Geology Reference
In-Depth Information
and petrogenic sources. The semi-volatile polycyclic aromatic hydrocar-
bons (PAHs) and their alkylated derivatives are generally less abundant
than the alkanes and terpanes of similar carbon number, but are present
in oil and liquid fuels and are also produced during combustion processes
(pyrolysis), and are present in soil and sediments from both petrogenic
and pyrolytic sources. The five-ringed benzo[a]pyrene is a notable exam-
ple, and is classed as a probable human carcinogen, present in urban
environments chiefly from the combustion of fossil fuels.
The involatile, complex polymeric organic matter present in soils and
sediments is commonly referred to as 'humic substances', and probably
forms through a series of oxidation and condensation reactions between
polyphenols, polysaccharides and polyamino acids of plant and microbial
origin.
7
There is no commonly accepted structure of humic substances, with
molecular weights and molecular formulae open to debate, and varying
widely between different soil and sediment types. Extraction of soils with
water and/or organic solvents generally yield only a small fraction of soil-
carbon present as humic substances, but the accepted practice of extracting
soils with aqueous alkali (0.5% NaOH), followed by centrifugation of the
soil/sediment suspension with subsequent acidification of the supernatant,
yields the acid-insoluble humic fraction known as humic acid. Any organic
material left in solution is referred to as fulvic acid and the remaining
insoluble material is referred to as humin.
8
These operationally defined
fractions prove very useful for conducting sorption and fate experiments
for organic pollutants present in soil/sediment systems, where humic sub-
stances play an important role in the soil chemistry of organic compounds.
Humic and fulvic acids give rise to dissolved organic material in water, and
are complex organic macromolecules with a wide variety of functional
groups, primarily carboxylic and phenolic groups,andtoalesserextent
amino (e.g.-NH
2
) and thiol (-SH) groups. These macromolecules have a
wide range of molecular weights (
E
o
500 to 410,000 g mol
1
)and
various humic acid fractions have been separated by gel-permeation chro-
matography (or size-exclusion chromatography) with their structural and
chemical characteristics investigated using
1
H- and
13
C-NMR, and pyr-
olysis-GC-MS.
9
The large surface areas of these molecules provide ideal
surfaces for sorption by other organic molecules, including semi-volatile
organic contaminants such as PAHs and pesticides, providing active sites
for surface-based, heterogeneous chemical reactions.
6.2.1
Identifying Sources of Hydrocarbons
Distinguishing between different sources of hydrocarbons including al-
kanes and PAHs for example, is of fundamental importance for source
