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Chapter 20 – Organic Pollutants

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1 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

2 1. natural vs. anthropogenic
There are many different organic contaminants that are spilled into the environment. Points of concern: natural vs anthropogenic Cl OH O – CH2 - COOH Cl Natural vs. Anthropogenic Domestic waste Herbicides/pesticides Paper Plastics Acid mine drainage Detergents Oil Chlorinated solvents Metals 2. quantity added or spilled - carrying capacity or self purification

3 Extent of problem 300 million metric tons/yr > 1,200 Superfund sites Cleanup costs estimated to exceed 1 trillion $ Metals Pesticides Petroleum Chlorinated solvents Emerging Contaminants Radionuclides

4 Factors affecting biodegradability
1. Bioavailability low water solubility sorption 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 C7H complete C16H ~ 64% C40H very, very low ~ 5%

5 How do microbes increase bioavailability in the environment?
Scenario –an ocean oil spill A - Uptake of solubilized hydrocarbon B – Uptake of hydrocarbon at the oil-water interface C – Uptake of dispersed droplets of oil D – Production of biosurfactants to increase the oil-water interfacial area

6 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 branching R - C - CH3 CH3 aromatic ring systems high molecular weight -(CH2 – CH2 – CH2 – CH2)n-

7 How different substituents influence biodegradation of phenol

8 Given a pair of structures you should be able to predict which of the pair will degrade more rapidly. CH3 - C - CH2 - CH2 - C - CH3 CH3 2,2,5,5-tetramethylhexane vs. CH3 – CH2 – CH2 – CH2 – CH2 – CH3 hexane

9 Benzene vs. Benzo(a)pyrene

10 CH2 - COOH Cl 2, 4- D vs. CH2 - COOH Cl 2, 4, 5 - T

11 C = C H Cl TCE vs. CH3 - CH2 - OH ethanol

12 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.

13 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 N NH - CH CH 3 - CH 2 - NH Atrazine mineralization CO2 + cell mass + H2O Cl N NH - CH CH 3 H - CH 2 NH2 + transformation product is not degraded further

14 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. Cl OH COOH lack of enzyme specificity detoxification

15 Biosynthesis - partial or incomplete degradation can also result in polymerization or synthesis of compounds more complex and stable than the parent compound. Cl NH – C – CH2 – CH3 = O Propanil Abiotic/biotic polymerization – C – CH2 – CH3 = O HO CO2 + cell mass + H2O Mineralization Binding to humus Cl NH2 Cl N = N Cl N = N H tetrachloroazobenzene dichloroanilino - trichloroazobenzene

16 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. Aliphatics: CH3 – (CH2 – CH2 )n – CH3 OH Alicyclics: Aromatics: 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.

17 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. From Atlas and Bartha studying degradation constraints in an oil spill: Treatment in seawater % biodegradation 1. oil alone 2. oil + microorganisms 3. oil + micro. + P 4. oil + micro. + N 5. oil + micro. + N + P

18 In subsurface terrestrial environments, there are many options
In subsurface terrestrial environments, there are many options. These include both in situ and ex situ treatment. 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: Is intrinsic activity fast enough? Will the plume impact human or ecological health?

19 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

20 Example 1 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.

21 Example 2 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.

22 Example 3 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.

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