Page 241 - Partition & Adsorption of Organic Contaminants in Environmental Systems

232 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

Hinman and Klaine (1992) studied the time dependence of the uptakes of
atrazine, lindane, and chlordane from overlying water by small rooted aquatic
vascular plants (Hydrilla verticillata Royle). It was found that the levels of
more water-soluble atrazine (logK ow = 2.71) in shoots and roots of the plant
approached apparent equilibrium within 1 and 2h, while the levels of increas-
ingly less water-soluble lindane (logK ow = 3.75) and chlordane (logK ow = 5.58)
in roots and shoots reached apparent equilibrium within 24 and 144h, respec-
tively. The corresponding contaminant concentration factors with rooted
hydrilla (i.e., C pt/C w) were 9.62, 38.2, and 1061 for atrazine, lindane, and chlor-
dane. Although it is arguable whether all three contaminants in their
plant–water systems have truly come to full equilibrium, because the plant
composition was not available for calculating their theoretical C pt/C w values,
the time dependence and the order in C pt/C w as observed are consistent with
their relative uptake capacities according to their K ow values.
Consider now some likely scenarios in phytoremediation of contaminated
soils or water. Let us start with highly water soluble compounds, in which the
K pom values would be small with practically all plants. If the compounds are
fairly resistant to biodegradation (i.e., if the metabolism is slow) and if they
have low vapor pressures, there will be little driving force for the continuing
plant uptake of these compounds from external water once the in-plant con-
centrations approach saturation values (a pt 1). In this case, the approach
to saturation (or near saturation) should be relatively fast, as shown for
solutes with small K ow values in Table 8.1. Thus, for such compounds, it would
be nearly improbable to achieve effective remediation of polluted soil or water
by plantings. Here, although the plant growth would in effect dilute the plant
contaminant level, and thus create a driving force for continuing uptake, the
uptake would be fairly limited. On the other hand, if the water-soluble com-
pounds have instead high vapor pressures, such as methyl t-butyl ether
(MTBE) and TCE, the high volatilization rate of the sorbed chemicals through
plant leaves and surfaces should then exert a continuing driving force for the
removal of these contaminants from the external water phase, despite the fact
that the contaminant level in the plant may either approach saturation (a pt
1) or maintain a steady-state value. This expected consequence is supported
by the finding that a sizable quantity of MTBE is removed from a groundwa-
ter site by planted poplar trees, which exhibit a very large transpiration rate
(Hong et al., 2001). In a similar study of MTBE uptake by young poplar trees
from a hydroponic solution, Rubin and Ramaswami (2001) found that the
MTBE concentration in plant water ascends rapidly to about the same con-
centration as in external water (i.e., to approach the limit of a pt = 1).
For compounds with low water solubility (i.e., the lipid-soluble com-
pounds), the situation is more straightforward. This is because the large par-
tition coefficient with plants enables the system to maintain a high and
continuing driving force for contaminant uptake. This is true whether or not
the contaminant dissipation by either metabolism or volatilization is efficient
and whether or not the plant has a high lipid content. Naturally, the rate of