Phytoremediation of Nickel: Mechanisms, Application and Management

Introduction

In recent decades, significant amounts of metals (including nickel (Ni)) have being added to the environment through various agricultural activities (such as agrochemicals usage and long–term application of metal contaminated sewage sludge in agricultural soils) and industrial activities (such as waste disposal, waste incineration, vehicle exhausts, and coal–fired power plants, as well as natural geological sources). All these pollution sources cause accumulation of toxic metals and metalloids in agricultural soils and water bodies, which poses a serious threat to food safety and potential health risks due to accumulation of toxic metals in agricultural products (Khan, 2005). In particular, many of these pollutants are also known carcinogens (Ensley, 2000). Widespread agricultural soil contamination has significantly decreased the extent of arable land available for cultivation worldwide (Grêman et al., 2003).

Unlike organic pollutants, toxic metals and radionuclides cannot be eliminated by chemical or biological transformation. Although it is possible to reduce their toxicity by alternating their chemical speciation, metal pollutants are generally persistent in the environment. The costs associated with the clean–up of organic and inorganic pollutants can be overwhelming, even in developed countries. Given the nature and extent of contamination worldwide as well as high costs associated with physical and chemical remediation, biotechnologies (such as microbial biotransformation, and phytoremediation) have become a much attractive and environmental–friendly strategy for the clean–up of a broad spectrum of hazardous organic and inorganic pollutants (Mrak et al., 2008; Pilon–Smits, 2005). The plant–based environmental remediation has been widely pursued by governments and industries as a favorable low impact cleanup technology applicable in both developed and developing nations (Raskin & Ensley, 2000; Robinson et al., 2003a). This chapter specifically provides an overview of the environmental chemistry, speciation and toxicity of Ni, and then elaborates on the removal of Ni by phytoremediation technology. Hyperaccumulation mechanisms involved in the uptake of Ni by plants, the use of chelating agents (or chelators) in enhancing Ni uptake and translocation in plants, and the application of genetic engineering technology in Ni phytoremediation will be addressed.