Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 64

Hydroxyl

Carboxyl

OH -

COOH -

Macromolecule

CO +

Carbonyl

+
x

NH

Amine

OH -

Phenolic

FIGURE 2.13
Some typical functional groups associated with organic chemicals. The macromolecule shown in the diagram
is an organic chemical.

2. Nitrogen-bonding functional groups such as the amine and nitrile groups. The
amino group NH 2 is found in primary amines. The amines may be aliphatic, aro-
matic, or mixed, depending on the nature of the functional groups, and are classi-
ied as:

a. Primary, e.g., methylamine (primary aliphatic); aniline (primary aromatic).

b. Secondary, e.g., dimethylamine (secondary aliphatic); diphenylamine (second-

ary aromatic).

c. Tertiary, e.g., trimethylamine.

Surface acidity is very important in the adsorption of ionizable organic molecules of
clays. The chemical properties of the functional groups of the soil fractions contribute
appreciably to the acidity of the soil particles. Surface acidity is an important factor in clay
adsorption of amines, s -triazines, amides, and substituted urea. This is due to the proton-
ation on the carbonyl group, as demonstrated by the hydroxyl groups in organic chemical
compounds. As shown at the beginning of this section, there are two broad classes of these
compounds: (1) alcohols (ethyl, methyl, isopropyl, etc.) and (2) phenols (monohydric and
polyhydric). In addition, there are two types of compound functional groups: (1) a C-O bond
(carboxyl, carbonyl, methoxyl, etc.) and nitrogen bonding (amine and nitrile). Amine, alco-
hol, and other organic chemicals that possess dominant carbonyl groups, which are posi-
tively charged by protonation can be readily sorbed by clays. In amines, for example, the
NH 2 functional group of amines can protonate in soil, thereby replacing inorganic cations
from the clay complex by ion exchange. The extent of sorption of these kinds of organic
molecules depends on (a) the CEC of the clay minerals, (b) the composition of the clay
soil (soil organics and amorphous materials present in the soil), (c) the amount of reactive
surfaces, and (d) the molecular weight of the organic cations. Because they are longer and
have higher molecular weights, large organic cations are adsorbed more strongly than
inorganic cations. Polymeric hydroxyl cations are adsorbed in preference to monomeric