Page 200 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 189

rhizosphere by the release of root exudates, enzymes, and the buildup of organic
carbon in the soil. Different species of plants have been used in various applications
including Salix spp. (hybrid poplars, cottonwoods, and willow), grasses (rye, Bermuda
grass, sorghum, fescue, bulrush), and legumes (clover, alfalfa, and cowpeas). The
main limitations of the technique are the following: (1) limited regulatory acceptance,
(2) long time required for cleanup below acceptable thresholds, and (3) diffi culty in
establishing and maintaining vegetation at some sites.

Recent field tests of phytoremediation are reported on wastes containing petroleum
hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), and poly-
cyclic aromatic hydrocarbons (PAHs), pentachlorophenol, polychlorinated biphenyls
(PCBs), chlorinated aliphatics (trichloroethylene, tetrachloroethylene, and 1,1,2,2-tet-
rachloroethane), ammunition wastes (2,4,6-trinitrotoluene or TNT, and RDX),
metals (lead, cadmium, zinc, arsenic, chromium, selenium), pesticide wastes and
runoff (atrazine, cyanazine, alachlor), radionuclides (cesium-137, strontium- 90, and
uranium), and nutrient wastes (ammonia, phosphate, and nitrate). A list of plants
employed for phytoremediation of organic pollutants from soil is given below.
1. Arabidopsis thaliana (Campos et al. 2008 )
2. Cucurbita pepo (Trapp and Karlson 2001 )
3. Festuca arundinacea (Zand et al. 2010 )
4. Galega orientalis (Trapp and Karlson 2001 )
5. Glycine max (Njoku et al. 2009 )
6. Hamamelis virginiana (Barnswell 2005 )
7. Ipomoea batatas (Doty 2008 )
8. Leucaena leucocephala (Doty et al. 2003 )
9. Morus rubra L. (Trapp and Karlson 2001 )
10. Nicotiana tabacum L. (Campos et al. 2008 )
11. Oryza sativa L. (Kawahigashi et al. 2007 )
12. Populus deltoides (Barnswell 2005 )
13. Populus spp. (Campos et al. 2008 )
14. Quercus spp. (Barnswell 2005 )
15. Robinia pseudoacacia (Barnswell 2005 )
16. Salix spp. (Campos et al. 2008 )
17. Salix viminalis (Trapp and Karlson 2001 )
18. Senecio glaucus (Radwan et al. 1995 ).
19. Solanum tuberosum L. (Inui et al. 2001 )
20. Ulmus pumila (Barnswell 2005 )
21. Zea mays (Zand et al. 2010 )

Rhizoremediation

There is enhanced soil organic carbon, soil bacteria, and mycorrhizal fungi in the
rhizosphere. These factors encourage degradation of organic chemicals in soil.
Rhizosphere bioremediation is also a kind of plant-assisted bioremediation. Jordahl
et al. ( 1997 ) showed that the numbers of benefi cial bacteria increased in the root