Phytoremediation of heavy metals: A strategy for the removal of toxic metals from the environment using plants

Abstract

Heavy metals constitute a heterogeneous group of elements; a relatively high density of approximately 6 g cm-3 is their common characteristic with atomic weight more than that of iron (Alloway, 1997) [3]. Sources of heavy metal contaminants in soils include metal liferous mining and smelting, metallurgical industries, sewage sludge treatment, warfare and military training, waste disposal sites, agricultural fertilizers and electronic industries (Alloway 1995) [2]. Toxic heavy metals cause DNA damage, and their carcinogenic effects in animals and humans are probably caused by their mutagenic ability (Knasmuller et al., 1998; Baudouin et al., 2002) [46, 8]. Metal-contaminated soil can be remediated by chemical, physical or biological techniques (McEldowney et al., 1993) [56]. Chemical and physical treatments irreversibly affect soil properties, destroy biodiversity and may render the soil useless as a medium for plant growth. Phytoremediation involves the use of plants to remove, transfer, stabilize and/or degrade contaminants in soil, sediment and water (Hughes et al., 1997) [43]. This plant based technology has gained acceptance in the past ten years as a cheap, efficient and environment friendly technology especially for removing toxic metals. Plant based technologies for metal decontamination are extraction, volatilization, stabilization and rhizofiltration. Various soil and plant factors such as soil’s physical and chemical properties, plant and microbial exudates, metal bioavailability, plant’s ability to uptake, accumulate, translocate, sequester and detoxify metal amounts for phytoremediation efficiency. Use of transgenic to enhance phytoremediation potential seems promising. Despite several advantages, phytoremediation has not yet become a commercially available technology. Progress in the field is hindered by lack of understanding of complex interactions in the rhizosphere and plant based mechanisms which allow metal translocation and accumulation in plants.