Environmental Engineering Reference
In-Depth Information
been conducted on phytoremediation of PAHs. Recent studies have shown
that plants are capable of removing not just the lower-molecular-weight
PAHs, such as anthracene, but also HMW compounds, such as chrysene and
benzo(g,h,i)pyrene. In contrast to phytoremediation of metals, which is an
extraction process, large organic compounds such as the PAHs are degraded
in the rhizosphere (root zone) of the plant. The rhizosphere constitutes the
top 3 to 6 ft of soil. Various elements of the rhizosphere appear to play a role
in the degradation. First, the soil around the roots is very different in chem-
istry and physical structure from the bulk soil. The root system of the plant
is a moist, aerobic environment, which promotes microbial activity. Second,
an exudate is released from the roots into the surrounding soil. The exudate
is composed of simple sugars, amino acids, enzymes, aliphatic and aromatic
compounds, and vitamins and has been shown to increase the dissipation
of soil-bound PAHs when applied to soil as a single treatment. Third, there
are active microbial and fungal communities associated with the root system.
These are enhanced by the root exudate in ways that are not completely
understood. Growth, and the development of biomass, is certainly one
response, but in some plant species, the exudate appears to favor selection
of microorganisms that degrade the contaminant.
7.7 Technology transfer
Technologies for bioremediation and natural attenuation of organic com-
pounds have developed dramatically over the past 15 years due to extensive
programs carried out in the United States and Europe. The possible breadth
of application of bioremediation continues to be inhibited by significant data
gaps in our ability to apply the principles of biotechnology to chemical
contamination. There is a considerable amount of field experience available,
and protocols have been developed, for the bioremediation of easily degrad-
able compounds such as petroleum hydrocarbons and chlorinated aliphatic
solvents. However, until now, the development of similar state-of-the-art
protocols for bioremediation of the more recalcitrant PAHs (particularly at
high concentrations) has continued to lag. This demonstration and its pub-
lication of improved bioremediation of these more toxic HMW PAHs through
bioaugmentation and biostimulation will enhance the potential for more
frequent and successful use of biotreatment as a viable option for PAH
cleanup.
The use of known surfactant-producing bacteria in conjunction with
vermiculite-carried bioaugmentation, enhanced nutrient delivery, and
appropriate bulking agent application provides the basis for a variety of field
applications on higher concentrations of toxic PAHs not previously thought
possible. The advances described herein allow the vast body of knowledge
and experience in bioremediation and natural attenuation to be extended to
hydrophobic organic compounds previously thought recalcitrant, such as
the polychlorinated biphenyls (PCBs) and pesticides. The accompanying
SERDP report on PCB bioremediation, for example, describes the use of the
