Phytoremediation: Low input-based ecological approach for sustainable environment

In today's fast-paced society, environmental breakdown is an ecumenical concern. This dilemma is tangled up with the unaccountable anthropogenic activities that are causing significant public health issues. Because of the rapid pace of development, urban soil and water pollution is progressively acting as a sink for a variety of contaminants, including heavy metals, pesticides, petroleum waste, radioactive moieties, etc. Phytoremediation is a low-cost, high-efficient method of extracting or removing pollutants from the environment. Water is transpired by green plants in order to move nutrients from the soil to the site of photosynthesis. During this process, pollutants in the water are also taken up and sequestered, digested, or drained out. This technique has many benefits over conventional approaches which is aesthetically pleasing and has long-term applicability. Phytoremediation could be a viable option to decontaminate heavy metal-polluted sites, particularly when the biomass produced during the process could be economically utilized in the form of bioenergy. Depending on the method used and nature of the contaminant involved, phytoremediating areas where metals and other inorganic compounds exist may utilize one of several techniques as discussed in the present study which are phytodegradation, phytoextraction, phytostimulation, phytostabilization, phytofiltration, phytovolatalization, etc. Phytoremediation is an emerging technology that employs the natural, biological, chemical, or physical processes of plants to remove, detoxify, or immobilize environmental contaminants in a growth matrix. This approach is hence an innovative tool with a great potential to decontaminate soil and water. Phytoremediation is a promising technique for ensuring the sustainability of future generations and reducing pollution.


Introduction
Environmental degradation is the detrimental variation of our atmosphere as a result of natural and anthropogenic activities, through changes in energy patterns, radiation levels chemical and physical compositions, and a superfluity of organisms (Manisalidis et al. 2020). Humans may be harmed directly or indirectly as a result of these changes, which may affect their daily needs or their possibilities for recreation and appreciation of nature (Afzal et al. 2019). Human activities have been disrupting the environment in some way or another since the Iron Age, with release of contaminants by industrial effluents, coal mines, and agricultural wastes, to name a few (Tarla et al. 2020). Contamination is caused by a wide range of organic and inorganic compounds, including heavy metals, putrescible and combustible substances, precarious wastes, explosives, and so on (Yadav and Kumar 2019). These pollutants pose a threat to ecosystem and, ultimately, enhance human health risk ( Table 1).
Detoxification of effluents is accomplished using a variety of physical and chemical processes, however, rather than full degradation, they just change their forms. Even at very low concentrations, these altered versions are hazardous and can harm human health (Pant et al. 2021). Although many elements are necessary for plant growth, but are toxic at higher amounts, making the land unfit for plant growth and sabotaging biodiversity (Ayangbenro and Babalola 2017). Over traditional methods, phytoremediation is one of the most promising technologies for environmental cleanup. Phytoremediation is a cost-effective and practical 'natural' remediation approach for polluted soils and water. It is defined as the engineered use of green plants to remove, contain, and render environmental toxicants like organic compounds, heavy metals, trace elements, radioactive compounds, etc. (Hettiarachchi et al. 2019). Because of their ability to revive the degradation of organic compounds in the rhizosphere through the discharge of exudates, roots, and enzymes, green plants play an important role in environmental planning (Yaqoob et al. 2019). Brassica juncea L. Salix species, Populus deltoides, Sorghastrum nutans, Helianthus annuus L. are projected as phytoremedial promising plants (Kang et al. 2021). Since the Industrial Revolution, the amount of hazardous materials in the environment has increased significantly. Excessive use of insecticides and pesticides in modern agricultural techniques also contributes to the pollution of water resources. Phytostabilization, phytoextraction, phytovolatalization, and rhizoremediation are some of the phytoremediation strategies for polluted areas (Saleem et al. 2020). Various phytoremediation strategies are shown in Fig. 1 (Dineshkumar et al. 2019).
The mechanisms of phytost abilization and phytoimmobilization are the uptake of pollutants on the surface roof and the bonding of contaminants with organic soil materials or humus by enzymatic plants. It utilizes plants, often in combination with soil additives, to assist in mechanically stabilizing sites for reducing pollutant transfer to other ecosystem compartments and to the food chain. Plants restrict pollutants by creating a zone around the roots where the pollutant is precipitated and stabilized (Nedjimi 2020). Accessible plants for phytosequestration include Picea abies and Populus tremula. Besides, phytoextraction, also known as phytoaccumulation, removes metals by taking use of a few plants' unique ability to (hyper-) collect, absorb, or transmit metals or metalloids by concentrating them inside biomass. Phytoextraction may provide an attractive alternative for the cleanup of heavy metal-contaminated sites (Khalid et al. 2020). Chrysopogon zizanioides, Spirodela polyrhiza, Pistia stratiotes, Eichhornia crassipes, and Pennisetum purpureum are employed in routine for their phytoextraction potential (Yang et al. 2020). While, phytovolatalization is a technique that uses the metabolic capacity of plants and rhizospheric-related microorganisms to convert pollutants into volatile molecules that are released into the atmosphere. It is a unique method of dealing with metal that have a gaseous form such as Hg or Se. Some bacteria have devised ways of dealing with Hg in particular, by cleaving the methyl group from it that is present in nature and reducing the element from Hg + to elemental Hg. (Sagar et al. 2019). Rhizofiltration is when plant roots adsorb or accumulate pollutant from waste water. It is also used to treat industrial and agricultural runoff in a controlled manner. It might be utilized to improve the performance of the plant absorption by maximizing the variables (da Silva et al. 2018). This technique utilizes plants to absorb, concentrate, and/or precipitate contaminants in the aqueous system. Plant uptake-translocation mechanisms are likely to be closely regulated. Plants generally do not accumulate trace elements beyond near-term metabolic needs (Nedjimi 2020

