Geoscience Reference
In-Depth Information
t
1
=
2
¼ 0
:
03r
2
=
D
;
ð
5
:
17
Þ
where r is the effective radius (cm) and t
1/2
is in seconds (Helfferich
1962
).
For these entrapped contaminant ''spheres,'' the diffusion is rapid, on the order
of seconds rather than days. Kan et al. (
2000
) suggested a diffusion model for
xenobiotics with a slow desorption phase, with a half-life of years rather than
seconds, assuming that diffusion is hindered by the NOM matrix and occurs when
the dimensions of diffusing molecules approach those of the pores. Under these
conditions, hindrance from the wall becomes significant (Renkin
1954
) and the
drag factor F can be expressed as
þ
2
:
14 r
m
=
r
p
3
0
:
95 r
m
=
r
p
5
þ;
F ¼ 1
2
:
09 r
m
=
r
p
ð
5
:
18
Þ
where r
m
and r
p
are the radii of the molecule and the pore, respectively. To extend
the diffusion half-life from seconds to years would require a drag factor of around
10
-8
, in the case where no interaction occurs between the diffusing molecules and
the entrapping matrix.
Another process leading to irreversible retention involves chemical binding of
contaminant molecules to organic matter (Bollag and Loll
1983
). Fulvic and humic
acids are the compounds commonly involved in such binding. If binding on
organic matter matrix involves physical entrapment, van der Waals forces, or
charge transfer, significant release occurs only as a result of matrix-induced
degradation by microorganisms or plant enzymes. The reactions involved appear
to be the same as those responsible for humic substance formation. Phenol and
aromatic amines may bind to organic matter by oxidative coupling, while
substituted urea and triazines may not (Bollag et al.
1992
). Binding of toxic
organic molecules on an organic matter matrix can take place also during humic
substance formation by polymerization processes.
References
Abdul
AS,
Gibson
TL,
Rai
DN
(1990)
Use
of
humic
acid
solution
to
remove
organic
contaminants from hydrogeologic systems. Environ Sci Technol 24:328-333
Amirtharajah A, Raveendran P (1993) Detachment of colloids from sediments and sand grains.
Coll Surfaces A 73:211-227
Amitay-Rosen T, Cortis A, Berkowitz B (2005) Magnetic resonance imaging and quantitative
analysis of particle deposition in porous media. Environ Sci Technol 39:7208-7216
Ashton FM, Sheets TJ (1959) The relationship of soil adsorption of EPTC to oats injury in
various soil types. Weeds 7:88-90
Bailey GW, White JL (1964) Review of adsorption and desorption of organic pesticides by soil
colloids with implications concerning pesticide bioaviailability. Agri Food Chem 12:324-382
Barraclough D, Kearney T, Croxford A (2005) Bound residues: environmental solution or future
problem? Environ Poll 133:85-90
