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Bioremediation can be defined as any process that uses microorganisms or their enzymes to return the environment altered by contaminants to its original condition. Bioremediation may be employed in order to attack specific contaminants, such as chlorinated pesticides that are degraded by bacteria, or a more general approach may be taken, such as oil spills that are broken down using multiple techniques including the addition of nitrate and sulfate fertilizer to facilitate the decomposition of crude oil by bacteria.
Not all contaminants are easily treated through the use of bioremediation; for example, heavy metals such as cadmium and lead are not readily absorbed or captured by organisms. The integration of metals such as mercury into the food chain may make things worse as organisms bioaccumulate these metals.
However, there are a number of advantages to bioremediation, which may be employed in areas which cannot be reached easily without excavation. For example, hydrocarbon spills (or more specific: petrol) may contaminate groundwater well below the surface of the ground; injecting the right organisms, in conjunction with oxygen-forming compounds, may significantly reduce concentrations after a period of time. This is much less expensive than excavation followed by burial elsewhere, incineration or other ex situ treatment and reduces or eliminates the need for pumping and treatment, which is a common practice at sites where hydrocarbons have contaminated groundwater.
Generally, bioremediation technologies can be classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site while ex situ involves the removal of the contaminated material to be treated elsewhere. Some examples of bioremediation technologies are bioventing , land farming , bioreactor, composting, bioaugmentation and biostimulation.
The process of bioremediation can be monitored by measuring the Oxidation Reduction Potential or redox in soil and groundwater, together with pH, temperature and oxygen content. This table shows the (decreasing) biological breakdown rate as function of the redox potential.
|Process||Reaction||Redox potential (Eh in mV)|
|aerobic:||O2 + 4e- + 4H+ → 2H2O||600 — 400|
|denitrification||2NO3- + 10e- + 12H+ → N2 + 6H2O||500 — 200|
|manganese IV reduction||MnO2 + 2e- + 4H+ → Mn2+ + 2H2O||400 — 200|
|iron III reduction||Fe(OH)3 + e- + 3H+ → Fe2+ + 3H2O||300 — 100|
|sulfate reduction||SO42- +8e- +10H+ → H2S + 4H2O||0 — -150|
|fermentation||2CH2O → CO2 + CH4||-150 — -220|
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