Keywords

1 Introduction

Frequent emissions of pollutants from several industrial, commercial, and agricultural sectors have been the main topic of concern, as these are detrimental to human health and the entire planet. The dispersion of industrial and urban wastes caused by anthropocentric activities has had a detrimental effect on our ecosystem by releasing solid, liquid, and gaseous wastes containing heavy metals, inorganic and organic compounds (Miri et al. 2016; Sharma et al. 2020). Excessive deposition of toxic substances like heavy metals or hydrocarbons in marine and soil habitats fosters environmental degradation (Peng et al. 2015; Xi et al. 2018). The mobility method of heavy metal in the environment depends upon ores extraction and diverse processing purpose, resulting in releasing these elements in the environment. Biologically, “heavy metals” refer to those metals and metalloids that can pose detrimental effects on living organisms when exceeding the permissible limit. They are known for their detrimental effects on human beings, animals, and plants.

Though, heavy metals are reported to cause various health manifestations among living organisms. However, excessively absorbed heavy metals have been shown in numerous studies to alter cell membrane permeability, disrupt mineral nutrition, disturb the photosynthetic apparatus, and cause oxidative stress, all of which affect plant morphology and growth, and photosynthetic processes (Sharma and Kaur 2019). Elevated levels of heavy metals in plants induce cellular damage primarily through the formation of reactive oxygen species (ROS), including superoxide radical (O2−), hydroxyl radical (⋅OH), and hydrogen peroxide (H2O2). Unnecessarily increased synthesis of ROS is one of the organisms’ immediate reactions to various stressful events.

ROS can cause irreversible oxidation of lipids, proteins, chloroplast pigments, DNA, and RNA, which can compromise cell viability (Sharma et al. 2019; Rani et al. 2021). Furthermore, high levels of various harmful and toxic metals make the soil unsuitable for plant growth and deplete biodiversity. As a result, implementing efficient and eco-friendly remediation technologies is imperative for sustainable development. Regulation of heavy metal contamination in the soil can be achieved in many ways, including biological and physical, and chemical approaches.

Mechanical or physical pollutant isolation, acid leaching, electrocoagulation, electrokinetics, chemical treatment, thermal or pyrometallurgical separation, and biochemical methods can be used to treat contaminated soil. On the other hand, these techniques are costly and technically challenging to implement (Rajput et al. 2019). Furthermore, these chemical technologies could cause secondary pollution issues, as well as the generation of a large volume of sludge, raising the cost of sludge management. As a result, for heavy metals to be removed, an alternative solution is needed.

Bioremediation is a novel and promising technology available to exclude heavy metals and their revival from polluted and environmental matrices. This technique offers a clean, safer, low-cost, and environment-friendly method which requires microbes and plants to detoxify, degrade, transform, or mineralize pollutant concentration to a nontoxic (Azubuike et al. 2016). Furthermore, phytoremediation is a highly effective bioremediation tool that can be an alternative for removing heavy metals. Also, the phytoremediation technique is a cheap and environmentally sustainable treatment system that employs the utilization of plants/hyperaccumulators in order to eliminate toxic contaminants from the ecosystem (Ali et al. 2013). Since the contaminant-removal plant has no impact on the topsoil, this process is environmentally friendly and does not damage the ecosystem (Cristaldi et al. 2017). Because of its cost-effectiveness and unique characteristics, phytoremediation has been a promising technology for the remediation of contaminated soil in recent years. Thus, this chapter aimed to review the role of phytoremediation technology and the efficient exploitation of microbes to alleviate the environmental burden of toxic heavy metals. Furthermore, other integrated approaches for environmental management are also discussed. Our main goal is to emphasize more straightforward and economical remediation approaches with efficient results and easier preparation procedures.

2 Sources of Heavy Metals in the Environment

Naturally , in soil, metals are a very common component. Nevertheless, in high levels, metal for living organisms can be toxic and harmful. There are certain heavy metals that are primarily found in soil such as mercury (Hg), zinc (Zn), chromium (Cr), selenium (Se), cadmium (Cd), lead (Pb), arsenic (As), copper (Cu), uranium (U), cesium (Cs), and strontium (Sr). Many of them are micronutrients essential for plant growth and enlargements, such as nickel (Ni), cobalt (Co), Cu, manganese (Mn), and Zn, while others have a poorly understood biological function, like Hg, Cd, and Pb (Aksu 2015).

