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Fitopemulihan

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Fitopemulihan memerihalkan pengolahan masalah persekitaran (biopemulihan) menerusi penggunaan tumbuh-tumbuhan untuk mengurangkan masalah tersebut tanpa menggali bahan kontaminasi dan melupuskannya di tempat yang lain.

Etimologi perkataan itu berasal daripada bahasa Greek, φυτο (phyto) = tumbuh-tumbuhan, dan bahasa Latin « remedium » = memulihkan keseimbangan, atau pemulihan. Fitopemulihan bertujuan untuk mengurangkan kepekatan bahan pencemar di dalam tanah, air, atau udara yang dicemari dengan menggunakan tumbuh-tumbuhan yang berupaya mengawal, mendegradasikan, atau menghapuskan logam, pestisid, pelarut, bahan letupan, minyak mentah dan terbitan-terbitannya, serta pelbagai bahan kontaminasi yang lain, daripada bahatara yang mengandunginya.

Kegunaan

Fitopemulihan boleh digunakan apabila tanah dan persekitaran air statik dicemari atau mengalami pencemaran kronik yang berterusan. Contoh fitopemulihan digunakan dengan berjaya termasuk pemulihan lombong logam yang terbiar, pengurangan kesan buruk tapak-tapak yang PCB dibuang semasa pengilangan, dan pengurangan kesan buruk luahan dari lombong batu arang yang masih aktif.

Fitopemulihan merupakan kaedah yang bersih, cekap, murah, dan tidak menimbulkan gangguan persekitaran, bertentangan dengan proses-proses yang memerlukan penggalian tanah. Buku teks fitopemuliihan yang paling sahih diterbitkan pada 2003 dengan rencana-rencana disumbangkan dan diulas setara oleh kumpulan-kumpulan penyelidikan fitopemulihan yang utama (Phytoremediation: Transformation and Control of Contaminants, disunting oleh Steven C. McCutcheon dan Jerald L. Schnoor).

Kelebihan dan pengehadan

  • Kelebihan:
    • kos fitopemulihan lebih rendah daripada proses-proses tradisional, baik in situ dan ex situ;
    • tumbuh-tumbuhan boleh diawasi dengan mudah;
    • kemungkinan pemulihan dan penggunaan semula logam-logam yang bernilai (oleh syarikat-syarikat yang mengkhususkan "fitopelombongan"); dan
    • kaedah ini paling kurang menjejaskan kerana menggunakan organisma semula jadi dan oleh itu, dapat mengekalkan keadaan persekitaran yang semula jadi.
  • Pengehadan:
    • fitopemulihan hanya terhad kepada keluasan permukaan dan kedalaman yang dapat dijangkau oleh akar-akar;
    • pertumbuhan yang perlahan serta biojisim yang rendah memerlukan komitmen jangka panjang;
    • menerusi sistem pemulihan berasaskan tumbuh-tumbuhan, pelarutresapan bahan kontaminasi ke dalam air tanah tidak dapat dicegah dengan sepenuhnya (tanpa pemindahan seluruh tanah terkontaminasi yang pada dirinya tidak dapat mengatasi masalah pemalaan);
    • kemandirian tumbuh-tumbuhan dijejaskan oleh ketoksikan tanah terkontaminasi dan keadaan am tanah; dan
    • kemungkinan berlaku biopenumpukan bahan kontaminasi yang kemudian memasuki rantaian makanan daripada pengguna-pengguna peringkat primer ke atas.

Pelbagai proses fitopemulihan

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.
  • 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.

Fitopengekstrakan

Phytoextraction (or phytoaccumulation) uses plants or algae to remove contaminants from soils, sediments or water into harvestable plant biomass. Phytoextraction has been growing rapidly in popularity world-wide for the last twenty years or so. Generally 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.

Kelebihan: The main advantage of phytoextraction is environmental friendliness. Traditional methods which 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.

Kelemahan: As this process is controlled by plants, it takes more time than traditional soil clean up methods.


Dua versi fitopengekstrakan:

  • natural hyper-accumulation, where plants naturally take up the contaminants in soil unassisted, and
  • induced or assisted hyper-accumulation, in which a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily. In many cases natural hyperaccumulators are metallophyte plants that can tolerate and incorporate high levels of toxic metals.

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

  • Arsenic, using the Sunflower (Helianthus annuus), or the Chinese Brake fern ("Pteris spp"], a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.
  • Cadmium, using Willow (Salix viminalis),In the year of 1999,one research experiment performed by Maria Greger and Tommy Landberg suggested Willow (Salix viminlais) has a significant potentiality for using it as 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,huge amount of biomass production, can use also for production of bio energy in the biomass energy power plant.[1]
  • 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).

Fitopenstabilan

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 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 mainly focuses 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.[3]

Fitotransformasi

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, carbondioxide 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" [1] is used to describe phytotransformation, as plants behave analogously to the human liver when dealing with these xenobiotic compounds(foreign compound/pollutant). 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. Whilst 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 pancreas 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 which 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 [4].

Peranan genetik

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 [5]. 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 compunds, 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.[6] 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.

Jadual hiperpenumpuk

Rujukan

  1. ^ Greger M and Landberg T. Using of Willow in Phytoextraction. International Journal of Phytoremediation. 1999; 1(2):115-123.
  2. ^ 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.
  3. ^ 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.
  4. ^ Murali Subramanian, David J. Oliver, and Jacqueline V. Shanks. TNT Phytotransformation Pathway Characteristics in Arabidopsis: Role of Aromatic Hydroxylamines. Biotechnol. Prog., 22 (1), 208 -216, 2006.
  5. ^ Hannink N, Rosser SJ, French CE, Basran A, Murray JA, Nicklin S, Bruce NC. Phytodetoxification of TNT by transgenic plants expressing a bacterial nitroreductase. 1: Nat Biotechnol. 2001 Dec;19(12):1168-72.
  6. ^ A.J.M. Baker, R.R. Brooks. Terrestrial higher plants which hyperaccumulate metallic elements – A review of their distribution, ecology and phytochemistry. Biorecovery (1989), 1:81–126

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