Environmental Engineering Reference
In-Depth Information
What about the fate of VOCs in groundwater? Plants are
composed primarily of water and both living and previously
living organic compounds, much like the contaminated
groundwater where plants are used for phytoremediation.
Because even highly soluble organic contaminants have
the potential to partition to organic matter, these compounds
once inside the vascular transport system of a plant can
partition into the plant tissue itself in a manner similar to
the elements discussed above. However, the tendency for
such compounds to partition to nonpolar lipids or to polar
carbohydrate structures is low (Chiou 2002).
Newman et al. (1999a) reported the detection of TCE and
its metabolites in various compartments of plants exposed to
TCE during a 3-year highly controlled experiment in the
field. TCE and its metabolites such as dichloro- and
trichloroacetic acid were detected in all plant compartments
measured. Weathered leaf litter measured after leaf drop
the second year (fall 1996) revealed that while no TCE
was detected in the leaf litter sampled, dichloro- and
trichloroacetic acid and cis -1,2-DCE and trans -1,
2-dichloroethylene ( trans -1,2-DCE) were detected. Because
no units of measurement were provided, it is hard to com-
ment on the risk that the detection of these compounds
presents to human health and wildlife. Davis et al. (1996)
reported that most contaminants are not water soluble or
volatile enough, nor present at high enough concentrations,
to present a significant risk through high concentrations in
the atmosphere. Davis et al. (1996) reported preliminary
calculations of potential maximum TCE transfer rates to
the atmosphere, near 10 g TCE/m 2 /d.
The fate of explosive compounds in the presence of plants
has been investigated, and showed that for TNT and some of
its intermediate breakdown products, these compounds were
observed to be taken up into plants and to accumulate in the
roots, whereas RDX and HMX were found primarily in the
leaves of test plants (Groom et al. 2002) (Fig. 16.1 ).The fate
of these compounds in leaves after they fall was studied by
Yoon et al. (2006). Regulators are typically concerned about
the potential risk exposure to contaminants by exposure to
leaves that might contain contaminants taken up by the
plant.
To address these concerns, Yoon et al. (2006) added
radiolabeled 14 C-TNT, 14 C-RDX, and 14 C-HMX, to track
the fate of the compound in different parts of the plant, to
flasks that contained a solution of half-strength Hoagland
solution, TNT mixtures, and a prerooted hybrid poplar
cutting ( Populus deltoides
Fig. 16.1 Fate of TNT, RDX, and HMX in the phytoremediation of
explosives-contaminated groundwater. The TNT is predominately
degraded in the rhizosphere, and RDX and HMX are translocated to
the leaves. These compounds are degraded in the soil after leaf fall
(Modified from Yoon et al. 2006).
whereas between one-fifth and one-half of 14 C-HMX and
14 C-RDX, respectively, was found in the leaves (Yoon
et al. 2006).
Due to the detection in leaves, dried leaves were exposed
to deionized water to simulate exposure to precipitation
after leaf drop and then resampled. Very little TNT was
found in the leachate, but one-fourth to one-half of RDX
and HMX was found in the simulated leachate. Moreover,
when dried roots were exposed to deionized water, very
little of any compound was detected in the leachate
(Fig. 16.2 and 16.3 ).
P. nigra DN-34). Over
2 weeks, the removal of TNT from the solution was greater
than the removal of RDX, with little removal of HMX
(Fig. 16.2 ). After uptake, the distribution of these
compounds within the plant was depicted. Almost half of
the 14 C-TNT taken up was detected in the roots after 30 d,
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