BARR for Decontamination of Soil and Groundwater - Volume 1
This is the REPORT
Copyright 1993 by Larry Dieterich All Rights Reserved Duplication and
distribution is permitted, but credit must be given to the author and the
document must be distributed in its entire form. No fragmentation, editing
or deletions are permitted on copies for distribution.
This document is part of a 3 volume set.
Volume 1 (this document) BARR report
Volume 2 BARR Technical Appendix
Volume 3 BARR Bibliography
The BARR process was developed by Larry Dieterich
Larry Dieterich
405 E 7th Street
Davis, California 95616
USA
voice/fax (916) 758-9260
Internet email-
[email protected]
This edition of the BARR reports is made available in ASCII format for
distribution over the Internet.
Paper, disk copies or formatted Macintosh files with graphics are
available upon request. Contact the author at the above address.
BARR: For Decontamination of Soil and Groundwater
B Bio-
A Anaerobic
R Reduction &
R Re-oxidation
Bio-Anaerobic Reduction & Re-oxidation
BARR: For Decontamination of Soil and Groundwater
REPORT
ABSTRACT
A technique for in situ degradation of organic pollutants in the
subsurface is described. The BARR process (Bio Anaerobic Reduction &
Reoxidation) is a technique that utilizes a "treatment cocktail" to
stimulate microbial activity to form multiple and var iable catalysts and
create sequential thermodynamic gradients to induce the transformation and
degradation of the target chemicals. The "treatment cocktail" utilizes
biodegradable reductants and oxidants with diverse, broad-spectrum
inoculum in a buffered mixture that provides for the nutritional needs of
the microbial components of the process. Mineralization of the target
pollutant is achieved through a combination of direct metabolism,
cometabolism and abiotic physical degradation processes, which are used
again and again, until complete mineralization of the target pollutant has
been achieved.
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TABLE OF CONTENTS- The next few pages are a detailed table of contents for
the report.
ABSTRACT
INTRODUCTION
THE BARR PROCESS DESCRIBED -Underground Bioreactor The BARR process
capitalizes on existing Technology. The BARR process begins with location
and description of the contaminated area. The next step in the BARR
process is to assess the contaminant subsurface distribution. Approximate
3-dimensional mapping of the subsurface profile (soil and groundwater),
including contaminant distribution, soil types and strata will be
accomplished as part of the initial site assessment process. The chemical
and microbial parameters of the system will be investigated also. An
important product of site characterization process using probes is the
resulting access to the formation.
Contaminant Isolation
The contaminated zone may be isolated from the rest
of the formation by placement of materials to plug the porous strata at
the perimeter of the contaminant zone. Injection of clay solutions may
clog the transmissive strata permanently. Injection of substrate and
inoculum may be used to clog the porous strata and thereby isolate the
treatment zone. Injection of a wax/steam emulsion into the porous strata
surrounding the contaminant zone may be used to seal the system.
Treatment Cocktail Delivery
"Treatment cocktail" delivery techniques and timing are an important part
of the overall treatment scheme. Coupled Flow will be used to move the
treatment cocktail into the highly polluted formations and strata.
Inoculum
The "treatment cocktail" will contain inoculum capable of degrading both
the contaminant and added surfactant/substrate under reducing and
oxidizing conditions. Inoculum provisions are made to insure that suitable
organisms are present in the treatment zone. It may be possible to rely on
the native microbial population in some sites. In many sites, it will be
desirable to provide inoculum in the form of landfill leachate or other
material collected from sites where the native microflora has been
conditioned to degrade pollutants.
Specific Toxicity
Specific Toxicity = (growth inhibitory toxin concentration/population
density)
Treatment Cocktail Composition
Based on the chemical, physical and microbial parameters established in
the site analysis and characterization step, a first stage "treatment
cocktail" is prepared for the target pollutant and system.
The treatment cocktail will be miscible with the target pollutant.
The "treatment cocktail" will contain mineral nutritional materials.
Nitrogen may be provided as ammonia.
The "treatment cocktail" will contain biologically active reductant.
Catabolite Repression
The reduced carbon provision might take the form of sugars.
There is a wide variety of fats, oils and waxes that may find application
in the BARR process.
Simple Lipids
Compound Lipids
Phospholipids
Glycolipids
Nonsaponifable Lipids
Cellulose
Hemicellulose
Volatile Fatty Acids
Incubation
After the introduction of the "treatment cocktail", the system is warmed
for anaerobic microbial growth
Monitoring
The chemical and physical status of the treatment zone will be monitored
via in-situ probes accessing the contaminant area, or by solution
extraction for lab analysis.
REOXIDATION AND REDUNDANCY
The repetition of oxidation and reduction cycles will increase the
opportunity for degradation to occur.
CHANGES RESULTING FROM THE BARR PROCESS
Temperature
The BARR process creates temperature changes in the contaminated system.
Coupled Flow
Coupled flow will result from the BARR process
pE
Induced biological activity in the treatment zone will change the pE which
will be accompanied by radical changes in the system chemistry.
The changes in redox potential will tend to be buffered by the local
system composition.
The redox potential can be maintained at some desired level by the
presence of inorganic electron acceptors.
The electron status of the system, described as the redox potential will
flux as the supplies and nature of reductant and oxidant are varied.
pH
Soil colloids are negatively charged and accumulate hydrogen ions.
Bacteria are amphoteric and will acquire positive or negative charge
depending on the pH of the system.
The dissolution of minerals accompanying changes in pE and pH will also
create changes in the makeup of the system solution by increasing the
ionic strength of the solution.
The increase in the ionic strength of the solution will also cause a
collapse in the electrical double layer (multilayer) on charged surfaces.
This solid matrix may be stationary aquifer matrix or mobile colloids.
Introduction of water of low ionic strength will dislodge those particles
which are attached by high-ionic strength conditions.
When oxidation occurs, there will occur mineral reprecipitation.
Colloids
The large surface area associated with these colloids introduces an
increased capacity for surface catalysis.
The colloids formed from the precipitation of minerals are an important
part of the BARR process.
The creation of microbial biomass constitutes a biocolloidal phase that
represents a separate, semipolar phase in the system.
The changes in solution chemistry accompanying the BARR process will cause
repeated flux in the metal ions in solution.
Solution Chemistry
The "treatment cocktail" will serve to increase the solubility of the
pollutant by solvent/surfactant properties.
See Technical Appendix for a complete discussion of cosolvency
The solvent effect of the treatment cocktail will serve to dilute the
concentration of the contaminant, a factor which may enhance the
biodegradability of the pollutant.
Microbes
As the redox status of the system varies, different microbial consortia
will be favored.
The increase in the microbial biomass and the diverse consortia of the
system resulting from the BARR process will increase the amount of enzymes
in the system and thereby increase the opportunity for both direct
metabolism and cometabolism of the target pollutant.
Yeasts
Fungi
Protozoa
Changes in the system will influence the dissemination of microbes in the
treatment zone.
Nitrogen transformations resulting from BARR
The BARR method will be useful for treatment of inorganic as well as
organic contamination
The growth of microbial biomass will tend to diminish the transmissivity
of the aquifer.
Sealant effects may be very beneficial.
The BARR process is repeated until the desired degree of degradation is
achieved.
The BARR process may be used in conjunction with existing pump-and-treat
technology to enhance recovery of the target contaminant.
Aspects of the BARR process requiring special attention.
BARR requires high-class geotechnical & engineering capabilities.
BARR process requires high-class biotechnical capabilities.
BARR is undemonstrated
CONCLUSION
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This begins the actual report
BARR: for Decontamination of Soil and Groundwater
ABSTRACT
A technique for in situ degradation of organic pollutants in the
subsurface is described. The BARR process (Bio Anaerobic Reduction &
Reoxidation) is a technique that utilizes a "treatment cocktail" to
stimulate microbial activity to form multiple and var iable catalysts and
create sequential thermodynamic gradients to induce the transformation and
degradation of the target chemicals. The "treatment cocktail" utilizes
biodegradable reductants and oxidants with diverse, broad-spectrum
inoculum in a buffered mixture that provides for the nutritional needs of
the microbial components of the process. Mineralization of the target
pollutant is achieved through a combination of direct metabolism,
cometabolism and abiotic physical degradation processes, which are used
again and again, until complete mineralization of the target pollutant has
been achieved.
INTRODUCTION
Objective- This report provides a rationale and a procedure to achieve the
in-situ mineralization of organic chemical contamination of the
subsurface.
BARR (Bio-Anaerobic Reduction and Re-oxidation) creates accelerated
degradation of contaminants by creating repetitive redox flux in a high
surface-area biocatalytic environment.
BARR is not strictly bioremediation, rather it is a biochemically assisted
process of stabilizing and degrading contaminants in soil and groundwater.
The process can be carried out in-situ.
The application of bioremediation to contaminated soil and subsoil has
been explored extensively in past years. The promise of bioremedial
processes has been fostered by the observed ability of various microbes to
degrade a wide variety of contaminants. Microbially mediated aerobic
oxidation of organic contaminants has been the objective of nearly all
applications of bioremediation. For a number years, evidence has been
accumulating that anaerobic transformation of "recalcitrant" organic
materials is poss ible. Both aerobic and anaerobic degradation have
demonstrated limitations of capability; chiefly the inability to
completely mineralize recalcitrant compounds.
More recently, there has been a significant body of evidence presented
that indicates that sequential oxidation and reduction may hold the key to
effective bioremediation. Coupled anaerobic-aerobic process have been
reported to degrade recalcitrant organ ics. For example, reductive
dechlorination of PCB's produces less-chlorinated congeners which are
suitable substrates for oxidative degradation by a wide range of aerobic
organisms. (HARKNESS et. al. 1993)
Sequential environments are sometimes the best alternative for the
detoxification of organic compounds. For example, compounds that degrade
through a series of reductive and oxidative steps are most efficiently
biodegraded by sequential anaerobic-aerobic processes. Other benefits
relate to the detoxification of a broad range of chemicals. Aerobic and
anaerobic environments each have limitations in their biodegrading
abilities, but they often compliment each other when they are combined.
One limitation of aerobic processes involves the recalcitrance of highly
chlorinated chemicals such as hexachlorobenzene, tetracholoroethylene and
carbon tetrachloride, which appreciably degrade only under anaerobic
conditions. In contrast, conventionally cultured aerobes are efficient
degraders of aromatic compounds that are anaerobically recalcitrant.
Notable exceptions exist to these generalizations. For example, highly
chlorinated compounds such as 1,1,1-trichloroethane, trichloroethylene and
chloroform will biotransform under aerobic conditions if methane, phenol
or toluene is provided as primary source of carbon and energy for growth.
However, these reactions are cometabolic rather than direct metabolism. In
fact, the majority of highly chlorinated compounds, such as
1,2,4-trichlorobenzene, 1,2,4,5-tetrachlorobenzene, and hexachlorobenzene
are recalcitrant under aerobic treatments. In contrast to highly
chlorinated aliphatic compounds, aromatic compounds are more successfully
degraded under aerobic, rather than anaerobic conditions. Conventionally
cultured aerobic microorganisms are considered particularly successful
degraders of aromatic compounds because they often produce mixed function
oxidase enzymes, which initiate aromatic ring cleavage.
This report provides the technical background for this new, emerging
technology and describes a process whereby the biological and abiological
processes of sequential oxidation and reduction may be utilized to
effectively degrade contaminants of the surface and subsurface in-situ.
The BARR process turns the subsurface treatment zone into an in-situ
bioreactor. By manipulating properties such as pE, pH, temperature,
chemical composition of the system and mass flow, BARR creates fundamental
changes in the system. The presence of specialized enzymes and extremely
high surface areas, along with modifications to the polarity of the
aqueous phase, exert strong transformation pressures on contaminants. BARR
is a broad spectrum treatment.
The overall result of the BARR process is the dissolution and complete
degradation of the target contaminants to mineral components.
THE BARR PROCESS DESCRIBED
The BARR process utilizes thermodynamic gradients and multiple mechanisms
of transformation to degrade the target pollutant in a multiple step
process.
In the first step, the polluted system is characterized and mapped. Where
possible and desirable, the contaminated formation may be isolated by
injection of different materials to temporarily or permanently plug the
transmissive strata surrounding the pol luted zone. A "treatment cocktail"
is delivered into the contaminated system which creates biological
activity and causes a decrease in the pE and shifts in other fundamental
chemical and physical parameters of the system, such as pH, temperature
and chem ical composition. The chemical and physical status of the system
is monitored and incubated before a second "treatment cocktail" is
introduced to re-oxidize the system. The process is repeated. In
subsequent steps, the "treatment cocktail" is varied to cr eate sequential
reducing and oxidizing conditions in the treatment zone. The sequential
environments are repeatedly created until complete degradation is
achieved.
Coupled, or sequential anaerobic-aerobic process have been reported to
completely mineralize a wide range of recalcitrant organics.
