Environmental Engineering Reference
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from the attacking nucleophile into the lowest unoccupied molecular orbital.
The one-electron reduction of CO 2 is thermodynamically strongly disfavored
( E 0 '
-1.9 V), while the two-electron reductions to CO ( E 0 '
¼
¼
-0.52 V) and formate
( E 0 '
-0.43 V) [ 2 ] are more favorable. For a more comprehensive discussion of
the chemistry and biochemistry of CO 2 we refer the reader to the recent review by
Appel et al. [ 2 ].
¼
1.2 Carbon Monoxide in the Biosphere
CO is a component of the past and present atmosphere. It is a ubiquitous pollutant of
the present atmosphere [ 3 ], but may have facilitated early life in the primordial
atmosphere by serving as carbon source [ 4 , 5 ]. Ubiquitous volcanic emissions in the
early Earth raised global CO concentrations to approximately 100 ppm. Nowadays,
0.05-0.35 ppm of CO are found in non-urban environments, while in dense urban
regions with high traffic CO concentrations can reach 1.30 ppm [ 3 , 6 ]. Concentra-
tions of up to 5,540 ppm can still be encountered in volcanic environments [ 7 ],
nurturing present day life on CO in the vicinity.
1.2.1 Biological Cycle of Carbon Monoxide
1.2.1.1 Sources of Carbon Monoxide
Natural and anthropogenic processes generate CO. They are responsible for a CO
emission of 2,500-2,600 teragram (Tg) per year [ 3 , 8 ]. Most CO is emitted by
natural processes including atmospheric methane oxidation, natural hydrocarbon
oxidation, volcanic activity, production by plants and photochemical degradation of
organic matters in water, soil, and marine sediments [ 3 , 8 , 9 ]. Notable amounts of
CO originate from the enzymatic degradation of heme [ 10 , 11 ]. Anthropogenic
processes such as incomplete combustion of fossil fuels and various industrial
processes are responsible for the remaining annual 1,200 Tg of CO emitted to the
atmosphere [ 3 ].
1.2.1.2 Removal of Carbon Monoxide
Chemical and biological processes are also responsible for CO removal. The major
part of CO in the atmosphere becomes oxidized to CO 2 by rapid reaction with
hydroxyl radicals in the troposphere (2,000-2,800 Tg/yr), reducing the half-life of
CO in the troposphere to a few months [ 8 , 12 ]. Microbes consume CO by using it
as a source of energy and carbon (Section 1.2.2 ). Overall, soil and marine
microbes reduce the global budget of CO by 20 % per year [ 3 , 9 , 13 ], to which
soil microbes contribute with 200-600 Tg of CO removal per year [ 14 ].
CO-oxidizing bacteria have a natural enrichment in the top layer of burning
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