Elements with atomic numbers >20 and metallic properties are categorized under heavy metals. These are not soluble in the soil and are present in the form of colloids and ions. They are nonperishable, unlike organic pollutants (Bitew and Alemayehu 2017). As a result, they persist in the soil for a long time, with a half-life of more than 20 years (Sidhu 2016). Heavy metals concentration in soil ranges from 1 to 100,000 mg/kg (Karami and Shamsuddin 2010). One of the most severe threats to human health is transferring these pollutants into the food chain (Singh 2012; Rostami and Azhdarpoor 2019).

  • Natural sources —Minerals, Volcanic activity, weathering, and erosion are the most significant natural sources.

  • Anthropogenic sources (human intervention sources)—Common anthropogenic sources are electroplating, mining, smelting, fertilizers and biosolids, pesticides in agriculture, industrial discharge, sludge dumping, etc. (Suvarapu and Baek 2017; Liu et al. 2018). Various anthropogenic sources of heavy metal emissions in the environment are given in Fig. 10.1.

Fig. 10.1
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Anthropogenic sources of heavy metals in the environment

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3 Bioremediation

Hazardous waste material from industries releases organic and inorganic pollutants resulting in environmental pollution (Ojuederie and Babalola 2017). Accumulation of these metals in soil, water, and sediments has led to various environmental and human health concerns. There have been numerous reports, which confirm the presence of toxic heavy metals in soil, sediments, and groundwater (Rajput et al. 2019).

Numerous chemical and physical approaches have been developed to reduce the contamination level, but these techniques have many disadvantages of adding more chemicals to the polluted soil and water (Ayangbenro and Babalola 2017). Moreover, these methods are not effective in low metal toxicity areas (Akinci and Guven 2011). In recent years nonconventional techniques bioremediation has been developed, which is based on microorganisms’ usage.

Bioremediation is a clean, safer, cheaper, environmental friendly, and sanguine technology which involves biological mechanisms like detoxifying, degrading, transforming or mineralizing pollutant concentration to a harmless state by making use of the integral biological mechanism of microorganisms and plants (Ekperusi and Aigbodion 2015; Azubuike et al. 2016; Verma and Kuila 2019). Microorganisms can degrade heavy metals more rapidly due to microbial enzyme activities. Bioremediation reduces about 50–60% of the cost when used to treat lead polluted soil compared to some conventional approaches like landfill and excavation (Yang et al. 2017). However, various environmental conditions like pH, temperature, and moisture are important factors for the growth and metabolism of microorganisms to degrade pollutants (Chibuike and Obiora 2014; Verma and Jaiswal 2016). Bioremediation involves the basic principle of solubility reduction of these environmental contaminants by changing the pH, inducing redox reactions, and contaminants adsorption from the polluted site (Jain and Arnepalli 2019). The biosorption capability of Aspergillus niger and Mycobacterium chlorophenolicum to remove pentachlorophenol is widely reported in the literature (Bosso et al. 2015).

The bioremediation technique is mainly of two types: In situ (Natural attenuation and enhanced) and ex situ (Biopile, windrow, bioreactor, and land farming) (Table 10.1). Enhanced In situ is further divided into bioslurping, bioventing, biosparging, and phytoremediation (Azubuike et al. 2016).

Table 10.1 Different types of bioremediation techniques
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3.1 Ex Situ Bioremediation

  • Biopile: This method involves long-term accumulation of contaminated soil followed by nutrient variation, and further aeration process increases the microbial activity thus enhancing bioremediation. This technique’s main components are nutrition, aeration, irrigation, treatment bed , and leachate collection (Whelan et al. 2015).

  • Windrows: In the windrows technique , the polluted soil is turned periodically to enhance microorganisms’ activities, mainly hydrocarbonclastic bacteria (HCB). This helps in the increased aeration and uniform distribution of nutrients and pollutants as well as increased enzymatic activities. This leads to an increase in bioremediation rate through various processes like absorption, biotransformation, and mineralization (Barr et al. 2002).

  • Bioreactors: As the name indicates, the raw material is converted into valuable products through a chain of biological reactions in bioreactors . Batch, fed-batch, continuous, sequencing batch, etc., are the different types of modes of bioreactors.

  • Land farming: Because of its cost-effectiveness and minimal equipment requisites for operation, land farming is one of the simplest ex situ remediation techniques. Polluted sites are frequently mined or cultivated through inland farming. There is a debate going on whether land farming should be placed in in situ or ex situ bioremediation techniques, probably due to the site of remediation. Depth of pollutants plays a crucial role in the determination of ex situ or in situ land farming.