The BARR process capitalizes on existing Technology.
BARR uses existing geotechnical and site characterization methods.
Furthermore, the use of existing landfill experience is of great value to
the development of this process. Biotechnical expertise and experience
with contaminant microbiology are utilized by the BARR process. BARR also
makes use of existing steam technology and relies heavily on probe and
injection lance technology. (Contact Larry Dieterich (the author) for
information about existing lance and probe technology. The author may be
contacted via email; his Internet address is
[email protected]).
The BARR process begins with location and description of the contaminated
area. Standard methods are used to determine the depth and lateral extent
of the contaminated soil and water. These methods include surface
examination and site record analysis, including current and historical
records. Where possible, aerial photographs of the site may reveal
information useful to the remediation process.
In some cases, it may be necessary and practical to remove the
contaminated surface soil to reveal the location of the pollutant entry
into the subsoil. Where excavation is necessary, the contaminated soil
will be stockpiled and treated thermally, biologically or with BARR in an
on-site or off-site heap process.
The objective of the site analysis is to determine the nature of the
contamination and the location of the target pollutant in the subsurface.
The next step in the BARR process is to assess the contaminant subsurface
distribution. The subsurface investigation is carried out with available
geophysical tools. Where possible, the subsurface will be characterized
and the contaminant distribution ascertained by utilizing hydraulic
probes fitted with soil samplers.
Where possible, the BARR process relies on the use of probes for sampling
and contaminant location and characterization. The cost of investigation
with hydraulic probes is much less than with the use of drilled wells.
Where probes are used, soil strata and contaminant distribution may be
determined by groundwater and soil core recovery from hollow sampler
attachments. Soil core samples are taken to ascertain the subsurface
profile as well as contaminant distribution.
In addition to the location of the major zone of contamination, the
downslope areas are probed and the soil vapor phase is sampled to track
the distribution of the contaminant plume. This is practical for volatile
contaminants or volatile tracers that may indicate the location and nature
of the contaminant.
Soil texture is an important parameter in pollutant distribution because
hydrophobic contaminants tend to distribute themselves in the finer
textured sediments. Sands in the vadose zone will tend to pass liquid
contaminant downward. Clay layers (or other confining layers) will tend
to stop the material and impede its downward flow. Lateral flow along the
top of clay strata may be expected as well.
The behavior of the contaminant when it encounters the water table will
depend largely on the density of the contaminant. Liquids heavier than
water will tend to sink while those lighter than water will tend to float
on the water surface. Low-density contaminant liquids that float on top
of the water table and will tend to "smear" at phreatic surface. The
fringe zone gets coated as the water table rises and falls. The water
soluble fraction mixes with the groundwater and the insoluble fraction
floats. It floats downgradient as well, downslope under the influence of
gravity.
If the contaminant liquids are heavier than water, they move down, through
the groundwater. They are moved by the mass flow of the moving groundwater
as they move downward. If they encounter an impermeable layer, they will
flow downslope. Some of the material will dissolve in the water,
depending on the intrinsic water solubility and the chemistry of the
solution.
Approximate 3-dimensional mapping of the subsurface profile (soil and
groundwater), including contaminant distribution, soil types and strata
will be accomplished as part of the initial site assessment process.
The chemical and microbial parameters of the system will be investigated
also. In addition to the soil textural distribution, soil and pollutant
chemical parameters such as pH, pE and EC (electrical conductivity) of the
subsurface soil and groundwater will be ascertained as part of the
preliminary assessment in the BARR process.
Contaminant location and assessment will be accompanied by sampling to
assess the mineral nutritional status of the system as it regards
microbial mineral nutritional needs. This will include nitrogen,
phosphorous, potassium, etc.
As part of the contaminant analysis, the biodegradability or biocidal
properties of the contaminant will be estimated.
Microbial population assessment is part of the characterization and
analysis process as well. Indicators of the resident microbial population
will be determined.
All of these parameters will be estimated as part of the contaminant
distribution assessment which will be accomplished, where possible, by
probes and core recovery.
The nature of the target pollutant, and the chemical and microbial
composition of the subsurface system will help determine the required
inoculum provisions, the pH and pE buffering capacity of the system and
the composition of the "treatment cocktail".
An important product of site characterization process using probes is the
resulting access to the formation. The contaminated formation is accessed
by the sampling probes, which serve as injection lances into the ground.
Many or all of the probes are left in the ground to provide access to the
contaminated subsurface. The ground resembles a "pin cushion" after the
characterization and investigation process. These probes, fitted with
porous sections, serve as conduits which may be used to deliver a variety
of materials into the ground to isolate and treat the contaminant.
Contaminant Isolation
The contaminated zone may be isolated from the rest of the formation by
placement of materials to plug the porous strata at the perimeter of the
contaminant zone.
This presupposes detailed knowledge of the subsurface and the ability to
selectively place materials such as biomass, clay or wax to seal the
transmissive formation around the outside of the contaminated zone. If the
contaminated zone is isolated and contained, any liquid materials added in
the treatment cocktail will remain in the treatment zone. In such an
event, accumulated liquid in the treatment zone may have to be pumped out.
The requirement of liquid removal might necessitate a drilled well. In
most cases, or where predominately gas "treatment cocktail" is used, there
will be no such requirement for liquid removal.
Injection of clay solutions may clog the transmissive strata permanently.
This may or may not be desirable. It may also be difficult to effectively
plug the formation with clay slurries.
Injection of substrate and inoculum may be used to clog the porous strata
and thereby isolate the treatment zone. Given that there is a lag time
anticipated with microbial response to "treatment cocktail", it may be
possible to place a considerable volume of "treatment cocktail" in the
contaminated formation before significant microbial growth begins.
Biologically active oxidizing conditions tend to cause greater biomass
accumulation than biological activity under reducing conditions, because
anaerobic metabolism produces less usable energy than aerobic metabolism.
The amount of biomass, and hence the effectiveness of the plugging from
such a treatment, will largely be a function of the aeration status of the
system and will vary with changes in "treatment cocktail". Such a
formation-plugging process will be temporary, since the biomass will
decrease when the substrate is exhausted by microbial metabolism.
In addition to the production of microbial biomass, the oxidizing step
will result in the precipitation of iron and manganese compounds, which
will also tend to plug the formation under oxidizing conditions. This
mineral precipitate will be subject to manipulation in the interior of the
treatment zone, where the redox is controllable, but the precipitate will
persist at the perimeter of the treatment zone, where an oxic interface
exists with the ambient subsurface. Oxygen diffusion from the atmosphere
will tend to create oxidizing conditions in the subsoil in the absence of
some reducing agent.
Formation Sealing and Isolation
Injection of a wax/steam emulsion into the porous strata surrounding the
contaminant zone may be used to seal the system. Subsurface temperature is
relatively constant. In the absence of heat generated by the oxidation
steps of the BARR process, or intentional heat injection into the system,
the soil and groundwater surrounding the contaminated formation will be
relatively constant temperature. A paraffin wax, or hydrogenated vegetable
oil can be introduced into the formation as a heated emulsion. Once the
formation returns to its natural temperature, the emulsion will form a
solid in the transmissive strata. The wax/steam emulsion may be formulated
to contain catalysts, nutrients and inoculum to degrade the wax within a
known time frame, under predetermined conditions.
The isolation step assists in the containment of the pollutant against
migration out of the zone of treatment. It may be challenging to
effectively isolate a contaminated formation, however partial plugging of
connected transmissive strata may serve to minimize pollutant transfer
into sensitive groundwater resources nearby.
Treatment Cocktail Delivery
"Treatment cocktail" delivery techniques and timing are an important part
of the overall treatment scheme. Obviously, the ability to deliver
treatment cocktail to the pollutant will depend on a number of things,
notably the texture and layer structure of the subsurface. As well as the
depth and lateral extent of the contamination. Cobbles and stones in the
subsurface may also frustrate efforts to place injection lances to deliver
the "treatment cocktail". In some situations, in very difficult soils,
piledrivers may be used drive injection lance jackets. Drilled wells may be
needed, but for reasons of expense, should be minimized where possible.
Under good subsurface conditions, this technique is effective for small or
large areas of highly or slightly contaminated soil and groundwater. The
same probes used for sampling and contaminant distribution assessment can
also be used for cocktail delivery.
(ascii format does not support this graphic- see original report for
graph) (graphic inserted here showing lance installation and cocktail
delivery)
The "treatment cocktail" delivery method will be largely dependent on the
type of formation in the treatment zone. In areas where the cocktail can
be delivered right into the contaminated formation, it may be possible to
pump "treatment cocktail" directly into the contaminated soil.
Areas with accessible transmissive strata directly adjacent to high
concentrations of pollutant are ideal for cocktail delivery. For example,
a contaminated clay formation overlain by a porous layer could be treated
by injection of "treatment cocktail" into the porous layer. The "treatment
cocktail" would then migrate to cover and permeate the contaminated clay.
A hydrophobic "treatment cocktail" would be driven by hydrophobic forces
to diffuse into the finer textured matrix, in response to the same
gradient that drives hydrophobic contaminants to partition into fine
textured materials.
High pressure pumps may be able to deliver "treatment cocktail" into clay
formations where high concentrations of contaminants are detected. In such
"tight" formations, the use of gaseous "treatment cocktail" should be
maximized, applying liquid "treatment cocktail" only for those "treatment
cocktail" components having no gaseous phase options (e.g., phosphates &
inoculum).
In saturated treatment zones, the relative densities of the "treatment
cocktail" and the contaminated groundwater should be considered. If it is
desired that the "treatment cocktail" migrate downward in the treatment
zone after injection, then a "treatment cocktail" with a density greater
than water must be formulated.
There is a great potential for creative variability in the application of
"treatment cocktail". For example, it is possible to use multiple
injection points to create desired conditions at some point between the
two injection points. One lance might inject inoculum, while another
lance, at some distance away, injects steam and ammonia. Both injections
may be made into a formation previously injected with volatile fatty acids
(VFA's). Such a technique would cause a zone of high bacterial growth at
the interface of the two treatments.
The nature and composition of the "treatment cocktail" will vary in the
different treatment steps. For example, the initial step might involve
the injection of an aqueous emulsion of oils, sugars and inoculum with a
liquid suspension of cellulose and mineral nutrients. This step could be
followed, at some time later by injection of other types of "treatment
cocktail", such as ammonia. This step will be followed later by oxygen gas
and/or other reactive gasses, such as CO2.
It is possible to adjust the redox by injection of "treatment cocktail" as
well. Obviously, the injection of oxygen will raise the pE radically. The
injection of anhydrous ammonia, for example, will have an effect on the
processes taking place in the treatment zone. Injection of a mineral
solution such as nitrate or sulfate will have the effect of buffering the
redox at the potential where those materials are reduced; assuming
adequate reductant (bioavailable energy source) is present.
Coupled Flow may be used to move the treatment cocktail into the highly
polluted formations and strata. Coupled flow is mass flow that is induced
in the absence of a pressure gradient. It may be induced by heat, solute
concentration, or electrical gradients.
The presence of a temperature gradient can cause groundwater flow as well
as heat flow when hydraulic gradients do not exist. Water will tend to
flow outward from heated areas. This can be used to advantage by injection
of "treatment cocktail" in steam in the area surrounding the concentrated
contaminant. Such a "treatment cocktail" delivery strategy may be coupled
to a liquid removal process in the center of the contaminated zone, to
create an additional gradient for flow of "treatment cocktail" into the
contaminated zone.
Chemical gradients cause mass flow of water as the osmotic potential
varies spatially. Water naturally moves to dilute concentrations of
dissolved or suspended materials. The role of chemical gradients in the
movement of chemical constituents is of considerable importance in the
BARR process, because the process creates dissolution and precipitation of
minerals in the subsurface matrix. Additionally, the process introduces
solutes into the system, which will cause osmotic gradients. The impact of
these osmotic gradients alone on coupled flow is difficult to assess. It
is also likely to be variable with different treatment stages and ion
flux.
The presence of existing probe access to the formation offers the
opportunity for electrode placement to induce electrokinetic flow into or
out of the contaminated zone.
Inoculum
The "treatment cocktail" will contain inoculum capable of degrading both
the contaminant and added surfactant/substrate under reducing and
oxidizing conditions.
In natural environments a number of relationships exist between individual
microbial species and between individual cells. The interrelations and
interactions of the various microbial groups making up the soil community
however are in a continual state of change and this dynamic state is
maintained at a level characteristic of the flora. The composition of the
microflora of any habitat is governed by the biological equilibrium
created by the associations and interactions of all individuals found in
the community. Members of the microflora rely on one another for certain
growth substances, but at the same time they exert detrimental influences
so that both beneficial and harmful effects are evident.
