Biodegradation or removal of toxic contaminants from soil with the help of plants is known as phytoremediation. This method is cost-effective, less labor intensive and the most common and traditional method of treating polluted soils since past ceumries as compared to other bioremediation methods. In phytoremediation, plants either completely extract or remove toxic contaminants or completely transform or stabilize them into non-toxic forms. Plant species remove soil pollutants in a synergistic combination with soil microbiota inhabiting the rhizosphere zone (Ojuederie and Babalola 2017, Dixit et al. 2015). Phytoremediation is mostly reported as a sub-category of in situ bioremediation because polluted soil is treated at the site of pollution (Azibuke et al. 2016). Phytoremediation is known to improve physicochemical and biological properties of soil due to hyperaccumulating properties of the plants utilized (Jutsz and Gnida 2015, Mahmood et al. 2015). Plants remove or treat toxic soil contaminants by complete extraction, accumulation in plant parts, stabilization, enhancement of degradation by rhizosphere microflora, volatilization or complete degradation of contaminations, and filtration through roots; therefore phytoremediation is broadly categorized into the following subcategories (Ojuederie and Babalola 2017).
3.2.1 Phytoextraction orphytoaccumulation
Phytoextraction refers to the hyperaccumulating property of plants that help in rapid extraction of contaminants from polluted soils. Heavy metals such as Mn, Zn, As, Co, Cr, Cu, Ni, Pb, Sb, Se, Ti, Cd are extensively removed by phytoextraction method; these metals are extracted from soil and transported to various parts of the hyperaccumulating plants (Verbruggen et al. 2009). These plants have unique physiology and morphology that enable sequestration and detoxification of large amounts of pollutants from the contaminated soils. Some of the hyperaccumulating plants are Berkheyacoddii, Helianthus annuus, Minuartiaverna, Euphorbia cheiradenia, Astragalus racemosus, Pteris vittata, Viola boashanensis, Arabidopsis halleri, Thlaspigoesingense, Sedum alfredi, Thlaspicaerulescens, Nicotiana tabacum and Thymus praecox (Ojuederie and Babalola 2017, Liu et al. 2018, Jaffre et al. 2013, Yang et al. 2017).
This method involves immobilization or absorption of toxic contaminants by plant roots in the rhizosphere zone itself, followed by separation and stabilization without affecting the surrounding environment. Phytostabilization prevents translocation of toxic metals to the above ground parts of the plants and the food web. Stabilization of toxic metals can be enhanced by application of soil amendments involving optimization of soil pH. Addition of organic matter, biochar and compost documented positive response (Ojuederie and Babalola 2017. Radziemska et al. 2017). A grass species, Festuca rubra, also known as red fescue, shows good phytostabilization of copper (Radziemska et al. 2017).
This process involves active absorption of toxic metals by various plant parts such as roots (rhizofiltration), excised shoots (caulofiltration) and seedlings (blastofiltration) (Ojuederie and Babalola 2017, Dixit et al. 2015, Mesjasz-Przybylowicz et al. 2004). These plant parts have been identified as sites for faster absorption of metals from soil through chelation, exchange of ions, and adsorption. Metal absoiption process depends on plant physiology, its efficiency in intracellular uptake of metals, vacuolar deposition of metals and translocation of metals to above ground parts of the plant (Salt et al. 1995).
In this method, microbial activity is stimulated in the rhizosphere zone by root exudates to increase degradation of toxic soil contaminants (Ojuederie and Babalola 2017). Mucor sp. has been reported as endophytic phytostimulant fungus in the rhizosphere of Brassica campestris degrading heavy metals such as Mn, Co, Cu and Zn (Zalioor et al. 2017). Another study showed good proliferation of cyanobacteria in the presence of phytohormones that enhanced its nitrogen fixing activity (Hussain and Hasnain 2011).
Phytovolatilization mediates volatilization of toxic soil contaminants and releases them into the atmosphere (Ali et al. 2013). This may occur in two ways—direct or indirect. In the direct method, the contaminant vapors are transcribed from the plant body through vascular pathway and escape through cuticle and stomata of leaves and shoots. In indirect method, vapors escape from the rhizophere zone due to root activities, without being transcribed through the plant body (Limmer and Burken 2016). Methane gas released from wetlands is an example of direct volatilization of organic compounds from plants such as Phraginites australis and Scirpuslacustris in wetland soils (Van Der Nat et al. 1998). Nicotiana tabacum has the ability to volatilize highly toxic soil contaminant such as methyl mercury from Hg into elemental Hg vapors that escapes through cuticle and stomata of the leaves (Rayu et al. 2012, Mukliopadhyay and Maiti 2010). It is reported that phytovolatilization is a result of combined activity of plant and microbial metabolism in the rhizosphere zone (Ojuederie and Babalola 2017, Так et al. 2013).
