Lecture 6: In Situ Bioremediation and Natural Attenuation

Transcription

1 ENGI 7718 Environmental Geotechniques ENGI 9621 Soil Remediation Engineering Lecture 6: In Situ Bioremediation and Natural Attenuation Spring 2011 Faculty of Engineering & Applied Science 1

2 6.1 Introduction In situ bioremediation the application of biodegradation to the cleanup of hazardous chemicals present in the subsurface Biodegradation the microbial catalyzed reduction in complexity of chemicals 2

3 Biodegradation triangle Source: Suthersan,

4 The ease of biodegradation The ease of degradation of organic molecules is a function of their structure Branched molecules are very difficult to break down Compounds with carbon-carbon double bonds (olefins) breakdown relatively quickly Alkynes > Alkenes >Carboxylic Acids>Alcohols>Straight Chains > Aromatics > Chlorinated Aromatics> Branched Chains 4

5 Microorganisms include Bacteria (aerobic and anaerobic) Single Celled Prokaryotic (lack nuclear membrane) Microorganisms 0.5 ~ meters Surface to Volume Ratio is extremely large Bacteria are very effective at biodegradation Fungi Non photosynthetic organisms, fungi require O 2 (aerobic) Tolerant of higher concentrations of heavy metals Actinomycetes (filamentous bacteria) 5

6 Microorganisms growth 6

7 Effect of microorganisms in degradation Organic molecules are transformed (degraded) by enzymes that reside within the cell walls of microorganisms Each transformation is carried out by a specific enzyme, often associated with a unique microorganism Slight changes in molecular structure will greatly alter the ability to transform organic molecules 7

8 6.2 Microbial metabolism Metabolism modes (1) Aerobic reactions: aerobic respiration Aerobic transformations occurred in the presence of molecular oxygen, with molecular oxygen serving as the electron acceptor 8

9 (2) Anaerobic reactions occur only in the absence of molecular oxygen are subdivided into anaerobic respiration, fermentation, and methane fermentation Fermentation organic compounds serve as both electron donors and electron acceptors 9

10 Summary of metabolism modes Source: Suthersan,

11 6.2.2 Microbial reactions e.g. e.g. Dechlorination the chlorinated compound becomes an electron acceptor a chlorine atom is removed and is replaced with a hydrogen atom Hydrolysis frequently conducted outside the microbial cell by exoenzymes simply a cleavage of an organic molecule with the addition of water 11

12 Cleavage cleaving of a carbon carbon bond an organic compound is split or a terminal carbon is cleaved off an organic chain e.g. Oxidation breakdown of organic compounds using an electrophilic form of oxygen e.g. Reduction breakdown of organic compounds by a nucleophilic form of hydrogen or by direct electron delivery e.g. 12

13 Dehydrogenation an oxidation reduction reaction that results in the loss of two electrons and two protons, resulting in the loss of two hydrogen atoms e.g. Dehydrohalogenation results in the loss of a hydrogen and chlorine atom from the organic compound e.g. Substitution involves replacing one atom with another e.g. 13

14 6.2.3 Hydrocarbon degradation (1) Aliphatic hydrocarbons: straight-, branched-chain or cyclic hydrocarbons of various lengths families of alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, and acids bioremediation of aliphatic hydrocarbons aerobic biodegradation 14

15 A. Straight-chain or branched B. Cyclic Structures of aliphatic hydrocarbons 15

16 e.g. Eventually CO 2 and H 2 O 16

17 (2) Aromatic hydrocarbons Examples of single-ring aromatic compounds, BTEX compounds 17

18 Benzene Biodegradation under Various Electron Acceptor and Redox Conditions Source: Shanahan, Waste Containment and Remediation Technology,

19 e.g. Biodegradation of benzene Benzene ==> Catechol 19

20 Then microbial degradation of Catechol Eventually CO 2 and H 20 2 O

21 (3) Polynuclear aromatic hydrocarbons (PAHs) Mostly rapidly degraded PAHs (two to three rings) 21

