Encyclopedia of Environmental Science and Engineering, Volume I and II

1272 WATER CHEMISTRY

Because of these peculiarities in equilibria with N 2 , Figure

10(b) has been constructed for the metastable equilibria

between NO 3 , and NO 2  by assuming the transition between

bound nitrogen and N 2 to be hindered. This diagram shows that

the shifts in relative predominance of the three species, NH 4 ,

NO 2  and NH 3  occur within a rather narrow p range. That

each of these species has a dominant zone within this range

would seem to be a contributing factor to the highly mobile

characteristics of the fixed species of the nitrogen cycle. Note

that the relevant species of N (III) at pH  7 is NH 4  rather

than NH 3 since the ammonia system p K a  9.3.

The reduction of SO 42  to H 2 S or HS^ ^ provides a good

example of the application of equilibrium concepts to aquatic

relationships. Figure 10(d) shows that significant reduction

of SO 42  to H 2 S at pH  7 requires p  3. The biological

enzymes that mediate this reduction must therefore operate

at or below this p. Because the system is dynamic rather

than static only an upper bound can be set in this way, for

the excess driving force in terms of p at the mediation site

cannot be determined by equilibrium computations. Since,

however, many biologically mediated reactions seem to oper-

ate with relatively high efficiency in utilizing free energy, it

O 2

H 2 O

NO 3

NO 3

MnO 2

Mn(I)

NH 4 +

FeOOH

CH 2 O

Fe(II)

SO 4 2– CH 3 OH

HS–

CO 2

CH 4

H 2 O

H 2

8

–5

0

5

p

10

15 900

600

EH

(mv)

25 °

300

0

–300

a

Electron Transfer Capacity “Titration Curve”

(REDOX INTENSITY VS REDOX CAPACITANCE)

REDUCTION BY ORGANICS

OXIDATION BY OXYGEN

N 2

meq / liter

5 10 15

FIGURE 11 Electron transfer capacity “Titration Curve.” In a system with

excess organic material, the redox intensity falls as the electron acceptors are suc-

cessively reduced. This diagram was constructed from the redox intensities and

initial concentrations (expressed as electron equivalents) listed in Table 3. The

reactions indicated on the curve proceed sequentially during the stagnation peri-

od in the deeper waters of a polluted lake, vertically downward or temporally in

sediments, sequentially after starting an anaerobic digester and chronologically in

ground water contaminated with organic nutrients.

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