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United States Department of Agriculture

Agricultural Research Service

Research Project: RISK ASSESSMENT AND REMEDIATION OF SOIL AND AMENDMENT TRACE ELEMENTS Title: Phytoextraction of Heavy Metals with Hyperaccumulator Plants

Authors
item Chaney, Rufus
item Broadhurst, C Leigh - UNIV MD, COLLEGE PARK
item Mcintosh, Marla - UNIV MD, COLLEGE PARK
item Reeves, Roger - MASSEY UNIV, NEW ZEALAND
item Angle, J Scott - UNIV GEORGIA, ATHENS

Submitted to: International Bioavailability Workshop
Publication Type: Abstract Only
Publication Acceptance Date: May 23, 2006
Publication Date: September 10, 2006
Citation: Chaney, R.L., Broadhurst, C., Mcintosh, M.S., Reeves, R.D., Angle, J. 2006. Phytoextraction of heavy metals with hyperaccumulator plants [abstract]. International Bioavailability Workshop, Seville, Spain. September 11-14, 2006. pp. 65-66.

Technical Abstract: When soils contain metals at high enough levels to comprise risk thru food-chain or soil ingestion, some methods must be applied to alleviate the risk, or the land use must be constrained. One approach to remediate risks from some metals is phytoextraction using hyperaccumulator plants. These remarkable plant species accumulate about 100-times higher concentrations of an element than do normal crop plants when the crop plant suffers yield reduction from metal phytotoxicity. The metal tolerance and hyperaccumulation evolved on mineralized soils and are specific to the metals present where the exposed plants evolved these traits. It is important that the limits of phytoextraction be better understood in the environmental community. Some elements are so strongly bound by soils, or precipitated in plant roots, that plants do not accumulate high enough levels to either comprise risk to provide useful phytoextraction. Because Pb is a common soil contaminant comprising risk to children, many attempts to phytoextract Pb have been reported. However, no plant has been identified which accumulates 1% Pb in the aboveground biomass unless strong chelating agents are applied to the soil to dissolve Pb and prevent it from being precipitated in the plant roots with phosphate also accumulated by the roots. Added chelating agents cause extensive leaching toward groundwater and cannot be tolerated. Author estimates that there have been at least 100 useless papers on chelator-induced phytoextraction, or on species with no practical utility in Pb removal. For Pb, in situ inactivation using phosphate has proven successful in alleviating risk from much of the contaminated land known to exist. Very high Pb contamination requires removal. No plants have been found with useful Co phytoextraction even though tolerant plants are known. Phytoextraction now appears to be relevant for Cd, Zn, As, Tl, and Se. Our research team has developed practical phytoextraction technologies for Ni and Cd. For Ni (and Co), we have developed Alyssum murale and Alyssum corsicum cultivars and appropriate agronomic management; increased soil pH reduces soil solution Ni concentration but increases Ni uptake to shoots of these species. With proper management, these species can reach over 2.5% Ni in shoots and over 400 kg Ni/ha. Because Ni metal is about $15/kg, we consider growing these crops on Ni contaminated or mineralized soils to be phytomining. The biomass can be burned for energy; the ash is a high grade Ni ore easily converted to metal in an electric arc furnace. The same species can phytoextract Co, but at a low pH; one could phytoextract the Ni at pH 6.5-7 until it was no longer economic, or acceptable Ni levels were reached, then acidify to phytoextract Co. The other practical technology is for Cd. Nearly all Cd contamination is part of Zn-Pb ore wastes or smelter emissions. It has been shown that if the normal geogenic ratio of Cd to Zn (1:200) occurs in soil that Zn phytotoxicity to plants, and Zn inhibition of Cd absorption in the intestine protect humans and wildlife from the soil Cd risk. However, paddy rice and tobacco can accumulate enough Cd from such soils to comprise risk to highly exposed individuals. And non-orogenic sources of Cd (pigments; plating) may have high Cd:Zn ratio, which promotes both uptake into plants and bioavailability to animals. Fortunately, populations of Thlaspi caerulecens found in southern France have remarkable ability to phytoextract soil Cd. Yes, the plants are slow growing and small and hard to harvest. But agronomic management and hand harvesting can remove over 10 kg Cd/ha per year. No other plant species with the remarkable Cd phytoextraction capability and Cd:Zn enrichment ratio of southern France Thlaspi caerulescens has been found. Cd is hyperaccumulated at about 10-times higher Cd:Zn ratio than found in the soil, so Cd can be removed to alleviate risk from soil Cd. This system is presently being tested in rice fields in Thailand as a winter crop. Delay in setting limits for rice grain Cd in international trade has delayed remediation decisions, but Thlaspi is ready for deployment. Until governments order soil Cd risk alleviated, companies have not elected to proceed with Cd remediation. Others are attempting to identify all genes needed to achieve effective Cd phytoextraction and transfer them to high yielding species, but any such product is still many years away, and a host species for the genes highly debated.

Last Modified: 9/22/2014