J

i in sediments

FIGURE 16-14 Concentration profiles (chromatograms) for various PCB congeners of commercial Aroclor 1242 before and after biodégradation ¡different scales) in a sediment. The major components of the peaks correspond to the following positions of chlorine substitution: 2 = 2-chlorobiphenyl; 5 = 2,2' and some 2,6; 7 = 2,3'; 8 = 2,4' and some 2,3; 31 = 2,2', 5,5'; 46 = 2,4,4', 5; 47 = 2,3', 4',5; 48 = ■ 2,3', 4,4'. Notice that biodégradation produces ortho-substituted congeners that are present in low concentrations or absent in the original sample. iSource: D. A. Abramowicz and D. R. Olson, CHEMTECH (July 1995): 36-40.]

Thus the products of anaerobic treatment here are ortho-substituted congeners, ultimately 2-chlorobiphenyl and 2,2'-dichlorobiphenyl especially.

Since dioxin-like toxicity of PCBs requires several meta and para chlorines (see Chapter 11), the anaerobic degradation process significantly reduces the health risk from PCB contamination. Of course, once adjacent ortho and meta sites without chlorine are available, aerobic microorganisms—if available— could degrade the biphenyl structure, as already discussed. Figure 16-14 illustrates the change in composition of a commercial PCB sample after it has resided for some time in sediment that contains anaerobic bacteria.

Phytoremediation of Soils and Sediments

The technique of phytoremediation, the use of vegetation for the in situ decontamination of soils and sediments of heavy metals and organic

FIGURE 16-15

Mechanisms of phytoremediation by a plant.

pollutants, is an emerging technology. As illustrated in Figure 16-15, plants can remediate pollutants by three mechanisms:

• the direct uptake of contaminants and their accumulation in the plant tissue (phytoextraction),

• the release into the soil of oxygen and biochemical substances such as enzymes that stimulate the biodégradation of pollutants, and

• the enhancement of biodégradation by fungi and microbes located at the root-soil interface.

Advantages of phytoremediation include its relatively low cost, aesthetic benefits, and nonobtrusive nature.

Some pollutants

Some forms of contaminants are trapped in fibrous parts of the tree or other plant.

Enzymes in fungi and microbes released from the plant react with pollutants and break them down.

Some pollutants in the soil are broken down by oxygen and enzymes emitted from the roots.

Other pollutants are absorbed into the tree from the roots.

Some pollutants in the soil are broken down by oxygen and enzymes emitted from the roots.

Some pollutants

Some forms of contaminants are trapped in fibrous parts of the tree or other plant.

Enzymes in fungi and microbes released from the plant react with pollutants and break them down.

Other pollutants are absorbed into the tree from the roots.

Certain plants are hyperaccumulators of metals, i.e., they are able to absorb through their roots much higher than average levels of these contaminants (by a factor of at least 10-100, to yield a contaminant concentration of 0.1% or more) and to concentrate them much more than do normal plants. This ability probably evolved over long periods of time as the plants grew on natural soils that contained high concentrations of pollutants, especially heavy metals. In bioremediation, these plants are deliberately planted on contaminated sites and then harvested and burned. In some instances, the resulting ash is so concentrated in metal that it can be mined!

Phytoremediation is an attractive technique because metals are often difficult to extract with other technologies since their concentration is usually so small. For example, the shrub called the Alpine pennycress has the ability to hyperaccumulate cadmium, zinc, and nickel. Phytoremediation has been successfully used for extraction, e.g., of cadmium by both water hyacinths and various grasses; of lead and copper by alfalfa; and of chromium by Indian mustard, sunflowers, and buckwheat. In some cases, a chelating agent is added to the soil to enhance the accumulation of metals by the plant. Scientists are experimenting with various types of plants that can extract lead from soils. One difficulty with phytoremediation is that hyperaccumulators are usually plants that are slow to grow and therefore slow to accumulate metals. However, fast-growing poplar trees show promise of being efficient phytoremediators. One recently developed type of hybrid poplar efficiently absorbs TCE from, hazardous waste sites and from groundwater. In general, there is a need to harvest the plants before they lose their leaves or begin to decay, so that contaminants do not become dispersed or return to the soil.

Plants can efficiently take up organic substances that are moderately hydrophobic, with log Kow values (Chapter 10) from about 0.5 to 3, a range that includes BTEX components and some chlorinated solvents. Substances that are more hydrophobic than the upper limit of this range bind so strongly to roots that they are not easily taken up within the plant. Once taken up, the plant may store the substance by transformation within its lignin component or it may metabolize it and release the products into the air.

Substances that plants release into the soil include chelating ligands and enzymes; by complexing a metal, the former can decrease its toxicity, and the latter in some cases can biodegrade pollutants. For example, it has been found that the plant-derived enzyme dehalogenase can degrade TCE. Plants also release oxygen at the roots, thereby facilitating aerobic transformations. As previously discussed, the fungi that exist in symbiotic association with a plant also have enzymes that can assist in the degradation of organic contaminants in soil.

Bioremediation in general and phytoremediation in particular are rapidly emerging technologies. The long-term potential for these techniques to be used at many sites that require decontamination is apparent. Experiments at several test sites have shown that phytoremediation can be used successfully to degrade petroleum products in soils. The number of Superfund sites applying bioremediation technology is shown in Table 16-3,

Continue reading here: Hazardous Wastes

Was this article helpful?

0 0