3.2 In Situ Bioremediation

  • Bioslurping: To achieve water and soil remediation, this method involves three techniques, namely soil vapor extraction, vacuum boosted pumping, and bioventing (Gidarakos and Aivalioti 2007). This method is premeditated to recover light nonaqueous phase liquids, semi-volatile, and volatile organic compounds. This method utilizes a slurp that pulls up liquids from the free products layer to the surface in a similar manner to a straw entice liquid from the vessel. Following product removal, bioventing can be applied to the complete remediation process (Kim et al. 2014).

  • Bioventing: Bioventing comprises airflow stimulation in a controlled manner so that oxygen can be delivered to the polluted site’s unsaturated zone to enhance the indigenous microorganism’s activities. The addition of nutrients and moisture enhancement helps transform pollutants into a less harmful form (Philp and Atlas 2005). This method has attracted worldwide attention due to the restoration of petroleum spilled sites (Höhener and Ponsin 2014).

  • Biosparging: To stimulate the removal of pollutants , the air is introduced into the polluted site’s saturated zone to cause an upward movement of volatile organic compounds to the unsaturated zone for enhancing the activities of microbes resulting in the degradation of pollutants. Soil permeability and pollutant biodegradability are the two main important factors determining the efficacy of biosparging (Philp and Atlas 2005).

4 Phytoremediation

Phytoremediation is another technique that relies on the physical, biological, chemical, and microbiological plant interactions to remove the detrimental effects of these present pollutants. This technique exploits higher plants for the removal of heavy metals (Table 10.2). Phytoremediation is a plant-based technique that improves polluted land and water supplies using either raw or genetically modified plant species. Phytoremediation with hyperaccumulator plants is now commonly accepted as a cost-effective and environmentally safe technique of removing pollutants from the environment (Rai et al. 2020).

Table 10.2 List of the various plant species screened for their heavy metal removal ability
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Filtration, stabilization, accumulation, extraction, volatilization, etc., are the various mechanisms through which plants degrade pollutants. Heavy metals are mostly degraded through the process of removal, conversion, and sequestration. In contrast, organic pollutants such as hydrocarbons and chlorinated compounds can be removed through volatilization, stabilization, rhizoremediation, and degradation. However, plants like alfalfa (Medicago sativa) and willow are also used for phytoremediation and mineralization (Kuiper et al. 2004). Metals are indispensable for plants’ biological functions, but they obstruct the organism’s metabolic system at higher levels. Metals like Hg, Zn, Cr, Se, Cd, Pb, and As, etc., are not expedient to plants. Moreover, they reduced photosynthetic activities, enzymatic activities, and mineral nutrition (Nematian and Kazemeini 2013). Depth and toxicity of the pollutant, plant adaptability, growth and survival rate, plant root system, biomass, resistance to pests and diseases, time requirement, etc., are crucial factors for choosing a plant that can be used for phytoremediation (Lee 2013). Miguel et al. (2013) reported that in some polluted sites, phytoremediation takes place through uptake and translocation of pollutants aided by xylem vessels and gets accumulated in shoots of the plant. Accumulation and transpiration are further dependent on segregation between xylem sap and adjacent tissues and the transpiration rate of the plant. Plant type and nature of the pollutant are other prominent factors on which the phytoremediation process depends. Most of the native plants growing on the polluted site can act as good phytoremediators. Thus, the pace of phytoremediation mainly relies on increasing the remediation capability of native plants by augmenting them with exogenous or endogenous plant rhizobacteria or by biostimulation. The role of plant growth-promoting bacteria (PGPB) in phytoremediation has been widely reported in the literature (Yancheshmeh et al. 2011; Maqbool et al. 2012; de-Bashan et al. 2012; Almansoory et al. 2015; Tiecher et al. 2016; Grobelak et al. 2017; Ramakrishna et al. 2020). Some valuable metals can be bioaccumulated in certain plants that are recuperated/recycled by the phytomining technique. Other relevant advantages of phytoremediation include cost-effectiveness, easy operation, low maintenance costs, soil structure preservation, and mitigation of soil erosion and metals leaching in the ecosystem (Ali et al. 2013). Owing to the organic matter input, there might be an improvement in soil fertility after phytoremediation (Mench et al. 2009).

