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Phytoremediation

Phytoremediation process and principles diagram. (French)

Phytoremediation (from the Ancient Greek (phyto,plant), and Latin remedium(restoring balance orremediation) describes the treatment ofenvironmental problems(bioremediation) through the use ofplantsthat mitigate the environmental problemwithout the need to excavate the contaminant material and dispose of it elsewhere.

Phytoremediation consists of mitigating pollutant concentrations in contaminated soils,water, orair, with plants able to contain, degrade, or eliminatemetals,pesticides,solvents,explosives, crude oiland its derivatives, and various other contaminants fromthe media that contain them.

Application

Phytoremediation may be applied wherever the soilor static water environment hasbecome polluted or is suffering ongoing chronicpollution. Examples wherephytoremediation has been used successfully include the restoration of abandoned metal-mine workings, reducing the impact of sites wherepolychlorinated biphenyls have beendumped during manufacture and mitigation of on-going coal mine discharges.

Phytoremediation refers to the natural ability of certain plants called hyperaccumulatorsto bioaccumulate, degrade,or render harmless contaminants in soils, water, or air.Contaminants such as metals, pesticides, solvents, explosives, and crude oil and itsderivatives, have been mitigated in phytoremediation projects worldwide. Many plants

such as mustard plants,alpine pennycress andpigweed have proven to be successful athyperaccumulating contaminants attoxic waste sites.

Phytoremediation is considered a clean, cost-effective and non-environmentallydisruptive technology, as opposed to mechanical cleanup methods such as soil excavationor pumping polluted groundwater. Over the past 20 years, this technology has becomeincreasingly popular and has been employed at sites with soils contaminated with lead,uranium, and arsenic. However, one major disadvantage of phytoremediation is that itrequires a long-term commitment, as the process is dependent on plant growth, toleranceto toxicity, and bioaccumulation capacity.

Advantages and limitations

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

both in situ and ex situo the plants can be easily monitored

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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 naturalstate.

Limitations:o phytoremediation is limited to the surface area and depth occupied by the

roots.o slow growth and lowbiomass require a long-term commitment

o with plant-based systems of remediation, it is not possible to completely

prevent the leaching of contaminants into thegroundwater(without thecomplete removal of the contaminated ground, which in itself does notresolve 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 whichthen pass into the food chain, from primary level consumers upwards orrequires the safe disposal of the affected plant material.

Various phytoremediation processes

A range of processes mediated by plants or algae are useful in treating environmentalproblems:

Phytoextraction uptake and concentration of substances from the environmentinto the plantbiomass.

Phytostabilization reducing the mobility of substances in the environment, forexample, by limiting the leaching of substances from the soil.

Phytotransformation chemical modification of environmental substances as adirect result of plant metabolism, often resulting in their inactivation, degradation(phytodegradation), or immobilization (phytostabilization).

Phytostimulation enhancement ofsoil microbial activity for the degradation ofcontaminants, typically by organisms that associate with roots. This process isalso known as rhizosphere degradation. Phytostimulation can also involve aquaticplants supporting active populations of microbial degraders, as in the stimulationofatrazine degradation byhornwort.[1]

Phytovolatilization removal of substances from soil or water with release into

the air, sometimes as a result of phytotransformation to more volatile and/or lesspolluting substances.

Rhizofiltration filtering water through a mass of roots to remove toxicsubstances or excessnutrients. The pollutants remain absorbed in or adsorbed tothe roots.

Phytoextraction

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Phytoextraction (orphytoaccumulation) uses plants or algae to remove contaminantsfrom 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 twentyyears 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 themuch smaller volume of the plant matter than in the initially contaminated soil orsediment. 'Mining with plants', orphytomining, is also being experimented with.

The plants absorb contaminants through the root system and store them in the rootbiomass and/or transport them up into the stems and/or leaves. A living plant maycontinue to absorb contaminants until it is harvested. After harvest, a lower level of thecontaminant will remain in the soil, so the growth/harvest cycle must usually be repeatedthrough several crops to achieve a significant cleanup. After the process, the cleaned soilcan support other vegetation.

Advantages: The main advantage of phytoextraction is environmental friendliness.Traditional methods that are used for cleaning up heavy metal-contaminated soil disruptsoil structure and reduce soil productivity, whereas phytoextraction can clean up the soilwithout causing any kind of harm to soil quality. Another benefit of phytoextraction isthat it is less expensive than any other clean-up process.

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

Two versions of phytoextraction:

natural hyper-accumulation, where plants naturally take up the contaminants insoil unassisted, and induced or assisted hyper-accumulation, in which a conditioning fluid

containing a chelatoror another agent is added to soil to increase metal solubilityor mobilization so that the plants can absorb them more easily. In many casesnatural hyperaccumulators are metallophyte plants that can tolerate andincorporate high levels of toxic metals.

Examples of phytoextraction (see also 'Table of hyperaccumulators'):

Arsenic, using the Sunflower (Helianthus annuus), or the Chinese Brake fern

(Pteris vittata), a hyperaccumulator. Chinese Brake fern stores arsenic in itsleaves. Cadmium, using Willow (Salix viminalis): In 1999, one research experiment

performed by Maria Greger and Tommy Landberg suggested Willow (Salixviminlais) has a significant potential as a phytoextractor of Cadmium (Cd), Zinc(Zn), and Copper (Cu), as willow has some specific characteristics like hightransport capacity of heavy metals from root to shoot and huge amount of biomass

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production; can be used also for production of bio energy in the biomass energypower plant.[2]

Cadmium and zinc, using Alpine pennycress (Thlaspi caerulescens), ahyperaccumulator of these metals at levels that would betoxic to many plants. Onthe other hand, the presence of copper seems to impair its growth (see table for

reference).

