Biodegradation

From Wikipedia, the free encyclopedia

Yellow slime mold growing on a bin of wet paper

IUPAC definition

Degradation caused by enzymatic process resulting from the action of cells.

Note: Modified to exclude abiotic enzymatic processes.[1]

Biodegradation is the chemical dissolution of materials by bacteria or other biological means. Although often conflated, biodegradable is distinct in meaning from compostable. While

biodegradable simply means to be consumed by microorganisms and return to compounds found in nature, "compostable" makes the specific demand that the object break down in a compost pile. The term is often used in relation to ecology, waste management, biomedicine, and the

natural environment (bioremediation) and is now commonly associated with environmentally friendly products that are capable of decomposing back into natural elements. Organic material

can be degraded aerobically with oxygen, or anaerobically, without oxygen. Biosurfactant, an extracellular surfactant secreted by microorganisms, enhances the biodegradation process.

Biodegradable matter is generally organic material such as plant and animal matter and other

substances originating from living organisms, or artificial materials that are similar enough to plant and animal matter to be put to use by microorganisms. Some microorganisms have a naturally occurring, microbial catabolic diversity to degrade, transform or accumulate a huge

range of compounds including hydrocarbons (e.g. oil), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pharmaceutical substances, radionuclides, pesticides, and metals. Decomposition of biodegradable substances may include both biological and abiotic

steps. Products that contain biodegradable matter and non-biodegradable matter are often marketed as biodegradable.

Monona Rossol wrote that "biodegradable substances break down into more than one set of

chemicals, which are usually called primary and secondary degradation products. Any of these may be toxic."

Contents

1 Meteorology 2 Plastics

3 Biodegradable technology 4 Etymology of "biodegradable"

5 See also 6 References 7 External links

Meteorology

In nature, different materials biodegrade at different rates, and a number of factors are important in the rate of degradation of organic compounds.[2] To be able to work effectively, most

microorganisms that assist the biodegradability need light, water and oxygen. Temperature is also an important factor in determining the rate of biodegradability. This is because microorganisms tend to reproduce faster in warmer conditions. The rate of degradation of many

soluble organic compounds is limited by bioavailability when the compounds have a strong affinity for surfaces in the environment, and thus must be released to solution before organisms

can degrade them.[3]

Biodegradability can be measured in a number of ways. Scientists often use respirometry tests for aerobic microbes. First one places a solid waste sample in a container with microorganisms and soil, and then aerate the mixture. Over the course of several days, microorganisms digest the

sample bit by bit and produce carbon dioxide – the resulting amount of CO2 serves as an indicator of degradation. Biodegradability can also be measured by anaerobic microbes and the

amount of methane or alloy that they are able to produce. In formal scientific literature, the process is termed bio-remediation.[4]

Approximated time for compounds to biodegrade in a marine environment[5]

Product Time to Biodegrade

Apple core 1–2 months

General paper 1–3 months

Paper towel 2–4 weeks

Cardboard box 2 months

Cotton cloth 5 months

Plastic coated milk carton 5 years

Wax coated milk carton 3 months

Tin cans 50–100 years

Aluminium cans 150–200 years

Glass bottles Undetermined (forever)

Plastic bags 10–20 years

Soft plastic (bottle) 100 years

Hard plastic (bottle cap) 400 years

Plastics

Main article: Biodegradable plastic § Examples of biodegradable plastics

Biodegradable plastic is plastic that has been treated to be easily broken down by microorganisms and return to nature. Many technologies exist today that allow for such treatment. Currently there are some synthetic polymers that can be broken down by microorganisms such as polycaprolactone, others are polyesters and aromatic-aliphatic esters,

due to their ester bonds being susceptible to attack by water. Some examples of these are the bio-derived poly-3-hydroxybutyrate, the renewably derived polylactic acid, and the synthetic

polycaprolactone. Others are the cellulose-based cellulose acetate and celluloid (cellulose nitrate).