22 Slowly degraded or persistent PAHs (four or more rings) 22

23 Factors influencing biodegradation of PAHs under either aerobic or anaerobic conditions solubility of the PAH number of fused rings type of substitution number of substitution position of substitution, and nature of atoms in heterocyclic compounds structure biodegradability relationship 23

24 6.2.4 Chlorinated organics degradation (1) Chlorinated Aliphatic Hydrocarbons (CAHs) Transformation of CAHs mainly anaerobic the predominant mechanisms for anaerobic transformation of chlorinated aliphatic compounds reductive dechlorination 24

25 Anaerobic transformations of chlorinated aliphatic hydrocarbons 25

26 Anaerobic transformation of PCE (tetrachloroethylene) and TCE(trichloroethylene) 26

27 (2) Chlorinated aromatic hydrocarbons (PCBs etc.) Chemical structure of PCBs (Polychlorinated biphenyls, C 12 H 10-x Cl x ) Transformation of PCBs use the PCBs as a carbon source, or destruction takes place through reductive dechlorination, with the replacement of chlorine with hydrogen on the biphenyl skeleton 27

28 6.3 In situ bioremediation systems History of bioremediation application 28

29 Screening criteria 29

30 Requirements for soil bioremediation Source: Shanahan, Waste Containment and Remediation Technology,

31 (1) Raymond Process groundwater recovery wells aboveground treatment nutrients an electron acceptor reinjection of the amended groundwater 31 Source: Suthersan, 1997

32 (2) Injection-extraction wells injection and extraction wells are used together enhance the effectiveness of the subsurface bioreactor Source: Suthersan, 1997 Injection well configurations for introducing reagents for bioremediation 32

33 (3) Pure oxygen/methane injection Subsurface recirculation system for methane and O 2 injection Methane provides necessary material substrate for indigenous microorganisms to produce the enzyme methane monooxygenase degrade TCE Source: Suthersan,

34 (4) Anaerobic aerobic sequencing biodegradation Source: Suthersan, 1997 If the contamination plume to be remediated is large multiple anaerobic aerobic sequencing segments can be implemented to achieve faster cleanup times 34

35 (5) Oxygen release compounds Formulations of very fine, insoluble magnesium peroxide (MgO 2 ) release oxygen at a slow, controlled rate when hydrated increase the dissolved oxygen concentrations within contaminated plumes enhance the rate of aerobic biodegradation 35

36 In situ bioremediation applicable to both soil and groundwater remediation Source: Hardisty, 2005 Source: Suthersan, 1997 In-situ bioremediation implementation 36

37 6.4 Limitations to biodegradation 37

38 6.5 Natural attenuation (NA) Definition Also called as monitored natural attenuation reliance on natural processes to achieve site-specific remedial objectives act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or ground water May be physical, chemical or biological include biodegradation, dispersion &dilution, chemical reactions, volatilization and sorption processes NA is not no-action alternative 38

39 Prerequisite for NA (1) Source control (2) Site-specific characterization data and analysis Historical chemical data showing clear trend of decreasing mass or concentration Hydrogeologic or geochemical data that indirectly demonstrate natural attenuation processes Field or microcosm studies that directly demonstrate natural attenuation processes (3) Performance monitoring e.g. a site contaminated by fuel hydrocarbons is treated by NA 39

40 Potential processes Aerobic biodegradation: fuel hydrocarbons CO 2 + H 2 O Denitrification: NO 3 N Iron reduction: Fe (III) Fe (II) Sulfate reduction: sulfate sulfide Carbon dioxide neutralization increased alkalinity Analytical protocol during performance monitoring Biologically, Field dehydrogenase test confirm presence of aerobic bacteria Volatile fatty acids biodegradation byproduct of complex organic compounds Microcosm studies confirm biodegradation is occurring 40

41 Chemically, Total hydrocarbons confirm HC decrease Aromatic hydrocarbons confirm BTEX decrease Oxygen confirm utilization, redox state Nitrate confirm utilization Iron(II) confirm production Sulfate confirm utilization Alkalinity confirm CO 2 production and neutralization Oxidation-reduction potential confirm geochemical environment ph, temperature, conductivity, chloride confirmation of single ground-water system 41