In the article entitled ''Nightmare Soil'' written by Joel Bleifuss and
published IN OUR TIMES, the author writes:
"Each year about 4 million metric tons of municipal sludge--about half
of the total produced annually in the United States--are dumped on farm
land. That sludge is derived chiefly from human excreta and from the
water wastes of 130,000 industrial plants. Typically, municipal sludge
contains PCBs, dangerous pesticides such as chlordane, chlorinated
compounds such as dioxin, heavy metals such as arsenic and lead, viruses
such as Hepatitis A, eggs of parasitic worms, etc. Cornell University's
Toxic Chemical Laboratory recently tested 50 municipal sludges and found
that two-thirds contained asbestos. 'You test it and you find so much--
dioxin, PCBs, DDT, asbestos--it's an endless list,' says Cornell
toxicologist Donald Lisk. Urban sludge is a huge problem.'
"In fact, according to the Archives of Environmental Contamination and
Toxicology, of 30 municipal sludges analyzed in 1983, only seven were
considered suitable for land application. The sludge from the other 23
plants contained elevated levels of one of more heavy metals, such as
lead and cadmium. But that was using the older, more stringent
standards."
In 1984, according to Bleifuss, EPA began the linguistic detoxification
of sewage sludge when it issued a beneficial sludge use and disposal
policy that permitted the controlled use of treated sewage as
fertilizer. In 1993, new regulations governing sludge policy were
written into 'Part 503' of the Clean Water Act. "...EPA raised the
acceptable exposure limits to some pollutants so that most of the
nation's sludge could be classified as 'clean'..."
For example, writes Bleifuss, "the new regulations increased the amount
of lead that can be applied annually via fertilizer sludge from 111
pounds of lead per acre to 267 pounds per acre. The arsenic level was
raised from 12.5 pounds per acre to 36 pounds per acre. The allowable
amount of mercury jumped from 13.4 pounds per acre to 50 pounds per
acre. And the amount of chromium permitted ballooned from 472 pounds
per acre to 2,672 pounds per acre. In fact, under 503, sludge sold as
fertilizer can be so contaminated with toxins that, according to the
EPA, such sludge cannot be legally landfilled."
Meanwhile, biotechnology is in the works to use crops which we normally
eat, to clean up the metals which are being spread on agriculture as a
''beneficial use''. See the following article which is NO LONGER
ONLINE, by Dr Janet Cotter-Howells [Telephone: 01224 272702/ e-mail:
bot156@abdn.ac.uk] came initially from the URL of
<http://www.cclrc.ac.uk/Publications/SRAnnual/art9.html>. I reproduce
it here in its original form, as written by the author.
~Bunny Snow.
**Certain plants have been identified that take up heavy metals in their
various parts. Certain plants are tolerant to certain metals, whereas
others will cause death. The problem is that waste sites are not
contaminated with a single metal, but ususally a combination.
==========================================================
Delicate yellow flowers unique to the Greek island of Lesbos have
provided vital clues that could lead to the development of plants that
will clean up contaminated soil. Andrew Smith and Ute Krämer from the
Department of Plant Sciences at the University of Oxford and
Janet Cotter-Howells of the University of Manchester (now at the
University of Aberdeen) have discovered how the plant, Alyssum
lesbiacum (Figure 1), mops up nickel. Armed with this knowledge, they
hope to design plants that draw metals out of soil much more
cheaply and cleanly than is possible with existing techniques.
Figure 1. Alyssum Saxatile - a familiar relative of alyssum lesbiacum
which hyperaccumulates nickel. Could saxatile be engineered to have
similar properties?
A. lesbiacum absorbs nickel very effectively from the soil, shuttling
the metal upwards into its green tissues above ground. Like other
'hyperaccumulators', it absorbs levels of metal that would kill most
other plants. Therefore these plants must employ a special mechanism
for the uptake and storage of metals in a non-toxic form. No one knows
why some plants accumulate metals instead of keeping them out. One
theory is that the metals keep insect pests at bay by deterring them
from feeding.
