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ISSN: 2155-6199
Journal of Bioremediation & Biodegradation

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An Emphasis on Xenobiotic Degradation in Environmental Clean up

Y.M.Varsha1*, Naga Deepthi CH2 and Sameera Chenna3
1School of Chemical and Biotechnology, SASTRA University, India
2Department of Microbiology, Andhra University, Visakhapatnam, India
3Department of Biotechnology, SVKP & Dr. K.S.Raju Arts and Science College, India
Corresponding Author : Y.M.Varsha
School of Chemical and Biotechnology
SASTRA University, India
E-mail: [email protected]
Received September 02, 2011; Accepted October 03, 2011; Published October 05, 2011
Citation: Varsha YM, Naga Deepthi CH, Chenna S (2011) An Emphasis on Xenobiotic Degradation in Environmental Clean up. J Bioremed Biodegrad S11:001. doi: 10.4172/2155-6199.S11-001
Copyright: © 2011 Varsha YM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

Setting up of new industries or expansion of existing industrial establishments resulted in the disposal of industrial effluents, which discharge untreated effluents causing air, water, soil and soil solid waste pollution. These disposed materials have high persistence capacities and also can change in to toxic recalcitrant up on combining with other eco-materials or manmade products. Remediation is the only way to tackle these so called xenobiotic compounds and to reduce the hazards caused by them. Even though, several practices have been implemented for degrading these recalcitrants, bioremediation step is proved to show the significa t impact on them. Giving a brief note on types of xenobionts and their impact on the environment, this study attempts to highlight on different xenobiotic degradation methods like bacterial bioremediation, phycoremediation, phytoremediation, photoremediation etc.

Keywords
Remediation; Industrial effluents; Xenobiotics; Biosorption; Photoremediation; Eco-industrial parks
Introduction
Pollution consequence on environ
In early times, we had an unlimited abundance of land and resources; today, due to our carelessness and negligence in using them however, the resources in the world show, in lesser degree [1]. The quick growth of various industries in the past century has extremely increased the release of toxic waste effluents in to water bodies along with ground water [2]. Environmental pollution caused by the release of a these wide range of compounds (i.e. persistent organic pollutants, POPs) from industries are creating disturbance to the ecosystem [3], causing climatic changes, reduction of water levels in the ground as well as oceans, melting of icecaps, global warming, ozone layer depletion due to photochemical oxidation etc. [4,5] and this made ecologists to focus more on impacts of pollution and its reduction.
In early times, we had an unlimited abundance of land and resources; today, due to our carelessness and negligence in using them however, the resources in the world show, in lesser degree [1]. The quick growth of various industries in the past century has extremely increased the release of toxic waste effluents in to water bodies along with ground water [2]. Environmental pollution caused by the release of a these wide range of compounds (i.e. persistent organic pollutants, POPs) from industries are creating disturbance to the ecosystem [3], causing climatic changes, reduction of water levels in the ground as well as oceans, melting of icecaps, global warming, ozone layer depletion due to photochemical oxidation etc. [4,5] and this made ecologists to focus more on impacts of pollution and its reduction.
Sources of xenobiotics
Direct sources: The prime direct source of xenobiotics is wastewater and solid residual releases from the industries like chemical and pharma, plastics, paper and pulp mills, textile mills, agricultural (enhancement products like pesticides, herbicides etc.) (Figure 1). Some of the common residual compounds in the wastewater and other effluents are Phenol, hydrocarbons, different dyes, paint effluents, Pesticides and Insecticides etc.
