Ex situ on-site bioremediation using a bioreactor—
polycyclic aromatic hydrocarbonsA bioreactor is a silo containing a slurry of contaminated soil mixed with water.
The slurry is aerated and enriched with nutrients and microorganisms, as
necessary, to control and optimize decomposition. Bioreactors are usually
small-volume closed systems that allow collection and treatment of volatile
components. Their small volume usually limits their use to batches of soil treated
individually, such that on large sites bioremediation will be slow and expensive.
Bioreactors are particulary useful for the treatment of contaminant hot spots
at a site, deployed alongside other bioremediation techniques.
In the 1980s the US Environmental Protection Agency (EPA) embarked on a
programme known as Superfund, to clean up abandoned hazardous waste sites.
The Superfund site at Burlington Northern (Minnesota) has a historic burden of
creosote contamination. Creosote is principally a mixture of polycyclic aromatic
hydrocarbons (PAHs). PAHs are a group of compounds based on fused benzene
rings resulting in the formation of chains and clusters. The US EPA has listed
16 PAHs as ‘priority pollutants’ (Fig. 4.32) on account of their toxicity, typically
as carcinogens and/or mutagens. PAHs exhibit a wide range of physical and chem-
ical properties, governed principally by the number of fused benzene rings. Naph-
thalene, for example, the smallest PAH, comprises only two benzene rings (Fig.
4.32), has a molecular weight of 128, an aqueous solubility of 32 mg l-^1 and a log
Kowvalue of 3.37 (Box 4.14). By contrast, benzo[a]pyrene is composed of five
benzene rings (Fig. 4.32), has a molecular weight of 252, an aqueous solubility
of 0.0006 mg l-^1 and a log Kowvalue of 6.04 (Box 4.14). The heavier PAHs with
more than four rings are a problem for bioremediation, being relatively insoluble
and strongly bonded molecules that are difficult to degrade.
Biodegradation of PAHs is analogous to the biodegradation of benzene (Fig.
4.33). Initial ring oxidation yields 1,2-dihydroxybenzene (commonly known as
catechol). The benzene ring is then broken (cleaved). Ring-cleavage occurs either
between the –OH groups or adjacent to one of them, known as ortho- and meta-
cleavage, respectively (Fig. 4.33). Ring cleavage occurs at these positions because
–OH groups are involved as reaction sites. Furthermore, because the process is
enzymatically mediated the presence of adjacent –OH groups enables recognition
of the molecule by the enzymes responsible for the degradation. Ring cleavage
yields straight-chain products, namely cis,cis-muconic acid and 2-hydroxymuconic
semialdehyde (Fig. 4.33). These products are further degraded to yield simple
molecules, such as pyruvate, citrate and acetaldehyde, used in the tricarboxylic
acid (TCA) cycle through which the microbes derive energy.
In the case of naphthalene (two-ringed PAH) initial ring oxidation yields 1,2-
dihydroxynaphthalene (Fig. 4.34). Ring cleavage then occurs, followed by
removal of side-chains to yield salicylic aldehyde. Salicylic aldehyde is then con-
verted to catechol via salicylic acid (Fig. 4.34). Catechol is then degraded as illus-
trated for benzene in Fig. 4.33. For heavier PAHs the initial phases of degradation
yield a catechol analogue to the PAH containing one less benzene ring than the
original PAH. By way of illustration phenanthrene is converted to 1,2-
The Chemistry of Continental Solids 135