Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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COMPOSITION AND METHOD FOR DEGRADATION
OF NITROAROMATIC CONTAMINANTS
' FIELD OF THE INVENTION
This invention relates to the removal of
nitroaromatic orgtLnic chemical contaminants from soils,
waters, or sediments by making the contaminants more
readily degradable by microorganisms. More specifically,
this invention relates to the use of fibrous organic
matter together with certain multi-valent metal
particles, which, when added to soil or water containing
nitroaromatic organic chemical contaminants, creates an
anaerobic, reductive environment which promotes
degradation of the chemical contaminants.
BACKGROUND OF THE INVENTION
Many nitroaromatic organic chemical contaminants
present in the environment are known to be highly
resistant to degradation. Current research has
demonstrated that their persistence in the environment
may be overcome by initially subjecting these
contaminants to reducing conditions. Once the nitro
groups are reduced to amine groups, the organic
contaminants are usually degraded easily, generally by
aerobic microbial processes.
Degradation of organic contaminants in microbial
ecosystems occurs by both enzymatic and non-enzymatic
mechanisms. An example of a non-enzymatic mechanism is
the reductive dechlorination of DDT (1,1,1-trichloro-2,2-
' 30 bis(p-chlorophenyl)ethane) by an iron porphyrin redox
system wherein the DDT is reacted with a reduced iron
porphyrin such as hematin. Most enzymatic reactions
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involve whole microbial cells such as bacteria, fungi and
algae. Enzymatic reactions are usually more specific
than non-enzymatic reactions but their activity is
destroyed by harsh conditions such as exposure to high
temperatures.
Microbial activity can assist degradation of organic
contaminants either directly by enzyme production, or
indirectly, by maintaining the reducing conditions of the
environment and thereby enhancing the inorganic and
biochemical mechanisms.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a
method for degrading nitroaromatic organic chemical
contaminants in the environment.
It is another object of this invention to provide a
novel composition which is useful for degrading
nitroaromatic organic chemical contaminants in the
environment.
In accordance with the present invention, there has
been provided a novel method of decomposing nitroaromatic
organic chemical contaminants in water, sediment or soil
comprising adding to the water, sediment or soil a
mixture of fibrous organic matter which is capable of
supporting bacterial or fungal growth and certain multi-
metal particles in amounts effective to provide a
valent
negative redox potential which results in conditions
which promote reducting conditions and degradation of the
nitroaromatic organic chemical contaminants.
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Also provided in accordance with the present
invention is a novel composition useful for degrading
nitroaromatic organic chemical contaminants comprising a
mixture of fibrous organic matter which is capable of
supporting bacterial or fungal growth and certain multi-
valent metal particles in a weight ratio of 0.001:1 to
1:1 respectively.
DETAILED DESCRIPTION
It has now been discovered that a combination of
fibrous organic matter which is capable of supporting
bacterial or fungal growth_and certain multi-valent metal
particles, when added to soil, water or sediment which is
contaminated with nitroaromatic organic chemicals, can
provide an environment which has a stable negative redox
potential, i.e., a reducing environment, which is
conducive to the enhanced degradation or decomposition of
the nitroaromatic organic contaminants.
In accordance with the principles of this invention,
there has been provided a method for decomposition or
decay of nitroaromatic organic chemical contaminants
which are present in soil, water or sediment. The method
of this invention comprises mixing fibrous organic
matter, together with certain multi-valent metal
particles into the soil, water or sediment to be
decontaminated. This mixture is then incubated under
suitable temperature and moisture conditions which are
conducive to anaerobic microbiological growth, i.e.,
generally at a temperature between 0 to 60 C preferably
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between 10 C and 40 C and most preferably between 25 C to
37 C, and at a moisture content for soil and sediment
samples, generally greater than 50% moisture content and
preferably at 100% of the water holding capacity of the
soil or sediment.
Microorganisms must be present in the mixture, and
are generally indigenous to both the organic matter and
the contaminated soil, water or sediment being treated.
Alternatively, additional microorganisms may optionally
be added to the contaminated soil, water or sediment
prior to introduction of organic matter or before,
during, and after the subsequent incubation period.
During the incubation period the combination of organic
matter and multi-valent metal particles provide an
enhanced reducing environment wherein the nitroaromatic
organic chemical contaminants are easily degraded or
decomposed by microorganisms which are naturally present
in the soil, water or sediment.