Lead, using Indian Mustard (Brassica juncea), Ragweed (Ambrosiaartemisiifolia), Hemp Dogbane (Apocynum cannabinum), orPoplartrees, whichsequester lead in their biomass.

Salt-tolerant (moderately halophytic)barley and/orsugar beetsare commonlyused for the extraction ofSodium chloride(common salt) to reclaim fields thatwere previously flooded by sea water.

Caesium-137 andstrontium-90 were removed from a pond using sunflowers afterthe Chernobyl accident.[3]

Mercury,selenium and organic pollutants such aspolychlorinated biphenyls

(PCBs) have been removed from soils by transgenic plants containinggenes forbacterial enzymes.[4]

Phytostabilization

Phytostabilization focuses on long-term stabilization and containment of the pollutant.For example, the plant's presence can reduce wind erosion; or the plant's roots canprevent water erosion, immobilize the pollutants by adsorption or accumulation, andprovide a zone around the roots where the pollutant can precipitate and stabilize. Unlikephytoextraction, phytostabilization focuses mainly on sequestering pollutants in soil nearthe 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 avegetative cap to stabilize and contain mine tailings.[5]

Phytotransformation

In the case oforganicpollutants, such aspesticides, explosives,solvents, industrialchemicals, and otherxenobiotic substances, certain plants, such as Cannas, render thesesubstances non-toxic by theirmetabolism. In other cases, microorganisms living inassociation with plant roots may metabolize these substances in soil or water. Thesecomplex and recalcitrant compounds cannot be broken down to basic molecules (water,carbon-dioxide, etc.) by plant molecules, and, hence, the termphytotransformation

represents a change in chemical structure without complete breakdown of the compound.The term "Green Liver Model" is used to describe phytotransformation, as plants behaveanalogously to the human liver when dealing with these xenobiotic compounds(foreigncompound/pollutant).[6] After uptake of the xenobiotics, plant enzymes increase thepolarity 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 increasesthe polarity of drugs and foreign compounds (Drug Metabolism). Whereas in the human

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liver enzymes such as Cytochrome P450s are responsible for the initial reactions, inplants enzymes such as nitroreductases carry out the same role.

In the second stage of phytotransformation, known as Phase II metabolism, plantbiomolecules 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 processesoccurring in the humanpancreas where glucuronidation (addition of glucose moleculesby the UGT (e.g. UGT1A1) class of enzymes) andglutathione addition reactions occuron reactive centres of the xenobiotic.

Phase I and II reactions serve to increase the polarity and reduce the toxicity of thecompounds, although many exceptions to the rule are seen. The increased polarity alsoallows for easy transport of the xenobiotic along aqueous channels.

In the final stage of phytotransformation (Phase III metabolism), asequestration of thexenobiotic 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 thexenobiotic 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 assnails), and, hence, plants involved in phytotransformation may need to be maintained ina closed enclosure.

Hence, the plants reduce toxicity (with exceptions) and sequester the xenobiotics inphytotransformation. Trinitrotoluene phytotransformation has been extensivelyresearched and a transformation pathway has been proposed.[7]

The role of genetics

Breeding programs and genetic engineering are powerful methods for enhancing naturalphytoremediation capabilities, or for introducing new capabilities into plants. Genes forphytoremediation may originate from amicro-organismor may be transferred from oneplant to another variety better adapted to the environmental conditions at the cleanup site.For example, genes encoding a nitroreductase from a bacterium were inserted intotobacco and showed faster removal of TNT and enhanced resistance to the toxic effectsof TNT.[8] Researchers have also discovered a mechanism in plants that allows them togrow even when the pollution concentration in the soil is lethal for non-treated plants.Some natural, biodegradable compounds, such as exogenouspolyamines, allow the plantsto 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 minimumpercentage which varies according to the pollutant involved (for example: more than1000 mg/kg of dry weight fornickel, copper,cobalt,chromium orlead; or more than

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10,000 mg/kg forzinc ormanganese).[9] This capacity for accumulation is due tohypertolerance, orphytotolerance: the result of adaptative evolution from the plants tohostile environments through many generations. A number of interactions may beaffected by metal hyperaccumulation, including protection, interferences with neighbourplants of different species, mutualism (includingmycorrhizae,pollen and seed dispersal),

commensalism, andbiofilm.

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.

Phytoremediation

Phytoremediation combines the Greek word "phyton", (plant), with the Latin word"remediare", (to remedy) to describe a system whereby certain plants, working togetherwith soil organisms, can transform contaminants into harmless and often, valuable forms.This practice is increasingly used to remediate sites contaminated with heavy metals andtoxic organic compounds.

Planning, engineering and design with the ecological paradigm as our template is the workof Sustainable Strategies. For example, the ecological paradigm reveals how to safelyutilize all of the polluting components and water of human and animal wastewater toultimately grow plants that have economic value.

