**1. Introduction**

An increasingly urban population and industrialized global economy over the last century have serious consequences on the environment. Understanding the sources, pathways and contaminants in the urban environment is essential for making informed management decisions. Urban areas are major concentrators, repositories and emitters of a myriad of chemicals because of the wide range and intensity of human and anthropogenic activities. Common contaminants include petroleum hydrocarbons (PHCs), polycyclic aromatic hydro‐ carbons (PAHs), halogenated hydrocarbons, pesticides, solvents, metals, salt and the resulting stresses on human and ecosystem health are well documented [1]. Polycyclic aromatic hydrocarbons are a class of complex organic chemicals consisting of over hundred different organic compounds. PAHs are unique contaminants in the environment because they are generated continuously by incomplete combustion of organic matter, for instance in forest fires, home heating, traffic, and waste incineration [2]. PAHs are hydrophobic compounds and their persistence in the environment is chiefly due to their low water solubility [3]. Generally, solubility of PAHs decreases and hydrophobicity increases with an increase in number of fused benzene rings. In addition, volatility decreases with an increasing number of fused rings [4]. The major source of PAHs is from the combustion of organic material [5]. PAHs are formed naturally during thermal geologic production and during burning of vegetation in forest and bush fires [6]. PAHs and their alkyl homologous may also be derived from biogenic precursors during early diagnosis [7]. However, anthropogenic sources, particularly from fuel combus‐ tion, pyrolytic processes, spillage of petroleum products, waste incinerators and domestic heaters [8] are significant sources of PAHs in the environment. At depth 90-135 cm, only

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phenanthrene (1.4 mg/kg), pyrene (4.0 mg/kg), chrysene (0.9 mg/kg) and dibenzoanthracene (0.8 mg/kg) were found [9]. The concentration of PAHs in the environment varies widely, depending on the level of industrial development, proximity of the contaminated sites to the production source and the mode of PAH transport. Kanaly and Harayama [10] reported that in soil and sediment, PAHs concentrations vary from 1μg/kg to over 300 g/kg.

Phytoremediation, the use of plants to degrade toxic contaminants in the environment involves a number of processes including phytoextraction, phytotransformation, phytostabilization, phytovolatilization and rhizofiltration [26]. Phytoextraction (or phytoaccumulation) involves the uptake and concentration of pollutants into harvestable biomass for sequestration or incineration. Phytotransformation involves enzymatic modification resulting in inactivation, degradation (phytodegradation), or immobilization (phytostabilization) of pollutants. Phytovolatilization involves the removal of pollutants from soil and their release through leaves via evapotranspiration processes and rhizofiltration involves the filtering of water through a mass of roots to remove pollutants. While some success has been reported using plants alone in bioremediation, the use of plants in conjunction with plant associated bacteria

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The importance of plant microbe partnerships in the remediation of organic contaminants was confirmed in studies at the level of rhizosphere [28, 29], the phyllosphere and inside the plant [30, 31]. Rhizoremediation is considered as the most potential approach for PAHS remediation in soil [37]. Soil microflora play vitally important role during rhizoremediation of xenobiotics [32]. The interaction among microbial degrader, plant and PAHs in soil might be regulated

Rhizoremediation systems for PAHs rely on a synergistic relationship between suitable plants and their root associated microbial communities [32]. Degradation is facilitated through a rhizosphere effect where plants exude organic compounds through their roots and thereby increase the density and activity of potential hydrocarbon degrading microorganisms in the zone, surrounding the roots [35]. The biodegradation abilities of bacteria and the expression and maintenance of bacteria in the rhizosphere are extremely important for the effective removal of contaminants in rhizoremediation [36]. Thus, bioremediation, phytoremediation and rhizoremediation contribute significantly to the fate of hazardous waste and can be used

Rhizoremediation is proposed as the most potential approach for PAHs remediation in soil [39]. Plant associated bacteria, such as rhizospheric bacteria have been shown to contrib‐ ute to biodegradation of toxic organic compounds in contaminated soil and could have potential for improving phytoremediation [40]. The amalgamation of the activity of plant roots and rhizospheric microbial communities like secretion of root exudates (various organic acids and amino acids etc.), production of siderophores, HCN, phytoharmone and phosphatases by plant growth promoting rhizobacteria (PGPR) are also effective for ecorestoration of polluted sites [41]. The plant growth-promoting capability of *B. aryabhat‐ tai* strains may be utilized as an environmentally friendly means of revegetating barren lands [41]. The valuable effects of some rhizobacteria on plant growth are well known, and the so called PGPR have been utilized for several decades, although their mechanisms of plant growth promotion have not been completely elucidated [42]. Some of the important mechanisms include direct phytohormonal action, increase of plant nutrient availability and the enhancement of other plant beneficial microorganisms [43]. When a suitable rhizospher‐ ic isolated strain is introduced together with a suitable plant, it inhabits on the root along with indigenous population, thereby enhancing the bioremediation process [44]. In addition,

to remove these unwanted compounds from the biosphere [37, 38].

offers much potential for rhizoremediation [27].

through rhizosphere processes [39].