Inoculum provisions are made to insure that suitable organisms are present
in the treatment zone. Many toxic organic chemicals persist at underground
waste sites despite being readily biodegradable under laboratory
conditions. When this occurs, bacteria selected for their capacity to
degrade the contaminants, and to proliferate after injection in the
aquifer, may be added to enhance biodegradation.
It is apparent from the literature that microbial consortia, rather that
pure cultures, are most likely to degrade organic mixtures in contaminated
soil. Multiple-step degradation processes are undoubtedly the rule, where
the product of one reaction becomes the input reagent for succeeding
reactions. The literature commonly reports the involvement of a consortium
of organisms in the ultimate degradation of contaminants.
It is commonly found that biodegradation processes are characterized by a
"lag period" or an "acclimation period", where there is no observed
degradation taking place. This "lag period" is followed by active
degradation of the target pollutant. The "lag period" is most frequently
explained as a period of time when organisms capable of utilizing the
pollutant (direct metabolism) are selectively multiplying. Degradation of
the target is not observed until sufficient numbers of the degrading
organisms are present. It is often reported that the "lag period" is
shortened or eliminated by the provision of inoculum from "acclimated
populations" of inoculum taken from sites that have been previously
contaminated with the target pollutant. The explanation offered is that
such an "acclimated population" has sufficiently high numbers of adapted
organisms capable of degrading the target pollutant, so the lag period is
not observed.
It may be possible to rely on the native microbial population in some
sites. Conditioned, or acclimated organisms are frequently found in
environments where the target contaminant has been present for some time.
Old contaminated sites, therefore, may not need inoculum.
It is doubtful if introduction of inoculum would be harmful, however,
since well-adapted microbes would probably persist in spite of any
introduced inocula.
In many sites, it will be desirable to provide inoculum in the form of
landfill leachate or other material collected from sites where the native
microflora has been conditioned to degrade pollutants. The variability and
heterogeneity of soil and the discreteness of the microhabitats is very
great. Even over small distances, <1mm, the composition and size of the
particulates, the amounts and types of water, nutrients and gases and the
pH, pE, ionic strength and other physicochemical characteristics can vary
widely.
This variability of abiotic factors is reflected in simultaneous
occurrence in the same soil sample of autotrophs and heterotrophs, aerobes
and anaerobes, vegetative cells and spores, procaryotes and eucaryotes,
cells with different requirements for and tolerances to osmotic pressure,
pH, pE, temperature etc.
It may prove desirable to collect a wide range of inocula from diverse
environments, such as other contaminated sites, landfill leachate, healthy
soils, sewage sludge, mucks, sediments, etc to guarantee a diverse
inoculum base for the BARR process. There are reports of diverse inocula
being beneficial to degradation of xenobiotics. The source of the culture
greatly influences transformation ability of the microbial consortia. For
instance, 6% of 3,5- dichlorobenzoate disappeared in sewage sludge,
whereas 100% disappeared in methanogenic aquifer seed from a site
bordering a municipal landfill. (ZITOMER & SPENCE 1993). Lotter, et al.
1990 report a significant enhancement of the biological degradation of oil
contaminated soils can be achieved if compost of separately collected
household, yard and garden waste (biowaste) is mixed with the soil. This
undoubtedly is the result of diverse inocula, as well as diverse
substrate.
While it is commonly reported that bacteria are the most prevalent
organisms involved in biodegradation, there are reports of protozoans
living under methanogenic conditions as well. Fungal spores are also
likely to be present in the inoculum. Most fungus are obligate aerobes.
As the BARR process becomes more developed, new sources of inoculum may be
developed. It will probably arise that the BARR process itself will
develop inoculum for future treatments, i.e., the application of BARR to a
particular site will produce well-adapted inocula for future applications
of BARR to other contaminated sites.
Adequate care must be taken to collect and maintain the inoculum in a form
that preserves the viability of the inoculum. Anaerobic inoculum should be
handled anaerobically so as not to be inactivated by oxidizing conditions.
Certainly, some microbes are either facultative (able to live in oxic or
anoxic conditions) and many others are able to form resistant "resting
structures" to withstand toxic concentrations of oxygen. There are
undoubtedly others, some of which may be very useful to the BARR process,
which are unable to withstand oxygen at atmospheric concentrations.
Careful sample recovery techniques, utilizing air-tight samplers and inert
atmosphere storage and analysis conditions may be important to the
survival of some inocula.
Additionally, inoculum must be successfully delivered. For such an
approach to bioremediation to be successful, the introduced species must
be able to reach the contaminated zone and to move through the porous
material along a possible contaminant plume in a viable state. Hence the
importance of pE buffers in the "treatment cocktail".
Bacterial adhesion to the solid matrix may be enhanced by groundwater of
high ionic strength. Since this adhesion may limit the desired dispersal
of the microbes in the treatment zone, it may be of value to counteract
the adhesion, at least initially. Such attachment can be reversed by
injection of deionized water, or by changing the pE to cause dissolution
of the mineral matrix or precipitation of supersaturated solution. All of
these measures are easily accomplished in the BARR process by varying the
treatment cocktail.
Specific Toxicity
Specific toxicity is a descriptive parameter that explains the ability of
the BARR process to degrade high concentrations of toxic organic
materials. The use of high levels of varied inoculum and supplemental
carbon substrates can enhance absolute degradation rates of organic
compounds at concentration levels of pollutant that would be otherwise
toxic to the degrading organisms.
In order to objectively express the ability of such a system to perform
its function of degrading toxic pollutants, a quantifiable parameter
characterizing how much toxin the system can withstand is generated by a
parameter called Specific Toxicity. This is a division of lethal
concentration of contaminant, by the biomass concentration.
The use of a biomass-based metric for toxicity assessment is not uncommon
in toxicology. Toxicants are often evaluated and expressed in terms of the
quantity toxic to the unit mass of the organism in question. For example,
the lethal dose of a substance for 50 percent of a given population (LD50)
is typically expressed in units of mg toxin per kg biomass. The specific
toxicity of an organic compound can be calculated if one knows both the
inhibitory concentration and the population density at which that toxin
concentration proves fatal to the system.
While the amount of toxic compound necessary to kill an individual cell
may remain unchanged, upon addition to the supplemental carbon source, the
population increase effected by the supplement allows the population to
survive and metabolize concentrations of toxin which would kill a lower
amount of cells. The increase in cell number due to the supplemental
carbon decreases the amount of toxin available to each cell.
Increased absolute biodegradation rates are primarily the result of
augmented biomass generated through accelerated growth rates in the
presence of the supplemental carbon source.
Specific toxicity is second order; in other words, it depends not only on
the concentration of the toxic compound, but on the population density of
degraders as well. Analogous to specific degradation rate providing
predictive information on absolute degradation rates in carbon
supplemented systems, specific toxicity allows prediction of absolute
levels of toxin which would prove fatal to the overall system of
degraders.
Specific Toxicity = (growth inhibitory toxin concentration/population
density)
This quantity may be determined for organisms who have no known ability to
degrade the toxin. For example, algae in a bioreactor may be fed toluene,
causing a die off.
The use of a supplementary carbon source to stabilize a toxin-degrading
population at high toxin concentrations is based on the premise that a
higher population density of microbes can withstand higher concentrations
of toxin because the amount of toxin per cell is lower.
In the case of low organic toxin concentrations, the specific degradation
rate (i.e. mg toxin degraded per mg biomass) may remain unchanged, but the
secondary carbon source provides for more biomass and hence elevated
absolute biodegradation rates.
Treatment Cocktail Composition
Based on the chemical, physical and microbial parameters established in
the site analysis and characterization step, a first stage "treatment
cocktail" is prepared for the target pollutant and system.
The BARR process utilizes a "treatment cocktail", the composition of which
must be considered on a case-by-case basis, at least initially, until
enough experience has been gained to allow for generalizations.
In general, adding combinations of nutrient materials will result in
greater mineralization than the addition of any single nutrient. Since
several types of organisms may be required to degrade any single compound,
the concept of a single limiting nutrient may not be applicable to
heterogenous subsurface populations. The metabolic abilities and nutrient
requirements of subsurface microbes can vary substantially within a single
system. This variability may be a result of the existence of a variety of
micro habitats, the patchy distributionof microbes in the subsurface,
differences in microbial community structure over short distances, and
differences in nutrient requirements and availability in different
microhabitats within superficially uniform material.
This "treatment cocktail" will consist of: reduced carbon substrate,
microbial nutritional provisions, inoculum, and buffers to maintain eH and
pH within a desired range. The "treatment cocktail" will be provided as a
sequence of different materials delivered at different times to create the
desired conditions in the treatment zone.
The treatment cocktail will be miscible with the target pollutant. Part of
the objective of the "treatment cocktail" is the stimulation of biological
activity in the zone of the target pollutant. As far as is possible, the
"treatment cocktail", or some substrate component of it, will be miscible
with the target pollutant. Since many petrochemically derived pollutants
are hydrophobic (lipophilic), the polarity of the "treatment cocktail",
will be close to that of the target pollutant and so may contain lipids.
Lipids are organic compounds that are soluble in organic solvents such as
ether, benzene, acetone, chloroform, hexane, carbon tetrachloride and
petroleum ether, etc.; and are only sparingly soluble in water. Many of
the degradation products of high molecular weight organochlorine compounds
are lipophilic. The lipids used in the BARR process are also chosen for
their biodegradability.
The injected "treatment cocktail" will serve to modify the properties of
the solution. The "treatment cocktail" may contain surfactants capable of
forming micelles.
Humic acids have been observed to have surfactant properties at
concentrations well below any critical micelle concentration. The
"treatment cocktail" may therefore contain humic materials or other
biodegradable compounds capable of forming micelles.
The "treatment cocktail" will contain mineral nutritional materials.
Nutritional provisions are necessary to establish a favorable environment
for microbial growth. Adequate primary, secondary and micronutrients must
be available for the desired increase in biological activity. These
provisions for mineral nutrition will include nitrogen, phosphorous,
potassium, vitamins, as well as other micronutrients and microbial growth
agents such as amino acids, purines, pyrimidines, etc.
The assessment of mineral nutrient requirements is part of the initial
site assessment and characterization process described above.
Nitrogen may be provided as ammonia. Ammonia is the most readily utilized
of the inorganic forms of nitrogen. Its utilization requires no oxidation
or reduction, since the nitrogen in cellular constituents is at the same
oxidation state as ammonia; the valence being -3. The ability of an
organism to utilize ammonia as a source of nitrogen, i.e. to assimilate
ammonia, depends on its ability to undergo amination, the addition of an
amino group to the molecule.
Extracellular ammonia will be oxidized to nitrate during the oxidative
phase of BARR.
It may be desirable to provide nitrate in the "treatment cocktail", both
as a biological nitrogen source and as a pE buffer, since nitrate serves
as an electron acceptor at moderately reducing conditions.
The nitrogen metabolism of the BARR process creates N2 gas from oxidized
forms of nitrogen during the reducing phase of BARR, in a process known
commonly as denitrification. The reduction of NO3- (denitrification)
produces N2, which is stable and inert.
The "treatment cocktail" will contain biologically active reductant. The
injection of an electron-rich biodegradable material into a soil
environment that is biologically active will create reducing conditions.
Reduced carbon material serves as an electron donor in biological
processes.
The result of the addition of readily degradable reduced carbon is the
increase in the amount of biological activity and biomass and thereby
increases in the opportunity for metabolic and cometabolic transformation
of the target contaminant from biologically influenced catalysts.
Highly chlorinated compounds, which are some of the most commonly found
"recalcitrant" organic contaminants, are partially degraded by anaerobic
cometabolic transformation in tests utilizing an electron donor "treatment
cocktail". Reductive dehalogenation is a commonly reported process in
anaerobic cultures.
Secondary carbon sources are reported to increase biomass levels and
catalyze rates of pollutant transformation. Secondary carbon sources
provide both energy and cellular carbon for microbial growth. The compound
or compounds from which the carbon of the cellular material is derived is
the carbon source. Prototrophs can utilize as a sole carbon source either
an inorganic (i.e. carbon dioxide (autotrophic prototrophs) or an organic
compound (heterotrophic prototrophs). The ability of any species to use
any specific carbon source depends on its genetic information. Some
prototrophs, such as pseudomonas, are capable of utilizing any one of
approximately 100 different types of organic molecules.
The biological activity resulting from the introduction of a food source
will be a function of several variables, such as the inoculum and the
ambient conditions of pE, pH, specific toxicity, electron supply
(biodegradable reductant), and electron acceptors. If the inoculum or
resident organisms are not able use the introduced carbon source, there
will be no increase in the population. Broadly usable carbon energy
sources will have the most obvious application in the BARR process.
In general; lower aliphatic hydrocarbons, gases from methane to pentane,
are commonly used by microbial organisms as a sole source of energy.
