principles & applications of btex bioremediation pedro j.j. alvarez, ph.d., p.e., dee university...
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Principles & Applications of BTEX Bioremediation
Pedro J.J. Alvarez, Ph.D., P.E., DEE
University of Iowa
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Prospectus
What are BTEX and why care about them? What is needed to biodegrade them? How to exploit biodegradation for site cleanup? What are the more serious technical and political
challenges related to BTEX bioremediation? What is epistemology and how can it help us
address some of these challenges?
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“Water, water everywhere, nor any drop to drink”The Rime of the Ancient Mariner, Samuel Taylor Coleridge
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Contaminants of Concern: BTEX
Importance: • Relatively high solubility = High migration potential • Toxicity: Benzene can cause leukemia at 5 µg/l• Volatile, hydrophobic, biodegradable
CH3
CH3
CH3
CH3
CH3
CH2CH3
CH3
CH3
Benzene
m-Xylene p-Xyleneo-Xylene
Toluene Ethylbenzene
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Requirements for Biodegradation
1. Existence of organism(s) with required catabolic potential.
Xenobiotic will be degraded to an appreciable extent only if the organism has enzymes that catalyze its conversion to a product that is an intermediate or a substrate for common metabolic pathways.
The greater the differences in structure between the xenobiotic and the constituents of living organisms (or the less common the xenobiotic building blocks are in living matter), the less likelihood of extensive transformation or the slower the transformation.
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Requirements for Biodegradation (contd)
2. Presence of organism(s) in the environment.
BTEX degraders are commonly found, but differences in relative abundance of dissimilar phenotypes may lead to apparent discrepancies in the biodegradability of a given BTEX compound at different sites.
Depending on the relative abundance of different strains,
B could degrade earlier than T at one site, but the opposite may be observed at other sites.
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0
10
20
30
40
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60
70
80
90
100
B T E p-X m-X o-X N
% S
tra
ins
that
de
gra
ded
co
mp
oun
dFrequency Analysis of Biodegradation
Capabilities of 55 Hydrocarbon Degraders
Gülensoy and Alvarez (1999). Biodegradation. 10:331-340
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Requirements for Biodegradation (contd)
3. Compound must be accessible to organism:
a) Physicochemical aspects (bioavailability).
Desorption, dissolution, diffusion, and mass transport
b) Biochemical aspects.
Membrane permeability (important for intracellular enzymes), uptake regulation.
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Requirements for Biodegradation (contd)
4. If catabolic enzymes involved are not constitutive, they must be induced
Inducer(s) must be present above specific treshold (e.g., [T] > 50 g/L)
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Benzene Degradation by Pseudomonas CFS-215: Toluene enhanced enzyme induction
1510500
10
20
30
40
50
Time (days)
Ben
zen
e C
on
cen
trat
ion
(m
g/L
)
T = 0.1 mg/L
T = 50 mg/L
T = 0
ControlControl
Alvarez and Vogel (1991) Appl. Environ. Microbiol., 57: 2981-2985
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Cometabolic Degradation of o-Xyleneby Denitrifying Toluene Degraders
504030201000
2
4
6
8
10
Active
Controls
TOLUENE
days
mg/
l
504030201000.0
0.5
1.0
1.5
2.0
Active
Controls
o-XYLENE
days
mg/
l
Alvarez and Vogel (1995) Wat. Sci. Technol., 31: 15-28
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Requirements for Biodegradation (contd)
5. Environment conducive to growth of desirable phenotypes and functioning of their enzymes:
a) Presence of “recognizable” substrate(s) that can serve as energy and carbon source(s) (e.g., the BTEX) and limiting nutrients (N and P, trace metals, etc.).
b) Moisture (80% of soil field capacity, or 15% H2O on a weight basis, is optimum for vadose zone remediation. Need at least 40% of field capacity).
c) Availability of e- acceptors (e.g., O2 for oxidative reactions) or e- donors (e.g., H2 for reductive transformations). The e- acceptor establishes metabolism mode and specific reactions.
