Biochemistry of Metals/Metalloids Toward Remediation Process

Abstract

The use of phytoremediation for restoration or amelioration of soils polluted with heavy metals is a promising technique for the near future. Generally, if a plant can accumulate more than 1000 mg kg−1 (or 1000 ppm) of Cu, Co, Cr, Ni, or Pb, or more than 10,000 mg kg−1 (or 10,000 ppm) of Mn or Zn, it is defined as a hyperaccumulator. The discovery and elucidation of the mechanism used by hyperaccumulators to take up, translocate, and fix heavy metals in aerial parts is of great interest to understand the role of such plants in biorestoration. The performance of plants for heavy metals phytoremediation depends on several factors, starting from the bioavailability of metals in the soil, which is conditioned by cation exchange capacity, soil pH and organic matter content, the oxidation state of the metal, presence of both natural and synthetic chelants, interaction with soil microorganisms, etc. The mechanism used by roots to uptake bioavailable metals from the soil is also crucial for the success of bioaccumulation, in addition to mechanisms used to translocate metals from root to shoots after absorption. Significant progress in understanding the mechanisms governing metal hyperaccumulation has been made in the last decade through comparative physiological, genomic, and proteomic studies of hyperaccumulators and related non-hyperaccumulator plants. Parts of these studies are discussed here. Finally, the efficiency in detoxification and sequestration is a key property of hyperaccumulators, allowing them to concentrate huge amounts of heavy metals in aerial organs without apparent phytotoxic effect. This exceptionally high heavy metal accumulation becomes even more surprising considering that it mainly occurs in leaves where photosynthesis, essential for plant survival, is accomplished, and that the photosynthetic apparatus is a major target for most of these contaminants. Although extensive laboratory studies on phytoextraction by plants have been reported, less research has been dedicated to evaluate the performance in field studies, which seems necessary to both validate laboratory studies and fully demonstrate the usefulness of this technique for site restoration.