Under low oxygen conditions biodegradable plastics break down slower and with the production of methane, like other organic materials do. The breakdown process is accelerated in a dedicated

compost heap. Starch-based plastics will degrade within two to four months in a home compost bin, while polylactic acid is largely undecomposed, requiring higher temperatures.[6]

Polycaprolactone and polycaprolactone-starch composites decompose slower, but the starch content accelerates decomposition by leaving behind a porous, high surface area polycaprolactone. Nevertheless, it takes many months.[7]

Many plastic companies have gone so far even to say that their plastics are compostable, typically listing corn starch as an ingredient. However, these claims are questionable because the plastics industry operates under its own definition of compostable:

"that which is capable of undergoing biological decomposition in a compost site such that the

material is not visually distinguishable and breaks down into carbon dioxide, water, inorganic compounds and biomass at a rate consistent with known compostable materials." (Ref: ASTM D

6002)[8]

Using this new definition, "carbon dioxide, water, inorganic compounds and biomass" encompasses every substance in the known universe, it makes no restriction on what the plastic leaves behind after it has biodegraded. So, while plastic manufacturers may legally be on solid

ground, "compostable plastics" can not be said to be compostable in the traditional sense. However the word biodegradable does still apply.

Biodegradable technology

In 1973 it was proved for first time that polyester degrades when disposed in bioactive material such as soil. As a result, polyesters are water resistant and can be melted and shaped into sheets,

bottles, and other products, making certain plastics now available as a biodegradable product. Following, Polyhydroxylalkanoates (PHAs) were produced directly from renewable resources by

microbes. They are approximately 95% cellular bacteria and can be manipulated by genetic strategies. The composition and biodegradability of PHAs can be regulated by blending it with

other natural polymers. In the 1980s the company ICI Zenecca commercialized PHAs under the name Biopol. It was used for the production of shampoo bottles and other cosmetic products. Consumer response was unusual. Consumers were willing to pay more for this product because it

was natural and biodegradable, which had not occurred before.[9]

Now biodegradable technology is a highly developed market with applications in product packaging, production and medicine. Biodegradable technology is concerned with the

manufacturing science of biodegradable materials. It imposes science based mechanisms of plant genetics into the processes of today. Scientists and manufacturing corporations can help impact climate change by developing a use of plant genetics that would mimic some technologies. By

looking to plants, such as biodegradable material harvested through photosynthesis, waste and toxins can be minimized.[10]

Oxo-biodegradable technology, which has further developed biodegradable plastics, has also

emerged. Oxo-biodegradation is defined by CEN (the European Standards Organisation) as "degradation resulting from oxidative and cell-mediated phenomena, either simultaneously or

successively." Whilst sometimes described as "oxo-fragmentable," and "oxo-degradable" this describes only the first or oxidative phase. These descriptions should not be used for material which degrades by the process of oxo-biodegradation defined by CEN, and the correct

description is "oxo-biodegradable."

By combining plastic products with very large polymer molecules, which contain only carbon and hydrogen, with oxygen in the air, the product is rendered capable of decomposing in

anywhere from a week to one to two years. This reaction occurs even without prodegradant additives but at a very slow rate. That is why conventional plastics, when discarded, persist for a long time in the environment. Oxo-biodegradable formulations catalyze and accelerate the

biodegradation process but it takes considerable skill and experience to balance the ingredients within the formulations so as to provide the product with a useful life for a set period, followed

by degradation and biodegradation.[11]

Biodegradable technology is especially utilized by the bio-medical community. Biodegradable polymers are classified into three groups: medical, ecological, and dual application, while in terms of origin they are divided into two groups: natural and synthetic.[12] The Clean Technology

Group is exploiting the use of supercritical carbon dioxide, which under high pressure at room temperature is a solvent that can use biodegradable plastics to make polymer drug coatings. The

polymer (meaning a material composed of molecules with repeating structural units that form a long chain) is used to encapsulate a drug prior to injection in the body and is based on lactic acid, a compound normally produced in the body, and is thus able to be excreted naturally. The

coating is designed for controlled release over a period of time, reducing the number of injections required and maximizing the therapeutic benefit. Professor Steve Howdle states that

biodegradable polymers are particularly attractive for use in drug delivery, as once introduced into the body they require no retrieval or further manipulation and are degraded into soluble,

non-toxic by-products. Different polymers degrade at different rates within the body and therefore polymer selection can be tailored to achieve desired release rates.[13]

Other biomedical applications include the use of biodegradable, elastic shape-memory polymers.

