Page 100 - Soil and water contamination, 2nd edition

Solid phase constituents 87

+
CH O + 3 H + 2 MnO 2 Mn 2+ + HCO + 2 H O (4.5)
2 2 3 2
Next, iron -reducing bacteria reduce Fe(III) to Fe(II):
+
CH O + 7 H + 4 Fe ( OH ) 4 Fe 2+ + HCO + 10 H O (4.6)
2 3 3 2
Subsequently, other bacterial species use sulphate as oxidant and reduce sulphate to sulphide :

2CH 2 O + SO 4 2 2HCO 3 + H 2 S 2HCO 3 + 2 H + + S 2 (4.7)

The hydrogen sulphide , a colourless gas with a rotten-egg odour, may volatilise from the
system or dissociate. Many sulphides are barely soluble in water, so will precipitate soon as
a metal sulphide (see Section 5.11). Ultimately, if sulphate is also used, methane -producing
bacteria continue the decomposition under anaerobic conditions:
2CH O CO + CH (4.8)
2 2 4
The methane thus produced is also known as swamp gas. The above sequence of reactions
is accompanied by a decrease of the redox potential (Figure 4.10), which can be observed
as step-wise gradients in redox potential, reactants, and reaction products, in, for instance,
water-saturated organic soils, bed sediments of rivers and lakes, and along groundwater flow
paths .
It was mentioned above that the decomposition rate of organic matter may vary
widely, depending on the nature of the organic compounds. The breakdown of organic
matter adsorbed to mineral surfaces is slower. Moreover, the decomposition rate of organic
matter is governed by environmental conditions such as the nature and concentration
of the oxidant , temperature , and pH. The decomposition of organic matter under oxic
conditions proceeds faster and produces more energy for the bacteria than decomposition
in anoxic environments. In general, decomposition rates increase with increasing
temperature and increasing pH, though the pH effect may be small at pH above 5
(Scheffer and Schachtschabel, 1989). The rate of organic matter decomposition is often
largely controlled by the nitrogen content. Microbes need nitrogen to build proteins, and
nitrogen is often a limiting element for their growth. In this context, the mass ratio of the
total nitrogen to organic carbon or C:N ratio is an important parameter that determines
the decomposition rate. Bacteria require a C:N ratio of about 6:1; detritus with C:N ratios
of 20:1 or less (e.g. young green leaves, algal detritus) has sufficient nitrogen for a relatively
fast decomposition. Detritus with C:N ratios of more than 30:1 (e.g. straw, pine needles)
decomposes slowly.
In surface waters, the decomposition of organic matter causes the dissolved oxygen
concentration to decrease. This may adversely affect the aquatic ecosystem , especially if the
organic matter is from an anthropogenic source (e.g. effluents from wastewater treatment
plants or discharges of untreated sewage water). The depletion of oxygen is usually
measured using a standard biochemical oxygen demand (BOD ) test. The result of this

-3
test indicates the amount of dissolved oxygen expressed in g m used up by a water sample
when incubated in darkness at 20 °C for five days, and is a measure of the concentration
of easily biodegradable organic matter (both dissolved and particulate). In fact, the BOD
cannot be fully attributed to the decay of organic matter, because other oxidisable substances
+
also contribute to the BOD: oxidisable nitrogen (ammonium NH ; see Section 6.2) in
4
particular. Therefore, a distinction is often made between the biochemical oxygen demand
of the carbonaceous matter (CBOD) and that of the nitrogenous matter (NBOD). A BOD
test may be supplemented by a chemical oxygen demand (COD) test, which measures the

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