Page 234 - Partition & Adsorption of Organic Contaminants in Environmental Systems

UPTAKE BY PLANT SEEDLINGS FROM SOIL 225

With the relatively high K ow values of the parent compounds in Table 8.2 and
the assumed lipid content, the contributions to total barley uptake by plant
water and cellulose would be quite small or negligible relative to that by lipids,
as reasoned earlier. Although the levels of metabolites in soil and plants are
a useful indicator of the contaminant fate, no calculations could be performed
for the metabolites as their chemical identities are not known. The necessary
K som values of the parent compounds in Table 8.2 are taken from the litera-
ture to complete the calculation of the a pt values.
The a pt values calculated for the compounds, except for 1,2,4-
trichlorobenzene,are quite consistent with the expected countertrend between
a pt and K ow, despite the fact that the calculated a pt values for lipophilic com-
pounds depend sensitively on the accuracy of the K ow and K som values. The
noted results on a pt are consistent with the model approach of substituting
C som/K som for C w in soil interstitial water. For 1,2,4-trichlorobenzene in soil, the
system was recognized to be unstable because of its high volatility (Trapp et
al., 1990); the total recovery of this compound and its metabolites from soil
and plants was only 70%, whereas the recoveries for all other compounds
exceeded 96%. Based on the model calculations, the uptake of atrazine by
plant water and carbohydrates constitutes about 20% of the total, with the rest
of the atrazine being taken up by lipids. The total uptakes of other less water-
soluble contaminants are exclusively by the small amount of lipids, with the
different a pt values reflecting the relative efficiencies of the compounds inside
the plants for approaching equilibrium with external soil water. One notes
with interest that atrazine, with a moderate logK ow = 2.71, gives rise to a pt = 1
despite the fact that the amount of metabolites in the plant is more than that
of the parent species. This suggests that the metabolic process or formation of
metabolites in plants does not seem to retard the plant passive uptake of the
parent compound.
Although the a pt value of a contaminant is partly a function of the plant
water uptake and transport, it could also be affected by other mechanisms.
Consider here, for example, the a pt value (about 0.1) for DDT (with K lip K ow
6
= 2.3 ¥ 10 ) on barley seedlings containing about 1% lipids. Since the plant
is considered to be about 90% water, the total plant mass is about the same
as the plant–water mass. If all the DDT uptake by barley seedlings were to
come only from absorption of the external soil water through the plant vas-
cular system (i.e., as a consequence of the plant transpiration), the needed
transport mass of water for DDT at a pt = 0.1 would exceed 2000 times the
plant mass. Since this amount seems unreasonably high in a short-term exper-
iment, the DDT uptake is more likely also facilitated by mass diffusion from
interstitial water into or across the root surfaces of the plant. Moreover,
had such a high water transport mass been involved, the a pt values for
contaminants with much lower K ow values (e.g., dieldrin and tetrachloroben-
zene) would have been much closer to 1 than observed. This is because the
transport masses required to saturate the lipid phase are much less for these
contaminants.