For purposes of explanation and not limitation, it
is believed that the fibrous organic matter provides
nutrients for aerobic and facultatively anaerobic
microorganisms. The growth of these microorganisms
consumes oxygen which promotes anaerobic conditions which
lowers the redox potential of the environment. The redox
potential may also be lowered by reducing compounds such
as sulfur-containing amino acids and the like which may
be present in the organic matter and also by the reducing
power of the multi-valent metal particles. This
environment promotes the growth of anaerobic
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microorganisms whose activity lowers and maintains a
strong negative redox potential i.e., creates strong
reducing conditions which are conducive to the reduction
of nitro groups to amine groups. The resulting system
contains a wide spectrum of inorganic, biochemical, and
enzymatic redox systems; some or all of which promote the
degradation of the nitroaromatic organic contaminants.
After reduction, the organic contaminants tend to be more
readily degradable, and will thus rapidly decompose or
decay by natural processes in the environment,
particularly if aerobic conditions are subsequently
maintained.
The present invention is of general.applicability
with regard to the precise nature of the fibrous organic
matter, provided of course that it is fibrous, that it
can be readily mixed with the contaminated soil, sediment
or water and that it is capable of supporting bacterial
or fungal growth. It is considered an important feature
of this invention that the organic matter be fibrous. it
has now been discovered that the use of fibrous organic
matter permits absorption of the nitroaromatic organic
chemical contaminants into the fibrous structure which
enhances the rate of degradation. Suitable fibrous
organic matter is generally derived from plant matter
such as crops, crop residue, bushes or trees including
their byproducts (e.g. sawdust), grass and weeds and
algae. Depending on the bioavailable nutrient content
i.e., the level of soluble sugars, carbohydrates and/or
amino acids; the physical structure of the organic
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matter, i.e., surface area/particle size and/or the
chemical properties (i.e., its carbon:nitrogen ratio
which is generally less than 50:1, preferably less than
25:1 and is most preferably around 10:1), it may be
beneficial to blend different sources of plant matter
together. Plant matter which is high in nitrogen content
e.g., leguminous plant matter is particularly preferred.
Alternatively, the plant matter may be supplemented with
nitrogenous material such as amines, nitrates, etc.,
including but not limited to ammonium nitrate, urea,
calcium nitrate, and the like, and mixtures thereof. The
plant matter may also be supplemented with other fibrous
or non-fibrous organic matter such as simple carbon
sources including carbohydrates, such as sugars, organic
acids such as lactic acids, and the like and mixtures
thereof; as well as complex organic matter including
sewage sludge, potato processing waste, molasses, spent
distiller grains, and spent coffee grounds and the like
and mixtures thereof. The fibrous organic matter is
preferably cut or ground into small particles in order to
increase the exposed surface area and thereby enhance its
contact with the soil components and absorption of the
nitroaromatic organic chemical contaminants. The
particle size of the fibrous organic matter is not, per
se, critical to the invention provided of course that it
can be readily mixed with the contaminated soil and is
generally in a thickness range of from 0.001 mm to 25 mm.
The fibrous plant matter particles may be applied to the
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contaminated environment at a dosage rate of 1.0% to 50%
w/w dry soil, dry sediment or water.
Suitable multi-valent metal particles for use in
this invention include those multi-valent metals which
are capable of being oxidized and reduced back and forth
under normal environmental conditions and which have
average particle diameters ranging from 0.001 mm to 5 mm.
Iron, magnesium and mixtures thereof are most preferred
metals due to their moderately low toxicity and good
reduc_ing power. These metals may be applied at 50 mg to
500 mg per kg of water or kg of dry weight of soil or
sediment preferably 250 mg to 2500 mg per kg of water or
kg of dry weight of soil or sediment. Other preferred
multi-valent metals for use in this invention include
zinc, copper, cobalt, nickel, and mixtures thereof.
However, due to the relatively high toxicity of these
metals, they are generally added at a lower dosage level
than iron or magnesium, generally in the range of 1 to
10,000 mg per kg of water or kg of dry weight of soil or
sediment, preferably 100 to 2500 mg per kg of water or kg
of dry weight of soil or sediment.
Mixtures of metal particles may also be used
advantageously in this invention. For example, some
redox systems such as those based on porphyrins are
complexed with iron while others, such as corins are
complexed with cobalt. Thus, it may be advantageous to
treat some contaminated environments with a combination
of multi-valent metals such as, e.g., a mixture of iron
and cobalt.
