From: london@sunSITE.unc.edu (Larry London)
Subject: Re: plants, metals and contaminated sites
Followup-To: bionet.plants
Organization: University of North Carolina, Chapel Hill
Distribution: bionet
Date: Thu, 17 Dec 1992 06:14:58 GMT

In article <9212151555.AA07029@esds01.es.dupont.com> cunninsd@esvax.dnet.dupont.com writes:
>
>	This is my first time on this network, but I heard a
>discussion of heavy metals in plants was underway, so I 
>thought I'd join in. Here at DuPont we have an active 
>research program in using plants toremediate contaminated 
>soils. For this effort we  have borrowed a term I first heard
>used by Ilya Raskin at Rutgers' and called it 
>"Phytoremediation". 
>
>	We define phytoremediation as the use of green plants to
>remove, contain, or render harmless an environmental 
>contaminant.  This definition applies to all plant-influenced
>biological, chemical, and physical processes that aid in site
>remediation. Although our particular current research 
>emphasis is the remediation of lead-contaminated soils, we 
>are interested in most other metals and organics as well.  
>Simply described, we propose to farm hazardous waste sites, 
>biomine the metal contaminants, and reclaim the metals 
>through postharvest processing of the biomass. We consider 
>the entire process to have multiple, but interdependent 
>components. For the technology to be useful each component 
>must be sound technically and economically, and must be 
>acceptable from a regulatory perspective. Our efforts in 
>phytoremediation of lead-contaminated soils address all of 
>these areas. I thought I would venture some general comments
>on the area and see what reaction they brought.
>
>	Although metal tolerant plants are relatively common, 
>most do not accumulate significant quantities of metal in the
>above ground biomass. Our metal-removal goals are ambitious,
>paralleling removal rates of plant nutrients such as 
>nitrogen, potassium, and calcium.  For plant-based 
>decontamination to be sucessful, we must find, breed or 
>engineer plants that absorb, translocate and tolerate these
>metals. These three processes are separate, distinct and, in
>some plants, mutually exclusive.(ie tolerance can be obtained
>by lack of translocation etc.) We have found that 
>combinations of any two processes in one plant are relatively
>easy to find.  All three processes, working efficiently in a 
>single plant with sufficient biomass to acheive the necessary 
>metal-removal rates, will be more difficult to achieve. 
>
>	The discovery or development of such plants might be 
>assumed to be infeasible if it were not for the existence of
>the hyperaccumulators that have been mentioned on this 
>network. These naturally-occurring plants can be found 
>growing on ore outcroppings and have spectacular metal-uptake
>capacities. The sap of one tree has been mentioned previously
>to have concentrations of Ni in excess of 25% dry weight. 
>Alan Baker (Sheffield) lists plants with concentrations in 
>excess of 1%  Cu and Co and 3% Zn, Ni, and Mn on a dry weight
>basis.  Lead levels, although lower,  have been reported as 
>high as 8,200 ppm in these plants. We are looking at these 
>hyperaccumulators for potential remediation uses, however, 
>due to their low growth habits and small biomass, they would
>seem to be agronomically and climatically unsuited for 
>phytoremediation of most sites. A breeding program to 
>increase biomass and metals content is a long-term, crop-
>development strategy that could be undertaken. Molecular 
>biology, however, may offer valuable shortcuts !!! - (Check 
>with your patent attorney, before you release it as this 
>plant can have real and significant value)
>
>	Parallel to our efforts with hyperaccumulators, we have
>been exploring lead-contaminated sites for plants that 
>accumulate lead. Our goal is to find, manipulate, and extend
>the lead-uptake limits of these plants. We have collected and
>analyzed many plants from Superfund, mining, and other
>industrial sites in search of appropriate germplasm. Of the
>plants we have analyzed to date, two plants have shown 
>significant abilities to accumulate lead. These are hemp 
>dogbane (Apocynum sp.) and common ragweed (Ambrosia sp.). 
>Their lead accumulation abilities are considerable, but not 
>consistent, however, across soils. Most metals, and lead in 
>particular, have numerous forms in the soil, not all of which
>are equally available for plant uptake.
>
>	Manipulating the chemistry of the soil to maximize lead
>removal requires balancing plant-nutritional requirements for
>biomass production with the availiability of lead for uptake
>by plants. We have found these to be often competing 
>processes. Maximizing lead availability requires a lower pH 
>and low solution levels of phosphate and sulfate, which 
>directly impacts total plant biomass produced. The plant-
>nutritional status of the soil must be continuously balanced
>against the lead-availability status to maximize total lead 
>removal.
>
>	Pb occurs in all of the physicochemical forms measured
>in a sequential extraction of contaminated soils, including
>water-soluble, exchangeable, specifically adsorbed, 
>carbonate, oxyhydroxide, organic, and other forms.  
>Experiments have confirmed, however, that there are wide 
>differences between soils in these Pb forms and in the 
>ability of plants to pick up the metal in question.
>
>	I would be interested in general comments on the 
>approach, names of others working in the area, etc. etc..


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