An Introduction to Environmental Chemistry

The Chemistry of Continental Solids 121

destroyed or lost to other environments (e.g. the atmosphere or hydrosphere).

The volatility of a compound is controlled by the vapour pressure (Box 4.14),

while solubility is governed principally by polarity—a function of molecular

structure, molecular weight and functional groups. Degradation results from both

biological and abiological mechanisms, although given the profusion of micro-

organisms in soil (a gram of soil typically contains 10^6 –10^9 culturable micro-

organisms) the potential for biodegradation is high. Abiological degradation

occurs by hydrolysis, reduction, oxidation and photo-oxidation.

Organic contaminants interact mainly with either the mineral or organic com-

ponents of soils. Two types of non-reactive interaction are possible: (i) adsorp-

tion (a surface phenomenon); and (ii) entrapment within the soil minerals or

components (Fig. 4.28). The nature and extent of interaction is dependent on the

properties of the molecule—its aqueous solubility, vapour pressure and hydro-

phobicity (Box 4.14)—but also its concentration and the properties of the soil.

Soil factors include the amount and type of soil organic matter, the clay content

and mineralogy, pore size and pore structure, and the microorganisms present.

The net result of both adsorption and entrapment is a decrease in both the bio-

logical and chemical availability of soil-associated contaminants with time. Thus,

the proportion of compound in the ‘available’ fraction decreases with time (Fig.

4.29), while the proportion of compound in the ‘non-available’ fraction increases

with time (Fig. 4.29). This process known as ‘ageing’, may cause a decrease in

the rate, and the extent, of degradation an organic contaminant suffers in the soil

environment. Organic contaminants attached to soil particles exist in four frac-

tions distinguished by the ease with which they can be released or desorbed from

Pentachlorophenol

OH

Cl Cl

Cl Cl

Cl

Phenol

OH

Ethyl benzene

CH 2 CH 3

Toluene

CH 3

Chlorobenzene

Cl

Benzene

C=C

Cl

Cl

Cl

Cl

Tetra chloroethene

H 2 C=CH 2

Ethene

Polychlorinated biphenyl

Cl Cl

Cl Cl

Cl

Polychlorinated dioxin

Cl

Cl Cl

O Cl

O

PAHs

Naphthalene Phenanthrene Pyrene

Fig. 4.26Examples of organic contaminants.