Biodegradable implant materials can now be used for minimally invasive surgical procedures through degradable thermoplastic polymers. These polymers are now able to change their shape

with increase of temperature, causing shape memory capabilities as well as easily degradable sutures. As a result, implants can now fit through small incisions, doctors can easily perform complex deformations, and sutures and other material aides can naturally biodegrade after a

completed surgery.[14]

Etymology of "biodegradable"

The first known use of the word in biological text was in 1961 when employed to describe the

breakdown of material into the base components of carbon, hydrogen, and oxygen by microorganisms. Now biodegradable is commonly associated with environmentally friendly products that are part of the earth's innate cycle and capable of decomposing back into natural

elements.

See also

Sustainable development portal

Ecology portal

Environment portal

Anaerobic digestion Biodegradability prediction

Biodegradable electronics Biodegradable polythene film

Biodegradation (journal) Bioplastic – biodegradable, bio-based plastics Bioremediation

Decomposition – reduction of the body of a formerly living organism into simpler forms of matter

Landfill gas monitoring List of environment topics Microbial biodegradation

Bioremediation

From Wikipedia, the free encyclopedia

Bioremediation is a waste management technique that involves the use of organisms to remove or neutralize pollutants from a contaminated site.[1] According to the EPA, bioremediation is a

“treatment that uses naturally occurring organisms to break down hazardous substances into less toxic or non toxic substances”. Technologies can be generally classified as in situ or ex situ. In

situ bioremediation involves treating the contaminated material at the site, while ex situ involves the removal of the contaminated material to be treated elsewhere. Some examples of bioremediation related technologies are phytoremediation, bioventing, bioleaching, landfarming,

bioreactor, composting, bioaugmentation, rhizofiltration, and biostimulation.

Bioremediation may occur on its own (natural attenuation or intrinsic bioremediation) or may only effectively occur through the addition of fertilizers, oxygen, etc., that help encourage the

growth of the pollution-eating microbes within the medium (biostimulation). For example, the US Army Corps of Engineers demonstrated that windrowing and aeration of petroleum-contaminated soils enhanced bioremediation using the technique of landfarming.[2] Depleted soil

nitrogen status may encourage biodegradation of some nitrogenous organic chemicals, [3] and soil materials with a high capacity to adsorb pollutants may slow down biodegradation owing to

limited bioavailability of the chemicals to microbes.[4] Recent advancements have also proven successful via the addition of matched microbe strains to the medium to enhance the resident microbe population's ability to break down contaminants. Microorganisms used to perform the

function of bioremediation are known as bioremediators.

However, not all contaminants are easily treated by bioremediation using microorganisms. For example, heavy metals such as cadmium and lead are not readily absorbed or captured by

microorganisms. A recent experiment, however, suggests that fish bones have some success absorbing lead from contaminated soil.[5][6] Bone char has been shown to bioremediate small amounts of cadmium, copper, and zinc.[7] The assimilation of metals such as mercury into the

food chain may worsen matters. Phytoremediation is useful in these circumstances because natural plants or transgenic plants are able to bioaccumulate these toxins in their above-ground

parts, which are then harvested for removal.[8] The heavy metals in the harvested biomass may be further concentrated by incineration or even recycled for industrial use. Some damaged artifacts at museums contain microbes which could be specified as bio remediating agents.[9] In contrast

to this situation, other contaminants, such as aromatic hydrocarbons as are common in petroleum, are relatively simple targets for microbial degradation, and some soils may even have

some capacity to autoremediate, as it were, owing to the presence of autochthonous microbial communities capable of degrading these compounds.[10]

The elimination of a wide range of pollutants and wastes from the environment requires

increasing our understanding of the relative importance of different pathways and regulatory networks to carbon flux in particular environments and for particular compounds, and they will certainly accelerate the development of bioremediation technologies and biotransformation

processes.[11]

Contents

1 Genetic engineering approaches 2 Mycoremediation

3 Advantages 4 Monitoring bioremediation

5 See also 6 References

7 External links

Genetic engineering approaches

The use of genetic engineering to create organisms specifically designed for bioremediation has great potential.[12] The bacterium Deinococcus radiodurans (the most radioresistant organism

known) has been modified to consume and digest toluene and ionic mercury from highly radioactive nuclear waste.[13] Releasing genetically augmented organisms into the environment

may be problematic as tracking them can be difficult; bioluminescence genes from other species may be inserted to make this easier. [14] :135

Mycoremediation

Mycoremediation is a form of bioremediation in which fungi are used to decontaminate the

area. The term mycoremediation refers specifically to the use of fungal mycelia in bioremediation.