5 Mechanisms of Phytoremediation

The phytoremediation mechanism is influenced by many factors, such as the nature and type of pollutants, concentration, and soil characteristics (Sharma and Kaur 2020). Mineral and nutrients as well as other nonessential elements or contaminants in the soil are absorbed by the plant root, which offers an enormous surface area for absorption (Yang et al. 2017). In order to effectively remove heavy metal contaminants from the ecosystem, plants have many mechanisms, which includes phytoextraction or phytoaccumulation, phytodegradation, phytovolatilization, phytostabilization, and phytofiltration or rhizofiltration (Dhir 2013; Ashraf et al. 2019; Sharma and Kaur 2020; Rai et al. 2020) (Fig. 10.2).

Fig. 10.2
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Schematic mechanism of different processes involved in phytoremediation

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5.1 Phytoextraction/Phytoaccumulation

It is defined as the acceptance of contaminants via plant roots from the soil and translocation to the aboveground portions of the plant (Pajevic et al. 2016). This method applies to metallic and radionuclides. This type of phytoremediation is reported to remediate Cd from soil, water, and sediments (Van Nevel et al. 2007). Furthermore, harvestable parts of the plants, where the metals are deposited, can be recycled from the ash that remained after drying, ashing, and composting (Singh and Bhargava 2017). Moreover, crops having high-biomass content can be grown in the contaminated soil to bio harvest and recover heavy metals easily. The concentration of contaminants in the soil can be decreased with successive cropping and harvesting (Pajevic et al. 2016).

Moreover, phytoextraction is a cheaper method compared to other conventional methods. It has an application in mineral industries to commercially produced metals by cropping (Sheoran et al. 2009). Also, it is advantageous when rapid immobilization is required to conserve drinking water resources .

5.2 Phytodegradation

Phytodegradation is also known as phytotransformation. It is defined as the uptake of contaminants from the soil and degradation of complex organic components resulting in the formation of simpler molecules or incorporating these molecules into plant tissues (Dhir 2013; Kumar et al. 2018; Yadav et al. 2018). The plants utilize the broken byproducts of the contaminants for their development and growth. Fertilizers, pesticides, chlorinated solvents, and other organic compounds can be degraded by phytodegradation (Sharma et al. 2019; Sharma and Kaur 2020). Phytodegradation has been earmarked to fix a few of the organic pollutants, which include chlorinated solvents, pesticides, and herbicides (EPA 2000). There are various plant enzymes such as nitroreductase, lactase, dehalogenase, peroxide, and nitrilase, which helps in the breakdown of complex inorganic and organic contaminants into simpler forms in plants (Dhir and Srivastava 2013). For example, Myriophyllum aquaticum secretes nitroreductase, which helps in the degradation of TNT (trinitrotoluene) (Rajakaruna et al. 2006). Brassica juncea and yellow poplar Liliodendron tulipifera are genetically modified plants focused by biotechnologists because of their practical phytoremediation abilities (Karami and Shamsuddin 2010; Ashraf et al. 2019).

5.3 Phytovolatilization

It is defined as the uptake of pollutants by plants from the soil, conversion into volatile forms, and releasing them into the atmosphere via transpiration process (Zhao et al. 2016). Thus, contaminants are discharged into the atmosphere in a less toxic form (Sharma and Kaur 2020). Selenium (Se), Arsenic (As), and Mercury (Hg) are some of the hazardous metals which can be transformed into volatile forms such as dimethyl selenide and mercuric oxide and then transpired into the environment (Table 10.3). The transformed dimethyl selenide and mercuric oxide are less harmful to living organisms, so this is an efficient phyto technique for fixing contaminants (Sakakibara et al. 2010). Also, there is no trace of the transfer of contaminants to other matrices (Kumar and Gunasundari 2018). The advantage of phytovolatilization in converting toxic contaminants such as mercuric ion into a less toxic form , i.e., elemental mercury, is also reported in several studies (Kramer 2018).

