Page 112 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 99

rocks, magnesium also occurs as a constituent of chlorite and serpentine. The major
sedimentary form of magnesium is dolomite , in which calcium and magnesium are
present in equal amounts. Furthermore, in most carbonate rocks Mg occurs in significant
amounts as dolomite, magnesite (MgCO ), hydrated species of Mg carbonate (nesquehonite
3
(MgCO ⋅3H O), and lansfordite (MgCO ⋅5H O)), and brucite (Mg(OH) ). Obviously,
3 2 3 2 2
the dissolution of these carbonate rocks also brings Mg into solution, but the process is not
readily reversible. As all Mg carbonate species are more soluble than calcite , a considerable
degree of supersaturation may be required before they precipitate. Consequently, the
precipitate that forms from a solution that has attacked an Mg-bearing limestone may be
nearly pure calcite and, in turn, in Mg-bearing rock or sediment both the Mg concentration
and the Mg:Ca ratio tends to increase along a groundwater flow path (Hem, 1989).
Magnesium is also present between the lattices of montmorillonite . The cation exchange
behaviour of Mg is similar to that of Ca. Both ions are strongly adsorbed to negatively
charged sites of clay minerals and organic particles. In general, the content of exchangeable
Mg increases with increasing clay and silt content. Podsolic sandy soils and acidic leached
soils contain very little Mg.
Anthropogenic sources of Mg are generally rare. In the case of Mg deficiency in
agricultural crops, the Mg content of the soil is adjusted by applying dolomitic lime,
potassium magnesium sulphate , or magnesium sulphate.

5.6 IRON

Iron is a transition metal and, after aluminium , the second most abundant metallic element
in the Earth’s outer crust. Nevertheless, the Fe concentrations in natural water are generally
small. In plants, about three-quarters of the iron (Fe) is associated with chloroplasts (i.e. the
green globules inside plant cells responsible for photosynthesis ). It plays an important role
in the plants’ redox reactions . Plants that suffer Fe deficiency exhibit the typical interveinal
chlorosis . In animals, Fe is part of haemoglobin, the carrier of the oxygen in red blood cells.
Although Fe is non-toxic, it is an undesirable constituent of drinking water, because it has
an adverse effect on the taste, it may clog distribution systems and may produce stubborn,
rusty brown stains on textile and ceramics. Therefore, during drinking water preparation it is
usually removed by aeration (see below).
Relatively large amounts of iron occur in a wide range of igneous rock minerals,
including the pyroxenes, amphiboles, biotite, magnetite, and olivine. In these minerals, Fe
2+
is largely present in the ferrous (Fe or Fe(II)) oxidation state , but it may also be present in
3+
the ferric (Fe or Fe(III)) oxidation state . When these primary minerals are weathered, the
Fe released generally reprecipitates quickly. Under reducing conditions, Fe is in the ferrous
oxidation state; if sulphur is available, pyrite or marcasite (both FeS ) is formed. When
2
sulphur is less available, siderite (FeCO ) or vivianite (Fe (PO ) ⋅8H O) may precipitate.
3 3 4 2 2
Under oxidising conditions, Fe is in the ferric oxidation state and the precipitates consist of
ferric oxides, oxyhydroxides , or hydroxides. Fresh precipitates consist mostly of amorphous
ferric hydroxide (Fe(OH) ). In the course of time, this Fe(OH) undergoes ageing which
3 3
results in the formation of more stable, crystalline minerals of Fe(III) oxides/hydroxides, such
as haematite (α-Fe O ) or goethite (α-FeOOH) (see Section 4.2.2).
3
2
3+
2+
Fe is much more soluble than Fe . Thus, the most obvious control on the solubility
2+
3+
of Fe is the oxidation reaction from Fe to Fe and the reverse reduction reaction. The
oxidation reaction occurs particularly in zones of groundwater discharge , where groundwater
2+
containing Fe encounters dissolved oxygen from the free atmosphere, and proceeds very
3+
rapidly. The Fe precipitates primarily as amorphous ferric hydroxide. Consequently, when

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