An Introduction to Environmental Chemistry

eqn. 5.17
At pH values much above 3 the iron(III) precipitates as the common iron(III)
oxide, goethite (FeOOH):

eqn. 5.18

The precipitated goethite coats stream beds and brickwork as a distinctive yellow-
orange crust (Plate 5.2, facing p. 138), a very visible manifestation of the problem.
Bacteria use iron compounds to obtain energy for their metabolic needs (e.g.
oxidation of ferrous to ferric iron). Since these bacteria derive energy from the
oxidation of inorganic matter, they thrive where organic matter is absent, using
carbon dioxide (CO 2 ) as a carbon (C) source. Iron oxidation, however, is not an
efficient means of obtaining energy; approximately 220 g of Fe^2 +must be oxidized
to produce 1 g of cell carbon. As a result, large deposits of iron(III) oxide form
in areas where iron-oxidizing bacteria survive.
We should note that the common sulphide of iron (pyrite—FeS 2 ) often con-
tains significant amounts of the toxic semimetal arsenic, as impurities. As a result,
when iron sulphides are oxidized (eqn. 5.15) arsenic is released along with the
dissolved iron and sulphate. In very rare circumstances this arsenic release can
result in groundwater contamination (see also Section 5.7.2).
The acidity caused by mining operations can be treated (neutralized) by adding
crushed CaCO 3 to the system and by removing dissolved trace metals. At active
mines this is the responsibility of the mine operators. Abandoned mines, however,
create a bigger problem because the source of leakage from the mine area is
unpredictable, flowing out of various fissures and fractures in the rock as the mine
fills with water. Furthermore, in abandoned mines there is often no operator to
take responsibility for treatment. Moreover, as developed countries move away
from coal as an energy source, more coal mines are being abandoned increasing
the risk of acid drainage. There are various strategies being developed to create
passive, low-cost treatments for acid waste including phytoremediation (see
Section 4.10.4), where reed-beds are used to encourage oxidation and trap the
solid iron oxides that precipitate.

5.4.3 Recognizing acidification from sulphate data – ternary diagrams

ternary diagrams

We have already seen how the factors regulating river chemical composition can
be summarized using simple cross-plots of weathering and atmospheric (sea-salt)
inputs (Fig. 5.3). Recognizing the significance of acidification either from acid
rain or from acid mine drainage is aided using a ternary (triangular) diagram to
allow for three inputs—weathering, sea-salt and sulphuric acid (Fig. 5.8). The
diagram plots alkalinity, chloride and sulphate data to trace weathering, sea-salt
and sulphuric acid inputs respectively for river systems discussed elsewhere in this
chapter. Sulphate is a good tracer of acid mine drainage (eqns. 5.15 & 5.17),
although not totally unique to this system (see below).

Fe^3 ()aq+ +Æ 23 H O 2 ()lsFeOOH()+H()+aq

The Chemistry of Continental Waters 159