Aliphatic hydrocarbons from C5 to C16 are readily degraded by microbes. In
general, the longer the C chain, slower the rate of decomposition.
Extensively branched chains are more slowly degraded than linear chains.
High molecular weight compounds are degraded the slowest.
Multiple substrate metabolism will provide the opportunity to effect
biodegradation of toxic organic compounds at rates well above those
possible in systems where toxic substrates are the sole carbon and energy
sources available.
Catabolite Repression
Catabolite repression is the favored use of one substrate over another.
When a target pollutant is the intended substrate for the microbial
population, the presence of a more easily degradable substrate will
inhibit the degradation of the target, as the microbes preferentially
degrade the easier substrate. High concentrations of easily metabolized
carbon substrates have been shown to repress the catabolic enzyme systems
needed by bacteria to degrade less desirable organic substrates until no
easily metabolizable substrate remains.
Most of the work describing simultaneous multiple substrate utilization
involves growth-rate limiting substrate concentrations. It is reported
(LINDSTROM AND BROWN, 1989) that the addition of secondary carbon sources
was found to inhibit mineralization of toxic substrates, presumably due to
preferential utilization of the more easily degraded carbon supplements
(catabolite repression). Maintenance of carbon-limited conditions in
culture prevents catabolite repression and allows simultaneous multiple
carbon substrate metabolism by microbial populations that directly
metabolize toxic materials. Simultaneous direct metabolism of the toxic
and supplemental substrates requires maintenance of growth limiting carbon
concentrations in culture to avoid catabolite repression.
Catabolite repression is not a limitation to BARR because the process
relies on heavily on cometabolism and abiotic degradation processes as
well as direct metabolism. Catabolite repression is only a problem if
direct metabolism is the sole mechanism of pollutant breakdown.
Cometabolic degradation will be enhanced by biological activity in the
general sense.
Only direct metabolism is influenced by catabolite repression. BARR
utilizes cometabolism, which does not yield energy for the organisms
involved. And abiotic degradation mechanisms. Hence, catabolite repression
has diminished impact on the overall effectiveness of the BARR process.
The reduced carbon provision might take the form of sugars. Sugar is a
readily oxidizable substrate. Fermentation is a common event when sugars
are present in a biologically active environment lacking adequate oxygen.
In the fermentation process, the substrate molecule itself serves as both
reducing agent and oxidizing agent.
The injection of some sugar, or mixture of sugars, will be accompanied by
an increase in the populations of organisms that have the ability to
ferment or otherwise utilize the chemical energy contained in the sugar,
under the ambient conditions
Fermentation products such as alcohol will effectively increase the
solubility of hydrophobic pollutants in the aqueous system by a phenomenon
known as cosolvency.
See Tecnhical Appendix (a separated document) for a complete explanation
of cosolvency.
For reasons of polarity and bioreductant value, oils and fats will
probably find great utility in the BARR process.
There is a wide variety of fats, oils and waxes that may find application
in the BARR process. A general discussion of paramaters and properties of
fats and oils is provided to assist in making intelligent choices about
which oil, or mixture of oils, to use.
Fats and oils are esters of higher fat acids and the trihydric alcohol,
glycerol. Esters of glycerol frequently are called glycerides. The
difference between fats and oils is merely that fats are solid or
semisolid at room temperature, whereas oils are liquids. Vegetable fats
and oils usually occur in the fruits and seeds of plants and are extracted
by pressing or solvent extraction. The oils may be hydrogenated to provide
a fat. The extent of hydrogenation will influence the properties of the
fat; more extensive hydrogenation increases the melting point.
Unsaturation lowers the melting point of the fat. Hence saturated acyl
groups predominate in fats and unsaturated acyl groups predominate in
oils. Another factor that affects the melting point is the molecular
weight. The acids obtained from low melting fats such as coconut oil, palm
oil and butter contain relatively small amounts of unsaturated acids but
considerable amounts of lower fat acids. Although classified as fats
because they are solid in temperate zones, coconut oil and palm oil are
called oils because they are liquids in the tropics where they are
produced.
Oils autooxidize sometimes via peroxides that degrade the molecule to a
complex mixture of volatile aldehydes, ketones and acids. Rancidity is
microbially mediated. Vegetable oils are more resistant to autooxidation
than animal oils. Oils can be very unstable in an oxidizing environment.
Waste or rags containing unsaturated oils are subject to spontaneous
combustion if air is not excluded, or if there is not enough ventilation
to prevent a rise in temperature as the oil oxidizes. Any rise in
temperature increases the rate of oxidation, and the process is
accelerated until the material bursts into flame.
The classification of vegetable oils into nondrying, semidrying and drying
oils depends on the ease of autooxidation and polymerization, which
increases with increasing unsaturation. The chief unsaturated fat acids
from the nondrying oils contain only one double bond; those from
semidrying oils contain a higher percentage of the doubly unsaturated
acid, linoleic acid; the acids from drying oils contain very little oleic
acid but chiefly linoleic and the triply unsaturated linolenic,
eleostearic and licanic acids. However there is no distinct line of
demarcation between the various types of oils.
The characteristic chemical features of the fats are the ester linkages
and the unsaturation. As esters they may be hydrolyzed in the presence of
acids, enzymes or alkali to free fat acids, or their salts, and glycerol.
The saponification value defined as the number of milligrams of potassium
hydroxide required to saponify one gram of fat.
The extent of unsaturation is likewise characteristic of a fat and may be
determined by the amount of halogen that the fat can add.
Drying oils include safflower, hemp, linseed oil or tung oil. Oiticica oil
is also a drying, high iodine value oil. Castor oil has a high percentage
of ricinoleic acid, which contains a hydroxyl group. The acetyl value
measures degree of hydroxylation. Castor oil has an acetyl value of
142-150, but other common fats and oils range from 2 to 20. Castor oil has
a low degree of unsaturation and it is a nondrying oil.
Simple and compound lipids contain fatty acids and alcohols and, for the
most part, these are bonded with ester linkages. The fats and oils are the
most commercially important lipids. Glycerol is a fairly small molecule
and contributes little to the average weight of fat and oil molecules.
Generally, the fatty acids are even numbered, straight-chain molecules
containing 12 or more carbon atoms and one carboxyl group. It might be
expected then that the physical and chemical behavior of the lipids is
determined primarily by the fatty acids. Because of this and the fact that
they are metabolically important compounds in the aqueous environment,
they warrant some closer attention.
The fatty acids exist in nature as products of the microbial metabolism of
various compounds or as partial products of the breakdown of decaying
animal and vegetable fats. When an ester linkage is made, a hydrogen atom
from the alcohol and a hydroxyl group from the carboxyl group of the acid
are removed as water. When the reverse reaction occurs, i.e., when the fat
is broken down into its constituent parts of hydrolyzed, water is added.
Although lipids include a highly heterogeneous mix of organic compounds,
there is a common chemical characteristic: simple and compound lipids
contain fatty acids and alcohols and, for the most part, these are bonded
with ester linkages.
Simple Lipids
Fats and oils are esters of various fatty acids and the alcohol glycerol.
Waxes are esters of various fatty acids and various long-chain
monohydroxyl alcohols. These are simple lipids. Glycerol is easily
oxidized to the carbonyl level (ketone or aldehyde) and is readily
incorporated into metabolic schemes. The fatty acids are also metabolized,
but their orderly utilization is somewhat more involved, and the process
is complicated by the fact that all but the very short-chain fatty acids
are insoluble in water. Water solubility decreases sharply with chain
length.
Beta-hydroxybutyric is soluble, but a macromolecular polymer consisting of
chains of this compound, called poly-beta-hydroxybutyric acid (PHB) is
insoluble. Some microorganisms store PHB as a source of reserve carbon as
animals store fats.
The low molecular weight acids, particularly acetic, propionic, and
butyric are collectively known as volatile acids because they can be
vaporized at atmospheric pressure. They are also sources of food for
aerobic and anaerobic microorganisms.
The higher molecular weight fatty acids occur in microorganisms and in
higher plants and animals largely as constituents of simple and compound
lipids.
Compound Lipids
The compound lipids are of two general types: phospholipids and
glycolipids. The phospholipids are found in all microorganisms, plants and
animals, but the glycolipids are found primarily in animals and occur in
only a few types of microorganisms.
Phospholipids are esters of various fatty acids and glycerol; they also
contain phosphorous and nitrogen compounds. The phospholipids commonly
found in microorganisms are derivatives of glycerol phosphate. They
contain many different fatty acids, most frequently with 16 or 18 carbons,
saturated or unsaturated. In bacteria, the substituent attached to
phosphate by an ester bond is most frequently ethanolamine. Choline,
serine, and inositol are attached to phosphate in other types of
phospholipids.
Glycolipids are esters of various fatty acids and alcohols; they also
contain a carbohydrate. Glycolipids are a class of lipids that contain
sugars and fatty acids. One type of glycolipid contains no glycerol but
may contain a long chain alcohol. Glycolipids are suggested as enhancing
biodegradation by functioning as a biosurfactant which acts to emulsify
contaminant and facilitate biodegradation under oxidizing conditions
(HURTIG et al., 1991). A species of bacteria produces rhamnolipids. These
accelerate hydrocarbon degradation by solubility enhancement under
oxidizing conditions.
Nonsaponifable lipids are compounds that are soluble in organic solvents
but do not contain fatty acids (do not yield soaps when subjected to
hydrolysis with strong alkali. Hence, nonsaponifiable). Sterols, fat
soluble vitamins, and plant pigments are included in this subclass.
Cellulose may be a useful component of the "treatment cocktail". Cellulose
is a polysaccharide that makes up the cell membranes of the higher plants.
The insolubility of cellulose in water and alcohols despite the presence
of five oxygen atoms for six carbon atoms results from the strong hydrogen
bonding between the chains, while the long unbranched molecules account
for its fibrous nature and high tensile strength. Cellulose is soluble in
sulfuric acid and concentrated solutions of zinc chloride, but since a
scission of glycosidic links takes place in the presence of acids, the
product regenerated by dilution in water has a much lower molecular weight
than the original cellulose.
There are numerous organisms, both aerobic and anaerobic that are capable
of degrading cellulose. The products of cellulose degradation (glucose or
cellobiose) are highly degradable by a number of microorganisms.
Aerobic decomposition of cellulose is markedly increased by the
availability of inorganic nitrogen.
The production of large quantities of ethanol and organic acids such as
acetic, formic, lactic and butyric acid is typical of the anaerobic
cleavage of the cellulose molecule.
Anaerobic degradation of cellulose is frequently associated with the
creation of very strongly reducing conditions.
Hemicelluloses may be a useful component of the "treatment cocktail".
Hemicelluloses are heteroglycans containing one to four different sugar
units. Xylans, which are polymers of xylose, are among the most prominent
polysaccharides of seed plants. Hemicelluloses are subject to
decomposition by a wide range of microorganisms and their breakdown
products include uronic acids, as well as hexose and pentose
monosaccharides.
Mixed, short chain organic acids may be ideal energy yielding components
of the "treatment cocktail" Volatile fatty acids, (VFA's) such as acetic
or butyric acid may be ideal to stimulate biological activity in the
treatment zone. Volatile fatty acids are organic acids with short chain
hydrocarbon moieites. Common VFA's include methanoic, ethanoic, propanoic
and butanoic acids. These are readily degradable by numerous organisms and
may be vaporized at atmospheric pressure. The advantage of being volatile,
and therefore easily deliverable into the treatment zone makes the utility
of these reductants in the BARR process potentially very great. This may
be especially important in a situation where the transmissive strata of
the contaminated formation is plugged by biomass or precipitated minerals.
Acetic acid is directly utilizable by methanogens. Other anaerobic
substrates must be broken down via a multiple-step pathway.
Incubation
The ability to inject steam may be a very important component of the BARR
process. Partial sterilization can be brought about by use of heat.
Partial sterilization of soil may be compared with fertilization, because
there is a proliferation of organisms in the wake of the treatment. Steam
injection may also be used to incubate the treatment zone.
After the introduction of the "treatment cocktail", the system is warmed
to an optimum temperature for anaerobic bacterial growth. Anaerobic
degradation can occur over a wide range of temperatures which has
generally been subdivided into three separate ranges, psychrophilic (5-25
C), mesophilic (25-38 C) and thermophilic (50-70 C). The rate of
methanogenic metabolism is temperature dependent, with optimum gas
production at the higher temperature ranges.
In wastewater sludge applications at ambient temperatures in temperate
climates, the rate of digestion is so low that the residence time of a
bacteria needs to be of the order of 3-12 months. Suitable bacteria for
seeding such systems, acclimatized to these low temperatures, can be
obtained from marshlands.
Most of the literature reports indicate that anaerobic incubation is
carried out at room temperature or higher. Some work is reported at actual
groundwater temperatures. The importance of temperature in chemical
reactions is utilized and manipulated by the BARR process.
