Bioremediation

Bioremediation can be defined as any process that uses microorganisms, fungi, green plants or their enzymes to return the environment altered by contaminants to its original condition. Bioremediation may be employed to attack specific soil contaminants, such as chlorinated hydrocarbons that are degraded by bacteria, or a more general approach may be taken, such as oil spills that are broken down by multiple techniques including the addition of nitrate and/or sulfate fertilizers to facilitate the decomposition of crude oil by indigenous or exogenous bacteria.

Examples and problems

Naturally-occurring bioremediation and phytoremediation have been used for centuries. For example, desalination of agricultural land by phytoextraction has a long tradition. Bioremediation technology using microorganisms was reportedly invented by George M. Robinson. He was the assistant county petroleum engineer for Santa Maria California. During the 1960's he spent his spare time experimenting with dirty jars and various mixes of microbes.

Not all contaminants are easily treated through the use of bioremediation with microorganisms; 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 worsen matters. Phytoremediation is useful in these circumstances, as many plants can bioaccumulate these toxins in their above-ground parts, which are then harvested for removal. The heavy metals in the harvested biomass can be further concentrated by incineration.

Using genetic engineering to create organisms specifically designed for bioremediation has great potential. The bacterium Deinococcus radiodurans (the most radioresistant organism known) has been modified to consume and digest toluene and ionic mercury from highly radioactive nuclear waste.

There are a number of cost/efficiency advantages to bioremediation, which can be employed in areas that inaccessible without excavation. For example, hydrocarbon spills (specifically, petrol spills) or certain chlorinated solvents may contaminate groundwater and introducing the appropriate electron acceptor or electron donor amendment, as appropriate, may significantly reduce contaminant concentrations after a lag time allowing for acclimation. This is typically much less expensive than excavation followed by disposal elsewhere, incineration or other ex situ treatment strategies, and reduces or eliminates the need for "pump and treat", 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, landfarming, bioreactor, composting, bioaugmentation, rhizofiltration, and biostimulation.

Monitoring bioremediation

The process of bioremediation can be monitored indirectly by measuring the Oxidation Reduction Potential or redox in soil and groundwater, together with pH, temperature, oxygen content, electron acceptor/donor concentrations, and concentration of breakdown products (e.g. carbon dioxide). This table shows the (decreasing) biological breakdown rate as function of the redox potential.

Process	Reaction	Redox potential (E_h in mV)
aerobic:	O₂ + 4e^- + 4H⁺ → 2H₂O	600 — 400
anaerobic:
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

This, by itself and at a single site, gives little information about the process of remediation.

it is necessary to sample enough points on and around the contaminated site to be able to determine contours of equal redox potential. Contouring is usually done using specialised software, e.g. using Kriging interpolation.
if all the measurements of redox potential show is that electron acceptors have been used up, it's in effect an indicator for total microbial activity. Chemical analysis is also required to determine when the levels of contaminants and their breakdown products have been reduced to below regulatory limits.

References

External links

Bioremediation (Toxic Cleanup) News from Genome News Network (GNN)

Bioremediation Discussion Group (BioGroup)

Phytoremediation Website hosted by the Missouri Botanical Garden

Biotechnology | Microbiology | Environmentalism | Environmental soil science | Soil contamination

Gourt Home::Gourt :: Bioremediation

Examples and problems

Monitoring bioremediation

See also

References

External links