Geoscience Reference
In-Depth Information
provision of an exact, general structure does not seem feasible. Comprehensive
discussion on effects of soil-sediment organic matter heterogeneity on sorption
and desorption of organic contaminants is given by Huang et al. (
2003
).
A reevaluation of molecular structure of humic substances based on data
obtained primarily from nuclear magnetic resonance spectroscopy, X-ray
absorption near-edge structure spectroscopy, electrospray ionization mass spec-
trometry, and pyrolysis studies was presented by Sutton and Sposito (
2005
). The
authors consider that ''humic substances are collections of diverse, relatively low
molecular mass components forming dynamic associations stabilized by hydro-
phobic interactions and hydrogen bonds. These associations are capable of orga-
nizing into micellar structures in suitable aqueous environments. Humic
components display contrasting molecular motional behavior and may be spatially
segregated on a scale of nanometers. Within this new structural context, these
components comprise any molecules intimately associated with a humic sub-
stance.'' Sutton and Sposito (
2005
) conclude by stating that biomolecules bound
strongly within humic fractions are, by definition, humic components.
1.1.5 Electrically Charged Surfaces
The electrically charged surface of the solid phase is characterized by a net
(positive or negative) charge on the solid surface that is in contact with the liquid
or gaseous phase. These charged surfaces usually are faced by one or more layers
of counterions having a net charge separate from the surface charge. The
adsorption of charged solutes onto a solid phase surface is subject to both chemical
binding forces and an electric field at the interface, and it is controlled by an
electrochemical system. Considerable differences exist between the surface
properties of original minerals constituting the bulk solid phase and surface
properties of organic and inorganic colloids.
Some of the functional groups (e.g., OH) on the clay surface exhibit electrical
charges. The magnitude of the electrical charge, as well as its character, is con-
trolled by the properties of the surfaces to which the functional groups are bound
and by the composition of the surrounding liquid. Sposito (
1984
) classified the
surface charge density of soils as follows:
• Intrinsic surface charge density, defined by the number of Coulombs per square
meter bound by surface functional groups, either because of isomorphic sub-
stitutions or because of dissociation/protonation reactions.
• Structural surface charge density, defined as the number of Coulombs per
square meter, as a result of isomorphic substitutions in soil minerals.
• Proton surface charge density, defined as the difference between the number of
moles of complexed proton charges and of complexed hydroxyl charges per unit
mass of colloids.