Table 10.3 List of various plant species reported for their phytovolatilization ability
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5.4 Phytostabilization

Phytostabilization is also known as in-place inactivation and is primarily utilized to fix contaminants in soil, sediments, and sludges (Singh 2012; Zeng et al. 2018). In this process, contaminants are immobilized in the rhizosphere by agglomeration by roots via root hairs, adsorption onto the root surface, or precipitation (Khalid et al. 2017). Phytostabilization decreases contaminants’ mobility, and hence intercepts contaminants’ migration to groundwater and therefore enters into the food chain (United States Protection Agency 2000). This technique successfully helps in reestablishing vegetation in polluted sites (Kohler et al. 2014). Besides, this type of phytotechnology has been employed for the remediation of severe toxic metals such as As, Pb, Cu, Zn, Cr, and Cd and is also helpful in reducing soil erosion and runoff via fixation of soil by plant roots (Bandara et al. 2017; Ramanjaneyulu et al. 2017). Various plant species screened for their capability to phytostabilization of heavy metals are enlisted in Table 10.4.

Table 10.4 List of various plant species reported for their phytostabilization ability
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5.5 Rhizofiltration

Rhizofiltration is also called phytofilteration and is primarily used to remediate groundwater, wastewater, and surface water contaminated with a low concentration of pollutants (Ashraf et al. 2019; Javed et al. 2019). It is defined as the uptake of contaminants by plant roots and this technique helps alleviate contaminants present in natural wetlands. It involves aquatic as well as terrestrial plants for absorption and extraction of contaminants from contaminated water sources because of their extensive root mass (Uddin et al. 2016; Dhanwal et al. 2017; Yan et al. 2020). Toxic heavy metals which are adjourned within the roots are fixed by the technique of rhizofiltration. Some plants such as Indian mustard, tobacco, corn, spinach, and rye have been reported to remove Pb from the contaminated water resources. Among these, sunflower has the most remarkable ability to remediate contaminants (da Conceição Gomes et al. 2016; Yan et al. 2020).

6 Role of PGPRs in Phytoremediation

Plant growth-promoting rhizobacteria (PGPR) are mainly used to promote plant growth by assisting plants in uptaking nutrients from soils. However, incorporating the potential PGPRs with plants has been extensively used to remediate heavy metals (e.g., Cd, Cu, Cr, Hg, Pb, Zn, and Al) and organic pollutants in contaminated soils (Zhuang et al. 2007; Manoj et al. 2020). The potential PGPR strains used for heavy metal removal belong to the genus of Acinetobacter, Azospirillum, Azotobacter, Bacillus, Enterobacter, Klebsiella, Paenibacillus, Pseudomonas, Rhizobium, Serratia, and Variovorax (Abdelkrim et al. 2020).

PGPRs facilitate metal mobility and increase bioavailability to the plant by acidification, phosphate solubilization, and redox changes. They also produce iron chelators and siderophores that further aid in iron mobilizing and increase availability to the plant (Zhuang et al. 2007). They release numerous natural chelating agents and organic acids such as citric, oxalic, acetic, malic, succinic, gluconic, and 2-ketoglutaric acids, which reduces the soil’s pH and sequester soluble ions (Khan and Bano 2018) (Fig. 10.3).

Fig. 10.3
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PGPRs mediated heavy metals remediation mechanism

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Recently, Manoj et al. (2020) reviewed the molecular mechanisms excreted by PGPRs to promote plant growth and heavy metals remediation. Abdelkrim et al. (2020) studied the in situ effects of Lathyrus sativus PGPR to remediate Pb and Cd polluted soils. They use PGPR inoculum (Luteibacter sp. + Pseudomonas fluorescens + Variovorax sp. + Rhizobium leguminosarum) and found that the Pb accumulation in the aboveground tissue was 1180.85 mg/kg DW, at the same time, the total reduction in Pb (46%) and Cd (61%) was also noted in the contaminated soils. Zand et al. (2020) investigated the effects of joint application of TiO2 nanoparticles and PGPRs and reported that TiO2 NPs and PGPRs increased Trifolium repens growth and Cd accumulation in Cd-contaminated soil. Recently, Guo et al. (2020) showed the effect of EDTA given in joint combination with PGPRs (Burkholderia sp. D54 or Burkholderia sp. D416) on the growth and metal uptake potential of Sedum alfredii plant. It was revealed that the EDTA decreased shoot and root biomass by 50% and 43%, respectively, meanwhile the uptake of heavy metals (Zn, Cd, and Pb) was also reduced to some extent. Wu et al. (2019) investigated the combination of biochar (BC) and PGPR strain with vetiver grass (Chrysopogon zizanioides) and observed that the Cd content was enhanced by 412.35% and bioaccumulation factor of accumulator 403.41%, as compared to control.

