Bioremediation of Organic Pollutants

Abstract

Organic pollutants are a global problem because sediments act as sinks for hydrophobic, recalcitrant and hazardous compounds. Depending on biogeochemical processes these hydrocarbons are involved in adsorption, desorption and transformation processes and can be made available to benthic organisms as well as organisms in the water column through the sediment–water interface. Most of these recalcitrant hydrocarbons are toxic and carcinogenic, they may enter the food-chain and accumulate in biological tissue. Organic pollutants may be treated through physical and chemical processes, but these processes are toxic and not environmental friendly because their final product may still remain toxic till the very end. Hence, the biological approaches may be a suitable alternative towards bioremediation practices being not only cost effective but eco-friendly as well. Moreover, the final product happens to be less toxic as compared to other approaches. The microorganisms and plants (bioremediation) are used to remediate the polluted environments widely and is emerging as a promising and appealing area of environmental biotechnology. This paper provides a review on new approaches with emphasis on bioremediation, like Biostimulation, bio augmentation and phytoremediation applied to sediments. These new techniques promise to be of lower impact and more cost efficient than traditional management strategies.

Keywords: Bioremediation; Organic Pollutant; Phytoremediation; Bioremediation; Remediation technologies

Table of Content

• Introduction 
  • Origin and Occurrence of Organic pollutants
  • Organic pollutants: Types and properties
• Remediation strategies for organic pollutants
  • Physical processes
  • Chemical Processes
  • Biological processes
• Types of organic pollutants and Bioprocess for Remediation 
  • Petroleum hydrocarbons 
    • Hazards of Petroleum hydrocarbons contamination
    • Remediation strategies for Petroleum hydrocarbons
    • Physicochemical strategies
    • Bioremediation of Petroleum hydrocarbons
  • Poly Aromatic Hydrocarbon 
    • Bioremediation of Poly Aromatic Hydrocarbon 
  • Polychlorinated biphenyls 
    • Methods for PCBs Removal 
      • Bio augmentation
      • Anaerobic dehalogenation 
  • Pesticides
    • Harmful effects of pesticides on human health 
    • Bioremediation strategies for pesticides 
  • Plastics 
    • Bioremediation of Plastics 
    • Biofragmentation 
    • Biofilms and Plastic Biodegradation
    • Microbial degradation of plastic
    • Mealworms and Plastic Biodegradation
    • Plastic wastes control and management strategies
• Conclusion
• References 

1.    Introduction

Any unwanted substance introduced into the environment is referred to as a ‘contaminant’. Deleterious effects or damages by the contaminants lead to ‘pollution’, a process by which a resource (natural or human-made) is rendered unfit for use, more often than not, by humans. Pollutants are present since time immemorial, and life on the earth. With pollutant analogues from geothermal and volcanic activities, comets, and space dust which are about 100 t of organic dust per day, the earth is forever a polluted planet (Marcano et al., 2003). Bioremediation, a nondestructive, cost- and treatment-effective and sometimes logistically favorable cleanup technology, attempts to accelerate the naturally occurring biodegradation of contaminants through the optimization of limiting conditions. Biodegradation is the metabolic ability of microorganisms to transform or mineralize organic contaminants into less harmful, non-hazardous substances, which are then integrated into natural biogeochemical cycles(Margesin and Schinner 2001). Relative to the pre-industrialization era, industrialization and intensive use of chemical substances such as petroleum oil, hydrocarbons (e.g., aliphatic, aromatic, polycyclic aromatic hydrocarbons (PAHs), BTEX (benzene, toluene, ethylbenzene, and xylenes), chlorinated hydro-carbons like polychlorinated biphenyl (PCBs), trichloroethylene (TCE), and perchloroethylene, nitro aromatic compounds, organophosphorus compounds) solvents, pesticides, and heavy metals are contributing to environmental pollution(Megharaj, Ramakrishnan et al. 2011). Large-scale pollution due to human-made chemical substances and to some extent by natural substances is of global concern now. Seepage and run-offs due to the mobile nature, and continuous cycling of volatilization and condensation of many organic chemicals such as pesticides have even led to their presence in rain, fog and snow (Dubus et al., 2000). Every year, about 1.7 to 8.8 million metric tons of oil is released into the world’s water. More than 90% of this oil pollution is directly related to accidents due to human failures and activities including deliberate waste disposal (Zhu et al., 2001). 