We use the term Wastewater Garden to describe our phytoremediation and evapo-

transpiration approach to effluent management problems. The objective is to drainpretreated wastewater into an appropriately engineered gardens or forests ofphreatophytes: plants known for fast growth and high water usage rates. These plants andtheir microbially-active rhizosphere will transform pollutants, including the nutrientnitrogen, into valuable biomass and use up the remaining water via evaporation andtranspiration.

Phytoremediation takes advantage of plants' nutrient utilization processes to take in waterand nutrients through roots, transpire water through leaves, and act as a transformationsystem to metabolize organic compounds, such as oil and pesticides. Or they may absorband bioaccumulate toxic trace elements including the heavy metals, lead, cadmium, andselenium. In some cases, plants contain 1,000 times more metal than the soil in which theygrow. Heavy metals are closely related to the elements plants use for growth. "In manycases, the plants cannot tell the difference" says Ilya Raskin, professor of plant sciences

in the Center for Agricultural Molecular Biology at Rutgers University.

Phytoremediation is an affordable technology that is most useful when contaminants arewithin the root zone of the plants (top three to six feet). For sites with contaminationspread over a large area, phytoremediation may be the only economically feasibletechnology. The process is relatively inexpensive because it uses the same equipmentand supplies used in agriculture.

Soil microorganisms can degrade organic contaminants. This is called bioremediation and

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has been used for many years both as an in-situ process and in land farming operationswith soil removed from sites.

Dr. Raskin also demonstrated the utility of certain varieties of mustard plants in removingsuch metals as chromium, lead, cadmium and zinc from contaminated soil and usedhydroponic plant cultures to remove toxic metals from aqueous waste streams.

Plants can accelerate bioremediation in surface soils by their ability to stimulate soilmicroorganisms through the release of nutrients from and the transport of oxygen to theirroots. The zone of soil closely associated with the plant root, the rhizosphere, has muchhigher numbers of metabolically active microorganisms than unplanted soil. Therhizosphere is a zone of increased microbial activity and biomass at the root-soil interfacethat is under the interface of the plant roots. It is this symbiotic relationship between soilmicrobes that is responsible for the accelerated degradation of soil contaminants.

The interaction between plants and microbial communities in the rhizosphere is complexand has evolved to the mutual benefit of both organisms. Plants sustain large microbialpopulations in the rhizosphere by secreting substances such as carbohydrates and aminoacids through root cells and by sloughing root epidermal cells. Also, root cells secretemucigel, a gelatinous substance that is a lubricant for root penetration through the soil

during growth. Using this supply of nutrients, soil microorganisms proliferate to form theplant rhizosphere.

In addition to this rhizosphere effect, plants themselves are able to passively take up awide range of organic wastes from soil through their roots. One of the more importantroles of soil microorganisms is the decomposition of organic residues with the release ofplant nutrient elements such as carbon, nitrogen, potassium, phosphate and sulfur. Asignificant amount of the CO2 in the atmosphere is utilized for organic matter synthesisprimarily through photosynthesis. This transformation of carbon dioxide and thesubsequent sequestering of the carbon as root biomass contributes to balancing theeffect of burning fossil fuels on global warming and cooling.

Compounds are frequently transformed in the plant tissue into less toxic forms or

sequestered and concentrated so they can be removed (harvested) with the plant. Forexample, mustard greens were used to remove 45% of the excess lead from a yard inBoston to ensure the safety of children who play there. The sequestered lead was carefullyremoved and safely disposed of. Besides mustard greens, pumpkin vines were used toclean up an old Magic Marker factory site in Trenton, New Jersey. Hydroponically grownsunflowers were used to absorb radioactive metals near the Chernobyl nuclear site in theUkraine as well as a uranium plant in Ohio. The mustard's hyper-accumulation results inmuch less material for disposal. The composting of plant material can be another highlyefficient stage in the breakdown of contaminants removed from the soil.

When large plants such as willows, poplars and bamboo are used, the idea is to move asmuch water through them as possible so that they take up as much of the contaminants aspossible. In 1991 the Miami Conservancy District Aquifer Update, No. 1.1 reported that a

single willow tree can, on a hot summer day, transpire over 19 cubic meters of water (5,000gallons)!, One hectare of a herbaceous plant like saltwater cord grass evapotranspires upto 80 cubic meters (21,000 gallons) of water per day. Once the heavy metals are absorbed,they are sequestered in the plants' leaves and/or roots. Any organic compounds that areabsorbed are metabolized.

Absorption of large amounts of nutrients by plants (and only a small amount of planttoxins that might be harmful to them,) is the key factor. Plants generally absorb largeamounts of elements they need for growth and only small amounts of toxic elements thatcould harm them. Therefore, phytoremediation is a cost-effective alternative to

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conventional remediation methods. Cleaning the top 15 centimeters (six inches) ofcontaminated soil with phytoremediation costs an estimated $2,500 to $15,000 per hectare(2.5 acres), compared to $7,500 to $20,000 per hectare for on-site microbial remediation. Ifthe soil is moved, the costs escalate, but phytoremediation costs are still far below thoseof traditional remediation methods, such as stripping the contaminants from the soil usingphysical, chemical or thermal processes according to Dr. Scott Cunningham, a scientist atDupont Central Research for Environmental Biotechnology.

