Heavy Metal Accumulation in Urban Soil: a Phytoextraction Method Review

Lead (Pb), cadmium (Cd), and arsenic (As) are three common non-essential heavy metals found in urban soils and can prove toxic to animals, humans, and some plants at low concentrations. The main exposure pathways of heavy metals in humans are through ingestion and inhalation of soil particles and ingestion of contaminated food. When dealing with contaminated soil in urban environments, activities like urban gardening can increase the likelihood of these exposure pathways, so heavy metal toxicity from contaminated soil can become a greater risk with the increased interest in urban agriculture. The US EPA created target concentrations for these heavy metals in residential soil, industrial soil, and agricultural soil. If any of these soils exceed their designated concentration, the US EPA has deemed them hazardous to both human health and the surrounding ecosystem. Phytoextraction is being considered and tested as a method to remove heavy metal pollution in urban soils. Two popular forms of phytoextraction are 1) using hyperaccumulator plants and 2) chelate-assisted phytoextraction using metal tolerant species. Hyperaccumulating plants can bioaccumulate 100 to 1000 times the heavy metal concentration of non-hyperaccumulators but have low biomass production/growth rates and are heavy metal specific. Chelate-assisted phytoextraction has higher a growth rate and biomass production, but can be expensive, has a stronger potential for heavy metal trophic transfer, and can lead to leaching of heavy metals off of the contaminated site. Phytoextraction using hyperaccumulating plant species may pose less risk and be suited for smaller sites with specific heavy metal pollution whereas chelate-assisted phytoextraction