Page 123 - Soil and water contamination, 2nd edition

110 Soil and Water Contamination

deep aquifers, the initial CO from the root zone is rapidly depleted. In such environments, a
2
2+
low pCO occurs together with low Ca concentrations and a high pH.
2
-
2+
It is worth mentioning that calcite dissolution by carbonic acid yields a Ca :HCO
3
-
molar ratio of 0.5, i.e. for every mole CaCO that dissolves, two HCO ions are produced
3 3
(see Equation 5.14). If calcite is dissolved by another acid (e.g. sulphuric acid (H SO )
2 4
derived from atmospheric pollution, acidic mine wastewater, or pyrite oxidation (see also
-
2+
Section 5.11), the Ca :HCO molar ratio amounts to 1.0 (see Equation 5.11). This implies
3
2+
-
that a Ca :HCO molar ratio greater than 0.5 may be an indication of acidification in
3
2+
waters in which the Ca concentration is controlled by calcite dissolution and precipitation.
Obviously, in the case of dissolution of dolomite (Ca,MgCO ) or gypsum (CaSO ⋅2H O)
3
2
4
2+
also contributing to the dissolved Ca concentration, this relationship becomes more
complex.
5.11 SULPHATE AND SULPHIDE
2-
Sulphate (SO ) is the dominant dissolved species and most highly oxidised form of sulphur
4
(S). As sulphur can occur in oxidation state s varying from 2- to 6+ sulphur, its chemical
behaviour is strongly governed by the redox conditions. As well as occurring as sulphate ,
2-
sulphur also occurs naturally as sulphide (S , which is the 2- oxidation state ) and, to a lesser
0
2-
extent, in the neutral oxidation state (S ) and in the sulphite form (SO , the 4+ oxidation
3
state). Sulphur is a constituent of some amino acids and so forms an essential part of
proteins: the sulphur content of protein materials can be as high as 8 percent. It is therefore
an important nutrient for both plants and animals. Sulphate is also an oxidant in the
microbial decomposition of organic matter , if oxygen is depleted under anaerobic conditions.
In this process, sulphate is reduced to sulphide, which either precipitates or forms H S gas
2
which is toxic to many plants (e.g. the Akiochi disease in some paddy rice).
Sulphate-containing minerals occur most extensively in evaporite sediments. The most
important of these are gypsum (CaSO ⋅2H O) and anhydrite (CaSO ; without crystalline
4 2 4
water). In other sedimentary rocks and igneous rocks , sulphur occurs commonly as metal
sulphides. The most abundant and widely distributed form of metal sulphide is pyrite (FeS ),
2
which is common in coal and slate. If the metal sulphides come into contact with oxygenated
2-
water, they oxidise to SO that readily dissolves. In soils, the decomposition of soil organic
4
matter including proteins constitutes an important supply of S to plants.

Another substantial source of S in soil and water is the atmospheric deposition of
2-
SO from volcanic or anthropogenic sources. Volcanic eruptions, fuel combustion,
4
and ore smelting lead to the emission of sulphur dioxide (SO ) that readily oxidises to
2
sulphuric acid (H SO ), the main compound of sulphate aerosols , in the atmosphere.
2 4
Therefore, these aerosols also contribute significantly to acid deposition and have led to
the widespread degeneration of forest ecosystems in Europe and North America. In Central
Europe in particular, where sulphur-rich brown coal (lignite) has for long been used for
electricity generation and heating, acid deposition has caused massive die-back of forest
trees. The rate of S deposition in the mountainous areas most affected by forest die-back
-1
2-
-1
reached 150 kg S ha y . Atmospheric deposition of SO has also greatly enhanced the
4
2-
SO concentrations in runoff water from catchments in industrialised countries. Other
4
2-
anthropogenic sources of SO involve the enhanced oxidation of sulphides or organic S in,
4
for example, mine heaps and domestic waste disposal sites.
In contrast to sulphides, sulphates are generally soluble, having solubility product s
greater than 5. The exceptions are barium sulphate (K = 10 -9.97 ), lead sulphate (K = 10 -7.79 ),
s
s
-6.5
strontium sulphate (K = 10 ) and calcium sulphate (K = 10 -4.58 ). Sulphate does not adsorb
s s
to mineral and organic surfaces and is rather mobile in solution. It tends to form complexes
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