Transcript Metals

HEAVY METALS
Bioremediation of mercury
contaminated soil
Forms of mercury:
1. Free Hg(II),
2. Particle-bound Hg(II), and
3. Hg(0).
• Hg(II) and Hgp have atomospheric
residence times on the order of weeks
• Hg(0) can be retained approx. 0.5 to 2
years in the global atmospheric pool.
Toxicity
• Hg(0) us readily absorbed through the
lungs and passively diffuses through other
biological tissues, but has a relatively
short residence time.
• It neither accumulates nor reacts with
most biochemicals.
Toxicity
• Hg(II) is highly reactive, but does not
freely diffuse through cellular membranes
and is inefficiently absorbed through the
intestines.
• Studies have reported that 80 to 85% of
ingested doses are excreted in feces with
the rest being eliminated over a period of
days or months through urine.
• As Hg(II) is processed and filtered by the
body, it is likely to cause damage to the
kidneys, liver, and gastrointestinal tract.
Toxicity
• Methylmercury is lethal at much lower
doses than Hg(0) and Hg(II) because it is
reactive yet highly mobile. It is efficiently
absorbed from the gastrointestinal tract
with an estimated uptake of 95% in
humans.
• It is readily transported through blood
vessel walls and into tissues.
Why are we concerned about them?
Mercury and mercurial compounds are
nonnutritive heavy metals, which are
hazardous to all biological organisms.
Basis for bioremediation
• Bacteria have developed resistance to
colonize mercury-contaminated environment:
an operon of mercury resistant (mer) genes
encoding transporters and enzymes for
biochemical dotoxification. e.g. Pseudomonas
strain K-62.
• Some plants demonstrate moderate level of
Hg tolerance. However, studies have showed
high levels of Hg-tolerance and Hgdetoxification in transgenic plants.
Mechanisms
A. The mer operon contains a series of five or more
genes that collectively confer Hg resistance to
bacteria.
• MerA and merB encode the enzymes Hg
reductase and organomercurial lyase,
respectively.
• MerT and merP code for Hg(II) transport
proteins, used by bacteria to shuttle Hg to the
cytoplasm for enzymatic Hg detoxification.
• MerD and merR encode proteins that regulate
gene expression of the mer operon.
B. Organomercurial lyase (merB) detoxifies
organic Hg by catalyzing the cleavage of
the C-Hg bond
C. Mercuric reductase (merA) reduces Hg(II)
to volatile Hg(0).
Hg(II) is the predominant form in soil, usually
as stable HgS. Hg(II) may be reduced to
volatile Hg(0) abiotically or by mer+ bacteria,
or methylated, primarily by anaerobic
bacteria.
Mer genes -transgenic plants may help extract Hg
from polluted sites either by translocating to their
shoots or evaporating as Hg(0).
A. wild-type Arabidopsis.
B. merA transformed Arabidopsis
The growth media was spiked with 50 mM Hg(II)
Tobacco plants about 2 weeks from seeds. The
growth media was spiked with 50 mM Hg(II)
Yellow-poplar, germinated on selective medium
for about 2 weeks The growth media was
spiked with 50 mM Hg(II)
Arabidopsis seeds, about 4 wks on medium
containing organomercurial salts:
a. 1mM methlmercury;
b. 1mM phenylmercuric acetate (PMA)
Hg(0) volatilization for wild-type and merA
transformed Arabidopsis plantlet
Hg(0) volatilization for wild-type and merA
transformed yellow-poplar plantlet
Hg(II) reduction in wild-type and merAtransformed tobacco plants
Example:
The magic market site in Trenton,
New Jersey: A case study
Site description:
 An old abandoned urban industrial site
that has been designated as a
“Superfund” site.
 Typical of many of the so-called
brownfield sites in NJ and across the US
 Industrial uses including Gould Inc., a
battery manufacturer.
 Toxic lead was left around the old
buildings
Problem
 Neighborhood is left to cop with the
industrial contamination left behind
when the companies moved away.
Solution
 Researchers from Rutgers Univ. devised
and created a company, Phytotech, to
implement its remediation.
