Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 324

strongly than inorganic cations by clays because they are longer and have higher molec-
ular weights. Organic chemical compounds develop mechanisms of interactions at the
molecular level that include
• London-van der Waals forces : These have been described previously and include the
three types: Keesom, Deybe, and London dispersion forces.
• Hydrophobic reactions and bonding : Organic chemical molecules bond onto hydro-
phobic soil particle surfaces because this requires the least restructuring of the
original water structure in the pore spaces of the soil. For soil organic matter
(SOM), the fulvic acids are by and large hydrophilic. These kinds of organic matter
have the least inluence on the structuring of water. Meanwhile, the SOM humins
are highly hydrophobic. These have considerable inluence on the restructuring of
the water structure.
• Hydrogen bonding and charge transfer : hydrogen bonding is a special case of charge
transfer complex formation. These are complexes formed between the electron
donor and the electron acceptor. The bonding between the aromatic groups in
SOM and organic chemicals is an example of charge transfer.
• Ligand and ion exchange : The bonding process requires that the organic chemical
possesses a higher chelating capacity than the replaced ligand.
• Chemisorption : For soils and contaminants, the process of chemical adsorption
involves chemical bonding between the contaminant molecule or ion in the pore-
water and the reactive soil particle surfaces. The process is sometimes called spe-
ciic adsorption and the bonds are covalent bonds.
The functional groups for organic chemical compounds (organic chemical contami-
nants) are either acidic or basic. The characteristics and properties of these groups in
organic molecules, such as shape, size, coniguration, polarity, polarizability, and water
solubility are important in the adsorption of the organic chemicals by the soil fractions.
The various functional groups associated with organic chemical contaminants have been
described in Section 2.5.2 of Chapter 2. A summary of some of these groups is shown in
Figure 9.11. These include the hydroxyl group (alcohols and phenols), the carboxyl group
(COOH), carbonyl (CO), and the amines (primary, secondary, and tertiary). The carbonyl
group technically includes the carboxyl group, and is considered to be the most important
functional group in organic chemistry. Most of the organic chemical contaminants found
in the ground contain the carbonyl group. These chemicals are associated with production
of pharmaceuticals, synthetic chemicals, and synthetic materials.
Not shown in the illustrations in Figure 9.11 are the hydroxy (OH) compounds. These
are the compounds that contain the hydroxyl functional group. The two main groups are
(1) aliphatic and (2) aromatic. The aliphatic compounds are the alcohols and the aromat-
ics are the phenols. Alcohols are hydroxyl alkyl compounds (R-OH), with a carbon atom
bonded to the hydroxyl group. The more familiar ones are CH 3 OH (methanol) and C 2 H 5 OH
(ethanol). Adsorption of the hydroxyl groups of alcohol can be obtained through hydrogen
bonding and cation-dipole interactions. Most primary aliphatic alcohols form single layer
complexes on the negatively charged surfaces of the soil fractions, with their alkyl chain
lying parallel to the surfaces of the soil fractions. Phenols, meanwhile, are compounds that
possess a hydroxyl group attached directly to an aromatic ring. As with the carbonyl func-
tional group, the various hydroxy compounds are widely used and can be found in the