PAHs have been detected in a wide variety of environmental samples, including air [8], soil [11], sediments [19], water [12], oils, tars [13] and foodstuffs [14]. PAHs contamination on industrial sites is commonly associated with spills and leaks from storage tanks and with the conveyance, processing, use and disposal of these fuel/oil products [4]. PAHs are also a major constituent of creosote (approximately 85% PAH by weight) and anthracene oil, which are commonly used pesticides for wood treatment [15]. The main route for PAH transport is through the atmosphere. Results from ambient air monitoring programs have shown that PAH concentrations are usually of the order of a few nano-grams per cubic metre of air [16]. However, PAH concentrations may vary from season to season depending on emissions arising from the combustion of home heating products. Motor vehicles, including spark emission and diesel automobiles, trucks and buses, also contribute to atmospheric PAHs pollution through exhaust condensate and particulates, tyre particles and lubricating oils and greases [17]. During the combustion of fossil fuels, diesel powered vehicles are the major sources of lighter PAHs to the atmosphere, whereas gasoline vehicles are the dominant source of higher molecular weight PAHs [18]. The persistence of PAHs in the environment depends on the physical and chemical characteristics of the PAHs. PAHs are degraded by photooxidation and chemical oxidation [19], but biological transformation is probably the prevailing route of PAH loss [20]. The microbial metabolism of PAHs containing up to three rings (naphthalene, phenanthrene, anthracene, fluorene) has been studied extensively.

Heavy metals such as lead, mercury, and cadmium are ranked second, third, and seventh, respectively, on the 2003 Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, commonly known as Superfund) priority list for hazardous substances because they are toxic widespread pollutants. Soil contamination is a particularly serious environmental concern, as the majority of superfund sites are highly contaminated with heavy metals [21].

Therefore, for remediation of contaminated environmental soil, the traditional technologies routinely used are as excavation, transport to specialized landfills, incineration, stabilization and followed by coagulation filtration or ion exchange are expensive and disruptive to the sites [22]. However, there has been much interest in bioremediation technologies which use plants and microorganisms (including bacteria) to degrade toxic contaminants in environ‐ mental soil into less toxic and/or non-toxic substances [23]. With *in situ* techniques, the soil and associated ground water is treated in place without excavation, while it is excavated prior to treatment with *ex situ* applications [24].

Biosorption (using microbially produced metallothioneins (MTs) and phytochelatins (PCs) having heavy metal binding affinities) and immobilization are major mechanisms utilized by animals and plants to limit the concentrations of internal reactive metal species [25].

Phytoremediation, the use of plants to degrade toxic contaminants in the environment involves a number of processes including phytoextraction, phytotransformation, phytostabilization, phytovolatilization and rhizofiltration [26]. Phytoextraction (or phytoaccumulation) involves the uptake and concentration of pollutants into harvestable biomass for sequestration or incineration. Phytotransformation involves enzymatic modification resulting in inactivation, degradation (phytodegradation), or immobilization (phytostabilization) of pollutants. Phytovolatilization involves the removal of pollutants from soil and their release through leaves via evapotranspiration processes and rhizofiltration involves the filtering of water through a mass of roots to remove pollutants. While some success has been reported using plants alone in bioremediation, the use of plants in conjunction with plant associated bacteria offers much potential for rhizoremediation [27].

phenanthrene (1.4 mg/kg), pyrene (4.0 mg/kg), chrysene (0.9 mg/kg) and dibenzoanthracene (0.8 mg/kg) were found [9]. The concentration of PAHs in the environment varies widely, depending on the level of industrial development, proximity of the contaminated sites to the production source and the mode of PAH transport. Kanaly and Harayama [10] reported that

