Halogenated Solvents Chemical Structures
(Kirk-Othmer Encyclopedia of Chemical Technology
Published by John Wiley & Sons, Inc.)
Halogenated solvents are degraded by microorganisms in both aerobic and anaerobic conditions. The anaerobic process proceeds by reductive dechlorinization one chlorine atom at a time. Halogenated solvents have been shown to be the sole carbon source for many anaerobic organisms (see Kohler-Staub, 1995).
Reductive dechlorination of carbon tetrachloride and tetrachloroethylene
![[image]](images/clip_image001_0000.gif)
(Kirk-Othmer Encyclopedia of Chemical Technology
Published by John Wiley & Sons, Inc.)
Under aerobic conditions, halogenated solvents can be mineralized by bacteria, however they are not usually used directly by the bacteria for growth (they are co-metabmolized—see below.) The process relies on the presence of additional growth substrates and the halogenated solvents are co-oxidized as part of the metabolism process. The oxidation process can be catalyzed by two types of enzymes that utilize O2: monooxygenases and dioxygenases. Monooxygenases insert one oxygen atom into the halogenated solvent, and reduce the other to water. Dioxygenases insert both atoms into the halogenated solvent. (Prince, 1998)
Aerobic Oxidation of Tetrachloroethylene
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“COMETABOLISM- a phenomenon that occurs when a compound is transformed by a microorganism but the organism does not grow on the compound and does not derive any energy, carbon or other nutrients from the transformation. Cometabolic transformation occurs when an enzyme of a microorganism growing on compound A, recognizes as substrate compound B and then transforms it into a product. The transformation is limited, because the next enzyme of the organism that should attack in sequence has a higher specificity and does not recognize the product of B as a substrate. It is a dead end transformation without benefit to the organism.” (Atlas and Bartha, 1998). (However there is of course potential benefit to the environment, because the product may be subject to subsequent degradation by another organism.)