Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 63

between metal ions and inorganic ligands are much weaker than those complexes
formed with organic ligands.
• Precipitation : Accumulation of material (solutes, substances) on the interface of the
soil solids to form new (insoluble) bulk solid phases. Precipitation occurs when
the transfer of solutes from the aqueous phase to the interface results in accumula-
tion of a new substance in the form a new soluble solid phase. Gibbs phase rule
restricts the number of solid phases that can be formed. Precipitation can occur on
the surfaces of the soil solids or in the porewater.
2.6.2 Chemically Reactive Groups of Organic Chemical Contaminants
When organic chemical compounds come in contact with soil, the nature of the chemically
reactive groups in the organic molecules, their shape, size, coniguration, polarity, polariz-
ability, and water solubility are important factors in determining the adsorption of these
chemicals by the soil solids. These chemically reactive groups, which are also known as
functional groups populate the surfaces of both contaminants and soil solids. The chemi-
cal properties of the functional groups of the organic chemicals will inluence the surface
acidity of the soil particles. This is important in the adsorption of ionizable organic mol-
ecules by the soil solids (clays).
The mechanisms of interaction between organic chemicals and soil fractions include
(a) London-van der Waals forces, (b) hydrophobic bonding, (c) charge transfer, (d) ligand
and ion exchange, and (e) chemisorption. Sorption of organic chemicals is enhanced when
there is no hydration layer (of water) on the surfaces of soil particles. Further sorption of
other organic chemicals occurs through van der Waals type forces and hydrogen bond for-
mation between functional groups such as the hydroxyl (OH − ) group on the soil particles
and the carboxyl (COOH) group on the organic chemicals.
The hydroxyl group (OH − ) consists of a hydrogen atom and an oxygen atom bonded
together. This group is by far the most common reactive surface functional group for soil
fractions such as clay minerals, amorphous silicate minerals, metal oxides, and the other
oxides (oxyhydroxides and hydroxides). The hydroxyl group is also present in two groups
of organic chemicals:
1. Alcohols: Methyl, ethyl, isopropyl, and n -butyl. Alcohols can be considered as
hydroxyl alkyl compounds (R-OH) and are neutral in reaction since the OH group
does not ionize.
2. Phenols: Monohydric (aerosols) and polyhydric (obtained by oxidation of accli-
mated activated sludge (pyrocatechol, trihydroxybenzene).
The two other kinds of functional groups associated with organic chemical compounds
(Figure 2.13), in addition to the hydroxyl (OH − ) group, are:
1. Functional groups having a C-O bond. These include the carboxyl, carbonyl,
methoxyl, and ester groups. Compounds possessing the carbonyl group, called car-
bonyl compounds, include aldehydes, ketones, and carboxylic acids. The carboxyl
group, which combines the carbonyl and hydroxyl groups into a single unit to
form a new functional group, is the characteristic functional group of carboxylic
acids, e.g., benzoic, acetic acids.