Geoscience Reference
In-Depth Information
biomolecules, which are not part of humic substances, also may precipitate at a pH
of 1 or 2 with the humic acids. Furthermore, the more polar compounds may
precipitate with fulvic acids.
Dark-colored pigments extracted from earth materials result from multiple
reactions, the major pathway being through condensation reactions involving
polyphenols and quinones. According to Stevenson (
1994
), polyphenols derived
from lignin are synthesized by microorganisms and enzymatically converted to
quinines, which subsequently undergo self-condensation or combine with amino
compounds to form N-containing polymers. The number of molecules involved in
this process as well as the number of ways in which they combine is almost
unlimited, explaining the heterogeneity of humic materials.
The major atoms composing humic materials are C (50-60%) and O (30-35%).
Fulvic acid has lower carbon and higher oxygen contents. The percentage of
hydrogen (H) and nitrogen (N) varies between 2 and 6 % and that of sulfur (S)
varies from 0 to 2 %. The various fractions of humic substances obtained on the
basis of solubility characteristics are part of a heterogeneous mixture of organic
molecules, which originate from different earth materials and locations and might
range in molecular weight from several hundred to several hundred thousand. The
average molecular weight range for humic acids is on the order 10,000-50,000,
and a typical fulvic acid has a molecular weight in the range of 500-7,000. The
humic fraction in the near surface represents a colloidal complex, including long-
chain molecules or two- or three-dimensional cross-linked molecules whose size
and shape in solution are controlled by the pH and the presence of neutral salts.
Under neutral or slightly alkaline conditions, these molecules are in an expanded
state, as a result of the repulsion of the charged acidic groups, whereas at a low pH
and high salt concentration, molecular aggregation occurs due to charge reduction.
These large organic molecules may exhibit hydrophobic properties, which govern
their interaction with nonionic solutes.
Numerous structures have been proposed for humic substances in the past,
based mainly on speculative or ''in vogue'' research results over the years. A basic
structure for modeling humic substances was the preliminary concept of a two-
dimensional representation of humic acid (Schulten and Schnitzer
1993
,
1997
).
Recent models were developed on the basis of analytical and spectroscopic data
associated with computer calculations (e.g., Bailey et al.
2001
; Jansen et al.
1996
).
Schulten (
2001
) improved the initial model by trapping biological substances, such
as sugars and peptides, thus developing different models for terrestrial humic
acids, soil organic matter, and dissolved organic matter. Because organic matter is
such an important component of soil, development of models to describe sub-
surface behavior now incorporates chemical interaction studies, molecular
mechanism calculations, structural modeling, and geometry optimization.
An example of a structural model of organic matter is presented in Fig.
1.9
,
reproducing Schulten's (
2001
) model for a terrestrial humic acid tetramer in open
form (Fig.
1.9
a) and of a tetramer formed by a trimer trapping an additional
monomer (Fig.
1.9
b). In these models, molecular mechanics calculations were
used for geometry optimization and determination of total potential energy, bond,
