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Microbe-Assisted Phytoremediation
A Promising Technology for Remediation of Environmental Pollutants
1.1 Introduction
Rampant industrialization, urbanization, and anthropogenic activities are associated with a continuous build-up of environmental contaminants and pollution, posing considerable risk to humans and environmental health, leading to epidemics of cancers, lung diseases, and other degenerative diseases (Chandra et al. 2018a,b). Among the different sources of environmental contaminants, industries are considered the major source of soil, water, and air pollution. To obtain good-quality products within a short period of time, industries generally use poorly biodegradable or non-biodegradable chemicals and subsequently generate a huge quantity of hazardous and toxic waste. Industrial waste, containing a mixture of numerous organic and inorganic pollutants, is generally dumped on land and/or discharged into water bodies, and thus they become a large source of environmental pollution and health hazards. In wealthy industrialized countries, contamination is often highly limited to a small area, and the pressure to use contaminated land and water for agricultural food production or human consumption, respectively, is minimal. However, soil and water contamination is increasingly recognized as dramatic in large parts of the developing world. A large array of methods based on not only physical and chemical but also biological means have been available for the remediation of contaminated soil and water for decades; however, these methods are financially expensive and energy intensive, cause secondary air or groundwater pollution, and can destroy waste-degrading microbial communities of soil, and for many refractory pollutants, no feasible technologies are yet available. Therefore, environmental preservation requires the development of sustainable approaches that promise thorough, economical, and eco-friendly ways to make them safe for human habitation and consumption and to protect the functions of the life-supporting ecosystem. To safeguard both humans and the environment from the adverse consequences of organic and inorganic pollutants, novel approaches must be designed, and phytoremediation is one such approach. Phytoremediation is an ideal approach for the treatment and/or elimination of toxic organic and inorganic pollutants from the contaminated environment or to render them harmless (Khan et al. 2014a; Chandra et al. 2015). It is an emerging green approach where plants are grown in contaminated soil, sediment, and surface and groundwater to increase the decomposition or removal rate of inorganic and organic pollutants in planta as well as ex planta. Plants possess extremely efficient root systems that acquire and concentrate nutrients from the contaminated matrices as well as numerous metabolic activities, all of which are ultimately powered by photosynthesis, but the major constraint of this technology is that even plants that are tolerant to the presence of contaminants often remain relatively small, due to the toxicity of the pollutants that they are accumulating from contaminated matrices or the toxic end-products of their degradation (Kumar and Chandra 2018a). Besides, it is a time-consuming remediation technology, with slow degradation and limited uptake of organic and inorganic contaminants from the contaminated matrices. The toxicity of organic and inorganic contaminants on plants can be reduced by using a microbe in association with the plant (Weyens et al. 2009a,b; Glick 2012; Khan et al. 2014a,b). The combinatorial systems of plants and their associated microbes have been shown to contribute to biodegradation and detoxification of organic and inorganic compounds in polluted soil and water and could have potential for improving the phytoremediation efficacy of plants (Rajkumar et al. 2012; Redfern and Gunsch 2016; Fatima et al. 2018). Microbe-assisted phytoremediation is a promising, inexpensive, and eco-friendly rehabilitation approach that uses a broad range of plants and their associated microbes for remediating pollutants present in different environmental matrices (Juwarkar and Singh 2010; Nanekar et al. 2015; Ali et al. 2017; GarcĂa-SĂĄnchez et al. 2018). They can metabolize, detoxify and/or biotransform many refractory pollutants either to obtain carbon and/or energy for their growth or as co-substrates, thus converting them to simpler products such as carbon dioxide (CO2) and water (H2O).
This chapter presents an overview of the environmental pollution, toxicity profile, and health hazards of numerous organic and inorganic pollutants discharged from various industries and their management approaches using physical, chemical, and biological means. We also discuss the merits and demerits of physico-chemical and biological methods used for the remediation of organic and inorganic pollutants from contaminated sites. Further, we also describe various processes of phytoremediation, with special emphasis on microbe-assisted phytoremediation as a cleanup technique for remediation of contaminated sites and provide a concise discussion of how microbes could be exploited to enhance the phytoremediation efficacy of plants in contaminated environments.
