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Role of biotechnology in remediation of soil pollutants

Biotechnology is being applied in recent years for decontamination of soils polluted with both organic and inorganic contaminants. Bioremediation technology (also known as environmental biotechnology) is an ecologically sound emerging tool and can be defined as the elimination, attenuation or transformation of polluting or contaminating substances by the use of biological processes, i.e., the use of natural strains of bacteria (Pal et al. 2010). Dining bioremediation, microbes utilize chemical contaminants in the soil as an energy source and, through oxidation- reduction reactions, metabolize the target contaminant into useable energy for microbial growth. By-products (metabolites) released back into the environment are typically in a less toxic form than the parent contaminants. For example, petroleum hydrocarbons can be degraded by microorganisms in the presence of oxygen through aerobic respiration. The hydrocarbon loses electrons and is oxidized, while oxygen gains electrons and is reduced forming carbon dioxide and water (Nester et al. 2001). Because of then appearance in nature, the population of the strains explodes which drives the process of breakdown of hazardous wastes or pushes the bioremediation process forward. These bacteria increase in number when a food source, i.e., the waste is present. When the contaminant is degraded, the microbial population naturally declines with the production of less harmless products (Sen and Cliakrabarti 2009). Bioremediation of contaminated soil is carried out at the place of contamination or in a specially prepared place. Bioremediation techniques that are applied to soil at the site with minimal disturbance are referred to as in situ, whereas ex situ techniques are applied at the site which has been removed via excavation soil (Dzionek et al. 2016).

In situ bioremediation

This method facilitates treatment of polluted soils at the site of contamination, thereby minimizing the expenses for excavation, and many effects on the integrity of soil contaminants before treatment (Azubuike et al. 2016). It is cost-effective and less labor intensive as compared to other methods of bioremediation. In situ bioremediation is a promising eco-friendly solution for complete transformation of hazardous soil pollutants into nou-hazardous forms without affecting ecological communities (Perelo 2010).

This method is based on strategies that decide the best-fit bioremediation techniques for polluted soils, considering environmental conditions at the polluted site, bioavailability of the soil contaminants, and the limiting factors during the remediation processes (Brown et al. 2017). In situ bioremediation techniques are grouped as ‘intrinsic bioremediation’ and ‘engineered bioremediation’, the former takes place without any artificial manipulation or stimulation during the treatment and the latter occurs in a manipulated or stimulated treatment environment. In intrinsic method, natural monitored attenuation (NMA) plays a significant role in keeping track of degradation activities without human intervention hr a naurral and undisturbed treatment environment (Brown et al. 2017). These amendments are foundational basis of various bioengineering techniques applied during in situ biorenrediation. NMA is one such strategy that ensures active degradation of soil pollutants naturally in the polluted sites. It involves activities like allowing natural sedimentation, and biochemical transformation of soil contaminants at site itself, thereby reducing then bioavailability (Perelo 2010). However, extensive monitoring is required in NMA to obtain results within a pre-defined time period. Other strategies include incorporation of various amendments to the polluted soils, installation of injection w'ell for subsurface soil contaminants, spray irrigation for shallow' soil contaminants and recirculation of treated gr ound water from polluted sites (Brown et al. 2017). Both nutrients and mediator compounds that enhance degr adation of soil contaminants are used as amendments. Nutrient amendments also instigate effective degradation of soil contaminants by microorganisms (Brown et al. 2017). In engineered biorenrediation approach, the polluted topsoil is capped or encapsulated with moisture proof material to prevent spreading and diffusion of contaminants (Lee and Lee 1997, Liu et al. 2018). Modified conventional in situ biorenrediation technologies can effectively treat the polluted soils with minimal effort. It includes treatment of contaminated soils at pollution site in combination with interdisciplinary understanding of scientific and engineering techniques. These techniques encompass a wide array of multidisciplinary applications such as biotechnological, biophysical, biochemical and bioengineering. Some of the in situ biorenrediation techniques are broadly discussed below.

4.1.1 Biosparging

In this technique, subsurface soil (saturated zone) or ground water table is injected with oxygen (air sparging) and occasionally nutrients to improve natural degradation of soil contaminants (Johnson et al. 2010). Biosparging is used to treat petroleum waste or hydrocarbons in contaminated soil and ground water (Kao et al. 2008). Ah' sparging promotes volatilization of organic compounds and nutrients to obtain enhanced microbial degradation of the organic compounds in the subsurface soils. Biosparging enables vertical movement of volatile organic compounds from saturated to unsaturated zone in the soil for effective volatilization as well as microbial degradation (Azubuike et al. 2016). This technique does not much affect the treatment site and requires a time period of 6 months to 24 months for complete treatment of the contaminated soils under favorable conditions (EPA 2004). The efficacy of biosparging depends on soil texture and structure, soil biophysicochemical properties and density of microbial population in the subsurface soil (EPA 2004).

