Phytoremediation Crops and Biofuels

Abstract

Environmental decontamination is an integral part of sustainable development. In recent years there has been growing interest in using plants for decontamination. On the other hand, water, soil and air are increasingly contaminated. Large amounts of toxic waste have been dispersed in thousands of contaminated sites spread all over the globe. These pollutants belong to two main classes: inorganic and organic. The challenge is to develop innovative and cost-effective solutions to decontaminate polluted environment. Phytoremediation is emerging as an invaluable tool for environmental cleanup. Various strategies are being applied to reduce the accumulation of toxic metals in plants. Cultivation of edible crops in contaminated soils is a subject of human health concern if the contaminant concentration in the edible parts of crops plant exceed the permissible level. In such cases non-food crop production viz. value chain and value additions appears profitable. In this review: (1) the contamination due to industrial effluents in peri urban region of greater Hyderabad, and (2) the strategies to use contaminated soil and water for raising phytoremediation crops, and generation of value products. Crops and products include medicinal and aromatic plants, ornamental plants, biofuels, tree crops, fiber crops, dyes, and plants for carbon sequestration.

7.1 Introduction

Environmental (Soil, water and pollution) (conflict between agro-ecosystems and urbanization) is not only a subject of pan-Indian concern, but also for the entire world. Consequently, developing eco-innovative remediation technologies to re-use contaminated substrates (land and water) in line with biomass production for the bio-economy are gaining considerable global attention (Prasad 2015). Rapid industrialization and extraction of large quantity of natural resources is the main cause of environmental contamination and pollution. Thus, every one of us are being exposed to contamination from past and present industrial practices, emissions in natural resources (air, water and soil) even in the most remote regions. The risk to human and environmental health is rising and there is evidence that this cocktail of pollutants is a contributor to the global epidemic of cancers, and other degenerative diseases.

Plant based technologies are emerging as viable alternatives to conventional remediation techniques when time constraint is low, being more energy-efficient and less disruptive to contaminated sites. Based on small-scale studies, annual and perennial accumulators are suitable candidates for these plant based technologies (= Phytotechnologies). Plant-associated microbes are also implicated in enhancing plant performance, reducing contaminant phytotoxicity or modifying (degrading) (organic) contaminants. These innovative phytotechnologies can be in-situ or ex-situ (Conesa et al. 2012), remediate soil layers explored by roots and at the same time provide plant biomass, contributing towards achieving envisaged targets on the use of renewable plant-based feedstock for various purposes (renewable energy sources, ecomaterials, biomass for bioreffineries, green fine chemistry, bioplastics, etc. (Prasad 2014b)) in substitution to fossil fuels and other non-renewable raw materials. They can also reduce the diversion of croplands to bioenergy and other non-food crops (Liu et al. 2011, 2012; Sheng et al. 2012; Zhang et al. 2014a). Development of economically sound valorisation pathways for complete chain of phytoproducts of value addition and value chain from phytoremediated plants would go a long way (Grison 2015; Prasad 2015; van der Ent et al. 2015) (Fig. 7.1).

Fig. 7.1

Environmental crops boosting circular economy, carbon sequestration and co-generation of phytoproducts

Energy and environment are interlinked complex issues. Population explosion (~7.2 billion) on plant earth and excessive use of natural resources resulted in environmental pollution and contamination with inorganic and organics (Prasad 2011; Xu et al. 2006). Pollution in environment is an ever increasing phenomenon and often regulatory systems and cleaning operations are not commensurate with waste generation. It is therefore, necessary to search for effective technologies for decontaminating the natural resources. Biodiversity has immense potential not only for monitoring and abatement of environmental pollution but also in generating a wide variety of by-products. Breakthrough research innovations that are amenable for field applications of pollution abatement are discussed with specific examples in several papers (Conesa et al. 2012; Prasad 2011; Prasad and Freitas 2003; Prasad and Prasad 2012; Prasad et al. 2010; Prasad 2013; Puhui et al. 2011).

The growing need for biomass for conversion to biofuels, require lignocellulosic-rich raw materials. Phytoremediated phytomass is one such option used to produce fuel like methanol, biodiesel, synthetic gas, and hydrogen (using thermal and thermochemical processes by direct or indirect liquefaction or gasification) and ethanol (through hydrolysis and subsequent fermentation). Biorefinery processes (the sustainable processing of biomass to a spectrum of marketable products and energy) is an absolute necessity and it is the key to meet this vision towards bio-based economy.

Brassica juncea (Indian mustard) Helianthus annus (Sunflower), Prosopis juliflora, bamboo, and Pistia stratiotes (water lettuce), could be grown for different purposes including energy generation Annual and perennial crops, including algae in wastewater ponds are potential candidates. Management and production of phytoremediation crops in contaminated substrates serves as a sink for contaminants with possibility for co-generation of economic products. Economics and byproduct generation with the overall success level of integration in the bioremediation fostering circular economy (Prasad 2015) (Fig. 7.2).

Fig. 7.2

“Biorefinery” is the sustainable processing of conversion of phytoremediated phytomass into a spectrum of marketable products for e.g. chemicals, composite materials, and energy

The challenge for environmental decontamination of inorganic and organic pollutants and contaminants is to develop innovative and cost-effective solutions to decontaminate polluted environments. Biodiversity is the raw material for environmental cleanup and is an invaluable tool box with wider application in the field of pollution abatement (Witters et al. 2012). Phytoremediation includes variety of technologies using plants and microbes to remediate or contain contaminants in soil, groundwater, surface water, and/or sediments including air. These technologies have become attractive alternatives to conventional cleanup technologies due to relatively low capital costs and the inherently aesthetic nature (Fig. 7.3).

Fig. 7.3

Bioremediation and biobased economy. A biobased economy is a sustainable economy, optimising economic value through valorization of bimass. The new wave in this direction is to use phytoremediated phytomass as substrate in biorefinery for replacing fossil resources and to produce a variery of economic products

Phytoremediation is a sustainable environmental cleanup technology. This approach is based on a wide range of plants that include terrestrial, aquatic, weeds, annuals, perennials (agricultural, tree and field crops) and ornamentals (Poonam et al. 2014; Vamerali et al. 2011, 2014). Plants applied in phytoremediation have several beneficial uses. Nature cure mechanisms for natural resources and environmental protection viz. phytosequestration, rhizodegradation, phytohydraulics, phytoextraction, phytodegradation and phytovolatilization are well established with suitable examples (Shekhar 2012).

Arsenic, Mercury, Chromium, Fluoride, Cyanide, abandoned mines, fly ash disposed sites, engineered phytotreatment technologies, biological permeable barriers and degradation of Organics viz., petroleum hydrocarbons, pesticides and explosives are some of the contaminants subjected to bioremediation and promising results have been obtained.

Quite a variety of plants, natural, transgenic, and/or associated with rhizosphere micro-organisms are extraordinarily active in these biological interventions and cleaning up pollutants by removing or immobilizing. While diverse microbes are the most active agents, fungi and their strong oxidative enzymes are key players as well in recycling recalcitrant polymers and xenobiotic chemicals following systems biology and biotechnology interventions (de Lorenzo 2008; Dowling and Doty 2009; Wood 2008). Constructed wetlands are the result of human skill and technology integrating the geology, hydrology and biology (Prasad 2004a, b, 2007a, b). Constructed wetlands have been designed centuries ago by mankind to treat wastewater (Bi et al. 2007; Türker et al. 2013). Bioeconomy is a solution for clean air, water and environmental sustainability (Prasad 2015).

Bioremediation approach is currently applied to contain contaminants in soil, groundwater, surface water, or sediments including air. These technologies have become attractive alternatives to conventional cleanup technologies due to relatively low capital costs and the inherently aesthetic nature. Nature’s cure using plant resources (= Phytoremediation) is a sustainable solution for environmental decontamination (Bañuelos et al. 1997; Bitterli et al. 2010). As of now about 20,000 research papers have been published on various aspects of using biological resources for environmental cleanup starting with only 11 in 1989. Environmental sources of heavy metals are shown in Table 7.1.

Table 7.1 Environmental sources of heavy metals (Prasad 2011)

Biodiversity and ecosystem services offer means and ways to render the usage of natural resources more sustainably. This can be achieved by using the soil and ecosystem to generate solutions for modern-day challenges such as industrial pollution and the growing demand for natural resources (Aparna et al. 2010). Thus, there is a need to develop appropriate tools with like minded parties and allied disciplines. This kind of action plan would assist planners in calculating the social benefits of green missions in urban environments. The nexus of ‘Biodiversity vs Industry’ consortium and knowledge based systems for better understanding of production chains would contribute to sustainable development.

Industrial and urban activities impact our environment, especially in terms of polluting soil, air and water. Large numbers of sites are nowadays contaminated/polluted by inorganic and/or organics. Treating these pollutants represents an economic need, which often remains unanswered by conventional civil/chemical engineering methods, due to their inappropriateness, their environmental impact and costs (notably for large sites) (Figs. 7.4 and 7.5).

Fig. 7.4

Nature’s biotechnology involves biodiversity as raw mterial for pollution abatement. During the last two decades, we have witnessed the emergence of gentle soil remediation techniques using various plant species and the combination of microbial biotechnologies

Fig. 7.5

Utilization of contaminated substrates for boosting bioeconomy through cascading approach is a new innovation wave. The added value is the highest at the top of the pyramid and the lowest at the bottom. On the contrary, the volume of biomass needed for the application is the lowest at the top of the pyramid and the highest at the bottom of the pyramid

7.2 Study Area

Major contaminated sites in Greater Hyderabad = Tri cities (Hyderabad, Secunderabad and Cyberabad) are selected for this study (Figs. 7.6–7.12).