1. Phenol: The natural water sources from the effluents of various chemical and pharma industries like coal refineries, phenol manufacturing, pharmaceuticals, dying, petrochemical, pulp mill etc., include wide variety of organic chemicals like phenol and various substituted phenol. Phenol is the simplest aromatic compound with hydroxyl group attached to the benzene [8]. Phenol is one among the most prevalent chemical and pharma pollutants, due to its toxicity even at lower concentrations and formation of substituted compounds during oxidation and disinfection processes. Its direct effects on the environment include depletion of ozone layer, effect on the earth’s heat balance, reduced visibility and adding acidic air pollutants to the atmosphere [9]. Phenol removal from the industrial wastewaters is very much necessary, prior to the wastewater discharge, so as to decrease all these effects. Phenol being a carcinogenic compound requires biodegradation method which results in minimum secondary metabolites and harmless end products [10]. Several studies and extensive investigation on biodegradation of phenol and its derivative compounds have shown that phenol can be aerobically degraded by a wide variety of pure cultures of microorganisms [11].
2. Plastics: Derek Pullen, a conservator at the Tate Gallery, explains, “Plastics are giant molecules held together by forces which can be broken by attacking energy forces such as light” [12]. Plastics are durable and degrade very slowly due to the molecular bonds and interactions. Plastics are made of polystyrene and polyvinyl chloride, polyethylene and its derivatives. Nowadays plastics (from crude oil) are used as fuels in industries since it breaks down in to liquid hydrocarbons [13]. Microbial degradation of plastics gained importance in the last few years, but the fragmented compounds released by these also lead to further environmental issues. Hence there was a need of bioplastics, a form of plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, pea starch or micro biota, so that to degrade easily [14].
Types of bioplastics include Starch-based plastics, Cellulose-based plastics, Polylactic acid (PLA) plastics, Bio-derived polyethylene. Biodegradation can be achieved by microbial treatment, oxidation disintegration process, using UV rays or by Phytoremediation. Many combinations and trails in making these bioplastics have been tried recently for fast degradation of these, one such implemented is Oxo-biodegradable plastic. OBD plastic is polyolefin plastic to which has been added very small (catalytic) amounts of metal salts. Polyolefin’s is relatively inert due to its hydrophobic chain and high molecular weight, so its degradation is quite very difficult [15]. As long as the polyolefins are attached oxygen (as in a littered state), they catalyze the natural degradation process to speed it up so that the OBD plastic will degrade when subject to environmental conditions. Once degraded, they can interact with biological processes to produce water, carbon dioxide and biomass. The process is shortened from hundreds of years to months for degradation and thereafter biodegradation depends on the microorganisms in the environment.
3. Hydrocarbons: Petroleum effluents mainly contain polycyclic (polynuclear) aromatic hydrocarbons, saturated hydrocarbons and nitrogen-sulphur-oxygen compounds. Degradation of such compounds using physico-chemical treatments is cost effective and may lead to further disturbances in the environment, thus giving importance to biotreatments, which had an impact on reduction of these recalcitrants. Microorganisms that biodegrade these components are isolated from various environments, particularly from petroleum-contaminated sites [16]. Saturated hydrocarbons having the straight-chain (n-alkanes) are most susceptible to microbial attack than branched alkanes. The aromatic fraction is more difficult to degrade and susceptibility of biodegradation decreases as the aromacity increases in the compound [17].
4. Paints: Volatile organic compounds and additives like emulsifiers, texturizers in paint are considered harmful which can be degraded by different means like chemicals (water as solvent), hygroscopic stresses and microbial sources [18].
5. Dyes: Dye agglomeration is the major cause for the persistence of xenobiotics and their presence in aquatic bodies will affect photosynthetic activity in aquatic life due to reduced light penetration even at low concentrations [19,20]. Number of industrial processes, such as textile industries, paper printing and photography uses synthetic dyes extensively, which usually have complex aromatic molecular structures. Azodyes (Black B, Turq Blue GN, Yellow HEM, Red HEFB and Navy HER), anthraquinone and phthalocyanine dyes are commonly used dyes in these industries [21]. The degradation of these dyes produces aromatic amines, which may be carcinogenic, and mutagenic. Microorganism (living or dead biomass) has ability, not only to decolorize dyes but also detoxify it [22,23], by adsorption of dyes on microbial surfaces because of the presence of negatively charged ligands in cell wall components. Microbial degradation and decolorization of dyes is an environment friendly and cost-competitive alternative to chemical decomposition processes [24].
