Recent Applications of Bioremediation and Its Impact

Abstract

Socioeconomic concerns have increased the technology dependence to facilitate the increasing population on the earth. Number of anthropogenic sources are responsible for contaminating the natural environment. Effluents from industries contain many toxicants that cause lethal effects on human and animal life on earth. Many techniques are used so far for the abatement of such pollutants from the environment. As “nature heals itself” so dealing with such problems with bioremediation utilizing the invisible workers (microorganisms), plants and enzymes can help minimize and get rid of such pollutants. It is a greener way to conserve the environment and get rid of such awful substances. Bioremediation can help to get rid of contaminants either by in situ or ex situ approach. By using both ways, either ex situ or in situ, the decontamination of the environment can be successfully done. Using various plant materials and microorganisms by tailoring the surrounding environment to make it suitable for rectifying the contaminant issue is the main goal of bioremediation.

1. Introduction

Bioremediation is a process of converting harmful substances to environmentally safe substances by the action of the invisible workforce. This invisible workforce is the number of microorganisms working in sequence to degrade environmentally toxic substances. Bioremediation works with detoxification and eradication of chemically diverse and physically hazardous materials that cause a threat to the natural existence of the environmental setup.

Ecologically discussing bioremediation refers to the interaction between three factors; Contaminant, invisible workforce, and environment, as shown in Figure 1. Interaction of these factors in turn ensures the mobility of contaminant in the environment, the presence of suitable conditions to degrade the contaminant, and eradication or degradation of the contaminant by converting it into an environmentally friendly substance. Mobility or bioavailability of any contaminant is about the ease with which the contaminant is available to microorganisms. The microorganisms need a suitable set of conditions (like; as availability of electron acceptors, pH, and availability of nutrients) to function well and convert the environmentally harmful substances to environmentally benign substances. The biodegradability of the contaminant depends upon the presence of suitable microorganisms to eradicate that contaminant under the required conditions [2].

Figure 1.

Ecological interpretation of factors governing bioremediation [1].

Eradication of contaminants depends primarily on the nature of the contaminant, which may include pesticides, herbicides, heavy metals, hydrocarbons, sewage, plastics, etc. nature of the contaminant, degree of contamination, environmental factors, contaminated sites, cheap policies for conserving environment are important selection study criteria that are considered while choosing any bioremediation technique [3, 4]. Although it is important to properly plan the selection criteria but other factors that involve the aerobic and anaerobic nature of the area under study, pH, and moisture content are equally important to be considered. Bioremediation strategies make it possible to increase the efficacy of the contaminant removal process. These strategies may be ex situ or in situ. Mostly, the bioremediation techniques work for the removal of hydrocarbon contaminating species from soil or water [5, 6, 7, 8]. Various other cost-effective techniques can be efficiently applied to the contamination sites for the removal of hydrocarbons [9].

2. Types of bioremediation

Majorly bioremediation can be categorized as:

2.1 Ex situ bioremediation

Ex situ bioremediation involves the excavation of contaminants and transporting them to the treatment sites above the ground. Indigenous microorganisms in the soil act as the remediating agents provided other environmental factors are kept monitored. This method can be tailored by changing the decay conditions and maintaining the optimum conditions required for microorganisms to work efficiently. In some conditions, the amendments are added to the soil. There are various types of ex situ techniques that include, biopiles, composting, windrow, landfarming, and slurry reactors [10].

Demerits associated with ex situ bioremediation are its being expensive in terms of excavation and solid handling, fractionation and screening, and treatment till final disposal. The contaminant may be either solid or liquid. On the basis of phases of contaminant material, ex situ method can be a solid phase or slurry phase. In case of the solid phase method; waste in the form of solid-like agricultural waste or domestic, sewage sludge, industrial waste, and municipal solid waste are treated to get compost, which is further employed for the conditioning of soil. Treatment is done prior to compost formation to enhance the biological treatment potential. Various physicochemical and biological factors of the site under study are considered for this purpose. Organic material thus presents or added to the soil act as the source of carbon for microorganisms. Depending on the availability of oxygen and suitable working pH the enzymes secreted by microorganisms detoxify the surrounding area. Applying ex situ treatment to a site that has some compositional limitations or nutrient deficiency for microbial activity needs tailoring of the site by adding site-specific compost. Adjusting pH and water availability to the bioremediating site ensures the efficiency of microbial colonies at ex situ operating sites. The slurry phase method is applicable to municipal wastewater. In this method supply of air, maintenance of proper pH, temperature, micronutrients are needed for the growth of microbial colonies [11].