Injection lances deliver steam to increase the temperature in the
treatment zone. Electric heating elements may also be used to generate
heat to stimulate transformation reactions, but the ready presence of
injection lances makes the use of steam more likely.
It is likely that the oxidation step will generate heat from exothermic
reactions. Therefore, the oxidation step of the BARR process may be
self-heating and it is possible that steam may not be required for
incubation of the oxidation step.
Monitoring
The chemical and physical status of the treatment zone will be monitored
via in-situ probes accessing the contaminant area, or by water extraction
for lab analysis. For purposes of monitoring and progress evaluation,
additional core probes may be taken as the treatment progresses. Dissolved
oxygen, redox potential (pE), pH, electrical conductivity, the presence of
specific ions are some of the parameters which can be monitored in-situ by
sensors placed in the probe holes.
The pE measurement is of limited value A meaningful redox measurement can
be derived if and only if chemical and electrical equilibrium exists
within the solution and all redox reactions between the various redox
couples occur reversibly upon the surface of the redox electrode.
Unfortunately, these conditions are seldom, if ever met within natural
aqueous systems. This is due in part to the irreversibility of
biologically mediated reactions. For reasons of physical environmental
variability in the subsurface environment, kinetic limitations prevent the
establishment of equilibrium in the BARR process.
Given the spatial variability of the treatment zone with respect to
chemical parameters that may be measured, multiple measuring points will
be very useful.
REOXIDATION AND REDUNDANCY
The repetition of oxidation and reduction cycles will increase the
opportunity for degradation to occur. After the reduction step has been
accomplished and suitable time for incubation has been allowed, treatment
with an electron acceptor such as peroxide, oxygen or ozone, nitrate,
etc., will re-oxidize the reduced system and create a proliferation of
aerobes who will have ample reduced carbon available from the decomposing
anaerobes. This oxidation step will take place in a mixture of elaborated
enzymes, bacterial slimes, sugars, proteins, etc. in an amorphous, dynamic
colloidal suspension.
The buildup of biomass in the form of a large colony of anaerobic bacteria
will constitute a substantial number of enzymes. When the electron donor
is exhausted and the population of the primary digesters has peaked, the
ensuing decline in population will be accompanied by the lysing of
bacterial cell walls and the release of elaborated proteins, sugars and
other biochemically significant compounds. These elaborated constituents
will serve as substrate for another bacterial bloom which will grow and
then decline and another following and so on and so on until the
utilizable substrate is depleted or some other limiting endpoint is
reached.
At this point, more "treatment cocktail" is introduced and another pE
shift in the system is created. The second round of reduction of the
treatment zone may be accomplished by injection of vfa's which are gasses
and will be easier to deliver into a plugged formation than a liquid
"treatment cocktail". The creation of reducing conditions will cause
dissolution of the iron and manganese precipitate, as the pE drops again
in response to the addition of reductant.
Since the reduction step will deplete the available nitrogen, a subsequent
oxidation step may be induced by the introduction of oxygen gas and
ammonia. The process may be carried on and on until complete degradation
of the target pollutant is achieved.
CHANGES IN THE SUBSURFACE ENVIRONMENT AS A RESULT OF THE BARR PROCESS.
There will be a number of changes in the environment of the treatment zone
resulting from the BARR process. Fundamental forces such as temperature,
pH, pE and mass flow are manipulated by the BARR process. Changes in
system features including surface area, chemical composition, enzyme
activity, ionic strength, oxidation state of reactants, ion exchange
characteristics, and velocity of flow are subject to manipulation by BARR.
The addition of electron donors (biological reducing agents) to the
treatment zone will lower the pE of the system. This represents a
fundamental shift in thermodynamic gradients. Initially, the treatment
cocktail creates reducing conditions, which alters the thermodynamic
mineral stability of the system. The reducing zone will spread, but will
be kinetically limited. Mineral dissolution ensues as inorganic and
organic compounds both undergo reduction. There is an increase in the
biomass as the treatment cocktail is utilized as an energy source.
Different microbial consortia will develop; there will be considerable
variability both spatially and temporally as the BARR process proceeds.
Ultimately, the system will stabilize at a very low pE (approximately the
point at which water is reduced). This is a theoretical limit in the pE of
aqueous systems. The pE may be maintained at some higher level, if it is
desired, by the application of a buffer or oxidizing agent. After the pE
has stabilized, an oxidizing "treatment cocktail" is introduced into the
system and the system is reoxidized.
The oxidation step will be accompanied by the release of heat from the
exothermic oxidation reactions. Depending on the rate of oxidation
reactions, the heat generated may not increase the temperature of the
system by very much, because the ground has a large thermal mass, and so
will resist increase in the sensible heat. The amount of heat released can
be controlled by the volume and type of oxidant introduced into the
reduced system, as well as the stage in the process at which oxidant is
introduced.
Coupled flow may be electrically, or thermally induced to provide bulk
flow and thus tend to diminish the kinetic limitations to the process.
It has been reported that transformation in dechlorinating cultures
appears to be associated with the transition from oxidizing to reducing
conditions. (KAESTNER 1991.)
(graphic inserted here, showing the carbon and nitrogen flux in the BARR
process ) (ascii format does not support this graphic- see original report
for graph)
The microbial and inorganic and contaminant will all be subject to
reduction over an area determined by the reductant and bulk flow of
reduced species. At some distance away from the reductant, the pE will
return to equilibrium.
The reoxidation process will cause precipitation of biocatalytic colloidal
mixture with a very high surface area. The minerals which were soluble in
the reduced environment will tend to reprecipitate as the pE rises.
Because the BARR method is biologically coupled, the mineral dissolution
and reprecipitation takes place in an enzyme-rich, cell wall rich,
fertilized, contaminated area. When minerals reprecipitate, they
frequently form amorphous solids, rather than crystalline forms which form
slowly, under more gradual conditions. These high-surface area colloids
will have surface catalytic capacity, as well as cometabolic and metabolic
catalytic capacity imparted from the enzymes, enzyme fragments and
organisms associated with the precipitate.
The formation of a separate, lipophilic phase will cosolve with
hydrophobic contaminants and the treatment cocktail, when lipids are
included in the "treatment cocktail".
The formation of micelles will create a nonpolar pseudophase in the
system, which will sorb hydrophobic contaminants into biodegradable
micelles, thereby bringing the target contaminant into intimate contact
with active organism that are consuming the micelles as substrate.
When sugar, or other fermentable substrate is used, the fermentation step
will generate alcohols which will diminish the polarity of the aqueous
phase, imparting a greater solvent capacity for hydrophobic contaminants.
The reduction process is repeated, and another round of mineral
dissolution occurs, as well as a shift in the microbial character of the
system. There will be turnover of the varied microbial consortia as the pE
drops.
The reoxidation step carried out again and then the reduction step again
and so on.
The BARR process is repeated as often as is necessary to completely
mineralize the target pollutant.
(graphic inserted here showing shifts in microbial consortia as redox
fluxes) (ascii format does not support this graphic- see original report
for graph)
Temperature
The BARR process creates temperature changes in the contaminated system.
Temperature is a fundamental parameter influencing reaction rates. The
effect of the temperature on the rate of the reaction is most often
described by the Arrhenius equation.
k=Ae-Ea/RT
where Ea is the activation energy for the reaction, R is the gas constant
T is the absolute temperature and A is a constant.
Temperatures in natural water systems range from 0C in temperate zones in
winter to as high as 45C in shallow ponds and stagnant waters in summer
time. For reactions with an activation energy of as little as 10kcal/mol,
this temperature range corresponds to a 14-fold difference of the half
life of the pollutant. For a reaction with an activation energy of around
30 kcal/mol, this corresponds to a factor of about 2500-fold difference in
half life. (WOLFE et al, 1990).
Enzymatically catalyzed reactions proceed at a higher rate at elevated
temperatures. The rate of biological reactions approximately doubles for
every increase in temperature of 10C, within limits of enzyme and membrane
stability.
Anaerobic metabolic reactions yield less energy than oxidative metabolism.
The introduction of an oxidant into a strongly reduced formation will give
rise to exothermic reactions that will increase the temperature of the
treatment zone and the surrounding formation. The subsurface has a high
heat capacity (i.e. it can absorb a substantial input of heat without
increasing in temperature). This will be especially true if the treatment
zone is below the water table.
Coupled Flow
Coupled flow will result from the BARR process There may be mass flow (of
water) induced by the heat of the BARR process. Because of differences in
density and kinetic energy (energy of motion), coupled flow will cause
water to flow out of the heated region. It is difficult to estimate the
magnitude and importance of such mass flow at this time, but it may be
significant and useful.
In addition to the coupled flow resulting from heat, the changes in
solution chemistry will create coupled flow as water moves in response to
concentration gradients. BARR creates increases in concentration of
dissolved salts, and water from the surrounding aquifer will flow toward
the concentrated solution and serve to dilute the treatment zone.
Coupled flow induced by electro-osmosis is easily accomplished in the BARR
process.
Coupled flow can serve to move inoculum within the contaminated system.
Movement of even a single viable cell into an area suited for growth will
greatly influence the overall process of biologically mediated reactions
and degradation processes.
pE Induced biological activity in the treatment zone will change the pE
which will be accompanied by radical changes in the system chemistry. The
initial introduction of "treatment cocktail" will cause a drop in the pE
as the biologically active system responds to the presence of readily
degradable substrate. The demand for oxygen as an electron acceptor will
exceed the supply, since diffusion rates cannot keep up with the
consumption of molecular oxygen. The resulting increase in electron
activity will create shifts in the thermodynamic equilibrium as the demand
for electron acceptors creates fundamental changes in the system.
Mineral stability is a function of equilibrium, including pE and pH. As
the pE drops, solubility equilibria will shift and soil minerals will
dissolve in response to the lowering of pE. The dissolution of minerals
will, in turn, create other changes in the system, such as increases in
ionic strength, particle flocculation and dispersion of sorbed bacteria.
Oxides of metals are present in soils, usually in crystalline forms and
are insoluble in water. Organic compounds in the "treatment cocktail" will
have a significant effect on the solubility and mobility of the metal
oxides due to chemical and microbiological action. Under anaerobic
conditions, organic compunds can bring about reductivce dissolution of
metal oxides from a higher to a lower oxidation state. The reduction has a
dramatic impact on the solubility and speciation of metals. For example,
oxides of Mn(III, IV) Fe (III), Co(III) and Ni(III), when reduced to
divalent ions under anoxic conditions, show an increase in solubility by
several orders of magnitude. Microorganisms play an important role in
dissolution of metal oxides by direct and indirect action. Direct action
involves enzymatic reductive dissolution of the metal oxide, wherein the
oxide is used as the terminal electron acceptor, whereas indirect action
involves dissolution due to production of metabolites, such as organic
acids and chelating agents and lowering the pH of the medium.
The changes in redox potential will tend to be buffered by the local
system composition. Change in oxidation state of the aquifer solid
materials will change the solubility of the minerals. The use of the term
aerobic or anaerobic is important only in a relative sense. The utility of
different electron acceptors depends on the pE of the system.
If the system is initially oxidizing (Eh approximately 500mV), as the pE
drops, any nitrate present in the system will be denitrified to N2.
Following this, as the pE continues to drop, Mn(IV) will be reduced to
Mn(II) with a concomitant increase in solubility of manganese compounds.
As the pE continues to drop, there will be an increase in iron solubility
as Fe(III) is reduced to Fe(II).
(three- graphic sequence inserted here, showing the flux of Nitrogen,
Manganese and Iron in the BARR process) (ascii format does not support
this graphic- see original report for graph)
This process constitutes a buffering effect. Redox buffering, the tendency
of a solution to maintain redox stability, is conceptually similar to pH
buffering. The pE will tend to poise as an available electron acceptor is
reduced. Redox buffering occurs as electron acceptors (oxidizing agents)
are consumed and their concentration diminishes. When the concentration of
the most easily reduced compound decreases, the electron concentration
increases (pE drops) until the threshold of reduction is reached for the
next most easily reduced compound.
Redox, then, becomes a descriptive measure of the system. Change in the
redox potential is the result of actual chemical reactions that have
kinetic considerations. A measured redox at one point does not necessarily
reveal the redox potential of a larger area. It is commonly accepted that
a measured redox potential is not an accurate description of the system.
Kinetic limitations and spatial variability preclude the use of a measured
redox couple as a meaningful descriptor of the overall status of the
system.
Because it is so convenient, it is likely that pE measurements will be
taken for tracking and development, but in actual BARR practice,
significant chemical species (e.g., CH4, CO2, NO3-, SO4-2, H2S, etc.) that
are detectable at multiple sampling points will provide the best measure
of the overall system status.
