Bioremediation

Template:Short description Template:Cs1 config Template:Pollution sidebar Template:Use mdy dates Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi in mycoremediation, and plants in phytoremediation), living or dead, is employed for removing environmental pollutants from air, water, soil, fuel gasses, industrial effluents etc., in natural or artificial settings.<ref name="yuvraj 305 318">Template:Cite book</ref> The natural ability of organisms to adsorb, accumulate, and degrade common and emerging pollutants has attracted the use of biological resources in treatment of contaminated environment.<ref name="yuvraj 305 318"/> In comparison to conventional physicochemical treatment methods bioremediation may offer advantages as it aims to be sustainable, eco-friendly, cheap, and scalable.<ref name="yuvraj 305 318"/>

Most bioremediation is inadvertent, involving native organisms. Research on bioremediation is heavily focused on stimulating the process by inoculation of a polluted site with organisms or supplying nutrients to promote their growth. Environmental remediation is an alternative to bioremediation.<ref>Template:Cite web</ref>

While organic pollutants are susceptible to biodegradation, heavy metals cannot be degraded, but rather oxidized or reduced. Typical bioremediations involves oxidations.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> Oxidations enhance the water-solubility of organic compounds and their susceptibility to further degradation by further oxidation and hydrolysis. Ultimately biodegradation converts hydrocarbons to carbon dioxide and water.<ref name="EPA_2013" /> For heavy metals, bioremediation offers few solutions. Metal-containing pollutant can be removed, at least partially, with varying bioremediation techniques.<ref name="Kapahi_2019">Template:Cite journal</ref> The main challenge to bioremediations is rate: the processes are slow.<ref name="Norris_1993"/>

Bioremediation techniques can be classified as (i) in situ techniques, which treat polluted sites directly, vs (ii) ex situ techniques which are applied to excavated materials.<ref>Template:Cite journal</ref> In both these approaches, additional nutrients, vitamins, minerals, and pH buffers are added to enhance the growth and metabolism of the microorganisms. In some cases, specialized microbial cultures are added (biostimulation). Some examples of bioremediation related technologies are phytoremediation, bioventing, bioattenuation, biosparging, composting (biopiles and windrows), and landfarming. Other remediation techniques include thermal desorption, vitrification, air stripping, bioleaching, rhizofiltration, and soil washing. Biological treatment, bioremediation, is a similar approach used to treat wastes including wastewater, industrial waste and solid waste. The end goal of bioremediation is to remove harmful compounds to improve soil and water quality.<ref name="Canak_2019">Template:Cite journal</ref>

In situ techniques

Bioventing

Bioventing is a process that increases the oxygen or air flow into the unsaturated zone of the soil, this in turn increases the rate of natural in situ degradation of the targeted hydrocarbon contaminant.<ref>Template:Cite journal</ref> Bioventing, an aerobic bioremediation, is the most common form of oxidative bioremediation process where oxygen is provided as the electron acceptor for oxidation of petroleum, polyaromatic hydrocarbons (PAHs), phenols, and other reduced pollutants. Oxygen is generally the preferred electron acceptor because of the higher energy yield and because oxygen is required for some enzyme systems to initiate the degradation process.<ref name="Norris_1993">Template:Cite book</ref> Microorganisms can degrade a wide variety of hydrocarbons, including components of gasoline, kerosene, diesel, and jet fuel. Under ideal aerobic conditions, the biodegradation rates of the low- to moderate-weight aliphatic, alicyclic, and aromatic compounds can be very high. As molecular weight of the compound increases, the resistance to biodegradation increases simultaneously.<ref name="Norris_1993" /> This results in higher contaminated volatile compounds due to their high molecular weight and an increased difficulty to remove from the environment.

