Page 226 - Partition & Adsorption of Organic Contaminants in Environmental Systems

THEORETICAL CONSIDERATIONS 217

wet mass of the plant, can be equated with the contaminant concentration in
external water (C w) at the time of sample analyses:

w [
C pt = a pt C f pom K pom + f pw ] (8.1)
in which f pom + f pw = 1 and

K pom = Â i i
,
,
f pom f pom K pom i = 12 3, ..., n (8.2)
In Eq. (8.1), K pom is the contaminant partition coefficient between plant
organic matter and water, f pom is the total weight fraction of the organic matter
in the plant, and f pw is the weight fraction of water in the plant, either for the
whole plant or for a specific part of it. In Eq. (8.2), the f pomK pom term is
expressed as the sum of contributions from all plant organic components
i
according to their specific partition coefficients (K pom) and weight fractions
i
(f pom). The term a pt (£1), called the quasiequilibrium factor, expresses the
extent of approach to equilibrium of any absorbed contaminant in the plant
(or in a part of it) with respect to the same contaminant in the external water
phase. In this model it may be viewed as the ratio of the respective concen-
trations in plant water and external water. Thus, a pt = 1 denotes the attainment
of equilibrium.
If passive transport is the dominant uptake process, a pt should not exceed
1, except for highly unusual situations; if the uptake involves an active process,
a pt may however exceed 1. In principle, with passive transport, if the concen-
trations in whole plant and external water are at equilibrium, all parts of the
plant must be at equilibrium. However, when equilibrium is not attained with
the whole plant, the a pt value may vary with the local composition of the plant
and its proximity to the contaminant source. Further rationale for variations
in a pt will be presented later. The a pt value for a contaminant with a plant (or
a selected part of it) is thus determined with inputs of C pt and C w together with
the overall (or local) plant compositions and the respective partition coeffi-
cients. Once the value of a pt is known, it may then be used to predict C pt from
C w and associated parameters.
We now extend the model formulation to the more typical case of plants
in contaminated soils. For plant growth, the water content in soil must be well
above the water content at the plant’s wilting point. This means that with
plants in soil, the soil interstitial (pore) space contains bulklike water with dis-
solved nutrients and contaminants that are available for plant-root uptake.
Therefore, as before, water is the transfer medium. The approach to model the
contaminant uptake from soil is in essence to relate the contaminant concen-
tration in plants to the effective concentration in soil interstitial (or pore)
water.
The concentration of a contaminant in soil interstitial water (C w ) can in
principle be determined experimentally for a given loading of the contami-
nant in a soil (C s ), where C w is related to C s at equilibrium as