Phytodegradation involves enzymatic degradation of organic soil pollutants; phytoenzymes such as nitroreductases and dehalogenases are known to transform toxic organic pollutants into non-toxic forms (Ojuederie and Babalola 2017, Ali et al. 2013). Phytodegradation activity may be enhanced by microorganisms in the rhizosphere. known as rliizodegradation (Newman and Reynolds 2004, Ojuederie and Babalola 2017). Microbes in the rhizosphere receive ample amount of nutrients from the root exudates that help in additional degradation of organic pollutants (Ojuederie and Babalola 2017, Babalola 2010).
Role of organic matter din ing remediation of polluted soil has been recognized by many researchers (Rada et al. 2019, Urzelai et al. 2000). Improvement of physical, chemical and biological health of soil under application of organics was recognized long back. Microbes responsible for biodegradation can obtain an interrupted supply of carbon with addition of organic matter in polluted soil. Lee et al. (2003) have documented that carbon in the form of pyruvate enhance the growth of PAH degrading microbes. Composting is a method of improving organic carbon in soil and has been found to be helpful dining biostimulation. In this process, contaminated soil is mixed thoroughly with the primary ingredients of compost (Semple et al. 2001). Efficacy in removal of organopollutants such as PAH has been achieved with the addition of both mushroom compost and spent mushroom compost (SMC) in the polluted sites apart from enhanced microbial growth, enzymatic activity and nutrient contents hi the mixture (Lau et al. 2003). Addition of compost increases the functional diversity of native microbial community. Some other organic wastes such as banana skin, melon shell and brewery spent grain were found usefril during bioremediation of oil contaminated soils (Abioye et al. 2012). Use of poultry manure as organic fertilizer with alternate carbon substrate is found to be effective in biodegradation (Okolo et al. 2005).
Soil remediation of heavy metals is successfril with the addition of vermicompost, humic substances and biocliar (Wang et al. 2018a, Burlakovs et al. 2013). Application of vermicompost improves soil nutrient content and supports plant growth and development, which is crucial for phytoremediation process. Earlier studies documented positive role of biocliar and humic substances in retention of heavy metals in contaminated soil (Wang et al. 2018b, Piccolo 1989, Fonseca et al. 2013). More research is needed on organic materials as cheap sources of nutrients, which can support microbial growth and plant activities during bioremediation. Generation of organic wastes arising from economic and demographic growth can be a potent resource for nutrients. Organic amendments derived from off field sources such as solid wastes and animal manures are effective in revitalization of soil nutrient status (Reeve et al. 2016). Immobilization of heavy metals promote the diversity of Arbuscular myconhiza in metal polluted soil (Montiel-Rozas et al. 2016). Uses of animal manure and biosolids have side effects causing elevated content of both amonical and nitrate nitrogen, and emission of greenhouse gases such as methane and carbon dioxide (Sun et al. 2008, Petersen 2018, Alvarenga et al. 2015).
Higher concentration of heavy metals like Cu and Zn at 10-20 cm topsoil layer after continuous application of swine compost might be basically due to higher content of these metals in the compost (Zhao et al. 2005). This higher content of metal also leads to reduced activities of some important soil enzymes like dehydrogenase and urease (Yang et al. 2006). Use of technology for lowering metal concentration before composting of these materials can overcome this issue. Soil factors such as soil pH, cation exchange capacity, temperature, and moisture status of soil along with content of humic substances play vital role in binding of metals in soil. Production methods (aerobic/anaerobic) used for preparation of compost are important as they determine the degree of heavy metal complexion with the produced organics. Aerobic composting is found superior over anaerobic composting (Smith 2009). With long tune application, heavy metals were found to be detached from the complex, leading to contamination of soil profiles. Organic matter dynamics is reported to be responsible for this metal movement rather than the mineralization of applied heavy metal contaminated organics (Sukkariyah et al. 2005). Therefore, constant monitoring of heavy metal fractions and their contributions to total metal concentration is important.