Fig. 7.6

Major contaminated sites in Greater Hyderabad = Tri cities (Hyderabad, Secunderabad and Cyberabad)

H Hussain Sagar, J Jawaharnagar, K Katteda

M Musi river, N Nakkavagu, P Patancheru

Bholakpur area – Abdul et al. (2012)

Municipal solid waste dumping sites – Jawaharnagar – Ahmed et al. 2011; Parth et al. 2009, 2011

Industrial waste contaminated areas – Ahmed et al. (2011), Balanagar, Machender et al. (2011, 2012)

Kattedan – Sekhar et al. (2006), Govil et al. (2008, 2012)

Hussainsagar lake sediments – PTE – Gurunadha Rao et al. (2004, 2008), Jain et al. (2010), Suneela et al. (2008), Vikram Reddy et al. (2012)

Industrially contaminated sites of Hyderabad – Sekhar et al. (2003, 2005)

Patancheru Industrial Area – Dasaram et al. (2011), Govil et al. (2001)

Fig. 7.7

Hussainsagar metal contamination budget (a) Surface water and (b) sediment

Fig. 7.8

Jawaharnagar waste dump site (a) Soil and (b) ground water

Fig. 7.9

Kattedan industrial area metal contamination (a) Surface water and (b) ground water

Fig. 7.10

Musi river heavy metal contamination (a) Surface water (b) Ground water

Fig. 7.11

Nakkavagu – Metal contamination (a) Surface water and (b) Ground water (ppb)

Fig. 7.12

Patancheru – Metal contamination (a) Surface water (b) Ground water

7.3 Products from Plants Applied in Phytoremediation

The objective of this paper aims at highlighting the sustainable valorization of phytoremediated phytomass for production of value additives and value chain products for boosting bio-economy (Bañuelos 2002, 2006) (Fig. 7.13).

Fig. 7.13

Products from phytoremediated plants

India is forging ahead to attain high GDP growth rate to achieve the status of industrialized and developed nation by 2020. In order to achieve this, heavy consumption of natural resources is inevitable. The other side of the coin of this task is enormous waste generation. The waste management summit, held in 2012 has estimated that the poised industrial growth would generate 100 million tons of non-hazardous solid waste, 6–7 million tons of hazardous waste annually. Thus, soil pollution, land-use and land cover change (conflict between agro-ecosystems and urbanization) are of pan-Indian concerns. Consequently, developing eco-innovative soil remediation technologies to re-use contaminated lands in line with biomass production for the bioeconomy are priority objectives.

Land resources on a global perspective are under immense pressure. Soil remediation is a solution to reduce the pressure on land resources. Land degradation is due to: (a) Natural processes, (b) erosion, (c) nutrient depletion, (d) loss of organic matter, (e) structural losses, (f) Induced land degradation, (g) irrigation with waste waters, (h) atmospheric deposition of pollutants, (i) poor agricultural practices and (j) climate change increases the intensity of land degradation.

Of the 92 known elements in earth crust, 35 metals that concern us because of occupational or residential exposure, 23 of these are “heavy metals”: Ag, As, Au, Bi, Cd, Ce, Co, Cr, Cu, Fe, Ga, Hg, Mn, Ni, P, Pb, Sb, Sn, Te, Tl, U, V, and Zn (Glanze 1996). Heavy metals are among the contaminants in the environment. Beside the natural activities, almost all human activities also have potential contribution to produce heavy metals as side effects. Migration of these contaminants into non-contaminated areas as dust or leachates through the soil and spreading of heavy metals containing sewage sludge are a few examples of events contributing towards contamination of the ecosystems (Gaur and Adholeya 2004; Peña et al. 2014). From a chemical point of view, the term heavy metal is strictly ascribed to transition metals, metalloids, with atomic mass over 20 and specific gravity >5. Here the term “heavy metals” will be for these potentially phytotoxic elements. Some of these heavy metals do not perform any known physiological function in plants. However, others such as micronutrients and beneficial elements are required for plant growth and metabolism, but these elements can easily lead to phytotoxicity when their concentration rises above optimal ranges. Heavy metal phytotoxicity may result from alterations of numerous physiological processes caused at cellular/molecular level by inactivating enzymes, blocking functional groups of metabolically important molecules, displacing or substituting for essential elements and disrupting membrane integrity. A rather common consequence of heavy metal poisoning is the enhanced production of reactive oxygen species (ROS) due to interference with electron transport activities, especially that of chloroplast membranes (Pagliano et al. 2006; La Rocca et al. 2009). This increase in ROS exposes cells to oxidative stress leading to lipid peroxidation, biological macromolecule deterioration, membrane dismantling, ion leakage, and DNA-strand cleavage (NavariIzzo et al. 1998, 1999; Quartacci et al. 2001). Plants resort to a series of defence mechanisms that control uptake, accumulation and translocation of these dangerous elements and detoxify them by excluding the free ionic forms from the cytoplasm. One commonly employed strategy lies in hindering the entrance of heavy metals into root cells through entrapment in the apoplastic environment by binding them to exuded organic acids (Watanabe and Osaki 2002) or to anionic groups of cell walls (Dalla Vecchia et al. 2005; Rascio et al. 2008). Most of the heavy metals that enter the plant are then kept in root cells, where they are detoxified by complexation with amino acids, organic acids or metal-binding peptides and/or sequestered into vacuoles (Hall 2002). This greatly restricts translocation to the above-ground organs thus protecting the leaf tissues, and particularly the metabolically active photosynthetic cells from heavy metal damage. A further defence mechanism generally adopted by heavy metal-exposed plants is enhancement of cell antioxidant systems which counteracts oxidative stress (Navari-Izzo et al. 1998; Sgherri et al. 2003).

Several methods are already being used to clean up the environment from these contaminants, but most of them are costly and far away from their optimum performance. The chemical technologies generate large volumetric sludge and increase the costs (Rakhshaee et al. 2009). Both chemical and thermal methods are technically difficult and expensive, and can also degrade the valuable component of soils (Hinchman et al. 1995). Conventionally, remediation of heavy-metal contaminated soils involves either onsite management or excavation and subsequent disposal to a landfill site. This method of disposal solely shifts the contamination problem elsewhere along with the hazards associated with transportation of contaminated soil and migration of contaminants from landfill into an adjacent environment. Soil washing for removing contaminated soil is an alternative way to excavation and disposal to landfill. This method is very costly and produces a residue rich in heavy metals, which will require further treatment. Moreover, these physio-chemical technologies used for soil remediation render the land usage as a medium for plant growth, as they remove all biological activities (Gaur and Adholeya 2004).

Recent concerns regarding the environmental contamination have initiated the development of appropriate technologies to assess the presence and mobility of metals in soil (Shtangeeva et al. 2004), water, and wastewater. Since, these heavy metals cannot be degraded from the nature, at great extent we can take the advantage of natural solar driven process of plants for translocating these contaminants to various parts from the soil or water. Hence, this process is called phytoremediation. Presently, phytoremediation has become an effective and affordable technological solution used to extract or remove inactive metals and metal pollutants from contaminated soil. The term phytoremediation (“phyto” meaning plant, and the Latin suffix “remedium” meaning to clean or restore) refers to a diverse collection of plant based technologies that use either naturally occurring, or genetically engineered, plants to clean up the contaminated soil or water environment (Flathman and Lanza 1998). Phytoremediation is natural, simple, cost effective, non environmentally disruptive green technology and most importantly, its by products can find a range of other economical uses (Truong 1999, 2003). Phytoremediation takes the advantage of the unique and selective uptake capabilities of plant root systems, together with the translocation, bioaccumulation, and contaminant degradation abilities of the entire plant body (Hinchman et al. 1995).

The main objective of this work is to highlight the potential of phyto-products from various plants applied in remediation of heavy metal contaminated soils as well as the usage of the polluted water for co-generation of biofuels using algae (Samardjieva et al. 2011).

Phytoremediation is the use of plants to accumulate, remove or render harmless toxic compounds contaminating the environment. Plants absorb/exclude, translocate, store or detoxify inorganic and organic. Thereby they contribute significantly to the fate of these contaminants from the biosphere. Thus, contaminants (inorganic and organic) can enter the food chain when bio-available, which would cause unwanted effects (Abhilash and Yunus 2011). Many think that phytoremediation is a temporary solution and often the following questions are posed:

• (a) There has been a disbelief among scientists and regulators about phytoremediation

• (b) How does phytoremediation works?

• (c) How to select plants suitable for fostering remediation?

• (d) Will phytoremediation work on every heavy metal contaminated/polluted site?

• (e) How to dispose of the plants contaminated in this decontamination process?

• (f) How to manage the risk based phytoremediation?

Often policy and decision makers, academia and civic governance think that phytoremediation is a temporary solution of transferring the pollutants and contaminants are transferred from one place to another. Scientists and academia are no exception to this feeling. Regulators have expressed apprehensions about phyto-remediation due to lack of knowledge of contemporary knowledge of environmental sustainability. Often, it is generally believed that this kind of environmental decontamination is a temporary solution. How to dispose of the contaminated phytomass is a puzzling question by environmental managers and regulators.

Soil, water and air are the important natural resources that must be cleaned. Unfortunately, natural resources are polluted globally. Rapid industrialization and extraction of large quantity of natural resources including indiscriminate extraction of ground water have resulted in environmental contamination and pollution. Large amounts of toxic waste have been dispersed in thousands of sites spread across the globe resulting in varying degrees of contamination and pollution. Thus, every one of us are being exposed to contamination from past and present industrial practices, emissions in natural resources (air, water, and soil) even in the most remote regions. The risk to human and environmental health is rising and there is evidence that this cocktail of pollutants is a contributor to the global epidemic of cancers, lungs and other degenerative diseases. These pollutants belong to two main classes: inorganic and organic. The challenge is to develop innovative and cost-effective solutions to decontaminate polluted environments.

