Why is Phytoremediation important?

Phytoremediation is a more advanced form of the biocleaning process that involves the use of bushes and shrubs for rubbing off contaminants off the surface of soil, air, and groundwater. A simpler version of phytoremediation would be bioremediation. Phytoremediation is a fervent process that is spruced up once all the other methods of decontamination have flunked. The procedure is entitled to the use of plants in the form of absorbers of metal ions that would otherwise go unnoticed and stubbornly stuck to the surface of soil. One of the primary measures that are considered standard and essential before the commencement of this procedure would be of measuring the oxidation-reduction potential of soil and the underground water. Phytoremediation , in many countries is regarded as an essential protocol for maintaining the titres of nutrients in the underground soil and for negating the presence of any possible effect that might have entered into the soil by use of pesticides or solvents earlier. Mines can be considered as a staunching location for such procedures. Metal mine workings encounter a heavy exposure to all sorts of contamination especially the polychlorinated biphenyls. An extended use of phytoremediation can be appreciated in the neutralization of the radioactivity portions that might have been lent by the spare uranium pieces in any field. When used for the extraction of uranium, phytoremediation is more precisely known by the name of phytoextraction. Some of the common plant subspecies used for this purpose are Brassica juncea and Brassica chinensis.

Branches of Phytoremediation

Phytoremediation is an economical approach of gaining absolute control over all the toxic substances that occur in the quality of air, soil and groundwater. While this technique uses the thought of hyperaccumulation with respect to plants as its primary tool, there are 6 different ways it can go about it.

Phytosequestration - As the name suggests, this process involves sequestration or grouping of metal ions before their absorption. The prime purpose of this process would be of reducing the bioavailability of contaminants in the environment. The process of sequestration is begun by the selection of plants that would be accurate for the absorption of biochemicals from the soil. Also known by the name phytostabilization, phytosequestration involves the sucking in of metal ions by plants by the means of absorption as well as adsorption. For instance, plants that have the ability of sequestration target the carbon atoms in the land. The sequestered carbon atoms get released in the form of CO2. into the air and are readily consumed by the plants. Sequestration of carbon atoms can have a major impact on the issue of global warming and climatic uncertainty.

Rhizodegradation : As the name suggests, the route adopted by this variant of phytoremediation is of the Rhizomes. Once the plant has been placed in contact with the concerned soil surface and groundwater, the roots of the plants come into form and begin their job. In a manner of saying, the rhizophores on the surface of other roots of the plant trigger the development of bacteria that further the process of biodegradation. The plants are nurtured to the level of full decontamination. Once a satisfied level of toxins have been devoured, the plants are uprooted and disposed in a secure manner. The process of rhizodegradation broadens its expanse of decontamination by depolluting the aquatic environs too. The time duration and the apt level of growth required by the plant is determined by the amount of decontamination required. Under most circumstances, the contamination is an indigenous mix of organic compound and heavy metals. In the case of a chemical potpourri, rhizodegradation would be treated as an accessory to a main contamination procedure. As a security measure, the plants selected for this form of phytoremediation are fodder crops that cannot be consumed by wild animals. Some of the common contaminants that can be erased from the soil and underwater bases by the process of rhizodegradation are - Cu2+, Cd2+, Cr6+, Ni2+, Pb2+, and Zn2+ . A standard expense of 2$ to 6$ can be expected for improving 1000 gallons of water by the standard procedure of Rhizodegradation.

Phytohydraulics - As the name suggests, this variant of phytoremediation uses the power of hydraulics to complete the process of bioremediation. Since the roots of the plant serve as channels, the plants chosen for this task are the ones that give away deeper roots into the soil. For instance, MTBE (methyl-tert-butyl-ether) is a petro compound that disperses through soil into the underground water sources and is readily absorbed by the roots of balsam poplar tree. Poplar plant has the capacity of growth that can suck up to 300 gallons of water per day. The water consumed by poplar plant during the process of photo hydraulics would prevent the movement of the contaminants into the underground water and therefore into the drinking water supply of any city. Some of the commonest applications that involve the use of this process are:

Riparian corridors : The name suggests,the process's role would be of opening the corridors of decontamination for the basic water sources & supplies. It could be a stream or river bank that could serve as a base for the plant required for perming the process of phyto hydraulics.

Buffer strips : This phyto hydraulic variant would make an aggregate of the various phyto processes to take care of the underground water situation. For instance, phytodegradation, phytovolatilization, and rhizodegradation are used in conjunction with phyto hydraulics to create a buffer and prevent the entry of contaminants into the subsoil water.

Vegetative caps : Most often used in the landfills sites, this variant of phyto hydraulics would involve the use of bunchgrasses and shrubs with a lifespan of at least 5 years in order to take care of the situation of silting in the farm fields placed closed to rivers and lakes.

