EFFECTIVE PLANT GROWTH REGULATORS IN IMPROVING PHYTOREMEDIATION
Plant growth regulators are efficient in the removal of auxins, cytokinins, gibberellins, and salicylic acid. It is influenced by environmental stress and the physiological state of the plant. Each plant growth regulator has a significant role on the plant - auxins help in cellular growth (Ali et al. 2013, Israr and Sahi 2008), and cytokinins increases the adsorption process of the heavy metals (Cassina et al. 2011). Cytokinins reduce the heavy metals’ side effect in sunflowers and improve the plant to uptake the water and contaminants from the soil (Tassi et al. 2008). Gibberellin also acts effectively in water stress plants (Tuna et al. 2008). A combination of Ca and gibberellin increases the enzyme in a plant which aids in higher chlorophyll content, which can reduce the negative effects of Ni toxicity (Siddiqui et al. 2011). Salicylic acid brings down the copper toxicity of the plant, which leads to Cu phytoremediation (Afrousheh et al. 2015).
In Helianthus annus, cytokinins help in the accumulation of heavy metals (lead and zinc) in the stem and leaves, which can also contribute to increase in transpiration (Rostami and Azhdarpoor 2019, Tassi et al. 2008). Jasmonates acts as a messenger molecule to activate pathways involved in heavy metal stress condition in plants (Poschenrieder et al. 2008).
TECHNIQUES TO BOOST PHYTOREMEDIATION EFFICACY
Techniques are needed to improve phytoremediation efficiency to remediate high levels of accumulation. Ethylene diamine tetra acetic Acid (EDTA) is a chelating agent which increases the solubility of heavy metals which could increase the phytoremediation capacity of the plant (Bareen 2012, Nowack et al. 2006). The chelating agent is used in tough condition and when the accessibility of metal content is low in the soil, it helps in the absorption of metals. EDTA acts in the liquid form forming a soil-metal complex that transmits heavy metals to the aerial parts of the plant (Lasat 2002, Farid et al. 2013). The application of an electric field in the soil increases the heavy metal absorption capacity of the root (Kim et al. 2005). But it causes negative effects on the plant such as a change in the genetic material and physical characters. Apart from an external application, pH also contributes to the metal solubility, and low pH increases the solubility of metal in the soil. GMOs are also used in the phytoremediation of heavy metals, which could increase the efficacy of absorption. Bacteria can also degrade the contaminants present in the soil with the production of enzymes, and heavy metals are degraded by the production of chelating and acidifying agents by the bacteria which could possess high surface volume, enabling them to be used as a microbial chelating agent. Bacteria can also influence the pH of the soil, making it favourable for the degradation of the contaminants (Rostami and Azhdarpoor 2019). Plant growth-promoting hormones such as IAA, jasmonates, ethylene, gibberellins, and cytokinins can also increase the efficiency of phytoremediation with minimal side effects of the pollutants present in the plant.
ADVANCES IN PHYTOREMEDIATION TECHNOLOGY
Phytoremediation is a thriving field of lively research with its novelty and cost-effectiveness; being eco-friendly, aesthetic, and efficient, phytoremediation can be applied onsite, with remediation by solar-driven technology (Ali et al. 2013). Many researchers have used plant- assisted biosolids or microbes to enhance the efficacy of the phytoremediation process in removing heavy metals from contaminated sites (Kim et al. 2010, Rajkumar et al. 2012). Natural plants and genetically engineered plants with positive outcomes improve the remediation of heavy metals from contaminated soil (Chanu and Gupta 2016, Gomes et al. 2016). The analysis of the plants’ ability to tolerate and to accumulate metals and polyaromatic hydrocarbons (PAH), including their bioconcentration factors and correlations between pollutant types, suggests that P ter is vittata and P ter is cretica are suitable for the joint remediation of arsenic and PAHs; Boehmeria nivea for the simultaneous phytoremediation of lead, arsenic, and PAHs; and Miscanthus floridulus for the phytoremediation of copper and PAHs pollution. Ricimts communis plant is used for phytoremediation of metal-polluted sites as well as in phytoremediation of fly ash (Olivares et al. 2013, Pandey 2013). Eucalyptus globulus was found to phytoremediate cadmium polluted soil, which was subjected to nearly 30 years of e-waste disposal (Luo et al. 2015). The potential application of plant enzymes to enhance phytoremediation is greatly recognized by current world biotechnologists. Some of the challenges of phytoremediation, however, could be overcome through the development of the genetically engineered plants as well as endophytic microbes equipped with the property of overexpressing the enzymes competent of contaminant degradation (Tripathi et al. 2020).
