aerobic and anaerobic biodegradation

1. AEROBIC AND ANAEROBIC BIODEGRADATION

4. Biodegradation and Biotransformation Conversion of contaminants to mineralized (e.g. CO2, H2O, and salts) end-products via biological mechanisms Biotransformation refers to a biological process where the end-products are not minerals (e.g., transforming TCE to DCE) Involves the process of extracting energy from organic chemicals via oxidation of the organic chemicals

5. Biodegradation Aerobic and anaerobic degradation Reduces aqueous concentrations of contaminant Reduction of contaminant mass Most significant process resulting in reduction of contaminant mass in a system

6. Fundamentals of Biodegradation All organics are biodegradable, BUT biodegradation requires specific conditions There is no Superbug - not Volkswagon Contaminants must be bioavailable Biodegradation rate and extent is controlled by a �limiting factor�

9. What do Microbes Need to Grow? Like all living things, microorganisms need Food Supply carbon Supply energy Respiratory substrate Something to �breathe� Some use oxygen as the �electron acceptor,� others can use alternatives including chlorinated solvents. Otherwise there should be donor electron as energy source

11. Terminology and Definitions Electron donor A compound that donates electrons during its oxidation Simple organic compounds such as sugars, alcohols, or methane can be oxidized to carbon dioxide (CO2) Electron acceptor A compound that accepts electrons during its reduction Inorganic compounds like oxygen, nitrate, sulfate, oxidized metals, or CO2 can be reduced to water, dinitrogen gas, hydrogen sulfide, dissolved metals, or methane, respectively

12. Electron Acceptor Use � Preferred Order Not sure if this is worth using in the context of this course but it does help explain why you need to control conditions to get the degradation that you want.Not sure if this is worth using in the context of this course but it does help explain why you need to control conditions to get the degradation that you want.

14. Aerobic vs Anaerobic Biodegradation (A matter of terminal electron acceptor) If oxygen is the terminal electron acceptor, the process is called aerobic biodegradation All other biological degradation processes are classified as anaerobic biodegradation In most cases, bacteria can only use one terminal electron acceptor Facultative aerobes use oxygen, but can switch to nitrate in the absence of oxygen

15. Electron Acceptor Zone Formation

16. Microbes Obligate aerobes - Microbes for which the presence of oxygen is essential. Oxygen is the only electron acceptor that these species can employ. Facultative anaerobes - Can use oxygen if it is available but are able to switch to alternate electron acceptors when oxygen is depleted. Obligate anaerobes - Use alternate electron acceptors exclusively. Oxygen is toxic.

17. Electron Exchange

18. Metabolism and Oxidation

19. Biotransformation of Organic Substances If carbon is in oxidized form (positive valencies), biotranformation by reduction is more important: If carbon is reduced (negative valencies), biotranformation by oxidation is more efficient:

20. Aerobic Oxidation

21. Oxidation and Extraction of Energy Oxidation of organic matter provides energy for living organisms because such reactions are thermodynamically favored � CH2O + � O2 ? � CO2 + � H2O ?G? = -119.98 kJ/electron equivalent Microbes employ catalyzing enzymes to surmount kinetic barriers. Enzymes function by forming a complex with the reactants, bringing them in close contact.

22. Microbiology of Aerobic Oxidation A wide variety of microorganisms can carry out these oxidation processes Activity is believed to be ubiquitous; bioaugmentation is not likely to be required Activity can be stimulated by oxygen addition Oxygen solubility is limited so the treatable concentrations are low

23. Oxygen Utilization of Substrates Benzene: C6H6 + 7.5O2 ��> 6CO2 + 3H2O Stoichiometric ratio (F) of oxygen to benzene Each mg/L of benzene consumes 3.07 mg/L of O2

27. Aerobic Oxidation

28. Cometabolism (Aerobic) Fortuitous transformation of a compound by a microbe relying on some other primary substrate Generally a slow process - Chlorinated solvents don�t provide much energy to the microbe Most oxidation is of primary substrate, with only a few percent of the electron donor consumption going toward dechlorination of the contaminant Not all chlorinated solvents susceptible to cometabolism (e.g., PCE and carbon tetrachloride)

29. Methanotrophs, example of cometabolism Use methane as the primary substrate, but cometabolize chlorinated solvent compounds. They oxidize methane to methanol using methane monooxygenase (MMO). MMO is non-specific, and cometabolizes trichloroethene (TCE) to TCE epoxide: This eventually degrades to CO2, Cl- and H2O.

30. CCl2=CHCl Cl2C CHCl CO2, Cl , H2O

31. Aerobic Co-metabolism

32. Anaerobic Co-metabolism

33. Dehalogenation Dehalogenation refers to the process of stripping halogens (generally Chlorine) from an organic molecule Dehalogenation is generally an anaerobic process, and is often referred to as reductive dechlorination R�Cl + 2e� + H+ ��> R�H + Cl� Can occur via dehalorespiration or cometabolism Some rare cases show cometabolic dechlorination in an aerobic environment

34. Dehalogenation of PCE PCE (perchloroethylene or tetrachloroethylene) TCE (trichloroethylene) DCE (cis-, trans-, and 1,1-dichloroethylene VC (vinyl chloride)

35. Anaerobic Transformation Reductive dechlorination - Most common anaerobic process. A cometabolic process in which the solvent is reduced by the replacement of a chlorine atom with a hydrogen atom. Compounds that contain more than 1 Cl atom dechlorinate in a series of steps, each involving loss of a single Cl atom. Carbon tetrachloride degrades to chloroform via reductive dechlorination. The latter is more resistant and accumulates. PCE and TCE degrade with cis-1,2-DCE and vinyl chloride as intermediates. VC is highly mobile and toxic.

38. Dehalorespiration (Anaerobic) Certain chlorinated organics can serve as a terminal electron acceptor, rather than as a donor Confirmed only for chlorinated ethenes Rapid, compared to cometabolism High percentage of electron donor goes toward dechlorination Dehalorespiring bacteria depend on hydrogen-producing bacteria to produce H2, which is the preferred primary substrate

39. Dependence on Redox Condition

40. Biodegradation of Chlorinated Organics More resistant to biodegradation than aromatic hydrocarbons. Bacteria cannot use most of these compounds as a substrate. Most biodegradation occurs via cometabolism. Cometabolism is slower than heterotrophic metabolism and requires the presence of suitable primary substrates.

41. Stoichiometry Electron Donor to Electron acceptor ratios Hydrocarbon requirements for electron acceptor are well defined Electron donor requirements for dechlorination are poorly defined Cometabolic processes are not predictable Each Electron Donor/Electron Acceptor pair has a unique stoichiometric ratio