Most bioremediation processes involve oxidation-reduction reactions where either an electron acceptor (commonly oxygen) is added to stimulate oxidation of a reduced pollutant (e.g. hydrocarbons) or an electron donor (commonly an organic substrate) is added to reduce oxidized pollutants (nitrate, perchlorate, oxidized metals, chlorinated solvents, explosives and propellants).<ref name="EPA_2013">Template:Cite book</ref> In both these approaches, additional nutrients, vitamins, minerals, and pH buffers may be added to optimize conditions for the microorganisms. In some cases, specialized microbial cultures are added (bioaugmentation) to further enhance biodegradation.

Approaches for oxygen addition below the water table include recirculating aerated water through the treatment zone, addition of pure oxygen or peroxides, and air sparging.<ref name="Leeson_2002" /> Recirculation systems typically consist of a combination of injection wells or galleries and one or more recovery wells where the extracted groundwater is treated, oxygenated, amended with nutrients and re-injected.<ref name="EPA_2017" /> However, the amount of oxygen that can be provided by this method is limited by the low solubility of oxygen in water (8 to 10 mg/L for water in equilibrium with air at typical temperatures). Greater amounts of oxygen can be provided by contacting the water with pure oxygen or addition of hydrogen peroxide (H₂O₂) to the water. In some cases, slurries of solid calcium or magnesium peroxide are injected under pressure through soil borings. These solid peroxides react with water releasing H₂O₂ which then decomposes releasing oxygen. Air sparging involves the injection of air under pressure below the water table. The air injection pressure must be great enough to overcome the hydrostatic pressure of the water and resistance to air flow through the soil.<ref name="Leeson_2002">Template:Cite book</ref><ref name="EPA_2017" />

Biostimulation

Bioremediation can be carried out by bacteria that are naturally present. In biostimulation, the population of these helpful bacteria can be increased by adding nutrients.<ref name="Kapahi_2019" /><ref>Template:Cite journal</ref>

Bacteria can in principle be used to degrade hydrocarbons.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Specific to marine oil spills, nitrogen and phosphorus have been key nutrients in biodegradation.<ref>Template:Cite journal</ref> The bioremediation of hydrocarbons suffers from low rates.

Bioremediation can involve the action of microbial consortium. Within the consortium, the product of one species could be the substrate for another species.<ref>Template:Cite book</ref>

Anaerobic bioremediation can in principle be employed to treat a range of oxidized contaminants including chlorinated ethylenes (PCE, TCE, DCE, VC), chlorinated ethanes (TCA, DCA), chloromethanes (CT, CF), chlorinated cyclic hydrocarbons, various energetics (e.g., perchlorate,<ref>Template:Cite book</ref> RDX, TNT), and nitrate.<ref name="Kapahi_2019" /> This process involves the addition of an electron donor to: 1) deplete background electron acceptors including oxygen, nitrate, oxidized iron and manganese and sulfate; and 2) stimulate the biological and/or chemical reduction of the oxidized pollutants. The choice of substrate and the method of injection depend on the contaminant type and distribution in the aquifer, hydrogeology, and remediation objectives. Substrate can be added using conventional well installations, by direct-push technology, or by excavation and backfill such as permeable reactive barriers (PRB) or biowalls.<ref>Template:Cite web</ref> Slow-release products composed of edible oils or solid substrates tend to stay in place for an extended treatment period. Soluble substrates or soluble fermentation products of slow-release substrates can potentially migrate via advection and diffusion, providing broader but shorter-lived treatment zones. The added organic substrates are first fermented to hydrogen (H₂) and volatile fatty acids (VFAs). The VFAs, including acetate, lactate, propionate and butyrate, provide carbon and energy for bacterial metabolism.<ref name="Kapahi_2019" /><ref name="EPA_2013" />

Bioattenuation

During bioattenuation, biodegradation occurs naturally with the addition of nutrients or bacteria. The indigenous microbes present will determine the metabolic activity and act as a natural attenuation.<ref name="Ying_2018">Template:Cite book</ref> While there is no anthropogenic involvement in bioattenuation, the contaminated site must still be monitored.<ref name="Ying_2018" />