Environment pollution has become a big threat, and bioremediation can play a vital role in cleaning up the contaminated site. Bioremediation is a process of remediating the environment with the help of microbes. Remediation strategies, such as chemical and physical methods, are not enough to diminish pollution problems because of the continuous production of novel recalcitrant pollutants due to anthropogenic activities. As physical and chemical approaches are tedious, bioremediation using microbes is acceptable and it is an eco-friendly and socially acceptable alternative to conventional remediation approaches. Many microorganisms with a bioremediation potential have been isolated and characterized but, in many cases, cannot completely degrade the targeted pollutant or are ineffective in situations with mixed contaminations (Dangi et al. 2018). Plants and microbes including bacteria, fungi, and algae have a high neutralizing ability in remediating the soil. Bioremediation using microbes is more acceptable, which mainly relies on the enzymes produced by them and takes part in the metabolic pathways. These microorganisms attack the contaminant and degrade them completely or convert them into less harmful products (Sivaperumal et al. 2017). Bioremediation using microorganisms has resulted in successes as well as failures. In the cases of failure of this method, the causes are generally time-consuming, low adaptability, lack of competitiveness of the microbes, and low bioavailability to the target pollutants (Singh et al. 2017). Autochthonous (indigenous) microorganisms present in the contaminated environments play a vital role in solving most of the challenges associated with biodegradation and bioremediation of contaminated sites (Verma and Jaiswal 2016). Bioremediation techniques can be categorized as ex situ techniques and in situ techniques. Pollutant nature, depth and degree of pollution, type of environment, location, cost, and environmental policies are some of the selection criteria that are considered while choosing any bioremediation technique (Frutos et al. 2012, Smith et al. 2015).
Ex situ Bioremediation Technique
Ex situ bioremediation is a process where contaminated soil or water is excavated from the environment and subsequently transporting them to another site for treatment. Ex situ bioremediation can use bioreactors and nutrients can be added to speed up the breakdown of anthropogenic pollutants (Christopher 2016).
Biopile-mediated bioremediation involves the piling of excavated polluted soil, followed by nutrient amendment, and sometimes aeration is provided to enhance the bioremediation process by increasing microbial activities. The basic pile system consists of a treatment bed, an aeration unit, an irrigation/nutrient unit, and a leachate collection unit (Fig. 3). The advantage of this particular ex situ technique is increasingly being well-thought-out due to its constructive features including cost-effectiveness, which enables effective biodegradation on the condition that nutrient, temperature, and aeration are adequately provided and controlled (Whelan et al. 2015).
The suppleness of biopile allows remediation time to be shortened as the heating system can be integrated into biopile design to increase microbial activities and contaminant availability thus increasing the rate of biodegradation (Aislabie et al. 2006). Furthermore, heated air can be injected into biopile design to deliver air and heat in cycles to facilitate enhanced bioremediation. It is also studied that humidified biopile had a very low final total petroleum hydrocarbon (TPH) concentration compared to heated and passive biopiles as a result of optimal moisture content, reduced leaching, and negligible volatilization of less degradable contaminants (Whelan et al. 2015). Soil biopiles can be up to 20 feet high. Biopile can be covered with plastic to control leachate runoff, evaporation leachate and volatilization of compounds as well as to enhance solar heating. Treatment time of biopile is typically 3 to 6 months, after which the excavated soil material is either returned to its original location or disposed of. Biopile treatment has been applied in the treatment of non-chlorinated VOCs, fuel-contaminated soil, chlorinated VOCs, semi-volatile organic compounds (SVOCs), and pesticides can also be treated, but effectiveness against each varies.
Figure 3 Construction of biopile (adopted from Menendez-Л ega et al. 2007).