Aquatic sediments are repositories of physical and biological debris and act as sinks for a wide variety of organic and inorganic pollutants. Chemical contaminants present in the aquatic ecosystem may be immobilized and accumulated in sediments or may be subject to transformation and activation processes. Hydrocarbons may enter the aquatic ecosystem either directly, by effluents or spills, or indirectly by terrestrial runoff or atmospheric deposition. Their persistence in the environment depends mainly on their chemical and physical characteristics. The more complex their structure, the more halogenated and hydrophobic they are, the more these pollutants tend to accumulate in sediments associated to particulate material therein. Hydrocarbons can become dangerous especially if they enter the food-chain, since several of the more persistent compounds, as PAHs and PCBs are carcinogenic(Perelo 2010).

1.1.Origin and Occurrence of Organic pollutants

The late 1800s and early 1900, has witnessed a dramatic increase in the range of chemically synthesized products which include pesticides, plastics, hydrocarbon fuels, soaps, detergents and other useful substance. The effects of these chemical substances on the environment are a consequence of a sequence of processes that depend on the properties of individual chemical. Halogenated organic pollutants (HOPs), such as polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), polybrominated diphenyl ethers (PBDEs), dechlorane plus (DP), and decabromodiphenyl ethane (DBDPE), have been of great concern due to their persistence, bioaccumulation and potential toxicity to wildlife and human. PCBs were used primarily as dielectric electric fluids PBDEs (including Penta-, Octa and Deca-BDE commercial formulations), DP and DBDPE are some widely used same retardants in and coolant fluids in capacitors, transformers and foams and building materials. Among them, Penta- and Octa-BDE technical mixtures have been added to the list of emerging POPs by the Stockholm Convention in

2009 and Deca-BDE technical mixtures have been phased-out in Europe and America, while DecaBDE, DP and DBDPE are still widely used in China(Ratnakar 2016).

Organic pollutants originate from diverse sources, which can be summarized in the following categories:

• Anthropogenic (industrial chemicals);
• Petroleum inputs;
• Incomplete combustion of fuels;
• Forest and grass fires
• Biosynthesis of hydrocarbons by aquatic or terrestrial organisms
• Diffusion from mental, petroleum source rocks or reservoirs(Perelo 2010).

Fig.1 Major Sources of pollution

1.2.Organic pollutants:  Types and properties 

Organic pollutants are chemical compounds that contain carbon and have a demonstrably negative effect on one or more components of the environment. Organic pollutant can be placed into three general classes: (i) hydrocarbons, (ii) oxygen, nitrogen and phosphorus compounds and (iii) organometallic compounds. The major category of organic pollutants includes the hydrocarbons and related compounds, which contains such compounds as Dichloro Diphenyl Trichloroethane (DDT), the dioxins and the polycyclic aromatic hydrocarbons (PAHs). These compounds contain the elements of carbon and hydrogen, with some containing chlorine and oxygen as well. There are a limited number of types of chemical bonds present, which are principally C-H, C-C, C-Cl, C=C and C=C (aromatic). All of these bonds are relatively stable and have limited polarity and this property is then conferred onto the related compounds.

 Owing to low polarity, hydrocarbons, in general are lipophilic, poorly soluble in water and persistent in the environment. 

 The organometallic group is considered the least important in view of environmental perspective and includes compounds which may be combinations of metal, such as lead and tin, with organic components based on carbon(Ratnakar 2016).

2.    Remediation Strategies for Organic Pollutants

Organic chemicals that are introduced into the environment are subjected to various physical, chemical, and biological processes which act in an interconnected way in environmental systems to determine the overall fate of the compound. The neutralization when done through chemical means, a huge amount of acid is used, which is neither economical, nor safe and poses serious health hazard. There are many processes for the degradation of organic pollutants. Some processes for degradation of organic pollutant are listed below:

• Physical Processes

Physical processes have been used for the degradation of organic pollutant from many decades, which may include various processes like photocatalytic degradation by using Ag-modified Zn ₂ GeO ₄ nanorods, TiO ₂ /graphene oxide nanocomposite hydrogels, Bio-silica coated with amorphous manganese oxide etc. Decomposition of these organic pollutants via catalytic/ photocatalytic oxidation is considered to be the most efficient green method for organic waste management. TiO₂ used as a photo catalyst because of its low cost, chemical stability, non-toxicity. TiO₂ is preferred because it is a promising photo-oxidation catalyst and has strong oxidizing ability of photo-induced holes(Sharma , 2012).  