Plants are effective at remediating soils contaminated with organic chemical wastes, suchas solvents, petrochemicals, wood preservatives, explosives and pesticides. Theconventional technology for soil cleanup is to remove the soil and isolate it in a hazardouswaste landfill or incinerate it.

"Phytoremediation", says Dr. Ray Hinchman, botanist and plant physiologist at ArgonneNational Laboratory, is "an in-situ approach," not reliant on the transport of contaminatedmaterial to other sites. Organic contaminants are, in many cases, completely destroyed(converted to CO2 and H2O) rather than simply immobilized or stored.

Salt-tolerant plants, called halophytes, have reduced the salt levels in soils by 65% in onlytwo years in one project involving brine-damaged land from run-off from oil and gas

production in Oklahoma. After the salt was reduced, the halophytes died and nativegrasses, which failed to thrive when too much salt entered the soil, naturally returned,replacing the salt-converting plants.

The establishment of vegetation on a site also reduces soil erosion by wind and water,which helps to prevent the spread of contaminants and reduces exposure of humans andanimals.

Classes of organic compounds that are more rapidly degraded in rhizosphere soil than inunplanted soil include:

Total petroleum hydrocarbons; polycyclic aromatic hydrocarbons Chlorinated pesticides (PCP, 2,4-D)

Other chlorinated compounds (PCBs, TCE) Explosives (TNT, DNT) Organophosphate insecticides (diazanon and parathion) Surfactants (detergents) Nutrients (N,P,K) and organic compounds

Some plants used for phytoremediation are:

Alfalfa (symbiotic with hydrocarbon-degrading bacteria) Arabidopsis (carries a bacterial gene that transforms mercury into a gaseous state) Bamboo family (accumulates silica in it's stalk and nitrogen as crude protein in it'sleaves) Bladder campion (accumulates zinc and copper)

Brassica juncea (Indian mustard greens) (accumulates selenium, sulfur, lead, chromium,cadmi um, nickel, zinc, and copper) Buxaceae (boxwood) and Euphorbiaceae (a succulent) (accumulates nickel) Compositae family (symbiotic with Arthrobacter bacteria, accumulates cesium andstrontium) Ordinary tomato and alpine pennycress (accumulates lead, zinc and cadmium) Poplar (used in the absorption of the pesticide, atrazine)

What is Phytoremediation

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Phytoremediation is the use of living green plants forin situ risk reduction and/orremoval of contaminants from contaminated soil, water, sediments, and air. Speciallyselected or engineered plants are used in the process. Risk reduction can be through aprocess of removal, degradation of, or containment of a contaminant or a combination ofany of these factors. Phytoremediation is an energy efficient, aesthically pleasing method

of remediating sites with low to moderate levels of contamination and it can be used inconjuction with other more traditional remedial methods as a finishing step to theremedial process.

One of the main advantages of phytoremediation is that of its relatively low costcompared to other remedial methods such as excavation. The cost of phytoremediationhas been estimated as $25 $100 per ton of soil, and $0.60 $6.00 per 1000 gallons ofpolluted water with remediation of organics being cheaper than remediation of metals. Inmany cases phytoremediation has been found to be less than half the price of alternativemethods. Phytoremediation also offers a permanent in situ remediation rather than simplytranslocating the problem. However phytoremediation is not without its faults, it is a

process which is dependent on the depth of the roots and the tolerance of the plant to thecontaminant. Exposure of animals to plants which act as hyperaccumulators can also be aconcern to environmentalists as herbivorous animals may accumulate contaminateparticles in their tissues which could in turn affect a whole food web.

How Does It Work?

Phytoremediation is actually a generic term for several ways in which plants can be usedto clean up contaminated soils and water. Plants may break down or degrade organicpollutants, or remove and stabilize metal contaminants. This may be done through one ofor a combination of methods. The methods used to phytoremediate metal contaminants

like lead and mercury, are slightly different to those used to remediate sites polluted withorganic contaminants.

Metal

PhytoextractionRhizofiltrationPhytostabilisation

Organic

PhytodegradationRhizodegradationPhytovolatilisation

Phytoremediation of metal contaminated sites

Phytoextraction (Phytoaccumulation)

Phytoextraction is where plant roots suck up metal contaminants from the soil andtranslocate them to the parts of the plant that is above the soil. Different plants havedifferent abilities to suck up and/or survive various metals, so many different plants may

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be used. Especially in places that are polluted with more than one type of metal. Thereare certain species called Hyperaccumulator plants that absorb much higher amounts ofpollutants than most other species. These species are used on many sites due to theirability to thrive in highly polluted areas

Once the plants have grown and absorbed the metal they are harvested and disposed ofsafely. This process is repeated several times to reduce contamination to acceptablelevels.

In some cases it is possible to actually recycle the metals through a process known asphytomining, though this is usually reserved for use with precious metals. Metalcompounds that have been successfully phytoextracted include zinc, copper, and nickel,but there is promising research being completed on lead and chromium absorbing plants.

Understanding How It Works:

(Uptake, Translocation, and Accumulation in Shoot)Metal contaminants in the soil: are absorbed by the roots (uptake), move into the shoot(translocation), and are stored in the shoot (accumulation).

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Harvest the Shoot and Recover Metal

A plant that contains metal contaminants can be harvested and destroyed, allowing for the

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recovery of the metals.