2.2 In situ bioremediation

In situ bioremediation is the subsurface treatment of contaminants by the biological system of that area. These are considered sustainable methods as they do not require any excavation and transportation of contaminants. Some in situ bioremediation techniques like biosparging, phytoremediation, and bioventing, have been enhanced to get good outcomes for onsite decontamination while some other techniques like natural attenuation or intrinsic bioremediation proceed without any enhancement. In situ bioremediation techniques have been successfully used to treat chemically contaminated sites like; as industrial effluents dumping sites containing dyes, chlorinated solvents, hydrocarbons polluted sites, and heavy metals [12, 13, 14].

In situ bioremediation works with the abatement of contaminants by disrupting the minimum area and is a continuous and economical treatment method for soil and water. It can be intrinsic or engineered. Intrinsic bioremediation involves the conversion of contaminant to nontoxic form by the microbial communities naturally present in soil and water. The detoxifying potential of these microbial communities must be tested in laboratories so that outcomes can be configured accordingly. Various working conditions or requirements are there for intrinsic bioremediation to be fruitful. Annual water flow through the area understudy determines the presence of various minerals and pH of that soil which in turn tells about the working of microbes under such conditions. The presence of heavy metals hindered the growth of microorganisms present in the soil and water. The time of exposure of microorganisms to the contaminant is also an important parameter that should be studied at a pilot scale before conducting the bioremediation on a wide surface area. Although intrinsic bioremediation shows very promising decontamination results, but the limiting factor is when working conditions and environmental factors/site conditions do not favor microbial growth. In such cases, engineered bioremediation replaces intrinsic bioremediation. This type of bioremediation technique accelerates the growth of microbial colonies by providing suitable physicochemical growth conditions. The availability of oxygen, nutrients, and electron acceptors like sulfates and nitrates increase the onsite growth of microbes. In situ bioremediation is laborious as compared to other methods. The outcomes of this method are highly environment-dependent. Continuous availability and replacement of nutrients must be ensured for efficient working genetically engineered microorganisms [11].

3. Factors influencing bioremediation

There are various factors that needed to be optimized to ensure the success of bioremediation processes [15].

3.1 Nature and concentration of contaminant

The concentration and nature of the contaminant are among one of the limiting factors for the bioremediation process. The presence of heavy metal impurities inhibits the growth of microbes and as a result of which it tends to inhibit the bioremediation process. Similarly, increased concentration of contaminant in the particular bioremediating site effect the microbial colonies both in terms of growth and enzymatic functioning of the microbes.

3.2 Nutrient availability

The presence of essential nutrients required for the growth and working of microbes is necessary for the proper outcomes of bioremediation. Nitrates, phosphates, and various electron transport sources are needed to be present in the soil or water environment where bioremediation is to be carried out.

3.3 Factors associated with contaminated site

3.3.1 pH

The pH of the soil or water determines the nature of the species present in it, as it can cause a change in the chemical composition due to the basic and acidic nature of the site. Generally, the pH ranges from 5 to 9 is considered optimum for the working of various microbial colonies. As biological reactions are all pH-sensitive so existence and functioning of enzymes are highly pH-sensitive.

3.3.2 Temperature

It is one of the important factors that defines the moisture content and chemical composition of the site. Generally, the temperature between 20 and 40° is considered good for the efficient performance of microbes.

3.3.3 O₂ availability and moisture content

The availability of oxygen is a very important factor also, as it ensures the oxidative and reducing environment in both soil and water environment. The nature of soil affects the aeration of the bioremediating site. Soil rich in sand and gravel content helps to retain moisture as well as aerate the soil well. More heavy the clay form or soil rich in organic content reduce the availability of oxygen and thereby inhibiting the functioning of microbes.

4. Various applications of bioremediation

Due to the increased population that is ultimately leading to pollution, anthropogenic activities have negative effects on ecosystems. Bioremediation is a purification technique to remove toxic waste from a polluted environment. Bioremediation is specifically helpful for decomposition, eradication, immobilization, or detoxification of variable chemical wastes and physical hazardous materials from the surrounding through the all-inclusive and action of microorganisms. The main principle is degrading and converting pollutants to less toxic forms. There are two approaches for bioremediation, in situ and ex situ. In situ methods involve treatment of the contaminated material at the site, whereas when the material is physically removed to be treated elsewhere it is referred to as ex situ. Bioremediation can occur naturally or stimulated, e.g. by the application of fertilizers (biostimulation), by the addition of similar microbe strains, the effectiveness of the resident microbe population to degrade contaminants may be increased. Every type of contaminant cannot be disposed of by means of microorganisms. Heavy metal contaminants, e.g. Cd²⁺ and Pb^2+, tend to resist interception by microorganisms. Bioremediation is the most effective, economical, eco-friendly management tool to manage the polluted environment. All bioremediation techniques have their own advantage and disadvantage because it has their own specific applications.