6. Pesticides and Insecticides: A large number of pesticides and insecticides like organophosphorous compounds, benzimidazoles, methyl parathion and morpholine are widely used and has contributed to the pollution load due to its slow degradation [25]. Although a slow process, microbial degradation is considered a boon in this case, since these halogenated aromatic compounds adversely affect the environment and ecosystem both directly and indirectly.
7. Paper and Pulp effluents: Effluent release from paper mill industries also contributes environmental pollution and cannot be neglected. Many of the chlorinated organic compounds randomly synthesized during pulp bleaching are reason for this. The situation can often be made worse if pulp mill effluents are released to oxygen-limited or depleted (anaerobic) waters. The increased public awareness and more restrictive laws against polluting processes has forced paper industries to minimize release of adsorbable organic halides and to search technologies for cleaner productions [26]. Certain species of anaerobic bacteria can methylate chlorinated organic compounds which increases both the toxicity and lipophilicity of the compound to higher animals [27]. The toxic compounds from pulp and paper mill effluent are di-, tri-, tetra-, and pentachlorophenols, tetrachloroguaiacols and tetrachlorocatechols [28].
Indirect sources: Indirect sources of xenobiotics include NSAIDs, pharmaceutical compounds, pesticide residues etc.
Pharmaceutically active compounds, being an indirect source of xenobiotics are discharged directly by manufacturers of the pharmaceuticals or effluents from hospitals which have performed their biologically intended effect and are passed onto the environment in either their complete or a fragmented state. These mainly include hormones, anesthetics and antibiotics which bioaccumulate in an organism and passed on the other through the common food chain [29]. Biomaterials developed from the synthetic polymers have the biocompatibility but their degradation into toxic substances in the body is a cause for concern [30]. Even though they are the indirect sources, they cause adverse effect on the ecological cycle.
Pollution of aquatic and soil is a worldwide problem that can result in uptake and accumulation of toxic chemicals in food chains and also harm to the flora and fauna of affected habitats [31]. Studies of bioaccumulation characteristics of various ecosystem is essential for long term planning of industrial waste disposal in ecosystem [32]. Bioaccumulation of pesticides and biomagnification processes lead to toxic behavioral effect on animals and mankind. DDT, having a half life of 10 years and BHC are chemicals used in pesticides accumulate in the plant or in plant parts like fruits and vegetables [33].
Non Steroidal Anti-inflammatory Drugs (NSAIDs) are a large diverse chemical group of drugs used in humans and animals for the treatment of inflammation, pain and fever (analgesic) [34]. Diclofenac use in animals has been reported to have led to a sharp decline in the vulture population, 95% decline in 2003, 99.9% decline as of 2008 [35].
Environmental Protection Agency (EPA) is in action to reduce the bioaccumulation and biomagnifications of various such xenobiotics by some genetic modifications and biodegradation strategies.
The current study deals with importance of xenobiotic degradation giving preference to different types of remediation process like
• Microbial Remediation
• Phytoremediation
• Photoremediation
• Other techniques
Xenobiotic Degradation
Several methods like physico-chemical and biological methods have been employed in the treatment or removal of xenobiotics. The physico-chemical methods are costly and often produce undesirable products which are toxic, requiring further treatment steps [11]. Such type of techniques often add fragmented elements which cannot be degraded easily and will make the environment still worse. To overcome these problems, many other eco-friendly techniques have been reported such as Bioremediation, phytoremediation etc. (Figure 2).
Bioremediation
Microbial degradation of xenobiotics is one of the important way to remove the environmentally harmful compounds. The potential of microorganisms to metabolize xenobiotic compounds has been recognized as an effective means of toxic and hazardous waste removal [11,36].
Bioremediation can be defined as any process that uses microorganisms or their enzymes to return the environment altered by contaminants to its original condition [37]. It can also be described as “a treatability technology that uses biological activity to reduce the concentration or toxicity of a pollutant [38]. Bioremediation process involves detoxification and mineralization, where the waste is converted into inorganic compounds such as carbon dioxide, water and methane [39]. When compounds are persistent in the environment, their biodegradation often proceeds through multiple steps utilizing different enzyme systems or different microbial populations. Contaminated wastewater, ground or surface waters, soils, sediments and air where there has been either accidental or intentional release of pollutants or chemicals are the sites where bioremediation are employed [40,41].