Moreover, Asad et al. (2019) reviewed the current status of PGPR with hyperaccumulator plants in improving the growth and remediation of heavy metals in contaminated environments. Furthermore, Mishra et al. (2016) and Mousavi et al. (2018) reported that siderophore-producing PGPR strains increase Fe, Zn, and Pb accumulation in their host plants. However, nowadays, genetically engineered PGPR strains associated with hyperaccumulator plants are used to remediate the heavy metals from the contaminated soils. These PGPRs strains are responsible for metal uptake, chelation, transport, degradative enzymes , homeostasis, and biotic-abiotic stress regulation (Ullah et al. 2015).

7 Role of Chelators in Phytoremediation

Phytoextraction is a promising tool for extracting heavy metals from soil and decontaminating a wide soil area (Zeng et al. 2018; Liang et al. 2019). However, the lack of availability and restriction on the translocation of certain heavy metals to plant shoots is a limiting factor in the process of phytoextraction. As a result, various chelating agents are used to increase metal solubility in soil thus increasing their availability to plants (Liang et al. 2019). Chelators and acidifying agents are added in the soil polluted with less concentration. This will help in increasing the solubilization in soil, and, thus, readily absorbed by plants. By forming a complex of metals, ethylenediaminetetraacetic acid (EDTA) introduces metals into the soil solution system and is eventually absorbed by the plant roots and translocated to the plant’s aerial organs (Rostami and Azhdarpoor 2019). The most widely used phytoremediation agents are EDTA and citric acid (Lesage et al. 2005; Rostami and Azhdarpoor 2019). According to Lesage et al. (2005), the application of both chelating agents raised heavy metal concentrations in the soil, such as Cu, Zn, Cd, and Pb. In contrast to citric acid however EDTA is reported to increase the bioavailability of Cu and Zn to plants.

Chelating agents mainly follow the apoplastic pathway in plants (Duarte et al. 2011). Also, chelating agents can raise the concentration of soluble metals, adjust their transition pathway from symplastic to apoplastic in plants, and make it easier for them to transfer across the plant (Rostami and Azhdarpoor 2019). Additionally, the use of chelating agents improves a plant’s drought tolerance (Rostami and Azhdarpoor 2019). High levels of toxicity in plants, leaching of these metals into groundwater, and intervention in metal transport from the root to the plant shoot are some of the negative consequences of these chelating agents (Rostami and Azhdarpoor 2019). Ethylenediamine disuccinate (EDDS) is secreted naturally by microbes and facilitates the uptake of heavy metals (Sidhu et al. 2018; Ashraf et al. 2019). EDTA is reported to show an excellent potential to absorb Pb accumulated in corn and pea, and further improves Pb accumulation ability in corn shoots. However, the application of chelates to improve metal bioavailability has raised so many queries, such as increment in the formation of a metal-chelate complex in the soil. Some studies have also pointed out the risk of groundwater contamination (Balakrishnan and Velu 2015). The effect of EDTA toxicity on microbes present in the rhizosphere is an indicator of environmental stress. Moreover, EDTA has household and industrial applications, such as cosmetics, detergents, perfumes, pharmaceuticals, water purification, agro-industries, paper, and pulp industry, is widely present in the environment, raising concerns about its role in heavy metal mobilization (Oviedo and Rodrıguez 2003).

