Geoscience Reference
In-Depth Information
matter, no environmental risk occurs. In contrast, however, if over time, the bound
residues exhibit properties different than those of NOM, compounds having a toxic
character become a contamination risk for the subsurface.
Bound residues of anthropogenic origin, found in the soil layer, may be com-
pared with those of natural organic molecules released from plant and animal
debris and utilized as a source of energy by microbial populations. Parent mole-
cules and their metabolites may interact in the subsurface with the organic matter
and then be desorbed and develop further by long-time contact. In this process,
known as aging, molecules become more tightly bound or entrapped into organic
matter or clay fractions of the solid phase. Barraclough et al. (
2005
) noted that the
mass balance of xenobiotics in the subsurface exhibits the same variation as that
seen with natural products, in terms of their partitioning between evolved CO
2
and
their incorporation into the humic and fulvic substances. For example, more than
80 % of the labeled carbon from a number of xenobiotic compounds was still in
the soil several years following their application (Burauel and Fuhr
2000
). In a
different case, 73 % of the carbon originating from a labeled phenanthrene was
recovered as CO
2
after only 82 days (Richnow et al.
2000
). Note that the data on
carbon evolution may show the rate of incorporation of the labeled carbon from
xenobiotics into the subsurface solid phase, but such studies alone do not give
information on the bonding of parent compounds and their metabolites on
molecular levels. The type of interaction, however, is an important factor deter-
mining both the likelihood and rate of release and the form in which the molecules
are mobilized.
The mechanism of bound residue formation is better understood today due to
the use of advanced extraction, analytic, and mainly spectroscopic techniques
(e.g., electron spin resonance, ESR; nuclear magnetic resonance, NMR; Fourier
transform infrared spectroscopy), methods that are applied without changing the
chemical nature of the residues.
Physical entrapment following intraorganic matter diffusion (IOMD) or inter-
particle diffusion in clay minerals is another potential explanation for the forma-
tion of bound residues. Diffusion out of the solid phase may account partly for
hysteresis, particularly for molecules that diffused into the organic aggregates.
Entrapping in humic polymer aggregates, suggested by Kahn (
1982
) and further
examined by Wershaw (
1986
) and Kan et al. (
2000
), is a possible explanation for
hysteresis of substances compatible with the structure of humic substances. The
rapid desorption phase is a result of an entrapped pool of readily desorbed
material, and the slow phase is controlled by an entrapped or irreversible com-
partment inside the most hydrophobic part of humic aggregates.
To calculate the release through diffusion of an entrapped residue, Barraclough
et al. (
2005
) considered the size of organic matter particles (effective radius 10
-7
to 10
-9
cm) and the effective diffusion coefficient of small organic molecules in a
sorbing medium (D & 10
-9
cm
2
/s). The time for 50 % of the material in a sphere
to diffuse out is given by
