An Introduction to Environmental Chemistry

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 prolifera-

tion 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 translo-

cate them to their leaves where volatilization to the atmosphere occurs. This phy-

tovolatilization (Fig. 4.36) does not degrade or immobilize the contaminant, but

138 Chapter Four

Phenanthrene

Degradation continues

as for naphthalene in

Fig. 4.34

1,2-Dihydroxynaphthalene

OH

OH

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

Table 4.12Bioreactor 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 (mg kg-^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).