Microbial bioremediation: Taking the waste product of one process and using it as fuel or food for another process is one way to get done biodegradation; it makes intelligent use of resources decreasing the pollution and microbes does the same. They use these residual compounds as one of their substrate and grow on them, degrading or fragmenting them, which is highly valuable in case of bioremediation [42,43].
Effective Microorganism (EM) is the consortia of valuable and naturally occurring microorganisms which secretes organic acids and enzymes for utilization and degradation of anthropogenic compounds [44]. These days, microbes are collected from the waste water, residual sites and distillery sludges which are believed to have the resistance against the hazardous compounds. This is particularly due to their tolerance capacity even at the higher concentrations of xenobionts [45]. Heavy metals and toxic organic pollutants which are believed to have resistance towards some of the microbes can be degraded using these tolerant microbes [46]. Microbial consortium used in activated sludges and aerated lagoons are used recently for solid waste effluent removal [47]. Biofilter technology is used to remove the hazardous chemicals and heavy metals from the effluents which contain these microbes capable of utilizing the substrates rapidly due to its high surface to volume ratio and fixed cell nature [48,49].
Microbial biodegradation is carried out by different organisms like Bacteria, Fungus, and Algae (Figure 3).
1. Bacteria-Biotic actors in Xenobiotic degradation: The basic sequence followed by bacteria for biodegradation of xenobiotics compounds are (Figure 4). Bacteria which endure bio-fix, a wide range of xenobiotic chemicals include aerobic, anaerobic, Methanotrophic, methanogenic bacteria, cyanobacteria and Sphingomonads [50] (Figure 5).
a) Aerobic bacteria: Pseudomonas, Escherichia, Sphingobium, Pandoraea, Rhodococcus, Gordonia, Bacillus, Moraxella, Micrococcus
b) Anaerobic bacteria: Pelatomaculum, Desulfotomaculum, Syntrophobacter, Syntrophus, Desulphovibrio, Methanospirillum, Methanosaeta [51].
c) Methanogenic bacteria and Methanotrophic bacteria: The process of degrading hydrocarbons resulting methane gas and carbon dioxide as end product is called Methanogenesis [52]. Methanotrophs use oxygen to oxidize methane into carbon dioxide. Methane monooxygenase, enzyme generated by methanotrophs to react with methane can degrade a wide variety of chlorinated hydrocarbons.
d) Cyanobacteria: Cyanobacterial consortia are generally used for degradation of oil derivatives. The use of cyanobacterial mats for bioremediation will avoid the costly use of organic and inorganic fertilizers and their maintenance at large scale can take an advantage [53].
e) Sphingomonads: Sphingomonads have a high capacity to degrade wide range of xenobiotics, including synthetic polymers, aromatic compounds etc and due to its plasmidborne mechanism. Many Sphingomonads contain large plasmids responsible for xenobiotics degradation which also help them to adapt to new environment quickly. Sphingomonads show adverse effect on polyethylene glycol (PEG) and polyvinyl-alcohol (PVA) degradation [54].
f) New Technologies: Identification of gene responsible for specific compound degradation would be beneficial to develop the recombinant Genetically Modified Organisms (GMOs) for the bioremediation of complex waste. Such advancements will always be a helping hand to already existing techniques. Sequential aerobic - anaerobic treatments are implied to degrade some of compounds, which are now replaced by these biochemical and genetic engineering approaches specially for dehalogenation. These are termed as “Super Bugs” which contain special genes residing on their plasmids [55]. Many such efficient strategies are evolved to replace the less eco-friendly physicochemical approaches.
Many different types of bacteria are used now a days for common effluent treatment which is tabulated as below (Table 1).
