Remediation Technologies - Environmental Investigation and Remediation - page 334

7.5 PHYTOREMEDIATION
Phytoremediation refers to contaminant removal from the soil and groundwater through one or more
of several plant-mediated processes. Transpiration is a major phytoremediation process. Transpiration
involves the transfer of soluble groundwater contaminants through the plant's root, stem, and leaf
systems to the atmosphere. Additional documented phytoremediation mechanisms include stimula-
tion of microbial activity and biological degradation in the root zone through enzymatic action,
incorporation of contaminants into the plant material, and transformation of contaminants into less
toxic forms through plant metabolism. Obvious limitations of phytoremediation include depth to
groundwater, growing season, and soil conditions.

7.5.1 B ENCH -S CALE T ESTS
Imperial Carolina hybrid poplar tree cuttings were rooted hydroponically in a series of experiments by
Aitchison et al. (2000). Cuttings were tested in individual l asks containing an aqueous solution of
20,000

g/L 1,4-dioxane. Several variables were evaluated in this study, including plant uptake, tran-
spiration, incorporation into the stem material, sorption to roots, and degradation by microorganisms
associated with the root zone. The researchers used a carbon isotope ( 14 C) to label 1,4-dioxane and
assess its distribution among the plant roots, stems, and leaves. Between 30% and 79% (average 54%)
of the 1,4-dioxane was removed from the solution after eight days. However, the fact that only 10% was
removed from the solution in experiments in which the leaves had been detached from the stem suggests
that transpiration was an important process. Eight percent removal occurred in the control l ask that did
not contain a poplar cutting, coni rming that the 1,4-dioxane was not degrading or volatilizing from the
l asks by some other mechanism. The data indicate that transpiration was the dominant removal mecha-
nism and that volatilization of the 1,4-dioxane from the plant's leaves accounted for 77% of the 1,4-di-
oxane removed by the plant. Degradation within the root zone was considered to be minimal.
Researchers at the University of Iowa experimented with adding Amycolata sp . CB1190 to
planted and unplanted soil (Kelley et al., 2001). CB1190 has been demonstrated to effectively
degrade 1,4-dioxane in water (Parales et al., 1994). The effectiveness of bioaugmentation of poplar
( Populus sp .) root zones for the destruction of 1,4-dioxane was evaluated by using a known 1,4-
dioxane-degrading bacteria to enhance phytoremediation. Soil and basal salt medium (BSM) micro-
cosms were prepared and spiked with 100,000

μ

g/L of 1,4-dioxane prior to the introduction of
CB1190 bacteria, which were grown on 1,000,000

μ

g/L of THF. Several controls were also used,
including one in an aqueous solution with CB1190 and no soil, to demonstrate the viability of the
bacteria to degrade 1,4-dioxane. Additional controls demonstrated that neither sterile conditions nor
the native soil bacteria were effective in 1,4-dioxane degradation. Complete removal of the 1,4-
dioxane occurred in all CB1190-amended microcosms within 30 days in the BSM and within
45 days in a soil amended with poplar root extract.
Adding the THF apparently led to enhanced enzyme creation. Poplar root extract and 1-butanol
stimulated additional growth of CB1190. Planting hybrid poplar trees in 1,4-dioxane-contaminated
soils caused some of the dioxane to be evapotranspired through the trees. Adding CB1190 to planted
soils was less effective because evapotranspiration removed the dioxane, making it less available for
cometabolic biodegradation. 1,4-Dioxane was not directly degraded by poplar root exudates; the root
exudates only enhanced the growth of CB1190. Only THF produced a dioxane-specii c activity.
Unplanted soils amended with CB1190 at 10 7 cells/g of soil completely destroyed ~25%

μ

5% of
the 14 C-labeled 1,4-dioxane added to the soil. CB1190 can survive in the soil substrate and compete
with indigenous soil and rhizosphere microl ora. CB1190 could colonize the poplar rhizosphere,
which suggests that bioaugmenting poplar roots with CB1190 might enhance the remediation
process. However, cells grow slowly and the cell yield is relatively low. To bioaugment 1 acre of
contaminated soil over a 1-foot thickness at 10 mg of dry cell mass/kg of soil (i.e., ~10 7 cells/g),
~15,000 gallons of cell suspension would be needed (Kelley et al., 2001).

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