Phytoremédiation

see also: Etymology of Phytoremédiation

The phytoremediation includes any technology using of the vascular plants, the algae (phycoremediation) or the mushrooms (mycoremediation) to eliminate or control contaminations or to accelerate the degradation of made up by the microbial activity. The phytoremediation is often reduced to only the depollution by the vascular plants , and consists of the use of the plants (and by extension of the ecosystem S which support them) for the Dépollution of the grounds, the purification of used water or the cleansing of the interior air.

What the phytoremediation?

The etymology comes from the Greek “phyton” = plant, and from Latin “remedium” = re-establishment of balance, remediation. The phytoremediation is not a new concept since there are 300 years the men used already the capacities épuratoires plants for the water treatment. It is since years 1970 that this practice found an renewed interest in particular for the treatment of the pesticides and metals.

The phytoremediation is a whole of technologies using the plants to reduce, degrade or immobilize organic compounds (natural or syntheses) polluting of the ground, Eau or Air coming from human activities. This technique also makes it possible to treat inorganic pollution (metal elements traces (ETM), radionuclides).

Ground : This technique is used biologically to decontaminate the grounds polluted by Métaux and metalloids, Pesticide S, Solvant S, Explosif S, Pétrole gross and its derivatives, Radionucléide S and contaminants various.

liquid Waste water and effluents : The phytoremediation is used also decontamination as water charged out of organic matter or various contaminants (Métaux, Hydrocarbure S, Pesticide S). One then considers the treatments on ground in place (the effluent is épandu) or directly in moist environment.

Air : It can also be a question of cleansing the interior Air or of recycling the Eau thanks to the plants (according to the searchs for Bill Wolverton for NASA in the years 1980-90). This research orientation has developed in an important way for a few years. The program Phyt' air is a French project which carries out a feasibility study for the constitution of a simple system of bioepuration of the interior air.

Principle of phytoremediation

The phytoremediation rests primarily on the interactions between the plants, the ground and the micro-organisms. The ground is a complex matrix being used as support with the development of the plants and the micro-organisms which nourish organic compounds or inorganic the component. When some of these compounds are in excess compared to the initial state of the ground, this last is described as contaminated (that also applies to water and the air which with the difference are fluids). The compounds in excess can then be used as energy source by the plants and the micro-organisms. In the system plants - ground - micro-organisms, the bacterial biological breakdown is often upstream of absorption racinaire. Plants and micro-organisms have coévolué to have a strategy with mutual benefit to manage the phytotoxicity where the micro-organisms benefit from the exsudats racinaires whereas the plant profits from the rhizospheric capacities of degradation of the micro-organisms to reduce the stress of phytotoxicity. With final, the plant is the essential engine of the export of a contaminant.

Effect rhizospheric

The Rhizosphère indicates the volume of ground subjected to the influence of the racinaire activity. This volume of ground is more or less important and varies according to the plants and the ground. The processes which proceed in the rhizosphère are essential for the phytoremediation. The activity and the microbial biomass are much more important there than in a ground without roots. The roots release naturally from the substances in the ground where they develop, the exsudats racinaires. Those support and maintain the development the microbial colonies by providing from 10 to 20% the sugars produced by the photosynthetic activity of the plant (photosynthétats). Many compounds can thus be released, for example, Hormones, Enzyme S as well as Oxygène and Eau. The micro-organisms rhizospheric in return support the growth of the plant (reduction of pathogenic, placed at the disposal of nutrients…). In theory, plus the roots are abundant plus them provide an important surface of development for the microfaune and microflora rhizospheric. In fact, the exsudats racinaires support the biological breakdown of the organic pollutants by stimulating the microbial activity.

Principle of decontamination

Briefly, the plants either will absorb the contaminant to metabolize it or store it, or to even reduce to prevent the release of the contaminant in other compartments of the environment (Phytostabilisation). Generally, the organic compounds (xenobiotic or not) can be degraded and metabolized for the growth of the plant. The compound polluting is then eliminated. When they are inorganic compounds (metals, metalloids or radionuclides), there can be only phytostabilisation or phytoextraction because these types of pollutants are not biodegradable.