One of the primary roles of fungi in the ecosystem is decomposition, which is performed by the

mycelium. The mycelium secretes extracellular enzymes and acids that break down lignin and cellulose, the two main building blocks of plant fiber. These are organic compounds composed of long chains of carbon and hydrogen, structurally similar to many organic pollutants. The key

to mycoremediation is determining the right fungal species to target a specific pollutant. Certain strains have been reported to successfully degrade the nerve gases VX and sarin.

In one conducted experiment, a plot of soil contaminated with diesel oil was inoculated with

mycelia of oyster mushrooms; traditional bioremediation techniques (bacteria) were used on control plots. After four weeks, more than 95% of many of the PAH (polycyclic aromatic hydrocarbons) had been reduced to non-toxic components in the mycelial- inoculated plots. It

appears that the natural microbial community participates with the fungi to break down contaminants, eventually into carbon dioxide and water. Wood-degrading fungi are particularly

effective in breaking down aromatic pollutants (toxic components of petroleum), as well as chlorinated compounds (certain persistent pesticides; Battelle, 2000).

Two species of the Ecuadorian fungus Pestalotiopsis are capable of consuming Polyurethane in aerobic and anaerobic conditions such as found at the bottom of landfills.[15]

Mycofiltration is a similar process, using fungal mycelia to filter toxic waste and

microorganisms from water in soil.

Advantages

There are a number of cost/efficiency advantages to bioremediation, which can be employed in areas that are inaccessible without excavation. For example, hydrocarbon spills (specifically,

petrol spills) or certain chlorinated solvents may contaminate groundwater, and introducing the appropriate electron acceptor or electron donor amendment, as appropriate, may significantly

reduce contaminant concentrations after a long time allowing for acclimation. This is typically much less expensive than excavation followed by disposal elsewhere, incineration or other ex situ treatment strategies, and reduces or eliminates the need for "pump and treat", a practice

common at sites where hydrocarbons have contaminated clean groundwater.

Monitoring bioremediation

The process of bioremediation can be monitored indirectly by measuring the Oxidation

Reduction Potential or redox in soil and groundwater, together with pH, temperature, oxygen content, electron acceptor/donor concentrations, and concentration of breakdown products (e.g. carbon dioxide). This table shows the (decreasing) biological breakdown rate as function of the

redox potential.

Process Reaction Redox potential (Eh in mV)

aerobic O2 + 4e− + 4H+ → 2H2O 600 ~ 400

anaerobic

denitrification 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

fermentation 2CH2O → CO2 + CH4 −150 ~ −220

This, by itself and at a single site, gives little information about the process of remediation.

1. It is necessary to sample enough points on and around the contaminated site to be able to determine contours of equal redox potential. Contouring is usually done using specialised software, e.g. using Kriging interpolation.

2. If all the measurements of redox potential show that electron acceptors have been used up, it is in effect an indicator for total microbial activity. Chemical analysis is also

required to determine when the levels of contaminants and their breakdown products have been reduced to below regulatory limits.

See also

Sustainable development portal

Biotechnology portal

Fungi portal

Biodegradation Bioleaching

Biosurfactant Dutch standards

Folkewall List of environment topics Living machines

Living wall Microbial biodegradation

Mycoremediation Phytoremediation Pseudomonas putida (used for degrading oil)

US Microbics Xenocatabolism

References

1. http://ei.cornell.edu/biodeg/bioremed/ 2. Mann, D. K., T. M. Hurt, E. Malkos, J. Sims, S. Twait and G. Wachter. 1996. Onsite

treatment of petroleum, oil, and lubricant (POL)-contaminated soils at Illinois Corps of

Engineers lake sites. US Army Corps of Engineers Technical Report No. A862603 (71pages).

