Page 228 - Partition & Adsorption of Organic Contaminants in Environmental Systems

THEORETICAL CONSIDERATIONS 219

latter approach is especially merited for analysis of earlier published studies
where the K d data are usually unavailable.
With the foregoing considerations, the plant uptake of a relatively nonpo-
lar contaminant from a soil can then be formulated to a good approximation
by substituting C som/K som for C w in Eq. (8.1):

C pt = a pt ( C som K som )( f pom K pom + f pw ) (8.6)

Equation (8.6) is an alternative form of Eq. (8.1) that accounts for the effect
of soil sorption on the contaminant concentration in soil interstitial water. The
use of C som/K som in Eq. (8.6) circumvents the need for the experimentally cum-
bersome determination of C w in soil interstitial water. The value of a pt in Eq.
(8.6) can be determined as before with the additional inputs of C som and K som.
The water content and the organic composition of plants may vary consid-
erably either between plant types or between the different parts of a plant.
The partition limits for a given contaminant inside a plant from the water
phase to different plant parts would therefore vary with the overall and local
plant composition. Most root and leaf crops are composed of large amounts
of water and polar organic constituents, such as carbohydrates, cellulose, and
proteins, and lesser amounts of lipids. From the partition standpoint [Eq.
(8.2)], the most striking differences in plant contamination level would prob-
ably occur for relatively nonpolar, lipid-soluble contaminants between plants
that differ radically in their lipid contents. Such differences are anticipated
because the partition coefficients of these contaminants are much higher
with relatively nonpolar lipids than with polar organic matter (Chiou, 1985;
Rutherford et al., 1992). For the more water-soluble solutes, the partition
capacities from water to either nonpolar lipids or relatively polar carbohy-
drates and proteins should be small to moderate.
Because the value of a pt characterizes the extent to equilibrium of a con-
taminant between external water and the plant (or a part of the plant) at the
time of the analysis, it must depend in part on the contaminant partition capac-
ity of the plant organic matter. For a plant with a given lipid content, a polar
contaminant with a low K lip(lipid-water) value would tend to exhibit a higher
a pt than a relatively nonpolar contaminant with a significantly higher K lip value,
because the attainment of partition equilibrium for the latter requires a much
greater volume of water transport within the plant. The magnitude of a pt is
therefore expected to be a function of the contaminant partition coefficient,
the plant water-organic composition, and the plant water-transport rate.
Finally, the contaminant uptake by plants through direct diffusion to the outer
layers of plant roots should contribute to the rate of local and overall plant
uptake and would therefore affect the local and/or overall a pt value before the
contaminant in plant and external water reaches equilibrium. The significance
of contaminant uptake by diffusion relative to that by transport of external
water into plant’s vascular system depends on the plant system but should not
in principle make a pt > 1.