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This session involves a directed reading of Chapter 6 Wiedemeyer, beginning on page 241. The chapter begins with a very good introduction to contamination problems associated with chlorinated solvents in the subsurface. It will quickly become evident that biodegradation of chlorinated solvents is significantly more complex than biodegradation of petroleum hydrocarbons, as presented in the previous session. We will emphasize fundamental concepts as presented in Wiedemeyer while, simultaneously, attempting to provide some additional perspective comments. The format is very similar to the previous session. We will identify specific sections in Chapter 6, Wiedemeyer which should be read in detail. Comments and questions for these sections will be offered in chronological order.

The most common chlorinated solvents of environmental concern include PCE, TCE, TCA, CT. These solvents will be discussed individually in this chapter. Note that these solvents do not all biodegrade by the same processes. These solvents, as well as many of their degradation products, are highly toxic and/or carcinogenic. Microbial degradation of chlorinated solvents is not as well understood as biodegradation of petroleum hydrocarbons.

In the biodegradation process:

Not all solvents are biodegraded by the same processes. See Table 6.1, pg 243.

Aerobic cometabolism is discussed on page 242 and again on page 269. Usually this process must be engineered in the subsurface in order to occur and, therefore, we will not treat it further in this course. Anaerobic cometabolism will likewise not be treated herein.

Aerobic oxidation of Chlorinated solvents is discussed on page 268 and 269. Note that highly chlorinated compounds (PCE, TCE, TCA) are not known to be degraded aerobically, but less chlorinated compounds (VC, DCE, DCA) can be aerobically degraded if oxygen is present (again refer to Table 6.1, pg 243, Wiedemeyer). Aerobic oxidation of lesser chlorinated compounds is attractive at bioremediation sites because it occurs rapidly and usually results in complete mineralization of the contaminant. However, if sufficient oxygen is not present it must be supplied through some type of engineered system.

Anaerobic oxidation of chlorinated compounds is discussed on pg 269. Note that this process is still very much in the research stage and very little is known about it.

Halorespiration. Halorespiration of chlorinated solvents is a form of reductive dechlorination driven by hydrogen as the electron donor. Because Halorespiration is the most important process for intrinsic bioremediation of chlorinated solvents, considerable attention will be given to understanding the basic concepts. The discussion of Halorespiration/reductive dechlorination concepts occurs in Wiedemeyer pg 243 – 267. We will proceed by highlighting important concepts presented in Wiedemeyer.

Pg 243-245 Wiedemeyer. Reductive dechlorination is a reaction in which a chlorinated solvent acts as an electron acceptor and a chlorine atom is replaced with a hydrogen atom. If this reaction is biologically mediated (i.e. if microorganisms are involved) its termed "Halorespiration". In halorespiration hydrogen is used as the electron donor, while the chlorinated compound is used as the electron acceptor. (see Figures 6.1 and 6.2). Halorespiration probably accounts for the majority of chlorinated solvent natural attenuation at field sites.

Pg 247-248 Wiedemeyer. Three conditions are necessary for halorespiration to occur in the subsurface: 1) the subsurface environment must be anaerobic and have a low oxidation-reduction potential, 2) chlorinated solvents present must be amenable to halorespiration, and 3) there must be an adequate supply of fermentation substrates for the production of dissolved hydrogen. Halorespiration (reductive dechlorination) will not occur until the subsurface becomes sufficiently reduced to support fermentation. That is, halorespiration will not occur in either aerobic or nitrate reducing environments. Halorespiration will likely occur if sulfate is being consumed (sulfate reduction) or if methane is being produced (methanogenesis). See Figure 4.3 pg176 for redox conditions favorable for halorespiration. Although hydrogen is the most important electron donor in halorespiration there are other fermentation products which may also serve this function. The importance of hydrogen in halorespiration (reductive dechlorination) has only recently been recognized. Relevant research projects conducted prior to this realization are summarized on page 248.

Pg 249 Wiedemeyer. Generation of dissolved hydrogen by fermentation is discussed here. Also recall discussions of fermentation in Chapter 4, pg 169.