Rhizofiltration

Rhizofiltration is similar to Phytoextraction but is used to clean up contaminatedgroundwater rather than polluted soils. The contaminants are either adsorbed onto theroot surface or are absorbed by the plant roots. Plants used for rhizoliltration are notplanted directly in the site but have to be acclimated to the pollutant first.

Plants are hydroponically grown in clean water rather than soil, until a large root systemhas developed. Once a large root system is in place the water supply is substituted for apolluted water supply to acclimatise the plant. After the plants become acclimatised theyare planted in the polluted area where the roots uptake the polluted water and thecontaminants along with it. As the roots become saturated they are harvested anddisposed of safely. Repeated treatments of the site can reduce pollution to suitable levelsas proven at Chernobyl where sunflowers were grown in radioactively contaminatedpools.

Phytostabilisation

Phytostabilisation is the use of certain plants to immobilize poisons in soil and water. Toprevent the contamination from spreading and moving throughout the soil andgroundwater, they are absorbed and accumulated by roots, absorbed onto the roots, orheld in the rhizosphere (this is the area around roots which works like a small chemistrylab with microbes and bacteria and micro organisms that are secreted by the plants.) Thisreduces or even prevents migration into the groundwater or air, and also reduces thebioavailibility of the contaminant thus preventing spread through the food chain. Thistechnique can also be used to re-establish a plant community on sites that have been

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completely deadly to plants due to the high levels of metal contamination. Once acommunity of these tolerant plants gets rooted and growing, even wind erosion andleaching of the soil contaminants are also reduced.

Understanding How It Works:

Direct Transformation by Exudates

Organic contaminants in the soil are: absorbed by the plant roots and broken down intotheir component parts by exudates in the plant root system.

Phytoremediation of organic polluted sites

Phytodegradation (Phytotransformation)

Phytodegradation is the breakdown of organic contaminants by metabolic processesdriven by the plant. Ex planta metabolic processes hydrolyse organic compounds intosmaller units that can be absorbed by the plant. Some contaminants can be absorbed thenbroken down by plant enzymes. These smaller pollutant molecules may then be used asmetabolites by the plant as it grows, thus becoming incorporated into the plant tissues.

Plant enzymes have been identified that breakdown ammunition wastes, chlorinatedsolvents such as TCE (Trichloroethane), and other plants which break down organicherbicides.

Understanding How It Works:

(Uptake, Translocation and Metabolism)Organic contaminants in the soil:are absorbed by the roots (uptake), travel up the shoot to

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the leaves (translocation), where they are broken down into their component parts(metabolism) and stored in the leaves.

Rhizodegradation

Rhizodegradation (also called enhanced rhizosphere biodegradation, phytostimulation,and plant assisted bioremediation) is the breakdown of organic contaminants in the soilby soil dwelling microbes that like the root systems of certain plants. There are soil

dwelling microbes that digest fuels and solvents, producing harmless products through aprocess known as Bioremediation. Plant root byproducts such as sugars, alcohols, andorganic acids act as carbohydrate sources for the soil micro plants and will enhancemicrobial growth and activity. Some of these compound may also act as chemicallyattractive signals for fuel eating microbes. The plant roots also loosen the soil andtransport water to the rhizosphere stimulating this helpful microbial activity

Understanding How It Works:

Microbially Mediated (plant assisted microbial biodegradation)Organic contaminants in the soil are broken down by microbes that live in the soil near

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the plant roots.

Phytovolatilization

Phytovolatilization is where plants suck up contaminaints that are water soluble andrelease them into the atmosphere as they release the water from their leaves. A, the toxicmaterial may become modified as the water travels along the plants vascular systemfrom the roots to the leaves. Then the contaminants evaporate into the air surrounding

the plant. There are varying degrees of success with plants as phytovolatilizers with onestudy showing poplar trees to convert and disperse up to 90% of the TCE theyabsorb.

Understanding How It Works:

(Uptake, Translocation and Volatilization)Organic contaminants in the soil: are absorbed by the roots (uptake), travel up the shoot

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to the leaves (translocation), andare released into the air (volatilization).

Hydraulic control of Pollutants

Hydraulic control is the term given to the use of plants to control the migration ofsubsurface water through the rapid uptake of large volumes of water by the plants. Theplants are effectively acting as natural hydraulic pumps whichwhen a dense rootnetwork has been established near the water table can transpire up to 300 gallons of

water per day. This fact has been utilised to decrease the migration of contaminants fromsurface water into the groundwater (below the water table) and drinking water supplies.There are two such uses for plants:

Riparian corridors

Riparian corridors and buffer strips are the simultaneous use of many aspects ofphytoremediation along the banks of a river or the edges of groundwater plumes. Theyare basically long stretches that act as a filter and processing system of plants thatbreakdown, contain or extract the pollution.Pytodegradation, phytovolatilization, and rhizodegradation are used to control the spreadof contaminants and to remediate polluted sites.

Riparian strips are used along the banks of rivers and streams:Buffer strips are the use of such applications along the perimeter of landfills.

Vegetative cover

Vegetative cover is the name given to the use of plants as a cover or cap growing overlandfill sites. The standard caps for such sites are usually plastic or clay. Plants used inthis manner are not only more aesthically pleasing they may also help to control erosion,leaching of contaminants, and may also help to degrade the underlying landfill.