Various forms of phytoremediation

Phytoextraction : use of plants which absorb and often concentrate in their parts ready for harvesting (sheets, stems) the pollutants contained in the ground (of the metal Element-trace: ETM). One often uses accumulating plants and/or hyperaccumulatrices which are able to tolerate and to accumulate the ETM. It is possible to improve this extraction by the addition of Chélateur S on the ground. Generally the plants are collected and incinerated; ashes are stored (in THIS) or are developed to recover accumulated metals (one speaks then about phytomining ).
Phytotransformation , or Phytodégradation : certain plants produce Enzyme S (déhalogénase, oxygénase,…) who catalyze the degradation of the absorptive substances or adsorbed; those are transformed into less toxic substances or not-poisons by the metabolisation of the contaminants in fabrics of the plants or by the organizations of the Rhizosphère maintained by the plant (one speaks then about rhizodegradation ( degradation by the Rhizosphère ).
Phytofiltration or rhizofiltration : used for the depollution and the restoration of water surface and underground. The contaminants are absorbed or adsorbed by the root S of the plants in moist environment.
Phytovolatilisation : the plants absorb the water of the Lithosphère containing organic contaminants and other toxic products, transform those into elements birds, and slacken them in the atmosphere via their sheets. They can also in certain cases transform organic contaminants into elements birds before transferring them in the atmosphere - always via the sheets. The phytovolatilisation is not always satisfactory, because if it decontaminates the grounds it releases sometimes from toxic substances in the atmosphere. In other more satisfactory cases, the pollutants are degraded in components less - or not-poisons before being released.
Phytostabilisation : reduced the mobility of the contaminants simply. The most used technique is to make use of the plants by reducing the flows of surface and sub-surface, by limiting erosion and by reducing the underground flows towards the tablecloth. This just practice what one call commonly the hydraulic Control , or phytohydroregulation . The hydraulic pumping (translated literally of English) can be made when the roots reach subterranean water while taking broad volumes of water and by controlling the hydraulic gradient and the side migrations of contaminants within the aquifer. In two words, it is a question of using plants with strong evapotranspiration to reduce the movement of the pollutants by the flows (side or in-depth). Another practice consists in immobilizing the polluting compounds by binding them chemically. The plants adsorb the pollutants of the ground, of water or the air, retaining them locally (from where the use of the adsorbtion term instead of absorption) and reducing to them Biodisponibilité. The process sometimes is made possible, or is amplified and accelerated, by the addition of organic compounds or mineral, natural or artificial. It is an effective method to prevent the dispersion of the pollutants in water surface or underground.
Phytorestauration : this technique implies the complete restoration of grounds polluted towards a state close to the operation of an original ground (Bradshaw 1997). This subdivision of the phytoremediation uses indigenous plants of the area where work of phytorestauration is carried out. This with an aim of reaching the whole rehabilitation of the original natural ecosystem, the ground at the vegetable communities. As underlines it Peer and Al (2005), compared to other technique of phytoremediation, the phytorestauration clarifies the question of the level of decontamination necessary and sufficient. There exists a great difference between decontaminating a ground to reach a level legally satisfactory so that it is again exploitable and to restore a space completely so that he returns in conditions pre-contamination. When one refers to the phytorestauration of waste water, one speaks about a recent process having milked to the use of the natural properties of self-purification of plants (Dabouineau and Al , 2005). Used in this direction, the phytorestauration becomes synonymous with the term Phytoépuration . This type of process integrates in particular the purification of water by the macrophytes. In this case, it is to be stressed that they are the bacteria living in the zone racinaire macrophytes which are guarantors of depollution, the plants are used there simply as substrate of growth for the micro-organisms (see station of Honfleur).
Phytostimulation : located primarily in the Rhizosphère, it is stimulation by the plants of the microbial activities favorable to the degradation of the pollutants. This aspect, when he was studied, was noted among all hyperaccumulateurs.

Advantages and limits

the advantages :

the cost of the phytoremediation is quite less than that of traditional processes in situ and ex situ;
the plants can be easily supervised;
recovery and metal re-use of value (in the companies specialize in the “phytominage”);
it is the least destroying method because it uses natural organizations and preserves the natural state of the environment (contrary to the physico-chemical processes there are no impacts on the fertility of the grounds);
exploitation of the produced plants.

the limits :

the phytoremediation is limited to the surface and the depth occupied by the roots;
slow growth and weak biomass requires an investment in rather important time ou/et the addition the chelating ones or other substances (for inorganic pollution like the ETM);
one cannot, with systems of remediation containing plants, completely to prevent the flow of the contaminants in the ground water (this is not possible that at the price of the complete removal of the ground, which does not solve the problem of contamination of the known as ground and the problems related). An experiment in Iowa (the U.S.A.) shows however that poplars planted between a corn field and a brook considerably reduced the nitrate concentration in surface water: in edge of field this one contained 150 mg/l nitrates, while among the poplars the content nitrates was not that of 3mg/l 320 accumulating species list coming from 43 families. Their number is higher: to date (2006) approximately 300 nickel hyperaccumulateurs are known. Centers of diversity arise to Cuba (subtropical climate) and New Caledonia (tropical climate). But the majority of the species studied for accumulation of metals are of Brassicaceae (moderate and cold climate, northern hemisphere).