Pg 250-252 Wiedemeyer. It is very important to understand that petroleum hydrocarbons, including BTEX, can be fermented to produce hydrogen for reductive dechlorination. This means that Halorespiration can occur un aided in a region where a chlorinated solvent plume is mixed with a BTEX plume. The fermentation of BTEX is discussed on pg 250. Remember that fermentation is the first step in the oxidation of petroleum hydrocarbons via methanogenesis (pg 205, chapter 5, Wiedemeyer). Thus the presence of methane is clear evidence that fermentation is occurring. In summary hydrogen is generated by fermentation of nonchlorinated organics including BTEX, acetone, naturally occurring organic carbon other compounds. Hydrogen is a highly reduced molecule (Figure 4.4 pg 177) and there an excellent electron donor. A wide variety of bacteria can use hydrogen as an electron donor including denitrifiers, iron reducers, sulfate reducers, methanogens, and halorespirators. Thus the production of hydrogen through fermentation does not, by itself, guarantee that hydrogen will be available for halorespiration.

Pg 252-254 Wiedemeyer. The Monod kinetic model is useful for describing bacterial growth under substrate (hydrogen) limiting conditions. Monod parameters for halorespirators vs denitrifiers, sulfate reducers and methanogens are shown in Table 6.3 pg 253. These parameters show that halorespirators will outcompete methanogens and sulfate reducers while denitrifiers will likely outcompete halorespirators. This suggests that high nitrate concentrations in groundwater will very likely prove unfavorable for halorespiration. The likely "chain of events’ leading to the onset of halorespiration is summarized on page 254.

Pg 255-262 Wiedemeyer. Here is a discussion of an electron donor/electron acceptor model for characterizing oxidation-reduction potential at chlorinated solvent sites. Read this section paying particular attention to Table 6.4, Figure 6.5, and Table 6.5. The important concept here is that the more oxidized electron acceptors (i.e. nitrate, iron (III) ) are able to extract more energy per mole of hydrogen consumed and, therefore, are able to utilize hydrogen at relatively low concentrations. As can be seen in Figure 6.5,Table 6.5, and Figure 6.6 optimum hydrogen concentrations for halorespirators are on the order of 1 nM. Figure 6.7 shows the flow of donors to acceptors for competing anaerobic reactions. Figure 6.8 shows the thermodynamic flow of electron donor and electron acceptor pathways at chlorinated solvent sites undergoing halorespiration. This figure represents a very good summary of important concepts discussed previously. It is very important to remember that, in reality, aquifers are heterogeneous and poorly mixed; therefore the oxidation-reduction potential can vary greatly from point to point.

Pg 262-263 Wiedemeyer. The section describing "stoichiometry of reductive dechlorination" serves as a useful reference. The section describing the "microbiology and biochemistry of halorespirators" summarizes research on both pure and mixed cultures of halorespirators. It should be noted that complete transformation of PCE to ethene has been observed by mixed field cultures while only one pure cultures has been identified which carries out complete biotransformation.

Pg 264- 268 Wiedemeyer. Chlorinated solvents which are amenable to halorespiration are summarized on pg 264. Figures 6.9, 6.10 and 6.11 show the reaction sequences and relative rates for halorespiration of PCE, TCA and CT.

Pg 268-269 Wiedemeyer. Material related to oxidation (anaerobic and aerobic) of chlorinated solvents along with cometabolic degradation was covered at the beginning this session.

A proposed classification system for chlorinated solvent plumes in the field is presented ( pg 270-277) based on the amount and origin of fermentation substrates that produce hydrogen which drives halorespiration. Subsurface environments are classified into three types: Type 1, systems that are anaerobic due to anthropogenic carbon, Type 2, Systems that are anaerobic due to naturally occurring carbon, and Type 3, Aerobic systems due to the absence of fermentation products. This material provides valuable insight for interpreting the size, shape and migration of chlorinated solvent plumes under a variety of field conditions. It also illustrates how many of the fundamental concepts regarding halorespiration can be applied to interpret results from field studies or make predictions of future solvent plume behavior.

This sections summaries what is know regarding biodegradation kinetics of chlorinated solvents. As can be seen from Table 6.6 observations of rate coefficients and half-lives (both laboratory and field) vary considerably. Table 6.7 provides recommended guidelines for choosing first order rate coefficients or half lives for selected solvents.

Tables 6.9 – 6.11 provide characteristics and data for a large number of chlorinated solvent plumes observed under various field conditions

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