systems and the operational costs thereof. If the ir-
material and the further degradation of reduced
rigated plants could be sold, there would be an
HMX and RDX intermediates.
added benefit.
In this study, plants were allowed to accumu-
In previous research we studied the conjuga-
late explosives through two routes of exposure:
tion of TNT transformation products in bioreme-
from contaminated soil using clean irrigation water,
diation matrices resulting from composting and
and from clean soil irrigated with contaminated
water. Kenaf (Hibiscus cannabinus, var. Tainung 2)
slurry digestion (Thorn 1997; Thorne and Leggett
1997, 1999). We demonstrated that there is a time-
was chosen as the test plant because it uses large
course of conjugation strength. Amino transfor-
quantities of water and produces a considerable
mation products are initially conjugated through
amount of biomass with a high value both as fiber
bonds that are reversible under strong hydrolytic
from the stem cortex and biodegradable sorbent
conditions. As humification continues, either the
from the pith. It was hoped that this plant could
bonds change to forms that are resistant to hydrol-
remediate enough contaminated groundwater to
ysis or additional bonds form at different sites,
be a useful replacement for GAC treatment. If the
resulting in unhydrolyzable residues. These
kenaf grew in moderately contaminated soil, it
results apply to uncontaminated plant tissues
could be used to remediate large areas with heter-
that were mixed with contaminated soil at the be-
ogeneous distributions of explosives. Further-
ginning of remediation.
more, areas of inhibited plant growth could serve
Researchers have shown that extractable TNT
as indicators of "hot spots" within OB/OD areas.
and some bound amino transformation products
These small spots could then be targeted for exca-
remain in root tissue (Palazzo and Leggett 1986,
vation and ex-situ remediation. The accumulated
Rivera et al. 1998, Larson et al. 1999). We and oth-
biomass from the irrigation and soil remediations
ers have observed that the degradation of RDX in
could either be turned under in place for humifica-
composting and slurry remediations takes more
tion or the biomass could be processed into com-
time (Griest et al. 1993, Shen et al. 1998) and
mercial products, e.g., pulping for "treeless"
shows no evidence of proceeding through a re-
paper, provided that the treatments destroyed
versible binding of transformation products
residual explosives.
(Thorne and Leggett 1999). RDX is known to ac-
cumulate in leaf and shoot tissues of plants
MATERIALS AND METHODS
grown in contaminated soils or irrigated with
contaminated water (Harvey et al. 1991, Rivera et
Experimental methods
al. 1998), and a recent study (Larson et al. 1999)
For the irrigation experiment, kenaf seedlings
has shown that the majority of products of RDX
that were germinated in peat pots were trans-
transformation are incorporated into plant com-
planted into 2-gal. plastic pots (three plants per
pounds that have molecular weights in the range
pot) containing 4 kg of clean, sandy soil. Plants
of 350 to 1000 daltons. A different time-course of
were irrigated with water contaminated with 12.4
conjugation may result when humification be-
mg/L of TNT, 12.4 mg/L of RDX, and 2.43 mg/L
gins with tissues that were contaminated with
of HMX. Irrigation rates were maximized to keep
explosives by plant uptake during phytoreme-
soil moisture close to field capacity. The total
diation projects.
quantity of contaminated water applied per pot
The objective of this project was to test the hy-
was 12 L. After two months' growth, the kenaf was
pothesis that plant-accumulated HMX, RDX,
harvested, divided into root, stem, and leaf sam-
TNT, and their transformation products would be
ples, and diced with scissors to approximately 5-
rapidly remediated following in-situ humifica-
mm-sized pieces. Some diced pieces were air-
tion when soil conditions alternate between
dried for analysis. The remaining moist tissue was
anaerobic and aerobic. At high soil moistures,
turned into its own growth soil for in-situ humifi-
oxygen transport into the tissues would be limited,
cation studies. The soil moisture was cycled three
yet available carbon and microbial activity would
times between saturated (anaerobic) and unsat-
be high. The resulting anaerobic environment
urated (aerobic) over a three-week period. No
would encourage the reductive transformation of
leaching was allowed from these pots. Samples of
HMX, RDX, and TNT. When the soil dried to below
soil and humifying tissue were collected during
saturation, the return to an aerobic environment
the aerobic phase when the contents of the pots
would support the oxidative conjugation of amino
were stirred and homogenized with a spatula. A
transformation products to the developing humic
second set of soils with turned-in contaminated
2
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