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This session involves a directed reading of Chapter 5 Wiedemeyer, beginning on page 185. The chapter begins with a fairly general introduction covering fundamentals of biotransformation of organic compounds (largely a review of Sessions II-1, II-2,and II-3). Section 5.1 covers both the aerobic as well as anaerobic biodegradation of petroleum hydrocarbons and this is the material which provides a solid fundamental understanding of the biodegradation process in the context of petroleum hydrocarbons. This is followed by Section 5.2 "Patterns of Intrinsic Bioremediation at field sites", which offers insight into bulk changes in ground water chemistry affected by petroleum hydrocarbons along with the concept of "biodegradation capacity". Section 5.3 provides petroleum hydrocarbon degradation rate data and Section 5.4 describes petroleum hydrocarbon plume data bases.

Format. Our directed reading for this chapter will consist of a series of comments and questions for each section in Chapter 5. The comments and questions will be presented in chronological order and are intended to identify and amplify important concepts in each section.

Chapter 5, Wiedemeyer, page 189. Introduction to Intrinsic Bioremediation of Petroleum Hydrocarbons.

What type of petroleum hydrocarbon compounds are we most concerned with in the subsurface?

What is BTEX?

What is MTBE? Why is MTBE such difficult contaminant to remediate in the subsurface?

What are PAH’s?

Describe the role of microorganisms in producing crude oil. Is it any surprise that microorganisms will also degrade refined oil products?

Why might indigenous microorganisms have an advantage over injected microorganisms in terms of their ability to degrade petroleum products?

Hydrocarbon degrading microorganisms have been shown to be ubiquitous in subsurface environments. As a result studies show that most of the dissolved petroleum hydrocarbon plumes are at steady state equilibrium or else receding , probably due to intrinsic bioremediation. Explain what this statement means in terms of the down-gradient spreading of dissolved hydrocarbon plumes with time staring immediately after the release has occurred.

Explain each term in the conceptual biodegradation equation at the bottom of page 190.

What are the common electron acceptors in ground water systems?

Do BTEX compounds biodegrade both aerobically and anaerobically? Check out Table 5.1 page 192.

Which constituent generally will limit the rate of natural biodegradation?

What is the driving force for biodegradation of petroleum hydrocarbons?

What is Gibbs Free Energy useful for?

Fill in the blanks…By coupling the ________of an electron donor (i.e. benzene) with the ________ of an electron acceptor (i.e. oxygen) the overall reaction becomes energy_______ as indicated by a _________value of Gibbs Free Energy.

The biodegradation of petroleum hydrocarbons is mainly through primary metabolism, in which the hydrocarbon is used as a growth substrate and the biodegradation process yields energy and carbon (for growth) to the microorganism.

Petroleum hydrocarbons can be biodegraded via the aerobic pathway when indigenous populations of hydrocarbon-degrading microorganisms are supplied with dissolved oxygen and nutrients necessary for the microorganisms to utilize petroleum hydrocarbons as a carbon and energy source. Aerobic biodegradation requires the action of oxygenases (enzymes) and therefore the presence of free (dissolved) oxygen. Equation 5.1, pg 195 describes the stoichiometry for the oxidation (mineralization) of benzene (C6H6) to carbon dioxide and water.

Question. verify the calculation on page 195 that 3.08 grams of oxygen are required to mineralize 1 gr of benzene. What implications does this have in terms of maintaining aerobic conditions in the subsurface as the biodegradation process continues?

Similar calculations can be made for toluene, ethylbenzene, and xylenes. If the mass ratios are averaged the result is that 3.14 grams of oxygen is required to degrade 1 gr of BTEX. This is defined as the BTEX Utilization Factor for oxygen. BTEX Utilization Factors for other terminal electron acceptors are shown in Table 5.2, pg 196. Note that production of microbial cell mass is not considered in these calculation.

If cell mass production together with energy requirements for cell maintenance are considered (pg 196) the actual oxygen demand will range from approximately 1 to 3 gr of oxygen per gr of benzene mineralized via subsurface aerobic biodegradation..

The requirements for aerobic biodegradation to occur in the subsurface are listed on pg 197.

The material on these pages provides a useful of issues affecting anaerobic biodegradation of petroleum hydrocarbons in general. As you read this material please refer also to Figure 5.1, pg 208, Wiedemeyer. This figure gives a qualitative summary of how electron acceptor conditions change as we move from the aerobic exterior of a (BTEX) plume toward the anaerobic center.

We will now look at the oxidation of petroleum hydrocarbons by specific anaerobic electron acceptors beginning with nitrate.

In denitrification nitrate is reduced first to nitrite then, after other intermediate steps, to nitrogen gas. Each step I this sequence is catalyzed by different microorganisms. The stoichiometry for denitrification of benzene is given by equation 5.3 pg 200.

Question. Verify the calculation of the ratio of nitrate to benzene as 4.77 to 1. If this same calculation is done for toluene, ethylbenzene, and xylene, and the results averaged, the average will be the BETX utilization Factor for denitrification shown in Table 5.2.

Conditions necessary for denitrification to occur are given at the bottom of page 200.

These sections are almost identical in scope and organization to the material on denitrification.

Methogenisis in a petroleum contaminated aquifer is a two step process involving fermentation and respiration:

1. BTEX compounds are fermented by fermentative bacteria into compounds such as acetate and hydrogen (see equation 5.6, pg 205)

2. produced hydrogen/acetate are then oxidized by other organisms in the reactions shown in equations 5.7 and 5.8. In the first reaction acetate is oxidized to methane and carbon dioxide. In the second reaction hydrogen (electron donor with CO₂ as the electron acceptor) is oxidized to methane and water.

Note that .77 mol (gr) of methane is produced for each mol (gr) of benzene mineralized.

The conditions necessary for methanogenesis to occur are listed on pg 206, Wiedemeyer.

Figures 5.1 and 5.2 give a good conceptual model of ground water geochemistry along the axis of flow through a dissolved petroleum hydrocarbon plume.

Check out the definition and calculation of "Expressed Biodegradation Capacity" given in Equation 5.9, pg 219. Make sure you understand what this concept means then look over the information in Table 5.3. Also check out Figure 5.3 pg 216. Note the dominant role sulfate reduction plays in the intrinsic bioremediation of many field sites.

Figure 5.4, pg 217 lists recommended first-order rate coefficients for selected petroleum hydrocarbons. Similar rate coefficient data are given in Tables 5.5 and 5.6, pg218-219. Monod Kinetic Parameters for BTEX compounds are given in Table 5.7. Petroleum hydrocarbon data bases are discussed in section 5.4 form pg 223-231.