Environmental factors and phytoremediation
Phytoremediation has many factors which work in its favour, this technology has been receiving attention lately as an innovative, cost-effective alternative to the more established treatment methods used at hazardous wastes sites (Yadav and Kumar 2019). The regulation of the technology-depending upon the type of pollution-pivots on various environmental factors. Actual plant growth is not feasible without ingress of water, which controls transfer of many substances and compounds mandatory in the life processes of plants. Stress analogous with the availability of water leads to disarranging of water potential gradients, disruption of membrane probity, loss of turgor and proteins denaturation (Saxena et al. 2019). Soil is one of the natural resources being over exploited globally due to increased industrial, agricultural and other human activities and is also the most important environmental factor in the growth and development of plant life. Bioavailability of metal ions for plant uptake and higher plant biomass is both responsible for favourable results of phytoextraction technology (Manisalidis et al. 2020). The phytoavailability of metals is significantly authorized by soil-associated factors, such as redox potential, pH, temperature, cation exchange capacity, soil texture, and soil type, and factors associated with plants, such as root rhizosphere processes (microorganisms) and root exudates. Many of the soil constituents and parameters have a crucial influence on the efficacy of the phytoremediation process (Jobby et al. 2018).

Phytoremediation of heavy metals
Heavy metals have been identified as a primary concern pollutant as a result of increased urbanization and industrialization, according to the US Environmental Protection Agency (Manisalidis et al. 2020). Heavy metals and toxicants emitted in industrial effluents, such as Cd, Cu, Zn, Pb, Cr, and others, eventually find their way into aquatic water bodies such as ponds, lakes, and rivers. Because of its unpleasant effects, persistent toxicants (Heavy Metals) pollution poses a significant threat to all living beings in the environment (Ansari et al. 2020). Heavy metal contamination in our environment is a primary cause of industrial leathery-depleting water. Heavy metals are substances of economic value in industrial applications, yet they are used indiscriminately by humanity. Copper, lead, chromium, cobalt, nickel, zinc, cadmium, selenium, silver, antimony, arsenic, mercury, and thallium are examples of heavy metals that are commonly encountered (Yan et al. 2020). Ionic, colloidal, dissolved, and particulate heavy metals are all found in nature. Organoclays, humic acids, and oxides covered with organic materials show a high predilection for metals as well. Besides of anthropogenic activities, toxic heavy metals in our surroundings come from volcanic eruption, weathering of minerals and soil erosion, etc. (Sumiahadi and Acar 2018). The largest concern in today's fast-paced world is dealing with heavy metal contamination, which cannot be degraded and accumulates in various locations. Precarious trash has no economic value and is wasted or dumped into the air, water, or land, posing a risk to human and their surroundings (Lv et al. 2018). The Hazardous Waste (Management and Handling) Rule (1989) of the Environment (Protection) Act 1986 (EPA) elaborates on hazardous waste and the State's responsibility to ensure that hazardous waste material is administered in a manner that ensures and protects human health as well as environmental safety and sustainability against the drastic effects that such wastes may have (Calheiros et al. 2017).
