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The ease of biodegradation will depend somewhat on In many cases,the major limitation on aerobic biodeg-
<br /> the type of hydrocarbon. Moderate to lower molecular radation.in the subsurface is the low solubility of oxygen in
<br /> weight hydrocarbons(Cio to C24 alkanes,single ring aromat- water. For example,aerobic toluene(C6H5-CH3)mineral-
<br /> ics)appear to be the most easily degradable hydrocarbons ization can be represented by the reaction:
<br /> (Atlas, 1988). As the molecular weight increases, so-does C6Hs-CH3+9 02-• 7 CO2+4 H2O (1)
<br /> resistance to biodegradation. Gasoline contains primarily j
<br /> low to moderate molecular weight compounds while diesel Water saturated with air contains from 6,000 to 12,000µgel
<br /> and coal tars contain abundant higher molecular weight of dissolved oxygen. Complete conversion of toluene(and
<br /> compounds.Jamison et al.(1975)found that the vast major- many other hydrocarbons) to carbon dioxide and water
<br /> ity of gasoline components were readily degraded by a requires approximately 3 pg/1 of.oxygen for each Mcg/l of
<br /> mixed microbial population obtained from a gasoline- hydrocarbon. At this ratio, the oxygen present in water
<br /> contaminated aquifer. Many of the individual gasoline could result in the biodegradation of 2,000 to 4,000µg/1 of
<br /> components would not support microbial growth as a sole dissolved hydrocarbon by strictly aerobic processes. At a
<br /> carbon source but did disappear when a mixed substrate of greater hydrocarbon concentration,aerobic biodegradation
<br /> gasoline dissolved in water was used.This finding suggested may be hindered or degradation may partially occur via
<br /> that a mixed microbial population may be necessary for slower anaerobic processes.The extent of aerobic biodegra
<br /> complete degradation. dation will be controlled by the amount of contamination
<br /> In a study of the catabolic activity of bacteria from an released,the rate of oxygen transfdr into the subsurface,and
<br /> aquifer contaminated with unleaded gasoline,Ridgeway et the background oxygen content of the aquifer (Borden,
<br /> al.(1990)found that most isolates were very specific in their 1994).
<br /> ability to degrade hydrocarbons. Although all of the 15
<br /> hydrocarbons tested were degraded by at least one isolate, Biodegradation Using Nitrate
<br /> most organisms were able tp degrade only one of several When oxygen is depleted and nitrate is present (or
<br /> closely related compounds. Toluene, p-xylene, ethylben- other oxidized forms of nitrogen), some facultative anaer-
<br /> zene,and 1,2,4trimethylbenzene were most frequently uti-
<br /> lized obit microorganisms will utilize nitrate(NOa')instead of
<br /> whereas cyclic and branched alkanes were least
<br /> utilized' oxygen as a terminal electron acceptor. For toluene, this' Hydrocarbon biodegradation is essentially an oxidation-
<br /> process can be approximated by the reaction:
<br /> reduction reaction where the hydrocarbon is oxidized C6H5-CH3+7.2 H+' +7.2 NO3' --
<br /> (donates electrons)and an electron acceptor(e.g.,oxygen)is 7 CO2+7.6 H2O+3.6 N2 (2)
<br /> reduced(accepts electrons).There are a number of different
<br /> compounds that can act as electron acceptors including Over the past decade, researchers have found that toluene;
<br /> oxygen (02),.nitrate (NO3-1), iron oxides [e.g., Fe(OH)3], ethylbenzene; m-, p-, and o-xylene; naphthalene; and a
<br /> sulfate (SOS-2), water (H2O), and carbon dioxide (CO2). variety of other compounds can be biodegraded using
<br /> Aerobic bacteria use molecular oxygen as the electron nitrate as the terminal electron acceptor(Kuhn et al., 1988;
<br /> acceptor.Anaerobic bacteria use other compounds such as Hutchins et al., 1991; Mihelcic and Luthy, 1991). At this
<br /> NO3-1,Fe(OH)3,and SO4-2 as electron aoceptors.,Qxy e time,there is so e,cpptroversy about.the biodegradability
<br /> the mo, .St�9� lectr kcause microorgan- of benzene under nitrifyutg coa(imp. .Several investiga
<br /> isms gain more energy from aerobic reactions. Water and tors 'have reported,6 to,bc "Watian'i(not biode-
<br /> carbon dioxide are the least preferred because microorgan- gradable) under denitrifying conditions (Hutchins et al.,
<br /> isms gain the least energy from these reactions. I991;Kuhn et al., 1988)whereas other studies indicate that
<br /> benzene is degraded(Major et al., 1988;.Kukor and.Olsen,
<br /> Aerobic Biodegradation 1989)•
<br /> Almost all petroleum hydrocarbons are biodegradable
<br /> under aerobic conditions. Oxygen is a cosubstrate for the Biodegradation Using Ferric Iron
<br /> only known enzyme that can initiate the metabolism of Once the available oxygen and nitrate are depleted,
<br /> hydrocarbon(Young, 1984)and is later used as an electron subsurface microorganisms may use oxidized ferric iron
<br /> acceptor for energy generation. 'Under ideal conditions, [Fe(III)] as an electron acceptor. Microorganisms have
<br /> biodegradation rates for low-to moderate-molecular-weight been identified which can couple the reduction of ferric iron
<br /> aliphatic, alicyclic, and aromatic compounds can be very with the oxidation of aromatic compounds including
<br /> high• toluene, phenol, p-cresol, and benzoate (Lovley and
<br /> Alvarez and Vogel(1991)observed essentially complete Lonergan,1990;Lovley et al.,1989).Large amounts of ferric
<br /> removal of mixtures of benzene, toluene,.and p-xylene in iron are present in the sediments of most aquifers and could
<br /> aquifer slurries and pure cultures after 3 to 13 days incuba potentially provide a large reservoir of electron acceptor for
<br /> tion. Using aquifer material from a gas plant facility in hydrocarbon biodegradation. This iron may be present in
<br /> Michigan,Chiang ct al.(1989)found between 80 and 100% both crystalline and amorphous mineral forms. The forms
<br /> removal of BTEX (120-16,000 dug/l) in microcosms with that are most easily reduced are amorphous and poorly
<br /> sufficient oxygen. Half-lives for the biodegraded com- crystalline Fe(III)hydroxides, Fe(III)oxyhydroxides,and
<br /> pounds varied between 5 and 20 days. Fe(III)oxides(Lovley, 1991).A hypothetical reaction cou-
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