Geoscience Reference
In-Depth Information
passage of ions into the interlayer space. Vermiculite is characterized by a more
structured interlayer space because the region between layers of silicate is selec-
tive for certain types of cations like K
+
and NH
4
+
(Sparks and Huang
1985
).
Cation exchange also is affected by the particle size of the mineral fraction. For
example, it was reported (Kennedy and Brown
1965
) that, of the total Ca-Na
content of a sand layer, 90 % is composed of particles of 0.12-0.20 mm and only
10 % contains a 0.20-0.50 mm sand fraction. Similar behavior was observed on
silt materials where the exchange rates (Ba-K) on medium and coarse silt diminish
with increasing particle size.
The organic fraction composition may influence the exchange capacity. A key
contribution to the exchange capacity of humus is given by the carboxyl and
phenolic hydroxyl functional groups. Under appropriate pH conditions, uranic
acids in polysaccharides or carboxy-terminal structures in peptides can contribute
to the negative charge and CEC of the soil organic matter. The basic amino acids
lysine, arginine, and histidine are positively charged at pH = 6; the amino ter-
minal groups in peptides and polypeptides can be expected to be the principal
contributors to positive charges in subsurface organic materials, in an appropriate
pH environment (Talibuden
1981
). Based on a study of pH dependence of organic
contaminant sorption on soil organic matter, Bronner and Goss (
2011
) suggest that
protonation/deprotonation of carboxylic groups in humic matter has no significant
influence on the sorption process.
The CEC of the organo-mineral complexes is less than the sum of each of the
separate organic and mineral components. The CEC decrease may be explained by
changes occurring in the humus configuration following coating of the mineral
surface. A significant elucidation of the relative contributions of mineral and
organic colloids to the adsorption of organic contaminants was made through
studies with separated fractions and well-defined model materials (Gaillardon et al.
1977
; Kang and Xing
2005
; Celis et al.
1996
). A different approach was to study
and compare adsorption before and after organic matter removal (Saltzman et al.
1972
) to assess the relative importance of soil minerals in parathion uptake.
Although the removal of organic matter from soil by oxidation with hydrogen
peroxide (a commonly used, strong oxidation agent) could affect the properties of
an adsorbent, the results obtained may provide qualitative information about its
role and properties in the contaminant retention process. The reported results
showed that parathion has a greater affinity for organic adsorptive surfaces than for
mineral ones. The important finding from this approach suggests that adsorption is
dependent on the type of association between organic and mineral colloids, which
determines the nature and the magnitude of the adsorptive surfaces. Although the
importance of organic matter has been well established, the properties of organic
colloids relevant to the adsorption of contaminants remain to be characterized
thoroughly. The available information suggests that these properties could be
related to the ratio among humic acid, fulvic acid, and humin, as well as the
presence of active groups (e.g., carboxyl, hydroxyl, carbonyl, methoxy), high
CEC, and surface area.