(This section is mainly a summary of the article " The significance off metal hyperaccumulation for biotic interactions " by Robert S. Boyd and Scott NR. Martens.

A plant is known as hyperaccumulatrice if it can concentrate the pollutants according to a variable minimum percentage according to the pollutant concerned (example: more than 1 matter mg/g dries for the Nickel, Cuivre, Cobalt, Chrome or Plomb; or more than 10 Mg g/1 for the Zinc or the Manganese. The majority of the 215 hyperaccumulateurs quoted by them relate to nickel. They listed 145 nickel hyperaccumulateurs, 26 of cobalt, 24 of copper, 14 of zinc, four of lead, and two of chromium. This capacity of accumulation is due to the hypertolérance , or phytotolérance : result of the adaptive evolution of the plants to hostile environments during multiple generations. Boyd and Martens list 4 biotic interactions being able to be affected by the metal hyperaccumulation:

Protection
Interferences with the plants close to different species.
Mutualism
Commensalisme
the Biofilm

Protection

A growing number of results of experiments indicates that metals in the hyperaccumulateurs have a role of at least partial protection for the plants towards a certain number of organizations (Bactérie S, Fungi, Insecte S).

Defense against the viruses is not always improved by the presence of metals. Davis and Al compared two close species S. polygaloides Gray (hyperaccumulator of Nickel) and S. insignis Jepson (not-accumulator), inoculating them with the virus mosaic. They thus showed that the presence of nickel weakens the response of the plants to the virus.

Elemental defenses of the plants are thwarted by their predatory in three ways and Klerks (1990) showed it for the animal species; Brown & Hall for the species fongales; and Schlegel & Al (1992) and Stoppel & Schlegel (1995) for the species bactériales.

Streptanthus polygaloides ( Brassicaceae ) can be parasitized by Cuscuta californica VAr. breviflora Engelm. ( Cuscutaceae ). Seedlings of Cuscuta thus discovered.

Associations mycorhizales are the symbiotic relations between the fungi and the roots of the plants. Certain hyperaccumulateurs can form associations mycorhizales, and in certain cases the mushroom mycorrhizal can play a part in the treatment of metal. On another side some mycorrhizae increase the tolerance with metals by decreasing the absorption of metal at certain not-accumulating species. Thus association mycorhizale assistance Calluna to avoid the toxicity of copper and zinc. The majority of the roots require approximately 100 times more carbon than the hyphae of the fungi associated to cover the same volume of ground. This is why it is easier for the hyphae than for the plants to acquire elements with reduced mobility, like the cesium-137 and strontium-90.

The mycorhizaux mushrooms depend on the plants hosts for their carbon, while making it possible the plants to absorb the nutrients and water more effectively. The fungus facilitates the catch of nutrients for the plants, while those provide them organic compounds rich in energy. Certain species of normally symbiotic plants with micorhizaux mushrooms can exist without association; but the mushroom largely improves the growth of the plant. From the point of view of energy spent, to lodge mushrooms is much more effective for the plant than to produce roots.

The family of the Brassicaceae would form few associations micorrhizales. The serpentine grounds are populated mushrooms tolerating the rate of metal generally high in these grounds. Some of these fungi are micorhizaux.

The absorption of radionuclides by the fungi depends on their nutritional mechanism (Mycorrhiza L gold saprophyte). Pleurotus eryngii absorbs Cs better than Sr and Co, while Hebeloma cylindrosporum supports Co. But increasing the amount off K increases the uptake off Sr (chemical similar to Ca) goal not that off Cs (chemical similar to K). Moreover, the Cs content decrease with Pleurotus eryngii (mycorrhizal) and Hebeloma cylindrosporum (saprophyte) if the Cs amount is increased, but that of Sr increases if the Cs amount is increased - this would indicate that absorption is independent of the nutritional mechanism.