PAHs have been detected in a wide variety of environmental samples, including air [8], soil [11], sediments [19], water [12], oils, tars [13] and foodstuffs [14]. PAHs contamination on industrial sites is commonly associated with spills and leaks from storage tanks and with the conveyance, processing, use and disposal of these fuel/oil products [4]. PAHs are also a major constituent of creosote (approximately 85% PAH by weight) and anthracene oil, which are commonly used pesticides for wood treatment [15]. The main route for PAH transport is through the atmosphere. Results from ambient air monitoring programs have shown that PAH concentrations are usually of the order of a few nano-grams per cubic metre of air [16]. However, PAH concentrations may vary from season to season depending on emissions arising from the combustion of home heating products. Motor vehicles, including spark emission and diesel automobiles, trucks and buses, also contribute to atmospheric PAHs pollution through exhaust condensate and particulates, tyre particles and lubricating oils and greases [17]. During the combustion of fossil fuels, diesel powered vehicles are the major sources of lighter PAHs to the atmosphere, whereas gasoline vehicles are the dominant source of higher molecular weight PAHs [18]. The persistence of PAHs in the environment depends on the physical and chemical characteristics of the PAHs. PAHs are degraded by photooxidation and chemical oxidation [19], but biological transformation is probably the prevailing route of PAH loss [20]. The microbial metabolism of PAHs containing up to three rings

in soil and sediment, PAHs concentrations vary from 1μg/kg to over 300 g/kg.

332 Applied Bioremediation - Active and Passive Approaches

(naphthalene, phenanthrene, anthracene, fluorene) has been studied extensively.

metals [21].

treatment with *ex situ* applications [24].

Heavy metals such as lead, mercury, and cadmium are ranked second, third, and seventh, respectively, on the 2003 Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, commonly known as Superfund) priority list for hazardous substances because they are toxic widespread pollutants. Soil contamination is a particularly serious environmental concern, as the majority of superfund sites are highly contaminated with heavy

Therefore, for remediation of contaminated environmental soil, the traditional technologies routinely used are as excavation, transport to specialized landfills, incineration, stabilization and followed by coagulation filtration or ion exchange are expensive and disruptive to the sites [22]. However, there has been much interest in bioremediation technologies which use plants and microorganisms (including bacteria) to degrade toxic contaminants in environ‐ mental soil into less toxic and/or non-toxic substances [23]. With *in situ* techniques, the soil and associated ground water is treated in place without excavation, while it is excavated prior to

Biosorption (using microbially produced metallothioneins (MTs) and phytochelatins (PCs) having heavy metal binding affinities) and immobilization are major mechanisms utilized by

animals and plants to limit the concentrations of internal reactive metal species [25].

The importance of plant microbe partnerships in the remediation of organic contaminants was confirmed in studies at the level of rhizosphere [28, 29], the phyllosphere and inside the plant [30, 31]. Rhizoremediation is considered as the most potential approach for PAHS remediation in soil [37]. Soil microflora play vitally important role during rhizoremediation of xenobiotics [32]. The interaction among microbial degrader, plant and PAHs in soil might be regulated through rhizosphere processes [39].

Rhizoremediation systems for PAHs rely on a synergistic relationship between suitable plants and their root associated microbial communities [32]. Degradation is facilitated through a rhizosphere effect where plants exude organic compounds through their roots and thereby increase the density and activity of potential hydrocarbon degrading microorganisms in the zone, surrounding the roots [35]. The biodegradation abilities of bacteria and the expression and maintenance of bacteria in the rhizosphere are extremely important for the effective removal of contaminants in rhizoremediation [36]. Thus, bioremediation, phytoremediation and rhizoremediation contribute significantly to the fate of hazardous waste and can be used to remove these unwanted compounds from the biosphere [37, 38].

