Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluents etc., in natural or artificial settings.[1] 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.[1] In comparison to conventional physicochemical treatment methods bioremediation may offer considerable advantages as it aims to be sustainable, eco-friendly, cheap, and scalable.[1] 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 the growth. In principle, bioremediation could be used to reduce the impact of byproducts created from anthropogenic activities, such as industrialization and agricultural processes.[2][3] Bioremediation could prove less expensive and more sustainable than other remediation alternatives.[4]

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.[5]

Organic pollutants are generally more susceptible to biodegradation than heavy metals. Typical bioremediations involves oxidations. 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.[6] For heavy metals, bioremediation offers few solutions. Metal-containing pollutant can be removed or reduced with varying bioremediation techniques.[7] The main challenge to bioremediations is rate: the processes are slow.[8]

Bioremediation techniques can be classified as (i) in situ techniques, which treats polluted sites directly, vs (ii) ex situ techniques which are applied to excavated materials.[9] 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 or reduce harmful compounds to improve soil and water quality.[10]

In situ techniques

Visual representation showing in-situ bioremediation. This process involves the addition of oxygen, nutrients, or microbes into contaminated soil to remove toxic pollutants.[10] Contamination includes buried waste and underground pipe leakage that infiltrate ground water systems.[11] The addition of oxygen removes the pollutants by producing carbon dioxide and water.[7]
Visual representation showing in-situ bioremediation. This process involves the addition of oxygen, nutrients, or microbes into contaminated soil to remove toxic pollutants.[10] Contamination includes buried waste and underground pipe leakage that infiltrate ground water systems.[11] The addition of oxygen removes the pollutants by producing carbon dioxide and water.[7]


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.[12] 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.[8] 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.[8] 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).[6] 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.[13] 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.[14] 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.[13][14]


An example of biostimulation at the Snake River Plain Aquifer in Idaho. This process involves the addition of whey powder to promote the utilization of naturally present bacteria. Whey powder acts as a substrate to aid in the growth of bacteria.[15] At this site, microorganisms break down the carcinogenic compound trichloroethylene (TCE), which is a process seen in previous studies.[15]
An example of biostimulation at the Snake River Plain Aquifer in Idaho. This process involves the addition of whey powder to promote the utilization of naturally present bacteria. Whey powder acts as a substrate to aid in the growth of bacteria.[15] At this site, microorganisms break down the carcinogenic compound trichloroethylene (TCE), which is a process seen in previous studies.[15]

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.[7][16]

Bacteria can in principle be used to degrade hydrocarbons.[17][18] Specific to marine oil spills, nitrogen and phosphorus have been key nutrients in biodegradation.[19] 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.[20]

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,[21] RDX, TNT), and nitrate.[7] 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. 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 precipitate certain metals (e.g., zinc, cadmium). 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.[22] 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.[7][6]


Numerous chemical, physical, and biological processes that lessen the bulk, toxicity, volume, or concentration of pollutants are referred to as bioattenuation. These procedures involve the transformation of pollutants as well as aerobic and anaerobic biodegradation, sorption, volatilization, and chemical or biological stabilization. Since there are no other remedial treatments that may be used, the duration is not a limiting issue and is typically applied to sites with low concentrations of contaminants. [23] 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.[24] While there is no anthropogenic involvement in bioattenuation, the contaminated site must still be monitored.[24]


A useful method for removing volatile organic chemicals is biosparging. By slowly infusing air into the aquifer beneath the contaminated zone, biosparging works. The injected air encourages oxygenation of the aquifer at a reasonably near well spacing, which is critical to encourage aerobic biodegradation. [25] Indigenous bacteria are encouraged to accelerate their rate of destruction when oxygen is introduced.[26] However, biosparging focuses on saturated contaminated zones, specifically related to ground water remediation.[27]

Ex situ techniques


In order to improve bioremediation by essentially boosting microbial activity, biopile-mediated bioremediation requires piling excavated tainted soil above ground, followed by nutrient supplementation and occasionally aeration. Aeration, irrigation, nutrient and leachate collecting systems, and a treatment bed are the elements of this technique. Due to its beneficial qualities, such as cost effectiveness, which enables effective biodegradation under the condition that nutrient, temperature, and aeration are sufficiently managed, the application of this specific ex situ technique is being studied more frequently. [28]


In order to improve ex situ bioremediation, windrows shift the piled polluted soil on a regular basis. This increases the degradation activities of the native and/or transitory hydrocarbonoclastic bacteria present in the polluted soil. It is possible to do bioremediation by assimilation, biotransformation, and mineralization. Periodic turning of polluted soil and the addition of water promote aeration, uniformly distribute pollutants, nutrients, and microbial degradative activities. [29]


Due to its low cost and little equipment needs, land farming is one of the simplest bioremediation processes. Ex situ bioremediation is what it is generally referred to as, however in other circumstances, in situ bioremediation technology is what it is referred to. The treatment site is at the center of this argument. Whether or whether land farming may be done in situ or ex situ depends in large part on the depth of the pollution. In land farming, dirty soils are often excavated and tilled, although it appears that the method of bioremediation depends on the treatment site. Excavated polluted soil might be considered in situ when it is treated there; otherwise, it is ex situ because it shares more characteristics with other ex situ bioremediation approaches. [30]