Bioremediation, an appropriate method, can be applied to different states of matter in the environment.

• Soils, sediment, and sludge as solids.

• Groundwater, surface water, and industrial wastewater as liquids.

• Industrial air emissions as gases.

• Saturated and vadose zones as sub-surface environments.

The biological community exploited for bioremediation generally consists of the native soil microflora. However, higher plants can also be manipulated to enhance toxicant removal (phytoremediation), especially for remediation of metal contaminated soils.

There are different types of bioremediants used for bioremediation. We can classify its applications on basis of its bioremediants.

4.1 Mycoremediation

It is an important form of bioremediation by the use of fungi. It is a cheaper method of remediation, and it does not usually require expensive equipment. Fungi are an excellent source to remove toxic pollutants from the environment and easily colonize both biotic and abiotic surfaces [16]. The most suitable fungi to be used in soil remediation are basidiomycetes and the ecological groups of saprotrophic and biotrophic fungi. Treu and Falandysz [17] various steps are involved in mycoremediation as shown in Figure 2.

• Fungi freely present in the soil, or in symbiotic association with plant roots (ectomycorrhizal and endomycorrhiza).

• Fungi being decomposers, decompose dead organic matters.

• Fungi being saprotrophs, feed on dead organic matters.

• Fungal hyphae produce and secrete special acids and enzymes that decomposed lignin (White-rot fungi) and cellulose (brown-rot fungi).

• Fungal mycelium by microfiltration removes toxic substances.

• Fungi are useful in the degradation of oils, petroleum compounds, hydrocarbons, aromatic compounds, and pollutants in soil and water.

• Mushrooms Agaricus, Amanita, Cortinarius, Boletus, Leccinum, Suillus, and Phellinus are used for mobilization/complexation of different heavy metals in soil [19].

Figure 2.

Steps involved in Mycoremediation [18].

4.2 Phytoremediation

It is a process involving plants for environmental cleanup. There is different process involved as follow; (summarized in Figure 3)

• Phytovolatilization: Plants absorb contaminants from the soil and release them into the gaseous atmosphere in an unstable form through the process of transpiration.

• Rhizodegradation: It is the symbiotic relationship between plants and microbes. It is the breakdown of the contaminants due to the presence of protein and enzymes by plants or soil organisms in the rhizosphere.

• Phytoextraction: Plants take up the contaminants from water and pass them from the roots to the plant’s upperparts.

• Phytostabilization: Certain plant species are used to bring contaminants from water and soil.

Figure 3.

Components of phytoremediation [20].

4.3 Phytoextraction

This technique involves the usage of different algae to extract pollutants from soils, sediments, or water into harvestable plant biomass (hyperaccumulators i.e. those organisms that take larger-than normal amounts of contaminants from the soil). Phytoextraction is more effective for extracting heavy metals than for organic contaminants. The plants translocate contaminants through their root systems to stems and leaves. Different plants absorb different elements and accumulate them into different organs of the plant, as shown in Figure 4. For example,

• Sunflower (Helianthus annuus), Chinese Brake fern (Pteris vittata) are effectively used for the removal of Arsenic. Chinese Brake fern, act as a hyperaccumulator, and accumulates arsenic in its leaves.

• Willow, a common plant, has significant potential as a phytoextractor of cadmium (Cd), zinc (Zn), and copper (Cu). This plant has some unique characteristics like a high transport capacity for heavy metals from root to shoot and large biomass production. Willow can also be used to produce energy in a biomass-fueled power plant.

• Alpine pennycress (Thlaspi caerulescens), a hyperaccumulator, is effective for the removal of metals Cadmium and Zinc, although its growth appears to be inhibited by copper.

• Indian Mustard (Brassica juncea), Hemp Dogbane (Apocynum cannabinum), Ragweed (Ambrosia artemisiifolia), or Poplar trees, are useful for the removal of Lead, which sequester lead in their biomass.

• Barley and/or sugar beets, Salt-tolerant (moderately halophytic) varieties are commonly used for the extraction of sodium chloride (common salt) to reclaim saline fields that were previously flooded by high groundwater.

• Selenium, mercury, and other organic pollutants including polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants containing genes for bacterial enzymes.

Figure 4.

Phytoextraction for environmental remediation [21].

4.4 Phycoremediation

It is an excellent form of remediation in aquatic ecosystems. Microalgae, “wonder organisms,” are capable of accomplishing bioremediation efficiently by two mechanisms, namely, bioassimilation and biosorption. They have the capability to grow in polluted water as “algal blooms” and assimilate various pollutants. Industrialization has led to increased emission of pollutants into ecosystems. Metal pollutants can easily enter the food chain if heavy metal-contaminated soils are used for the production of food crops.

