Remediation of PAH-Contaminated Soils and Sediments: A ...

The addition of inorganic nutrients compensates for limiting nutrients values in
the waste material. The optimum C:N:P ratio for degrading hazardous wastes is
reportedly between 100:10:1 (Straube et al., 2003) and 300:10:1 (Eweis et al.,
1998). Adding too much of any nutrient can greatly increase costs. Mixing,
associated with tillage, breaks apart soil aggregates, exposes PAHs once
entrapped within the aggregates to biotic and abiotic degradation and
homogenizes PAH levels in the soil. During landfarming, it is possible to
enhance the bioavailability and degradation rate of the rapidly desorbable
fraction in large pores, but not the slowly desorbable fractions associated with
organic matter and smaller pores (Harmsen, 2004). Microorganisms cannot enter
into pores smaller than their own size, so PAHs strongly sorbed to organic matter
are slowly degraded. The biodegradation of the slowly degradable fractions is
thus controlled by diffusion (Harmsen et al., 2007).
Reducing PAHs to acceptable levels through landfarming can be a lengthy
process and often does not result in levels low enough to meet EPA standards.
In one study in Holland, rapid degradation of PAH compounds (550 mg kg -1 ) in
dredge sediment occurred within the first year, followed by slow degradation for
the next seven years. After 15 years, PAH levels (22 mg kg -1 ) were similar to the
background soils in the same area (Harmsen et al., 2007).
Phytoremediation
Phytoremediation is an on-site remediation strategy that uses plants to reclaim
contaminated areas mainly through increasing microbial activity in the
rhizosphere while breaking down organic compounds in contaminated soils by
metabolic processes (Salt et al., 1995; Barter, 1999; Liste and Alexander, 2000;
Binet et al., 2000; Dzantor and Beauchamp, 2002). The rhizosphere is the small
volume of soil immediately surrounding plant roots which has much higher
concentrations of root exudates, CO2 pressures, and microbial activity (20x) than
surrounding bulk soil (Rovira and Davey, 1974; Hutchinson et al., 2003). Root
morphology of various plant species impact phytoremediation success. Some
important root morphological properties that affect the success of
phytoremediation include: length, surface area, mass, depth of penetration,
quantity and composition of dead roots and exudates, root hairs, and bacterial
and fungal associations (Hutchinson et al., 2003). Selecting species with varying
root properties can be beneficial during phytoremediation efforts.
Plants are thought to enhance PAH degradation mainly through mechanisms
such as mobilization into the rhizosphere (thus increasing bioavailability) and
enhancing bacterial populations in the rhizosphere (Liste and Alexander, 2000;
Binet et al., 2000). Plant roots provide an easily degradable source of carbon to
the soil (both through root turnover and exudates) which encourages microbial
activity in the rhizosphere and promotes biodegradation of organic contaminants
(Olson et al., 2003; Yu et al., 2006), but can also decrease PAH bioavailability
through sorption (Hutchinson et al., 2003). Root contributions of carbon to the
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