Phytoremediation, also known as green remediation, is a novel, cost-effective, high-efficiency, and beneficial approach for removing toxic heavy metals and restoring heavy metals in contaminated lands and water (Obeng-Gyasi and Obeng-Gyasi 2020). It is defined as the enhanced remediation of ecological niches such as water, soil, and sediments and is a good substitute for conventional cleanup approaches that can be carried out through biologically concealed changes in the oxidation state (Amari et al. 2017). Several plants have been identified as having phytoremediation capability for removing various heavy metal toxicants from contaminated sites ( Table 2).
The search for innovative technology to remove hazardous heavy metals from contaminated water has led to the discovery of a process called biosorption, which relies on the heavy metal binding capacities of diverse living materials (Leao et al. 2017). Biosorption to cellular walls, extracellular polysaccharides, pigments, and intracellular aggregation contributes to microbial biomass contributing to a metal sink. Biosorption or accumulation is the two methods for removing metals. A solid phase (biosorbent) and a liquid phase (solvent) containing a dissolved species to be sorbed are required in the biosorption process (Jobby et al. 2018). In addition to the removal of heavy metal for polluted site, studies revealed potential of plants for the possible metal recovery (Table 3).

Phytoremediation of petroleum contaminant
Petroleum oil contamination has become a severe environmental problem that has the potential to affect individuals and the environment catastrophically. This pollution is expected to pose a global threat to all developing nations. Toxic substances called petroleum hydrocarbons are discharged into river waterways (Burroughs Peña and Rollins 2017). For the cleanup of oil pollution, a variety of physio and biochemical techniques have been used. Phytoremediation is a potential biotechnology that entails the use of plants to breakdown harmful contaminants into less-toxic or non-toxic compounds, thereby reducing the negative impacts on human health and the environment. Luffa cylindrica, Prototheca zopfi, Branchiaria decumbens, Typha domingensis, and Vetiveria zizanioides are some of the plants that show promise for this technique (Abplanalp et al. 2017). Crude oil is a complex mixture of organic molecules and hydrocarbons, some of which are suspected of posing environmental and health hazards, such as polycyclic aromatic hydrocarbons (PAH) and benzene. Water and soil contaminated with crude oil may show signs of harmful crude oil chemicals, as well as bioaccumulation and agricultural products. Oil poisoning in oceans, rivers, and lakes can have both long-and short-term consequences. The short-term consequences include oil poisoning, a reduction in dissolved oxygen, and light transmission, all of which affect marine life's photosynthetic activities (Tang and Angela 2019). Besides, the long-term consequences include changes in biogeochemical processes such as food disappearance, reproduction, and migration. It also poses genetic threats to marine life, especially when polluted with PAH (Singh and Singh 2017). Local green plants are favoured for oil cleanup because they adapt well to local climate and socioeconomic conditions, increasing the chances of success in redesigning and broadcasting on damaged soils and water (Tang and Angela 2019). Various studies exhibit phytoremediation technology and its unremarkable potential for cleanup of crude oil contaminants and their toxic by-products (Table 4).