Windrows is one of the ex situ technique which relies on the periodic turning of piled contaminated soil to enhance bioremediation by increasing degradation activities of indigenous and/or transient hydrocarbonoclastic bacteria present in polluted soil. The intermittent turning of polluted soil, together with the addition of water, boosts up aeration, and uniformly distributes contaminants, nutrients, and microbial degradative activities, thus enhancing the rate of bioremediation, which can be accomplished through assimilation, biotransformation and mineralization of pollutants. When compared to biopile treatment, windrows showed a higher rate of hydrocarbon removal; however, the higher efficiency of the windrow towards hydrocarbon removal depends on the soil type, which was reported to be more friable (Coulon et al. 2010). The employment of windrow treatment has been implicated in CH4 (greenhouse gas) emission due to intensification of the anaerobic zone within piled polluted soil, which usually occurs following reduced aeration (Hobson et al. 2005). Phytoremediation is an upcoming field in the research as it is cost-effective and eco-friendly (Ali et al. 2013). Plant-assisted biosolids are used or microbes are employed to increase the efficacy of phytoremediation (Kim et al. 2010, Rajkumar et al. 2012). GMOs and wild variety plants have reported being positive in remediating metal contaminated soil (Chanu and Gupta 2016, Gomes et al. 2016). The ability of Pteris vittata and Pteris cretica to tolerate and remediate hydrocarbons and heavy metals with their bioconcentration proved to be the best remediation. On the other hand, Boehmeria nivea remediates Pb, Ar and PAHs, and Miscanthus floridulus remediates Cu and PAHs. Ricinus communis is used for phytoremediation of metal-polluted sites as well as in phytoremediation of fly ash disposal site (Olivares et al. 2013, Pandey 2013). Eucalyptus globulus can remediate e-waste disposal contaminated with Cd which is 30 years old (Luo et al. 2015). Apart from these enzymes secreted by the plants, microbes also play a vital role in phytoremediation of the contaminated soil and it is greatly recognized in the field of biotechnology (Tripathi et al. 2020).
The bioreactor is an ex situ technique which contains a closed vessel which contains the raw materials and is converted into the desired products using biological reactions. There are different types of bioreactors used for different operations. It can support the growth of cell mimicking the natural environment. There are different parameters used in the bioreactor for the effective reduction of bioremediation time, important being the restricted bioaugmentation, the addition of nutrients, pollutant bioavailability in large amount and limited mass transfer for effective bioremediation. Benzene, VOCs and BTEX can be remediated in the bioreactor. GMOs can be used for bioremediation processes and can be destroyed after the process is over; however, biosurfactant cannot be treated in the bioreactor (Mustafa et al. 2015). The bioreactor cannot treat a very large amount of contaminants; therefore, the volume is restricted, and bioreactor-based bioremediation technique involves more capital and manpower (Philp and Atlas 2005). Another factor is that it requires monitored conditions; if it is not maintained properly, then the microbial activities decrease causing a decrease in the bioremediation process. But the main problem with the bioreactor-based bioremediation is the cost involved in pilot-scale bioremediation.
Land farming is one of the simplest ex situ bioremediation technique, which is cost-effective and requires fewer operations. It is widely used in the disposal of oil sludge, drill cuttings, and petroleum wastes. It uses clay at the bottom to prevent the pollutants from contaminating the water table (Kumar and Yadav 2018). Contaminants are tiled periodically for aeration; when it is tilled on the site, then it is called in situ treatment, otherwise it is an ex situ technique. The bioremediation process is carried out by the indigenous microbes present in the soil. While tilling the laud takes place, nutrients can be added into the soil, which can enhance the microbial activities to enhance the bioremediation process. In the absence of nutrients, it is found that heterotrophic bacteria take over remediating the diesel contaminants (Silva-Castro et al. 2015). Contaminants are degraded, transformed, and immobilized by biological processes by the action of microorganisms with pH, aeration and moisture content being monitored. There are a few disadvantages such as large working space and high capital. The major advantage of this method is that it does not need any preliminary testing of the land, is less laborious and has a faster remediation process (Christopher 2016).