Environmental Engineering Reference
In-Depth Information
1000
800
600
Nitrate - N2
Pyrolucite - Mn2+
Goethite - Fe2+
Sulfate-Sulfide
CO2-CH4
NTCHS Tech Std
400
200
0
-200
-400
4
5
6
7
8
pH
Fig. 7.2 Eh-pH stability phase diagram illustrating the redox conditions under which various
common electron acceptors in soils become reduced
matter because the diffusion of O
2
into and through liquid water is quite slow.
Such conditions where O
2
has become depleted are referred to as anaerobic.
In order for microorganisms to continue to oxidize organic materials under
anaerobic conditions, they must use some alternate electron acceptor to O
2
. Chemi-
cal thermodynamics determine which compounds that are common in soils can
most easily accept electrons and be reduced. Those chemical ions and compounds
readily found in soils that function as electron acceptors once O
2
is depleted are:
NO
3
; manganic manganese (Mn
4+
); Fe
3+
; sulfate (SO
4
2
); and CO
2
. These are
listed in order of their ease of being reduced which means that the soil environment
must become increasingly reduced in order to proceed through this list of electron
acceptors. This is sometimes shown in an Eh-pH stability diagram such as Fig.
7.2
where higher Eh values represent more oxidizing conditions and lower Eh
represents more reducing conditions. Each line in Fig.
7.2
represents a “redox
pair” and above the line, the oxidized form is stable and below the line, the reduced
phase is stable. Thus, as the soil becomes progressively more reducing (lower Eh),
various reduced phases would be predicted to be stable. First, NO
3
would be
expected to be reduced (function as an electron acceptor), and then Mn oxides (such
as pyrolusite), Fe oxides (such as goethite), SO
4
2
and eventually (under highly
reducing conditions) CO
2
can be reduced to methane.
Although this stepwise change or progression seems very systematic and
orderly, soil systems are typically highly complex and also highly variable. Not
all soils contain all these compounds. Also, the various proportions among these
compounds can be very different among soils or ecological settings. Small scale
variability in redox potential (Eh) in soils can be very great over short distances
