Chapter 15 - Phytoextraction: The Use of Plants to Remove Heavy Metals from Soil

Abstract

The continuous increase in industrial activities has contaminated the soil with heavy metals such as cadmium, chromium, copper, mercury, lead, arsenic, nickel, and zinc and are nondegradable and hazardous to life. Many physiochemical methods have been proposed to remove metals from soil, but no method is completely safe and satisfactory. Phytoextraction or phytoaccumulation has emerged as a promising technique for soil remediation that can readily absorb heavy metals and purify the soil of its contaminants. Plants have a natural mechanism to take up and store nutrients according to their bioavailability in soil and the plant’s requirement. Among them, hyperaccumulators have the tendency to take up even nonessential elements by 100-fold greater than nonhyperaccumulators. Because of their larger biomass, they can gather heavy metals using ion channels and metal transport proteins through roots and store them in the above-ground organs where they are either stored in vacuoles and cell walls or detoxified. More than 450–500 plant species have been identified as hyperaccumulators including Thalaspi and Arabidopsis and members from families such as Brassicaceae, Cyperaceae, Poaceae, Fabaceae, and several others. For the plants to effectively take up heavy metal contaminants, the contaminants need to be converted into water-soluble compounds. This technique is called induced phytoextraction in which chelating agents are added in soil that desorb the toxic metals and allow easy uptake by roots. Most common chelates are ethylene diamine tetra acetate, ethylene diamine disuccinate , and other organic acids. However, they can increase toxicity of ground water and affect soil microfauna; therefore, environmental-friendly chelates need to be developed. Another strategy to detoxify soils is to create transgenic plants with increased hyperaccumulation activity against a particular metal. Hence, phytoextraction can be a perfect technique for soil purification because of its minimal limitation. This chapter will focus on the phytoextraction mechanism, metal uptake, and accumulation techniques in plants. Further, the impact of phytoextraction on environmental health will also be explained.