• Chemical Processes

The chemical methods for bioremediation include electrochemical dehalogenation of chlorinated benzenes, in this the chlorine is eliminated step by step from the highly chlorinated benzenes to yield less-chlorinated benzenes and finally transform to benzene. The catalytic degradation of organic molecules through MnO₂ nanostructures are concerned, several groups reported the mineralization of various organic compounds/ dyes, such as Rhoda mine B (RhB), MB, Benzyl alcohol (BA) in the presence of strong oxidizing agents at elevated temperature(Ratnakar 2016).

• Biological Processes

Bioremediation of organic pollutant contaminated soil offers a cost-competitive treatment for many sites that are currently facing costly incineration or the extended liability of land disposal. In the field, under conditions of full-scale site remediation, this technology has been shown to be cost effective. Different types of biological processes include bio-attenuation, bio-stimulation and bio augmentation (Seech et.al, 2008).

3.  Types of Organic Pollutants and Bioprocess for Remediation

There are different types of organic pollutants and their bioremediation technologies for their degradation are listed below:

3.1.  Petroleum Hydrocarbons  

Petroleum hydrocarbons are one of the most frequently encountered pollutants in soil habitats due to the increased usage of petroleum products and the seemingly increasing probability of accidents. The most noticeable sources of contamination are releases from manufacturing and refining installations, oil-tanker spills and accidents during transportation of the oil. Some major accidental spills in last few decades worldwide starting with Torrey Canyon in 1969; Exxon Valdez in Prince William Sound, Alaska in 1989, history biggest ever oil spill during Gulf War in 1991 (Fayad and Overton, 1995) and British Petroleum Deep Water Horizon Oil spill in 2010 happened and damaged aquatic environment as well as terrestrial environment. In Pakistan scenario, oil pollution has not been well reported due to one or other reasons except the major accidental spills which comes under the eyes of electronic or publishing media. Examples of such major accidental spill in Pakistan may be led by Tasman Spirit, a Greek Ship, which off grounded in 2003 and contaminated not only aquatic environment but also several kilometers of coastal area of Karachi. The citizens of the same city, the Karachi, again faced a major accidental oil spill five years later in 2008 while pipeline of Pak Arab Refinery Corporation (PARCO) rupture bathed the streets and homes of Korangi Town Karachi with several tons of crude oil (Alam, 2008) and in the following year collision of two cargo trains spilled 18 tankers of crude oil in the same province (Khan, 2009).   Most recently, another huge oil spill happened when two NATO oil tankers each bearing 60,000 liters’ gasoline toppled on Khojak Pass near Chaman border, Pakistan. Other than accidental spills, sources of contamination may be storage tanks due to non-maintenance and absence of proper vandalism (Adam, 2001) and oily sludge unavoidably produced by refining industries throughout the country (National Environmental Policy of Pakistan, 1999). Other source of petroleum hydrocarbon contamination spreaded all around the country are petrol pumps and service stations which has not been reported by any environment regulating agency. 

3.1.2. Hazards of Petroleum Hydrocarbons Contamination 

Petroleum hydrocarbons contamination affects almost all kind of life adversely. As for as human health is concerned, petroleum hydrocarbons and its various fractions have deleterious effects. Oil spills can occur in marine and terrestrial environments and threat the ecosystem and human health (Cheng et al. 2017). This environmental contamination can pollute the drinking water, cause fire and risk of explosion, ruin the water and air quality, destroy the recreational areas, waste the nonrenewable resources, and have huge economic costs. Oil spill’s negative impacts have different economic, environmental, and social aspects. For instance, upon exposure to gasoline, one of the distillate of crude oil, causes serious health hazards to human being such as ethylbenze cause irritation in respiratory system, skin and eyes. Upon inhalation, gasoline may cause drowsiness, nausea and numbness (Baniasadi and Mousavi 2018).