3. Sims, G.K. 2006. Nitrogen Starvation Promotes Biodegradation of N-Heterocyclic Compounds in Soil. Soil Biology & Biochemistry 38:2478-2480.

4. O'Loughlin, E. J, S. J. Traina, and G. K. Sims. 2000. Effects of sorption on the

biodegradation of 2-methylpyridine in aqueous suspensions of reference clay minerals. Environ. Toxicol. and Chem. 19:2168-2174.

5. Kris S. Freeman (January 2012). "Remediating Soil Lead with Fishbones".

Environmental Health Perspectives. 6. http://coastguard.dodlive.mil/2012/07/battling- lead-contamination-one-fish-bone-at-a-

time/ 7. Huan Jing Ke Xue (February 2007). "Chemical fixation of metals in soil using bone char

and assessment of the soil genotoxicity".

8. Meagher, RB (2000). "Phytoremediation of toxic elemental and organic pollutants". Current Opinion in Plant Biology 3 (2): 153–162. doi:10.1016/S1369-5266(99)00054-0.

PMID 10712958. 9. Francesca Cappitelli; Claudia Sorlini (2008). "Microorganisms Attack Synthetic

Polymers in Items Representing Our Cultural Heritage". Applied Environmental

Microbiology 74. 10. Olapade, OA; Ronk, AJ (2014). "Isolation, Characterization and Community Diversity of

Indigenous Putative Toluene-Degrading Bacterial Populations with Catechol-2,3-Dioxygenase Genes in Contaminated Soils". Microbial Ecology. Epub ahead of print. PMID 25052383.

11. Diaz E (editor). (2008). Microbial Biodegradation: Genomics and Molecular Biology (1st ed.). Caister Academic Press. ISBN 1-904455-17-4.

http://www.horizonpress.com/biod.

12. Lovley, DR (2003). "Cleaning up with genomics: applying molecular biology to bioremediation". Nature Reviews Microbiology 1 (1): 35–44. doi:10.1038/nrmicro731.

PMID 15040178. 13. Brim H, McFarlan SC, Fredrickson JK, Minton KW, Zhai M, Wackett LP, Daly MJ

(2000). "Engineering Deinococcus radiodurans for metal remediation in radioactive mixed waste environments". Nature Biotechnology 18 (1): 85–90. doi:10.1038/71986. PMID 10625398.

14. Robert L. Irvine; Subhas K. Sikdar. "Bioremediation Technologies: Principles and Practice".

15. "Biodegradation of Polyester Polyurethane by Endophytic Fungi". Applied and Environmental Microbiology. July 2011.

External links

Phytoremediation Website hosted by the Missouri Botanical Garden

Toxic cadmium ions removal by isolated fungal strain from e-waste recycling facility (Kumar et al., 2012)

Removal of Cu2+ Ions from Aqueous Solutions Using Copper Resistant Bacteria (Rajeshkumar and Kartic 2011)

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Phytoremediation

From Wikipedia, the free encyclopedia

Phytoremediation (from Ancient Greek φυτο (phyto), meaning "plant", and Latin remedium, meaning "restoring balance") describes the treatment of environmental problems

(bioremediation) through the use of plants that mitigate the environmental problem without the need to excavate the contaminant material and dispose of it elsewhere.

Phytoremediation consists of mitigating pollutant concentrations in 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 media that contain them.

Contents

1 Application

2 Advantages and limitations 3 Various phytoremediation processes

o 3.1 Phytoextraction

o 3.2 Phytostabilization o 3.3 Phytotransformation

4 Role of genetics 5 Hyperaccumulators and biotic interactions

o 5.1 Table of hyperaccumulators

6 Phytoscreening 7 See also

8 References 9 Bibliography 10 External links

Application

Phytoremediation may be applied wherever the soil or static water environment has become polluted or is suffering ongoing chronic pollution. Examples where phytoremediation has been

used successfully include the restoration of abandoned metal mine workings, reducing the impact

of contaminants in soils, water, or air. Contaminants such as metals, pesticides, solvents, explosives,[1] and crude oil and its derivatives, have been mitigated in phytoremediation projects

worldwide. Many plants such as mustard plants, alpine pennycress, hemp, and pigweed have proven to be successful at hyperaccumulating contaminants at toxic waste sites.

