Chapter 20 – Organic Pollutants

Objectives• Be able to give examples of pollutants that have unique structures

and structures similar to naturally-occurring organic compounds• Be able to define carrying capacity• Be able to list the four factors that affect biodegradability of an

organic compound in the environment• Be able to list properties of a molecule that can make it difficult to

degrade• Be able to define biodegradation terminology including

transformation, mineralization, biosynthesis, and cometabolism• Be able to list the various approaches to bioremediation

Natural vs. Anthropogenic

Domestic waste Herbicides/pesticidesPaper PlasticsAcid mine drainage DetergentsOil Chlorinated solventsMetals

Points of concern:

1. natural vs. anthropogenic

ClOH

Cl

O – CH2 - COOHCl

Cl

There are many different organic contaminants that are spilled into the environment.

2. quantity added or spilled

- carrying capacity or self purification

Extent of problem

• 300 million metric tons/yr

• > 1,200 Superfund sites

• Cleanup costs estimated to exceed 1 trillion $

MetalsPetroleumPesti

cides

Radionuclides

Chlorinated solvents

Emerging Contaminants

In most cases there are two steps required for biodegradation:

1) uptake and transport of the contaminant into the cell and 2) metabolism. Compounds with low solubility and/or high sorption are not in the aqueous solution surrounding the cell and therefore their uptake is limited.

Example:

Compound Solubility (mg/L) Biodegradation in 5 days

C7H16 2.93 complete

C16H34 0.0063 ~ 64%

C40H82 very, very low ~ 5%

Factors affecting biodegradability

1. Bioavailabilitylow water solubility

sorption

A

B

C

D

Hydrocarbon phase

Aqueous phase

How do microbes increase bioavailability in the environment?

A - Uptake of solubilized hydrocarbonB – Uptake of hydrocarbon at the oil-water interfaceC – Uptake of dispersed droplets of oilD – Production of biosurfactants to increase the oil-water interfacial area

Scenario –an ocean oil spill

2. Genetic makeup - lack of appropriate degrading genes

Each step in a biodegradation reaction is catalyzed by an enzyme. If the appropriate enzymes are not present, biodegradation will not occur. Since each enzyme is encoded by a gene, the genetic makeup of the microbial population is a critical factor in determining whether biodegradation will occur.

3. Contaminant structure (steric hindrance or unusual functional groups)

The presence of the following structures generally inhibit biodegradation

unusual atoms (halogens) R - CH2 - Cl

aromatic ring systems

high molecular weight -(CH2 – CH2 – CH2 – CH2)n-

branching R - C - CH3

CH3

CH3

Increasing biodegradation rate (mg/g sludge dry solids/ hr)

94.8 55.0 55.0 25.0 13.9

COO - CH 3 OH Cl NO 2

How different substituents influence biodegradation of phenol

2,2,5,5-tetramethylhexane

CH3 - C - CH2 - CH2 - C - CH3

CH3

CH3

CH3

CH3

CH3 – CH2 – CH2 – CH2 – CH2 – CH3

vs.

hexane

Given a pair of structures you should be able to predict which of the pair will degrade more rapidly.

Benzene

Benzo(a)pyrene

vs.

CH2 - COOH

Cl

Cl

CH2 - COOH

Cl

Cl

Cl

2, 4- D

2, 4, 5 - T

vs.

ethanol

TCE

CH3 - CH2 - OH

C = CH

Cl Cl

Cl

vs.

4. Environment (biotic and abiotic)

• moisture content (too much limits oxygen availability, too little

inhibits microbial activity in general)

• oxygen (required for rapid biodegradation processes)

• pH (extremes limit microbial activity)

• nutrient availability (includes mineral nutrients and organic matter)

• competition (are the microbes of interest active, do added microbes

survive?)

All of these need to be with acceptable ranges to allow optimal

biodegradation activity.

Biodegradation terminology

Transformation - any single biodegradation step in a pathway is a transformation reaction. A transformation can result in partial or complete detoxification of a contaminant or can create a compound even more toxic than the parent compound.