Biosparging

Biosparging is the process of groundwater remediation as oxygen, and possible nutrients, is injected. When oxygen is injected, indigenous bacteria are stimulated to increase rate of degradation.<ref name="Disadv-Overview" /> However, biosparging focuses on saturated contaminated zones, specifically related to ground water remediation.<ref>Template:Cite journal</ref>

UNICEF, power producers, bulk water suppliers, and local governments are early adopters of low cost bioremediation, such as aerobic bacteria tablets which are simply dropped into water.<ref>Template:Cite web</ref>

Ex situ techniques

Biopiles

Biopiles, similar to bioventing, are used to remove petroleum pollutants by introducing aerobic hydrocarbonsTemplate:Citation needed to contaminated soils. However, the soil is excavated and piled with an aeration system. This aeration system enhances microbial activity by introducing oxygen under positive pressure or removes oxygen under negative pressure.<ref>Template:Cite journal</ref>

Windrows

Windrow systems are similar to compost techniques where soil is periodically turned in order to enhance aeration.<ref>Template:Cite book</ref> This periodic turning also allows contaminants present in the soil to be uniformly distributed which accelerates the process of bioremediation.<ref>Template:Cite journal</ref>

Landfarming

Template:Main Landfarming, or land treatment, is a method commonly used for sludge spills. This method disperses contaminated soil and aerates the soil by cyclically rotating.<ref>Template:Cite book</ref> This process is an above land application and contaminated soils are required to be shallow in order for microbial activity to be stimulated. However, if the contamination is deeper than 5 feet, then the soil is required to be excavated to above ground.<ref name="EPA_2017">Template:Cite web</ref> While it is an ex situ technique, it can also be considered an in situ technique as Landfarming can be performed at the site of contamination.<ref>Template:Cite journal</ref>

In situ vs. Ex situ

Ex situ techniques are often more expensive because of excavation and transportation costs to the treatment facility, while in situ techniques are performed at the site of contamination so they only have installation costs. While there is less cost there is also less of an ability to determine the scale and spread of the pollutant. The pollutant ultimately determines which bioremediation method to use. The depth and spread of the pollutant are other important factors.<ref>Template:Cite journal</ref>

Heavy metals

Heavy metals are introduced into the environment by both anthropogenic activities and natural factors.<ref name="Kapahi_2019" /> Anthropogenic activities include industrial emissions, electronic waste, and mining. Natural factors include mineral weathering, soil erosion, and forest fires.<ref name="Kapahi_2019" /> Heavy metals including cadmium, chromium, lead and uranium are unlike organic compounds and cannot be biodegraded. However, bioremediation processes can potentially be used to minimize the mobility of these material in the subsurface, lowering the potential for human and environmental exposure.<ref>Template:Cite journal</ref> Heavy metals from these factors are predominantly present in water sources due to runoff where it is uptake by marine fauna and flora.<ref name="Kapahi_2019" />

Hexavalent chromium (Cr[VI]) and uranium (U[VI]) can be reduced to less mobile and/or less toxic forms (e.g., Cr[III], U[IV]). Similarly, reduction of sulfate to sulfide (sulfidogenesis) can be used to immobilize certain metals (e.g., zinc, cadmium).

The mobility of certain metals including chromium (Cr) and uranium (U) varies depending on the oxidation state of the material.<ref>Template:Cite book</ref> Microorganisms can be used to lower the toxicity and mobility of chromium by reducing hexavalent chromium, Cr(VI) to trivalent Cr(III).<ref>Template:Cite book</ref> Reduction of the more mobile U(VI) species affords the less mobile U(IV) derivatives.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> Microorganisms are used in this process because the reduction rate of these metals is often slow in the absence of microbial interactions<ref name="Palmisano_2003">Template:Cite book</ref> Research is also underway to develop methods to remove metals from water by enhancing the sorption of the metal to cell walls.<ref name="Palmisano_2003" /> This approach has been evaluated for treatment of cadmium,<ref name="source 3">Template:Cite journal</ref> chromium,<ref name="Durán_2018">Template:Cite journal</ref> and lead.<ref name="source 1">Template:Cite journal</ref> Genetically modified bacteria has also been explored for use in sequestration of Arsenic.<ref name = "Yam et al, 2021">Template:Cite book</ref> Phytoextraction processes concentrate contaminants in the biomass for subsequent removal.