3.1.3. Remediation Strategies for Petroleum Hydrocarbons

Numerous technologies for the remediation of petroleum hydrocarbons have been developed. Mainly three types of technologies; physical, chemical and biological have been used for cleaning up of petroleum contaminated soils. The schematic diagram is given below.

Fig.2 Remediation Strategies of hydrocarbons

3.1.4. Physicochemical Strategies

Physicochemical strategies are means of destroying or separating contaminants abiotically from the environment. Although these techniques are well established and have higher efficiency in recovering minutes, most of them are too costly or add secondary pollutants in the environment. For example, incineration or thermal destruction removes contaminants with 99% efficiency but it is not only costly but also become the source of secondary pollution due to incomplete combustion and releasing volatilized compounds into air. Similarly, solvent extraction, an ex situ chemical method for the removal of organic and non-aqueous contaminants by separating and concentrating and subsequent incineration has recovery efficiency as high as 10,000:1, but costly and generate secondary pollutant in the environment (Helmy et.al 2015). Soil washing is done by using scrubbing action of water supplemented sometimes with surfactants. This technique is only pretreatment which is followed by successive treatments such as incineration and thermal desorption that make the cleaning process too costly and laborious. As these techniques bear drawbacks of economics and public non-acceptance, so the scientific community shifted its attention towards seeking of some alternatives and biological approaches has been under focus since 1990’s extensively (Somayeh Emami 2012).

3.1.5. Bioremediation of Petroleum Hydrocarbons

Bioremediation approaches, i.e. using selected indigenous microbial organisms to degrade hydrocarbons, are currently receiving favorable publicity because bioremediation is environment friendly and cost effective. Among those microorganisms, the genus Pseudomonas, particularly P. putida F1 is one of the most well-studied hydrocarbon degrading bacterial strains and having approved the capability to utilize organic compounds from the generic group aliphatic, cyclo aliphatic, aromatic and polynuclear aromatic hydrocarbons (Farhadian et al., 2008).

• A strain of Bacillus subtilis is a good degrader of both hydrocarbons with degradability of 98% n-hexadecane and 75% naphthalene(Wang, Zhang et al. 2011). White rot fungi are reported to be able to degrade compounds such as polychlorinated biphenyls (PCBs) and PAHs (Baniasadi et al. 2018). Some yeasts including Candida sp., Pichia sp., and Yarrowia sp. also reported to have the potential to degrade the compounds available in oil contaminants (Jain et al. 2011).
• Natural attenuation processes involve contaminant attenuation to harmless products through natural processes, such as microbial degradation, volatilization, sorption and immobilization. The natural attenuation process is contaminant specific and commonly employed for petroleum hydrocarbon contaminated sites(Megharaj and Naidu 2017).
• Bio augmentation is the process in which specific microorganisms are introduced to decontaminate the soils when indigenous microbes are not efficient is considered a more acceptable approach to remediate the contaminated soils. Bio augmentation has been proven to be successful for a wide range of pollutants including pesticides such DDT, lindane, endosulfan, pentachlorophenol (PCP), polyromantic hydrocarbons (PAHs) and total petroleum hydrocarbons(Megharaj and Naidu 2017).
• Land farming, also known as land treatment or land application, is an upper-ground remediation technology for soils to reduce concentrations of petroleum constituents during biodegradation. This technology usually involves spreading excavated contaminated soils in a thin layer on the ground surface and stimulating aerobic biological activity in the soils during aeration and the addition of minerals, nutrients, and moisture. The enhanced biological activity results in degradation of adsorbed petroleum product constituents during microbial respiration(Somayeh Emami 2012).
• Composting is a process through which organic materials are degraded, or eaten, by microorganisms, consequential in the production of organic and inorganic by yield and energy in the form of heat. Composting by using hydrocarbon contaminated soil cocomposted with cow manure and mixed vegetable waste showed that more than 90% of the hydrocarbons including some of the recalcitrant components were removed(Pontes, et.al. 2013).
• Bio surfactants are produced by a variety of oil-degrading microorganisms. These bio surfactants can be of low molecular weight, acting by decreasing the oil–water interfacial tension, or high molecular weight and act as bio dispersants by preventing coalescence of oil drops in water. The high molecular weight bio emulsifiers are hetero polysaccharides, and the active components are lipids or proteins. The activity of bacterial bio surfactants in bioremediation stems from their ability to increase the surface area of hydrophobic waterinsoluble substrates and to increase the solubility and bioavailability of hydrocarbons. They can be added to bioremediation processes as purified materials or in the form of bio emulsifier-overproducing bacteria. In either case, they can stimulate the growth of oildegrading bacteria and improve their ability to utilize hydrocarbons(Ron and Rosenberg 2002).

