Page 231 - Partition & Adsorption of Organic Contaminants in Environmental Systems

222 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

partition contribution by carbohydrates becomes unimportant relative to that
by the lipids for compounds with logK ow > 3.0.
The a pt values calculated for the compounds are listed in the last column
of Table 8.1. They are generally consistent with the overall hydrophilic-to-
lipophilic trend of the solutes in that the water-soluble compounds have a pt
values close to 1 and that the a pt values for lipophilic compounds (high K ow
values) are less than 1. This relatively smooth transition is reflective of the
passive transport of contaminants into the different plant-root organic matri-
ces and of the spatial uniformity of the contaminant concentration in external
water. As seen for both O-methylcarbamoyloximes and substituted ureas with
K ow £ 500 or so, the a pt values are, within the uncertainties from all sources,
essentially 1, suggesting that the passive uptake of these relatively water-
soluble compounds by barley roots comes close to equilibrium within 24 to
48h. For compounds with increased lipophilicity (i.e., those with K ow > 1000),
the a pt values are clearly below 1. With the assumed lipid content in barley
roots (1%), the calculation shows that for compounds with K ow £ 10, the con-
taminant level in root water accounts for more than 85% of the total root
uptake; for compounds with K ow = 100, the uptake by the root water and the
root lipid each contributes about 50% to the total uptake; for compounds with
K ow > 1000, the total root uptake is predominated by the lipid uptake.
The a pt values calculated depend sensitively on the assumed lipid content
and the accuracy of the K ow values for compounds having K ow > 100; this sen-
sitivity increases proportionately with increasing K ow of the compound. As for
the K ow, it is not uncommon for the reported value to be in error by a factor
of 2 to 3, especially for compounds with large K ow values (Leo et al., 1971).
Since the variation of the a pt values in Table 8.1 is supportive of passive trans-
port, one may verify the relative K ow values of the compounds in terms of their
measured RCF (or C pt/C w) values. Here, for example, the RCF values for 4-
bromophenylurea, 3,4-dichlorophenylurea, and 4-phenoxyphenylurea are
3.17, 5.86, and 7.08, respectively; the corresponding measured K ow values are
95, 436, and 631. Thus, based on the measured RCF values, the K ow for 4-
bromophenylurea seems to be too low, by nearly a factor of 2, relative to the
values for the other two compounds. The relatively high a pt = 1.63 value for
4-bromophenylurea, compared to the values for the other two compounds
(where a pt 1), may be an artifact of the calculation rather than a manifesta-
tion of active uptake. Similarly, the a pt value for 3-(3,4-dichlorophenoxy)ben-
zaldehyde O-methylcarbamoyloxime could be somewhat too low, as the
estimated K ow value seems somewhat too high.
The less-than-1 a pt values (i.e., the very small RCFs) for highly polar aldoxy-
carb (0.74) and 3-methylphenylurea (0.82) in Table 8.1 are noted with inter-
est, since their uptake by root lipids and carbohydrates would be small relative
to that by root water. Briggs et al. (1982) attributed the small RCFs of highly
polar chemicals to difficulties in passing through the lipid membranes in the
root, thus resulting in selective rejection of the chemicals at the membrane
barriers. The low a pt values could also result from the high ionic strength in