Bioremediation of Hydrocarbons

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Bioremediation of Hydrocarbons
Dr. Joseph Hughes Rice University
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Introduction

• Oldest and most mature application for the
  bioremediation field
• Range of contaminants
• Fuels, refinery wastes, coal gasification wastes,
  etc.
• Processes include in situ and ex-situ
• Aerobic processes dominate engineered
  bioremediation of hydrocarbons
• Anaerobic reactions may be critical in natural
  attenuation

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Fuel Components
Aliphatics
Aromatics
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Polynuclear Aromatic Hydrocarbons
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Aerobic Biodegradation of Hydrocarbons
Hungry Microbe
Pollutants
Over simplified - and dangerous!
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Example - Benzene

• Component of fuels
• Known carcinogen
• Excellent growth substrate
• Must be dissolved!
• Metabolism requires dissolved oxygen!!!
• Metabolism requires specific enzyme!!!

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Benzene Example (cont.)
NAPL
Dissolved
Cytoplasm
oxygenase
O2
Dissolution
Diffusion
Metabolism
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Bioavailability of Hydrocarbons

• Contaminants exist in forms not directly
  accessible to microorganisms
• non-aqueous phase
• sorbed phase
• gas phase
• Rates of contaminant transfer controlled by
  concentration gradients
• Slow or incomplete transfer may limit
  biodegradation

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Example Sorbed Contaminants
Sediment
Solution
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Adsorption Normalization
solution(µg/ml)
Sorbed (µg/g)
Organic carbon content (fraction)
Linear partition coefficient (ml/g)
Organic carbon normalized partition
coefficient (ml/g)
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Intra-Particle Desorption

• Soil grains contain pores
• Contaminants diffuse and sorb in
• pore spaces
• Must desorb and diffuse out before
• being accessible to bacteria

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Concentration Gradients During Biodegradation
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Potential Effects on Bioremediation

• May control rates observed
• May result in non-degraded residual
• Concentrations too low to support growth
• Desorption resistant compounds
• May limit application to less sorptive or
  soluble compounds

?
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Partitioning between Water and Soil/Sediment
define
then
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Mass Distribution in a Soil-Water System
In-Situ/Landfarming
Slurry Reactor

• 0.3
• ? 2.6

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Kd Relationships
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Desorption Resistance

• Sorption and Desorption are biphasic (i.e., two
  different compartments)
• First compartment
• Contains larger mass of contaminant
• Proportional to foc
• Rapid desorption rate
• Second compartment
• Limited and finite capacity
• Proportional to foc
• Kinetics of release are slow
• Not available for surface reactions

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Lab Observations
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PAH Desorption
Naphthalene
Solid Phase
Aqueous Phase
Isotherm
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Solution Phase Conc. (mg/L)
Desorption Step No.
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Qualitative Summary of Key Observations

• Sorption and desorption are both biphasic,
  consisting of two compartments, each with a
  finite capacity and unique equilibrium and
  kinetic characteristics.
• At low exposure, partial filling of both
  compartments occurs.
• Upon high exposure, partitioning into compartment
  1 accounts for the bulk of contaminant sorption.
• The soil/water partition coefficient for the 1st
  compartment is proportional to the fraction of
  organic carbon, fOC, and to the organic carbon
  normalized partition coefficient.
• Sorption to the 1st compartment is typically
  found to be linear and desorption is generally
  rapid.
• Sorption and desorption to and from the 2nd
  compartment is distinctly different from that of
  the 1st compartment in, at least, four well
  characterized ways.
• The 2nd compartment has a well-defined maximum
  sorption capacity, proportional to fOC, and
  secondarily to compound-specific constants.

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Characteristics of Second Compartment

• All hydrophobic organic compounds partitioning to
  this 2nd compartment are characterized by a
  single organic-carbon normalized partition
  coefficient.
• The 2nd compartment can be saturated in one
  single high exposure.
• The sorption kinetics of this 2nd compartment are
  generally slower than the 1st.
• Contaminants sorbed in the 2nd compartment are
  not available for reactions, but once desorbed
  they can undergo chemical and biological
  reactions as expected from the aqueous
  concentration and solution conditions.
• Also, The impacts of co-solvents alter the
  aqueous phase activity, and these effects are
  thereby predictable.

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Model Developed
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Net Effect
log Kd
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Fit to Field Observations
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From Desorption to Bioavailability

• Desorption rate and extent from desorbed
  sediments is over-predicted by linear isotherm.
• If biological response controlled by the
  dissolved concentration of contaminant, then
  SQLs may be significantly overprotective.

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Conceptual Model of Bioavailability
Sediment
Aqueous
Organism
qlab
Klw
Biological Response
C
Ksw
qres
Uptake
Desorption
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Microbial Availability of Desorption-Resistant
PAHs

• Desorption resistance may explain the observed
  recalcitrance of biodegradable hydrocarbons in
  sediments and soils

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Laboratory Studies

• Sediment preparation
• naphthalene, phenanthrene, fluoranthene
• Biodegradation studies
• Enriched culture augmentation
• Control studies
• enhanced solubility, mass balance methods

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Contaminants
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Model Predictions andExperimental Results
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T 15 days
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Surfactant Aided PAH Bioremediation

• Surfactants can increase aqueous phase
  contaminant concentration
• Many surfactants are toxic
• Results of laboratory and field tests either
  negative (toxicity) or inconclusive

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Surfactant Example
From Tsomides, Hughes, Thomas, and Ward (1995).
Effect of Surfactant Addition on
Phenanthrene Biodegradation in SedimentsEnv.
Toxicol. and Chem., 14(6)953-959.
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Current State of Bioavailability Issue

• Incomplete biodegradation often observed in field
  systems
• Laboratory systems usually exhibit high extents
  of degradation
• Two factors, mixing and particle size
  distribution appear to be critical
• Surfactant addition questionable
• Most critical for low solubility, highly sorptive
  contaminants

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