Geoscience Reference
In-Depth Information
humate approached the CEC of the polyelectrolyte. This can be explained, in part,
by the preference of the ion exchanger for mineral ions of higher valence. How-
ever, the effects of macromolecules are more important in this instance than
consideration of valence. Adsorbed organo-cations bridge negative charges within
strands and between strands, and the interstrand binding causes the macromole-
cules to shrink; water is then excluded from the matrix and precipitation of the
paraquat-humate complex may occur. This behavior was observed for Na
+
-
smectite uptake of paraquat by Na
+
-humate, approaching the CEC value of the
polyelectrolyte. Adsorption by the Ca
2+
- and H
+
-exchanged humic acids was
significantly less than the CEC of the preparation. Comparisons of the isotherms in
Fig.
8.31
indicate a high affinity for adsorption of paraquat by the H
+
-humic acid
in water and by the Ca
2+
-humate. These isotherms are representative of adsorption
occurring on readily available sites or close to exteriors of macromolecular
structures. The presence of CaCl
2
reduces the adsorption, due to the competition
from the excess Ca
2+
ions in solution.
In a review on the effects of OM heterogeneity on sorption of organic con-
taminants, Huang et al. (
2003
) emphasized the possible presence of matured
kerogen and black carbon or soot particles as a fraction of soils and sediments. For
example, analyzing the naturally occurring total organic matter (TOC) in a sedi-
ment, Song et al. (
2002
) found 8 % humic acid, 52 % black carbon, and 40 %
kerogen materials. The molecular structures of kerogen and black carbon are very
different from those of humic acids, so their interaction with organic contaminants
in the subsurface are different. Kerogen, for example, has a three-dimensional
structure with aromatic nuclei cross-linked by aliphatic chainlike bridges. The
nuclei appear to be formed mainly from clusters (about 100 nm in diameter) of 24
or more parallel aromatic sheets, separated by gaps or voids of 30-40 nm, and
therefore are capable of trapping small hydrophobic organics. The aromatic sheets
contain up to 10 condensed aromatic homocyclic or heterocyclic rings. Bridges are
linear or branched aliphatic chains or O- or S-containing functional groups (Engel
and Macko
1993
). Absorption capacities of TOC components in a pond sediment
vary, for example, with a lower sorption capacity for phenanthrene compared to
that calculated only for humic acid (Xiao et al.
2004
). The lower sorption capacity
is explained by the presence of particulate kerogen and black carbon associated
with the sediment; because these surfaces (in soil/sediment aggregates) are coated
with metal oxides and hydroxides, the TOC components are not fully accessible by
phenanthrene.
The charge characteristics of many pesticides are pH dependent. Some anionic
species are formed through dissociation of protons, and cationic compounds may
be formed by the uptake of protons. Compounds with carboxylic acid groups are
characteristic of anionizable compounds, although phenolic groups rise to anionic
species in alkaline subsurface conditions. The s-triazine family of herbicides
applied to soil is considered the classical representative of the cationizable species.
Several compounds containing carboxylic acid groups enter the subsurface when
sprayed on the vegetation canopy or directly onto the land surface. These act as
neutral molecules when pH values are well below their pK
a
values but become