Rhizoremediation is proposed as the most potential approach for PAHs remediation in soil [39]. Plant associated bacteria, such as rhizospheric bacteria have been shown to contrib‐ ute to biodegradation of toxic organic compounds in contaminated soil and could have potential for improving phytoremediation [40]. The amalgamation of the activity of plant roots and rhizospheric microbial communities like secretion of root exudates (various organic acids and amino acids etc.), production of siderophores, HCN, phytoharmone and phosphatases by plant growth promoting rhizobacteria (PGPR) are also effective for ecorestoration of polluted sites [41]. The plant growth-promoting capability of *B. aryabhat‐ tai* strains may be utilized as an environmentally friendly means of revegetating barren lands [41]. The valuable effects of some rhizobacteria on plant growth are well known, and the so called PGPR have been utilized for several decades, although their mechanisms of plant growth promotion have not been completely elucidated [42]. Some of the important mechanisms include direct phytohormonal action, increase of plant nutrient availability and the enhancement of other plant beneficial microorganisms [43]. When a suitable rhizospher‐ ic isolated strain is introduced together with a suitable plant, it inhabits on the root along with indigenous population, thereby enhancing the bioremediation process [44]. In addition, such capability for root colonizing, pollutant degrading bacteria utilize the growing root system and hence this acts as an injection system to spread the bacteria through soil [45].Plant root performs certain specialized roles, including the ability to synthesize, accumulate and secrete a diverse array of nutrient compound consequently no require‐ ment of exogenous carbon source, roots may regulate the soil microbial community in their immediate vicinity, cope with herbivores, encourage beneficial symbioses, change the chemical and physical properties of the soil and inhibit the growth of competing plant species [46]. PGP bacteria may facilitate plant growth either directly or indirectly [47]. Though the rhizoremediation process takes place naturally, but it can be modified by premeditated exploitation of the well-equipped rhizospheric microorganisms whereas it can be proficient by using suitable plant microbe pairs.

**3. Bioaugmentation**

habitat [55].

**4. Phytoremediation**

cheaper than excavation and reburial) [60].

up or enable the degradation of pollutants [48, 55]

Bioaugmentation is the introduction of microorganisms with specific catabolic abilities into the contaminated environment in order to supplement the indigenous population and to speed

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Bioaugmentation has proven successful for remediation of PAHs in sediments with poor or lacking intrinsic degradation potential [17], while other studies demonstrated that bioaug‐ mentation did not enhance biodegradation significantly compared to natural attenuation [56]. One of the main problems in applying bioaugmentation is to ensure the survival and activity of the introduced organisms in the environment [55]. Bioaugmentation can be inhibited by a variety of factors including pH and redox, the presence of toxic contaminants, concentration and bioavailability of contaminants or the absence of key co-substrates [48]. However, the key factor for the success of bioaugmentation process is the selection of the appropriate bacterial strain. When selecting the strain for augmentation purposes, the kind of microbial communi‐ ties present in the source habitat should be considered [57]. Bioaugmentation strategies may prove successful especially in the remediation of manmade contaminants, where specialized bacteria with the appropriate catabolic pathways may not be present in the contaminated

Some workers quantified and compared the responses of soil microbial communities during the phytoremediation of PAHs in a laboratory trial [15]. A recent publication of some workers describes the development of transgenic poplars (*Populus* sp.) over expressing a mammalian cytochrome P450, a family of enzymes commonly involved in the metabolism of toxic com‐ pounds. The engineered plants showed enhanced performance about the metabolism of trichloroethylene and the removal of a range of other toxic volatile organic pollutants, including vinyl chloride, carbon tetrachloride, chloroform and benzene. Some workers suggested that transgenic plants might be able to contribute to the wider and safer application of phytoremediation [58]. Widespread phytoremediation field trials research was performed *in vitro* condition and many of the works explored the effects of plants on removal of contam‐ inants from spiked soil and soil excavated from contaminated sites [7] and most of these experiments provided valuable insights into the specific mechanisms of phytoremediation of organic contaminants [29]. Previously, numerous organic pollutants such as TCE (trichloro‐ ethylene), herbicides such as atrazine, explosives such as TNT (trinitrotoluene), PHC, BTEX (mono aromatic hydrocarbons) and PAHs, the fuel additive MTBE (methyl tertiary butyl ether), and PCBs (polychlorinated biphenyls) have been successfully phytoremediated. [59] Major advantages of phytoremediation *viz*., cost of the phytoremediation is lower than that of traditional processes both *in situ* and *ex situ*, plants can be easily monitored, possibility of the recovery and re-use of valuable products, use of naturally occurring organisms and preser‐ vation the natural state of the environment, low cost of phytoremediation (up to 1000 times

Incorporation of plant and PGPR having the pollutant degrading activity may be performed. Similarly, Kuiper *et al.* [48] described the pair of a grass species with a naphthalene degrading microbe which protected the grass seed from the toxic effects of naphthalene and the growing roots exploited with the naphthalene degrading bacteria into soil.

Previously, several researchers have also used this symbiotic relationship of plant and microbes for degradation of hazardous and xenobiotic compounds like PCBs, PAHs and TCE [49]. Mechanical injection of contaminated sites with pollutant degrading bacteria has been used to clean polluted sites in an inexpensive and less labor intensive way than the removal and/or combustion of polluted soils [50].