Bioremediation includes variety of technologies using plants and microbes to remediate or contain contaminants in soil, groundwater, surface water, and/or sediments including air. These technologies have become attractive alternatives to conventional cleanup technologies due to relatively low capital costs and the inherently aesthetic nature.

Bioremediation is the use of biological interventions of biodiversity for mitigation (and wherever possible, complete elimination) of the noxious effects caused by environmental pollutants in a given site. It operates through the principles of biogeochemical cycling. If the process occurs in the same place affected by pollution then it is called in-situ bioremediation. In contrast, deliberate relocation of the contaminated material (soil and water) to a different place to accelerate biocatalysis is referred to as ex-situ bioremediation. Bioremediation has been successfully applied for cleanup of soil, surface water, groundwater, sediments and ecosystem restoration. It has been unequivocally demonstrated that a number of xenobiotics including nitro-glycerine (explosive) can be cleaned up through bioremediation. Bioremediation is generally considered to include natural attenuation (little or no human action), bio-stimulation or bio-augmentation, the deliberate addition of natural or engineered micro-organisms to accelerate the desired catalytic capabilities. 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 (Rajkumar et al. 2009, 2010, 2012).

Hibiscus cannabinus (Kenaf), Brassica juncea (Indian mustard), Helianthus annus (Sunflower), Ricinus communis (Castor), Vetiveria zizanioides (Khus Khus grass) and Prosopis juliflora (Velvet Mesquite) are considered as environmental/ phytoremediation crops. Therefore, phytoproducts (value additives) from the plants that are applied in phytoremediation and safe disposal of contaminated phytomass (risk based remediation) would propel bioeconomy.

7.3.1 Products from Phytoremediated Crops

Due to rapid globalization of the current century, the demand for energy is steeply increasing. Use of fossil fuels have an adverse environmental impact although economic viability and efficiency are in favour of fossil fuels. To reduce the negative impact of fossil fuels, use of contaminated substrates for bioenergy is being researched globally (Meers et al. 2010). This strategy avoids competition over land use for food crops. Further, vast resources that can be harvested from contaminated substrates, conversion through efficient biorefineries via research and development is being tapped in many countries. Technically, biomass and the conversion products can be used as supplemental sources for conventional fuels (fossil fuels) and chemical feedstock for various industries. This approach is promising in the area of alternate sources of energy and dependency on fossil fuels can be minimized to a considerable extent. The production of biomass involves the use of abundantly available and rapidly growing non-agricultural plants, preferably with good coppicing and nitrogen fixing capabilities to produce energy products by suitable conversion technologies (Table 7.2).

Table 7.2 Metal accumulation reports with reference to Brassica juncea (Indian mustard)

The annual photosynthetic storage of energy in biomass is eight times more that of energy use from all sources. This estimate clearly illustrates the immense potential of biomass resources, if harnessed and managed sustainably. Further, biomass derived constituents serve as analogues of fossil fuel derivatives. Sunflower and Indian mustard are popular energy and environmental crops (Fozia et al. 2008; Madejón et al. 2003).

Large tracts of contaminated sites are available in different agro-climatic zones of our country. A number of multiuse plant species with energy rich chemicals have been identified. It is therefore, necessary to integrate such energy plantations into a system of rotational cycle to suit socio-economic aspects of the people. Broad areas of biomass energy sources.

Energy crops include corn, sugar cane, sugar beet, cassava, soyabean and other sugar and starch producing crops. In Brazil, sugar cane cultivation was expanded to some extent at the expense of food crop. Brazil, USA, Philippines, Germany use blended mixture of gasoline and alcohol. Such a blended fuel was termed Gasohol or alcogas. Brazil’s National alcohol and pure alcohol programmed “Proalcool” aims at running cars on gasohol and pure alcohol. Cassava (mandioca, tapioca), grown in many developing countries is rich in starch. It can be cultivated in wide range of agroclimatic situations and does not require high dosage of fertilizer and pesticides to produce high yields. The notable examples of biomass conversion to biofuels are sweet sorghum and Miscanthus (Babu et al. 2014; Brosse et al. 2009; Chami et al. 2014; El Hage et al. 2010; Pavel et al. 2014; Płażek et al. 2014; Sharmin et al. 2012).

Bioenergy is renewable alternative fuel/energy produced from materials derived from biological sources such as plants, animal oils or fermentation. Biomass is any organic material which has stored sunlight in the form of chemical energy. As a fuel it may include wood, wood waste, straw, manure, sugarcane, and many other byproducts from a variety of agricultural processes. Energy is a key to socio-economic development (Figs. 7.14–7.16).

Fig. 7.14

Sun flower as environmental crop for cultivation on inorganic and organics contaminated environment. Biodiesel, fiber and industrial feed stock are the phytoproducts. BETX Benzene, Ethylbenzene, Toluene and Xylenes, Cs Cesium, Ra Radium, Sr Strontium, TCE Trichloroethylene, Th Thorium, TNT Trinitrotoluene, TPH Total petroleum hydrocarbons, U Uranium

Fig. 7.15

Biodiesel and cogeneration of industrial feed stock from sun flower from phytoremediated crop produce

Fig. 7.16

Brassica juncea (Indian mustard) as environmental crop for cultivation on inorganic and organics contaminated environment. Biodiesel is the main byproduct (Marillia et al. 2014; Tomé et al. 2009). BETX Benzene, Ethylbenzene, Toluene and Xylenes, Cs Cesium, Ra Radium, Sr Strontium, TCE Trichloroethylene, Th Thorium, TNT Trinitrotoluene, TPH Total petroleum hydrocarbons, U Uranium

Sunflower crop can be grown in the pollution affected areas. The root system of the plan develops well in varying soil pH conditions. Moistening and acidification of the rhizosphere could enhance the nutrient uptake mechanism of the crop, inducing a ‘fertilising’ effect (Junkang Guo et al. 2014; Juwarkar et al. 2008; Khouja et al. 2013; Kötschau et al. 2014; Liphadzi and Kirkham 2006; Madrid et al. 2008; Rajkumar et al. 2009; Stanbrough et al. 2013). Sunflower is a multipurpose crop producing a variety of industrial feed stock.

In recent years environmental crops for e.g. Indian mustard, sunflower and sweet sorghum etc. have gained considerable attention. The primary goal of this exercise is to cultivate these crops using contaminated substrates with two objectives. (a) to combat environmental pollution, and (b) to produce beneficial and regenerable products.

The biofuel generations are the following:1st generation – Jatropha based2nd generations – lignocellulosic ethanol3rd generation – algal biofuels4th generation – Bio-butanol, bio-hydrogen, biomethyl furan etc.

7.3.2 Medicinal and Aromatic Plants on Contaminated Soils

Aromatic crops, used for production of essential oils as opposed to food or feed, may be suitable alternative crops in heavy metal contaminated agricultural as well as non-agricultural soils. Essential oils are low-volume, high value products that are widely used as aromatic agents in various non-food industries, such as perfumery, cosmetics, and aromatherapy suggesting the possibility that such plants could be used in phytoremediation of contaminated soils. The use of aromatic plants for phytoremediation may have several advantages over other crop plants in that the harvested foliage is a source of essential oils, which are the marketable revenue-generating products of aromatic crops. Growing of these aromatic crops in metal contaminated areas may not introduce heavy metals into the food chain and may not result in an economic penalty compared to most other edible crops. In the process of oil extraction by distillation, heavy metals remain in the extracted plant residues, limiting the quantities of heavy metals in the commercial oil product (Zheljazkov and Nielsen 1996; Scora and Chang 1997; Zheljazkov and Warman 2004). Thus, significant amounts of heavy metals could be removed from the soil through proper disposal of the metal contaminated plant residues, while the metal-free, extracted oils could be safely marketed. High-value aromatic crops may be a better alternative for heavy metal contaminated agricultural soils than the suggested woody species such as Salix and Betula (Hammer et al. 2003; Rosselli et al. 2003), or other plants like Sesbania drummondii that have been shown to hyperaccumulate Pb (Sahi et al. 2002). For disposal of metal contaminated phytomass, several approaches such as composting, incineration, ashing, pyrolysis, direct disposal and liquid extraction have been proposed (Sas-Nowosielska et al. 2004; Keller et al. 2005).

Some aromatic plants appear capable of accumulating heavy metals from contaminated soil (Chand et al. 2012; Zheljazkov and Nielsen 1996; Zheljazkov and Warman 2004; Zheljazkov et al. 2008b; Chaiyarat et al. 2011; Aziz et al. 2011; Amirmoradi et al. 2012), and some might demonstrate significant phytoremediation potential if coupled to other means for increasing bioavailability and uptake of Cd, Pb, and Cu, such as chelates (Schmidt 2003) or biosurfactants (Mulligan et al. 2001) (Fig. 7.17).

Fig. 7.17

Commiphora wightii, a source of guggul steroids capable of growing on lime stone quarry waste

The effects of these metals on growth, essential oil production, and metal accumulation of most commercially important essential oil producing aromatic crops, such as coriander (Coriandrum sativum L.), dill (Anethum graveolens L.), chamomile (Chamomilla recutita (L.)K.), peppermint (Mentha x piperita L.), basil (Ocimum basilicum L.), hyssop (Hyssopus officinalis L.), lemon balm (Melissa officinalis L.), and sage (Salvia officinalis L.) are, however, largely unknown. Coriander, dill, chamomile, peppermint, basil, hyssop, lemon balm, and sage are aromatic crops that have been traditionally grown as cash crops in Europe (Topalov 1962), and North America. Coriander, dill, chamomile, peppermint, basil, hyssop, lemon balmand sage have very high phytoremediation potential (Brown et al. 1998) (Table 7.4).

