Environmental Engineering Reference
In-Depth Information
Fig. 16.2
The differential removal of TNT from solution relative to
RDX and HMX after exposure to poplar cuttings in the laboratory.
Much of the TNT removed entered the roots where it stayed, and the
RDX and HMX taken up was translocated to the leaves (Modified from
Yoon et al. 2006).
16.1.4 Plant Detoxification Reactions
The products of Phase I reactions often further react with
substances present in plant cells to form larger molecules
which become essentially nonextractable. Conjugation of
one chemical with another often results in the decrease of
the toxic effect of the chemical. This process differs from
bioaccumulation in that the parent compound taken up is
changed into a less harmful form, and the process is regarded
as beneficial. In plants, this occurs after the toxicant reacts
with water-soluble cell components such as glutathione,
amino acids, and sugars. This conversion renders the toxi-
cant more water soluble and, therefore, reduces toxicity.
Rather than be excreted in a way similar to that of animals,
however, plants tend to store the compound, possibly in
vacuoles.
Once a xenobiotic is taken up into a plant, the process of
detoxification can occur. As was detailed in Chap. 11, the
initial step is usually interaction with cytochrome P-450
monooxygenases or peroxidases (POX). After this oxida-
tion, conjugation reactions occur where various sugars (glu-
cose) or amino acids interact with the activated xenobiotic to
glycoside compounds; these reactions are mediated by
glycosyltransferases (Schroder and Collins 2002). The result
is an inactivated xenobiotic of a glycoside. All these pro-
cesses (Phase I and II) act to protect the plant by removing
the xenobiotic as quickly as possible by increasing polarity
and making it innocuous after storage in vacuoles or in other
organic matter in the plant.
Fig. 16.3
HMX and RDX were preferentially taken up by plants
relative to TNT in this laboratory experiment (Modified from Yoon
et al. 2006).
As detoxification reactions occur at a site over time, it is
possible that the plants should be evaluated in terms of
becoming a hazardous waste. To meet the definition of a
hazardous waste as defined under 40 CFR Part 261.24,
the medium in question, such as a soil sample or biomass
sample, must meet guidelines established for toxicity
characteristics. One of these is the Toxicity Characteristics
Leaching Procedure (TCLP). In this test, environmental
samples are evaluated in the laboratory for the potential
for contaminant mobilization after exposure to leaching
solutions of low pH. However, because plants tend to store
detoxified contaminants in the form of nonextractable
compounds, the potential for plant materials to fail a TCLP
is remote. Such issues of plant toxicity may be encountered
at the end of the life cycle of phytoremediation projects, and
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