Over the past 20 years, this technology has become increasingly popular and has been employed

at sites with soils contaminated with lead, uranium, and arsenic. While it has the advantage that environmental concerns may be treated in situ; one major disadvantage of phytoremediation is that it requires a long-term commitment, as the process is dependent on a plant's ability to grow

and thrive in an environment that is not ideal for normal plant growth. Phytoremediation may be applied wherever the soil or static water environment has become polluted or is suffering

ongoing chronic pollution. Examples where phytoremediation has been used successfully include the restoration of abandoned metal-mine workings, reducing the impact of sites where polychlorinated biphenyls have been dumped during manufacture and mitigation of ongoing coal

mine discharges.

Phytoremediation refers to the natural ability of certain plants called hyperaccumulators to bioaccumulate, degrade,or render harmless contaminants in soils, water, or air.

Advantages and limitations

Advantages: o the cost of the phytoremediation is lower than that of traditional processes both in

situ and ex situ

o the plants can be easily monitored o the possibility of the recovery and re-use of valuable metals (by companies

specializing in “phyto mining”) o it is potentially the least harmful method because it uses naturally occurring

organisms and preserves the environment in a more natural state.

Limitations :

o phytoremediation is limited to the surface area and depth occupied by the roots. o slow growth and low biomass require a long-term commitment

o with plant-based systems of remediation, it is not possible to completely 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) o the survival of the plants is affected by the toxicity of the contaminated land and

the general condition of the soil. o 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.

Various phytoremediation processes

A range of processes mediated by plants or algae are useful in treating 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.[2]

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

See also: Phytoextraction process

Phytoextraction (or phytoaccumulation) uses plants or algae to remove contaminants from soils, sediments or water into harvestable plant biomass (organisms that take larger-than-normal

amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been growing rapidly in popularity worldwide for the last twenty years or so. In general, 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.

Advantages: The main advantage of phytoextraction is environmental friendliness. Traditional

methods that are used for cleaning up heavy metal-contaminated soil disrupt soil structure and reduce soil productivity, whereas phytoextraction can clean up the soil without causing any kind

of harm to soil quality. Another benefit of phytoextraction is that it is less expensive than any other clean-up process.

Disadvantages: As this process is controlled by plants, it takes more time than anthropogenic soil clean-up methods.

Two versions of phytoextraction:

natural hyper-accumulation, where plants naturally take up the contaminants in soil unassisted.

induced or assisted hyper-accumulation, where 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 (see also 'Table of hyperaccumulators'):

Arsenic, using the Sunflower (Helianthus annuus),[3] or the Chinese Brake fern (Pteris vittata),[4] a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.

Cadmium, using willow (Salix viminalis): In 1999, one research experiment performed

by Maria Greger and Tommy Landberg suggested 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 bio energy in the biomass energy power plant.[5]

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 (see table for reference).

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.

Caesium-137 and strontium-90 were removed from a pond using sunflowers after the Chernobyl accident.[6]

Mercury, selenium and organic pollutants such as polychlorinated biphenyls (PCBs) have

been removed from soils by transgenic plants containing genes for bacterial enzymes.[7]

Phytostabilization

Phytostabilization focuses on long-term stabilization and containment of the pollutant. 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 focuses mainly 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.[8]

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, carbon-dioxide, 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 analogously to the human liver when dealing with these

xenobiotic compounds (foreign compound/pollutant).[9] 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 that the human liver increases the

polarity of drugs and foreign compounds (Drug Metabolism). Whereas in the human liver enzymes such as 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 where glucuronidation (addition of glucose molecules by the UGT (e.g. UGT1A1) class of enzymes) and glutathione addition reactions occur on reactive centres 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. 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 develop a complex structure that is sequestered in the plant. This ensures that the xenobiotic is safely stored, 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.