Traditionally, peppermint, basil, and sage tissue wastes after distillation are used as feed for sheep (Topalov 1962). Results of experiments demonstrated that if peppermint, basil, and sage are grown in highly heavy metal contaminated medium, Cd, Pb, and Cu may accumulate in shoots and wastes from distillation above the maximum permissible concentrations for these elements in animal feed (NRC 1985), making these waste products unsuitable as animal feed. To reduce the amount of Cd, Pb, and Cu and produce usable final product, metal contaminated distillation wastes could be composted after mixing with wastes from other low metal feedstock.

Trace elements in the healing plants can act as remedies (Prasad 2008). Trace elements accumulated in medicinal plants have healing power in numerous ailments and disorders. Herbal preparation known as ‘bhasmas’ (ash of the polyherbals and specific plants and or their parts) are popular in Ayurveda, Indian traditional medicinal system. According to this medicinal system, metal based drugs known as ‘bhasma’ involve the conversion of a metal into its mixed oxides. Plants that accumulate metals and metalloids have gained considerable significance and are implicated in healing function. Plants that accumulate essential trace elements are implicated in propelling metabolic processes (metallomics) (Prasad 2008).

7.3.3 Ornamentals for Environmental Moderation and Toxic Trace Metal Cleanup

Several ornamental plants have been successfully applied in environmental toxic cleanup (Figs. 7.17–7.25) (Abad et al. 2001; Bosiacki 2009a, b; Ding and Hu 2012). For e.g. Lemon-scented geraniums (Pelargonium sp. ‘Frensham’, or scented geranium) accumulated large amounts of Cd, Pb, Ni and Cu from soil in greenhouse experiments (Bosiacki 2008; Dan et al. 2000; Saxena et al. 1999). Biotechnological interventions through hairy root regenerants are useful in floriculture (Giri and Narasu 2000; Giovanni et al. 1997). Pellegrineschi et al. (1994) improved the ornamental quality of scented Pelargonium spp. This plant has pleasant odor that adds scent to the toxic metal contaminated soil.

Fig. 7.18

Cultivation of ornamentals on contaminated soils in peri urban Greater Hyderabad. (a) Chrysanthemum sp. (pink flowers) (b) Chrysanthemum (Yellow flowers), (c) Gaillardia sp. and (d) Rosa sp. (Colour figures online)

Fig. 7.19

Cultivation of ornamentals on contaminated soil in perdi urban Greater Hyderabad (a) and (b) Jasminum officinale L. with Musi river contaminated water. (c) and (d) Tagetes sp. (Patel and Patra 2014)

Fig. 7.20

Cultivation of (a) Crossandra infundibuliformis (L.) Nees with Musi river contaminated water (b) flowers of C. infundibuliform (c) Asparagus setaceus (Kunth) Jessop and (d) Origanum majorina

Fig. 7.21

Ornamentals in commercial market (a) Chrysanthemum sp. (b) Alternanthera sp.

Fig. 7.22

Ornamentals in commercial market (a) Rosa sp. (b) Lilium sp. (c) Jasminum officinale L.

Fig. 7.23

Water ornamentals. (a) Pistia stratiotes L. dry phytomass. (b) Nymphaea odorata Aiton. (c) N. nouchali Burm. f. (d) N. pubescens Willd.

Fig. 7.24

(a and b) Vetiveria zizanioides and (c and d) Water lilies (Nymphaea) in commercial market

Fig. 7.25

Ornamental Canna indica in soil scape filtration for removal of nutrients and colouring material from waste water (Huiping Xiao et al. 2010)

Vetiveria zizanioides (Vetiver grass): It is known to have multiple uses. This plant had several popular names such as ‘the miracle grass’, ‘a wonder grass’, ‘a magic grass’, ‘an unique plant’, ‘an essential grass’, ‘an amazing plant’, ‘an amazing grass’, ‘a versatile plant’, ‘a living barrier’, ‘a living dam’, ‘a living nail’, ‘a living wall’, ‘an eco-friendly grass’. This extraordinary grass is adaptable to multiple environmental conditions and it is globally recognized as an easy and economical alternative to control soil erosion and to solve a variety of environmental problems. It has been used for restoration, conservation and protection of land disrupted by man activities like agriculture, mining, construction sites, oil exploration and extraction, infrastructure corridors, as well as used for water conservation in watershed management, disaster mitigation and treatment of contaminated water and soil. Research at the global level has proved the relevance of vetiver in multiple applications. Vetiveria zizanioides exhibited high metal tolerance (Andra et al. 2009, 2010, 2011; Chintakovid et al. 2008; Chen et al. 2012; Chiu et al. 2006; Chong and Chu 2007; Danh et al. 2011; Rotkittikhun et al. 2007; Makris et al. 2007; Pang et al. 2003; Truong 2000; Wilde et al. 2005) and is used in stabilization of mine tailings in different parts of the world (Frérot et al. 2006; Pérez-López et al. 2014; Rocio et al. 2013).

The debris generated from the ornamentals containing the toxic metal residues can be treated easily as the biomass would be relatively less in view of high water content. Appropriate techno-economic feasible options based on integrated model systems was recently suggested for the appropriate use of Eichhorinia crassipes (water hyacinth) (Malik 2007). Similar solutions need to be worked out for the ornamentals proposed for toxic metal celanup.

Ornamental plants have an added advantage of enhancing environmental aesthetics besides cleaning the environment. This approach has several advantages for environmental moderation, cleanup and generation of revenue. Therefore, this approach will add new dimension to the field of phytoremediation of contaminated environment. The compost generated from the phytoremediated ornamental plants serves as a medium for use as growing media for production ornamentals (Page et al. 2014).

“Scent the soil” with aromatic plants is a fascinating proposition. Several potential and promising options exist for environmental management of contaminated soils using medicinal and aromatic plants. Yet certain bottlenecks are to be investigated for wider applications. The regulatory bodies and biosafety issues might complicate this approach. The compost generated from the ornamentals plants used in remediation serves as a compost and can be reused as growing media for production ornamentals. The quantum of biomass generated would be relatively less in view of high water content in several of these ornamentals. Integrated model and appropriate techno-economic feasible options were suggested for the use of Eichhorinia crassipes (water hyacinth) (Malik 2007). Similar solutions need to be worked out for the ornamental proposed for toxic metal celanup.

7.3.4 Non-edible Oil Plants for Biofuels on Contaminated Soil

Non-edible vegetable oils or the second generation feed stocks have become more attractive for biodiesel production. These feed stocks are very promising for the sustainable production of biodiesel (Agbogidi and Eruotor 2012; Agbogidi et al. 2013; Ahmadpour et al. 2010; Jamil et al. 2009; Liang et al. 2012; Luhach and Chaudhry 2012; Majid et al. 2012; Mangkoedihardjo and Surahmaida 2008).

Taking the advantage of second generation biodiesel plants and obtaining the biodiesels from these plants as most of these plants naturally grown in highly contaminated with variety of inorganic and organic pollutants. Most of the these plants grow widely in contaminated substrates. Our field work survey reveled that some of the potential plants are Ricinus communis, Jatropha curcas, Azadiracta indica. The world’s dependence on fossil fuels is a perfect example. Therefore, biodiesel feedstock should be as diversified as possible, depending on geographical locations in the world. Therefore, exploring alternative biodiesel feedstocks like non-edible vegetable oils should be an important objective for biodiesel industries in near future.

In developing countries like India, biofuel program of Government of India has launched alcohol production from sugar/starch based feedstock the Jatropha curcas plant has been planted on massive scale on waste/degraded or other lands to open new avenues for producing biodiesel from Jatropha curcas oil thereby reducing/saving foreign exchange needed to import the petroleum fuel (Abioye et al. 2010; Acharya et al. 2012; Gao et al. 2009; Ghavri and Singh 2012; Kumar et al. 2008, 2013; Singh et al. 2013, 2014; Wu et al. 2011; Yadav et al. 2009). In biodiesel production, India is on 8th position with 0.03 billion liters of biodiesel production per year.

The above problem can be solved by using cheapest, low cost several potential tree borne oil seeds (TBOs) and non-edible crop source have been identified as suitable feed stock for biodiesel. However, it must be pointed out that global biodiesel feed stocks should not rely on certain sources as it could bring harmful influence in the long run. The worlds’ dependence on fossil fuels is a perfect example.

Therefore, biodiesel feed stock should be as diversified as possible, depending on geographical locations in the world. The determined properties beside the engine performance and emission characteristics of non-edible biodiesel covered in this review indicated that there is a huge chance to produce biodiesel from non-edible sources in the future.

Since, India is net importer of edible vegetable oils, these oils are therefore not available for conversion to biodiesel. India has the potential of becoming the world’s leading producer of biodiesel, as it can be “harvested,” and produced from non-edible oils like Jatropha curcas, Pongamia pinnata, Madhuca indica plants etc (Awwokunmi et al. 2012; Bauddh and Singh 2012a, b).

The increasing use of biodiesel in India will provide green cover to wasteland, support the agricultural and rural economy, reduce the dependency on imported crude oil and improve the environmental emissions. Even though, 12 Jatropha species are reported by several Indian floras, research has been confined to nine species only. Among all the Jatropha species, J. curcas is the most primitive form and has the potential for biodiesel and for medicinal (Abd El-Kader et al. 2012).

