An Introduction to Environmental Chemistry

atmospheric CO 2 into the soil (or CO 2 that has been generated as the result of
biological activity) the rather insoluble FeCO 3 can be converted to a reduced
aqueous species Fe(HCO 3 ) 2 and mobilized:

eqn. 4.19

As water moves through the soil, either by fluctuating groundwater levels or capil-
lary action, this mobile aqueous iron species is dispersed. If the water table drops,
however, the reduced Fe(HCO 3 ) 2 is exposed to more oxygenated condition, result-
ing in the formation of oxidized (Fe^3 +) minerals such as Fe(OH) 3 (Fig. 4.25).

4.10 Contaminated land

In exceptional cases, the rocks, minerals and soils of the land surface contain com-
pounds that generate natural chemical hazards. Uranium (U) and potassium (K),
common elements in granitic rocks, are inherently unstable because of their
radioactivity (see Section 2.8) and radioactive decay of isotopes of uranium to
form radon (Rn) gas can be a health hazard (Box 4.13). Some chemicals, such as
herbicides and pesticides, are present in soils because we put them there inten-
tionally. Other chemicals arrive in soils because of unintentional or unavoidable
releases, for example the byproducts of combustion in car engines. Exotic or syn-
thetic chemicals (see Section 1.4) are ubiquitous in the environment today. In the
USA alone, 20¥ 109 kg of ethylene and 1¥ 109 kg of benzene were produced in

1996. These compounds are the feedstock chemicals for synthetic processes that
      generate a huge array of synthetic organic compounds. Significant proportions
      of these compounds are released, usually by accident, into the atmosphere,
      hydrosphere and soils. There are a number of routes through which these
      compounds might reach soils, including aerial deposition, spillage, leaching and
      movement in groundwater. Thus, it is possible for some chemicals to reach areas
      remote from the site of compound production or use (see Section 7.4).
      Where a substance is present in the environment at a concentration above
      natural background levels the term ‘contaminant’ is used. The term ‘pollutant’ is
      used when a contaminant can be shown to have a deleterious effect on the envi-
      ronment. Contaminants are broadly divided into two classes: (i) organic conta-
      minants with chemical structures based on carbon, for example benzene; and (ii)
      inorganic contaminants, for example asbestos or lead, which may be in com-
      pound, molecular or elemental form. In the following section we concentrate on
      organic contaminants in soils; the inorganic contaminants mercury and arsenic
      are discussed in the context of water chemistry in Sections 5.6 and 5.7.2. This is
      an artificial division because organic and inorganic contaminants affect both soils
      and water. However, the chemical principles involved are similar in each case.

4.10.1 Organic contaminants in soils

Organic contaminants are compounds with a carbon skeleton, usually associated
with atoms of hydrogen, oxygen, nitrogen, phosphorus and sulphur (see Section

FeCO 32 ()sla++ ÆH O() CO 2 ()qFe HCO() (^32) ()aq
The Chemistry of Continental Solids 119