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Where has Phytoremediation Been Used?

Location Application Pollutant Medium plant(s)Ogden, UT Phytoextraction & Rhizodegradation Petroleum & Hydrocarbons Soil &

Groundwater Alfalfa, poplar, juniper, fescueAnderson, ST Phytostabilisation Heavy Metals Soil Hybrid poplar, grassesAshtabula, OH Rhizofiltration Radionuclides Groundwater SunflowersUpton, NY Phytoextraction Radionuclides Soil Indian mustard, cabbageMilan, TN Phytodegradation Expolsives waste Groundwater Duckweed, parrotfeatherAmana, IA Riparian corridor, phytodegradation Nitrates Groundwater Hybrid poplar

Pros & Cons of Phytoremediation

As with most new technologies phytoremediation has many pros and cons. Whencompared to other more traditional methods of environmental remediation it becomesclearer what the detailed advantages and disadvantages actually are.

Advantages of phytoremediation compared to classical remediation

QuoteIt is more economically viable using the same tools and supplies as agriculture

It is less disruptive to the environment and does not involve waiting for new plantcommunities to recolonise the site

Disposal sites are not needed

It is more likely to be accepted by the public as it is more aesthetically pleasing thentraditonal methods

It avoids excavation and transport of polluted media thus reducing the risk of spreadingthe contamination

It has the potential to treat sites polluted with more than one type of pollutant

Disadvantages of phytoremediation compared to classical remediation

Quote

It is dependant on the growing conditions required by the plant (ie climate, geology,altitude, temperature)

Large scale operations require access to agricultural equpment and knowledge

Success is dependant on the tolerance of the plant to the pollutant

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Contaminants collected in senescing tissues may be released back into the environment inautumn

Contaminants may be collected in woody tissues used as fuel

Time taken to remediate sites far exceeds that of other technologies. Of course this isbalanced out in any project involving governmental or public bodies, as they generallyhave more paperwork and process than those required by a private firm on privateproperty.

What is Phytoremediation

Phytoremediation is the use of living green plantsfor in situ risk reduction and/or removal of

contaminants from contaminated soil, water,sediments, and air. Specially selected or engineered

plants are used in the process. Risk reduction can bethrough a process of removal, degradation of, orcontainment of a contaminant or a combination ofany of these factors. Phytoremediation is an energyefficient, aesthically pleasing method of remediating sites with low to moderatelevels of contamination and it can be used in conjuction with other moretraditional remedial methods as a finishing step to the remedial process.

One of the main advantages of phytoremediation is that of its relatively lowcost compared to other remedial methods such as excavation. The cost of

phytoremediation has been estimated as $25 - $100 per ton of soil, and $0.60 -$6.00 per 1000 gallons of polluted water with remediation of organics beingcheaoer than remediation of metals. In many cases phytoremediation has beenfound to be less than half the price of alternative methods. Phytoremediationalso offers a permanent in situ remediation rather than simply translocating the

problem. However phytoremediation is not without its faults, it is a processwhich is dependent on the depth of the roots and the tolerance of the plant tothe contaminant. Exposure of animals to plants which act as hyperaccumulators

can also be a concern to environmentalists as herbivorous animals mayaccumulate contaminate particles in their tissues which could in turn affect awhole food web.

How Does It Work?

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Phytoremediation is actually a genneric term for several ways in which plants

can be used to clean up contaminated soils and water. Plants may break downor degrade organic pollutants, or remove and stabilize metal contaminants. Thismay be done through one of or a combination of the methods described in thenext chapter. The methods used to phytoremediate metal contaminants areslightly different to those used to remediate sites polluted with organiccontaminants.

Metal Organic

Phytoextraction Phytodegradation

Rhizofiltration Rhizodegradation

Phytostabilisation Phytovolatilisation

Methods of Phytoremediation

Phytoremediation of metal contaminated sites

Phytoextraction (Phytoaccumulation)

Phytoextraction is the name given to the process where plant roots uptake metal

contaminants from the soil and translocate them to their above soil tissues. Asdifferent plant have different abilities to uptake and withstand high levels of

pollutants many different plants may be used. This is of particular importanceon sites that have been polluted with more than one type of metal contaminant.Hyperaccumulator plant species (species which absorb higher amounts of

pollutants than most other species) are used on may sites due to their toleranceof relatively extreme levels of pollution.

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Once the plants have grown and absorbed the metal pollutants they areharvested and disposed of safely. This process is repeated several times toreduce contamination to acceptable levels. In some cases it is possible torecycle the metals through a process known as phytomining, though this isusually reserved for use with precious metals. Metal compounds that have beensuccessfully phytoextracted include zinc, copper, and nickel, but there is

promising research being completed on lead and chromium absorbing plants.

Rhizofiltration

Rhizofiltration is similar in concept toPhytoextraction but is concerned withthe remediation of contaminated groundwater rather than the remediation of

polluted soils. The contaminants are either adsorbed onto the root surface or areabsorbed by the plant roots. Plants used forrhizoliltration are not planteddirectly in situ but are acclimated to the pollutant first. Plants are

hydroponically grown in clean water rather than soil, until a large root systemhas developed. Once a large root system is in place the water supply issubstituted for a polluted water supply to acclimatise the plant. Afer the plants

become acclimatised they are planted in the polluted area where the rootsuptake the polluted water and the contaminants along with it. As the roots

become saturated they are harvested and disposed of safely. Repeatedtreatments of the site can reduce pollution to suitable levels as was exemplifiedin Chernobyl where sunflowers were grown in radioactively contaminated

pools.

