Environmental Engineering Reference
In-Depth Information
(augmented natural attenuation) or engineered (in-place treatment), a prag-
matic solution is to focus active biotreatment on the available contaminant
fraction. Research is needed to identify the factors affecting the bioavailabil-
ity of PAHs on soil and how these factors affect treatment rates and accept-
able toxicological endpoints. Research is then needed to use the bioavailabil-
ity information to develop a technical base for enhancing natural recovery
processes involved in
in situ
biotreatment of PAH-contaminated soils. Such
research could result in providing guidelines for the assessment and predic-
tion of the bioavailability of PAHs for
in situ
biotreatment.
Beyond availability, the issue of residual contamination is still unre-
solved in the case of biological treatment of PAHs, where an understanding
of the complex interactions between hydrophobic organic contaminants and
soil is key to establishing realistic risk assessment criteria. Cleanup criteria
based only on the chemical properties of the contaminant lead to an over-
prediction of the risks associated with the contamination. However, leaving
an immobile, bound residue in the soil after cleanup needs to be justified
scientifically. On the other hand, if residuals are released, the rate should be
sufficiently slow to allow consumption by the microbial community. Thus,
an alternative environmental endpoint (cleanup level) may be appropriate
rather than basing decisions solely on total concentration of contaminant in
the soil or sediment. Thermal programmed desorption (TPD) is one new
technology available to study the first of these issues, the mechanism of
PAH binding to the soil, the process that affects their bioavailability in the
environment.
Phytoremediation (defined as the use of green plants to remove, contain,
or decrease toxicity) is a cost-effective technology with many advantages
over highly engineered solutions, including public acceptance. Only in the
last decade has research 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 com-
pounds, such as chrysene and benzo(g,h,i)pyrene. In contrast to phytoreme-
diation 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 comprises 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 chemistry 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 com-
munities associated with the root system. These are enhanced by the root
exudate in ways that are not completely understood. Growth, and the devel-
opment of biomass, is certainly one response, but in some plant species the