There are several tree species that are suitable for Biodiesel production like Jatropha sps., Pongamia pinnata (Millettia pinnata), Moringa, Simarouba glauca, Calophyllum inophyllum, Citrullus colocynthis, Simmondsia chinesis, Helianthus tuberosus, Garcinia indica, Madhuca indica, Azadirachta indica, Linum usitatissimum etc. (Fig. 7.26).

Fig. 7.26

Cultivation of biodiesel and oil producing crops contaminated soil (a) Ricinus communis (b) Vetiveria zizanioides (c) Jatropha curcas and (d) Moringa oleifera

Advantages of non-edible oil as Biodiesel has the following advantages:

• Mitigate Environmental Global Warming

• Energy Security

• Rural and Urban Economic Development

• Reduce Sulfur/Aromatics

• Clean Air

• Health Effects

• Biodiesel Exhausts reduce Carcinogen Emissions

• High Cetane and Lubricity

• Biodegradable

• Non-Toxic

• The cancer causing potential of diesel exhaust largely a function of:

  • Amount, size, and composition particulates

  • Mutagenicity of exhaust gases

  • High exhaust mutagenicity can cause:

  • shortened life, birth defects

  • asthma and respiratory problems

  • specially in infants or the elders

7.3.5 Tree Crops

The following are some examples Casurina equisetifolia, Leucaena leucocephala (Subabul, koobabul, ipil-ipil, lead tree, tan tan, white popinac etc.) Parkinsonia aculeata (Jerusalem thorn), Pithecellobium dulce (Jangil jailebi), Bauhinia variegata, Cassia siamea, Prosopis juliflora (Mesquite), P. chiliensis (Paradeshi Babul), P. cineraria (Khejri), Peltophorum pterocarpum (Copper pod), Sesbania gradiflora (Avisa), and S.bispinosa (Jeelgu) (Yang et al. 2003; Zalesny et al. 2009). However, only a few like Eucalyptus, Leucaena and Casuarina are mostly grown (monoculture). In such plantations, wood production is about 20 cu m/ha/year/ to 60 cu m/ha/year. Eucalyptus wood has industrial value in pulp, rayon charcol, methanol distilleries (Fig. 7.27).

Fig. 7.27

Stabilization of mine tailings with Cymbopogon citratus (lemon grass) miracle grass, Vetiveria zizanioides and Jatropha curcas (a) Nyveli lignite mine (b) Mine over burden (c) Jatropha curcas plantation in mine over burden and (d) Jatropha biodiesel

In the contemporary time energy and environment security are at stake in our situation. Land resources are under immense pressure glocally (globally and locally). The pressure on available land resources is increasing due to land degradation, population explosion, global economic development and urbanization. Land degradation is mainly due to erosion, nutrient depletion, loss of organic matter, structural losses, induced land degradation, faulty irrigation, atmospheric deposition of pollutants, poor agricultural practices. Climate changes increases the intensity of land degradation. Therefore, utilization of contaminated environment (terrestrial and aquatic) following the principles of phytoremediation for biofuel production would be one of the sustainable options (Núñez-López et al. 2008). Although this strategy has immense scope and limitations (Tables 7.3 and 7.4) considerable progress and successful demonstration projects have provided convincing evidence for environmental moderation, cleanup and co-generation of revenue. Therefore, this approach will add new dimension to the field of sustainable development.

Table 7.3 Metal accumulation reports with reference to Helianthus annus (sun flower) (Ruiz et al. 2007)

Table 7.4 Medicinal and aromatic plants capable of growing on contaminated soils

Natural vegetation of Prosopis juliflora in Patancheru Industrial Development Area contaminated with highly above permissible limits of heavy metals in soil and surface and ground water. Prosopis juliflora is an evergreen, fast growing, drought resistant, widely distributed phreatophyte not only in India but also in other arid and semi-arid tropical countries. A valued tree for shade, timber and forage It is a thorny, deciduous, large crowned and deep rooted bush or tree which grows up to 10 m height or more, depending on the variety and climatic conditions. It is widely distributed in the dry tropical and sub-tropical regions of Central America and Northern South America. Also, it is widely propagated in Africa and Asia. It is the only exotic species capable of growing on a wide variety of soils and climatic conditions. It is an ideal species for afforestation and helps in the reclamation of waste lands, stabilization and prevents soil erosion. It has tremendous field potential for reclamation of a variety of mine soils, fly ash land fills and disturbed ecosystems.

P. juliflora is an ideal species for stabilizing the pegmatitic tailings of mica mines in Nellore district of Andhra Pradesh. Research findings revealed that it is helpful for reclamation of copper, tungsten, marble, dolomite mine tailings and is a green solution to heavy metal contaminated soils. This is an appropriate species for rehabilitation of gypsum mine spoil in arid zone restoration of sodic soils. It out-performed all other tree species in sand dune stabilization. Arbuscular mycorrhizal inocula have been isolated from its rhizosphere (low cost agrotechnology) were found to accelerate the growth of their agroforestry and social forestry legumes in perturbed ecosystems (Patrick Audet 2014).

Prosopis juliflora colonized the industrial effluent produced by textile, paper products, tannery, chemical products, basic metal products, machinery parts and transport equipments industry. P. juliflora and Leptochloa fusca association was successful for revegetating salt laden lands (Singh 1995; Shelef et al. 2012) (Figs. 7.28, 7.29, 7.30, 7.31, and 7.32).

Fig. 7.28

Prosopis juliflora in industrial areas contaminated with heavy metals. is the feed stock in biomass based power plants (a) Prosopis juliflora (b) Prosopis wood (c) Biomass fired power plant

Fig. 7.29

Prosopis juliflora – charcoal production (a) Prosopis wood (b) Controlled burning to produce charcoal (c and d) Charcoal

Fig. 7.30

Prosopis juliflora wood is used as fuel and also for production of charcoal which is also used as a carrier of biofertilizing microbes (a) Charcoal production (b) Charcoal (c) Charcoal is used as carrier for plant growth promoting bacteria (d) Application of biofertilizer using charcoal as carrier

Fig. 7.31

Tree – grass – legume association was found to be the best combination for restoration of mica, copper, tungsten, marble, dolomite, limestone, and mine spoils of Rajasthan State and else where in India

Fig. 7.32

Prosopis juliflora benefits and products

Grass-legume-tree association need to be tested on different sites for remediation, if necessary with biotic and abiotic amendments. Restoration of fly ash landfills with P. juliflora following different amendments and Rhizobium inoculation yielded promising results (Dwivedi et al. 2008). Mycorrhizae improved the growth of P. juliflora on high pH soils. P. juliflora seedlings growing in gypsum mine had high frequency arbuscular mycorrhizal fungal infection. However, P. juliflora has some biological characters that foster invasion, hence appropriate management practices needed to be developed for recommending it for phytoremediation.

Industrial and urban activities impact our environment, especially in terms of soil pollution. Large numbers of sites are nowadays contaminated by pollutions of chemical or organic origin. Treating these pollutions represents an economic need, which often remains unanswered by conventional civil engineering methods, due to their inappropriateness, their environmental impact and costs (notably for large sites). During the last two decades, we have witnessed the emergence of gentle soil remediation techniques using various plant species and the combination of microbial biotechnologies. Several phytotechnologies can be considered and applied to polluted soils Conesa et al. (2012): (1) phytostabilisation, which uses perennials able to sorb and immobilize potentially toxic trace elements (PTTE) in the root zone, avoiding their transfer toward groundwater and aerial parts and preventing their bioaccumulation in the food chain as well as dispersion by natural agents (wind erosion, water, etc.) (2) phytoextraction, based on root-to-shoot transfer and storage of PTTE in harvestable plant parts. P. juliflora produced phytoproducts are shown in Figs. 7.33–7.40).

Fig. 7.33

Cultivation of Gossypium arboreum in polluted (a and b) (c) Cotton ready for hearvest (d) Cotton godown

Fig. 7.34

(a) Cultivation of Vetiveria, (b) Hibiscus cannabinus, (c) H. sabdariffolia and (d) Jatropha gossipyfolia on contaminated lands. Variety of non-edible products are produced from these plants

Fig. 7.35

Novel composites containing polyaniline coated short kenaf (Hibiscus cannabinus) bast fibers and polyaniline nanowires

Fig. 7.36

Variety of handi crafts made out of vetiver roots (Source: www.vetiver.org/IND_handicrafts.pdf; www.vetiver.org and Dr Paul Truong)

Fig. 7.37

Venetian blinds and door curtains made out of vetiver roots (Source: www.vetiver.org/IND_handicrafts.pdf; www.vetiver.org and Dr Paul Truong)

Fig. 7.38

Variety of household items made out of vetiver roots (Source: www.vetiver.org/IND_handicrafts.pdf; www.vetiver.org and Dr Paul Truong)

Fig. 7.39

Garlands and a wide variety of household items made out of vetiver roots (Source: www.vetiver.org/IND_handicrafts.pdf; www.vetiver.org and Dr Paul Truong)

Fig. 7.40

Soft sandals and wide variety of household items made out of vetiver roots (Source: www.vetiver.org/IND_handicrafts.pdf; www.vetiver.org and Dr Paul Truong)

7.3.6 Products from Phytoremediated Fiber Crops

Since ancient times, plants were of considerable help in satisfying man’s necessities in respect of food, clothing and shelter. In those days, man also required some form of cordage for his snares, bow-strings, nets, etc. and also for better types of covering for his shelter. Tough, flexible fibres obtained from stems, leaves, roots, etc., of various plants served the above purposes very well. With the advancement of civilization, the use of plant fibres has gradually increased and their importance today is very great. Fibre-yielding plants have been of great importance to humans and they rank second only to food plants in their usefulness. Although many different species of plants, roughly about two thousand or more, are now known to yield fibres, commercially important ones are quite small in number (Tables 7.2 and 7.3). Fibers are long narrow tapering cell, dead and hollow at maturity, thick cell wall composed mostly of cellulose and lignin. They are rigid for support, found mainly in vascular tissue. Most importantly natural fibers are biodegradable (Figs. 7.33–7.40).