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Benzene degradation to CO2 and CH4 under methanogenic conditions
0
-0.25
-0.50
0.25
0.50
0.75O2H2O
NO3-N2
HS-
CH4
benzene
H2
SO42-
CO2
CO2
H+
EH°´volts
Half ReactionReduction Potential Hierarchy
OxidizedReduced
C6H6 + 4.5 H2O 2.25 CO2 + 3.75 CH4
Go’ = -(30 e-/mol) (96.63 kJ/volt) (-0.24 -(-0.29) volts)Go’ = -133 kJ/mol of benzene, or -4.5 kJ/e- equiv transferred(barely feasible)
Benzene degradation to CO2 under aerobic conditionsC6H6 + 7.5 O2 6 CO2 + 3 H2OGo’ = -(30 e-/mol) (96.63 kJ/volt) (+0.82 -(-0.29) volts)Go’ = -3,200 kJ/mol of benzene, or -107 kJ/e- equiv transferred (24 x more feasible)
Electron T
ower
The electron tower concept
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Aerobic BTEX Degradation
BTEX are hydrocarbons (highly reduced) so their Oxidation to CO2 is highly feasible thermodynamically (fuel)
Aerobic BTEX biodegradation is fast (O2 diffusion is often rate-limiting)
Aerobic BTEX degraders are ubiquitous (e.g., Pseudomonas)
Need oxygenase enzymes (i.e., enzymes that “activate” O2 and add it to carbon atoms in the BTEX molecule)
The ring must be dihydroxylated before ring fission. Once the ring is
opened, the resulting fatty acids are readily metabolized further to CO2.
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Anaerobic BTEX Degradation
Rates are much slower because anaerobic electron acceptors (e.g., NO3-, Fe+3,
SO4-2, and CO2) are not as strong oxidants as O2.
Benzene, the most toxic of the BTEX, is recalcitrant under anaerobic conditions (i.e., it degrades very slowly – after TEX, or not at all)
Anaerobic degradation mechanisms are not fully understood. Benzoyl-CoA is a common intermediate, and it is reduced prior to ring fission by hydrolysis. The oxygen in the evolved CO2 is from water.
Anaerobic BTEX degradation processes (e.g., denitrifying, iron-reducing, sulfidogenic, and methanogenic) are important natural attenuation mechanisms.
CS-CoAO
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Source: Wiedemeier et al., 1999
In aquifers, electron acceptors are used in sequence. Those of higher oxidation potential are used preferentially:
O2 > NO3- > Mn+4 > Fe+3 > SO4
-2 > CO2
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Requirements for Biodegradation (contd)
5. Favorable environment (continued):
d) Adequate temperature (rates double for ∆T = +10°C).
e) Adequate pH (6-9).
f) Absence/control of toxic substances (e.g., precipitation of heavy metals, dilution of toxic conc.).
g) Absence of easily degradable, non-target substrates that could be preferentially metabolized (ethanol?).
6. Time. Without engineered enhancement, benzene half-lives on the order of 100 days are common in aquifers.
Want degradation rate > migration rate
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What is Bioremediation? It is a managed or spontaneous process in which biological,
especially microbiological, catalysis acts on pollutants, thereby remedying or eliminating environmental contamination present in water, wastewater, sludge, soil, aquifer material, or gas streams. (a.k.a. biorestoration).
Ex Situ (Above ground)
In Situ (In its original place, below ground)
Engineered Systems (biostimulation vs. bioaugmentation)
Natural Attenuation (intrinsic/passive)
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Why Use Bioremediation? Can be faster and cheaper (at least 10x less expensive
than removal & incineration, or pump and treat) Minimum land and environmental disturbance (e.g.,
generation of lesser volume of remediation wastes) Can attack hard-to-withdraw hydrophobic pollutants Done on site, eliminates transportation cost & liability Environmentally sound (natural pathways) Does not dewater the aquifer
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When is engineered bioremediation feasible?
6 5 4 3 2 1
- log Kh (cm/s)
2
1
0
-1
-2
-3
log
k; (
per
day)
Feasible
with
Enhancement Feasible
Not feasible
Feasibility depends on:
1) Kh distribution of nutrients and e- acceptors (Kh > 10-5 m/s)
2) Adsorption bioavailability (depends on Kow and foc, problem for PAHs)
3) Potential degradation rate (half life < 10 days)
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Bioventing Used to bioremediate BTEX trapped above water table
Vacuum pumps pull air through unsaturated soil
Need to infiltrate water (with nutrients) to prevent desiccation
Source: MacDonald and Rittmanm (1993) ES&T, 27(10) 1974-1979
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Water Circulation Systems Used to bioremediate BTEX in saturated zone (Raymond)
Contaminated water is extracted, treated (air-stripping, activated carbon, or biodegradation), and recycled.
Some is amended with nutrients and reinjected (pulsing is better).
