Page 242 - Partition & Adsorption of Organic Contaminants in Environmental Systems

TIME DEPENDENCE OF CONTAMINANTS IN PLANTS 233

removal by plants should increase with the plant lipid content and water tran-
spiration rate. As shown earlier, even for plants with a lipid content of 0.1%
by weight, the potential toward accumulation by the plant lipid of a relatively
water-insoluble compound (e.g., PCBs) is tremendously large.
Dissipation of a contaminant in the plant, which leads to a reduction in a pt,
will in principle increase the driving force for plant uptake in situations where
the contaminant level in plant could otherwise approach saturation (i.e., where
the a pt value is close to 1). If the dissipation loss is small, there will be a sharper
dependence of the plant contaminant level with time. If the dissipation loss is
high, an apparent steady-state trend may occur if the rate of contaminant
uptake from external water happens to offset the rate of contaminant dissi-
pation. The uptake of hexachlorobenzene, a relatively chemically stable,
poorly volatile, and lipid-soluble compound, from a hydroponic solution by
young rye grass shows a continuous rise of its level in plant over a long period
of time (Li et al., 2001). By contrast, the uptake of tetrachloroethylene, a
relatively volatile and far more water-soluble compound, by the same grass
quickly approaches a steady-state level following a short period of exposure
(Li et al., 2001). These results are consistent with the model expectation.
Based on the available plant-uptake data, the partition-limited model
appears to give a satisfactory account of the passive transport of various con-
taminants from soil and water into thus far a small number of plants and crops.
The plant uptake of nonionic contaminants through active transport does not
appear to be significant for the systems examined. According to the model
analysis, it may be generally concluded that for plants with a high water
content, highly water-soluble contaminants occur mainly in the plant–water
phase, as their uptakes by lipids and other plant matters are either small or
insignificant. By contrast, the plant uptake of highly water-insoluble contam-
inants is predominated by partition into the plant–lipid phase, even though the
lipid content may be very low. These conclusions, which appear to make good
sense, serve as a useful guide to the problem of crop contamination and to the
proper selection of plants in relation to contaminant type for intended biore-
mediation of contaminated soils and groundwater. More extensive experi-
mental data are yet to be furnished to substantiate the range, and to define
the limits, of the model applicability.