Phytostabilisation

Phytostabilisation is the use of certain plants to immobilise soil and watercontaminants. Contaminant are absorbed and accumulated by roots, adsorbedonto the roots, or precipitated in the rhizosphere. This reduces or even preventsthe mobility of the contaminants preventing migration into the groundwater orair, and also reduces the bioavailibility of the contaminant thus preventingspread through the food chain. This technique can alos be used to re-establish a

plant community on sites that have been denuded due to the high levels ofmetal contamination. Once a community of tolerant species has beenestablished the potential for wind erosion (and thus spread of the pollutant) isreduced and leaching of the soil contaminants is also reduced.

Phytoremediation of organic polluted

sites

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Phytodegradation (Phytotransformation)

Phytodegradation is the degradation or breakdown of organic contaminants byinternal and external metabolic processes driven by the plant.Ex plantametabolic processes hydrolyse organic compounds into smaller units that can

be absorbed by the plant. Some contaminants can be absorbed by the plant andare then broken down by plant enzymes. These smaller pollutant moleculesmay then be used as metabolites by the plant as it grows, thus becomingincorporated into the plant tissues. Plant enzymes have been identified that

breakdown ammunition wastes, chlorinated solvents such as TCE(Trichloroethane), and others which degrade organic herbicides.

Rhizodegradation

Rhizodegradation (also called enhanced rhizosphere biodegradation,

phytostimulation, and plant assisted bioremediation) is the breakdown oforganic contaminants in the soil by soil dwelling microbes which is enhanced

by the rhizosphere's presence. Certain soil dwelling microbes digest organicpollutants such as fuels and solvents, producing harmless pproducts through aprocess known asBioremediation. Plant root exudates such as sugars, alcohols,and organic acids act as carbohydrate sources for the soil microflora andenhance microbial growth and activity. Some of these compound may also actas chemotactic signals for certain microbes. The plant roots also loosen the soiland transport water to the rhizosphere thus additionaly enhancing microbialactivity.

Phytovolatilization

Phytovolatilization is the process where plants uptake contaminaints which arewater soluble and release them into the atmosphere as they transpire the water.The contaminant may become modified along the way, as the water travelsalong the plant's vascular system from the roots to the leaves, whereby thecontaminants evaporate orvolatilize into the air surrounding the plant. Thereare varying degrees of success with plants as phytovolatilizers with one studyshowing poplar trees to volatilize up to 90% of the TCE they absorb.

Hydraulic control of Pollutants

Hydraulic control is the term given to the use of plants to control the migrationof subsurface water through the rapid upltake of large volumes of water by the

plants. The plants are effectively acting as natural hydraulic pumps which when

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a dense root network has been established near the water table can transpire upto 300 gallons of water per day. This fact has been utilised to decrease themigration of contaminants from surface water into the groundwater (below thewater table) and drinking water supplies. There are two such uses for plants:

Riparian corridors

Riparian corridors and buffer strips are the applications of many aspects ofphytoremediation along the banks of a river or the edges of groundwaterplumes. Pytodegradation, phytovolatilization, and rhizodegradation are used tocontrol the spread of contaminants and to remediate polluted sites. Riparianstrips refer to these uses along the banks of rivers and streams, whereas bufferstrips are the use of such applications along the perimeter of landfills.

Vegetative cover

Vegetative cover is the name given to the use of plants as a cover or capgrowing over landfill sites. The standard caps for such sites are usually plasticor clay. Plants used in this manner are not only more aesthically pleasing theymay also help to control erosion, leaching of contaminants, and may also helpto degrade the underlying landfill.

Where has Phytoremediation Been Used?As it is a relatively new technology phytoremediation is still mostly in it'stesting stages and as such has not been used in many places as a full scale

application. However it has bee tested successfully in many places around theworld for many different contaminants. This table shows the extent of testingacross some sites in the USA

Location Application Pollutant Medium plant(s)

Ogden, UTPhytoextraction &Rhizodegradation

Petroleum &Hydrocarbons

Soil &Groundwater

Alfalfa, poplar,juniper, fescue

Anderson,ST

Phytostabilisation Heavy Metals SoilHybrid poplar,grasses

Ashtabula,

OH

Rhizofiltration Radionuclides Groundwater Sunflowers

Upton, NY Phytoextraction Radionuclides SoilIndian mustard,cabbage

Milan, TN Phytodegradation Expolsives waste Groundwater Duckweed,parrotfeather

Amana, IARiparian corridor,phytodegradation

Nitrates Groundwater Hybrid poplar

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Pro's & Con's of PhytoremediationAs with most new technologies phytoremediation has many pro's and cons.When compared to other more traditional methods of environmentalremediation it becomes clearer what the detailed advantages and disadvantagesactually are.