Clogging near injection well screens and infiltration galleries can be a problem (bacterial growth, mineral precipitation) but pulsing reduces clogging (may need occasional Cl2, H2O2)
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Air Sparging Injection of compressed air directly into contaminated zone stimulates
aerobic degradation, strips BTEX into unsaturated zone to be removed by vapor-capture system
Not effective when low-permeability soil traps or diverts airflow
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Air Curtain
Treated Water
Biobarriers Containment method that prevents further transport (hydraulic or
physical controls on groundwater movement may be required to ensure that BTEX pass through barrier
Biologically active zone is created in the path of the plume by injecting nutrients and electron acceptors (could use oxygen-releasing compounds, or inject compressed air and form an air curtain)
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Benzoate addition as auxiliary substrate (1 mg/L) stimulated benzene attenuation through 1-D “biobarrier”
0
50
100
150
200
Effl
uen
t Be
nze
ne
(µ
g/L
)
0 1 2 3 4 5 6 7 8 9 10
Time (days)
with benzoate
Sterile control
Not amended
Alvarez P.J.J., L. Cronkhite, and C.S. Hunt (1988). Environ. Sci. Technol. 1998; 32(5) 634-639
COO-
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Bioremediation Market
According to the Organization for Economic Cooperation & Development), the global market potential for environmental biotechnology doubled in the past 10 years to $75 billions in the year 2000
In USA, we have 400,000 highly contaminated sites, and NRC estimates the cleanup cost to be on the order of $1,000 billions
In USA, the current bioremediation market is only about $0.5 billions
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Bioremediation experienced many up- and downturns
1950’s: Microbial infallibility hypothesis (Gayle, 1952)
1970’s: Regulatory pressure stimulates development. Adding bacteria to contaminated sites becomes common practice. Failure to meet expectations (e.g., DDT accumulation) prompts a major downturn.
1980’s: It becomes clear that fundamental processes need to be understood before a successful technology can be designed. This realization, along with the fear of liability and Superfund, stimulates the blending of science and engineering to tackle environmental problems.
1990’s: Many bioremediation and hybrid technologies are developed. However, decision makers insist on pump and treat, and Superfund is depleted. Poor cleanup record and high costs stimulate paradigm shift towards natural attenuation and RBCA.
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VolatilizationVolatilization
Aerobic Unsaturated ZoneAerobic Unsaturated Zone
Oxygen ExchangeOxygen Exchange
Aerobic Aerobic uncontaminateduncontaminatedgroundwatergroundwater
DissolutionDissolution
Aerobic ProcessesAerobic Processes
Anaerobic coreAnaerobic core
Mixing, DilutionMixing, Dilution
AdvectionAdvection
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PE
Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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PE
Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
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Fluxo da água subterrânea
Atenuação Natural
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Atenuação Natural
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Fluxo da água subterrânea
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Fluxo da água subterrânea
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Fluxo da água subterrânea
Atenuação Natural
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Plume
Source
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What is Monitored Natural Attenuation?MNA is the combination of natural biological, chemical and physical
processes that act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of the contaminants (e.g., biodegradation, dispersion, dilution, sorption, and volatilization).
Success depends on adequate site characterization, a long-term monitoring plan consistent with the level of knowledge regarding subsurface conditions at the site, control of the contaminant source, and a reasonable time frame to achieve the objectives.
MNA should not be a default technology or presumptive remedy. The burden of proof (e.g., loss of contaminants at field scale, and geochemical foot-prints) should be on proponent, and evidence of its effectiveness should emphasize biodegradation.
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Plume Dimensions Reflect Natural Attenuation
MEDIAN PLUME DIMENSIONS
132 ft
1000 ft
500 ft
700 ft
200 400 600 800 10000
BTEX Plumes(604 Sites)
TCE Plumes (88 Sites)
Other chlorinated solvent plumes(29 Sites)
Salt Water Plumes (chloride)(25 Sites)
Feet
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Co
nce
ntr
atio
n
“safe”
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Co
nce
ntr
atio
n
“safe”
What is Risk-Based Corrective Action?
Clean source only to a level that will result in an acceptable risk at the potential receptor’s location (e.g., property boundary)
Need a mathematical model to calculate the required Co
Co =?
receptor
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Models are useful analytical tools, and can be used to demonstrate that natural attenuation is occurring
Limited predictive capability (order-of-magnitude accuracy): groundwater flow and microbial behavior rarely follow simplifying assumptions.
