Bioremediation of oil spills

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Bioremediation of petroleum contaminated environments is a process in which the biological pathways within microorganisms or plants are used to degrade or sequester toxic hydrocarbons, heavy metals, and other volatile organic compounds found within fossil fuels. Oil spills happen frequently at varying degrees along with all aspects of the petroleum supply chain, presenting a complex array of issues for both environmental and public health.[1] While traditional cleanup methods such as chemical or manual containment and removal often result in rapid results, bioremediation is less labor-intensive, expensive, and averts chemical or mechanical damage.[2][3] The efficiency and effectiveness of bioremediation efforts are based on maintaining ideal conditions, such as pH, RED-OX potential, temperature, moisture, oxygen abundance, nutrient availability, soil composition, and pollutant structure, for the desired organism or biological pathway to facilitate reactions.[4] Three main types of bioremediation used for petroleum spills include microbial remediation, phytoremediation, and mycoremediation. Bioremediation has been implemented in various notable oil spills including the 1989 Exxon Valdez incident where the application of fertilizer on affected shoreline increased rates of biodegradation.[5]

Oil spills

Petroleum contamination of both terrestrial and marine environments results from

ecosystem dynamics leading to algae blooms and a mass die-off of marine life.[10] It is estimated that over 1000 sea otters, along with many birds, died from the Exxon Valdez spill.[11] Oil spill clean up efforts commonly employ multiple methods in tandem. Controlled burning and barriers were both used as manual remediation efforts following the Exxon Valdez incident.[12] Chemical solvents and dispersants were briefly used by Exxon in water surrounding the Valdez although discontinued as they required specific conditions and contained carcinogenic compounds.[12] Bioremediation techniques used in the Exxon Valdez spill included nitrogen and phosphorus seeding along coastline increasing available nutrients for indigenous petroleum degrading microorganisms doubling rates of decomposition.[13] Across all remediation techniques less than ten percent of the oil released from Exxon Valdez tanker was recovered.[12] Many genera of plant, microbes, and fungi have demonstrated oil remediating properties including Spartina, Haloscarcia, Rhizophora, Nocardioides, Dietzia, and Microbacterium.[14][15][16][17]

Bioremediation

Bioremediation refers to the use of specific microorganisms or plants to metabolize and remove harmful substances. These organisms are known for their biochemical and physical affinity to hydrocarbons among other pollutants. Various types of bacteria, archaea, algae, fungi, and some species of plants are all able to break down specific toxic waste products into safer constituents. Bioremediation is classified by the organism responsible for remediation with three major subdivisions: microbial remediation, phytoremediation, and mycoremediation.[18] In most cases, bioremediation works to either increase the numbers of naturally occurring microorganisms or add pollutant-specific microbes to the area. Bioremediation can involve using many varieties of microorganisms as well, either synergistically or independently of each other. The costs and environmental impacts of bioremediation are often negligible when compared to traditional manual or chemical remediation efforts.[citation needed]

Bioremediation of petroleum

Due to their ubiquity across environments, many organisms have evolved to use the hydrocarbons and organic compounds in petroleum as energy while simultaneously denaturing toxins through molecular transfer mechanisms.[19]

Microbial bioremediation uses

molecular-weight, such as polycyclic aromatic hydrocarbons (PAH's). These microbes require a wide array of enzymes for the breakdown of petroleum, and very specific nutrient compositions to work at an efficient rate.[21]

Microbes work in a step-wise fashion to breakdown and metabolize the components of petroleum.[21]

  1. Linear Alkanes
  2. Branched Alkanes
  3. Small
    aromatic compounds
  4. Cyclic Alkanes

Treatments that use these breakdown processes most commonly use heat and chemicals to extend the efficacy.[22] Later, more biological systems are used for specific ecosystems that use specific mechanisms.[22]

macrophytes.[citation needed] Phytoremediation is most effective when used in conjunction with microbial remediation and Mycoremediation.[25][26]

crude oil displays toxic levels of various heavy metals such as lead, zinc and magnesium. Application of mycoremediation techniques to crude contaminated soils have shown significant reductions of heavy metal concentrations.[29]

Mechanisms involved in bioremediation of toxic compounds.

Bioremediation parameters

The efficiency and efficacy of each method of remediation has limitations. The goal of remediation is to eliminate the environmental pollutant as quickly as possible; only inefficient processes require human intervention.[30] Environmental factors such as requirements of reaction, mobility of substances, and physiological needs of organisms will affect the rate and degree that contaminants are degraded.[31] Over time, many of these requirements are overcome. This is when petroleum degrading bacteria and archaea are able to mediate oil spills most efficiently. Weathering and environmental factors play large roles in the success of bioremediation. Interacting soil and pollutant chemicals truly account for the work that can be completed by these microorganisms. These processes change the soil composition and layering, along with the biochemistry of the ecosystem. These chemical and biological changes require adaptation from soil microbes to bioremediate.[30] The susceptibility of the pollutant is also important to consider. Properties such as solubility, temperature, and pH will affect bioremediation and affect the process.[32] Pollutants that are more soluble will be easier for microbes to transform into the environment. Otherwise, pollutants with rigid molecular structures extend bioremediation as they are harder to convert into innocuous substances. Bioaccessibility, the amount of pollutant available for absorption, and bioavailability of pollutant will affect efficiency as well.[32] In many instances, needed nutrients are collected and allocated for petroleum degrading microorganisms in order to maximize the efficiency of the process.[30] Providing microorganisms with the nutrients and conditions they need allow them to thrive.

Factors that affect bioremediation

[30]

Bioremediation mechanisms

Microorganisms use many unique mechanisms to convert molecules and transfer electrons.[31]
Bioremediation Technique Conversion Products
Aerobic Respiration
 Petroleum substrate with molecular oxygen Nitrogen Gas, Hydrogen Sulfide,

Methane, Metals, Carbon Dioxide, Water

Inorganic Electron Donation Ammonium, Nitrite, Iron, Manganese are oxidized. Nitrate, Nitrite, Iron, Manganese, Sulfate
Fermentation Toxic petroleum compounds of organic nature Harmless Compounds, Fermentation Products
Demobilization Iron, Sulfate, Mercury, Chromium, Uranium Ferric Hydroxide, Sulfide, Pyrite, Reduced Chromium,

Uraninite

Reductive Dehalogenation
 Halogen compound with electron donor Reduced contaminant

Listed above, the chemicals required and products formed in petroleum degradation are shown. These microbes will reduce,

oxidize, ferment, and demobilize the constituents of oil spills over time, and create innocuous compounds. Bioremediation techniques[33]
involve using these mechanisms to reduce pollutant amounts and are dependent on pollutant aspects:

Ex situ bioremediation

Ex situ remediation refers to reactions performed outside the natural habitat of these organisms.

  • Increased microbial activities through aeration, irrigation, and creation of bio-piles.
  • Increased degradation activities via turning of polluted soils and addition of minerals and water.
  • The use of bioreactors, to enhance and speed up the biological reactions of microorganisms to decrease bioremediation time.
  • Farming techniques that call for addition of nutrients in soil to stimulate microbial mechanisms

In situ bioremediation

In situ remediation refers to reactions performed inside a reaction mixture.

  • Bio-venting
    , using moisture and nutrients to enhance the transformation of pollutants to more innocent substances.
  • Bio-slurping
    , using pumping to apply oxygen and water, thus separating and compiling soils to increase remediation of microbes.
  • Bio-sparging
    , where air is pushed into soil to stimulate microbial bioremediation.
  • Phytoremediation, uses the mechanisms of plants to decrease efficacy of pollutants

See also

References

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