3.2.    Poly Aromatic Hydrocarbon 

PAHs are much toxic and have potential risk hazards to human life and the surrounding environment. Therefore, they need to be mineralized to basic building blocks. However, since PAHs are recognized as persistent pollutants, so degradation is very to negligibly slow. Tough several bacteria have been isolated to degrade PAHs, the degradation rate is much lower than the production of PAHs from different sources(Mougin 2002). Having high toxicity, various conventional degradation methods of PAHs are electro-ultra-sonic remediation, thermal incineration, landfilling, and natural and synthetic surfactants. However, biodegradation of PAHs is gaining much attention due to the extravagant cost of the aforementioned conventional methods and environmental concerns(Ahmad, Zhu et al. 2020).

3.2.1. Bioremediation of Poly Aromatic Hydrocarbon

• Bacterial strains are used for degrading PAHs that taxonomically diverse, belonging to genera Pseudomonas, Mycobacterium, Sphingomonas, Alcaligenes, and Bacillus(CastroGutiérrez and M. 2012).
• Rhizodegradation is a promising alternative in remediating PAHs in soil due to its low cost and low environmental burden. This complex process capitalizes on the ability of plant to stimulate the growth and activity of rhizosphere bacteria in order to degrade PAHs in the soil(Dominguez, Inoue et al. 2020).
• Phenanthrene (PHE) is a polycyclic aromatic hydrocarbon (PAH) which was produced from incomplete combustion of hydrocarbons and fossil fuels and can cause harmful biological effects. A halo-tolerant bacterium Bacillus kochii strain AHV-KH14 isolated from municipal compost are used for the bioremediation of PHE from the contaminated soil(Feizi, et al. 2020).
• PAHs degradation with ligninolytic fungi by an incorporation of epoxide hydrolases, cytochrome P450 monooxygenases, and ligninolytic enzymes, which led to complete mineralization of the hazardous compound (Bezalel, et al. 1997).

3.3.  Polychlorinated Biphenyls

Polychlorinated biphenyls (PCBs) are a group of xenobiotic composed of biphenyl molecule containing from one to ten chlorine. They are oily fluids with high boiling point, high chemical resistance, low electrical conductivity, flame resistance, and high refractive index. Because of these properties, they have been used mainly as insulators in electrical transforms and capacitors, as heat exchange fluids, plasticizers, adhesives and lubricant (Tomotada, et.al. 2001). Due to their inertness, PCB contamination is still widespread in all types of ecosystems and affects both natural environments and wildlife. They have been found in human’s tissues, blood, and breast milk and are introduced via the consumption of meat, fish, and dairy products. Consequentially, they have been linked to chronic effects in humans including immune system damage, decreased pulmonary function, bronchitis, and interferences with hormones leading to cancer. Therefore, the clean-up of PCB- contaminated sites has become a priority of great relevance due to the teratogenic, carcinogenic and endocrine-disrupting features of these xenobiotic(Stella, Covino et al. 2017). 

3.3.1. Methods for PCBs Removal

• Biostimulation

Biostimulation is a process to add the nutrients or substrates to a contamination site to stimulate the activities of microorganisms. For the cases of anaerobic degradation of PCBs,

Biostimulation of PCB indigenous dechlorination bacteria can be achieved by halopriming with halogenated aromatic compounds such as halogenated benzoates. The chlorine atoms of PCB congener are replaced with hydrogen during this process. Biostimulation can also apply to aerobic bioremediation that the microorganisms use oxygen to breakdown the low-chlorine content PCBs. 

During the aerobic degradation, the benzene ring with less chlorines of the PCB molecular is destructed(Jing R 2018).  