7.3.7 Dye Yielding Plants

Dye yielding plants are shown is Table 7.12.

Table 7.5 Metal accumulation in selected Medicinal and aromatic plants

7.3.8 Phytoremediation Crops for Carbon Sequestration

Biofuel production, carbon sequestration are the two sides of the coin of the “Phytoremediation crops” (Aggarwal and Goyal 2007). Before the industrialization revolution (1760–1830 A.D), the amount of greenhouse gases (GHGs) such as Carbon dioxide (CO₂), Carbon monoxide (CO), Nitrous oxide (N₂O), Methane (CH₄), Ozone (O₃), Chlorofluorocarbon (CFC) i.e., Hydrofluoro-carbons (HFCs), Perfluorocarbons (PFCs), Sulfur hexafluoride (SF₆) in the atmosphere remained relatively constant. Except for slow changes on a geological time scale, the absorption through photosynthetic process by green plants and release of carbon from various sources was at equilibrium.

The worldwide economic growth and development basically requires energy. Among the many human activities that produce GHGs, the use of energy represents by far the largest source of emissions of GHGs, direct combustion of fossil fuels dominates the GHGs emissions from the energy sector. CO₂ results from the oxidation of carbon in fuels (www.iea.org). According to available data 2011 A.D, total CO₂ emissions from the consumption of energy is 32,578.645 Million metric tonnes (http://www.eia.gov). The world’s current data for atmospheric CO₂ is measured at the Mauna Loa Observatory in Hawaii. Measurements are made and reported independently by two scientific institutions: Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration (NOAA). According to the recent data, the upper safety limit for atmospheric CO₂ is 350 parts per million (ppm) has crossed in 1988 itself and in July 2013 the atmospheric CO₂ raised to 398.58 ppm and projected to cross the 400 ppm mark (http://co2now.org) (Table 7.6).

Table 7.6 Ornamental plants capable of accumulating metals

Although some of the effects of increased CO₂ levels on the global climate are uncertain, most scientists agree that doubling atmospheric CO₂ concentrations may cause serious environmental consequences. The ten indicators of warming world are increasing the humidity, air temperature near surface (Troposphere), temperature over oceans, sea surface temperature, sea levels, temperature over land, ocean heat content, decreasing the glaciers, snow cover, and sea ice. Increasing global temperatures could raise sea levels, change precipitation patterns and affect both weather and climatic conditions. Currently, we are experiencing all the effects of globalwarming and in future our next generations have to face.

Anything that removes carbon from the atmosphere is a ‘sink’. In order to be effective in combating climate change, the sink must be large and the carbon must stay in the sink. Carbon is continuously exchanged between atmosphere, soil, ocean and life, which is predominately, plants. Since, CO₂ is important GHGs, one strategy that can partially combat global warming and climate change is to increase the amount of carbon stored in plants. By increasing the amount of plant life on earth, or altering it to plant types that store the most carbon, more carbon dioxide may be pulled out of the atmosphere and stored for a period of time through photosynthetic process. It is presumed that only 26 % (9.5 million metric tonnes) of CO₂ is trapped again in the photosynthetic process by green plants. However, real time data reveals the rapid land changing pattern i.e., clearing the forest lands, increasing the agricultural lands (http://www.iaees.org) and burning fossil fuels more rapidly. Due to shrinking of plant life in due course of time the present percentage of carbon sequestration is still further reduces. This phenomenon forced to increasing the global temperature and rise of sea levels, which are significant effects of GHGs. In the process of human development further, industries are necessary and release of pollutants is obvious end products. Hence to combat and control the GHGs and other pollutants (inorganic and organic), huge plantation programme should be conducted and see that its survival in industrial zones as marginal lands. This helps in carbon sequestration as well as pollutants remediation from those industrial zones. The latest carbon sequestration method is digging the deep wells and release into the injection zone (i.e., 7,000 ft below the surface) (http://www.epa.gov). Taking the advantage of latest carbon sequestration method integrated approach of phytoremediation techniques in the vicinity of industrial zones is both way getting benefited i.e., release of GHGs are significantly reduced and effectively implement the phytoremediation.

Selection and plantation of right choice of plants for effective phytoremediation as well as carbon sequestration is important for obtaining the phyto products in industrial zones and contaminated abandoned lands. Revegetating with right choice of plants on former mining sites can provide phytoremediation services as well as carbon sequestration. Though, the process is cost effective it requires considerable time and should be employed at sites where remediation can occur over a long period of time. Generally, long term carbon sequestration can be achieved when carbon from above ground biomass transfers to the roots and enters the pool of Soil Organic carbon (SOC) or Soil Inorganic Carbon (SIC) which is possible for perennial trees and herbaceous plants with extensive root systems (Jansson et al. 2010) (Fig. 7.41).

Fig. 7.41

Simplified carbon cycle. Unlike fossil fuels, biomass does not increase atmospheric green house gases when burned. Closed carbon cycle

Photosynthesis

$$ 6{\mathrm{C}\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}+674\kern0.24em \mathrm{Kcal}\underset{\mathrm{Solar}\kern0.22em \mathrm{energy}}{\overset{\mathrm{C}\mathrm{hlorophyll}}{\to }}\underset{\mathrm{C}\mathrm{arbohydrate}\kern0.36em 1\kern0.22em \mathrm{mol}}{{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2} $$

One of the important limiting factor of solar energy conversion by plants is their photosynthetic efficiency

$$ \mathrm{IS}\to \mathrm{P}\to \mathrm{H}\to \mathrm{C}\to \mathrm{T}\mathrm{C} $$

• IS = Insolation (in coming solar radiation)

• P = Producer, H = Herbivore, C = Carnivore

• TC = Top carnivore

• PN = Q.β.ε-R (PN= Net Productivity)

• Q = PAR, the quantity of incident light

• β = the proportion of that light intercepted by green plant organs (canopy size, structure, pigments)

• ε = the efficiency of photosynthetic conversion of the intercepted light into biomass

• R = respiration

$$ {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}+\mathrm{light}\left(8\;\mathrm{quata}\right)\to \left({\mathrm{CH}}_2\mathrm{O}\right)+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\mathrm{where}{\mathrm{CH}}_2\mathrm{O}=1/6\;\mathrm{glucose}\;\mathrm{molecule} $$

Gibbs free energy stored per glucose is 477 KJ and 8Q of light (400–700 nm monochromatic) of 575 nm is required.

8Q of light have energy content of 1,665 KJ, which gives maximum photosynthetic efficiency of 477/1,665 = 0.286

Maximum photosynthetic efficiency of PAR constitute about 43 %

Canopy absorb only 80 % available PAR

Respiratory losses and energy requirement for life accounts for 1/3 of stored energy = 66.7 %

Hence, over all photosynthetic efficiency of conversion of solar energy into stored chemical energy by terrestrial plants is 0.43 × 0.8 × 0.286 × 0.667 = 6.6 %

Therefore, in view of the above explained limitation, the only way is to use contaminated land for phytoremediation crops and thereby enhance carbon capture.

• 1. Strategies of wasteland development (NWDB)

  Remediation have been frequently used in the literature.

  Restoration: replication of site conditions prior to disturbance

  Reclamation: rendering a site habitable to indigenous organisms

  Rehabilitation: disturbed land will be returned to a form and productivity in conformity with a prior to land use plan

  Full restoration: Restoration of a site to its pre-damaged condition

  Partial restoration: Restoration of selected ecological attributes of the site and Creation of an alternative ecosystem type the latter though often desirable, is not to be called restoration.

• 2. Biotic and Abiotic stress factors

• 3. Species available for sustained yields

• 4. Ecological constraints/Soil management for

  • nutrient depletion

  • annual crop production

  • pasture livestock production

  • perennial and tree crops

• 5. Role of biofertilizers in restoring soil fertility, Azobacter, Azospirillum Rhizobium, Legume green manure

Also stated that, bioenergy crops (bioethanol, biogas, biodiesel, feedstock for electricity, charcoal) occupy a distinctive position in future terrestrial carbon sequestration and the vast areas of bioenergy cultivation envisioned for sustainable biofuel production (Smith et al. 2013), especially from perennial grasses and woody species, offer the potential for substantial mitigation of GHGs emissions both by displacing fossil fuels and through phyto-sequestration through extensive root systems. It is well known that most of these plants can accumulate pollutants by any of the phytoremediation process (Fig. 7.42).

Fig. 7.42

Scheme showing carbon sequestration by phytoremediation crops

Promising short rotation woody energy crops (SRWEC) are Populus ssp. (Imada et al. 2009; Sebastiani et al. 2004; Wu et al. 2010), Salix spp. (Lewandowski et al. 2006), Liquidambar styraciflua, Platanus occidentalis, Robinia pseudoacacia, Acer saccharinum L. and Eucalyptus can be grown for other uses also such as paper production and the waste can be utilized for energy (Capuana 2011; Claudia et al. 2012; Delplanque et al. 2013). Hibiscus cannabinus (Kenaf), Brassica juncea (Indian mustard), Helianthus annus (Sunflower), Ricinus communis (Castor), Vetiveria zizanioides (Khus Khus grass) Prosopis juliflora (Velvet Mesquite), are potential candidates for phytoremediation and carbon sequestration. Conversion of phytoremediation- borne biomass for (1) green-fine chemistry (catalyst production from metal accumulating biomass) (2) biorefinery (prehydrolysis and organosolv pre-treatment from metal accumulating woody lignocellulosic biomass) and (3) by increasing the panel of plant species cultivated on metal contaminated soils for value chain and value added products enhancing bioeconomy are emerging fields (Fig. 7.43).