Hence, the plants reduce toxicity (with exceptions) and sequester the xenobiotics in

phytotransformation. Trinitrotoluene phytotransformation has been extensively researched and a transformation pathway has been proposed.[10]

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.[11] Researchers have also

discovered a mechanism in plants that allows them to grow even when the pollution concentration in the soil is lethal for non-treated plants. Some natural, biodegradable compounds,

such as exogenous polyamines, allow the plants to tolerate concentrations of pollutants 500 times higher than untreated plants, and to absorb more pollutants.

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).[12] This capacity for accumulation is due to hypertolerance, or phytotolerance: the result of adaptative evolution from the plants to hostile environments through many generations. A number of interactions may be affected by metal hyperaccumulation, including

protection, interferences with neighbour plants of different species, mutualism (including mycorrhizae, pollen and seed dispersal), commensalism, and biofilm.

Table of hyperaccumulators

Hyperaccumulators table – 1 : Al, Ag, As, Be, Cr, Cu, Mn, Hg, Mo, Naphthalene, Pb, Pd, Pt, Se, Zn

Hyperaccumulators table – 2 : Nickel

Hyperaccumulators table – 3 : Radionuclides (Cd, Cs, Co, Pu, Ra, Sr, U), Hydrocarbons, Organic Solvents.

Phytoscreening

As plants are able to translocate and accumulate particular types of contaminants, plants can be

used as biosensors of subsurface contamination, thereby allowing investigators to quickly delineate contaminant plumes.[13][14] Chlorinated solvents, such as trichloroethylene, have been

observed in tree trunks at concentrations related to groundwater concentrations.[15] To ease field implementation of phytoscreening, standard methods have been developed to extract a section of the tree trunk for later laboratory analysis, often by using an increment borer.[16] Phytoscreening

may lead to more optimized site investigations and reduce contaminated site cleanup costs.

See also

Phytoremediation plants

Phytotreatment Biodegradation Bioremediation

References

1. Phytoremediation of soils using Ralstonia eutropha, Pseudomas tolaasi, Burkholderia fungorum reported by Sofie Thijs

2. Rupassara, S. I.; Larson, R. A.; Sims, G. K. & Marley, K. A. (2002), Degradation of Atrazine by Hornwort in Aquatic Systems, Bioremediation Journal 6 (3): 217–224,

doi:10.1080/10889860290777576. 3. Marchiol, L.; Fellet, G.; Perosa, D.; Zerbi, G. (2007), Removal of trace metals by

Sorghum bicolor and Helianthus annuus in a site polluted by industrial wastes: A field

experience, Plant Physiology and Biochemistry 45 (5): 379–87, doi:10.1016/j.plaphy.2007.03.018, PMID 17507235

4. Wang, J.; Zhao, FJ; Meharg, AA; Raab, A; Feldmann, J; McGrath, SP (2002), Mechanisms of Arsenic Hyperaccumulation in Pteris vittata. Uptake Kinetics, Interactions with Phosphate, and Arsenic Speciation, Plant Physiology 130 (3): 1552–61,

doi:10.1104/pp.008185, PMC 166674, PMID 12428020 5. Greger, M. & Landberg, T. (1999), Using of Willow in Phytoextraction, International

Journal of Phytoremediation 1 (2): 115–123, doi:10.1080/15226519908500010. 6. Adler, Tina (July 20, 1996). "Botanical cleanup crews: using plants to tackle polluted

water and soil". Science News. Retrieved 2010-09-03.

7. Meagher, RB (2000), Phytoremediation of toxic elemental and organic pollutants, Current Opinion in Plant Biology 3 (2): 153–162, doi:10.1016/S1369-5266(99)00054-0,

PMID 10712958. 8. Mendez MO, Maier RM (2008), Phytostabilization of Mine Tailings in Arid and Semiarid

Environments—An Emerging Remediation Technology, Environ Health Perspect 116 (3):

278–83, doi:10.1289/ehp.10608, PMC 2265025, PMID 18335091. 9. Burken, J.G. (2004), "2. Uptake and Metabolism of Organic Compounds: Green-Liver

Model", in McCutcheon, S.C.; Schnoor, J.L., 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

10. Subramanian, Murali; Oliver, David J. & Shanks, Jacqueline V. (2006), TNT Phytotransformation Pathway Characteristics in Arabidopsis: Role of Aromatic

Hydroxylamines, Biotechnol. Prog. 22 (1): 208–216, doi:10.1021/bp050241g, PMID 16454512.