Metal extractions can in principle be performed in situ or ex situ where in situ is preferred since it is less expensive to excavate the substrate.<ref>Template:Cite journal</ref>

Bioremediation is not specific to metals. In 2010 there was a massive oil spill in the Gulf of Mexico. Populations of bacteria and archaea were used to rejuvenate the coast after the oil spill. These microorganisms over time have developed metabolic networks that can utilize hydrocarbons such as oil and petroleum as a source of carbon and energy.<ref>Template:Cite journal</ref> Microbial bioremediation is a very effective modern technique for restoring natural systems by removing toxins from the environment.

Pesticides

Of the many ways to deal with pesticide contamination, bioremediation promises to be more effective.<ref>Template:Cite journal</ref> Many sites around the world are contaminated with agrichemicals.<ref>Template:Cite journal</ref> These agrichemicals often resist biodegradation, by design.<ref name = "Mohapatra_2022">Template:Cite journal</ref><ref>Template:Cite journal</ref> Harming all manners of organic life with long term health issues such as cancer, rashes, blindness, paralysis, and mental illness.<ref name = "Mohapatra_2022" /> An example is Lindane which was a commonly used insecticide in the 20th century. Long time exposure poses a serious threat to humans and the surrounding ecosystem. Lindane reduces the potential of beneficial bacteria in the soil such as nitrogen fixation cyanobacteria. As well as causing central nervous system issues in smaller mammals such as seizures, dizziness, and even death. What makes it so harmful to these organisms is how quickly distributed it gets through the brain and fatty tissues. While Lindane has been mostly limited to specific use, it is still produced and used around the world.<ref>Template:Cite journal</ref>

Actinobacteria has been a promising candidate in situ technique specifically for removing pesticides. When certain strains of Actinobacteria have been grouped together, their efficiency in degrading pesticides has enhanced. As well as being a reusable technique that strengthens through further use by limiting the migration space of these cells to target specific areas and not fully consume their cleansing abilities. Despite encouraging results, Actinobacteria has only been used in controlled lab settings and will need further development in finding the cost effectiveness and scalability of use.<ref>Template:Cite journal</ref>

Limitations of bioremediation

Bioremediation is rarely employed to remediate pollutants. Heavy metals and radionuclides generally cannot be biodegraded. In some cases, these metals can be bio-transformed to less mobile forms.<ref name="Disadv-Comprehensive">Template:Cite journal</ref><ref name="Disadv-Factors">Template:Cite journal</ref><ref name="Disadv-EncTox">Template:Cite book</ref> In some cases, microbes do not fully mineralize the pollutant, potentially producing a more toxic compound.<ref name="Disadv-EncTox" /> For example, under anaerobic conditions, the reductive dehalogenation of TCE may produce dichloroethylene (DCE) and vinyl chloride (VC), which are suspected or known carcinogens.<ref name="Disadv-Comprehensive" /> However, the microorganism Dehalococcoides can further reduce DCE and VC to the non-toxic product ethene.<ref>Template:Cite journal</ref> The molecular pathways for bioremediation are of considerable interest.<ref name="Disadv-Comprehensive" /> In addition, knowing these pathways will help develop new technologies that can deal with sites that have uneven distributions of a mixture of contaminants.<ref name="Disadv-Overview">Template:Cite journal</ref>

Biodegradation requires microbial population with the metabolic capacity to degrade the pollutant.<ref name="Disadv-Overview" /><ref name="Disadv-Factors" /> The biological processes used by these microbes are highly specific, therefore, many environmental factors must be taken into account and regulated as well.<ref name="Disadv-Overview" /><ref name="Disadv-Comprehensive" /> It can be difficult to extrapolate the results from the small-scale test studies into big field operations.<ref name="Disadv-Overview" /> In many cases, bioremediation takes more time than other alternatives such as land filling and incineration.<ref name="Disadv-Overview" /><ref name="Disadv-Comprehensive" /> Another example is bioventing, which is inexpensive to bioremediate contaminated sites, however, this process is extensive and can take a few years to decontaminate a site.<ref name="Sharma_2019">Template:Cite journal</ref>>