Furthermore, despite their lower efficacy than other chelates such as phosphate and amino acids, EDTA-Zn and Zn sulfate chelates are widely recommended (Wang et al. 2013; Laware and Raskar 2014). Some chelates aid in absorbing more zinc; citrate, for example, has been documented to assist Thlaspi caerulescens, to absorb more zinc (Singh and Bhargava 2017). Citrate is also a metal chelator in the xylem tissue, indicating that it can play a pivotal role in the translocation of absorbed heavy metals in aerial regions (Singh and Bhargava 2017). Besides this, amino acids (AA), amino polycarboxylic acids (APCAs), and organic acids with low molecular weight (LMWOA) are the organic compounds used in phytoremediation of several heavy metals. For example, nicotianamine increased Cu presence by Lycopersicon esculentum, histidine increased Ni uptake by Brassica juncea, phytochelatins upregulate Cd uptake by Helianthus annuus, and cysteine is reported to increase the uptake of various metals by Helianthus annuus. These are some of the most widely used amino acids as per the reports (Meers et al. 2005). When plants are under extreme heavy metal stress, then free AA acts like nitrogen donor ligands and help the plant to adapt well in such condition by accelerating tolerance and detoxification mechanisms. Also, a significant increase in AA in plant tissues helps in adjusting osmotic stress and maintaining water potential in plants. Accumulation of heavy metals such as Uranium (U) by Berberis vulgaris, Cu, Zn, U, Pb, Cd, Zn by Brassica juncea, Cu by Nicotiana tabacum, Eubleekeria splendens, and Trifolium repens is assisted by LMWOA. These organic acids are exudates from plant roots that can control ion dispersibility and absorption owing to their metal-chelating features, indirect influence on microbial action, and alteration in rhizosphere region (Ashraf et al. 2019). Numerous genera of rhizobacteria (Pseudomonas, Rhizobium, Sinorhizobium, Bacillus, etc.) are used to enhance the pace of metal accumulation when phytoextraction is assisted by microbes (Meers et al. 2005; Ullah et al. 2015). Many symbiotic and nonsymbiotic bacterium can enable the absorption of heavy metals by plants such as Pityrogramma calomelanos, Cu by Elsholtzia splendens, Ni by Eichhornia crassipes, Pb by Alyssum murale, and Se by Hordeum vulgare (Ullah et al. 2015). Several factors such as pH, microbes, and the oxidation state of the metal (Ullah et al. 2015) affect the accumulation potential of plants. For example, a microbial strain has been reported to reduce the mobility and toxic nature of Cr(VI) to the nontoxic and immobile Cr(III), as well as minimize the mobility of other toxic ions in the soil system (Ullah et al. 2015). Furthermore, many beneficial bacteria in the rhizosphere are capable of improving plant growth, suppressing the activity of phytopathogens, and synthesizing plant hormones even under stress (Ullah et al. 2015). The metallophytes, which can withstand and survive in toxic and metal-polluted soil, are the most commonly used plant to stabilize metals. In nonsaline and saline soils of semiarid and arid areas, several studies have shown that halophytes are ideal for heavy metal phytoremediation. Experimental evidence indicates that resistance to salt and heavy metals depends on common mechanisms of physiology (Przymusiński et al. 2004).

8 Other Pragmatic Approaches for an Environmental Clean-up

Besides phytoremediation, there are some other approaches employed for the management of the environment. The electrokinetic assisted microbial approach is utilized efficiently to convert the organic substances to useful products electrochemically. The generation of bioelectricity can be made possible via different microorganisms’ activities (Selvi et al. 2019). Literature survey revealed the use and successful operation of this integrated approach in the remediation of various heavy metals such as Hg, Co, Fe, As, Cu, As, Pb, etc., by various researchers (Azhar et al. 2016; Sharma et al. 2018; Cai et al. 2019). Likewise, to obtain efficient phytoremediation results, an electrokinetic assisted phytoremediation approach can be used. This integrated approach has been reported to alleviate the increased levels of Cr, Cd, and Cu in contaminated soil (Bhargavi and Sudha 2015: Azhar et al. 2016). Furthermore, the electrokinetic remediated soil was used to grow various plants.

A phytobial remediation approach is an integrated approach vastly explored and involves the simultaneous use of plants and microorganisms to remove excessive toxic metals from groundwater and soil. In this method, plants are used to uptake contaminants from soil or water, and microbes are utilized to break down these contaminants into less toxic forms (Selvi et al. 2019). Unlike other invasive technologies, this phytobial remediation approach is considered an environment-friendly and cheap process. Nevertheless, certain microbes (bacteria and fungi) are present in the plant’s rhizosphere region and play a pivotal role in the immobilization of certain toxic metals by secreting enzymes. This remediation technique can further be investigated for proteomics, genomics, and metabolomics (Rai et al. 2020).

Practical application of hairy root biotechnology was also reported to remediate the areas contaminated with the mixture of organic and inorganic pollutants. However, this technique has the potential to remove the contaminants present at a very lower concentration. Thus, the technique has limited scope for large-scale phytoremediation (Ibañez et al. 2016). According to Song et al. (2019), the pace of phytoremediation can further be enhanced using nanoparticles to treat various contaminants. It was revealed that the remediation potential of ryegrass to remediate Pb from contaminated sites could be accelerated by the addition of Nano-hydroxyapatite (Huang et al. 2018; Song et al. 2019). Similarly, nanoparticles’ addition augmented the phytoremediation ability of various plants such as Phragmites australis, Salix alba, etc. (Fernandes et al. 2017; Mokarram-Kashtiban et al. 2019).

