Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 378

Because of their cyclic structure, cycloalkanes are not as degradable as alkanes, and they
become less degradable as their number of rings increases. Pitter and Chudoba (1990) attri-
bute some of this to their decreasing solubility. Species of Nocardia and Pseudomonas are
able to use cyclohexane as a carbon source. Oxidation of the cycloalkanes with the oxidase
enzyme leads to production of a cyclic alcohol and then a ketone (Bartha, 1986).
10.6.2.2 Gasoline Components BTEX and MTBE
Benzene, toluene, ethylbenzene, and xylene (BTEX) are volatile, water-soluble, hazardous
components of gasoline. Aerobic degradation of all components of BTEX occurs rapidly
with available oxygen. Under anaerobic conditions, degradation is less reliable and is
slower than under aerobic conditions. Bacterial metabolism proceeds through a series of
steps depending on the availability of electron acceptors.
The gasoline additive methyl tert -butyl ether (MTBE) is believed to be highly resistant to
biodegradation since it is reactive with microbial membranes. Some believe that it is slowly
biodegraded (Borden et al., 1997), whereas others believe that it partially degrades to tert -
butyl alcohol (TBA), a health hazard (Landmeyer et al., 1998). More recently, it has become
generally accepted that MNA of MTBE is an acceptable remediation scheme. Sorption and
volatilization are limited. However, mechanisms such as uptake by plants, and abiotic
degradation by oxidation and hydrolysis are likely. In addition biodegradation and its by-
products are biodegradable under aerobic and anaerobic conditions (Bradley et al., 2001).
Guidance documents have been prepared by the API for the natural attenuation of MTBE
(Zeeb and Wiedemeier, 2007).
10.6.2.3 Polycyclic Aromatic Hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs, C 4 n +2 H 2 n +4 ), are components of creosote. As with
cycloalkanes, they are dificult to degrade and as the number of rings increases, the com-
pounds become more dificult to degrade—a result of their decreasing volatility and solu-
bility, and increased sorption. They are degraded one ring at a time. As an example, the
pathway for biodegradation of anthracene is from anthracene cis -1,2-dihydrodiol to salicy-
late with at least six intermediates beginning with 1,2-dihydroxy anthracene onward to
1-hydroxy-2-naphthoic acid as the last intermediate before salicylate.
10.6.2.4 Halogenated Aliphatic and Aromatic Compounds
Halogenated aliphatic compounds include (a) pesticides such as ethylene dibromide (DBR)
or CHCl 3 , CHCl 2 Br and (b) industrial solvents such as methylene chloride and trichlo-
roethylene. Because of the presence of halogen, the lower energy and higher oxidation
state make aerobic degradation more dificult to achieve than anaerobic biodegradation.
Methylene chloride, chlorophenol, and chlorobenzoate are the most aerobically biodegrad-
able. Removal of the halogen and replacement by a hydroxide group is often the irst step
of the degradation process, particularly when the carbon chain length is short. An example
of this is methylene chloride, with formaldehyde, 2-chloroethanol, and 1,2-ethanediol as
intermediates and carbon dioxide as the inal product (Pitter and Chudoba, 1990).
Biodegradation of chlorinated ethenes involves formation of an epoxide and hydrolysis
to carbon dioxide and hydrochloric acid. Reductive dehalogenation can occur anaerobi-
cally and involves replacement of the halogen with hydrogen or formation of a double
bond when two adjacent halogens are removed (dihalo-elimination). This is the particular