Mineralization - the parent compound is completely degraded to CO2, new cell mass, and water. This is a highly desirable result for toxic contaminants.

Cl

Cl

N

NH - CH

CH3

CH3

CH3 - CH 2 - NH N

N

Atrazine transformation product is notdegraded further

Cl

Cl

N

NH - CH

CH 3

CH 3

N

N

H

CH 3 - CH 2 -NH2

+ mineralization

CO2 + cell mass + H2O

Biodegradation terminology (cont.)

Cometabolism - Sometimes an enzyme can act nonspecifically on a substrate leading to a transformation reaction that does not provide energy to the microbe. A good example is oxidation of TCE by methane-utilizing microbes.

lack of enzyme specificity

detoxification

COOH

Cl

OH

OH

Biosynthesis - partial or incomplete degradation can also result in polymerization or synthesis of compounds more complex and stable than the parent compound.

Cl NH2

Cl

Cl NH

Cl

– C – CH2 – CH3

=

O

Propanil

Abiotic/biotic polymerization Binding to humus

Cl N

Cl

Cl= N

Cl

Cl N

Cl

= N

Cl

Cl

Cl

N

H

tetrachloroazobenzene dichloroanilino - trichloroazobenzene

– C – CH2 – CH3

=

O

HO

CO2 + cell mass + H2O

Mineralization

CH3 – (CH2 – CH2 )n – CH3Aliphatics:

Alicyclics:

Aromatics:

OH

Biodegradation pathways

Most contaminants can be categorized into one of three structure types, all commonly found in petroleum products. Some contaminants contain a combination of these structures.

Note to instructors: No actual pathways are presented in this slide show. You will have to decide what pathways (aerobic and anaerobic) you want to present.

Bioremediation

For successful and cost-effective bioremediation, there need to be degrading microbes, adequate bioavailability, and suitable environmental conditions. For petroleum spills, there are normally degrading microbes present so the issues become bioavailability and environmental conditions.

In ocean oil spills, access to the oil is limited to the surface area between the oil-water interface. In general oxygen is not limiting but as shown below, nitrogen and phosphorus are limiting.

1. oil alone 0

2. oil + microorganisms 5

3. oil + micro. + P 5-10

4. oil + micro. + N 5-10

5. oil + micro. + N + P 75

Treatment in seawater % biodegradation

From Atlas and Bartha studying degradation constraints in an oil spill:

In the subsurface, the most limiting factor is generally oxygen. Therefore, addition of oxygen is one of the most common approaches to cleanup of subsurface contamination.

In addition, nutrients such as N and P may be added.

In some cases, natural activities are fast enough to control the contaminant plume. This is called intrinsic bioremediation or natural attenuation. This approach is desirable because it requires only monitoring of the contaminant plume. Must address the questions:

In subsurface terrestrial environments, there are many options. These include both in situ and ex situ treatment.

Is intrinsic activity fast enough?

Will the plume impact human or ecological health?

If a more aggressive approach is required, there are several options available:

In situ treatments

• Bioventing• Air sparging• Permeable reactive barriers

Ex situ treatments

• Biofiltration• Soil vapor extraction and treatment• Groundwater extraction and treatment

In situ bioremediation in the vadose zone and groundwater. Nutrient and oxygen are being pumped into the contaminated area to promote in situ processes. Water is being pumped to the surface for ex situ treatment in an aboveground bioreactor. Following treatment, an injection well is returning the contaminant-free water to the aquifer.

Example 1

Bioventing and biofiltration in the vadose zone. Air is slowly drawn through the contaminated site (bioventing) which stimulates in situ aerobic degradation. Volatile contaminants removed with the air can be treated biologically using a biofilter as shown or by adsorption on activated carbon, or by combustion.

Example 2

Bioremediation in groundwater by air sparging. Air is pumped into the contaminated site to stimulate aerobic biodegradation Volatile contaminants brought to the surfaced are treated by biofiltration, activated carbon, or combustion.

Example 3