Page 28 - Soil and water contamination, 2nd edition

2

Basic environmental chemistry

2.1 INTRODUCTION

Molecules, ions, and solid particles in the environment undergo physical and chemical
transfer processes which distribute mass within or among phases; for example, the
volatilisation of a chemical from water to air, or the dissolution of calcite (CaCO ).
3
Knowledge of the basic principles of chemistry is essential to be able to quantify these
transfer processes and to assess the distribution of concentrations of chemicals in space and
time and their chemical speciation . Speciation refers to the occurrence of different forms,
whether chemical or physical, of an element. The speciation of an element determines its
bioavailability and toxicity and controls its transport and fate in soil and water.
This section recapitulates the fundamentals of environmental chemistry which will be
applied in subsequent chapters. Only the chemical interactions between relatively simple ions
and molecules and between phases will be covered in this chapter. Chemical interactions on
solid surfaces are dealt with in Chapters 4 and 13.
Chemical reactions result in the breakdown of one chemical and the formation of
another. This process inevitably involves chemical bonds being broken or formed between
atoms. The various types of chemical bonds include covalent bonds, in which atoms share
one or more electrons, ionic bonds, in which opposite electric charges on neighbouring
atoms result in the bonding force, and hydrogen bond s, in which a slightly positively charged
hydrogen atom at the edge of a molecule loosely bonds with a slightly negatively charged
atom of another molecule. In addition, weak, attractive Van der Waals forces between
molecules may contribute to bonding as well.
Most inorganic chemicals are electrolytes which to varying extent can dissolve in water to
+
2+
form ions. Positively charged ions (e.g. Na or Ca ) are called cations and negatively charged
-
-
ions (e.g. Cl or HCO ) are called anions . Organic substances can also dissolve in water
3
to form organic cations or anions. However, most organic liquids are non-electrolytes and
dissolve in water as non-ionic molecules.
2.2 UNITS OF ANALYSIS

We have already encountered the term concentration in the previous chapter, being used
to specify how much of a substance is present in a given amount of soil or water. The
concentration of chemicals in soil and water can be presented in various units. Though the
-1
-3
official SI units for concentration are moles m or moles kg , these are not commonly used.
The most common units are listed in Table 2.1.
-1
-1
For dilute fresh water samples, the unit mg l is numerically equal to mg kg , since
-3
the density of the water is 1 g cm . For samples of higher density, for example seawater
-3
(density ≈ 1.023 g cm ), recalculation is necessary to convert the results from concentration
-1
-1
by volume to concentration by weight. To recalculate the results from mg l to mmol l ,

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