• Bio augmentation

Bio augmentation, a feasible, in-situ, PCB transformation pathway, is defined as the addition of bacterial cultures to accelerate the degradation rate of a contaminant. Bio augmentation is mainly applied in sediment, soil, and solid phases.

• Phytoremediation

Phytoremediation has been recognized as an ecologically responsible alternative for removing organic pollutants in soils, water and sediments. Phytoremediation processes is the biodegradation of organic pollutants in soil or groundwater and uptake into plant tissues through their roots followed by transformation by plant enzymes or direct volatilization into the atmosphere. Plant species used for PCB phytoremediation including Medicago sativa (alfalfa), Lespedeza cuneate (Chinese bushclover), Lathyrus sylvestris (everlasting pea), Phalaris arundinacea (reed canary grass), Cucurbitaceae (cucurbits), Sparganium (bur-reed), Salix alaxensis (Alaska willow).

• Microbial Degradation

Microbial degradation is a natural biological process that relies on microorganisms (e.g., bacteria, fungi) to degrade, break down, transform, and remove contaminants or hazardous materials. Many bacterial strains are capable of oxidative degradation of PCBs such as Pseudomonas, Burkholderia, Comamonas, Rhodococcus and Bacilus. Fungi that are used for biodegradation of PCBs are Aspergillus niger that degrade lower chlorinated PCB’s, and White rot fungi degrade lignin (PCB) at low concentration with the help of ligninases. Marine bacteria, Pseudomonas aeruginosa are able to degrade various congeners of PCBs and can efficiently remove 70% of cadmium from the growth medium along with mercury from freshwater(Sarkar , 2006).  

• Anaerobic Dehalogenation

Degradation of highly chlorinated PCB congeners is generally achieved by organohalide respiration under anaerobic conditions. Organohalide respiration of PCBs is a biological process that potentially decreases the toxicity of PCBs through the removal of chlorines. During this process the chlorine substituent is replaced with hydrogen. The PCB congeners serves as the terminal electron acceptor with three potential chlorine substituent positions; para, meta, and ortho. Substitution of a chlorine with a hydrogen atom preferentially occurs at the first two sites. The potential pathway for anaerobic dehalogenation of a highly chlorinated PCB congener is illustrated in Figure 3 (Jing R 2018).

Figure.3: Possible dechlorination pathway of 2,3,4,5,6-Pentachlorobiphenyl and the distribution of product.

3.4.  Pesticides 

Pesticides are the chemical substances use to kill or manage pests at tolerable levels. The extensive use of pesticides has resulted in serious environmental as well as health problems besides has effected biodiversity as well. The pesticide contamination of surface and ground water pose a serious threat to surrounding ecosystems (Cynthia G, et.al 2004).  Pesticide consumption in world has reached to 2 million tons, from these 2 million tones Europe utilizes 45% followed by USA 24% and rest 25% in rest of the world. Pesticide consumption in Asia is also alarming. China uses highest percentage followed by Korea, Japan and India. (Uqab B 2016). At present, India is the largest producer of pesticides in Asia. (Randhawa and Kullar 2011).

3.4.1 Harmful Effects of Pesticides on Human Health

Ideally a pesticide must be lethal to the target pests, but not to non-target species, including humans. Pesticides constitutes potential occupational hazards and environmental risks for ecosystems. Besides, pesticide residues retained in crops can directly influence public health via food consumption. According to their toxicity, the most toxic pesticides are organophosphorus (OP), carbamate (CB), and OC pesticides. These compounds are powerful chemicals that act primarily by disrupting nervous system function.People with chronic OP exposures develop a pesticide-related illness, which includes symptoms as nausea, headache, dizziness, blurred vision, abdominal pain, vomiting.Exposure to CB insecticides is associated with the development of respiratory diseases.Organochlorine insecticides constitute a serious environmental problem and considerable risks for human health. This is because its biological degradation is difficult, they are highly soluble in lipids (and consequently bio magnify in the food chain), and they are persistent in the environment because of its chlorinated nature(Alvarez, et al. 2017).

3.4.2. Bioremediation Strategies for Pesticides

The level of toxicity caused by the pesticides leads to the great need for bioremediation. No doubt in some cases intrinsic bioremediation occurs because of microbes that are already present in polluted ecosystems, but it is also true that in some cases intrinsic bioremediation is not adequate (Anderson, et al.1994). 