Advantages of phytoremediation compared to classical remediation

It is more economically viable using the same tools and supplies as agriculture It is less disruptive to the environment and does not involve waiting for new plant

communities to recolonise the site Disposal sites are not needed It is more likely to be accepted by the public as it is more aesthetically pleasing

then traditonal methods It avoids excavation and transport of polluted media thus reducing the risk of

spreading the contamination It has the potential to treat sites polluted with more than one type of pollutant

Disadvantages of phytoremediation compared to classical remediation

It is dependant on the growing conditions required by the plant (ie climate,geology, altitude, temperature)

Large scale operations require access to agricultural equpment and knowledge Success is dependant on the tolerance of the plant to the pollutant Contaminants collected in senescing tissues may be released back into the

environment in autumn Contaminants may be collected in woody tissues used as fuel Time taken to remediate sites far exceeds that of other technologies Contaminant solubility may be increased leading to greater environmental

damage and the possibility of leaching

The low cost of phytoremediation (up to 1000 times cheaper than excavationand reburial) is the main advantage of phytoremediation, however many of the

pro's and cons of phytoremediation applications depend greatly on the locationof the polluted site, the contaminants in question, and the application of

phytoremediation.

Phytoremediation & BiotechnologyThe first goal in phytoremediation is to find a plant species which is resistant toor tolerates a particular contaminant with a view to maximising it's potential for

phytoremediation. Resistant plants are usually located growing on soils with

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underlying metal ores or on the boundary of polluted sites. Once a tolerantplant species has been selected traditional breeding methods are used tooptimize the tolerance of a species to a particular contaminant. Agriculturalmethods such as the application of fertilisers, chelators, and pH adjusters can beutilised to further improve the potential for phytoremediation.Genetic modification offers a new hope for phytoremediation as GMapproaches can be used to overexpress the enzymes involved in the existing

plant metabolic pathways or to introduce new pathways into plants. RichardMeagher and colleagues introduced a new pathway intoArabidopsis to detoxifymethylmercury, a common form of environmental pollutant to elementalmercury which can be volatilised by the plant.

The genes originated in gram-negative bacteria MerB encodes a protein organomercurial lyase converts methylmercury

to ionic mercury MerA encodes mercuric reductase, which reduces ionic mercury to the

elemental form Arabidopsis plants were transformed with eitherMerA orMerB coupled

with a consitutive 35S promoter The MerA plants were more tolerant to ionic mercury, volatilised

elemental mercury, and were unaffected in their tolerance ofmethylmercury

The MerB Plants were significantly more tolerant to methylmercury andother organomercurials and could also convert mthylmercury to ionic

mercury which is approximately 100 times less toxic to plants MerA MerB double transgenics were produced in an F2 generation.

These plants not only showed a greater resistance to organic mercurywhen compared to the MerA, MerB, and wildtype plants but alsocapable of volatilising mercury when supplied with methylmercury.

The same MerA/MerB inserts have been used in other plant speciesincluding tobacco(Nicotiana tabacum), yellow poplar(Liriodendrontulipifera).

Wetland species (bulrush and cat-tail) and water tolerant trees (willowand poplar) have also been targetted for transformation.

Risk AssessmentThe use of phytoremediation in the field is subject to many environmentalconcerns, especially in the light of the recent pulic hysteria about the release ofGM crops into the environment. Even if non GM strains of plants are usedthere are still many concerns:

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It is unknown what ecological effects hyperaccumulator plants may haveif ingested by animals

Fallout from senescing tissues in autumn may also re-enter the foodchain

Do volatilized contaminants remain at 'safe' levels in the atmosphere Exposure of the ecosystem to contaminants is prolonged as

phytoremediation is a relatively slow process

However there are other issues that affect the risk assesment for the use oftransgenic organisms as phytoremediators. Not only do such organisms havethe same risks as witld type remediators but they also have the same risks asreleasing any GM organism into the field have:

The potential genetic pollution of native species Potential for the gene to recombine with other genes possibly leading to

the hyperaccumulation of non-contaminant compounds Reporter/marker genes may also escape into the environment The GM plants may revert to a wild type genotype

GlossaryThis section is intended as a basic glossary for some of the more technicalterms used in these pages. For a more comprehensive glossary check out thelinks section

ADAPTATION: Changes in an organism or population through which theybecome more suited for living in the current environment

BIOACCUMULATION: The intracellular accumulation of environmentalpollutants by a living oragnism

BIOAVAILABILITY: The availability of chemicals to degradativemicroorganisms

BIODEGRADATION: The breakdown of organic substances by

microorganisms

BIOREMEDIATION: The process whereby living organisms degrade orconvert organic contaminants

BIOTRANSFORMATION: The metabolic alteration to the chemical structureof a compound by a living organism or enzyme

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ORGANIC PUMP: The uptake of vast quantities of water by plant rootswhereby the water is transpired by the plant into the atmosphere.

PHYTODEGRADATION: The process where plants are able to metabolicallydegrade organic pollutants

PHYTOEXTRACTION: The use of plants to extract contaminants from theenvironment

PHYTOMINING: Use of plants to extract metal compounds of high economicvalue

PHYTOREMEDIATION: Use of plants to remediate polluted soil and/orgroundwater

PHYTOSTABILISATION: Use of plants to reduce bioavailability andmigration of contaminants

PHYOTVOLATILISATION: The use of plants to volatilise pollutants frompolluted soils and water

RHIZOFILTRATION: The uptake of contaminants by the roots of plants whichare immersed in water

RHIZOSPHERE: The soil area immediatley surrounding the plant root surface.Typically up to a few millimetres from the root surface