Phytoextraction - Also goes by the name of phytoaccumulation. Plants kept as a part of this process take up the toxins from the earth and store them in their stem and leaves. Unlike other forms of phytoremediation, Phytoextraction doesn't degrade the the toxin but keeps it as an essence in the bark of the stem and in the capillaries of leaves. Phytoextraction can be used for the metal ions that are intended to be used again. The process of re-initiating extraction of metal ions from the plants is known as phytomining. The. terrific combination of phytoextraction & phytomining Phytomining is essential for the collection of minerals that exist in their rarest forms on the planet. Phytoaccumulation is a preferred method for extraction of heavy metal ions and not the organic compounds. Once the plant is saturated with heavy metal ions, it is replaced by another plant of the same variant for extraction of any metal remains that might be irritating the surface of earth. It's like the cycle needs to be repeated to ensure complete and thorough clean up of the soil and the underground water of any entities that could spoil their nature in the first place. Phytoaccumulation procedure can be further categorized into 2 forms:

Natural hyper-accumulation - Phytoaccumulation accomplished with the sole use of a reliable plant variety

Induced hyper-accumulation - Phytoaccumulation is achieved by readying the soil before the implantation. This would require the use of a chelating agent to make the process of phytoremediation go easy for the plant.

Phytovolatilization : Volatile contaminants can be tough to capture from the atmosphere. Phytovolatilization sticks to the use of plants that can absorb volatile compounds. Most of these volatile compounds exist in the underground water. Water is absorbed by the capillaries of the plant, and the compounds get altered along the way. By the time the volatile compounds make it to the process of transpiration, they have been converted into harmless metabolites by the plant’s system. Oftentimes, it is the mercurial toxicity of the soil and hence in the underground water that can be gotten rid of by this process. Primarily, the process of phytovolatilization is used for getting rid of mercury ions. Once these ions have been absorbed by the plant, they are readily converted into a lesser toxic form of elemental mercury. Tritium is yet another compound that can be reduced to its non-productive form of helium by the process of phytovolatilization. The released compound can be contained by the process of precipitation and coolant, and then returned to the ecosystem in their original form.

Phytodegradation : This form of bio cleaning process would involve degradation of the chemical compounds absorbed by the concerned plant species. The compounds that are mostly contained by the use of this process of decontamination would be the organic compounds like PAH polycyclic aromatic hydrocarbons, TPH total petroleum hydrocarbon. Trinitrotoluene, more commonly known as TNT is known amongst many as an explosive pollutant. And therefore, the disposal of this hazardous contaminant can be quite a challenge especially moneywise. While there are numerous plant species that can be used for the disposal of TNT, the challenge of growth potential would circumvent most of them during their large-scale use in elimination of TNT. Entero cloacae however is a gram negative bacteria that can be used for taking care of the massive amounts of TNT due to any possible reason. Irrespective of the source of TNT, the fervenly working enzymes of this bacteria do not have any performance issues and be relied on for elimination of toxins in no time. PETN (pentaerythritol tetranitrate) reductase, is an NADPH dependent enzyme that works by reducing TNT into harmless nitrates and them intermixing them with the soil’s bacteria for further degradation. During a study, a tobacco plant (implanted with the aforementioned enzymes) and a wild plant were used for affirming the role of aforementioned enzymes in TNT degradation. The above mentioned plant species were exposed to 0.25mM of TNT. On exposure, the wild plant withered while the transgenic plant survived. At the end of the study, a heightened rate of TNT metabolism was confirmed from the remains of the tobacco plant.

Indoor plants can do Phytoremediation too!

Higher plant species like spider plants can be counted on for degradation and detoxification of the various indoorsy air pollutants. Most of the air pollutants indoors are particulate matter. For the purpose of the study, five rooms were selected with completely different indoor environments - a dental chamber, a bottling room for perfumes, a random suburb house, a workplace and a random apartment). Spider plants performed well and ruled out any chances of sparing the particulate matter in the form of liquid or wax. The waxy matter was the one that was deposited on the surface of the leaves whereas the water-washable matter was the one that got accumulate on the aluminium plate on which the plant was placed. The results proved that higher amounts of waxy particulate matter were retrieved by the end of the study, thus indicative of the high-end potency of spider plants over the regular gravitational force that worked in the case of aluminium plate.

Phytoremediation Pluses

  • Phytoremediation process is a cost-effective process of getting rid of environmental toxins.
  • Since the process involves the use of just plants, keeping a 24-hour vigil over them might not be necessary.
  • In some scenarios, the process of Phytoremediation accentuates the process of phytomining
  • The process of Phytoremediation is environment-friendly
  • Trees used as a part of the process of Phytoremediation are sturdy and barely give in to the natural disasters. For instance, willows and poplars are the trees that known for extending their lifetime to a decade.

Phytoremediation pullbacks

  • Phytoremediation is a slow process and mostly limited to the area occupied by the roots of the plants and trees
  • The survival instinct of the plants varies and is based on the level of toxicity
  • Crucialness of the disposal method of plants is a noteworthy point. The toxins absorbed by plants aren’t supposed to enter the food chain by any means.