• Biological remediation is an attractive technology that results in the complete conversion of organic compounds into less harmful end products such as CO2 and H2O. There are basically three types of bioremediation with microorganisms: remediation through improved natural attenuation (taking advantage of the natural capacities of the microorganisms present in the matrix), bio augmentation (introduction of non-native and genetically modified microorganisms), and bio stimulation (addition of electro acceptors or nutrients are used for degradation of pesticides (Marican, 2018).
• The minor structural changes that fungi do to degrade pesticides and render them into nontoxic substances and release them into soil where it is susceptible to further degradation. White-rot fungi are also used for degradation of Heptachlor atrazine, terbuthylazine, lindane, metalaxyl, chlordane mirex, dieldrin, diuron, Aldrin, DDT (Uqab B, 2016). Bacteria species that degrade the pesticides belongs to genera Flavobacterium, Arthobacter, Aztobacter, Burkholderia, and pseudomonas(Radhika 2014). The various fungi and bacterial strain which have shown ability to degrade pesticides are listed in table. 
• Aquatic plants Leman minor, Elodea Canadensis and Cabomba aquatic, to remove and assimilate three pesticides: copper sulphate (fungicide), flazasulfuron (herbicide) and dimethomorph (fungicide). Monoraphidium braunii, green algae, appeared to be a promising species for the phytoremediation of waters from biphenyl A and also at high levels of contamination found in surface waters.(Esteban Sánchez, et.al. 2014).
• Composting involves the mixing of the contaminated soil in a pile with a solid organic substrate, which serves as a carbon source for the indigenous aerobic soil microorganisms. Composting is a means for the remediation of pesticide contaminated sites and several large companies, such as W.R. Grace and Astra Zeneca, have developed and patented successful composting technologies. The piles may also be kept anaerobic by covering them with plastic sheets and encouraging the aerobic microorganisms to utilize all of the oxygen remaining underneath. Once the oxygen in the pile has been depleted, anaerobic microorganisms will become active, degrading the organic pollutants that were nondegraded by the aerobic microbial population(Tarla, et al. 2020).
• Land farming of soils containing pesticides involves spreading the solids over a land area that is designed for bioremediation of the pesticide. Land farming of pesticides were established to enhance the remediation. The spreading of the contaminants over a large area dilutes the pesticides. Biodegradation is the desired result, but there may also be loss by volatilization. This method is appropriate for pesticides that are well known to be biodegradable. It should not be used for persistent pesticides(Tarla, et al. 2020).
• Pseudomonas plecoglossicida is a novel organism for bioremediation of hazardous compounds like cypermethrin and chlorpyrifos (Organophosphate insecticide) by Pseudomonas aeruginosa. These microorganisms obtained from cow dung though have the ability for bioremediation in laboratory setups, can also be applied in pesticide contaminated soil and water (Randhawa, et.al. 2011).

3.5.  Plastics

Plastic has become an indispensable part of every sphere of human life due to its high quality, durability and inexpensiveness. Each year, approximately 140 million tonnes of synthetic polymers are estimated to be produced and several studies aim at investigating their global impact and interactions with organisms at several trophic levels. Plastic contamination of aquatic environments from waste discharges, industrial raw materials, manufactured pellets or fragments of fishing nets, is becoming an emerging global threat for its multiple (social and environmental) implications(Sharma, et al. 2017).

Figure 4. The most widely used plastics in the market 

3.5.1 Bioremediation of Plastics

Bio-deterioration is a natural process in which plastic gets modified chemically, physically and mechanically by the superficial degradation caused by microbes and decomposer organisms. Plastics biodegradability is determined by many factors, such as molecular weight, morphology and hydrophobicity. High molecular weight and crystalline morphology generally relates to lower degree of biodegradability. Polymers with highly crystalline (ordered) structures, such as polyethylene, tend to be more resistant than more amorphous polymers such as polycarbonate. The amorphous regions of polymers, where the molecules are less densely packed, are preferentially attacked by plastic-degrading enzymes. Tus, the more amorphous a polymer is, the more access the enzymes have and the faster the degradation can proceed.

