Page 225 - Partition & Adsorption of Organic Contaminants in Environmental Systems

216 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

Most current models for plant uptake of contaminants from soil, water, or
air are formulated on a differential mass-balance basis in terms of the rates of
contaminant interface transfer, plant growth and transpiration, and contami-
nant metabolism, along with some estimated transfer coefficients (Riederer,
1990; Trapp et al., 1990; Paterson et al., 1994; Trapp and Matthies, 1995; Tam
et al., 1996). Although these models are intended primarily for delineating the
rates of contaminant uptake by plants (or their specific parts) with time from
given external source(s), the model calculations are very sensitive to the accu-
racy of assumed contaminant interface-transfer rates and coefficients. Alter-
natively, equilibrium models have been utilized in some studies to assess
contaminant levels in plants (or their parts) after their exposure to chemicals
in water over a certain period of time (Briggs et al., 1982; Trapp, 1995).
However, as shown later, the actual state of a contaminant in plants may or
may not be at equilibrium with the external source.
A quasiequilibrium partition model has recently been developed by Chiou
et al. (2001) to account for the passive plant uptake of contaminants from their
external sources in soil or water. The model takes explicit account of the plant
contaminant level in relation to the source level and plant composition. More-
over, the model contains both equilibrium and kinetic features and sets the
upper (equilibrium) limit for the level of a contaminant in a plant with respect
to the external-source level, against which the actual approach to equilibrium
of the contaminant in the plant at the time of analysis can then be estimated.
Although in the initial model testing by Chiou et al. (2001) the partition co-
efficients of contaminants with certain plant components have had to be esti-
mated, the observed consistency of the plant-uptake data with the conceived
model parameters is stimulating to warrant further investigation. The essen-
tial features of the model are presented below.

8.3 THEORETICAL CONSIDERATIONS

Consider first the simpler case of a partition-limited model for the plant
uptake of nonionic contaminants from a soil-free nutrient solution by passive
transport through the plant vascular system. Here water is both the solvent
for the contaminant and the medium that carries it to plant roots (and to other
water-contacted surfaces) and eventually to other parts of the plant via
the plant vascular system. The overall plant uptake process is driven by the
external-water concentration and is considered to consist of a series of parti-
tion uptakes, with the understanding that the contaminant concentrations
within the plant may or may not come to full equilibrium with the external
water solution. On the other hand, for any given volume element inside the
plant, local equilibrium is assumed for a contaminant between sap water and
the various organic constituents within that volume element. With these con-
siderations, the concentration of a contaminant either in the whole plant or in
a specific part of the plant (C pt ), expressed as the mass of contaminant per unit