of print]
(8) Heider,
J. and Rabus, R. (2008) "Genomic Insights in the Anaerobic Biodegradation
of Organic Pollutants." Microbial Biodegradation: Genomics and
Molecular Biology. Caister Academic Press. ISBN 978-1-904455-17-2.
(9) Cupples, A. M., Sanford,
R.A. and Sims, G.K. (2005) “Dehalogenation of Bromoxynil
(3,5-Dibromo-4-Hydroxybenzonitrile) and Ioxynil (3,5-Diiodino-4-Hydroxybenzonitrile)
by Desulfitobacterium chlororespirans.” Appl. Env. Micro. 71, 3741.
(10) Villemur, R. (2013) “The
pentachlorophenol-dehalogenating Desulfitobacterium
hafniense strain PCP-1.” Phil. Trans.
Roy. Soc. B. 368, 1616.
(11) Ding, C., Chow, W.L., Lee,L.K. and He,J.(2013)”Isolation of Acetobacterium sp.
Strain AG, Which Reductively Debrominates Octa- and Pentabrominated Diphenyl Ether
Technical Mixtures.” J. Appl. Environ. Microbiol. 79,1110.
(12) O'Loughlin, E. J, Traina, S.J. and Sims, G.K. (2000) “Effects of sorption on the
biodegradation of 2-methylpyridine in aqueous suspensions of reference clay minerals.”
Environ. Toxicol. and Chem. 19, 2168.
(13) Parales, R.E. et al. (2008). "Bioavailability, Chemotaxis, and Transport of Organic
Pollutants." Microbial Biodegradation: Genomics and Molecular Biology. Caister Academic
Press. ISBN 978-1-904455-17-2.
(14) http://www.biotecharticles.com/Applications-Article/Potential-Hydrocarbonoclastic-Bacteria-730.html
(15) Dracopoulos, D.C., and Piccoli, R.
(2010) “Bioreactor control by genetic programming”, Parallel
Problem Solving from Nature, PPSN XI Lecture Notes in Computer
Science, 6239, 181.
(16) http://en.wikipedia.org/wiki/Mycoremediation
(17) http://www.fungi.com/blog/items/helping-the-ecosystem-through-mushroom-cultivation.html
(18) Burken,
J.G. (2004), "2. Uptake and Metabolism of Organic Compounds: Green-Liver Model",
in McCutcheon, S.C.; Schnoor, J.L. (Eds.), Phytoremediation: Transformation
and Control of Contaminants, A Wiley-Interscience Series of Texts and
Monographs, Hoboken, NJ: John Wiley, p. 59, doi:10.1002/047127304X.ch2,
ISBN 0-471-39435-1
(19)
http://en.wikipedia.org/wiki/Phytoremediation
(20)
http://www.unep.or.jp/Ietc/Publications/Freshwater/FMS2/2.asp
(21) http://tede.ibict.br/tde_busca/arquivo.php?codArquivo=431
(22) Dushenkov, S. (2003) “Trends in phytoremediation
or radionuclides.” Plant and Soil, 249, 167.
(23) http://en.wikipedia.org/wiki/Rhizofiltration
(24) EPA,
(1998) “A Citizen's Guide to Phytoremediation,.” U.S. Environmental Protection
Agency, Office of Solid Waste and Emergency Response,” EPA 542-F-98-011,
August.
(25) Dushenkov, V., Motto, H., Raskin, I. and
Nanda Kumar, P.B.A. (1995) "Rhizofiltration: the Use of Plants to Remove
Heavy Metals From Aqueous Streams." Environmental
Science Technology 30, 1239.
(26) Raskin, I., Smith, R.D. and Salt, D.E.
(1997) "Phytoremediation of Metals: Using Plants to Remove Pollutants from
the Environment." Current Opinion in
Biotechnology. 8, 221.
(27) Cooney, C. M. (1996) "Sunflowers
Remove Radionuclides From Water in Ongoing Phytoremediation Field Tests." Environmental Science and Technology 30, 194.
Subscribe to:
Posts (Atom)