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Microorganisms which are known to degrade
nitroaromatic organic chemical contaminants including
their byproducts may optionally be added to further
enhance the degradation reactions. Effective
concentrations of organisms range from 102 to 109 cells
per kg water or kg of dry weight of soil or sediment.
Another embodiment of the present invention is to
pre-incubate a mixture of organic matter and metal
particles, and, if desired, microorganisms, to enhance
the initial reducing power of the mixture and provide
higher microbial content and then introduce this mixture
into the contaminated environment. This embodiment is
particularly advantageous for treating contaminated
environments in which the contaminants are toxic to
microorganisms by increasing the content of desired
microbial species prior to introduction into the
contaminated environment.
It may be convenient to treat the contaminated
water, sediment or soil with a mixture of fibrous organic
matter which is capable of supporting bacterial or fungal
growth and multi-valent metal particles. Thus, in
accordance with this aspect of the invention, there has
been provided a composition which is useful for degrading
nitroaromatic organic chemical contaminants in water,
soil or sediment comprising a mixture of fibrous organic
matter which is capable of supporting bacterial or fungal
growth and multi-valent metal particles wherein the
weight ratio of metal particles to organic matter ranges
from 1:1 to 1:500,000 respectively.
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The following examples are provided to illustrate
the invention in accordance with the principles of this
invention, but are not to be construed as limiting the
invention in any way except-as indicated in the appended
claims. All parts and percentages are by weight unless
otherwise indicated. - -
Example 1
Both microbial and biochemical reductive degradation
processes require the establishment of a reducing
environment. An experiment was designed to show the
ability of the different soil treatments to produce and
maintain a negative redox potential. Soil treatments
were thoroughly mixed into dried, sieved soil which had
been brought to 100% water-holding capacity. The results
in the table below demonstrate that the combination of
iron and organic matter maintain a lower negative redox
potential for a longer duration than iron alone or
organic matter alone. The combination also permits less
treatment to be used.
Effectiveness of Fibrous Organic Matter and Multi-Valent Metals in
Establishing and
Maintaining Reducing Environments in Soil Lil
v~
REDOX POTENTIAL (mV)
TREATMENT 1 day incubation at 25 C 62 days incubation at 25 C
None 333 393
Iron (2.5 gm/kg soil) -414 263
NLA (100 gm/kg soil) -464 108
NLA + Iron (50 g/kg + 0.25 g/kg) -488 -58 o
1 ~
NLA + Iron (50 g/kg + 2.5 g/kg) -545 -135
NLA + Iron (100 g/kg + 0.25 g/kg) -494 -102
NLA + Iron (100 g/kg + 2.5 g/kg) -517 -87
LA (50 g/kg soil) -395 -223
ILA + Iron (25 g/kg + 0.25 g/kg) -512 -540
LA + Iron (25 g/kg + 2.5 g/kg) -520 -339
LA + Iron (50 g/kg + 0.25 g/kg) -484 -371 n I
LA + Iron (50 g/kg + 2.5 g/kg) -521 -272
~=
M
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NLA = non-leguminous additive (wheat straw)
LA = leguminous additive (alfalfa)
ti
Exam-Dle 2
The effectiveness of different treatments on the
degradation of soil samples contaminated with
nitroaromatics was evaluated. The treatments were
thoroughly mixed into dried sieved soil samples which had
been'brought to 100% water-holding capacity. The table
below shows the results after 31 days of incubation at
ambient laboratory temperatures.
Amount of Nitroaromatics Remaining After
31 days of Treatment
Nitroaromatic Control LA & Iron
2,4,6-TNT 98% 19%
1,2,5-TNB 10043%
Control = No treatment
LA = leguminous plant additive (alfalfa), added
at 2.5% (W/W, dry soil)
TNT = Trinitrotoluene
TNB = Trinitrobenzene
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Iron was added at 1 g/kg soil with LA amendment.
The initial concentration of 2,4,6-TNT was 7,200 ppm
while initial level of 1,2,5-TNB was 23 ppm.
A negative redox potential was measured in the
treated soil indicating anaerobic conditions and a
reducing environment. The first step in degradation
under anaerobic conditions is usually reduction of nitro
groups to amine groups. Since the loss of nitroaromatics
could not be attr_ibuted to complete mineralization, as
indicated by experiments using 14C labelled
nitroaromatics, the original nitroaromatic compounds must
have been substantially degraded.
