Green Chemistry and the Ten Commandments

Thermocline

Sediment

CO 2 from

respiration photosynthesisO^2 from

{CH 2 O} + O 2 CRespiration O 2 + H 2 O

O 2 from

atmosphere

CO 2 from

atmosphere

Sunlight,

photosynthesis

Acid-base reaction:

CO^23 - + H 2 O → HCO 3 - + OH-

Precipitation:

Ca2+ + CO 32 - → CaCO 3 (s)

Microbial oxidation-reduction:

NO- 3 + 2{CH 2 O} + 2H+ → NH 4 + + 2CO 2 + H 2 O

2

Leaching Uptake

Epilimnion,high dissolved O

2

Hypolimnion,low dissolved O

2

Fe2+

Metal species

2HCO 3 {CH 2 O} + O 2 + CO 3 -

Figure 7.4. Stratification of a lake affects its physical characteristics, which in turn influences chemical
phenomena in the body of water. The formula {CH 2 O} is a generic expression representing degradable
biomass.

The photosynthesis produces glucose, C 6 H 12 O 6 , which is converted to other
forms of biomass. This reaction produces elemental oxygen and, during daytime when
photosynthesis occurs, can contribute substantially to the dissolved oxygen content of
the epilimnion. Whereas algae and plants are producers that generate biomass, other
heterotrophic organisms, usually bacteria in water, metabolize biomass,

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O (7.4.2)

consuming oxygen. Normally, biomass is produced by photosynthesis near the surface
of the water in the epilimnion. The biomass sinks to the hypolimnion and is consumed
by heterotrophic organisms, consuming oxygen and lowering the levels of dissolved
oxygen in the hypolimnion, often to around zero. Specialized bacteria in water can
utilize oxidized chemical species with high oxygen contents other than molecular O 2 for
oxygen sources. One such source is nitrate ion, NO 3

-

, which acts as an oxidizing agent
in the bacterially-mediated biodegradation of biomass,

C 6 H 12 O 6 + 3NO 3

-

+ 6H

+

→ 6CO 2 + 3H 2 O + 3NH 4

+

(7.4.3)

producing ammonium ion (or ammonia, NH 3 ).

166 Green Chemistry, 2nd ed