Phytoremediation of Soil Trace Elements

This chapter summarizes research progress in development of phytoremediation technologies. Some soils have become contaminated by trace elements enough to kill plants, inhibit soil organisms, and/or threaten wildlife, humans or the environment. Traditional remediation by dig and haul methods are very expensive and disruptive. Phytoremediation is an alternative approach which can alleviate the environmental risk at lower cost and disruption. Phytoremediation includes phytoextraction, phytomining, phytovolatilization, rhizofiltration and phytostabilization. Phytoextraction uses plants which can accumulate high levels of trace elements in their shoots, coupled with biomass harvest, to remove metals from the soil. If the value of metals in the biomass is high enough, this is an alternative method of mining, and the ash of the biomass is a high grade ore. Phytovolatilization can occur for mercury and selenium where solid forms in soils are converted to gaseous forms by plants and soil microbes. Phytostabilization uses applications of limestone to make the soil calcareous plus organic matter soil amendments to improve soil fertility and trace element binding strength of the soil, and cover plants to reduce plant- and bio-availability of trace elements in the soil, and to limit exposure to the soil by animals which might ingest soil. For several elements, phytomining has been demonstrated (Ni, Au, Tl, Co). For some other elements, phytoextraction can alleviate risk at much lower cost than dig and haul, but no one will pay for the biomass ash as an ore (As, Cd). For some others, the element is so insoluble in soil or in plant roots than the element is not practically accumulated in plant shoots so phytoextraction cannot be achieved. Fortunately, phytostabilization can be remediate soils rich in Pb and Cr, Zn, and many other trace elements which could comprise risk in the soil were not amended to reduce bioavailability.