)(2
)(
)(2
)(
)(2
)2/(
)(2
2)/(
)(2
)/41(8
),( 2/12/12/12/12/1
2/1
x
Zerf
x
Zerf
x
Yyerf
x
Yyerf
vt
vvtxerfcCtxC
zzyyx
xo
)/(exp4
112
exp2/1
vxtkv
xs
x
x
Analytical Solution of the Advection-Dispersion-Sorption Equation with First-Order Decay, for Constant Rectangular
Source (Domenico, 1987)
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Variable Baseline Value Lp (%)
(day-1) 0.0005 -24
Co (ppb) 25,000 +7
Z (m) 3 +7
Y (m) 10 +7
x (m) 10 -1
foc 0.01 -17
n 0.3 +17
b (g/cm3) 1.86 -17
Vw (m/day) 0.044 +33
Sensitivity Analysis:
Effect of Doubling a Variable on Plume Length (Lp)
Lovanh, N., Y.-K. Zhang, and P.J.J. Alvarez (1999). Proc. 6th International Petroleum Environmental Conference, Houston, TX.
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Frequency Distribution for (n=79)
0.100.050.00
100
50
0
(day-1)
Den
sity
Mean = 0.0112 day -1
Median = 0.005 day-1
(t1/2 = 139 days)
How variable are biodegradation rates in the field, and
What are “reasonable” parameters for RBCA models?
Lovanh, N., Y.-K. Zhang, and P.J.J. Alvarez (1999). Proc. 6th International Petroleum Environmental Conference, Houston, TX.
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Current Status of Bioremediation
We have made significant advances towards understanding the biochemical and genetic basis for biodegradation. However, bioremediation is still an underutilized technology.
Bioremediation is not universally understood, or trusted by those who must approve it. To take full advantage of its potential, we need to communicate better the capabilities and limitations of bioremediation, and answer:
What is being done in the subsurface, Why, How, and Who is doing what?
How fast is it being done, and can we control it and make it go faster?
When can we meet cleanup standards in a cost-effective manner?
Can we reasonably predict that what we want to happen, will happen?
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EPISTEMOLOGY OF BIOREMEDIATIONepisteme = knowledge
Theory of the method and basis we use to acquire knowledge, including the possibility and opportunity to advance fundamental understanding, sphere of action, and the philosophy of the scientific disciplines that we rely upon.
Reductionism: System analysis through separation of its components
(eliminates complexity to enhance interpretation).Based on the premise that a system can be known by studying its components, and that an
idea can be understood if we understand its concepts separately. Used increasingly in bioremediation research to investigate mechanisms.
Holism: The totality of a system is greater than the sum of its parts
(synergism & antagonism)
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Epistemology’s Uncertainty PrincipleReductionism simplifies the system, enhances hypothesis testing, and interpretation It also augments lab artifacts and hinders the relevance of the information we obtain
Holism Reductionism
High
LowExpt. c
ontrol,
Lab artifactsComplexity, Relevance
High
Low
Scale: Field Microcosms Cells Extracts Genes Disciplines: Ecology
BiogeochemistryPhysiology
Biochemistry Genetics
Molecular Biology
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ImplicationsQuantitative extrapolation from the lab to the field is taboo.
(interpolate but do not extrapolate)
Rely more on holistic disciplines (e.g., ecology, biogeochemistry) and iterate more between the field and the lab, between basic and applied research.
Multidisciplinary Research (interstices)
Aurea mediocridad (San Ignacio de Loyola)
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Bioremediation is seldom a straight line to an imagined goal (many branching decision points requiring flexibility and versatility)
Remedial technologies are rapidly evolving. Be committed to life-long learning, and be aware that imagination and creativity could more important than knowledge
Pay attention to detail. You never know who is watching your work, and where your next promotion or demotion will come from.
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Conclusions
Indigenous microorganisms can often destroy BTEX and other common groundwater contaminants, making bioremediation (often) technically feasible.
The pendulum recently swung towards natural attenuation. This can save money but take much longer to achieve cleanup and appear as if officials are walking away from contaminated sites. Early public involvement is critical to minimize such controversy.
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Lets Take a Break!
