In Situ Bioremediation When Does it Work? (1993) / Chapter Skim
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2 Principles of Bioremediation
2 Principles of Bioremediation
Pages 16-46
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From page 16... ...
In situ bioremediation can be regarded as an extension of the purpose that microorganisms have served in nature for billions of years: the breakdown of complex human, animal, and plant wastes so that life can continue from one generation to the next. Without the activity of microorganisms, the earth would literally be buried in wastes, and the nutrients necessary for the continuation of life would be locked up in detritus.
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From page 17... ...
Whether the application is conventional or novel by today's standards, the same principles must be applied to stimulate the right type and amount of microbial activity. Basics of Microbial Metabolism Microbial transformation of organic contaminants normally occurs because the organisms can use the contaminants for their own growth and reproduction.
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Microbes get energy from the contaminants by breaking chemical bonds and transferring electrons from the contaminants to an electron acceptor, such as oxygen. They "invest" the energy, along with some electrons and carbon from the contaminant, to produce more cells.
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Common "substitutes" for oxygen are nitrate, sulfate, and iron. Fermentation: The process whereby microorganisms use an organic compound as both electron donor and electron acceptor, converting the compound to fermentation products such as organic acids, alcohols, hydrogen, and carbon dioxide.
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Engineered bioremediation: A type of remediation that increases the growth and degradative activity of microorganisms by using engineered systems that supply nutrients, electron acceptors, and/or other growth-stimulating materials. ing water.
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inorganic Compounds as Electron Donors. In addition to organisms that use inorganic chemicals as electron acceptors for anaerobic respiration, other organisms can use inorganic molecules as electron donors.
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A typical bacterial cell is 50 percent carbon; 14 percent nitrogen; 3 percent phosphorus; 2 percent potassium; 1 percent sulfur; 0.2 percent iron; and 0.5 percent each of calcium, magnesium, and chloride. If any of these or other elements essential to cell building is in short supply relative to the carbon present as organic contaminants, competition for nutrients within the microbial communities may limit overall microbial growth and slow contaminant removal.
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These chemical changes follow directly from the physiological principles of microorganisms outlined above. Microbial metabolism catalyzes reactions that consume well-defined reactants contaminants and O2 or other electron acceptors converting them to well-defined products.
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Similarly, under anaerobic conditions, concentrations of other electron acceptors NO3-, SO42-, Fe3+, Mn4+ will decrease, with a corresponding increase in the reduced species of these compounds (N2, H2S, Fez+, and Mn2+, respectively)
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Sequestering of organic contaminants can occur when the contaminant is dissolved in a nonaqueous-phase liquid-a solution that does not mix easily with water and therefore travels through the ground separately from the ground water. Sequestering of organic contaminants can also occur if the contaminant is strongly adsorbed to soil surfaces or is trapped in pores too small for circulating ground water to penetrate easily.
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From page 26... ...
Organic contaminants can be mobilized by adding surfactants. When only small surfactant concentrations are applied, the surfactant molecules accumulate at solid surfaces, reduce the surface tension, and, in principle, increase the spreading of organic contaminants.
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From page 27... ...
Presence of Multiple Contaminants and Natural Organic Chemicals Frequently, contaminated sites contain a combination of several man-made organic contaminants and naturally occurring organic chemicals from decayed plant and animal matter. When such mixtures of organics are present, microbes may selectively degrade the compound that is easiest to digest or that provides the most energy.
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From page 28... ...
Aquifer Clogging Stimulating the growth of enough microorganisms to ensure contaminant degradation is essential to in situ bioremediation. However, if all the organisms accumulate in one place, such as near the wells that supply growth-stimulating nutrients and electron acceptors, microbial growth can clog the aquifer.
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The fourth column describes contaminant properties that may limit bioremediation. The key limiting properties are the contaminant's tendency to sorb to subsurface solids and to partition into a nonaqueous phase that travels separately from the ground water.
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For example, PAHs are released when crude oil is refined and from the manufacture of petroleum products such as plastics. Creosote is used in wood preservatives.
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Halogenated Aliphatics Halogenated aliphatic compounds are compounds built from straight chains of carbon and hydrogen with varying numbers of hydrogen atoms replaced by halogen atoms. Halogenated aliphatics are effective solvents and degreasers and have been widely used in manufacturing and service industries, ranging from automobile manufacturing to dry cleaning.
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However, as discussed above for the aliphatic compounds, anaerobic microbes can remove chlorine atoms from the highly halogenated aromatics. As the halogen atoms are replaced by hydrogen atoms, the molecules become susceptible to aerobic attack.
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Figure 2-2 illustrates the differences between the two. Intrinsic bioremediation manages the innate capabilities of naturally occurring microbial communities to degrade environmental pollutants without taking any engineering steps to enhance the process.
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Consequently, contaminant degradation occurs more quickly in the engineered system. Intrinsic bioremediation requires extensive monitoring to ensure that the contaminant does not advance more quickly than the native microbes can degrade it.
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Third, concentrations of the crude oil components benzene and ethylbenzene, which are susceptible to aerobic degradation but less susceptible to anaerobic degradation, remained relatively stable in the anaerobic portion of the plume but decreased dramatically at the outer edges of the plume, where mixing with oxygenated water allowed aerobic degradation to occur. The evidence from this site shows that, in hydrologic settings where intrinsic bioremediation rates are fast relative to hydrologic transport rates, native microbes can effectively confine contaminants to near the spill source without further human intervention.
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pictured here. Thus, at this site, intrinsic bioremediation has effectively confined the contamination to a small region.
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Other terms used to describe engineered bioremediation include "biorestoration" and "enhanced bioremediation." As summarized in Box 2-3 and described below, the site conditions that influence a bioremediation project's success differ for intrinsic and engineered bioremediation. Site Conditions for Engineered Bioremediation Because engineered bioremediation uses technology to manipulate environmental conditions, the natural conditions are less important for engineered than for intrinsic bioremediation.
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The process requires thorough site monitoring to demonstrate that contaminant removal is occurring. Key characteristics of sites amenable to intrinsic bioremediation are: O Consistent ground water flow (speed and direction)
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Carbonates can also occur as shell material in beach deposits. Intrinsic bioremediation is more extensive when the ambient ground water surrounding the spill has high concentrations of oxygen or other electron acceptors.
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Also necessary for intrinsic bioremediation is the presence of the elemental nutrients that microbes require for cell building, especially nitrogen and phosphorus. Although nutrients must be present naturally for intrinsic bioremediation to proceed, the quantity of nutrients required is much less than the quantity of electron acceptors.
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FURTHER READING While this chapter has briefly reviewed the principles underlying successful bioremediation, the references listed in Table 2-2 provide more thorough coverage of the key disciplines related to bioremediation. The list is not exhaustive.
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