Similarly, nanosized zerovalent iron , TiO2 nanoparticles, and salicylic acid nanoparticles are reported to enhance the phytoremediation of Trinitrotoluene (TNT), Cd, and As, respectively (Gong et al. 2017; Souri et al. 2017; Song et al. 2019). Also, the presence of humic acid can further enhance nanoparticles assisted phytoremediation (Le et al. 2019). Moreover, some algae, fungi, and aquatic plants are also reported for their tolerance, sequestration, and heavy metals’ detoxification (Sharma et al. 2019). Literature survey unravels that those integrated technologies that involve the use of electrokinetics and bioremediation and phytoremediation had shown better results in alleviating the deleterious consequences of heavy metals. These techniques can be further utilized for their practical applicability in affected regions. Furthermore, there is an urgent need for a hyperaccumulators database to achieve site-specific phytoremediation (Reeves et al. 2018). For the proper recognition and acceptance of environmental friendly technologies, an integrated forum for ecologists, private/government agencies, environmental scientists, and engineers should be made (Rai et al. 2020; Sharma and Kaur 2020).

9 Advantages and Disadvantages of Phytoremediation

Plants and microbes have the potential to extract toxins from the environment and accumulate it in their bodies. Researchers compiled a list of the benefits, drawbacks, and limitations of phytoremediation technologies. These have shown the advantages of being ideal for a range of pollutants (organic compounds, metals, and metalloids), being cheap, and not requiring energy (energy is obtained from solar radiation) (Selvi et al. 2019). Phytostabilization is the first mechanism of phytoremediation, in which pollutants in the soil and groundwater were immobilized by using plants to avoid their movement by adsorption or accumulation onto the roots or precipitation within the root region (Awa and Hadibarata 2020). Through the process of phytostabilization, migration of heavy metals in the aerial parts of the plant can be prevented in those plants that have shown excellent tolerance potential under heavy metal stress (Sumiahadi and Acar 2018). When rapid immobilization is needed to save land and surface waters, it is highly effective. The deterioration or decay of heavy metals and organic contaminants in the soil is known as rhizodegradation. The application of microorganisms further aided this method (Mahar et al. 2016; Awa and Hadibarata 2020). It has a low start-up and maintenance rate.

The main benefits of phytoremediation technology are its low-cost equipment requirements, low labor costs, and cost-effectiveness. This technology may be used in situ, or on-site, to remove pollutants, whether in the soil, groundwater, or elsewhere. They are esthetically appealing and widely accepted by the general public. It has a relatively low environmental effect on soil and water because it is nondestructive, nonintrusive, and highly biologically active. They reduce soil erosion, make inorganic soils thinner, reduce particulate matter leaching, and disperse toxicants. Contaminants can be extracted from plant tissues, and plant biomass can be used for generating thermal energy and biogas. This technology shows the best results in the regions contaminated with low levels of heavy metals. It can be used for phytoremediation of non-agriculturally productive soils (Muthusaravanan et al. 2018). On the other hand, phytoremediation is slow and depends on soil profile composition, pH, salt concentration, and other toxins. There is limited applicability to various types of wastes, especially wastes with high levels of toxicity (Dhanwal et al. 2017).

10 Conclusion and Future Recommendations

Although it is evident that phytoremediation delivers as an economic tool for in situ mitigation of polluted sites, it is not feasible to get it into practice due to certain constraints. Moreover, field studies should be conducted in vivo rather than in vitro to analyze plants’ efficiency to alleviate the environment’s toxic metals. More investigations are required to understand better interactions existing among metals, soil, microbes, and plant roots and their mechanisms to degrade or detoxify the toxic metals. The application of various transgenic plants in the remediation of heavy metals can also help the elucidation of the phytoremediation mechanism adopted by the plants. Recent advances in plant biotechnology in genome editing can further play a pivotal role in accelerating phytoremediation. Also, elucidation of detoxification processes adopted by plants under heavy metal stress must be explored and explicitly studied. Exploring various hyperaccumulators, endophytes, algae, and their potential to remove high concentrations of toxic metals present from soil/water is required.

Lastly, in perspective to expand the horizon of phytotechnologies and other approaches for environment management, nanoparticles’ role in enhancing the pace of phytoremediation needs to be understood. Other international agencies should be developed to conduct regular meetings to address the challenges and barriers in the path of the evolving technology of phytoremediation.