Pseudomonas sp. has been characterized for complete and partial mineralization of organophosphorous pesticides and fungicides, morpholine and methyl parathion. Pseudomonas sp. also involved in characteristic aromatic and aliphatic hydrocarbon degradation of oils [25]. Pseudomonas pseudomallei is used for efficient removal of phenols from aqueous solutions. Hexavalent Chromium which is water soluable and toxic is converted in to trivalent Chromium (less toxic) by Pseudomonas sp. [56]. Pseudomonas fluorescens SM1 strain is reported to be a good candidate for remediation of some heavy metals and phenolics in heavily polluted sites [57]. P. aeruginosa is used for the reclamation of oil/metal contaminated soils by producing surfactants and tolerate to certain heavy metals [58] and this strain is also used in decolorization and degradation of reactive dye Remazol Black B (RBB) [59].
Bacillus sp. have been characterized and documented for their ability to degrade benzimidazole compounds [25].
Azotobacter sp. as the biomatrix is used to remove the less toxic trivalent Cr through biosorption.
• Thermophilic bacteria Anoxybacillus rupiensis isolated from the hot water springs of Unhavre situated in the western coast of Maharashtra for its ability to degrade a local textile effluent containing dyes [3]. Synthetic dye Reactive Black 5, from the wastewater was degraded upto 80% by a thermophilic Anoxybacillus pushchinoensis, Anoxibacillus kamchatkensis and Anoxibacillus flavithermus [3]. Azo dyes degradation generally needs microbial consortia or combinations of anaerobic and aerobic steps. This time consuming process is now replaced by using lactic acid bacteria which is efficient under both anaerobic and aerobic conditions [60].
• Polyethylene used for manufacturing plastics are degraded by Brevibaccillus borstelensis and Rhodococcus ruber which can degrade the CH2 backbone and use polyethylene as its sole carbon source due to the hydrophobic nature of the cell membranes [61].
• Microbial enhancing oil recovery processes (MEOR), uses biosurfactants which reduces the interfacial tension between oil and water interfaces. Some gram positive, spore forming bacteria like Bacillus subtilis releases surfactin, a biosurfactant which acts up on oil spills and degrade them easily [62,63].
• Microbial mats develop on the accidental oil spillages at the water-sediment interface, which contains microbial consortium. Evidences have been presented that these mats mainly contain phototrophic Cynobacteria which have capability to actively degrade petroleum derivatives [64].
• Staphylococcus can form biofilms, a self produced polymer matrix. This property makes the organism unique, helping in fragmenting the compounds [65].
• It has been reported that urease producing bacteria like Proteus mirabilis, Pseudomonas aeruginosa and Micrococcus luteus produces “Microbial Concrete”, novel metabolic byproduct of biomineralization which can remediate and restore the building structures [66].
Rhodococcus erythropolis in vertical rotating immobilized cell reactor is used as biocatalyst to carry out biodesulfurization of crude oil [67].
1. Mycoremediation: The process of using fungi for bioremediation of contaminated soils (usually) is termed as mycoremediation, coined by Paul Stamets. Mycoremediation plays a pivotal role in breaking down numerous toxic substances like petroleum hydrocarbons, polychlorinated biphenyls, heavy metals (by biosorption), phenolic derivatives, persistent pesticides etc. Fungi utilize some of these hazardious compounds as the nutrient source and convert them to simpler fragmented forms. Microalgae have a potential to be used as a substrate for the bioenergy production on a commercial scale.
Different classifications can be explained under this like
• Ligninolytic fungal degradation
• Fungal biosorption
• Mycorrhizal fungal degradation
a) Ligninolytic fungal degradation: Many fungal species like basidiomycetes, ascomycetes have the potential to degrade lignocelluloses materials present in dead wood, paper and pulp effluents [68].