But whatever the reason, hyperaccumulators could be very useful in
cleanup operations, because they take metals out of the soil and store
them in parts of the plant above ground. Metal-rich shoots and leaves
could simply be harvested and disposed of, in landfill sites for
example. It might even be possible to extract and recycle the metals.
In research that could accelerate the development of these techniques,
Smith and Krämer have discovered that A. lesbiacum mops up nickel using
the amino acid histidine. Sap was sampled by collecting the exudate from
de-topped plants. Analysis of the sap indicated that concentrations of
the amino acid, histidine, were highly elevated, over ten-fold that of a
related non-hyperaccumulator, Alyssum montanum. This suggested that
histidine was the primary chelator of Ni.
In collaboration with scientists at Daresbury, EXAFS was used to obtain
information on the co-ordination chemistry of Ni (Figure 2). A distinct
advantage of EXAFS is that it is a non-invasive technique, allowing
fresh material to be analysed without any prior treatment. The
experiments gave direct evidence that Ni binds to histidine in leaves,
roots and sap of A. lesbiacum. Furthermore, EXAFS showed that Ni was
bound to one of the N atoms within the histidine molecule. Nitrogen
atoms in the amino acid carry spare electrons which they donate to the
electron-hungry nickel ion, forming a strong bond as they do so. The
nickel is then imprisoned inside the histidine's molecular cage. This is
significant as it rules out binding of Ni to the sulphur of
metallothioneins and low molecular weight peptides (phytochelatins)
which were previously suggested to have an affinity for heavy metals. To
their surprise, the researchers also found that the histidine roves
freely in the plant's roots. Usually, histidine only functions as part
of much larger biological molecules such as peptides or proteins.
Figure 2. Extended X-ray absorption fine structure (EXAFS) analysis of
nickel in xylem sap from A. lesbiacum. (a) Normalised oscillatory
EXAFS amplitude (%), weighted by k3, plotted against the photoelectron
wave vector (k). (b) Associated Fourier transform (R is the distance of
scattering atoms from the primary absorber). Continuous lines are
experimental data and broken lines the best theoretical fit.
The group are now trying to track down the genes that orchestrate
hyperaccumulation. The problem with all these hyperaccumulators is that
they are slow-growing and could take many years to decontaminate a site.
The process might be speeded up if the genes could be made to work in
brassicas, including sprouts and cauliflowers (Figure 3), that grow fast
and have plenty of foliage to soak up the metal. If the researchers find
the genes responsible, they will try to transfer them into fast-growing
plants that will decontaminate soils more quickly.
Figure 3. Cauliflowers may be genetically engineered to hyperaccumulate
heavy metals.
Alan Baker, an ecologist from the University of Sheffield, is
collaborating with Smith's team, tracking down plant species that
hyperaccumulate metals other than nickel. The team is to begin work soon
on plants from the copper-mining belt in southern Zaire that accumulate
copper and cobalt. Another candidate is alpine penny-cress (Thlaspi
caerulescens), a plant native to Britain that hyperaccumulates zinc.
One problem that the researchers face is that polluted land is usually
contaminated with several metals. Over 400 different hyperaccumulator
species have been identified. Many of these plants will only be tolerant
to one or two metals and will die through exposure to the rest. This
suggests that different mechanisms exist for different metals. By
understanding the genetic machinery controlling hyperaccumulation,
Smith's team hopes to genetically engineer plants to survive this
exposure, or to mop up several metals at once. Current studies are
investigating the uptake and storage mechanisms of species that
hyperaccumulate Cu, Zn, Pb and Co.
Current decontaminating methods involved trapping the metals in the soil
to make them harmless, or removing soil from the original site and
exposing it to strong acids that leach out the metals. But the acids
kill micro-organisms, leaving the soil barren and sterile. Also, the
process costs as much as £1 million per hectare. Decontamination with
plants could be much cheaper as well as being more environmentally
friendly. **
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