In a bioreactor, basic materials are transformed into specified products through a sequence of biological reactions. The bioreactor can be operated in a variety of ways, such as batch, fed-batch, sequencing batch, continuous, and multistage. The market economy and capital expenditure have a major role in determining the operating mode. The environment of a bioreactor mimics and preserves the natural environment of cells to provide the best circumstances for growth. In either instance, using a bioreactor to clean polluted soil has various benefits over alternative ex situ bioremediation methods. Polluted samples can be put into a bioreactor as dry matter or slurry. [31]

Heavy metals

Heavy metals become present in the environment due to anthropogenic activities or natural factors.[7] Arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), and mercury (Hg) are five heavy metals that pose risks to environmental ecology and human health because they are carcinogenic and poisonous even in small concentrations.[32] Anthropogenic activities include industrial emissions, electronic waste, and ore mining. Natural factors include mineral weathering, soil erosion, and forest fires.[7] Heavy metals including cadmium, chromium, lead and uranium are unlike organic compounds and cannot be biodegraded. However, bioremediation processes can potentially be used to reduce the mobility of these material in the subsurface, reducing the potential for human and environmental exposure.[33] Heavy metals from these factors are predominantly present in water sources due to runoff where it is uptake by marine fauna and flora.[7]

The mobility of certain metals including chromium (Cr) and uranium (U) varies depending on the oxidation state of the material.[34] Microorganisms can be used to reduce the toxicity and mobility of chromium by reducing hexavalent chromium, Cr(VI) to trivalent Cr (III).[35] Uranium can be reduced from the more mobile U(VI) oxidation state to the less mobile U(IV) oxidation state.[36][37] Microorganisms are used in this process because the reduction rate of these metals is often slow unless catalyzed by microbial interactions[38] Research is also underway to develop methods to remove metals from water by enhancing the sorption of the metal to cell walls.[38] This approach has been evaluated for treatment of cadmium,[39] chromium,[40] and lead.[41] Genetically modified bacteria has also been explored for use in sequestration of Arsenic.[42] Phytoextraction processes concentrate contaminants in the biomass for subsequent removal. Different strains of microalgae showed a range of responses and tolerances as well as the capacity to bioaccumulate heavy metals. Proteins and peptides, as well as various functional groups, are in charge of metal binding. Heavy metal bioremediation and biosorption involve a variety of mechanisms, including extracellular adsorption, reduction, volatilization, complex formation, ion exchange, intracellular accumulation, chelation, and bio-methylation. [43]


Recent technological advancements have exposed the ecology and people to a variety of chemical toxins, particularly (pesticides, herbicides, insecticides, and fungicides). Pesticides are synthetic chemical compounds that are used to manage pests in a variety of settings, including agriculture. Therefore, in integrated pest control systems, pesticides are viewed as efficient, economical, and effective tools (IPMs). Unchecked pesticide use results in their bioaccumulation in food chains, which puts mammals and other non-target animals at serious risk. Additionally, pesticides' direct or indirect effects on organisms other than their intended targets cause an environmental imbalance. Furthermore, pesticide residues persist in plant tissues, soil, air, and even water. These residues are among the most harmful dangers to the ecosystem since they can linger in the environment for a very long period and have carcinogenic consequences. [44]

Limitations of bioremediation

Bioremediation can be used to completely mineralize organic pollutants, to partially transform the pollutants, or alter their mobility. Heavy metals and radionuclides are elements that cannot be biodegraded, but can be bio-transformed to less mobile forms.[45][46][47] In some cases, microbes do not fully mineralize the pollutant, potentially producing a more toxic compound.[47] For example, under anaerobic conditions, the reductive dehalogenation of TCE may produce dichloroethylene (DCE) and vinyl chloride (VC), which are suspected or known carcinogens.[45] However, the microorganism Dehalococcoides can further reduce DCE and VC to the non-toxic product ethene.[48] The molecular pathways for bioremediation are of considerable interest.[45] In addition, knowing these pathways will help develop new technologies that can deal with sites that have uneven distributions of a mixture of contaminants.[26]

Biodegradation requires microbial population with the metabolic capacity to degrade the pollutant.[26][46] The biological processes used by these microbes are highly specific, therefore, many environmental factors must be taken into account and regulated as well.[26][45] It can be difficult to extrapolate the results from the small-scale test studies into big field operations.[26] In many cases, bioremediation takes more time than other alternatives such as land filling and incineration.[26][45] 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.[49]>

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.[50] Altering the pH and temperature of the contaminated soil is a resolution to increase bioavailability which, in turn, increased degradation of harmful compounds.[50]

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.[51]

Since the experience with harmful contaminants are limited, laboratory practices are required to evaluate effectiveness, treatment designs, and estimate treatment times.[49] Bioremediation processes may take several months to several years depending on the size of the contaminated area.[52]

Genetic engineering

The use of genetic engineering to create organisms specifically designed for bioremediation is under preliminary research.[53] Two category of genes can be inserted in the organism: degradative genes which encode proteins required for the degradation of pollutants, and reporter genes that are able to monitor pollution levels.[54] Numerous members of Pseudomonas have also been modified with the lux gene, but 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.[55]

There are concerns surrounding release and containment of genetically modified organisms into the environment due to the potential of horizontal gene transfer.[56] Genetically modified organisms are classified and controlled under the Toxic Substances Control Act of 1976 under United States Environmental Protection Agency.[57] 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.[56] In addition, the tracking of modified organisms can be made easier with the insertion of bioluminescence genes for visual identification.[58]

Genetically modified organisms have been created to treat oil spills and break down certain plastics (PET).[59]

See also


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