Basidiomycetes species are considered to be a very interesting group of fungi, considered as natural lignocellulose destroyers and include very different ecological groups such as white rot, brown rot, and leaf litter fungi. Polycyclic aromatic hydrocarbons from natural oil deposits are degraded by laccases, a copper containing enzyme found in these basidiomycetes [69]. White-rot fungi degrades PVC under aerobic conditions secreting extracellular enzymes which act on the polymers. An edible rot fungus, Pleurotus pulmonarius is known for its ability to degrade crude oil [70]. Ligninolytic fungi show higher potentials to degrade organ pollutants including synthetic dyes. Nowadays immobilized fungal cultures in semisolid state-, trickling-bed- and rotating disk reactors are used for efficient biodegradation of textile dyes, because of the advantages which include long retention time of biomass in the system, ease of use in a continuous reactor and their ability for scale up [3]. Mascoma demonstrated that Saccharomyces cerevisiae can convert 85% of paper sludge to ethanol without the addition of commercial enzymes. Trichoderma harzianum (also a fungicide) is well known producer of cellulolytic enzymes that extensively used for the degradation of cellulose in textile and paper industries [71].
b) Biosorption by Fungi: Biosorption is a physiochemical process that occurs naturally in certain biomass (abundant (seaweed) or wastes from other industrial operations) which allows it to passively concentrate and bind contaminants onto its cellular structure [72]. Biosorption have an upper hand over bioaccumulation process since accumulated metal or waste can be desorbed easily by simple physical methods without damaging the biosorbent’s structural integrity [73].
Fungus is considered as most promising adsorbant, whose cell walls and their components have a major role in biosorption. It has been reported that fungal biomass can also take up considerable quantities of organic pollutants from aqueous solution by adsorption or a related process, even in the absence of physiological activity [74].
Many fungal species such as Mucor sp, Aspergillus carbonaruius, Aspergillus niger, Rhizopus sp, Saccharomyces cerevisiae, Botrytis cinerea, Neurospora crossa, Phanerochaete chrysosporium and Lentinus sajor-caju have been extensively studied for heavy metal biosorption [75]. Saccharomyces cerevisiae was found to be the efficient biosorber of heavy metals like Pb, Au, Co, Cu [76].
Along with fungus, plant and its parts like solid residues of oil mill products, sawdust, straw xanthate, aquatic plants and seaweeds also plays a pivotal role in adsorbing waste compounds. New techniques have been evolved which implement the fungal symbiont residing in the plants where both fungal and plant part help in bioadsorption [77].
c) Mycorrhizal fungal degradation: Mycorrhiza, a symbiotic association of fungi and actinomycetes with the root zone of the vascular plant which increases soil organic carbon. Mycorrhizal fungi, growing as a symbiont encourage degradation of organic contaminants in soil. The typical mycorrhizons which naturally biodegrade the organic pollutants are Morchella conica and Tylospcno fibrilnsa [78].
d) Phycoremediation: Phycoremediation is defined as the use of macroalgae or microalgae to remove or biodegrade pollutants from the environment. Algae possess the ability to take up toxic heavy metals from the environment, resulting in higher concentrations than those in the surrounding water [79]. Special polysaccharides are present in the algae cell wall contained potential metal ion binding sites.
Commonly used algal groups for degradation are Chlorococcum sp, Chroococcus sp, Desmococcus sp, Dactylococcopsis sp, Chlamydomonas sp [80]. Blue Green Algae is extensively used for wastewater treatment. Recent studies have quoted that, Spirulina platensis, a photosynthethis blue-green algae is used as biosensor to detect the mercury concentration accumulated in the solids waste residues and soil. Biosorption using Blue Green Algae has distinct advantages over the conventional methods because of reasons like
• easy to operate
• cost effective for the treatment of large volumes of waste waters
• it could be selective
• More efficient [81].
Phytoremediation: Phytoremediation (also called as green remediation, botano-remediation, agro remediation, and vegetative remediation) is method that use green or higher terrestrial plants for treating chemically polluted soils [82], reducing the amount of hazardous compounds [83]. USEPA (2000) defines phytoremediation as “the use of plants for containment, degradation or extraction of xenobiotics from water or soil substrates” [84]. Using green plants as weapons, phytoremediation is one of most eco-friendly technique to target the organic and inorganic pollutants in the water, soil and air simultaneously.
