Page 232 - Partition & Adsorption of Organic Contaminants in Environmental Systems

UPTAKE BY PLANT SEEDLINGS FROM SOIL 223

root water that increases the activity (and thus decreases the solubility) of
neutral compounds (Trapp, 2000). However, the expected high a pt value (1.02)
for equally polar and soluble oxamyl in Table 8.1 suggests that an effect other
than membrane barrier and ionic strength may be relevant. Overall, the results
indicate that the concentrations of compounds with K ow £ 1000 in barley roots
are reasonably close to equilibrium with external water after 24 to 48h; for
compounds with K ow > 1000, accurate K lip values are required to determine the
approach to equilibrium.

8.5 UPTAKE BY PLANT SEEDLINGS FROM SOIL

Although there are numerous reports in the literature on contaminant up-
take by plants or crops from soils, only a few provide the corresponding soil
contaminant levels; even in studies where the levels in soil are reported, the
requisite data on the SOM (or K d) values are often not available. In other
cases, the incorporation of contaminants into soils has not reached a stable
condition, because either of insufficient time of incorporation or of instability
of the compounds (e.g., large dissipation by vaporization). The following
model calculations are performed for those systems that are considered to be
relatively stable and where the data on SOM contents are reported.
Trapp et al. (1990) measured the concentrations of a herbicide (atrazine)
and other chlorinated hydrocarbons in barley seedlings (Hordeum vulgare)
germinated from a contaminated soil. The crop uptake was separately
14
investigated for each of the C-labeled contaminants in a closed aerated lab-
oratory system. After the barley seedlings had been in contact with the soil
for 1 week, plant and soil samples were taken for analysis of the parent com-
pound and its metabolites in both soil and plants; however, the metabolites
were not identified. The soil contained 2.06% organic carbon, or approxi-
mately 3.5% in SOM, and was maintained at 20% water during the experi-
ment. The measured concentrations of the parent compounds and their
metabolites in both soil and plants (roots + shoots) are given in Table 8.2. For
our model calculations, the concentrations in soil have also been normalized
to the SOM.
For whole barley seedlings, the authors assumed the composition to be
87.5% water and 1% lipids. As before, we further assume that the remaining
11.5% consists essentially of carbohydrates and cellulose, on which the con-
taminants exhibit the same partition coefficients. With these assumptions
together with Eq. (8.5), Eq. (8.6) may then be expressed as

C pt = a pt C som ( f pw + f K + f K ) K som (8.10)
ch
ch
lip
lip
or
.
a pt = K som C ( pt C som ) (0 875 0 115. + . K + 0 01 K ow ) (8.11)
ch