Fig. 7.43

Carbon sequestration by aquatic macrophytes and byproduct generation (Prasad 2012, © Springer Science+Business Media, LLC 2012)

7.3.9 Non-edible Oil Plants for Bio-Fuels on Contaminated Soils

Our world runs on energy – it’s fundamental to our way of life and growing our economy. Majority of the world’s energy needs are supplied through petrochemical sources, coal and natural gases (non-renewable), with the exception of hydroelectricity and nuclear energy. World primary energy consumption grew by 1.8 % in 2012 and oil remains the world’s leading fuel, at 33.1 % of global energy consumption (http://www.bp.comstatisticalreview). Today, global transportation sector is almost entirely dependent on petroleum-derived fuels and its escalating supply and consumption is dramatically increasing. While supply and consumption is increasing, its associated problems such as greenhouse gases especially CO₂ emission is also increasing (http://www.eia.gov/). Petroleum-based products are one of the main causes of anthropogenic carbon dioxide (CO₂) emissions to the atmosphere. Moreover, the increase in pollutants emissions from the use of petroleum fuel will affect human health, such as respiratory system, nervous system and skin diseases etc. The conventional source of energy is non-renewable resources and in the recent times the world has been confronted with an energy crisis due to rapid depletion of natural resources, increased social and environmental problems. It is important to develop suitable long-term strategies based on utilization of renewable fuel that would gradually substitute the declining fossil fuel production. Various groups of biomass generated and its uses are shown in Table 7.7.

Table 7.7 Metal accumulation capacity in selected high biomass producing ornamentals (Kaiser et al. 2009)

Biodiesel is considered to be a possible substitute for conventional diesel getting the interest of scientists and workers all over the world. Biodiesel, mixture of fatty acid methyl esters (FAME), is generally produced from a varied range of edible (First generation feedstock) and non-edible (Second generation feedstock) vegetable oils, algae (Third generation feedstock), animal fats, used frying oils, and waste cooking oils and waste soap stocks from the oleo-chemical industries have been identified as a source of biodiesel feed-stock.

The biodiesel is quite similar to conventional diesel fuel in its physical characteristics and can be used as a direct substitute for petrodiesel and is technically called B100. The preferred ratio of mixture ranges between 5 % (B5) and 20 % (B20). Up to 20 % blending of biodiesel with diesel has shown no problems.

There are a great number of advantages as the primary feedstock can grow season after season and it provides a market for excess production of vegetable oils, and animal fats, thus enhancing the rural economies. Also for using biodiesel apart from reduces the country’s dependence on imported petroleum and specifically it is biodegradable, non-toxic, renewable and closed carbon cycle. Also it reduces emission of CO, CO₂, SO₂, particulate matter, volatile organic compounds and unburned hydrocarbons as compared to conventional diesel. In addition it has higher cetane number and flash point greater than 423 K as compared to 350 K for petroleum based diesel fuel. World production of various sources of vegetable oils in 2012–2013 was 160.22 Million metric tonnes and mostly used for consumption purposes (www.fas.usda.gov). However, in different countries of the world, same vegetable oils resources are used for biodiesel production. The source for biodiesel production is chosen according to physico-chemical properties, production cost, transportation and policy. In USA, Europe, Brazil other parts, 95 % used type of biodiesel fuel is first generation feedstock i.e., edible oils like soybean, rapeseed, sunflower, safflower, canola, palm, Coconut and fish oils are used to reduce air pollution and dependency on fossil fuel, which are limited and localized to specific regions (Jeonng and Park 2008; Darnoko and Cheryman 2000; Vicente et al. 2004; Ramadhs et al. 2004; Cheng et al. 2004; de Oliveira et al. 2005; El Mashad et al. 2008; Sarin and Sharma 2007; Meka et al. 2007).

Although, biodiesel are mainly produced in many regions recently, environmentalists have started to debate on the negative impact of biodiesel production from edible oils (Butler 2006). Recently, the use of edible vegetable oils or the first generation feed stocks has been of great concern and the major obstacle for commercialization of biodiesel is its cost from the feed stocks (Canakci and Van Gerpen 2001). Cost of edible oils is very higher than petroleum diesel and we use edible oils for biodiesel production leads food oil crisis. This is because they raise many concerns such as food versus fuel debate that might cause starvation especially in the developing countries and other environmental problems caused by utilizing much of the available arable land. This problem can create serious ecological imbalances as countries around the world began cutting down forests for plantation purposes and perfect examples is in countries like Malaysia, Indonesia and Brazil. In fact, the real time world land alterations can be glanced at on http://www.iaees.org.

Taking these factors into consideration, the above problem can be solved by using cheapest, low cost non edible oils which are not suitable for human consumption because of the presence of some toxic secondary metabolites in their oils. Non edible oil crops can be grown in waste lands and the cost of cultivation is much lower because these crops can still sustain reasonably high yield without intensive care.

Exploring the availability of non-edible oil seed as alternative biodiesel feedstock in the transportation sector is critical towards achieving higher self-reliance energy security. This situation offers a challenge as well as an opportunity to look for replacement of fossil fuels as well as first generation feedstock of biodiesels for both economic (Food verses Fuel) and environmental benefits. The following are the advantages of non-edible oil:

Non-edible vegetable oils are not suitable for human consumption due to the presence of some toxic compounds in their oils. Non-edible oil plants are well adapted to arid, semi-arid, high rain fall zone conditions and require low fertility and moisture and to grow. Moreover, Non-edible biodiesel crops are expected to use lands that are largely unproductive and those that are located in poverty stricken areas and in degraded forests which can fix upto 10 t/ha/year CO₂ emissions. They can also be planted on cultivators’ field boundaries, fallow lands, and in public land such as along railways, roads and irrigation canals. They do not compete with existing less farm land and agricultural resources. Hence they eliminate competition for food and feed. Non-edible biodiesel plant cultivation as mentioned above could become a major poverty alleviation program for the rural poor apart from providing energy security. Moreover, they can be propagated through seed or cuttings. Most of the non-edible oils are highly pest and disease resistant. Non-edible feed stock can produce useful by-products during the conversion process, which can be used in other chemical processes or burned for heat and power generation.

7.3.10 Bioenergy from Phytoremediated Phytomass via Gasification

Phytomass gasification is basically the conversion of wood chips and wood waste into a combustible gas mixture called as producer gas (Low Btu gas). This process involves partial combustion of phytomass. Given that phytomass contains carbon, hydrogen and oxygen, complete combustion would produce carbon dioxide and water vapour. Partial combustion produces carbon monoxide and hydrogen both of which are combustible gases. The gas thus generated in a “Gasifier” could be used for captive power generation (a few KWs to several hundred KWs). Fuelwood direct combustion has several disadvantages owing to its limited applications and is of low efficiency. Direct thermal applications for power generation require steam boilers with steam engine or turbine along with necessary equipments. Such a conversion technology is not only capital intensive but is of low conversion efficiency. Conversion of the same phytomass (wood chips/ wood waste) to combustible gases in gasifier and its utilisation for power generation has innumerable advantages. Phytomass gasification (using gasifiers of various capacities) generates electricity, and the gas can be utilised for direct thermal energy and for shaft power (Fig. 7.44).

Fig. 7.44

(a–d) Harvested phytoremediated aquatic weed biomass is used as feed stock for biogas in anaerobic digester

7.3.10.1 Aquatic Biomass

Biomass of Eichhornia crassipes (water hyacinth), Pistia statoides (water lettuce) and other aquatic weeds can be used for gasification (Figs. 7.45–7.49). Macrophytes spread rapidly and clog the aquatic systems (Bi et al. 2011). Therefore, utilization of such freely available biomass for bioenergy would be a happy solution. Aquatic weed biomass when subjected to anaerobic digestion optimum yield of gas was recovered when the C/N ration was between 20 and 30. One hectare of water hyacinth grown on sewage can purify the wastewater and produces 0.8 ton of dry matter per day which can be converted to 200 cu m gas that is enough for generating 250 kw power.

Fig. 7.45

(a–c) Profuse growth of water hyacinth (Eichhornia crassipes) in polluted water. Harvested biomass is used as feed stock for biogas (please see also Fig. 7.22)

Fig. 7.46

Phytofilteration of toxic metals in aquaculture ponds using Pistia stratiotes (Prasad and Prasad 2012, © Walter de Gruyter, Berlin, Boston)

Fig. 7.47

After harvesting Pistia stratoites the water body is used for fish culture – white arrows show feed bags suspended with the help of rope across the water body

Fig. 7.48

Arsenic accumulating ornamentals are being used for treating arsenic contaminated water e.g. Pteris vittata, Colocasia esculenta and Cyperus sp.) (Kurosawa et al. 2008; Nakwanit et al. 2011; Prasad and Nakbanpote 2015, © Springer India 2015)

Fig. 7.49

Productivity and relative proximate dry matter production of potential macrophytes being applied for phytoremediation (Prasad et al. 2001)

It is possible for generating 100 L of biogas from 1 kg of dry matter under controlled conditions. In China and Japan, large scale sea weed farms are under operation. Macrocystis (Giant kelp) is grown for fuel production. The kelp yields are represented to be of the tune of 90 tons/ha/year. In USA, California expects an area of 40,000 ha of ocean energy farms by the turn of the century. Each hectare of cultivated kelp would yield about 10 million kilo calories of oil and about a 100 kilo calories of methane energy per year.

