Phytoremediation and Bioremediation of Soils and Waters

Abstract

Toxic metals are problematic in soils because not only are they adsorbed into the zeolite soil structure, but they are also absorbed into the humus and biomass present in soils. These humic and fulvic substances in soils contain compounds that act as chelating agents to these metals, thereby contributing to the difficulty of their removal. In addition, these substances often contain redox active agents that can convert the adsorbed metals into ones that have different oxidation states, or reduce metal ions down to the metallic state. Although the removal of metals from soils poses several challenges, the presence of good chelating agents in humic and fulvic substances is one of the obstacles that must be overcome if phytoremediation or bioremediation is to be the method of choice for metal removal. Phytoremediation is the use of green plants to remove pollutants from the environment. Two recent reviews have been written on this subject.^1,2 Among the types of phytoremediation currently in use are phytoextraction and rhizofiltration. Phytoextraction is defined as the use of metal-accumulating plants that concentrate them into the harvestable parts. Rhizofiltration is the use of plant roots to absorb metals from aqueous waste streams. Phytoextraction can be carried out either with or without added chelate complexant to assist in removing the metals. In certain cases the addition of chelating agents enhances the accumulation of metals by plants, especially if the chelate has a strong affinity for the targeted metal. Nevertheless, a consideration when using this method is the requirement that the chosen chelate must be biodegradable or readily removed from the contaminated site. Alternatively, phytoremediation can rely only on the physiological processes that allow plants themselves to accumulate metals. A disadvantage of this approach is that growth rates are slow, and the selectivity for particular metals is likely to be low. In the future, however, genetic engineering could be useful in producing plants that have both higher growth rates and metal selectivities. Bioremediation involves the use of biological remedies for pollution reduction.³ For metals this detoxification process must involve processes such as the oxidation or reduction of the metal center to make it either more water soluble, so that it precipitates and can be removed in solid form, or converted to a more volatile form that can be removed in the gas phase. In choosing a bioremediation strategy for metals, the biological system must be able to tolerate the concentration of metal that is present at the site.