11. Hannink, N.; Rosser, S. J.; French, C. E.; Basran, A.; Murray, J. A.; Nicklin, S.; Bruce,

N. C. (2001), Phytodetoxification of TNT by transgenic plants expressing a bacterial nitroreductase, Nature Biotechnology 19 (12): 1168–72, doi:10.1038/nbt1201-1168,

PMID 11731787. 12. Baker, A. J. M.; Brooks, R. R. (1989), Terrestrial higher plants which hyperaccumulate

metallic elements – A review of their distribution, ecology and phytochemistry,

Biorecovery 1 (2): 81–126. 13. Burken, J.; Vroblesky, D.; Balouet, J.C. (2011), Phytoforensics, Dendrochemistry, and

Phytoscreening: New Green Tools for Delineating Contaminants from Past and Present , Environmental Science & Technology 45 (15): 6218–6226, doi:10.1021/es2005286.

14. Sorek, A.; Atzmon, N.; Dahan, O.; Gerstl, Z.; Kushisin, L.; Laor, Y.; Mingelgrin, U.;

Nasser, A.; Ronen, D.; Tsechansky, L.; Weisbrod, N.; Graber, E.R. (2008), "Phytoscreening": The Use of Trees for Discovering Subsurface Contamination by

VOCs, Environmental Science & Technology 42 (2): 536–542, doi:10.1021/es072014b.

15. Vroblesky, D.; Nietch, C.; Morris, J. (1998), Chlorinated Ethenes from Groundwater in Tree Trunks, Environmental Science & Technology 33 (3): 510–515,

doi:10.1021/es980848b. 16. Vroblesky, D. (2008). "User’s Guide to the Collection and Analysis of Tree Cores to

Assess the Distribution of Subsurface Volatile Organic Compounds".

Bibliography

“Phytoremediation Website” — Includes reviews, conference announcements, lists of companies doing phytoremediation, and bibliographies.

“An Overview of Phytoremediation of Lead and Mercury” June 6 2000. The Hazardous Waste Clean-Up Information Web Site.

“Enhanced phytoextraction of arsenic from contaminated soil using sunflower” September 22 2004. U.S. Environmental Protection Agency.

“Phytoextraction”, February 2000. Brookhaven National Laboratory 2000.

“Phytoextraction of Metals from Contaminated Soil” April 18, 2001. M.M. Lasat July 2002. Donald Bren School of Environment Science & Management.

“Phytoremediation” October 1997. Department of Civil Environmental Engineering. “Phytoremediation” June 2001, Todd Zynda. “Phytoremediation of Lead in Residential Soils in Dorchester, MA” May, 2002. Amy

Donovan Palmer, Boston Public Health Commission. “Technology Profile: Phytoextraction” 1997. Environmental Business Association.

Vassil AD, Kapulnik Y, Raskin I, Salt DE (June 1998), The Role of EDTA in Lead Transport and Accumulation by Indian Mustard, Plant Physiol. 117 (2): 447–53, doi:10.1104/pp.117.2.447, PMC 34964, PMID 9625697.

Salt, D. E.; Smith, R. D.; Raskin, I. (1998). "Phytoremediation". Annual Review of Plant Physiology and Plant Molecular Biology 49: 643–668. doi:10.1146/annurev.arplant.49.1.643. PMID 15012249. edit

External links

Look up phytoremediation in Wiktionary, the free dictionary.

Missouri Botanical Garden (host): Phytoremediation website — Review Articles, Conferences, Phytoremediation Links, Research Sponsors, Books and Journals, and Recent Research.

International Journal of Phytoremediation — devoted to the publication of current laboratory and field research describing the use of plant systems to remediate contaminated environments.

Using Plants To Clean Up Soils — from Agricultural Research magazine New Alchemy Institute — co-founded by John Todd (biologist)

Categories:

Bioremediation Phytoremediation plants

Environmental soil science Environmental engineering

Environmental terminology Pollution control technologies Conservation projects

Ecological restoration Soil contamination

Biotechnology Sustainable technologies

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