Geoscience Reference
In-Depth Information
Humic substances are also able to enhance abiotic transformation of organic
substances, such as anthropogenic pollutants, at the solid-liquid interface. The
transformation of organic pollutants adsorbed on organic matter surfaces occurs
because the natural organic fraction contains many reactive groups that are known
to enhance chemical changes in several families of organic substances, and humic
substances provide a strong reducing capacity (Stevenson
1982
). The presence of
relatively stable free radicals in the fulvic and humic acid fractions of subsurface
organic matter further supports enhanced abiotic transformations of many organic
contaminants.
Transformation of toxic organic chemicals by humic substances at the solid-
liquid interface may occur mainly through hydrolysis, as discussed in the reviews
of Senesi and Chen (
1989
) and Wolfe et al. (
1990
).
Hydrolysis of triazine herbicides, for example, describes humic-induced trans-
formation of toxic organic molecules. The catalytic effects of the chloro-s-triazine
herbicides on dechloro-hydroxylation were determined by Armstrong et al. (
1967
).
They found that the formation of H-bonding between the ring or side-chain
nitrogen of s-triazine and the humic acid surface causes electron withdrawal from
the electron-deficient carbon atom that already is surrounded by electronegative
nitrogen and chlorine atoms. As a consequence, the weak nucleophile, water,
replaces the chlorine atom and increases the rate of hydrolysis. The formation of
H-bonding between humic acids and atrazine was suggested as being responsible
for the observed decrease in the activation energy barrier of the reaction. The
catalytic effect of humic acids depends not only on the number of effective acid
groups but also on their arrangement in the humic acid molecule (Li and Felbeck
1972
). Based on knowledge of the type, number, and pK
a
values of the acidic
functional groups in a quantitatively characterized fulvic acid, Gamble and Khan
(
1985
) confirmed that hydrogen ions and undissociated carboxyl groups are the
only catalytic agents for atrazine hydrolysis.
Purdue and Wolfe (
1983
) proposed a general mechanism for the effects of
subsurface organic matter on the hydrolysis of hydrophobic organic contaminants.
The suggested mechanism is derived from a combination of processes that
describe, separately, partitioning equilibrium, acid-base catalysis, and micellar
catalysis. The resulting model indicates that the overall reaction rates of toxic
organic chemicals can be attributed almost totally to partitioning equilibrium and
micellar catalysis.
Nonspecific enhanced surface transformation is observed on charged surfaces
found in the vicinity of the surface that are not specific adsorption sites. Probably
the most significant phenomenon in the interfacial region, near the surface of
charged solids, is the strong dependence of the concentration of charged solutes on
the distance from the surface. The concentration in the interfacial region of
charged inductors, catalysts, reactants, and products therefore can be different from
their concentration in the bulk solution. Another factor that can affect organic
transformations in the interfacial region near charged particles is the aforemen-
tioned influence of the electric field on the polarization and dissociation of the
solute and solvent.