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TYPES OF MICROBES USED
A. Indigenous MicroorganismsUsed in most applications (99%)Pseudomonas have wide catabolic capacityMay need to enhance proliferation/enzyme induction
B. Acclimated StrainsPreselected naturally occurring bacteriaGenerally not needed for BTEX Often fail to function in situ; common reasons:
- Conc. of target compound too low to support growth- Other substances and organisms inhibit growth- Microbe uses other food than target contaminant- Target compound not accessible to microbe
C. Genetically Engineered Microbes (GEMs)Could combine desirable traits from different microbes:
- Ability to withstand stress & degrade recalcitrant compounds
- Not needed for BTEX, many technical & political constraints
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H o w d o e s o n e p r o v e b i o d e g r a d a t i o n i s o c c u r r i n g i n s i t u ?
1 . D o c u m e n t l o s s o f c o n t a m i n a n t s a t f i e l d s c a l e .
S h o w t h a t d e c r e a s e i n c o n c e n t r a t i o n s i s n o t s o l e l y t h e r e s u l t o f p l u m e m i g r a t i o na n d d i l u t i o n . e . g . , s h o w t h a t c o n t a m i n a n t f l u x d e c r e a s e s a l o n g f l o w p a t h , u s i n gw e l l t r a n s e c t s .
2 . G e o c h e m i c a l i n d i c a t o r s t o d e m o n s t r a t e i n d i r e c t l y t h e t y p e o f d e g r a d a t i o np r o c e s s e s a c t i v e a t t h e s i t e . L o o k f o r O 2 , N O 3
- , a n d S O 42 - l e v e l s b e l o w b a c k g r o u n d i n t h e c o r e o f t h e p l u m e ,
a n d F e ( I I ) a n d C H 4 l e v e l s a b o v e b a c k g r o u n d . A l s o , h i g h e r C O 2 a n d a l k a l i n i t y .
H 2 l e v e l s c a n r e f l e c t d o m i n a n t r e d o x p r o c e s s e s :0 . 1 n M d e n i t r i f i c a t i o n0 . 2 – 0 . 8 n M F e ( I I I ) r e d u c t i o n1 . 0 – 4 . 0 n M s u l f a t e r e d u c t i o n> 5 . 0 n M m e t h a n o g e n e s i s
M u l t i l e v e lW e l lC l u s t e r
F l u x= v C
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Stable Isotope Analysis: Carbon atoms in BTEX are mainly 12C with some heavy isotopes (13C) Isotope fractionation results because light (12C) isotope bonds are preferentially biodegraded compared to heavy isotope (13C) bonds No isotope fractionation results from abiotic processes (dilution, sorption, etc.). Thus, biodegradation results in isotopical depletion of 13C for dissolved inorganic carbon and daughter products (13C = -20 to –30 per mil), and get and enrichment of 13C for residual contamination of parent compound. 3. Laboratory or in situ microcosms showing BTEX degradation
(look for mineralization or accumulation of metabolites)
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Análisis de varianza de las interacciones BTEXN
Las capacidades de degradación fueron mas amplias cuando los BTEXN fueron alimentados como mezcla que separadamente (particularmente cuando el T estaba presente)
Las interacciones negativas (e.g., inhibición competitiva, toxicidad) fueron estadísticamente significativas cuando se alimentó 1 mg/L a cada una.
Por estadística de Kappa se encontró una correlación significativa entre las habilidades para degradar T y E, p-X y m-X, y p-X y o-X. La falla de degradar B fue correlacionada con la inhabilidad para degradar o-X.
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k
KS Contaminant Concentration, C
Spe
cific
deg
rada
tion
rate
dC
/dt/X
2k
CKX C k
dtdC
S-
Monod’s Equation
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Why First-Order Degradation Rates?Monod’s Equation, When C << KS
SS K C X k- CKC X k - dt
dC
SK X k
dt
dCC
(not constant)
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conc
ent
ratio
n
cell
distancecell
cell
CK
Cqq
2/1maxdeg
cellbulkt CCkq
0.0
0.2
0.4
0.6
0.8
1.0
05
1015
2025
30
05
1015
Also, Mass Transfer Limitations Are Conducive to
First-Order Kinetics (even if C > Ks)
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0
20
40
60
80T
OL
UE
NO
(m
g/L
)
0 30 60 90 120 150
Tiempo (Días)
102 células/mL
107 células/mL
Simulaciones empleadas:k = 0.28 g-T/g-células/díaKS = 8.6 mg-T/LY = 0.6 g- células/g-T
Alta concentración microbiana = Taza más rápida
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¿Por qué es tan difícil limpiar acuíferos?
Detectar la contaminación en aguas subterráneas es como buscar una aguja en un pajar. Los puertos de muestreo pueden ser demasiado profundos, no muy profundos o en un lugar equivocado.