• Biofragmentation

Biofragmentation of plastics occurs, which happens mainly via two major mechanisms, hydrolysis or oxidation. Hydrolysis is the more common process used by organisms, but oxidation has the potential to degrade aliphatic materials such as polyethylene and polypropylene. Hydrolytic degradation of plastic occurs more easily in polymers with ester groups. Hydrolysis typically occurs in two steps: first, the enzyme binds to the polymer substrate and subsequently catalysis a hydrolytic cleavage; second, polymers are degraded into low molecular weight oligomers, dimers and monomers and finally mineralized. Organisms that degrade plastics by hydrolysis usually do so by producing lipases and esterases. Such organisms release the hydrolytic enzymes, maintain them at the cell surface, or display a combination of both to maximize degradation efficiency. Usually, the microbial cells adhere to the substrate, secrete enzymes that degrade the substrate into smaller fragments which are then further hydrolyzed into monomers by cell-surface enzymes. The microorganisms that are able to degrade such substances are diverse and exist in multiple taxa, ranging from Gram-negative to Gram-positive bacteria and fungi(Jenkins and Batool 2019).

• Biofilms and Plastic Biodegradation

Biofilm forms over the surface of plastic. Biofilm formation causes the release of acidic compounds such as nitrous acid (Nitrobacter sp.) (Nitrosomonas sp.), or nitric acid sulphuric acid (Thiobacillus sp.) by chemo lithotrophic bacteria. The formation of acid leads to the change in the pH inside the pores that result in progressive degradation of plastic causing changes in their microstructure contributing to the chemical deterioration of plastic(Bunty Sharma , 2017). 

• Microbial Degradation of Plastic

Microbial degradation of plastic is a promising eco-friendly strategy which represents a great opportunity to manage waste plastic materials with no adverse impacts. There are different types of microbes that are used for plastic degradation are given in table (Caruso,  2015). The Actinomycetes Rhodococcus rubber and the fungus Penicillium simplicissimum, thermophilic bacterium Brevibacillus and Streptomyces sp. are able to degrade plastic(Caruso, 2015). 

• Mealworms and Plastic Biodegradation

The larvae of yellow mealworms (Tenebrio molitor Linnaeus) have demonstrated promising Polystyrene biodegradation performance. Mealworms have demonstrated the ability to chew and ingest Plastic foam as food and are capable of degrading and mineralizing Plastic into CO2 via microbe dependent activities within the gut in less than the 12-15 hrs. gut retention time. These mealworms have revealed a potential for microbial biodegradation and bioremediation of plastic pollutants(Yang, et al. 2018). 

3.5.2. Plastic Wastes Control and Management Strategies 

Except for the risks and hazards to the human health and the environment safety, the improper management and disposal of plastic wastes impact the aesthetic of the environment, badly affect the beauty of urban and remote environments(Jafarinejad,2017). The generation of plastic wastes can be mainly classified into two categories (Figure 5): (1) preconsumer or industrial plastic wastes; and (2) postconsumer plastic wastes. Preconsumer or industrial plastic wastes referred to the discarded plastics, which are produced during the plastic production and product fabrication processes.  These kinds of plastic wastes can be directly recycled and re-used by the processing and manufacturing departments(Yang, et al. 2018).

Figure 5. Sources of plastic wastes and strategies for the management of plastic wastes

 

4.    Conclusion 

Environmental effects of many organic substances could not be anticipated earlier, which is exemplified by the ‘hero to villain’ status of DDT. Considerable researches on organic pollutants now provide the necessary body of knowledge to understand their recalcitrance and toxic nature. Regulations that limit the disposal of chemicals and escalation in the costs of physical and chemical treatments make bioremediation technologies more attractive. Wider usage of bioremediation technologies desires new governmental regulations which include risk-based criteria in cleanup treatments. Each strategy of bioremediation process has certain specific advantages and disadvantages, which need to be considered for each organic pollutant degradation. For future improvement, more field data and pilot plot scale experiments are essential in order to make bioremediation a reliable option of remediation activities. Improving the knowledge about microbial diversity in natural environments may lead to the development of a culture collection of well-characterized organisms with degradation abilities and high tolerance to a broad range of environmental chemical and physical stresses in the future. This ultimately may lead to an increasingly robust and reliable bioremediation, capable of replacing more invasive techniques.

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