General Aspects/Case Studies on Sources and Bioremediation Mechanisms of Metal(loid)s 155

9.5.4 Phytoextraction

Phytoextraction, better known as phytomining or phytoaccumulation, is the practice of raising a

certain crop that has been known for collecting toxins within its shoot system and leaves (hyper

accumulator or tolerant plant), harvesting them followed by elimination of contaminants from the

affected areas. Unlike destructive degrading mechanisms, this process produces a consolidated

plant and contaminate (mostly metallic) mass that must be disposed of or recycled. Correlated to

landfilling and excavation, this technology is based on the concentration of pollutants which leaves

a significantly smaller bulk of pollutants to be disposed off (Wani et al. 2012). Chelation is the

process by which soil-borne metal pollutants are transported by roots to tissues. By translocation

from roots to stems and leaves, metal(oids) are removed from the soil in Figure 9.2.

Figure 9.2. Metals uptake by Phytoextraction: Metal(oids) are eliminated from the soil by translocation into plant roots,

stem, and leaves (Mishra et al. 2017).

A lot of studies have been undertaken on phytoextraction, including novel phytoextractors

(Baker and Brooks 1989), for a better understanding of plant-metal absorption, metabolism,

transport, excretion and tolerance (Krämer et al. 1996, Rauser 1995, Salt et al. 1999, Lasat et

al. 1998) and genetic changes to improve plant metal accumulation and tolerance (Rauser 1995,

Krämer et al. 1996, Kärenlampi et al. 2000, Clemens et al. 2002, Krämer 2005). An alternative

option to advance this concept was to combine plants producing high biomass with chemical agents

that boost the solubility of metals for plant uptake (Blaylock et al. 1997, Huang et al. 1997). Hyper-

accumulators are plants that abnormally uptake metals. Over 400 varieties of plants have been

found to improve the quality of soil and water (Lone et al. 2008). Since distinct plants have distinct

capacities to absorb significant levels of contaminants, a variety of plants can be introduced for

the phytoremediation process to see their effectiveness. One of the methods utilized to create a

phytoremediation plant is the genetic engineering of improved hyperaccumulators. Arabidopsis sp.,

Sedum alfredii sp., and Thlaspi sp. are the hyper accumulators that have been examined the most

by scientists (Brassicaceae and Alyssum are members of the Brassicaceae family) (Milner and

Kochian 2008).