1.2 Pollutants and Their Fate in the Environment
Solid and liquid waste discharged after various industrial operations is considered a major source of hazardous, toxic, and refractory pollutants in the environment (Calheiros et al. 2009; Chandra and Kumar 2015a,b, 2017a,b; Enazy et al. 2017; Mesa et al. 2017). In general, there are two types of environmental pollutants (i) organic pollutants and (ii) inorganic pollutants.
1.2.1 Organic Pollutants
Organic pollutants may include various compounds, such as petroleum hydrocarbons (i.e., benzene, pyrene, toluene, and xylene), chlorinated solvents (i.e., polychlorinated biphenyls [PCBs], trichloroethylene [TCE]), polycyclic aromatic hydrocarbons (PAHs), persistent organic pollutants (POPs) (i.e., aldrin, chlordane, dichlorodiphenyltrichloroethane [DDT], dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene, chlordecone, lindane, hexachlorobenzene, pentachlorobenzene, Îą-hexachlorocyclohexane, β-hexachlorocyclohexane, perfluorooctane sulfonic acid and its salts, perfluorooctane sulfonyl fluoride, and tetrabromodiphenyl), endocrine-disrupting chemicals (EDCs) (i.e., disulfide; o-phenylphenol; tetrabrominated diphenyl ether; 4-chloro-3-methylphenol; 2,4-dichlorophenol; resorcinol; 4-nitrotoluene; 2,2â˛-bis(4-(2,3-epoxypropoxy) phenyl) propane; 4-octylphenol; estrone [E1]; 17Îą-ethinylestradiol [EE2]; and 17β-estradiol [βE2]), azo dyes, melanoidins, organophosphorus compounds (i.e., glyphosate, chlorpyrifos, parathion, monocrotophos, dicrotophos, diazinon, dimethoate, fenitrothion), volatile organic carbons (VOCs), and explosives (i.e., nitroglycerine [NG]; 2,4,6-trinitrotoluene [TNT]; hexahydro-1,3,5-trinitro-1,3,5-triazine [Royal Demolition ExplosiveâRDX]; octahydro-1,3,5,7-tetranitro-1,3,5-tetrazocine [HMX]; and pentaerythritol tetranitrate [PETN]). One of the primary concerns about the environmentâs exposure to organic compounds is their potential to contaminate aquatic and terrestrial ecosystems and consequently posea risk to humans and other organisms associated with the food chain of the aquatic and terrestrial eco-biota (Ying and Rai 2003; Adeola 2004; Katrin et al. 2005; Singh and Walker 2006; Taioli et al. 2007; Carpenter 2011; Manzetti 2013; Igbinosa et al. 2013; Chandra and Kumar 2015a, 2017a,b; Faroon and Ruiz 2016; Hussein and Scholz 2017; Carnevali et al. 2018; Kumar and Chandra 2018a; Yaseen and Scholz 2019; Kumar et al. 2020).
1.2.2 Inorganic Pollutants
Inorganic pollutants may include non-biodegradable heavy metals, metalloids, and nonradioactive or radioactive nuclides such as uranium (U), vanadium (V), wolfram (W), strontium (Sr), and cesium (Cs). Inorganic pollutants also include various nutrients like ammonia, chloride, sodium, nitrate, and phosphate (Chandra and Kumar 2015a,b, 2017c). Heavy metals (HMs) and metalloids are the main groups of inorganic contaminants generated through natural sources such as weathering of minerals, erosion and volcanic activities, and forest fires, and particles released by vegetation and/or anthropogenic sources include human activities such as mining, smelting, ore processing, irrigation by sewage water, injudicious use of chemical fertilizers and pesticides, pile-up of municipal waste, automobile exhaust, electroplating, leather tanning, textiles and dyeing, distilleries, pulp and paper industries, and other industrial and domestic activities that pour directly or indirectly into the environment, and a considerably large area of land and water is contaminated with them (Wuana and Okieimen 2011; Tchounwou et al. 2012; Chandra et al. 2018a,b). The term âHMsâ refers to any metallic element that has a relatively high atomic weight (>20) and high density (>4 g/cm3, or 5 times or more than water) and is toxic to organisms even at very low concentrations (Ali et al. 2013, 2019). HMs include mercury (Hg), cadmium (Cd), chromium (Cr), zinc (Zn), aluminum (Al), cobalt (Co), copper (Cu), iron (Fe), molybdenum (Mo), manganese (Mn), lead (Pb), nickel (Ni), magnesium (Mg), selenium (Se), and silver (Ag). On the other hand, a metalloid is a chemical element tha...
