Page 99 - Soil and water contamination, 2nd edition

86 Soil and Water Contamination

However, the decay rates of the various organic compounds may differ considerably and the
complete breakdown may be a very slow process taking several decades or – in the case of
recalcitrant humic substances – thousands of years. Because of its complex composition,
there is no general chemical formula for organic matter. However, it is possible to write a
general simplified equation of the reaction, which only involves the initial reactants and the
final products:
CH O + e. a. CO + H O + inorganic N, P, S (4.2)
2 2 2
where CH O refers to organic matter and e.a. to the various electron acceptors (oxidants )
2
-
2-
available in the system (e.g. oxygen (O ), nitrate (NO ), and sulphate (SO )). In well-
2 3 4
aerated soils and surface waters, oxygen is the predominant electron acceptor :
CH O + O . CO + H O HCO + H + (4.3)
2 2 2 2 3
In many environments, however, the amount of oxygen is limited, and the other oxidants
are used consecutively. Figure 4.10 shows the sequence of redox reactions which operate
at successively lower redox potential s. Firstly, if oxygen is depleted, nitrate -reducing
microbe species take over the decomposition of organic matter , using nitrate as oxidant .
-
Nitrate is reduced via nitrite (NO ) , nitric oxide (NO ) , and nitrous oxide (N O) to free
2 x 2
nitrogen (N ). This process is called denitrification (see also Section 6.2) and this overall
2
transformation can be written as:
.
0
0
.
CH O + 8 NO HCO + 4 N + 4 H O + 2 H + (4.4)
.
0
0
.
2 3 3 2 2
Not all nitrate is transformed into free nitrogen gas, because both nitric oxide) and nitrous
oxide) are also gases, which can escape prematurely from the system. If nitrate has been
depleted, manganese -reducing bacteria use Mn(IV) as the principal electron acceptor
(oxidant ):
900
O H O
2 2
NO 3- N
2
600
MnO Mn 2+
2
300
Eh (mV)
0
Fe(OH) Fe 2+
3
SO 2- H S
4 2
CO CH
-300 2 4
(H O H )
2 2
6642 6642 6642 -600
Organic matter consumption

Figure 4.10 Evolution of the redox potential (Eh) as function of organic matter consumption .

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Soil and Water.indd 98 10/1/2013 6:44:23 PM