Another major drawback is finding the right species to perform bioremediation. In order to prevent the introduction and spreading of an invasive species to the ecosystem, an indigenous species is needed. As well as a species plentiful enough to clean the whole site without exhausting the population. Finally the species should be resilient enough to withstand the environmental conditions.<ref>Template:Cite journal</ref> These specific criteria may make it difficult to perform bioremediation on a contaminated site.

In agricultural industries, the use of pesticides is a top factor in direct soil contamination and runoff water contamination. The limitation or remediation of pesticides is the low bioavailability.<ref name="Odukkathil_2013">Template:Cite journal</ref> Altering the pH and temperature of the contaminated soil is a resolution to increase bioavailability which, in turn, increased degradation of harmful compounds.<ref name="Odukkathil_2013" />

The compound acrylonitrile is commonly produced in industrial setting but adversely contaminates soils. Microorganisms containing nitrile hydratases (NHase) degraded harmful acrylonitrile compounds into non-polluting substances.<ref>Template:Cite journal</ref>

Since the experience with harmful contaminants are limited, laboratory practices are required to evaluate effectiveness, treatment designs, and estimate treatment times.<ref name="Sharma_2019" /> Bioremediation processes may take several months to several years depending on the size of the contaminated area.<ref>Template:Cite web</ref>

Genetic engineering

The use of genetic engineering to create organisms specifically designed for bioremediation is under preliminary research.<ref name=pmid15040178>Template:Cite journal</ref> Two category of genes can be inserted in the organism: degradative genes, which encode proteins required for the degradation of pollutants, and reporter genes, which encode proteins able to monitor pollution levels.<ref name="Menn et al 2001">Template:Cite book</ref> Numerous members of Pseudomonas have been modified with the lux gene for the detection of the polyaromatic hydrocarbon naphthalene. A field test for the release of the modified organism has been successful on a moderately large scale.<ref>Template:Cite journal</ref>

There are concerns surrounding release and containment of genetically modified organisms into the environment due to the potential of horizontal gene transfer.<ref name=Davison2005>Template:Cite journal</ref> Genetically modified organisms are classified and controlled under the Toxic Substances Control Act of 1976 under United States Environmental Protection Agency.<ref>Template:Cite journal</ref> Measures have been created to address these concerns. Organisms can be modified such that they can only survive and grow under specific sets of environmental conditions.<ref name=Davison2005/> In addition, the tracking of modified organisms can be made easier with the insertion of bioluminescence genes for visual identification.<ref name="Irvine">Template:Cite book</ref>

Genetically modified organisms have been created to treat oil spills and break down certain plastics (PET).<ref>Template:Cite web</ref>

Additive manufacturing

Additive manufacturing technologies such as bioprinting offer distinctive benefits that can be leveraged in bioremediation to develop structures with characteristics tailored to biological systems and environmental cleanup needs, and even though the adoption of this technology in bioremediation is in its early stages, the area is seeing massive growth.<ref name=":0">Template:Cite journal</ref>

References

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Bioremediation

Contents

In situ techniques

Bioventing

Biostimulation

Bioattenuation

Biosparging

Ex situ techniques

Biopiles

Windrows

Landfarming

In situ vs. Ex situ

Heavy metals

Pesticides

Limitations of bioremediation

Genetic engineering

Additive manufacturing

See also

References

External links

Navigation menu

Bioremediation

In situ techniques

Bioventing

Biostimulation

Bioattenuation

Biosparging

Ex situ techniques

Biopiles

Windrows

Landfarming

In situ vs. Ex situ

Heavy metals

Pesticides

Limitations of bioremediation

Genetic engineering

Additive manufacturing

See also

References

External links

Navigation menu

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