Phytoremediation of pesticides
Pesticide-polluted terrestrial soils and aquatic water are worldwide concern due to cleaning of spray equipments, spills, agricultural wastes to urban living and disinfecting of empty utensils. Soil erosion and flood waters may move pesticides from both managed land and storage site to soils down-gradient and in the flood plain (Gupta et al. 2018). Pesticides contaminate the soils and underground water, which is eventually used to make drinking water. This poses a risk to human health and may result in a number of diseases. For 11 European countries and 6 farming   Effendi et al. (2017) systems, there are 76 pesticides wastes in the top layer of soil (Alaboudi et al. 2018). According to (Baoune et al. 2019), the United Nations' Agriculture and Food Organization (FAO) estimates that 70 per cent of food manufacturing will need to be increased by 2050 to support 2.3 billion people. This demonstrates the fast impulse of development of agricultural applications is the feasible option to uplift the food production worldwide. Pesticides play a critical role in the economic development of a wide range of fruit, forage, cereal, fibre, and oil crops, which are now an important element of many countries' agricultural industries (Ndubueze 2018 (Chander et al. 2018). Because of their tendency for dispersion, bioaccumulation, and transit in the food chain, organochlorine pesticides (OCPs) pose a global concern to environmental issues. As a result, developing transgenic plants is critical in order to meet potential approaches to improving phytoremediation capacities (Tirgar et al. 2020). The possibility of phytoremediation to remove pesticides has been suggested by a number of research (Table 5).

Phytoremediation of radionuclides
As a result of a variety of human-induced activities, radioactive dangerous substances have been released into our Ecosphere. Unlike industrial effluent, radionuclide is released into the atmosphere as a result of nuclear power tests, metallurgical mining, nuclear reactor discharge, and other sources (Singh and Singh 2017). Radioactive resources have a wide range of applications, including agricultural, scientific, medicinal, industrial, and energy generating, and play an important role in human society's everyday life. As a result, it is expected that such diverging behaviours will result in the formation of radioactive waste (Neels et al. 2019). The radioactive isotopes, Uranium (U), Strontium (S), Caesium (Cs), and Plutonium (Pu), are the most common examples that are present in the atmosphere as an outcome of nuclear activities and are the radionuclides of most concern (environment quality and human health). Nuclear precarious remains containing short radiation, generally of little concern as it dispels immediately by natural radioactive decay (Vardhan et al. 2019). Table 6 demonstrates several strategies and potential of phytoremediation to remove radionuclides.
To handle radioactive hazardous waste, innovative new procedures have been developed, with phytoremediation proving to be one of the most promising. The goal of phytoremediation access is to partially restore waste areas for eventual use by native plants and animals, as well as to reduce or eliminate radioactive entity off-site transportation and filtration (Poty et al. 2018).

Conclusion
Urbanization and industrialization enhance the toxicant levels beyond the permissible limits and are the forefront subject of interest in terms of health and environmental  1 3 issues. The green economy focuses on managing the line of the horizon between recent technologies and environmental fortification. With rising human demands, the most pressing environmental issue is dealing with toxicants including heavy metal, petroleum, pesticide, and radioactive waste which are intentionally harming the ecosystem and altering the equilibrium of our planet. As a result, raising knowledge about the sources of contaminants and developing effective remediation methodologies with real-world applications is critical for achieving a safe and sustainable environment. Although physical and chemical remedies are available but they are unsatisfactory in terms of complete removal of toxicants and higher cost. Phytoremediation, on the other hand, is a cutting-edge, eco-friendly, cost-effective technique that has shown to be a "natural", realistic, green, and practical option for a more sustainable future. this article.
Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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