• Algal blooms have the capability to grow in polluted water and assimilate various pollutants.

• The algal biomass, after harvesting and lipid/protein extraction used as an efficient biosorbent.

• Algal blooms are excellent to remove pesticides from water bodies.

4.5 Bioremediation by microorganisms

Microorganisms are the beneficial source for removing pollutants from soil and water. Microorganisms remove pollutants from water by passive as well as active approaches as shown in Figure 5.

• Genetic engineers are working to produce genetically modified microorganisms for bioremediations e.g. Deinococcus radiodurans. It is helpful in the absorption of mercury and aromatic hydrocarbons like toluene. Oil spills in water make water unfit and cause the death of life. A large number of marine lives are lost due to these oil spills. Hence causing the disturbance in food chains and ecosystems. These microorganisms are good tools to remediate these oil spills to conserve the environment.

• Protozoa, mites, isopods, and collembolan are used in bioremediation in soil, air, and water.

• Bioaugmentation is the addition of microorganisms to the soil, where biostimulation is the modification, addition, reduction, or genetic engineering of the microbial colonies to degrade pollutants

Figure 5.

Steps involved in environmental remediation using microorganisms [22].

4.6 Bioremediation by nematodes

Nematode parasites are a sensitive indicator of heavy metals in the aquatic ecosystem showing sharing of more burden of environmental pollution of the sea and also act as bioremediator of heavy metals in fish. In rhizosphere, they are involved in cleaning, nutrient mobilization, nitrification, enzyme activation, etc.

For heavy metal removals, nematodes are being used.

• Bioaccumulation of heavy toxic metals in muscles and guts of fish can be done in the Echinocephalus sp. and Ascaris sp. which are reported as natural bioremediator of heavy metals in Liza vaigiensis [23].

• Nematodes (Caenorhabditis elegans, Plectus acuminatus, Heterocephalobus pauciannulatus) are indicators of pollution. They are excellent bioremediators of heavy metals in aquatic habitats.

5. Advantages and disadvantages of bioremediation

5.1 Advantages of bioremediation

The use of naturally available sources for bioremediation increases the efficacy of this process. It imparts socioeconomic as well as environmental benefits to ecosystems.

1. It is a natural waste treatment process. The treatment products are commonly harmless including cell biomass, water, and carbon dioxide.

2. It needs a very less laborious and can commonly carry out on-site, regularly without disturbing normal microbial activities. This also eradicates the transport amount of waste off-site and the possible threats to human health and the environment.

3. It is a cost-effective process in comparison to other conventional methods that are used for clean-up of toxic hazardous waste regularly for the treatment of oil-contaminated sites. It also supports in complete degradation of the pollutants; many of the toxic hazardous compounds can be transformed into less harmful products and disposal of contaminated material.

4. It is chemically benign. Enzymes of microorganisms decontaminate the environment without the addition of toxicants in the environment.

5. This way of remediating the environment is an ecofriendly and economically sustainable approach.

5.2 Disadvantages of bioremediation

Various limitations or disadvantages are associated with bioremediation despite the research and development going on in this field.

1. The process of bioremediation is only applicable to those materials that are biodegradable and cannot be applied as the generalized treatment method for all types of wastes

2. Research is going on to find out more about the persistent and toxic nature of the products of bioremediation

3. As it is a biological process involving the microbial communities so more site-specific environmental specifications are needed for the microorganism to work. In order to maintain that environment, the cost-effectiveness remains there no more.

4. Often it becomes difficult to conduct the pilot-scale bioremediation study in a field study.

5. Genetic engineering of the microbes is needed in order to enhance the efficacy of the bioremediation process [24].

6. Conclusion

Bioremediation is nature’s self-healing process by utilizing the hidden workforce capable of decontaminating the environment. The decontaminating ability and efficiency of the biological agents like algae, bacteria, fungi, etc. depends on various factors like oxygen, nutrients, moisture, pH, and temperature. In different regions of the globe, the practice of bioremediation can be made successful by ensuring different factors like cost, the concentration of contaminant, and composition of the degrading site. These factors ultimately ensure the applicability of the ex or in situ bioremediation technique to be implemented. As it has been discussed that ex situ techniques are expensive due to excavation and transportation, although they can treat the large number of contaminants as compared to in situ. The use of various bioremediating agents like bacteria, fungi, algae, and nematodes and further involvement of modern technology including nanoscience is helping to develop new ways to genetically study and engineer the microorganism for need-based functions.