Geoscience Reference
In-Depth Information
We see from Fig.
8.50
that both sorbents exhibit adsorption-desorption hysteresis.
Compared to the humic acid, the kerogen exhibits a greater degree of hysteresis, as
indicated by a higher hysteresis index.
The quality of desorption water is another factor affecting the release of organic
contaminants from adsorbed surfaces. For example, Barriuso et al. (
1992
) dem-
onstrated that DOM in desorbing aqueous solutions increases the release of atra-
zine and carbetamide adsorbed on soils. Desorption of diquat and paraquat
herbicides was also affected significantly by the salt concentration of the aqueous
extract (Kookana and Aylmore
1993
). Both Ca
2+
and Na
+
cations compete for
sorption sites with these herbicides, but Na
+
is not as influential as Ca
2+
.
Desorption of diquat was higher than that of paraquat for all salt concentrations.
8.6 Bound Residues
Formation of bound residues is related mainly to the fate of crop protection
chemicals and other toxic waste organics in the biologically active soil surface,
during and after chemical redistribution in the subsurface over long periods of
time.
Bound residues are those chemicals retained in the subsurface matrix in the
form of the parent organic contaminant or its metabolites; these residues remain
after subsequent extractions, during which the nature of the compound or of the
matrix is not altered by the extraction procedure. An example of sequential
extraction of pesticide residues from soil, to define the nonextractable contaminant
or the bound residue, is described in the laboratory experiment protocols presented
by Mordaunt et al. (
2005
). Six pesticides (atrazine, dicamba, isoproturon, lindane,
paraquat, and trifluralin) with various properties were added to an agricultural soil
from Terrington, United Kingdom (17 % clay, 2-2.5 % OM, pH 8), and kept
under controlled environmental conditions for 90 days. The soil was sampled six
times, submitted to sequential solvent extraction procedure, and analyzed for
pesticide content during the incubation period. The following steps for sequential
extraction were performed:
1. Extraction in 0.01 M CaCl
2
, and shake for 24 h.
2. Acetonitrile: water (9:1) shake extraction for 24 h.
3. Methanol shake extraction for 24 h.
4. Dichloromethane shake extraction for 24 h.
5. Added
14
C-activity combusted to
14
CO
2
.
Step 1 simulates the readily available soil fraction, steps 2-4 indicate poten-
tially available soil fractions, and step 5 yields the unextracted residue and com-
pletes the mass balance. Note that the solvent used becomes increasingly nonpolar
during the extraction sequence. Summary data for the six studied compounds are
presented in Fig.
8.51
.