Plants have exposed the capacity to withstand relatively high concentrations of organic xenobiotic chemicals without toxic effects [85] and also have capacity to take up and convert chemicals quickly to less toxic metabolites [86]. Deep roots, luxuriant leaves have special sorptive properties and the associated bacteria in root zones allow plants to absorb, take-up, accumulate, metabolize and/or degrade the pollutants from water, soil and air.
Phytoremediation can be classified in to subcategories depending up on the type of remediation (Figure 6).
1. Rhizodegradation: Rhizodegradation is the enhancement of naturally-occurring biodegradation in soil through the influence of plant roots, and ideally will lead to destruction or detoxification of an organic contaminant. A wide range of organic contaminants are candidates for rhizodegradation, such as petroleum hydrocarbons, PAHs, pesticides, polychlorinated biphenyls (PCBs), surfactants and chlorinated solvents.
2. Phytodegradation: Phytodegradation, also called as phytotransformation, is the uptake, metabolizing, and degradation of contaminants within the plant, or the degradation of contaminants in the soil, sediments, sludges, ground water, or surface water by enzymes produced and released by the plant. The term “Green Liver Model” is used to describe phytotransformation, as plants behave analogously to the human liver when dealing with these xenobiotic compounds (foreign compound/pollutant). Chlorinated solvents like TCE, some organic herbicides and trinitrotoluene can be degraded using this method [87].
3. Phytoextraction: Phytoextraction (also known as phytoaccumulation, phytoabsorption, and phytosequestration) is contaminant uptake by roots with subsequent accumulation in the aboveground portion of a plant, generally to be followed by harvest and ultimate disposal of the plant biomass. Phytoextraction has also been referred to as phytomining or biomining. Phytomining is the use of plants to obtain a gain from hyperaccumulated metals extracted by a plant, whether from contaminated soils or from soils having naturally high concentrations of metals. This is particularly useful for removing metals from soil and, in some cases; incorporation of plant incinerations will help metal reuse [87]. These processes extract both metallic and organic constituents from soil by direct uptake into plants and translocation to aboveground biomass using metal- (hyper) accumulating plants. Brassica juncea, Berkeya coddii, Allysum bertolonii, Thlaspi caerulescens and Thlaspi goesingense are some of the plants involved in phytoextraction [88]. The main advantage of phytoextraction is the process is eco- friendly but will take more time than anthropogenic soil clean-up methods.
4. Rhizofiltration: Rhizofiltration (also known as phytofiltration) is the removal by plant roots of contaminants in surface water, waste water, or extracted ground water, through adsorption or precipitation onto the roots, or absorption into the roots. Here accumulation can occur in root or can be retained in any portion of the plant. Plants used for rhizofiltration are not planted directly in situ but are acclimated to the pollutant first, which makes the process little tedious and time consuming. Sunflowers grown in radioactively contaminated pools exemplify this process.
5. Phytovolatilization: Phytovolatilization is the uptake of a water soluble contaminant by a plant, and the subsequent release of a vol atile contaminant, a volatile degradation product of a contaminant, or a volatile form of an initially non-volatile contaminant. Plants as phytovolatilizers have to be studied still for better utilization.
6. Phytohydraulics (hydraulic plume control): Phytohydraulics is the use of deep-rooted plants to degrade ground water contaminants that come into contact with their roots. Ground water plume of methyl-tert-butyl-ether (MTBE) has been recovered using this technique [89].
7. Phytostabilization: Phytostabilization (also called as phytoimmobilization) is the use of plants to immobilize soil and water contaminants. Some organic contaminants or metabolic byproducts of these contaminants can be attached or incorporated into plant components such as lignin and such type of phytostabilization is called phytolignification [90]. Indian mustard appeared to have potential for phytostabilization.
8. Recent trends used in phytoremediation: A recent strategy to improve phytoremediation and detoxification of contaminants is the use of endophytic bacteria which are often found genera in soil like Pseudomonas, Burkholderia, Bacillus, and Azospirillum [91].
