Page 240 - Partition & Adsorption of Organic Contaminants in Environmental Systems

TIME DEPENDENCE OF CONTAMINANTS IN PLANTS 231

reported range for PCB partition equilibrium between sediments and plants.
To ascertain whether the C lip and C som values are truly at equilibrium, the
observed C lip–C som correlation may be evaluated against the model calculation
using Eq. (8.13), as presented below.
To begin with, one recalls from Table 7.4 that the ratios of K ow to soil K som
(i.e., K om) for four PCB congeners (with 1 to 3 chlorines) are relatively
comparable at 16.5 ± 2.49. Using these values as the approximate ratios for
all PCBs, the estimated ratios of K ow to sediment K som would then be about
8.3 ± 1.2, because the sediment organic matter is about twice as efficient a
partition medium as the soil organic matter for low-polarity solutes (see
Chapter 7, section 7.3.2). Thus, with the correlation of Vanier et al. (2001) (i.e.,
C lip = 3.74 C som for all samples), the calculated average a pt values would be
0.45 ± 0.07; that is, the PCB levels in plant shoots are about half the equi-
librium capacities with respect to the PCB levels in in-situ bed sediments. For
the entire set of data, in which C lip is 2.6 to 4.9 times C som, the a pt falls into the
range 0.30 to 0.59. Thus, although the C lip values are reasonably close to their
equilibrium values in some samples, the averaged values for all samples are
only about one-half the equilibrium values. Despite these variations, practi-
cally all the PCB levels observed in shoots relative to their levels in bed sed-
iments comply with the upper limits imposed by the partition-limited model,
Eq. (8.6) or (8.13). In the calculation above, it is assumed that the bed-
sediment samples from different geographic locations have comparable
organic-matter compositions.

8.9 TIME DEPENDENCE OF CONTAMINANTS IN PLANTS

A subject of considerable interest to the plant uptake of a contaminant is the
time dependence of the contaminant level in a specific part of the plant. In
the practice of bioremediation by planting, this would determine the efficiency
of a plant in taking up a target contaminant with time from an external source.
Although not well established at this time, the preceding data analysis leads
to the expectation that the change with time of the in-plant contaminant level
should be a function of the contaminant partition capacity and the specific
plant physiology (e.g., plant growth and water uptake rate). On a given plant
species (or a specific part of it), one expects a major difference between a
water-soluble compound and a lipid-soluble compound. For the water-soluble
compound, not only will the water-to-plant concentration factor be low under
any conditions, but the in-plant level should also approach an apparent steady-
state value in a shorter time than is the case for a lipid-soluble compound.
Thus, a plot of the in-plant contaminant level with time should display a small
time-dependent region for a highly water-soluble contaminant and a pro-
nounced time-dependent region for a lipid-soluble contaminant. This differ-
ence should especially be prominent with small plants, where the transport
path of contaminants with water inside the plants is relatively short.