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�i N Engineered Approaches to In Situ Bioremediation of Chlorinated Solvents 2.4.1 Aerobic Oxidation rli hi aerobic zones of the subsurface (zones of the subsurface where oxygen is present), certain CAHs can be oxidized to carbon dioxide, water, and chloride by direct and cometabolic mechanisms (Hartman and 1 DeBont 1992; McCarty and Semprini 1994; Malachowsky and others, 1994; Gerritse and others, 1995; Bielefeld and others 1995; Hopkins and McCarty 1995). Direct mechanisms are more likely to occur with the less chlorinated CAHs (mono- and di-chlorinates). In general, the more chlorinated CAHs can be oxidized by cometabolic mechanisms, but no energy is provided to the organism. Incidental oxidation is caused by enzymes intended to carry out other metabolic functions. Generally, direct oxidation mechanisms degrade CAHs more rapidly than cometabolic mechanisms (McCarty and Semprini 1994) (refer to the following case studies in Appendix A: Aerobic Degradation in Field Demonstration at Moffett Naval Air Station, Mountain View California [Moffett Field]; Aerobic Degradation Field Demonstration at Site 19, Edwards Air Force Base, California/Edwards AFBJ; Methane Enhanced ;I Bioremediation Using Horizontal Wells at Savannah River Site, Aiken, South Carolina[SRS] ; and Cometabolic Bioventing at Building 719, Dover Air Force Base, Dover, Delaware[Dover Building 7191). i !� Aerobic Oxidation (Direct) Aerobic oxidation (direct) is the microbial breakdown of a compound in which the compound serves as an electron donor and as a primary growth substrate for the microbe mediating the reaction. Electrons that are generated by the oxidation of the compound are transferred to an electron acceptor such as oxygen. In addition a microorganism can obtain energy for cell maintenance and growth from the oxidized i' compound (the compound acts as the reductant). In general, only the less chlorinated CAHs (CAHs with one or two chlorines) can be used directly by microorganisms as electron donors. CAHs that can be oxidized directly under aerobic conditions include DCE, DCA, VC, CA, MC, and CM (Bradley 1998 ; RTDF 1997; Harkness and others 1999). The CAHs are oxidized into carbon dioxide, water, chlorine, and electrons, in conjunction with the reduction of oxygen to water. Exhibit 2-11 shows an example of aerobic oxidation (direct) of a CAH. Exhibit 241 : Aerobic Oxidation (Direct) OXIDIZED CO + H 0 CAH + CI + energy Bacterial Enzymes OZ REDUCED H2O Source: Modified from Har mans and DeBont 1992 2- 13