Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER CHEMISTRY 1273

appears likely that the operating p p value does not differ

greatly from the equilibrium value. The diagram also indi-

cates the possibility of formation of elemental sulfur within

a narrow p range at neutral and low pH.

The major feature of the carbon system, shown in

Figure 10(e), is simply the interconvertibility of C(IV) to

C (IV): i.e., the reduction of carbon dioxide bicarbonates

and carbonates to methane and the reverse oxidation reac-

tions. Formation of solid carbon is thermodynamically pos-

sible close to p  4 but its inclusion does not change

other relationships. Other carbon compounds exist under

equilibrium conditions only at very small concentrations

( 10 ^9 M). The existence of myriad synthetic and bio-

chemical organic compounds at ambient p levels is due to

their exceedingly slow rates of equilibration to CO 2 or CH 3

or, in the case of the complicated organic structures in living

systems, to the constant input of energy.

Microbial Mediation and Free Energy of

Redox Reactions

To survive and hence reproduce, microorganisms must not

only capture a significant fraction of the thermodynamically

available energy but must also acquire this energy at a rate

compatible with the maintenance of life. Thus the salient

capability is that of power production per unit biomass and

therefore kinetics (or the rate of movement toward equilib-

rium) should be considered as well as thermodynamics. To

provide for such energy production microorganisms have

developed highly efficient and specific biological com-

pounds (enzymes) which catalyze energy yielding reac-

tions and cell constituent producing processes.

Organisms do not oxidize organic substrates or reduce

O 2 or SO 42  ; they only mediate those reactions which are

thermodynamically possible, or more specifically, the elec-

tron transfer occurring in these reactions. The p range in

which certain oxidation or reduction reactions are possible

may be estimated by calculating the equilibrium concen-

trations of the relevant species as a function of p. Since,

for example, SO 42  can be reduced only below a given p

or redox potential, an equilibrium model can characterize

the p ranges in which reduction of sulfate is possible and

is not possible. Such models are graphically presented in

Figure 11, where all the reactions are amenable to micro-

bial mediation. These diagrams manifest the use of p as

a parameter that characterizes the ecological milieu in a

restrictive fashion.

The data in Table 4 permit the calculation of combina-

tions of the listed half reactions to give complete redox

reactions. Those that are thermodynamically possible are

always accompanied by a decrease in free energy. The free

energy change of the complete redox reaction, G, is

easily calculated from the p values through rearrange-

ment of Eq. (34):

GRT

nn



23.

[( ) ( ) ].

∑

ppεεfor reduction ∑ for oxidation

(42)

If  G is negative, the reaction can occur. Combinations that

lead to energetically possible reactions are given in Table 3.

All of these reactions are mediated by microorganisms.

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