From: cunninsd@esvax.dnet.dupont.com
Newsgroups: bionet.plants
Subject: plants, metals and contaminated sites
Date: 15 Dec 92 15:55:43 GMT
Distribution: bionet


	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..