Dispersion of pollen and seeds

Certain animals obtain food of the plants (nectar, pollen, or pulp fruit - Howe & Westley 1988). The animals nourishing hyperaccumuleurs with high metal concentration must either be tolerant or to dilute the metal concentration by mixing food with other sources with content of less metal. Alternatively, the hyperaccumulateurs can depend for dispersion on their seeds on abiotic vectors or animal vectors not-mutualists animal vectors, but we miss information on these mechanisms of dispersion with regard to the hyperaccumulateurs.

Jaffré & Schmid 1974; Jaffré and Al 1976; Reeves and Al 1981; studied the metal rate of fruits and flowers entireties. They generally found metal rates important in those. Baker and Al (1992) found an exception with Walsura monophylla Elm. ( Meliaceae ), originating in Philippines and showing 7000 mg/kg Nor in the sheets but only 54 mg/kg in the fruits. Certain plants can thus have a mechanism which excludes metals or other contaminants from their reproductive structures.

Commensalisme

It is a beneficial interaction at an organization while having a neutral value for another . Most probable for the hyperaccumulateurs is the epiphytism. But one most usually finds this phenomenon in the tropical forests, and the studies led in such habitats related only more or less of attention to this point. (e.g., Proctor and Al 1989; Baker and Al 1992). Proctor and Al (1988) studied the tree Shorea tenuiramulosa , which can accumulate to 1000 Mg Nor /kg of dry weight in its sheets. They estimated the cover of épiphytes in Malaysia, but did not bring back the values for the individual species. Boyd and Al (1999) studied the occurrence of épiphytes on the sheets of the tropical bush hyperaccumulatior in Nor Psychotria douarrei (Beauvis.). The quantity of épiphyte S increases considerably with the age of the sheet, up to 62% for the oldest sheets. A specimen epiphytic of liverworts coming from a sheet of P. douarrei , contained 400 Mg Neither /kg dry weight (much less than the plant host, of which the oldest sheets - those épiphitisées - contained a median value of 32,000 Mg Nor /kg dry weight). High amounts of Nor thus do not prevent the colonization of Psychotria douarrei by the épiphytes.

The chemical compounds which intervene in the interactions host-épiphyte are more likely to be localized in the most external fabrics of the host (Gustafsson & Eriksson 1995). Moreover, the majority of metal accumulate in the walls of the cell S or Vacuole S épidermales or subépidermales (Ernst & Weinert 1972; Vazquez and Al 1994; Mesjasz- Rzybylowicz and Al 1996; Gabrielli and Al 1997). This suggests that the épiphyte S would undergo rates of higher metal concentration when they push on sheets of hyperaccumulateurs. But Severne (1974) measured the evacuation of metal by the sheets for the nickel hyperaccumulator Hybanthus floribundus (Lindl.) F. Muell. ( Violaceae ), originating in Western Australia; he concludes that the sheets do not slacken nickel easily.

In theory another interaction commensale could exist if the metal high rate in the ground were necessary so that another species of plants can settle. No obviousness is until now alley in this direction.

The Biofilm

This paragraph is under development. See the corresponding articles on the Biofilm and Pseudomonas aeruginosa.

Count hyperaccumulateurs

Ms Stevie Famulari, born in New York of Italian origin, teaches Architecture landscape designer in Landscape Architecture Department of the University of New Mexico City. It uses the phytoremediation in a project with its students in Los Alamos, New Mexico City, concerning the canyon of drainage for Manhattan Project. For this purpose it began a list of varied contaminants: Radionuclide S, metals, hydrocarbons and others, and of the plants used for their treatment; list that it made public here. This now increased list was divided into several sections:

Table of hyperaccumulateurs - 1: Al, Ag, Ace, Be, Cr, Cu, mn, Hg, Mo, Pb, Pd, Pt, Zn, Naphthalene
Table of hyperaccumulateurs - 2: Nickel.
Table of hyperaccumulateurs - 3: Organic radionuclides, Hydrocarbons and Solvents.

Practitioner cities partially or completely the phytoremediation

Honfleur (France, Normandy)

Rønnede (Denmark)

Suzhou (China, Jiangsu)

Fuyang (China, Zhejiang)

It is also possible to use these techniques on an individual scale, in a garden, in condition of good Trier its waste and to eat bio rather. This practice is in particular used individually in Australia and France in autonomous houses.

Sources and References

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