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2 Meta cleavage

Cis, c/'s-muconic acid

COOH COH

2-Hydroxymuconic semialdehyde

Fig. 4.33 Aerobic biodegradation pathways for benzene.

Naphthalene

OH /Ring cleavage OH

1,2-Dihydroxynaphthalene

HOOC

HOOC

HOOC

Naphthalene

1,2-Dihydroxynaphthalene

Catechol

COOH Salicylic acid

Catechol

COOH Salicylic acid

"CHO Salicylic aldehyde

"CHO Salicylic aldehyde

Pyruvic acid

HOOC

HOOC

HOOC

Pyruvic acid

HOOC

Fig. 4.34 Biodegradation pathways for naphthalene.

dihydroxynaphthalene (Fig. 4.35). Degradation then proceeds as shown for naphthalene (Fig. 4.34).

At the Burlington Northern site the creosote-contaminated soil was sieved and then ball milled to reduce particle size, a process that increases contaminant availability by increasing reactive surface area. The milled creosote-contaminated soil was then slurried with water and placed in five separate (to allow comparisons) 64-litre stainless steel bioreactors, equipped with aeration, agitation and temperature controls. An inoculum of PAH-degrading bacteria was then added, along with an inorganic supplement, containing nitrogen as NH4, potassium, magnesium, calcium and iron. Conditions within the reactors were controlled to optimize degradation for 12 weeks.

Average initial concentrations in the PAH-contaminated soil and the residual concentrations after 12 weeks in the bioreactor are shown in Table 4.12. Although degradation was clearly greater for the lighter PAHs (98%) it was still extensive for the heavier compounds (70%). Furthermore, these extents of degradation were consistently achieved in the five replicated systems. Thus, while the use of bioreactors is technically more demanding, and more expensive, biodegradation is extensive and reliable.

4.10.4 Phytoremediation

Phytoremediation is the use of plants and trees to clean up metals, pesticides, solvents, explosive hydrocarbons, PAHs and leachates at contaminated sites.

O

jO

OH

OH

if]

Degradation continues as for naphthalene in Fig. 4.34

^JL^J

Phenanthrene

1,2-Dihydroxynaphthalene

Fig. 4.35 Biodegradation pathway for phenanthrene as an example of a heavier PAH.

Fig. 4.35 Biodegradation pathway for phenanthrene as an example of a heavier PAH.

Table 4.12 Bioreactor remediation of creosote (PAH) contaminated soil, adapted from US Environmental Protection Agency technology demonstration sheet EPA/540/S5-91/009.

Residual PAH

PAH

Initial PAH

concentration after 12

reduction

concentration (mgkg-1)

weeks' treatment (mg kg-1)

(%)

Two- and three-ring PAHs

1500

30

98

Four- through six-ring PAHs

960

280

70

Total PAHs*

2460

310

87

* Sixteen PAHs listed as priority pollutants by the US EPA (see Fig. 4.32).

* Sixteen PAHs listed as priority pollutants by the US EPA (see Fig. 4.32).

While microbial bioremediation is usually the fastest and most widely applied clean-up technique, phytoremediation can prolong or enhance degradation over longer timescales on sites where microbial techniques have been used first. It is also useful at remote sites missed during the main remediation campaign, and can be aesthetically pleasing.

Plants may accumulate contaminants within their roots, stems and leaves. This is called phytoaccumulation (Fig. 4.36) and is known to remove a variety of heavy metals (see Section 5.6) from soils, including zinc (Zn), copper (Cu) and nickel (Ni). Once the plants have had sufficient time to accumulate contaminants they are harvested and usually incinerated to leave a metal-rich ash. The ash typically represents about 10% of the original mass of the contaminated soil, and is either landfilled or processed as a metal ore (bio-ore) if economically viable. Some plants exude enzymes that are capable of transforming organic contaminants into simpler molecules, used directly by the plants for growth, a process known as phytodegradation (Fig. 4.36). In some plants, degradation of contaminants occurs when root exudates (e.g. simple sugars, alcohols and acids) stimulate proliferation of microbial communities in the soil around the root (rhizosphere). This is known as phyto-enhanced or rhizo-enhanced degradation (Fig. 4.36). Roots also de-aggregate the soil matrix, allowing aeration and promoting biodegradation. Some plants take up volatile and semivolatile compounds from soil and translocate them to their leaves where volatilization to the atmosphere occurs. This phy-tovolatilization (Fig. 4.36) does not degrade or immobilize the contaminant, but

Plate 3.1 (a) Flying into Los Angeles in the early morning before an intense photochemical smog has developed. (b) Later in the day further north on the west coast (USA). Lengthy exposure of primary pollutants to sunlight has induced the photochemical formation of a brown smog. Photographs courtesy of P. Brimblecombe.

facing p. 138

Plate 4.1 Red bauxite-bearing oxisol (ferralsol) overlying Tertiary limestone (white) in south Jamaica. The soil is piped into solution-enlarged hollows of the limestone surface. Cliff face approximately 6 m high. Photograph courtesy of J. Andrews.

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