Plants endophytes relationships and the microbial communities play a key role in degrading the hazardous contaminants in rhizosphere to varying extents (Reichenauer and Germida 2008; Husain et al. 2009; Scow and Hicks 2005).

Arsenic contamination in Bengal Delta region is prevalent. The first known arsenic hyper accumulating plant is Pteris vittata. It is also known as Chinese brake fern, was discovered from an arsenic-contaminated site that was contaminated from pressure-treating lumber using chromated-copper-arsenate (CCA). P. vittata is reported to accumulate 22 g kg⁻¹ of arsenic in its fronds (Carrier et al. 2011, 2012; Shelmerdine et al. 2009; Ye et al. 2011).

In addition to P. vittata and P. cretica, several other arsenic hyperaccumulating plants have been reported recently including Pityrogramma calomelanos, Pteris longifolia and Pteris umbrosa. Pilot-scale demonstration of phytofiltration for treatment of arsenic has been demonstrated in New Mexico for production of drinking water (McCutcheon and Schnoor 2003) (Fig. 7.24).

Bioconcentration factor: Bioconcentration of heavy metal by water weeds is described as the bioconcentration factor (BCF), which is the ratio of heavy metal accumulated by plants to that dissolved in the surrounding medium. For this, two bioconcentration factors were computed from the plant compartment concentrations as

$$ {\mathrm{BCF}}_{\mathrm{R}} = {\mathrm{C}\mathrm{R}}_{\mathrm{root}}/{\mathrm{C}}_{\mathrm{water}} $$

(7.1)

$$ {\mathrm{BCF}}_{\mathrm{L}}={\mathrm{C}}_{\mathrm{leaf}}/{\mathrm{C}}_{\mathrm{water}} $$

(7.2)

The translocation of heavy metal from the roots to harvestable aerial part is generally expressed as the translocation factor (TF) (Li et al. 2009, 2011, 2014). It was calculated on a dry weight basis by dividing the heavy metal concentration in aerial parts by the heavy metal concentration in root. Based on the above two equations (7.1) and (7.2), the translocation factor can be expressed as:

$$ \mathrm{Translocationfactor}\left[\mathrm{T}\mathrm{F}\right]={\mathrm{BCF}}_{\mathrm{leaf}}/{\mathrm{BCF}}_{\mathrm{root}} $$

(7.3)

Plants with efficient phytofiltration of toxic metals are beneficial for cleanup of contaminated fish ponds (Pistia stratiotes = Water lettuce/Water cabbage). For metal contaminants, plants with high potential for phytoextraction (uptake and recovery of contaminants into above-ground biomass) are desirable (Cao et al. 2007; Prasad 2007a, b) (Fig. 7.50)

Fig. 7.50

Schematic view of photobioreactor for cultivation of algae in hazardous waste water (Chisti 2008)

Hazardous wastewater utilization for high rate algal ponds for production of biodiesel is being considered. Algae growing in wastewater treatment, high rate algal ponds [HRAPs] assimilate nutrients and thus subsequent harvest of the algal biomass recovers the nutrients from the wastewater (Figs. 7.51–7.54).

Fig. 7.51

Algae based photobioreactor for cultivation of algae in hazardous waste water for production of bio fuels, industrial feedstock and carbon dioxide sequestration. Leachate from hazardous waste dump sites are being used to establish High Rate Algal Ponds [HRAPs] for production of biodiesel (Park et al. 2011). The harvested phytomass of aquatic plants used for bioremediation serves as a valuable feedstock for biogas production (Abbasi et al. 1991) (Fig. 7.53)

Fig. 7.52

Nutrient rich and sewage water treatment in constructed engineered wetland using high biomass produced macrophytes viz. Typha and Phragmites. Production of low cost roofing material and compost from macrophytes used in nutrient rich and sewage water treatment in constructed engineered wetland (Zhang et al. 2014; Zheng et al. 2013)

Fig. 7.53

Waste water treatment in High Rate Algal Pond [HRAP] using hazardous waste water (Lim et al. 2013)

Fig. 7.54

Biodiesel production using algae and waste water at lab scale (Source: Mr M.V. Bhaskar “Kadambari Consultants Pvt Ltd - Nualgi”)

7.3.11 Stabilization of Contaminated Lands by Sodding

Mulberry is an economically important tree, used for feeding the silkworm, Bombyx mori. Mulberry exhibits spatial localization and serves as sink for Sr, Zn and Cd in leaves along with Ca and Si accumulation. It is the sole food source for the silk worm (Ashfaq et al. 2009; Hegde and Fletcher 1996; Katayama et al. 2013). The productivity of leaf biomass on coal mine over burden is quite high and serves as a resource. An estimate of 40 tons/ha/year (approximately 10 tons of dry matter) has been reported. Based on the yield data, the amount of Sr deposition in leaves is roughly estimated to be 300 kg/ha/year. Mulberry leaves posses not only a high Sr sink but also yield high biomass, indicating that mulberry is one of the most efficient bioaccumulator plants for Sr and can be used in cleaning or rehabilitating soil contaminated by radiostrontium (Fig. 7.55).

Fig. 7.55

Luxurient growth of Morus alba (mulberry) on coal mine over burden of west bokaro coal field

Sodding is a bioengineering technique that uses vegetation mats for mine soil stabilization and erosion control. This technique has been used with success with a variety of grasses (Bonanno 2012; Bonanno et al. 2013), Perennial grasses with sturdy adventitious root mat anchor and stabilize mine soils. The following is a list of recommendations for using vegetation mats as bioengineering materials (Fig. 7.56)

Fig. 7.56

Sodding, a bioengineering technique that uses grass mats for soil stabilization and erosion control. Perennial grasses are preferred. Grasses with sturdy adventitious root mat stabilize metalliferous soil. To anchor perennial solid binding grass mats to a slope, mats are cut to desirable sizes (1 × 1 foot). This technique would beneficial to stabilize the contaminated soil (Thangavel and Sridevi 2015, © Springer India 2015) (a) Cultivation of Zoysia grass with contaminated substrated (soil and water) (b) Harvested grass mats are ready for sale (c) and (d) road cutting are stabilized with grass mats

• To anchor perennial solid binding grass mats to a slope, mats can be cut to form any shape desired. A shallow, narrow trench built along the contour of a slope and planted with a vegetation mat may become an effective terrace.

• The mat should remain attached to stable vegetation and thus be held in place from the top. The mat can be pegged to prevent ripping and sliding. This technique could be used to stabilize the contaminated soil.

• Vegetation mats can be used as building bricks. Slice the mats into rectangular pieces and use them to construct a very steep, living wall. The bricks can be pegged to each other and to the underlying substrate. This technique may be useful around culverts or sunken walkways and controls erosion.

Tables 7.8–7.17 show various examples of biomass generation in phytoremediation and their possible economic uses.

Table 7.8 Metal accumulation in Ricinus communis (Castor bean)

Table 7.9 Prosopis juliflora, a phreatophyte and its capability for accumulation of heavy metals

Table 7.10 Fiber yielding plants that can be grown on contaminated sites (Bjelková et al. 2011; Griga and Bjelkova 2013; Linger et al. 2002; Smykalova et al. 2010)

Table 7.11 Metal accumulation in Vetiver zizanioides

Table 7.12 Dye yielding plants capable of growing on contaminated soils

Table 7.13 Various groups of biomass resources and their possible scope for utilization as energy sources.

Table 7.14 Metal accumulation and adsorption capacity of Eichornia crassipes (Water hyacinth)

Table 7.15 Metal accumulation in Alternanthera philoxeroides (Alligator weed)

Table 7.16 Metal accumulation in Pistia stratiotes (Water lettuce/Water cabbage)

Table 7.17 Advantages and limitations of bioremediation (Prasad 2011)

7.4 Conclusions

Although phytoremediation is environmentally friendly, powerful and low-cost technology, one general belief is that it might take long time to clean-up inorganics in soil. However, it is relatively convenient to clean small water bodies and waste waster. Most of the previous researches have focused only on wild selected species (non-economic crops) that can tolerate and takeup large amounts of inorganic contaminants to increase the efficiency of phytoremediation. Much progress has been made in countries like UK, USA, Canada, Australia, Japan and many European countries. In developing country like India, it is difficult to convince environmental regulators and local agencies to grow metal accumulators (the data on hyperaccumulators is scanty) in the contaminated areas for the sole purpose of removing pollutants from their environment unless financial renumeration or expenses are subsidized.

The present study highlights the potential of ornamentals, fiber and energy crops for phytoremediation, because they are non-edible and income generators. Further studies should focus on combining phytoremediation with enhancing soil fertility and maximizing pest management, together with investigation of additional species for phytoremediation which may also enhance economic benefits. The management of plant biomass applied in phytoremediation should be also studied. Use of phytoremediated phytomass for phytoproducts and boosting bioeconomy via cogeneration of value additions and value chain products has tremendous scope and should be a priority area of research.

Grison (2015) pioneered in turning metalliferous waste from plants into a resource through innovative technologies, processes and services. She and her team discovered an unprecedented concept in chemistry, namely ‘ecocatalysis’. The development of this new concept created a paradigm shift in sustainable and green chemistry i.e. the metallic wastes are becoming new ecofriendly and efficient catalytic systems. Grison feels that combining phytoextraction and ecocatalysis has opened up a new vistas (E4) in greener chemistry i.e. Environmental, Ecological, Ethic and Economic (E4) opportunity.