Genetic modification offers a new hope for phytoremediation as they can be used to over express the enzymes involved in the existing plant metabolic pathways or to introduce new pathways into plants [91,92]. With increased understanding of the enzymatic processes involved in plant tolerance and metabolism of xenobiotic chemicals, there is new potential for engineering plants with increased phytoremediation capabilities [93,94]. Transgenic plants that over express mercury-resistance genes have been reported to be highly resistant to organic mercury and are effective for degradation, thus bringing a new advancement in the phytodegradation process [95].
Richard Meagher introduced a new pathway into Arabidopsis to detoxify methyl mercury, a bioaccumulative organometalic cation, to the elemental mercury which can be volatilized by the plant [92].
9. Pros and cons of phytoremediation: Phytoremediation is considered a clean, cost-effective and non-environmentally disruptive technology, as opposed to mechanical cleanup methods such as soil excavation or pumping polluted groundwater [96]. Over the past 20 years, this technology progressed and has been employed at sites with soils contaminated with arsenic, lead and uranium. On the other hand, one major disadvantage of phytoremediation is that it requires a long-term commitment, as the process is dependent on plant growth, bioaccumulation capacity and tolerance to toxicity.
Although phytoremediation is a promising technique to remove pollutants, it is still an immature and developing technology to deal with pollution problems. However, it is clear that phytoremediation already plays an important role in removing pollutants from the environment we just need to find the right plant for the right pollutant [97].
Photodegradation
Photodegradation is degradation of a molecule which has capability to absorb photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. The solution of plastic ecological problem is achieved by this particularly, due to the development of photodegradable and biodegradable polymer with controlled lifetime. Many new strategies have been introduced in order to make this technique applicable for wide range of xenobiotic degradation. Transition metals like Cr, Mn, Fe, Co act as antioxidant or bioactive elements in photodegradation of polymer degradation [98]. Congo red dye used in the cellulose industries (cotton textile, wood pulp & paper) has long been abandoned, primarily because of its tendency to change color and its toxicity. Recent advances to degrade this Congo red include photocatalytic degradation using ZnO/ UV-A [99,100].
Initiators like Ketones, quinones and peroxides are added in some cases to carry out photo-degradation reactions. Nowadays photodegraded films are used to evaluate biodegradation using microorganisms such as Aspergillus niger and Pencillium funculosum for degradation of both natural and synthetic plastics. Heterotrophic microorganisms uses polymers especially plastics are potential substrates and degrade them first converting them to simpler monomeric forms [101].
Although many such techniques are implied to reduce these xenobiont in the environment, each one have their own drawback. Biodegradation actually means complete elimination, but this is not the case in most of the above process. Fragmental release due to these steps also effect the environ in the long run. Eco-Industrial Parks (EIP) have been built which is actually an association between local community and business corporate to solve the environmental problems. The benefits Eco-Industrial Parks may serve as incentives for companies to improve their environmental performance in terms of management of hazardous waste, raw materials, conservation of energy use. The EIP prevent further degradation of our environment and remediate damage caused by increasingly industrialized society via phytotechnologies. Such EIP offer efficient and environmentally friendly solutions to clean up contaminated soils, sediments, brown fields and wastewater, to enhance food chain safety and to develop renewable energy sources [102].
Conclusion
Environmental problems caused by the industrial effluents is mainly due to accumulation of pollutants and other fragmented compounds, which in turn form into other substitutes (natural or manmade), finally forming a xenobiont. There is a quick need to degrade these xenobiotic compounds in an eco-friendly way. Various techniques like microbial remediation, phytoremediation and photoremediation and their subtypes have been discussed. Each having their own ways of degrading these xenobionts, also have negative impact on the environ (side effects due to fragmentations and bioaccumulations). Photoremediation, a novel equipment based technique which is rapid but also have a negative impact on the environment. Being a solar driven technique, phytoremediation is restricted to particular sites containing contaminants. Although slow, on the whole microbial bioremediation was found to cover wide range of recalcitrant degradation and is known to be a better choice because of its nature of degradation.
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