Phytoremediation & the 4 Grasses

Once the plants were found to be successful in the extraction of toxins from the soil and underground water, it was the grass that was given a chance to prove its worth in the process of phytoremediation. Scientists at the University of Tokyo, Japan went through with the preparations of the experiment that involve the use of 4 forms of grass - Bermuda grass, Lawn grass, Bent grass and White clover. There were 2 land areas that were chosen for this purpose. The control land area was sprinkled with a chemical name DBF - dibenzofuran. The other land area was kept clean. In order to keep a track of the progress, the microbial numbers and the land mass ratio of the roots were measured from time to time. Furthermore, an insightful study into the potential of the 4 grass forms was carried out. By the end of the study, it became relevant that dibenzofuran could be degraded and filtered out of the soil with uttermost ease by white clover. While the other forms of grass had an equal opportunity to perform in the same manner, their potency were proven to be low when compared with the quality work provided by the white clover.

Phytoremediation at Molecular level

We would now be looking the degradation process involved in Phytoremediation from a molecular level.

Aerobic pathway : The genomic pathway followed by the bacteria for the reduction of compounds in an aerobic manner is identical to the one followed by the pseudomonads for the reduction of xenobiotics. These pathways have been distinctly mannered as Box and Paa pathways that partake in the use of non-oxygenolytic strategies for their smooth run. For instance, the catabolism process of certain terpenoids and steroids can be owed to the dioxygenases. Nevertheless, the recent reports suggest that an upgrading pattern in the species of LB400 has been witnessed during the degradation process of pollutants.

Anaerobic degradation : Anaerobic degradation can be mostly seen in involved in the cases of organic compounds. There are a wide variety of environmental reactions involved in this one. For instance, the iron-reducing species of bacteria namely the Dehalococcoides and ethenogenes were found capable of anaerobically reducing the halogenated hydrocarbons.

Electron transfer : This process is mostly used by the bacteria in scenarios that include the pollutants found on the surface of the soil instead of deep within. The pollutants during the subsurface transfer are carried out by Geobacter species. A quantitative analysis performed on the in situ items of Geobacter species in the presence of uranium substantiated the use of Geobacter in projects that involve the phytoremediation of underground water that were contaminated with heavy amounts of uranium or any other organic contaminant.

Modified catabolic reactions : Over a period of time bacteria tend to get adapted to their use in elimination of compounds like carbon, nitrogen and several other energy sources. Some of the common compounds that can be included in this category are chloroalkanes, dichlorophenoxyacetic acid, lindane and chlorobenzenes. At some level, the bacteria exhibit copious amounts of genetic rearrangements for expansion of their metabolic capabilities. The adaptation criterion of bacteria can be owed to their repeated use in a process.

Marine systems : Petrol products can be held as a major reason for pollution in most countries. A stitch provided by the use of phytoremediation could save the marine eco-environments from becoming a region of oil spills. Alcanivorax borkumensis is a distinct species that is used for degradation of oil and its by products. Some of the important functions that can be carried out by Alcanivorax borkumensis are:

  • Degradation of n-alkane alongwith production of a protective biofilm
  • Serving as a nutrient scavenger in an oligotrophic marine environment
  • Providing stress-free marine habitats
  • In all aspects one can be sure of gaining some control over marine environment with the use if such species at random intervals of time.
  • Studies that are furthering the use Phytoremediation are…

    One of the most former of agronomists to initiate a rebel against the pollutants in the 1980’s was Rufus L. Chaney. Chaney reported that the control over pollutants shouldn't be limited to the naturally occurring plant forms and therefore supported the growth of a new plant variant name alpine pennycress. According to Chaney, alpine pennycress is capable of eliminating even the tiniest amounts of zinc and cadmium from the soil and subsoil surfaces. Cadmium and zinc are some of the common contaminants that can be found in the industrial waste. Based on his studies about the new plant variety, Rufus L. Chaney claimed that none of the other plant variants in use possessed the capability of getting rid of heavy amounts of cadmium at once.

    Yet another scientist who used the path of lab to further the use and advancement of phytoremediation was Michael P.Russell. Michael developed a renewed form of alfalfa that could handle copious amounts of nitrogen in one take. This altered form of alfalfa can be used in land as well as aquatic environment. His research didn’t stay indoor unlike many experiments but explored its way to the northern region of Dakota after a massive spillage occurred and dispersed enormous quantities of liquid nitrogen fertilizer.

    Though the researchers are taking their own sweet time to catch up with the updates of phytoremediation, it won’t be too late before phytoremediation would be declared as an affordable and green biotechnology unit for maintaining the strength of the environment.

    References

    • https://www.sciencereviews2000.co.uk/blog_v2/view/applications-of-bioremediation-and-phytoremediation/732#.XL_zqOgzbIU
    • https://pubs.acs.org/doi/pdf/10.1021/es971027u#
    • https://www.thebalance.com/six-types-of-phytoremediation-375529
    • https://pubs.acs.org/doi/full/10.1021/acs.est.5b04113
    • https://www.ncbi.nlm.nih.gov/pubmed/19321258
    • http://www.unep.or.jp/Ietc/Publications/Freshwater/FMS2/2.asp
    • https://www.caister.com/biod
    • https://agresearchmag.ars.usda.gov/2000/jun/soil
    • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4449931/
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