There is a fairly predictable sequence of reactions that will occur as
inorganic compounds are consumed as electron acceptors (reduction step),
or electron donors (oxidation step). The treatment cocktail provides the
reducing and oxidizing agent for these steps. The sequence of electron
acceptors consumed under increasingly reducing conditions is most commonly
suggested as O2, NO3-, Fe+3, Mn+4, SO4-2, CO2, H2O. Hence, the pE is
governed by the available concentration of reducible/oxidizable compound.
The chemical may be naturally present as part of the system or may be
introduced in the "treatment cocktail".
The redox potential can be maintained at some desired level by the
presence of inorganic electron acceptors. The mineralogy of the aquifer or
subsurface matrix will have a definite effect on the course of the
biological and chemical reactions of the BARR process. The aquifer (or
vadose zone) solids will become reduced as the pE drops. The abundance of
electron acceptor (oxidizing agent) will determine the amount of reductant
(carbon source) which is required for a given level of redox achieved.
Such concerns may become relevant as specific organisms or consortia are
identified which have the ability to metabolize the contaminant (or a
metabolite) and require a certain redox potential to thrive. For example,
if methanogenic conditions (very strongly reducing) are required to host
an organism with the direct or cometabolic ability to degrade a chemical
of concern, an aquifer with a lot of sulfate minerals would be difficult
to reduce to a low enough potential for methanogenic conditions to
manifest. Such a site would require more reductant than a site where
sulfate was in less abundant supply. Similarly, an efficient degrading
consortia that requires sulfidogenic, but not methanogenic conditions, may
be fostered by a treatment cocktail which contains a lower amount of
reductant in a sulfate pE buffer.
The solution chemistry will be a function of the geochemistry of the solid
matrix and the regional geohydrology. This means that some of the most
important governing parameters are relatively quantifiable and may be
determined as part of the initial investigation step. The mineralogy of
the aquifer, and thus the ionic composition of the groundwater, will
greatly affect the behavior of the system under the influence of an
introduced reducing or oxidizing agent.
There is much information available which may be applied to the BARR
process which has been gained from experience with landfill leachate.
Leachate from landfills containing organic matter has a content of
dissolved organic carbon (DOC) in the range of thousands of mg/l C. The
DOC, including fatty acids, and humic compounds, will act as a substrate
for microbial processes in the aquifer, potentially inducing major changes
in the governing redox environment. Close to the landfill, the aquifer
will be methanogenic, similar to the methanogenic conditions that exist in
most landfills.
Depending on the interaction between migrating leachate rich in organic
carbon, the kinetics of the actual redox processes and the availability of
the electron acceptors, a sequence of redox zones will develop
downgradient from the landfill, ranging from methanogenic, sulfidogenic
(sulfate reduction), ferrogenic (Fe(III) reduction), manganogenic (Mn(IV)
reduction), nitrate-reducing to aerobic conditions in the most diluted
part of the plume farthest away from the landfill. This sequence assumes
that the oxidized species are present in the aquifer in significant
quantities: free oxygen, nitrate and sulfate in the groundwater and
oxidized Fe and Mn compounds in solids associated with the aquifer
sediment. If some of these electron acceptors are missing, the
corresponding redox zones, of course, are also missing. This redox zone
sequence is believed to be key in controlling the fate of reactive
pollutants leached from the landfill.
Aerobic conditions are identified by free oxygen (O2) concentrations in
excess of 1.0 mg/l and very low concentrations all reduced species. The
value of 1.0 mg/l is defined in order to minimize the presence of
nitrate-reducing microenvironments in the aerobic aquifer. The possibility
exists for wide fluctuations in redox potential within microenvironments.
In the nitrate reducing zone, the concentration levels must be low for
oxygen as well as for the more reduced species, but no criteria are
associated with the nitrogen compounds. Nitrate concentrations could be
high in the nitrate reducing zone, if the organic carbon is the limiting
factor, but low if nitrate is the limiting factor.
The manganogenic and ferrogenic zones are defined by the presence of
substantial concentrations of dissolved Mn and/or Fe. In their oxidized
state, Mn and Fe are practically insoluble at neutral pH and dissolved
concentrations are considered to represent reduced species. The
sulfidogenic zone contains some sulfide. However, the presence of reduced
Fe and Mn may cause precipitation of metal sulfides and may lead to very
low concentrations of sulfide (sulfides tend to be insoluble). At the same
time sulfate may still be high if the reduction process is limited by
organic carbon.
The methanogenic zones and the sulfidogenic zones may be very closely
related, due to their similar redox potentials and a strict distinction
may be difficult. However, sulfate concentrations should not be too high
and methane should be present at significant levels. The applied criteria
are consistent with thermodynamic principals, but concentration-based
criteria are established since a meaningful calculation of the redox
potential from the Nernst equation is not warranted in such a complex
system. The actual criterion concentrations may be operationally defined
by determining the water chemistry of the compounds, and experiences on
sampling and water sample analysis.
Electrochemical determination of redox potentials is mechanically very
simple, but according to experience often yields results difficult to
interpret, or in some cases even misleading. Identifying redox zones is
assumed to be a key to understand biological and mineral behavior
associated with BARR. These realizations, and the fact that determination
of an exact redox potential is not needed but rather an identification of
the dominating redox processes as expressed by the electron acceptor being
reduced by the reductant, lead to the choice of assigning redox status
based on analysis of groundwater samples for redox-sensitive compounds. It
may be possible to use ion-specific electrodes in-situ in some situations
to monitor the BARR process.
Approximate criteria for redox parameters used for assigning redox status
to groundwater samples (all values in mg/l).
Parameters O2 NO3- NO2- NH4+ Mn(II) Fe(II) SO4+2 S(II) CH4
Aerobic >1.0 n.a. <0.1 <1.0 <0.2 <1.5 n.a. <0.1 <0.1
Nitrate Redn. 1.0 n.a. n.a. n.a. <0.2 <1.5 n.a. <0.1 <0.1
Manganogenic <1.0 <0.2 <0.1 n.a. >0.2 <1.5 n.a. <0.1 <1.0
Ferrogenic <1.0 <0.2 <0.1 n.a. n.a. >1.5 n.a. <0.1 <1.0
Sulfidogenic <1.0 <0.2 <0.1 n.a. n.a n.a. n.a. >0.2 <1.0
Methanogenic <1.0 <0.2 <0.1 n.a. n.a. n.a. <40 n.a. >1.0
Organic compounds also function as electron acceptors, but the potential
variety makes assumptions and projections difficult without actual
experience.
There can be widely varying redox subenvironments distributed in the
system. Due to kinetic limitations and heterogeneity of "treatment
cocktail" distribution, substrate, inoculum, soil mineral composition,
temperature, contaminant distribution and induced mass flow, there will be
spatial variation in the pE.
The presence of water in the system has a buffering effect, or damping
effect by virtue of its "reducibility" and "oxidizability"(i.e., water
itself can be oxidized and reduced). It is questionable if biological
activity or even uncoupled enzyme activity would continue beyond the pE
stability ranges of water. It is an interesting exercise to consider the
behavior of a system in the vadose zone, above the water table, where a
strongly reducing "treatment cocktail" is introduced into a vigorous
reducing inoculum. Would the pE drop below the stability limits of water?
Such an event would likely cause coupled flow of water in the vapor phase
or any capillary network of the soil.
The electron status of the system, described as the redox potential will
flux as the supplies and nature of reductant and oxidant are varied. The
initial reducing treatment cocktail is followed by different treatments,
at different times, and potentially in different zones, to manipulate the
pE, pH, ionic strength, inoculum composition, temperature, etc. There is
potential for application of different "treatment cocktail" to become
somewhat of an art as well as a science.
pH
The values of the pE and pH parameters are strongly dependent on each
other. Degradation of the "treatment cocktail" will likely result in the
formation of organic acids as well, which will tend to lower the pH. The
pH of the solution will change as the chemical reactions (both biological
and abiological) consume or produce hydrogen ions.
Hydrogen ion activity affects transformation kinetics in two distinct
ways. The most common is in acid-base mediated hydrolysis reactions. The
rate of the reaction is directly proportional to the proton and hydroxide
concentration. In the case of base-catalyzed hydrolysis, the rate equation
is
(-dP/dT) = k[P][OH-]
where k = second order rate constant (mol-1s-1),
[P] = pollutant concentration and
[OH-] = hydroxide ion concentration.
For compounds that have acidic or basic functional groups, the pH of the
water also governs the relative ratios of the associated and disassociated
species. Because associated and disassociated species react differently,
small changes in pH can effect large changes in half-lives of contaminants
that are susceptible to hydrolytic transformations.
Additionally, the changes in pH will alter the solubility of soil
minerals, causing dissolution and reprecipitation as the pH varies
spatially and temporally in the system.
Denitrification, organic nitrogen breakdown and sulfate reduction are
examples of biological reactions that can cause in increase in pH. A
decrease in pH may be caused by sulfate oxidation, nitrification, and
organic carbon oxidation. However, the actual changes in pH are influenced
and determined by the buffer capacity of the system and the amount of
substrate utilized by the microorganisms.
The presence of surface functional groups which are capable of exchanging
a proton creates pH dependent charge, whereby the ionic character of the
surface increases with pH.
The molecular configuration of polyelectrolytes may be influenced by pH as
the molecules coil and uncoil as the pH decreases or increases. In such a
situation, charged sites such as acidic hydroxyl groups or amines can lose
or acquire charge as a result of changes in solution pH. In a large,
flexible, polyfunctional molecule, intramolecular self-association is
thought to occur in the absence of electrostatic repulsion. The tendency
to form such intramolecular bonds will vary as charged sites are created
or satisfied by pH changes. In such a situation, decreases in pH will
satisfy the charge on the surface of the molecule, thereby lowering the
hydrophilicity of the surface and also decreasing the coulombic repulsion
of the molecular chain for itself and permitting intramolecular bonding.
The evolution of CO2, the most common end product of the reduction of O2,
has considerable influence on the system pH. When a system that was
previously under reduced conditions becomes oxidized, its pH may drop
drastically due to the oxidation of iron to Fe(III) and the subsequent
hydrolysis of the iron or the oxidation of sulfite to sulfate, which is
accompanied by the release of protons. Lowering the pE of the system often
will result in a rise of pH because many reduction reactions (such as the
reduction of sulfate to sulfite) involve the uptake of protons or the
release of hydroxyls. When the reaction of a couple that controls the
redox potential of a given system involves protons or hydroxyls, a change
in pH of the solution will directly cause a change in its pE. The pH may
affect the rate and direction of a redox couple in the soil solution.
Acidification of the soil, for example, is likely to strongly increase the
solubility of trivalent iron and of other oxidized transition metal
species, but will have a smaller effect on the solubilities of the reduced
species of these metals.
Elevated concentrations of calcium, magnesium, and bicarbonate have been
identified in a landfill leachate contaminant plume, indicating that
carbonate mineral dissolution is an important pH buffering mechanism in a
carbonate mineral aquifer.
The following reactions illustrate the dissolution of calcite and dolomite
by reaction with acid.
CaCO3 + H+ = Ca+2 + HCO-3
CaMg(CO3)2 + 2H+ = Ca+2 + Mg+2 + 2HCO-3
Soil colloids are negatively charged and accumulate hydrogen ions. Because
of isomorphic substitution, clay particles tend to have a negative charge.
This charge may be neutralized by protons. At low pH, the effect of the
electrostatic charge on clay particles will be minimized.
Bacteria are amphoteric and will acquire positive or negative charge
depending on the pH of the system. Bacteria have a variable surface
charge. They can be neutral, positive or negative, depending on the nature
of the cell and the pH of the solution.
At solution pH values equal to the pHZPC (zero point of charge), the
surface has no net electrostatic charge. At solution pH-values lower than
the pHZPC, the surface is positively charged and becomes increasingly
positive at lower pH-values. At solution pH-values higher than the pHZPC,
the surface is negatively charged and likewise becomes more negative at
higher pH-values. The precipitate coating on the mineral grains affects
not only the electrostatic charge expressed at the interface, but the
physical surface as well.
Bacterial adhesion to the mineral matrix will be affected by the chemical
surface of mineral grains. The surface of mineral grains is commonly
coated with ferric hydroxides and oxyhydroxides. Such a surface will
change as dropping redox reduces ferric iron compounds to form the more
soluble ferrous form. When mineral grain surfaces undergo such
dissolution, any adsorbed bacteria will be released into the solution and
allowed to migrate.
The mobility of microbes is very important to the success of the BARR
technique. If the bacteria and reducing agent are not transported to the
extent of the contamination, the desired direct and cometabolic
degradation cannot occur. Extensive advective transport to depth in the
subsurface may be limited to macropores or fractures. Similarly, in zones
of low permeability, non-advective processes such as growth or motility
may play the most important role in widespread dispersal of degrading
bacteria.