4.1.2 Bioventing

This technique may be considered as a part of or modified biospargiug technique that adds or injects oxygen (an sparging) under a controlled flow rate. Oxygen is injected in the unsaturated zone, also known as vadose zone, to improve and sustain microbial degradation of contaminants (Azubuike et al. 2016). Bioventing involves ah injection with a maintained flow rate vis-a-vis high rate or low rate that helps in obtaining uniform distribution of air hi the vadose zone. Therefore, bioventing may be applied to treat recalcitrant compounds like chlorinated hydrocarbons through partial oxidation under aerobic conditions (Azubuike et al. 2016, Philp and Atlas 2005). Bioventing is used to treat soils contaminated with petroleum waste, hydrocarbons, pesticides and organic chemicals, mostly organic compounds that could be degraded under aerobic conditions (Hoeppel et al. 1991, Hellekson 1999, EPA 2004). Bioventing depends on soil physicochemical factors such as moisture, permeability, stratification, temperature, etc. (Brown et al. 2017).

4.1.3 Bioaugmentation

This technique involves introduction or addition of natural microbial consortium or genetically modified microorganisms into the contaminated soils (Mrozik and Piotrov'ska-Seget 2010). Bioaugmentation is useful hi degrading a wide range of inorganic and mostly organic compounds such as aromatic hydrocarbons, chlorinated compounds, and petroleum hydrocarbons, and is also most effective in degrading recalcitrant compounds that are highly resistant to degradation (Mrozik and Piotrov’ska-Seget 2010, Nzila et al. 2016). It has some major challenges which include (i) survival of microorganisms or acclimatization of the microorganism in the new' environment after injection, and (ii) uniform distribution of microorganisms, availability of oxygen and nutrients or interference by umvanted growth near the injection site (da Silva and Alvarez 2010). New'er approaches to bioaugmentatiou have been developed by combined bioengineering techniques which include encapsulation of microbial consortium in alginate, inoculation of microbial strains in the rhizospliere of plants or genetically modified plants incorporating microbial genes (Gentry et al. 2004). Overall bioaugmentation is a feasible in situ technique with minimal complexity that leads to faster degradation of soil contaminants. It depends on the nature of microbial strains, soil physicochemical properties and bioavailability of the contaminants (da Silva and Alvarez 2010, Brown et al. 2017, Federici et al. 2012).

4.1.4 Bioslurping

This technique involves bioventing and vacuum extraction of free products of ground water and subsurface soils (Brown and Ulrich 2014). Basically, it is a vapor extraction process that also removes slugs and liquid droplets with the help of a sluip tube from vadose zone. Bioshirping is accompanied by bioventing to provide aeration for degradation of various soil contaminants (Miller 1996). It is very effective in removing petroleum hydrocarbons, primarily light non-aqueous phase liquids (LNAPL), that are found on the top of ground water table and capillary fringe, i.e., the region immediately above the saturated zone holding water in soil capillaries (Place et al. 2003). Combined bioshupiug and bioengineering methods have utilized DNA microarrays to identify and investigate genes that are responsible for degradation of aliphatic and aromatic hydrocarbons. Based on this genetic information, bio slurping and bioventing are performed (Kim et al. 2014). Bioshupiug depends on soil physicochemical properties, height of ground water table and soil porosity. Therefore, it has certain limitations like inadequate soil moisture and soil permeability tend to diy the soil, slow down biodegradation of the contaminants and reduce effectiveness of bioventing. Biolsurping focuses on removal of toxic products and most of the extraction products are removed or discharged from vadose zone without treatment. It is less effective in aerobic degradation of recalcitrant chlorinated compounds in absence of a co-metabolite. However, bioslurping is considered as a cost-effective technique as it pumps lower quantity of ground water, minimizing additional costs of storage, treatment of vapor and free product and disposal of other extraction products (Pliilp and Atlas 2005, Held and Doit 2000, Azubuike et al. 2016).

4.1.5 Phytoremediation

In this method, removal and degradation of soil contaminants are carried out by plant uptake at the pollution site. One of the following actions take place during this process: (i) toxic soil pollutants are extracted by the plants and translocated to different parts (ii) degraded into less toxic forms (iii) volatilized as vapors (iv) stabilized in the rhizosphere zone degraded by phytoeuzymes (Ojuederie and Babalola 2017). Phytoremediating plants have excellent morphology and physiology for liyperaccumulating heavy metals and degrading toxic organic pollutants. Some of these plants are—Aeolanthusbioformifollus, Lenina minor, Vigna radiata, Larreatridan-tata, Bacopa monnieri, Brassica jimcea, Alyxiarubricaulis, Macademianeurophylla, Thlaspicaerulescens, Lenina minor, Pistia stratiotes, Eichhorniacrassipes, Hydrilla rerticillata, Phyllanthus serpentines, Lenma minor, Salvinia molesta, Spirodelapolyrhiza, Hanmaniastrumrobertii, etc. (Ingle et al. 2014). Phytoremediation is comprehensively discussed hi Section 3.2.

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