Bioremediation is a waste management technique that
involves the use of organisms to remove or neutralize pollutants from a
contaminated site. According to the EPA, bioremediation is a “treatment that
uses naturally occurring organisms to break down hazardous substances into less
toxic or non toxic substances”. Technologies can be generally 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 related
technologies are phytoremediation, bioventing,
bioleaching,
landfarming,
bioreactor,
composting,
bioaugmentation,
rhizofiltration,
and biostimulation.
Bioremediation may occur on its own (natural attenuation or
intrinsic bioremediation) or may only effectively occur through the addition of
fertilizers, oxygen, etc., that help encourage the growth of the
pollution-eating microbes within the medium (biostimulation). For example, the US Army Corps of Engineers demonstrated
that windrowing and aeration of petroleum-contaminated soils enhanced
bioremediation using the technique of landfarming.
Depleted soil nitrogen status may encourage biodegradation of some nitrogenous
organic chemicals, and soil materials with a high capacity to adsorb pollutants
may slow down biodegradation owing to limited bioavailability
of the chemicals to microbes. Recent advancements have also proven successful
via the addition of matched microbe strains to the medium to enhance the
resident microbe population's ability to break down contaminants. Microorganisms
used to perform the function of bioremediation are known as bioremediators.
However, not all contaminants are easily treated by
bioremediation using microorganisms. For example, heavy
metals such as cadmium and lead are not readily absorbed or captured by microorganisms. A
recent experiment, however, suggests that fish bones have some success
absorbing lead from contaminated soil. Bone char has been shown to bioremediate
small amounts of cadmium,
copper, and zinc. The assimilation
of metals such as mercury into the food chain
may worsen matters. Phytoremediation is useful in these circumstances
because natural plants or transgenic
plants are able to bioaccumulate these toxins in their above-ground parts,
which are then harvested for removal. The heavy metals in the harvested biomass
may be further concentrated by incineration or even recycled for industrial
use. Some damaged artifacts at museums contain microbes which could be
specified as bio remediating agents.
The elimination of a wide range of pollutants and wastes
from the environment requires increasing our understanding of the relative
importance of different pathways and regulatory networks to carbon flux
in particular environments and for particular compounds, and they will certainly
accelerate the development of bioremediation technologies and biotransformation
processes.
Genetic engineering approaches
The use of 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.
Mycoremediation
Mycoremediation is a form of bioremediation in which fungi are used to
decontaminate the area. The term mycoremediation
refers specifically to the use of fungal mycelia in
bioremediation.
One of the primary roles of fungi in the ecosystem is decomposition,
which is performed by the mycelium. The mycelium secretes extracellular enzymes and acids that break down lignin and cellulose,
the two main building blocks of plant fiber. These are organic compounds
composed of long chains of carbon and hydrogen, structurally similar to many organic pollutants.
The key to mycoremediation is determining the right fungal species to target a
specific pollutant. Certain strains have been reported to successfully degrade
the nerve
gases VX and sarin.
In one conducted experiment, a plot of soil contaminated
with diesel
oil was inoculated with mycelia of oyster
mushrooms; traditional bioremediation techniques (bacteria) were used on
control plots. After four weeks, more than 95% of many of the PAH (polycyclic aromatic hydrocarbons)
had been reduced to non-toxic components in the mycelial-inoculated plots. It
appears that the natural microbial community participates with the fungi to
break down contaminants, eventually into carbon dioxide and water.
Wood-degrading fungi are particularly effective in breaking down aromatic
pollutants (toxic components of petroleum), as well as chlorinated compounds (certain
persistent pesticides;
Battelle, 2000).
Two species of the Ecuadorian fungus Pestalotiopsis are
capable of consuming Polyurethane in aerobic and anaerobic conditions such as
found at the bottom of landfills.
Mycofiltration is a similar process, using
fungal mycelia to filter toxic waste and microorganisms
from water in soil.
Advantages
There are a number of cost/efficiency advantages to
bioremediation, which can be employed in areas that are 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 long 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 practice common at sites where
hydrocarbons have contaminated clean groundwater.
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 (Eh in mV)
|
|
O2 + 4e− + 4H+ → 2H2O
|
600 ~ 400
|
|
|
|
||
|
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− +10 H+
→ H2S + 4H2O
|
0 ~ −150
|
|
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 that electron acceptors have been used up, it is 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.
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