The dissolution of minerals accompanying changes in pE and pH will also
create changes in the makeup of the system solution by increasing the
ionic strength of the solution. The ionic strength will increase as
minerals dissolve, which will cause a further increase in the solubility
of slightly soluble mineral components by changes in the activity
coefficients of the processes.
The common ion effect will cause precipitation of some salts as the
solubility is exceeded.
The increase in the ionic strength of the solution will also cause a
collapse in the electrical double layer (multilayer) on charged surfaces.
This collapse of the double layer will allow flocculation of charged
species via van der Waals forces. There is greater tendency for dissolved
and suspended particles to sorb and flocculate at higher ionic strength,
because the high ion concentration compresses the electrostatic field on
the charged surfaces, and allows for closer approach of charged solution
components. This closer approach allows van der Waals attractive force to
overcome electrostatic repulsion and flocculation results.
Bacterial adhesion to charged surfaces increases with increasing ionic
strength. Theoretical explanation is the close approach resulting from
the collapse of the electric multilayer, allowing close enough approach
for van der Waals forces to take over.
This solid matrix may be stationary aquifer matrix or mobile colloids. The
attachment and detachment of bacteria for the solid matrix will occur in
repeated cycles as a result of BARR. The dissolution and reprecipitation
caused by the fluctuations in the pE and pH will cause the ionic strength
and chemical makeup of the solution to flux. In addition to growth as a
mechanism of bacterial spread throughout the contaminated formation,
bacterial attachment to mobile colloids in solution will also spread
bacteria through the formation with mass flow. The bacterial cells will
attach to solid materials and then detach as the surface charge of the
solid changes, or the solid dissolves in response to changes in ambient
pE/pH. Thus, even strongly sorbed bacteria will be spread through the
formation as a result of the BARR process.
Filtration theory contains both physical and chemical components to
describe retention of colloids in porous media. The physical controls of
filtration are based upon particle size characteristics of the porous
media and of the advected colloid.
Chemical filtration theory is described by the Derjaguin-Landau and
Verwey-Overbeek (DLVO) theory, which states that initial contact of
colloidal particles with surfaces is determined by the additive effect of
the attractive and repulsive forces at the interface (van der Waals and
electrostatic forces). The balance of these forces may result in adhesion
of particles at some distance (a few nm) from the surface. In addition to
the forces mentioned, hydrophobic and steric forces can also contribute to
binding of cells to solid surfaces.
In some cases, the precipitation and dissolution may entrain bacterial
cells in mineral ion precipitate. These colloids may be held in solution
and moved with bulk flow.
Introduction of water of low ionic strength will dislodge those particles
which are attached by high-ionic strength conditions. This may be used as
a step in the BARR process to stimulate the spread of degrading bacteria.
When oxidation occurs, there will occur mineral reprecipitation. Low pE
will cause dissolution of soil mineral grains containing Fe(III) and Mn
(IV). At some point in the system, either spatially or temporally, the pE
will increase to the point of reprecipitation of these minerals as
insoluble compounds. They will certainly reform as the pE raises at the
oxic interface, or upon the implementation of an oxidative cycle in the
BARR process.
When it does occur, precipitation can sometimes result in different
allotropic modifications, ranging from amorphous to crystalline and can
have variations within each form .
Coprecipitation can occur when materials in solution get trapped or
caught-up in a precipitation event. This can cause scavenging of solution
constituents when a precipitate forms. Scavenging can occur on different
scales. On the molecular level, dissolved species can become entrapped or
bonded in the crystalline lattice, if it forms. This may result in
phenomena such as isomorphic substitution in clay minerals or simply the
existence of "impurities" in the resulting solid. On the macroscopic
level, scavenging can occur when dissolved or suspended solution
components are taken out of solution by becoming entrapped in a
precipitate. This sort of coprecipitation or scavenging event is
intentionally created in the use of coagulants in water treatment
operations where a slightly soluble salt is rapidly added to water in
sufficient amounts to create a saturated solution. When solubility is
exceeded, the precipitation event scavenges materials in solution which
are responsible for undesirable turbidity. These are trapped in the
amorphous matrix and settled out by gravity, thus removing them from
solution.
Colloids
Since these amorphous precipitates will be forming and dissolving in a
rich and varied, biochemically active solution, the precipitate suspension
will contain extracellular enzymes and enzyme fragments and whole
bacteria. The creation of such lipophilic colloidal particles wil tend to
sorb free enzymes and thus increase the cometabolic reactivity of the
colloids.
Proteins (such as enzymes) adsorb more to hydrophobic surfaces than to
hydrophilic surfaces. Adsorption to hydrophobic surfaces initially
inhibits protein degradation, which results in low bacterial growth rates.
During long incubations, surface bacterial growth rates increase with
surface hydrophobicity because of increasing amounts of adsorbed protein,
which serves as substrate.
Such a colloidal phase will be catalytic, both from the properties of the
enzymes as well as from the large mineral surface area.
The large surface area associated with these colloids introduces an
increased capacity for surface catalysis. Colloid particles interact
strongly with the fluid, but the individual particles retain their
structural integrity, so they cannot be said to dissolve. The colloidal
mixture behaves so distinctively because of the large surface area of
interaction between the two phases.
Amorphous precipitates with a very large surface area will enhance the
apparent solubility of hydrophobic organics. Hydrophobic bonding will tend
to drive insoluble components onto the colloidal phase as well as onto the
solid matrix.
The colloids formed from the precipitation of minerals are an important
part of the BARR process. The dissolution and precipitation of soil
mineral components resulting from BARR will tend to create a colloidal
phase in the system.
See Technical Appendix (a separate document) for a complete discussion of
colloids.
The creation of microbial biomass constitutes a biocolloidal phase that
represents a separate, semipolar phase in the system. The lipid content of
bacteria varies from 10-15% of the dry weight, it is slightly higher in
fungi. Cell walls contain both hydrophobic and hydrophilic surface groups.
This presents the opportunity for partitioning of hydrophobic organic
contaminants into this nonpolar phase, which is also active catalytically
from the enzymes in the organisms.
In addition to the lipid content of the organisms, many bacteria produce
extracellular semipolar slimes that will tend to sorb slightly soluble
organic materials. These slimes will act as surfactants in the system when
dissociated from the bacteria which formed them. In addition, these slimes
are biodegradable and will serve as substrate for other organisms.
In actual practice, these two colloidal phases (mineral colloids and
biocolloids) will be intermingled to form a variable composition
biomineral colloidal phase with a highly active catalytic surface and a
substantial nonpolar phase which will tend to solve and sorb hydrophobic
contaminants in a reactive milieu. This will accelerate the degradation of
the target contaminants and is really at the heart of the BARR process.
The changes in solution chemistry accompanying the BARR process will cause
repeated flux in the metal ions in solution. The metals most often
proposed to catalyze the hydrolysis of pesticides in natural waters are
Cu+2, Fe+3, Mn+2, Mg+2 and Ca+2. These ions will be active in redox
couples in the BARR process; repeatedly dissolving and reprecipitating,
forming various compounds with components of the contaminated system.
Solution Chemistry
The "treatment cocktail" will serve to increase the solubility of the
pollutant by solvent/surfactant properties. Lack of water solubility is
frequently cited as a reason for low degradation rates of degradable
pollutants.
The increase in the concentration of polyatomic ions in solution resulting
from the BARR process will tend to increase the solubility of hydrophobic
organics. Van der Waals forces will create associations between
hydrophobic species and large ions. Such an association will have the
effect of increasing the solubility of hydrophobic system components.
The presence of biodegradable surfactants will increase the water
solubility of insoluble pollutants and thereby serve to increase the
degradation of the target. Surfactants modify the interfacial behavior in
liquid systems. The use of surfactants has been observed to increase the
biodegradation in toxic organic contaminants. This is related to the
increased solubility of the target.
The mobility and degradation rate of hydrophobic organics will be enhanced
by the BARR process through the creation of colloids and micelles.
The use of nonpolar components of the "treatment cocktail" will create a
nonpolar micellar phase to enhance the solubility of the target, as well
as dilute it with biodegradable substrate and nutrient materials.
Hydrophobic bonding will be diminished by the treatment cocktail The
"treatment cocktail" will decrease the polarity of the solution in a
contaminated aquifer, which will diminish hydrophobic forces which keep
insoluble organic pollutants out of solution.
The presence of a miscible or partially miscible organic solvent will give
rise to a phenomenon known as cosolvency.
Partially miscible organic solutes modify the solvent properties of the
solution to decrease the interfacial tension and give rise to an enhanced
solubility of organic chemicals in a phenomenon often called "cosolvency".
The interfacial tension of a solution decreases logarithmically as the
concentration of cosolvent increases linearly. Cosolvent effect is
especially apparent when both cosolvents are very hydrophobic.
Cosolvent effects differ depending upon the nature of the cosolvent.
Nonpolar cosolvents (such a lipids) have a very large effect on the
sorption of hydrophobic material to the aquifer matrix. While polar
cosolvents only have cosolvent effect (i.e. log-linear solubility
increase) when the concentration is high (>10%). Increase in hydrophobic
organic chemical solubility in the presence of cosolvents is reflected by
decreased sorption by the solid matrix and increased mobility of
hydrophobic organic chemicals. The cosolvency of partially miscible
organic solvents is expected to be most pronounced in systems where a
variety of cosolvents is present in high concentrations.
For miscible cosolvents, such as alcohols, the log/linear solubility
relationship only applies at cosolvent concentrations above about 10%. At
cosolvent concentrations below 10%, the solubility enhancement is linear.
See Technical Appendix for a complete discussion of cosolvency
The solvent effect of the treatment cocktail will serve to dilute the
concentration of the contaminant, a factor which may enhance the
biodegradability of the pollutant. The effective concentration of toxic
pollutants is diminished by dilution. A cosolvent "treatment cocktail"
will dilute and disperse the contaminant in a biologically active milieu,
thereby enhancing the degradation rates. Solubilization can increase the
amount of sparingly soluble hydrocarbons or other organic material taken
up by aqueous phases by orders of magnitude. Evidence exists that
surfactants can influence the uptake and consumption of insoluble
substrates. Many microorganisms produce extracellular surface active
compounds. The generally accepted purpose of these compounds is to enhance
biodegradation.
The three mechanisms traditionally proposed to explain how bacteria take
up sparingly soluble substrates are as follows: 1) interaction of cells
with hydrocarbon dissolved in the aqueous phase; 2) direct contact of
cells with hydrocarbon drops considerably larger than the cells 3)
interaction of cells with "solubilized", "pseudosolubilized" or
"accommodated" hydrocarbon in entities much smaller than the cells.
Note that pseudosolubilization or accommodation need not be put into a
classical surfactant micelle, but instead into the lipophilic regions of
proteins or other polymeric molecules produced and excreted by cells.
Adding surfactants not only facilitates emulsification, with a resulting
increase in interfacial area, but also provides micelles for
solubilization. When hydrocarbon is solubilized in small micelles of
surfactants, its rate of biodegradation can substantially increase.
Microbes
As the redox status of the system varies, different microbial consortia
will be favored. There will be a variety of redox environments created by
the BARR process, ranging from strongly oxidizing to strongly reducing.
These redox environments will variably favor oxidizing, reducing and
fermentative organisms, thereby increasing the pool of enzyme catalysts
and the potential for catalytic degradation of the contaminant.
The surface texture changes in the soil/mineral matrix accompanying the
changes in pE and pH will affect microbial microhabitat. In addition to
the dissolution and dispersal mechanisms mentioned above, the surface
texture of a mineral grain determines the amount of surface area to which
cells can attach and can also affect colonization of grain surfaces. If
the surface irregularities are on a scale similar to bacterial size, a
larger area of the cell wall may be able to interact with the surface
during initial adhesion. Larger surface irregularities may provide
protection from shear forces associated with water flow, grazing by
protozoa, etc.
The increase in the microbial biomass and the diverse consortia of the
system resulting from the BARR process will increase the amount of enzymes
in the system and thereby increase the opportunity for both direct
metabolism and cometabolism of the target pollutant. It is important to
remember the role of enzymes in this process. There are a lot of organisms
with the ability to degrade a wide array of materials. The BARR process
stimulates the development of a wide array of enzyme capabilities and
cometabolic opportunities. Since the BARR process is carried out in-situ
in the contaminated formation, the microbial community is constantly under
pressure to accommodate the presence of the contaminant either by
resistance or by utilization. The presence of the treatment cocktail
creates an strong driving force for the bacterial biomass to increase.
The wide diversity of organisms created by the BARR process will provide a
great variety of metabolic capability for the treatment system.
Microbial communities behave differently from pure strains. Waste products
and exudates from a mixed consortia will provide substrate and habitat for
the succeeding consortia.
Local conditions may be more or less unique, depending upon such factors
as inoculum, temperature, mineral composition of the system, etc.. But the
types of organisms will mainly be limited to the availability of viable
inoculum.
