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United States Department of Agriculture

Agricultural Research Service

Title: Using Hyperaccumulator Plants to Phytoextract Soil Ni and Cd

item Chaney, Rufus
item Mcintosh, Marla - UMD DEPT NAT RESOURCES
item Synkowski, Evaclaire - UMD DEPT NAT RESOURCES

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: April 28, 2004
Publication Date: September 9, 2004
Citation: Chaney, R.L., Angle, J.S., Mcintosh, M.S., Synkowski, E. 2004. Using hyperaccumulator plants to phytoextract soil ni and cd [abstract]. OECD Phytoremediation Workshop. p. 18.

Technical Abstract: In order to achieve practical phytoextraction of soil metals, hyperaccumulator plants are necessary. Induced hyperaccumulation using synthetic chelating agents has not been found to be acceptable to the environment due to leaching of metal chelates. Thus natural hyperaccumulators offer an important tool for inexpensive soil decontamination for those elements which plants hyperaccumulate. To date, useful natural hyperaccumulators of Ni, Co, Zn, Cd, As, and Se have been identified which give enough annual yield to achieve soil remediation goals. Cu accumulator plants have yet to be domesticated, and limited yields of As accumulating ferns have slowed development of phytoextraction for these elements. Pb cannot be phytoextracted without addition of chelators; Cr may be phytostabilized by reduction of chromate to chromic, but phytoextraction potential is not evident to date. Plants which phytovolatilize Hg0 have been bioengineered, but public acceptance is weak; further development of plants which accumulate Hg in shoots may offer Hg phytoextraction acceptable to the public. Different elements have different value in the market. Thus Ni phytoextraction from contaminated or mineralized (geogenic) soils can offer economic return greater than producing most crop plants, especially when considering the low fertility or phytotoxicity of Ni rich soils. The other elements may be phytoextracted at lower cost than soil removal and replacement, especially if biomass energy production is practiced with the phytoextraction crop. In our development of a commercial Ni phytoextraction/phytomining technology, we had to domesticate a new species. We selected tall Alyssum species (e.g., Alyssum murale) for development; seed were collected from diverse sites across southern Europe and evaluated under uniform conditions in the field. Laboratory and field experiments were conducted to identify agronomic management practices needed to produce high yields in severely infertile serpentine soils, or in smelter contaminated soils at Port Colborne, Ontario, Canada. Low fertilizer requirements greatly increased annual yield without decreasing shoot Ni levels. Remarkably, plant accumulation of Ni was improved at higher pH which caused lower soluble Ni in the soil; but plant accumulation of Zn, Mn and Co followed the usual pattern of reduced shoot levels at higher soil pH. High soil Fe oxides limited this effect to about pH 6.5, while smelter contaminated farm soils had maximum annual Ni uptake at pH 7.5. Biomass was cut at early flowering to prevent loss of leaf biomass (richer in Ni than stems); biomass was baled mechanically. Subsequently ash of Alyssum biomass was charged into an industrial smelter and Ni recovered very effectively. Alyssum biomass appears to be the best ore for Ni ever produced; elements in plant biomass do not interfere with recovery of Ni by smelter operations. This technology was licensed to Viridian LLC which is marketing the Ni phytoextraction at several locations. Improved risk assessment has indicated that most Cd+Zn contaminated soils will not require Cd phytoextraction because the Zn limits practical risk from soil Cd. But in rice paddies, and in soils with Cd contamination without corresponding 100-fold greater Zn contamination, Cd can more readily enter food plants and diets. Clear evidence of human renal tubular dysfunction from soil Cd has only been obtained for subsistence rice farm families in Asia. Because of historic metal mining and smelting, Cd contaminated rice soils have been found in Japan, China, Korea, Vietnam and Thailand. Phytoextraction using southern France ecotypes of Thlaspi caerulescens appears to be the only practical method to remove the soil Cd without soil removal and replacement. The southern France ecotypes accumulate 10-20-fold higher Cd in shoots than most Thlaspi ecotypes such as those studied from Belgium and the UK. Addition of fertilizers to maximize yield does not reduce Cd in shoots; and soil acidification promotes annual Cd phytoextraction. The value of Cd in the plants is low, so the remediation service must pay the costs of Cd phytoextraction plus profits to the parties who conduct phytoextraction. Some other plants have been studied for Cd phytoextraction, but annual removals are lower than the best Thlaspi. Improved Thlaspi cultivars with higher yields and retaining this remarkable Cd phytoextraction potential are being bred using normal breeding techniques.

Last Modified: 4/23/2014
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