Page 229 - Partition & Adsorption of Organic Contaminants in Environmental Systems

220 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

We now evaluate the validity of the model against the pertinent literature
data on crop/plant contamination. The present analysis is restricted to systems
with relatively nonreactive nonionic compounds in water or soil. Ionizable
compounds are excluded from consideration because of the possibility that
their plant uptakes may involve active transport to certain plant organic
constituents.

8.4 UPTAKE BY SMALL PLANT ROOTS FROM WATER

Starting with the simplest system, consider the uptake, by the roots of barley
(Hordeum vulgare cv. Georgie), of various O-methylcarbamoyloximes and
substituted ureas from nutrient water solution, as reported by Briggs et al.
(1982). The experiments measured the root concentration factor (RCF =
14
C pt/C w) for each of these C-labeled compounds individually in replicated
laboratory systems after the 10-day-old barley plants were transferred to
nutrient solution with the test compound for 24 and 48h. The authors indi-
cated that the RCF values for the parent compounds alone, or for the parent
compounds plus their metabolites, were very similar after 24 and 48h. The 24-
to 48-h averaged RCF values of the parent compounds (with reported data
uncertainties of ±5%) and their octanol–water partition coefficients (K ow ’s) are
presented in Table 8.1.
The K ow values for many of the compounds in Table 8.1, such as the more
lipophilic benzaldehyde O-methylcarbamoyloximes, were obtained either by
indirect experimental methods or by empirical calculations. No information
on the water–organic composition of the barley roots was provided. However,
an approximate composition for barley roots (Hordeum vulgare) is given by
Trapp et al. (1990); it comprises 87.5% water and 1% lipids by weight. It is
assumed that the remainder consists mainly of carbohydrates and cellulose,
with traces of proteins and nutrients, for a total of 11.5% by weight. We assume
further that the partition coefficients of the compounds with the relatively
polar carbohydrates, cellulose, and proteins are practically the same. Since
octanol is known to mimic biological lipids closely in contaminant partition
(see Table 5.5), the lipid–water partition coefficients (K lip’s) are assumed to be
the same as the corresponding K ow’s.
On the premise of the assumptions above, the f pomK pom term in Eq. (8.2) for
barley roots can be simplified as the sum of the contributions by carbohydrates
and lipids:

(8.7)
f pom K pom = f ch K ch + f lip K lip

where the subscripts “ch” and “lip” designate carbohydrates and lipids, respec-
tively. Substituting Eq. (8.7) into Eq. 1 with the assumed barley root compo-
sition (Trapp et al., 1990) leads to