Viability becomes an issue when temperature, pH and pE are variables. Many
microbes have the ability to form resting structures, or resistant spores
that may enable them to withstand adverse chemical conditions.
In addition to the bacteria, there are also a number of other organisms
involved in the BARR process.
Yeasts
Yeasts are commonly responsible for anaerobic fermentation. It is very
likely that native or introduced inoculum will contain yeast spores.
Fungi
Fungi are almost all obligate aerobes. Many fungi are involved in
degradation of toxic materials. The BARR process provides wide spectrum
inoculum which contains fungal spores.
Protozoa
Protozoa are important to the microbial ecology. Protozoa are mobile and
they eat bacteria. They are said to "graze" on bacteria. Protozoan grazing
of bacteria is also likely to influence aquifer transmissivity, by
diminishing bacterial populations when formation-plugging populations
occur (SINCLAIR et. al. 1993). Protozoans have been found in both
oxidizing and strongly reducing environments and are able to form resting
structures to persist when conditions become unfavorable.
Some protozoans are also host to bacteria. They are reported to have
methanogen symbionts living inside (FINLAY and FENCHEL.1991). The ciliated
protozoa encyst in response to shortages of food or water, and the
methanogens remain viable within the cysts.
Unlike free living methanogens, these are not particularly sensitive to
oxygen, the symbiotic methanogens remain viable following exposure of the
consortium to atmospheric oxygen. Dispersal of methanogen-bearing
protozoan cysts through oxygenated environments is a potential mechanism
of transfer between different anaerobic environments. Anaerobic protozoan
consortia are theoretically capable of making a significant contribution
to methane generation under anaerobic conditions.
Changes in the system will influence the dissemination of microbes in the
treatment zone. Microbial dissemination in the system will occur through
several mechanisms; chiefly through growth, motility, symbiotic
association with mobile protozoans, attachment to mobile colloids, or mass
flow as free cells. All of these processes are stimulated by the BARR
process.
Nitrogen transformations resulting from BARR
The nitrogen transformations resulting from the BARR process are an
interesting and important part of the process. Nitrogen is a necessary
component of biometabolism necessary for the overall process. Nitrate is
an undesirable contaminant of groundwater.
BARR utilizes denitrification processes to generate N2 from the nitrogen
in the system. The BARR process thereby effects a net removal of available
nitrogen. Bioavailable nitrogen in the form of ammonia, will be necessary
as a component of the treatment process. Ammonia, in the gaseous form,
will also act as a strong base and may be used to adjust the pH of the
system, as well as provide nitrogen for the desired biological activity.
Cellular or inorganic nitrogen will be oxidized to nitrate during the
oxidative phase of the BARR process. Added nitrogen, that is not
sequestered in biological organisms, will be subsequently lost upon the
implementation of reducing conditions in the reduction cycles of the
process.
The BARR method will be useful for treatment of inorganic as well as
organic contamination. While it is not possible to degrade inorganic
contaminants such as heavy metals, it is possible to form and unform
stable compounds with them (such as chelates) and it is also possible to
utilize changes in pE to influence the solubility and mobility of heavy
metals in aqueous subsurface environments.
The behavior of heavy metals in soil depends on many factors such as
molecular status of the metal, pH of the soil solution, organic matter
content, soil cation exchange capacity (CEC) and pE.
Organic matter is often regarded as a major factor in the sorptive
behavior of metals. There is firm evidence that organic materials have
relatively high stability constants for metals. Additionally, considerable
evidence has been accumulated indicating that the hydrous metal oxides
play a major role in the sorption of heavy metals in mineral soils. These
oxides, particularly those of Fe, Mn, and Al are common in soils. They may
occur as discrete crystalline minerals or as coatings on other soil
minerals. Many are of indefinite structure (amorphous) and composition.
They have high surface areas in relation to their weight, are highly
reactive, and the Fe and Mn oxides are quite labile since they are formed
and dissolved in oxidizing and reducing conditions, respectively, in the
soil. The latter is of importance since the BARR process manipulates the
formation of these compounds and thus the mobility of heavy metals in
soils.
Where Cd, Pb and Zn are fixed and immobilized in a carbonate phase,
significant mobilization is caused by solution of iron hydroxides and
their heavy metal coprecipitates under moderately reducing conditions.
In systems rich in organic matter, the role of Fe/Mn oxides is much less
important because of competition from the more reactive humic acids,
organo-clays, and oxides coated with organic matter.
Of prime importance, however, is whether or not the heavy metals have been
introduced into the soil as inorganic salts or in forms bonded to organic
matter. Both inorganic and organic matter constitute ligands present in
the soil solution. In this respect, humic substances, may play the most
important role in the behavior of heavy metals in the soil. Copper forms
strong complexes with humic substances. Metals complexed by humic
substances become unavailable to form sulfides, hydroxides, and carbonates
and thus prevent the formation of insoluble salts. The stability of
humic/metal associations is as follows: Cu+2>> Pb+2>> Ni+2>> Zn+2> Cd+2.
The behavior of Cr(III) in soil is strongly correlated with pH because at
neutral and basic pH, Cr(III) forms insoluble oxides and hydroxides.
The generalized sequence of the capacity of solids to sorb heavy metals is
as follows; MnO2>humic acid> Iron oxide> clay minerals.(GUY and
CHARKRABARTI 1975):
The solubility of metals in waters is principally controlled by: (1) pH,
(2) type and concentration of ligands and chelating agents, (3) oxidation
state of the mineral components and the redox environment of the system.
Redox affects metal speciation in two ways: (1) by direct changes in the
oxidation state of the metal ions and (2) by redox changes in available
and competing ligands or chelates.
It is reported that upon waterlogging of soils containing sufficient
organic matter to effect a decrease in pE (i.e. bioavailable electron
donor), the availability of many metals is increased. Increased
mobilization of Cr, Ni, Cu, Zn, and Co have been reported, while
waterlogging of low organic matter content soils has shown little effect
on increased mobilization of metals.
Since BARR manipulates the redox of the system, changes in oxidation state
may be utilized to increase mobility of heavy metals in conjunction with
pump and treat schemes to effect enhanced recovery of heavy metals in the
subsurface.
The growth of microbial biomass will tend to diminish the transmissivity
of the aquifer. The buildup of biomass will affect aquifer permeability as
bacterial cells plug the pores of the saturated soil. Biofouling is a
commonly reported problem with groundwater injection wells. The ability of
the bacteria to plug the aquifer and modify transmissivity will be
governed by the pore size of the media. This pore size is frequently
spatially variable as a function of soil deposition and morphogenesis.
Such changes in diffusivity of the solution components will also alter the
reaction kinetics.
Because of the microbial energy metabolism, the oxidative step will result
in a greater accumulation of biomass than will the reductive metabolic
phases of BARR, hence the problem of biofouling will tend to be greatest
during the oxidative treatment.
The problem of biofouling will be minimized by the fact that the BARR
process utilizes gas phase "treatment cocktail" under most steps. In the
event that biofouling is a problem which restricts introduction of
"treatment cocktail" or removal of recovery leachate, it is possible that
steam injection or other biocidal treatment will temporarily alleviate the
blockage to enable treatment to proceed.
Sealant techniques may be very beneficial. The BARR process will greatly
advance the ability to place materials into and around contaminant zones.
Where BARR is used in conjunction with a pump-and-treat scheme, the
intentional placement of a plugging treatment at the perimeter of the
contaminated formation may be used to contain the contaminant. Accurate
sealing of the transmissive formation may help to protect sensitive water
sources as a preliminary step to the BARR process.
The BARR process is repeated until the desired degree of degradation is
achieved. The redundant nature of the BARR process is key to the success
of the technique. Direct and cometabolic transformation is fostered by the
wide diversity of microbes and enzymes induced by BARR. Additionally, the
wide swings in pE and pH, and the variations in temperature and the
opportunity for surface catalysis all contribute the degradative
capabilities of this process.
Sequential reduction-oxidation-reduction etc. is carried out by
manipulating the chemical balance of the treatment zone. The creation of
repetitive reduction and oxidation increases the opportunity for
cometabolism to transform the pollutant. Neither the reduction nor the
oxidation step is to be overdone. Too much reducing substrate will create
too much growth and result in excessive biofouling of the transmissive
strata and difficulty in subsequent "treatment cocktail" delivery.
Likewise, over oxidizing a reduced treatment step may generate excessive
heat or excessive biomass. The process may be controlled by the amount and
type of "treatment cocktail" used.
Declining levels of hydrocarbon in an biologically oxidative environment
will place increased populations of microbes in proximity to the
pollutant, thus increasing the likelihood of transformation.
The dynamic nature of the environmental forces created by repeated cycles
of oxidation and reduction greatly increase the opportunity for
non-enzymatic degradation to occur.
The BARR process may be used in conjunction with existing pump-and-treat
technology to enhance recovery of the target contaminant. Pump-and-treat
technology aims to remove the pollutant from the soil matrix in the
aqueous phase which is then treated to destroy (or recover) the
contaminant after the aqueous phase is pumped out. The limitation to this
is the attraction of the contaminant for the solid phase. The contaminant
is reluctant to enter the solution phase where it can be removed and
recovered via mass flow toward a recovery well. A further limitation
exists in that the contaminant is typically concentrated in the finer
textured materials of the solid phase where solution transport is
extremely low. The BARR process influences both of these limitations to
enhance the recovery of the contaminant. The changes in the solution
chemistry accompanying the BARR process will tend to diminish the
solvophobic force that drives nonpolar contaminants from solution and into
the solid phase. In fact, the aqueous phase will acquire sufficient
cosolvent to create a logarithmic increase in the solubility of the
contaminant in the soil solution. The introduction of a nonpolar treatment
cocktail will tend to permeate the finer textured soils and dilute the
nonpolar contaminant with a readily biodegradable substrate.
Reduction of the native soil minerals will create an increase in the ionic
strength of the soil and thereby collapse the diffuse electrical
multilayer on the clay surfaces. This will allow the closer approach of
negatively charged bacterial particles which can attach to the colloidal
precipitate. This will allow greater attachment of bacterial biomass to
the coilloidal & solid phases and enhance biological activity in the
treatment zone, as well as pollutant recovery in the pumped solution.
Aspects of the BARR process requiring special attention. There are
limitations to the effectiveness of this process. These limitations are
not unique to the BARR process, but are commonly shared by other
degradation processes as well. Some of the limitations are in the realm of
knowledge, while others are in the realm of technology. This technique is
not proposed for amateurs.
BARR requires high-class geotechnical & engineering capabilities. The
ability to assess the location of the contaminant and to deliver treatment
cocktail into the proximity of the contaminant is key to the BARR process.
There may be limitations to the BARR process in very deep contaminated
formations, due to the limited ability to deliver treatment cocktail into
the proximity of the contaminant.
Injection lance delivery techniques are limited usefulness in cobbly soil.
It may be likewise be difficult to deliver treatment cocktail into clays
or tight soils.
BARR process requires high-class biochemical and biotechnical
capabilities. The location and recovery of viable inoculum from diverse
sources and the ability to assess the activities of microbes in the
treatment zone are a challenge that must be met by high-class biochemical
and biotechnical skills. The transformations that will occur in the BARR
process will challenge the best biochemist. Additionally, the formulation
of the "treatment cocktail" requires better than average biochemical
knowledge and a great deal of creativity. This process is undemonstrated
and it is unlikely that complete mechanisms are known.
It is of concern that the transformation of the pollutant may not result
in complete mineralization. Most of the literature which describes
sequential environments indicates that complete mineralization of organic
contaminants is the product of such a treatment scheme.
Conclusion
BARR is a remediation technique to treat subsurface contamination
resulting from organic chemicals. The technique is in-situ, bio-assisted
remediation. This technique, is a multiple stage process utilizing direct
metabolism, cometabolism and abiotic degradation to transform
contaminants.
The BARR process has significant advantages over existing bioremedial
techniques, chiefly effectiveness and broad applicability to a wide array
of contaminants.
The BARR process utilizes a potentially numerous array of tools to bring
about transformation Enzymes, free radicals, and surfaces catalyze
transformations over a relatively short time period.
This is a multiple-stage process which consists of injection of a
specially formulated "treatment cocktail" which contains substrate,
inoculum and mineral nutritional provisions to create biological activity
in the proximity of the target pollutant. The treated system responds to
create variable and controllable reducing conditions along with an
increase in microbial biomass. The second-stage treatment creates
oxidizing conditions and more biomass. The entire process is repeated as
often as is desired, by injection of treatment cocktail to sequentially
and redundantly reduce and oxidize the contaminated area. Contaminant
mineralization occurs because of thermodynamic conditions favor it.
The impact on the target pollutant is subject to considerable manipulation
in terms of solution chemistry, temperature, pH, pE, biological activity,
etc.
This process offers a great tool for decontamination of soil.
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