Approaches
Starting with a 200 x 30-ft plot of
freshly tilled earth, they sowed the tract
with seeds of the Indian mustard plant
that grows quickly and has the ability to
accumulate lead from the soil in which
it is planted.
A “Northwest Community Improvement
Association” was established. From the
collaboration among private industries,
government agencies and local residents,
the City of Trenton expanded its
involvement in neighborhood
remediation and successfully garnered an
EPA grant for $200,000 to provide funds
for the next step.
 Mustard plants uptake and store Pb in
their leaves rather than roots. Their roots
typically reach about 20 in into the
ground, which is well below 6-8 in depth
where lead contamination usually resides.
 The plants reach maturity in about 6
weeks. The researchers were able to grow
and harvest several cycles in the garden
during the spring and summer of 1996.
 Lead levels in the soil and in the plants
were tested between each planting.
 Later in the year, they planted pumpkins
on the site and harvested pumpkins for
Halloween. All these were done very
publicly involving Trenton and
environmental agency officials and
community members.
Result
At the time harvesting pumpkin
(Oct. 22, 1996), Lead concentrations
in the soil have been reduced to
established safety levels in 75% of
the area they planted mustard plants
earlier in the year.
Phytostabilization--A superfund site
• location: in the Black Hills of western
South Dakota
• A 18-m stretch of Whitewood Creek was
contaminated with arsenic and cadmium
(surface and groundwater)
• Cause: 130 years of gold mining activity
Remediation:
• The university of Iowa planted 3100 hybrid
poplar trees on one acre of the site to a depth
of 1.6 m in old mine tailing along Whitewood
Creek in April 1991.
• A commercial N/P/K fertilizer was used to
ensure vigorous early growth of the cuttings
• Roots formed along the entire length of the
cutting in the soil, and a root mass was
established to prevent leachate generation
and percolation to groundwater
Concentration mg/kg
Concentration of As, Cd, Pb, and Zn in
subsuface soil at various soil depths.
Remediation:
• As was the element of concern according to
risk assessments at the site, but cadmium was
monitored because of its potential for uptake
and translocation to leaves, thus creating
pathway of exposure at the site.
• At the end of the first growing season, the
trees had grown to four feet in height. There
were evidence of phytoxicity of the tailings in
the field and in laboratory pot studies, but
vegetation was growing.
Remediation:
• Poplar leaves in the field did not take up very
much Cd or As (1.2 and 27 mg/kg dry weight).
• These concentrations are below most limits
(e.g. EPA 503 for biosolids) established for filed
application of municipal sewage sludge or
compost.
• Results indicated that uptake of metals by the
poplar trees was not a serious concern relative
to published guidelines and native vegetation at
the site.
• There was a weak relationship between
the total digestible As concentration in
soils at the site and the concentration
that was monitored in the stem tissue of
young poplar trees.
• There was a correlation between Zn
and Cd concentration in leaves of
poplar trees.
In general, relationships between
total digestible metal in soil (or
chelate extractable fractions) and
the concentration of metals found
in leaves were not statistically
significant.
• At the Whitewood Creek site, South Dakota,
approximately 150 poplar trees were still living
in 1995 from a total of 6000 trees planted
between 1991 and 1993.
• Overall survival rates were very low due to
possible phytotoxicity, harsh climate, and
animal browse.
• The site is rather remote, and there was not one
to maintain it on a daily basis.
• Fencing was need to prevent animal browse
(deer and cattle) that was very heavy at times.
• Severe ice storm killed many of the trees.
Conclusion
• It was difficulty to establish vegetation at
the Whitewood Creek site given multiple
stresses on the trees.
• At a second site established in 1994 at an
abandoned smelter in Dearing, Kansas,
the addition of aged manure and straw
allowed success in establishing a
phytostabilization system with greater
than 50% survival of the trees.
Processes influencing free metal ion activities in soil
solution of a phytostabilization system
• What would you do different if you
were in charge of remediating this site?
• Do you think phytostabilization is the
most viable strategy for this site? Any
other feasible remediation strategy ?